United States
            Environmental Protection
            Office of Marine
            and Estuarine Protection
            Washington DC 20460
EPA 503/3-88-001
September 1988
\U/cilfcJI	_^_____			
Linking Estuarine Water
Quality and Impacts on
Living  Resources:
Shrinking Striped Bass
Habitat in Chesapeake Bay
and Albemarle Sound



                          AND ALBEMARLE SOUND

              Project Title: Prioritization of Pollution Control in Estuaries
                Through Analysis of Temperature and Dissolved Oxygen
                             Habitat Space for Biota
                               Charles C. Coutant
                                Denise L. Benson
                          Environmental Sciences Division
                               Publication No. 2972
                                   August 1987
                                   Prepared for
                       Office of Marine and Estuarine Protection
                         U.S. Environmental Protection Agency
                                401M. Street, S.W.
                               Washington, DC 20460
                            Kim Devonald, Project Officer
                     Interagency Agreement EPA DW 89931605-01-0
                                  DOE 40-1629-85
                                   Prepared by the
                       OAK RIDGE NATIONAL LABORATORY
                              Oak Ridge, Tennessee 37831
                                     operated by
                       for the U.S. DEPARTMENT OF ENERGY
                          under contract DE-AC05-84OR21400

Although the research  described in this report has  been funded wholly or in part by  the

mOP fSSSS t  tf^T ^^ (EPAl thrUgh InteraSency Agreement No. DW89931605-01-0
t?PL?"H    ?        U.S Department of Energy, it has not been subjected to EPA review and,
therefore, does not necessarily reflect the views of EPA, and no official endorsement should be inferred

   !                                                                    	vii
RESULTS	:  ' '
   !    I.   Retrospective Confirmation of Striped Bass Upper Avoidance Temperature	   5

   i                                                                 	   8
   i   II.   Chesapeake Bay	

   i        SummerBayResidencybySubadultandAdultStripedBass .  .  . . '	

           Temperatures in Chesapeake Bay	
   i                                                                            	18
           Dissolved Oxygen in Chesapeake Bay	
   i        shrinking Habitat for Striped Bass Due to Temperature-Oxygen Squeeze	

                                                                    :	23
   i        Physiological Effects	
           Other Species	
   |        Remedial Action	
   i                                                               	28
      III.   Albemarle Sound	'
   '         striped Bass in Albemarle Sound-Roanoke River	

            A New Hypothesis for Striped Bass Decline in the Albemarle-Roanoke System	30

   REFERENCES	'  "  "


                                       LIST OF FIGURES

Sampling locations for July data in the EPA Chesapeake Bay Program computerized
Stations in the Pamunkey and York rivers and in the Chesapeake Bay monitored
for temperature	
May-October water temperatures at surface (broken line) and bottom (solid line)
in the York River estuary and adjacent Chesapeake Bay	

Chesapeake Bay and its tributaries divided into 5 nautical mile segments and showing
features prominent for evaluating striped bass habitat	

Temperature and salinity along the longitudinal axis of the Chesapeake Bay for
 May-October 1968	'




 Selected depth contours for the Chesapeake Bay	
 Average summer temperatures along the longitudinal axis of the Chesapeake Bay	    17

 Contrasting summer water temperature conditions along the longitudinal, axis of
 the Chesapeake Bay	
 Area of Chesapeake Bay bottom affected by low dissolved oxygen levels 3n the
 summers of 1950 (A) and 1980 (B)	


   I 12

   ;  14
  Dissolved oxygen concentrations with depth along the longitudinal axis of the
  Chesapeake Bay	
  Hypothesized chronology of typical seasonal changes in distributions of temperature,
  oxygen, and subadult and adult striped bass along a longitudinal axis of the
  Chesapeake Bay	
  Hypothesized cycle of reproductive impairment of striped bass due to summer
  temperature and dissolved oxygen habitat limitations	
   Map of Albemarle Sound and its major tributaries	

   Hypothesized summer striped bass distribution in Albemarle Sound .  ..





This proiect seeks to develop strategies and priorities for arresting habitat  deterioration and restoring
?  in estuaries  through identification of critical zones for  maintaining  hvmg resources   It
uses as an example one representative and important estuarme species,  the striped bass (Morone
s2tms)   DatT on summe? water temperatures, dissolved  oxygen  concentrations, and striped  bass

suitability for adults and subadults were those identified in freshwater reservoirs (<25 C and >2 mg/L
dissolved oxygen).

T^  Chesapeake Bay two  key areas were identified: (1) a zone of residual cool water  (<25C) in the

 River where warm surface waters (>25C) in summer impinge on the bottom and may block egress ol
 SedbSs  subadults and adults from the bay. Increasing  anoxia in  the bay rn recent years, especially
 ^rStodSfiSr, has reduced the  amount of suitable habitat  available.  Severe deoxygenation
 m W summer  of 1980 and 1984,  which would have affected resident adults  and newly  maturing
 subadults, is linked to record low values for the Maryland Juvenile Striped Bass Index for 1981 and U.985
 in the upper bay.  The Bay Bridge and sill  areas  are suggested as high-priority zones for pollution
 monitoring and control.

 A more limited analysis of Albemarle Sound suggests one key area, a zone  of generally deeper water in
 Ae westemSound, broadly defined at this  time as lying between Pleasant Grove and River ^Neck  but
 also possibly including parts of the Roanoke River delta.  Progressive deoxygenation of  this deeper
 wStern zoL  sTnC,  f^ mid  1970s is  suspected  from reported  algal blooms and other signs of
 SopWcation.   Most aspects  of the historical changes  in striped bass population structure, inc^g
 sever! reductions in viability of eggs spawned in the Roanoke River since 1974, are consistent with
 SSioroTSorica^  important habitat in summer and resultant physiological stresses and potentially
  enhanced toxicant exposure that affect reproductive competence.


There  is a need to systematically evaluate the impacts of water quality degradation on the biota of
estuaries and to develop  strategies and  priorities for arresting habitat deterioration  and restoring lost
habitats.  Estuaries throughout  the United States are experiencing  the pressures of increasing human
population,  including  domestic wastes (or the nutrients  resulting  from wastewater treatment); toxic
discharges;  power  plant  cooling water  use; and non-point runoff  of pesticides, acid deposition, and
fertilizers.   Notable improvements have been made in the quality of some systems [e.g., the Hudson
River,  despite continuing PCB contamination (Smith hi press)].  Other systems, such  as the Chesapeake
Bay, are exhibiting alarming trends toward progressive degradation of both water  quality and living
resources (Officer et al. 1984; Seliger et al. 1985; Boreman and Austin 1985).

Two problems continually plague implementation of good  intentions to clean up the nation's estuaries:
(1) an unclear  relationship between water quality parameters (e.g.,  temperature, dissolved  oxygen,
nutrients, chemical toxicants) and the  viability of valued populations of the living resources inhabiting
the water, and (2)  the need to  place priorities on cleanup  efforts because of limited financial resources.
The cost of a general cleanup of a whole estuarine system such  as the  Chesapeake Bay would be
immense, and thus a method for  detecting and prioritizing areas of water  quality degradation that are
most significant for populations of important organisms seems essential.

This project addresses and links  these  problems.   The overall  project seeks to  develop a method to
prioritize pollution  control  in  estuaries  through analysis of  two water quality parameters-temperature
and | dissolved oxygen-found to  be especially important for one  key estuarine apecies, the striped  bass
(Morone saxatilis).   The Chesapeake Bay, where water quality degradation and decline in populations of
striped bass are concurrent  concerns, is taken as an  initial example  for  analysis.   Preliminary
considerations toward generalizing the concepts have been  made through  study of  another estuary in
which striped bass populations are threatened, Albemarle Sound, North Carolina.

This; report presents a summary of initial results.  Analyses and conclusions are tentative  and  subject to
revision.  Nonetheless, important conclusions about linkage of water  quality  and critical zones for striped
bass are emerging.  These tentative conclusions will be refined further in subsequent work.

A recent synthesis of ecological  data  on striped bass in both  fresh and  saltwater environments has
concluded that  distribution and population declines of this  species  can be related to habitat selection
according to thermal preferences alone or in  concert with dissolved oxygen  (Coutant 1985).   The
physiologically optimum  temperature  range shifts to  lower temperatures  as striped bass grow.   The
subadult and adults of the  species are limited to zones of a water body that are sufficiently cool and
well; oxygenated  during critical times of the year, such as  summer.  The size (volume) of the thermally
suitable habitat with  sufficient oxygen may be a small  portion of the water body; thus the annual
carrying capacity of the whole system may be restricted.

A number  of direct and secondary detrimental effects have been  seen  in striped bass populations in
which adults and subadults are crowded into these "thermal refuges" in summer (Coutant 1985).   They
include  direct mortality of those that  can not find  the  refuge, increased disease due to  crowding,
deteriorating body condition throughout the summer as food resources  are exhausted, overfishing,
catch-and-release  mortality, and diminished reproductive competence of females  the  following year
(presumably due to a bioenergetic deficit during egg development).  Although evidence is not fully
conclusive that  this type of habitat restriction is important for striped bass  declines in estuaries such as
the Chesapeake Bay, the evidence is strongly suggestive that it may be a factor.

We emphasize summer habitat because during this season  suitable habitat space can be most limiting for
marly species.   Seasonal warming of  surface  waters, either  alone or in combination with density
stratification due to salinity differences, can create  thermal  zones that may match the species' thermal

niche in only limited areas or not at all.  Microbial respiration diminishes oxygen resources most rapidly
in density stratified, warm temperatures of summer. Toxic materials can be most rapidly bioaccumulated
and have their most rapid  effect in the active summer times  of feeding  and growth  of organisms.
Toxicants released into the zones of fish concentration hi summer can have  effects  disproportionate  to
those they might  have if they and the fish were dispersed throughout the water body.   Energy stores
accumulated in body tissues during  the warm seasons are often vital for maturation of gonads  in
preparation for the next year's spawning.  We feel that limitations on summer habitat space can be  as
critical for population survival as more commonly identified critical areas, such as spawning grounds.

The premise of this work is that identifying habitat limitations for key species in  estuaries in summer,
due to  temperature  preferences  and seasonal  patterns of temperature and dissolved oxygen, will be a
productive  tool for focusing attention of water quality investigations.   This focus should help reduce the
cost and effort of estuary study and cleanup to more manageable levels.

This work consisted of careful evaluation of existing reports  and data sets that may be relevant (no
original field study was included).  We sought all available historical data on  temperatures, dissolved
oxygen concentrations, and striped bass distributions in the Chesapeake Bay. We made use of original
reports whenever  possible.   The computerized  water quality data base being developed by the
U.S^ Environmental Protection Agency (EPA) Chesapeake Bay Program is a resource that we attempted to
utilize for automated  analyses, although coverage  of the bay is not uniform  in either space or time
(Fig.  1).  Recent data not yet summarized in reports are not covered adequately.  In the case of
Albemarle  Sound, we conducted a less thorough  search of the literature  and are  indebted to
Mr.! Anthony W. Mullis,  Coastal Research Coordinator for the North Carolina Wildlife Resources
Commission, Division of Inland Fisheries, for use of his unpublished data.  We consider  this report to be
interim because we are not confident that our review of available data is yet comprehensive.

We [ first sought to estabh'sh water temperature patterns in the Chesapeake Bay that could direct habitat
selection  by subadult and adult striped bass based on our results in reservoirs.  We assumed  that
juveniles would occupy shallow, warm zones  (Coutant  1985),  and they were not included in the purview
of this study.  The fisheries literature was examined somewhat  concurrently fox information on striped
bass distribution  that could be correlated with the thermal regimes. We then sought to describe the
spatial  and temporal  patterns of dissolved oxygen in the bay.  We did this, from two perspectives:
(1) characterizing the  changing pattern of summer  oxygen resources for the whole bay over the period
of record (since about 1950), which includes both high and declining abundances of striped bass (a task
that has been attempted by others), and (2)  focusing on quantitative changes in dissolved oxygen in
specific zones we estimated from  temperature analyses to be important habitats  for large striped bass.
We also attempted to use the computerized water quality data base of the EPA Chesapeake Bay Program
to quantitatively  graph seasonal and interannual changes in suitable striped bass habitat, as has been
done for  at least one reservoir (Virginia Power  1986).  Following the  Chesiipeake Bay analysis, the
process was repeated in less detail for Albemarle Sound.
A few comments on the success of the process may be fruitful.  Despite what  seems to have been a
large amount of research and monitoring on Chesapeake Bay, the water quality data relevant to striped
bass  populations are frustratingly spotty.  Reasonable hypotheses  can  be developed based on current
understanding, but  there are insufficient  data in the critical  places and times to provide rigorous tests
of them  (Heinle et al. 1982  also observed  this difficulty).  A surprisingly large amount of time was
required  to search the relevant  literature  and to retrieve reports (many of which  were  laboratory
documents  with  limited distribution).  The laudable computerized data base at the Chesapeake Bay
Program  offered  another slow and sometimes frustrating learning curve to be surmounted before useful
information could be  retrieved.  From both hard copies and computer printouts, we realized that the
available  data sets are but a sparse and discontinuous sampling of the processes believed to be relevant
and important to the  dynamic striped bass populations in the  bay.  Perhaps tliis study will provide the
impetus to monitor selected zones in the future especially well and with unbroken time sequences.

Fifties Chesopeake Bay Points
                                                                      ORNL-DWG 87-9536

                                                          Sixties Chesopeake Boy Points
                             77'   Tt'u'W   '*   TJ'WW    77-W  77
                         Seventies Chesapeake Boy Points         Eighties Chesapeake Boy Points
                        	c	t	    w>l)   =       -T	1	
Fig. 1.  SampUng locations for July data in the EPA Chesapeake Bay Program computerized data base, by
decade from 1950 to 1980.  Numbers are the last  digit of the year when samples were taken.  Locations
appearing on land are for tributaries not drawn.

I. Retrospective Confirmation of Striped Bass Upper Avoidance Temperature

Is 'the upper avoidance temperature of striped bass in an estuary the same as in freshwater reservoirs,
where  it has been  determined precisely with temperature telemetry?   Although there have been no
detailed studies  of temperature selection by  this species in any estuary, the existing,  independent
literature on water temperatures and on seasonal fish distributions can provide a reasonable retrospective
test.                                                                    !

The upper avoidance temperature for adult striped bass in freshwater reservoirs in Tennessee has been
estimated to be near 25C (Coutant 1985; Cheek et al. 1985).  The temperature range in which subadult
striped bass spent 75% of their time in a small Tennessee quarry lake in summer was 20-24 C (Coutant
arid Carroll 1980)  There was clear avoidance of relatively large volumes of otherwise acceptable water
ini summer when  they exceeded these upper temperature ranges.  The volumes of water that were avoided
included surface  layers (Lambert Quarry, Cherokee Reservoir) and main reservoir reaches when cooler
tributaries were  available (Watts Bar Reservoir).  Several  other telemetry studies  of striped  bass in
reservoirs have confirmed this general pattern of temperature selection in fresh water, although  warmer
temperatures up  to 29C have been occupied for short periods when no cool water was available (e.g
Virginia Power 1986).   The thermal niche of striped  bass has been  shown  to change with age, with
juveniles preferring about 26C, thus  creating habitat  partitioning  (Coutant 1985, 1986)   Earlier
research, primarily on juveniles, has misled our view of the habitat requirements of larger striped bass.

We tested the  upper avoidance temperature for striped  bass of the Chesapeal.ee region by examining the
York-Pamunkey River-estuary system.  There, striped bass distribution was analyzed in a 1968-69 tagging
 study by  Grant et al. (1970) and in 1967-71 trawl catches  by  Grant (1974).  Monthly water
 temperature-depth profiles  were available along the length of the system and into the  main bay tor
 1956-59 (Massman 1962) (Fig. 2).  Additional temperature data were also indicated in Grant (1974), and
 Byooks (1983a, 1983b) provided extensive data on temperature, dissolved oxygen, and salinity for 1970-80.
 The 1956-59 data  set is a useful example of the general water temperature conditions (Fig. 3). Water
 temperatures at  the surface and bottom were generally higher in the upper teaches in May-August and
 fairly isothermal or slightly  cooler upriver in September and October.  Headwater temperatures were in
 the 75% occupancy range of 20-24C in May, whereas lower reaches were cooler. As seasonal  warming
 progressed, the  upper  reaches warmed above the preferred temperature range and by July, preferred
 temperatures occurred  only in the lower reaches or in  the main bay.   The August pattern was  variable:
 in a cool year (1957) the entire York system was in the upper part of  the preferred range, whereas in a
 Warm year (1959)  all temperatures were above those preferred.  The  entire system cooled to within or
 below the preferred range in September and October.

 Brooks (1983a 1983b) confirmed that summer temperatures are above 25C most of the time in  the York
  estuary and that,  on the whole, dissolved oxygen is not a problem for fish distribution.   Values were
  almost always above 3-4 mg/L, even in summer.  Temperatures seemed to grade smoothly from  the York
  River mouth to the headwaters with no special anomalies.

  If striped bass  were  to follow the preferred temperature range through the seasonally  changing
  temperatures (e.g., crosshatched  range in Fig. 3), the fish would move up and down the estuary.
  Movement would  be to the upper reaches in May, and shift downstream hi summer while vacating the
  upper reaches  entirely.  The  striped bass would seek  refuge in the deeper parts  of  the mam  bay.
  September or  October  would see the  whole system in the preferred range, thus  allowing widespread

                              ORNL-DWG 86-9566
                            Chesapeake Bay
                                    for temperature

                                                                             ORNL-DWQ M-9SI7
     I   ' . II	1	1
   I	1	1II
                                                                           NO DATA

                                                                        L_J	I	I	1	1	1
                                               l   l  I	I	I	1	1	
                         I  I   I  I   I  I   I  .1
                                        ' *
                                                  II _ I _ I - 1 - 1
                            ~l. I  I - 1  T~
                             __ -  .. ------
                                             i   i  ii	I	III
                      l  I   I  I	I	1	1
                            L_I	I	1	L_l	
i   i  II	L_J	1	L
                      j	L
                           i_J	II	1L
  16 14 12 10  8  6 4  2    16 14 12 10 8  6  4  2

                STATION NUMBER
                       i  l   I  .1	I	1	1	L_

                      16 14 12  10  8  6   4  2
  _l__l	I	II	1	L.
                                                                           I	1	1
                                                                      16 14 12 10 8  6  4
                                    STATION NUMBER
Fie 3  Mav-October water temperatures at surface (broken line) and bottom (solid line) in the York

1980). Station numbers are shown on Fig. 2.

  dispersal.  Grant .et al.  (1970) observed  that the 2- and 3-year-old  striped bass in  his tagging  study
  appeared to move from the York River into Chesapeake Bay in warmer months, but return in the fall
  Grant (1974) found that mature striped bass caught in the fishery rarely appeared in the river in warmer
  months.  There were anomalously more striped bass in the York River in the summer of 1969- this  was -a
  year, however, when there was rapid deoxygenation in sthe main bay (Taft et al. 1980)  and the main bay
  was warm in summer and  had low dissolved oxygen in the  deep channel  (Price et al. 1985)   Habitat
  restriction in die bay in 1969 may have forced more fish to remain in the York River estuary in summer
  Massman (1962) caught few striped bass and provided no fish sizes for catches that correspond directlv
  with his temperature observations.

  To the extent possible  by matching  sketchy fish distribution data in the York estuary to more abundant
  temperature data, the general range of preferred and  avoided temperatures for larger striped bass seems
  confirmed  Further correlations among existing data sets in the Chesapeake Bay would  be desirable and
  more  confidence would  be gained through temperature telemetry studies of Chesapeake Bay' fish
  Additional confidence in  the view that estuarine stocks foUow the same temperature cues that we have
  '/JSSrr ", \eshwater st"Ped bass comes from the Connecticut River.   There,  Kynard and Warner
  S^rV1?       the aCtivity f subadult  striPed bass a* fi^ lifts was related to temperature and that
  72% of fish passage over  a  7-year period occurred from 20C to 24C. These results seem sufficient to
  accept the published upper  avoidance  temperature as the basis for a working hypothesis about iabitat
  suitability for subadult and adult striped bass in the Chesapeake Bay and Albemarle Sound  systems.

  It. Chesapeake Bay

  Summer Bay Residency by Subadult and Adult Striped Bass

 Much attention has been paid to the contribution of Chesapeake Bay striped bass to coastal waters
 where this stock has been dominant (e.g., Kohlenstein 1981) and to seasonal migratory movements.  Much
 less attention has been paid to the seasonal concentrations  of fish that remain in the bay  Summer
 records  are particularly  scarce because it is not the season of an intense commercial fishery.  However
 the literature does indicate a historical record of declining residency in the  bay as striped bass age and
 also significant summertime catches of large fish in  certain areas,  both of which may be  correlated
 retrospectively with water temperature and dissolved oxygen conditions.

 From the early tagging and recovery studies of Vladykov and  Wallace (1938) onward  it has been
 recognized that the younger  ages of  female striped bass (through about age 4) and most of the  males
 tend  to  remain in the bay throughout the year, whereas the larger females tend  to  leave. However
 departure  seems to  depend  on population density (Goodyear 1978; Kriete et al. 1979).  This density'
 dependence suggests  a limitation on the amount of suitable habitat for subadults and adults.   Goodyear
 based his conclusion  on  regressions between New York landings and young-of-the-year  densities in  the
 Maryland portion of the Chesapeake 3-6  years  earlier.  Kriete et al. found  that  when abundance is
 average, an insignificant  proportion of 2-year-olds (<3%) join the  coastal migration;  when population is
 high, more do so.

 Substantial numbers (perhaps half) of the females in the year before their first spawning at age 5 remain
 in the bay  (Kohlenstein 1981).  This observation could  be important  for successful spawning in the next
 year.  Mansueti and  Hollis (1963) concluded that the  principal contribution to natural  reproduction is
 probably from  the smaller females between 5 and  15  Ib (2.3-6.8 kg) because of their  greater relative
 abundance  compared  to those larger, even though larger fish produce more eggs per female (Jackson and
Some sites of summer residence have been suggested for subadults and adults that could be  important
spawners  in  the  following year.   The information from catch records and  personal  observations by

authors is biased, however, by preponderance of data on smaller  sizes and  ailure of some studies  to
Indicate clearly the sizes  of fish.  Vladykov and Wallace  (1952)  indicated "summer feedmg  grounds
around Tilghman, Galesville, and Rock Hall (105, 115, and 132 nautica] mfes or 195, 213 and_245 km
from the mouth of the Chesapeake Bay, respectively), and that  large fish from 6  to  15 Ib(2.7-6SI kg)
ha'd been taken by sportsmen during summer months around Rock Hall and Tilghman (Fig. 4).  Mansueti
and Hollis (1%3) cited a June-September 1962 sport fishing survey in the bay bridge area near Annapolis
(120 nautical miles or 222 km from the  mouth) that  reported many large fish as  well as smaller  ones
be'ing  caught.   About 12%  (1300 fish) were >15 Ib  (6.8 kg).  Personal communications  from current
fisheries biologists in  Maryland confirm the importance of this reach of the. bay for rt?*^
of large fish.  Coker and Hollis  (1950) noted the disappearance of large striped bass (33.5-106  cm)  from
midday off the mouth of the Patuxent River (83 nautical miles or 154  km from the mouth) in late June
during Navy detonation testing conducted between early May and late August 1925C) in summer and vacate them.  This
  generalization comes from examination of temperature-depth profiles  through the summer season m the
  |pA Chesapeake Bay Program computerized data base  and  in original reports (Brooks and Fang 1983 tor
  James River; Brooks 1983c for the Mattaponi River).

  toata for  vertical profiles along the longitudinal  axis of  the main  bay were profoundly revealing of
  temperature patterns that would guide larger striped bass.   Seitz (1971) provided what  appears in
  iretrospect to be a reasonably typical pattern for seasonal changes in warm-season temperatures. Heako
  gave salinities that, along with  temperature, strongly influence  seasonal water  column stratification
  (Hg. 5).

                                          ORNL-DWG 87-1300
  0   6  10  15   20  25

                                                                                           ORNL-DWG 87-1304
120      100       80       60
                                          DISTANCE FROM ENTRANCE OF BAY d
! 45 with temperatures >25C (after Seitz 1971).

                                                                                                            ORNL-DWO 87-1305
                          160       140
                                              120        100
                                                                            60        40
                                                   DISTANCE FROM ENTRANCE OF BAY (nJttail mite,)
                                                      1UU        80         60        40
                                                 DISTANCE FROM ENTRANCE OF BAY (nautical mil,,)
Fig. 5.  (continued).

     DEPTH (It)
g    S   S   S
                                  DEPTH (m)
                                                             DEPTH (m)
                                                                                                          DEPTH (m)
                                                                                                                                      DEPTH (m)

Hall. This cool water
adults throughout the
Pooles Island, 140

                                                     '      f
                                                                    b""8 SUbadultS
         that is seasonally
            bass hav, pted
                                                                              U a
        off fc Mouttfepao*     ^5         f ^^ " *' " ta &
  would appear to provide an effective closure of tS ,,    k    T- ?      uig m many summers
  of large striped bL, which we Sd tto oM tf  y f    Ca" *" 'he Sea^d "iSratio
  there are similarly shallow sills father S?, h    te-Pratures m excess of about 25C.  Although
  of the aorthernmost sill  I sZed hT^h  "* T/  nf'5 Cler *" ^C-  UP
vacated (warm) sul and   
Rappahinock Rivers
from the Virginia tributaries (especially dJ
stocks noted throughout the striped bas li
       confirmation, but theyldicate that
                                          \movement by upper bay fish to repopulate the

                                            SST "* ^ "" 'r660 the Pt0mac and
                                           JamesTand' ^   ??!! t13^ f Striped bass
                                         for ^th         ^     POtmaC and UpPer
25C with the sill (in terms of how hkh the tlnTf     J cuonv;ergence of temperatures above
These differences are due pardy toTamofmJ t ^ S  ^ *"* ^ lonSitudinal ^stance covered).
true interannual differences due" t^ 4S Winter S  f ^ ^ f ^ ^ but 3re most ^
solar heating,  air temperatures  ocJaTt^S^T^^^^' i**** freshwater flows,
described some of these ^c^c^rTS^ ^^T" ^r^'  SeUger et d' W
discharge of the Susquehanna  Rve r for st a ificadon ?n Th %>    ' 'T^^ f Variatins ^
Pritchard (1986). The^hermal pattern oTl%8  KS 5Tis not a   " 7" T^** by Schubd and

                                             <*-< conditions with respect to probable

                                                          ORNL-DWG 87-1302
   30 ft CONTOUR
       (9.1 m)
Fig 6  Selected depth contours (A, 30 ft or 9.1 m; B, 40 ft or 12.2 m) for the Chesapeake Bay showing
the horizontal extent of deep water in the Pooles Island-Rock Hall-bay bridge vicinity (upper  circles on
A and B)  and the relatively shallow sill that  cuts off the  deep bay channel near  the mouth of the
Rappahannock River where maximum water temperatures occur (lower  circle on B)  (after  Hires  et al.

                                                            ORNL-DWG 87 1301
                           CHESAPEAKE BAY
                              40 ft CONTOUR
                                 (12.2 m)
Hg. 6. (continued).

                                                                                    QRNL-DWG 87-1307
          924QQ 918T  908        848E
20 -
                                                            7070 657W
                                                              I     I
  I           I
3900'      3840
 3820'      3800'
                                                                       3740'      3720'     3700' N
     Fig. 7.  Average summer
     Stroup and Lynn 1963).
 temperatures along the longitudinal axis of the Chesapeake Bay, 1949-61 (after


  filled with water  of 21-24C (Fig. 8B) (Stroup and Lynn 1963).  In 1958 (Fig. 8A), the 25C isotherm
  does not appear to have reached the bottom at the sill, thus maintaining a migration access.

  Interannual variability in temperature patterns, therefore, encompasses conditions that  could either
  stimulate or prevent migration out of the bay to coastal waters and conditions that  could cause severe
  or little crowding in the residual cool water (based on temperature alone).  It appears that strong
  density stratification, as seen in 1961 (Fig. 8B), produces both especially high  temperatures  at the siU
  (and thus strong blockage) and preserves cool temperatures in the refuge. Historically, the  two thermal
  effects may have compensated for each other somewhat in maintaining suitable striped bass habitat
  Year-to-year differences in temperature distributions  and thus striped bass  distributions  may have
  iunctional significance for widely varying year-class success of this species.

  There is only sketchy evidence to  suggest a change in thermal patterns in the bay that correlates with
  drastic population declines of striped bass since the early 1970s.  The EPA Chesapeake Bay Program data
  set suggests that surface waters in the uppermost reaches of the bay may have been more consistently
  above 25 C since 1969.  Thermal power stations such  as the  Calvert  Cliffs station or those  on  the
  Potomac River might be adding  sufficient heat to affect large areas  of the bay  in  marginal years
  (although we have not examined their influence rigorously).  Priority zones to consider when  examining
  long-term temperature records to establish if there has been a  general temperature change important to
  striped bass would be the cool refuge below 15 m in the bay bridge-Pooles Island reach and the bottom
  water over the sill just north of the mouth of the Rappahannock River. The sill area  near nautical mile
  45 would appear to  be an  especially  sensitive  area  for future anthropogenic heating, such as from
  thermal electric power plants.

 Dissolved Oxygen hi Chesapeake Bay

 As suitable as the deep, cool water zone near the bay bridge seems for summer  habitat of subadult and
 adult striped bass blocked upstream of the sill, its suitability is  compromised by depleted concentrations
 of dissolved  oxygen.   Telemetry  studies in reservoirs have shown  that water masses at acceptable
 SCrai"r.eS but wth dissolved oxygen values less than about 2 mg/L will be actively avoided (Coutant
 1985).  This response, coupled with temperature selection,  produces a summer habitat "squeeze" in  which
 surface warming and  deep-water deoxygenation by microbial respiration shrink  the volume of suitable
 habitat, often to relatively tiny "thermal refuges."  One can  assume from the evidence gathered in
 Ireshwater reservoirs that striped bass occupying the central basin of the Chesapeake and, in particular
 the  cool water mass  near the bay bridge in summer will be subjected to such a  habitat "squeeze-
 Avoidance  of unsuitably warm water at the surface (or horizontally, both laterally and along the
 longitudinal  axis of the bay) and  unsuitably low dissolved oxygen concentrations  in  the deeper  water
 shrinks the habitable water volume.

 Oxygen depletion is a major feature resulting from water pollution that regulatory  agencies seek  to
 control.   Microbial activity, largely in the sediment, reduces dissolved oxygen in the overlying  water
 mass by decomposmg (oxidizing)  organic wastes and  the accumulated remains of phytoplankton
 Phytoplankton in the water is  stimulated to grow in large  numbers by  discharge and runoff of nutrients'
Although water temperatures  may not have changed markedly in  recent decades, the  degree of
 deep-water deoxygenation certainly has,  both  in lakes and reservoirs undergoing eutrophication and  in
estuaries (e.g.,  Officer et  al. 1984; Price et al.  1985).   The central basin  of the  Chesapeake Bay
experiences  some summer oxygen reduction naturally (Newcombe and Home 1938- Taft et al  1980)
However  trends that suggest  increasingly  depleted oxygen resources,  mostly in summer, have aroused
considerable concern among scientists and  water quality regulators alike (Heinle et  al.  1982;  EPA  1983-
Officer etal. 1984; Price etal. 1985; Seligeretal. 1985).

                   924QQ 918T  908
    813D 804C
                                                                              ORNL-DWG 87-1309
                                                                 744B      724R   707O 657W
                        39-20-    39=00'    38-40-    38=20-     3800"    37'40"     37:>0-   37>00' N
                    924QQ  918T  908      848E
                           914S I904N 850DI    834G      I 813D 804C    744B     724R   707O 657W
                          3920'     3900'     3840'
   3820-     3800'
                                                                     3740"    3720"    3700' N
Fig. 8.  Contrasting summer water temperature conditions along the longitudinal axis of the Chesapeake
Bay (after Stroup  and Lynn 1963).   In a  cool year (A) the 25C isotherm does not reach the sill at
nautical mile 45; in a warm year (B) temperatures there exceed 27C.  Strong density stratification seems
to produce both especially high temperatures at the sill and cool temperatures in the residual pocket.


   It is clear from the analyses by Officer et al. (1984)  and Seliger et al. (1985) that two of the  areas
   where dissolved oxygen in summer has been degraded most severely between 1950 and the present lie
   (1) in the thermal refuge" near Baltimore and the  bay bridge and (2) in the reach near the mouth of
   the Potomac River upstream of the sill (Fig. 9).  Historically, the reach just upstream of the bay bridge
   seems to have maintained high dissolved oxygen values  in spite of oxygen depletion elsewhere, although
   the data are sparse (Hires et al.  1963) (Fig. 10).  Heinle  et al.  (1982) included the refuge area in the
   zone called "heavily enriched" with nutrients,  and the sill area  in the zone  where oxygen has shown
   marked change.  Much of the deep-water zone between these two areas has also shown expansion of
   both the bottom area and the water column thickness affected by low dissolved oxygen.

   Shrinking Habitat for Striped Bass Due to Temperature-Oxygen Squeeze

   Progressive restriction of habitat for subadult and adult striped bass in Chesapeake  Bay by the combined
   effects of high  temperature and low dissolved oxygen can be seen over the decade and a half (1965-80}
   for which most data were filed on the EPA Chesapeake Bay Program computerized data set when we
   conducted these  analyses.  For  our  analysis we  illustrated the suitability of habitat  during Julv on
  schematic water columns for the standard EPA bay zones (these drawings  are  too numerous to  be
  reproduced here)  The cool thermal refuge is generally represented by data from  zone  CB-3  in which
  most data were taken from the deep channel area.  Zone  CB-4 generally represents the upper part  of
  the central basin, and CB-5 the lower part terminating about at the sill.

  Although  the  data  are sparse and discontinuous, the  pattern for zone  CB-3 shows  suitable habitat
  through a large segment of the water column.  Habitat  limitation is mostly by high temperature at the
  surface.  In zone CB-4, however,  a pronounced squeeze is noticeable in most years, intensifying  with
  tuneover the years examined.  In CB-5, there are so  few data from deep water that a pattern cannot be

 A dynamic picture of the generally concurrent  seasonal warming and deoxygenation processes as  thev
 appear  to us to generate a  habitat squeeze  for large  striped bass has been outlined for the Chesapeake
 Bay by Schubel and Pritchard (1986).  The exact pattern  will vary from year to year as numerous
 climatic and other environmental factors (mentioned in  the temperature section) vary.

 The onset of deoxygenation  in the lower layers of the  bay is  ascribed by Schubel and Pritchard (1986) to
 (1) a sharp increase in stratification following the spring freshet,  (2) a change in the thermal  structure
 from near vertical homogeneity to a condition of warmer surface  and cooler depths, which adds to the
 density differences  due to vertical differences in salinity, and  (3)  a decrease in the intensity  and
 frequency of high winds that accompanies the transition  from spring to summer.

 Timing  and duration of high Susquehanna River flow in spring  have  major impacts on  initiation of
 oxygen  conditions   Early freshets (winter) are dissipated in the bay by strong winds, and oxygen
 deplehon is delayed until thermal input  causes stratification in summer.  Late freshets (April and May)
 cause strong salinity stratification  which, augmented by rising surface temperatures, isolates bottom
 waters and speeds  the onset of low dissolved oxygen  conditions.  Intensity of the low-oxygen condition
 (i.e.  longttudmal extent and vertical  thickness of layers with low dissolved  oxygen)  depends to a
 considerable extent on the accumulated freshwater discharge in May through July.

 The duration of hypoxia  in the upper bay is also affected  by the Susquehanna River  discharge  The  end
 ot the hypoac period is associated with a weakening of vertical stratification and a downward mixing of
 higher-oxygen surface waters in the face of autumn  winds  and cooling temperatures, a process that
 normal y occurs  in September but can occur in late  August or early October.  Citing Goodrich (1985)
Schubel  and Pntchard described how the influx of fresh water in this period can strengthen vertical
Stratification in opposrtion to the forces  that would otherwise weaken it.  Overturn of the  water column
is delayed and hypoxic conditions persist until some time in October.

                                                                              ORNL-DWG 87-1657
Fig. 9. Area of Chesapeake Bay bottom affected by low dissolved oxygen levels in the summers of 1950
(Ai) and 1980 (B)  (after Officer et al. 1984).  Two  areas showing marked decrease in summer oxygen
resources are off Baltimore (upper circle on B) and between the Rappahannock and Potomac rivers (lower
circle on B) that correspond to the bay bridge thermal refuge and upstream of the sill, respectively.

            924QQ 918T  908       848E           818P
         927SS  922Y 914S 904N 850D      834G       813D 804C
                                                                                       ORNL-DWG 87-1308
                                                                              724R    7070  657W
                                                                               J	I	L
        1 JULY-3 AUGUST 1949  ,
3920'      3900'     3840'
                                          3800'     3740'      3720'    3700'N
  Fig. 10.  Dissolved oxygen concentrations with depth along the longitudinal axis of the Chesapeake Bay
                    Wm * *& * ^ ^^ Cntent extendin to the bottom m the *&** <* the

We  have  translated these  processes into influences on striped bass adults and subadults (Fig. 11).  In
spring high freshwater flows from the Susquehanna River establish a strong demsity stratification.  The
greater the freshwater flow, the more intense is the stratification.  Striped bass overwinter in the  deep
basin  oxygenated by vertical  mixing in fall and  winter and by a densuy underflow from  the coast.
Especially cold, windy winters will probably have the coolest and most well oxygenated deep water.

Vertical stratification is intensified in spring by warm riverine flows (that attract spawning striped  bass)
and solar  heating of the bay surface.  Oxygen depletion begins  in the deep  basins.   By late spring or
early summer, riverine inflows including tributaries exceed 25 C and are avoided by large striped bass
whilib  the bay surface is near  25C. The warmest  surface waters lie above the lower main basin   Low
dissolved oxygen values occur in progressively shallower depths,  and  the density return flow is largely
anSc fron?decomposition in the downstream end of the central basin.   Stnped bass  subadults and
adults begin to be squeezed vertically, and some  escape  the  main basin over the  sill. Others  move
northward toward the residual cool water.

By  a  typical  midsummer, warm (>25C) water at the  surface has impinged  on the bottom  at the sill,
 closing the upper  and middle bay to emigration.  Low dissolved oxygen has overlapped  warm surface
 water in all but the uppermost reach of the basin, excluding large striped bass.  Subadults  and adults
 trapped upstream  of the sill crowd in the refuge near the bay bridge, while those downstream of the siU
 car? still  follow the  thermal gradient  toward cooler  coastal  waters.  Declining temperatures and
 wind-induced destratification in autumn  replenish striped bass habitat in the  mam basin and tributaries,
 and the fish respond by spreading out, particularly southward. As the bay becomes colder than the
 coast, some  fish continue past the sill,  toward the warmer ocean temperatures that are now closer to

 In  this hypothesized sequence of habitat changes, two  areas are critical zones  for striped bass survival:
 the! thermal refuge near the bay bridge  and the sill.  These areas most warrant special  attention  in
 monitoring and remedial action.

 Models  of the deoxygenation process (Taft et al. 1980;  Officer et al. l^Seliger et al 1985) could be
 combined with temperature and circulation  models (e.g.,  Elliot  1976;  Goodrich 1985) to more
 quantitatively estimate the timing and extent of habitat exclusion.  Such models have been developed for
 freshwater reservoirs (Brown 1983; Brown et al. 1985).  The models could illustrate the  annual variability
 in  habitat suitability and in fish movements and concentrations caused by variable climatic factors  In
  addition to the generalized features  averaged over several weeks, one should also consider the effects ot
  the prominent tilting of density dines  by winds, and  thus  shifting of temperature and oxygen regimes
  (Carter et al. 1978).  The models could  also include  the weather-related mechanisms of induction and
  destruction of density stratification that affect striped bass habitat (Goodrich 1985).

  Physiological Effects

  The long-term trend of progressively greater deoxygenation in  the cool refuge- must increasingly  require
  subadult and  adult striped bass  to occupy waters with warmer  temperatures  and/or lower concentrations
  of dissolved oxygen than normal, with resulting  physiological stress.  Physiological stress of this  type in
  freshwater reservoirs has led to reduced reproductive  capability (Coutant 1987).  This  reduced capability
  took two main forms:   (1)  reduced percentage  of adult females capable of spawning and (2)  reduced
  survivorship of eggs and larvae after successful  spawning.  These effects were demonstrated by  6 years
  of controlled hatchery spawning of  stocks derived from reservoirs with differing water quality.  Reduced
  survivorship in the early life stages has been characteristic of reproduction  in the Chesapeake Bay in
  recent years.

                                                 ORNL-DWG 87-1311

                   LATE WINTER - EARLY SPRING      OCEAN
  COLD    ^srS7fi\nrsTT*ATff7c^T/gjrp	
                                     WINTERING STRIPED BASS
                                       .*. '.  " '
                       HEAT  WARMING SURFACE  <25C
                        ty            ^ FLOW
 T^&c<^>;#i*$y* >

          >>* -^ANOXIC
SQUEEZE ,_ '\^==-x*S'    \FLOW-3f
BEGINS  ^ ;:'A?7 - '  ,>^^/ ,  ., ,

'  , " '*">. ,^a. '*"'' ,' i       ,  J"'  '   , ,'*,''
s?7t\,^   v! ,/!.,'  ;'   '   "' Vl ^ '''//'
"*'it'1]?1'f%''%            * '/%  f    ^ ^   "*
-J- _ . * ^   " X' %    \^       Sl' / '   ^  *,
r  "'"".,  ^  i       '  ;  v   s   ,   '
          ..  SOME STRIPED BASS

                                                         ORML-DWG 87-1310

                               HEAT    >25C
                       LOW OXYGEN
        SMALL TO "
        ZONE OF <25 C
        AND >2 mg/L OXYGEN " < vV~*
        - "-.       -    ,'     NO STRIPED BASS
           ,\- \    "- ; HABITAT IN CENTRAL BASIN
          I i,        -  . .-     { >25;<2mg/LO2)
                "\:,^j  ~  ..  ^'^
                  ~\,- ?   --
                  EGRESS BY STRIPED BASS
                  >25C.VIRGINIA FISH CAN
                                              STRiPEb BASS SPREAD
                                           SOUTHWARD IN COOLED AND
                                         REOXYGENATED BAY (INCREASING
                                            ATTRACTION TO WARMER
                                                COASTAL WATER)
Fig. 11. (continued).

                                                                               = BE
 (speculated to be from the York James  H,T     mJL984Jtnd ^mxgrahon  of a different stock
 Other Species

herrings" in the bay, displayed upger afoidante ^n thl SS                        T ? *? "^

important for other species as weU.'                         important for striped bass are  likely to be
Remedial Action
                                                                                   s , 
could be b&W ,o
                                                                             rOUtl! of

                                                                                         ORNL-DWQ 87-9516
                Cycle of Reproductive Impairment Due to Summer Habitat Limitations
        Low Spawning
            (R, H)
                         Poor Egg Fertilization
Stressed Subadults and Adults
  Crowding (.25C, .2 mg/L oxygen)
  Energetic Inefficiency
  Poor Gonad Maturation
                Poor Hatch
                 (R, H, A)
                                             Evidence Key:
                                                R reservoirs
                                                H hatchery
                                                C Chesapeake Bay
                                                A Albermarle Sound
                                 > \
"i_-;|     Normal Juvenile Survival?
 '  ~ '        (24-26C preferred)
Poor Larval Survival?
      (C?, A?)
                                          Poor Juvenile Index
       Fig. 12.  Hypothesized cycle of reproductive impairment of striped bass due to summer temperature and

       dissolved oxygen habitat limitations.


  Citizen Report, Spring 1987) A shnrf SnoTT  f    (HydroQual, Inc., as reported in Chesapeake

  HI. Albemarle Sound
  Striped Bass in Albemarle Sound-Roanoke River

                                                               at the
 spo Gsl,emen (M* and Guier % and
recovered m coastal tel, The nigra^ co,stal stocks a.              '             ""

    (RIVER MILE 120)
                               WILLIAMSTON   _
                               (RIVER MILE 375)
                 NORTH CAROLINA
                 SHOWING LOCATION
                 ALBEMAHLt Suun'D
                             Fig. 13. Map of Albemarle Sound and its major tributaries.


  Once hatched, larval striped bass
  found abnormally large numbers  aui-vivmg larvae with emnrv  where
                                             in               ^othesized avoidance of warm
                                               thejeastern sound is supported by resource maps
                                              SUnd m March-May and October-December, bu

It is SUSpected that striped bass vacating * 
an4 cooler zones (Fig. 14).  The deepesparts of
Pleasant Grove and River Neck.  S
tagging studies suggest not. Resource maps
Rrfer Neck in July-October and  the deeper
32Ybridge year-round.  ArmuaUepor s
Hassler  (summarized in Hassler et al            ^
Carolina Division of Marine Fisheries) show the genend

mouth of the Roanoke

                                                      historically occupying the deeper water off
                                                       between Pleasant Grove and the Highway
                                                             y 2- to 4-lb striped bass by W. W.
                                                                   data submitted to  the North
                                                                     June-September  to be the
                                                        <*                 ^        ^ 1985

                                                         ^ western found (although  stiU above

                                                             els dropped
 The suspected historic  refuge of relatively
 increasingly less suitable  by ^*
 than it was formerly (Mulhs and Guier 1981
 nutrient levels in the Chowan River
 fertilizer plant near Tunis (that

                                                       communication). This trend is due to high
                                                                  ^      ^ ^^ from a

                                                            ^ began appearing  in the Chowan
                                                    ofpOor striped bass reproduction.   Elevated
                                               adecompose i deeper reaches,  where
 restricted by salinity stratification.
  EvidMc. d- stressW
  comes from evaluations of bony
                                                       o &*> Betted in 1980 showed many take
                                                     otolith5' to oldeiTlsl1 ta partic"lar'
  an increased occurrence of false annuh.

  Reduced reproductive competence may be the
  reside through the summer m J^ .T
  mortalities, or deteriorating ^ ^
  reduction in egg viability recorded in detail for the
                                                                       they may still occur).  The
                                                                          spawners since the mid
                                                                             eutrophic Cherokee
                                                                          reinforces the ^ of
Reproductive impairment we have proposed (Fig. 12).

              of subadu,,  and a

| summer
            and receiving and  retaining effluent materials

 developing eggs (as suggested by Guier and Mullis 1982).
    The impact of a reduction in egg viability could

    in the Roanoke River ^^^^^
    first symptoms reported (Hassler et al. 1981 ,^ Hode 1 and
    quantities ol :eggs by a normal Dumber
    viability) (Hassler et al.  1981)
                                                                              production of normal
                                                                                  ^ ^ rf ^

                                                                 but  ander  stressed conditions.
                                                                            female ^ though

                   EDENTON,    HWY
                   PLEASANT    32
                   GROVE      BRIDGE    RIVER NECK

                                                                          ORNL-DWG 87-1388

                                                                         CROAT AN SOUNDS
                                             ^P!^B^?! - - HALCL,Ng
                                               STRIPED BASS
                                               FORCED TO OCCUPY > 25 r
                                                                         OREGON INLET

   !                                            33

population  Mullis and Guier (1981)  reported an increase in the average size of striped bass caught by
Semite' Sound sport fishermen from 1977 to 1980, attributed to fewer smaller fish bemg recnuted to
the population.

With increased population pressure from reduced habitat size for subadults sod adults  in the western
"reL^more fish could be expected to remain in the previously less suitable eastern zone.  These fish
it^eSsqueezed by low oxygen to the marginal temperature conditions there  Those >***?
mc/st able to tolerate this shift would be the younger  subadults   There has m  fact bee, ^jbfl M
distribution of the "nursery area"  to  the east in recent  years (Hodel  and Baklnge 1985).  Mulhs and
Scr0981) reported a shift in both fishing effort  and harvest to more eastern portions  of the
Sbemarie Sound'area. Also, the catch has been dominated by younger fish (1+ and  2+ ages contributog
an average of 69% of the annual sport harvest).   The necessity for occupymg  water at temperatures
SoJTSLd for growth would explain the numerous false annuli reported by Humphreys and Kornegay

 BJcause both temperatures and  estuary  mixing will  vary from year to year (low mixing creating
 oStions for enhanced deoxygenation), there will undoubtedly be considerabb mterannud ^bihty ni
 habitat  space available for subadult  and adult  striped bass.   This variability wdl be reflected ui the
 vbe effects on egg viability in the  Roanoke River  the  following  springs.   Egg viabihly has ;  been
 shown to vary amonfyears  (Kornegay and Mullis  1984) although there has been  no  attempt  to test
 statistical correlations with climatic conditions.
 If 'the population problems  with  striped bass in Albemarle Sound are due  to  a
 squeeze abated by increasing deoxygenation and possible toxicants m the refuges, the, ,  i J
 that this would be the only species  affected.  Many estuarine and freshwater  species have temperature
       eleTin the same range as those of subadult and adult striped bass. They could be expected to
        Som "he same shrinking habitat.  On the other hand, some other species prefer much warmer
                and they  would likely be less affected.  In fact, all species harvested from Albemarle
        exephe waJ-water largemouth bass and white catfish were declining in 1980 (Mulhs and G^x
      )  Stocks of the anadromous alosid stocks of the Atlantic coast have been in decline over  the same
  period as the general decline in striped bass stocks (Richkus and DiNardo 1984).
  In summary, current information suggests that a critical zone of especially cool and x^enated
  necessary in summer for maintaining a healthy population of striped bass in the A bemar e Sound      ^
  That zone appears to be a reach of deeper water in the western sound, now still poorly defined.  It is
  beSmTng mcreasingly eutrophic, and its suitability as the critical summer habitat for striped  bass  (as
  deS by thermal preferences) has been reduced because of progressively greater deoxygenation during
  the period of striped bass decline.  The restricted zone of suitable habitat m summer may also be the
  site of heightened toxicant exposures.


  What guidance has this study provided for the general problem of prioritizing pollution monitoring and
  control in estuaries?  Our evaluation of habitat space for striped bass in Chesapeake Bay and Albemarle
  bound as defined by temperature and dissolved oxygen concentration  might be viewed as  simply a
  parochial effort with the environmental biology of one fish species that happens to interest the authors.

  First,  the  notion that critical zones in an estuary have a predominant influence on the "health" of the
  system seems supported, if not confirmed.  These are not necessarily the places with greatest effluent
  discharge  (as water quality engineers might suspect)  or the places where  organisms spawn (as  biologists
  usually proffer).                                                                                ^

  The next lesson is that the zones will  be recognized by interaction among spheres of investigation that
  are all too often separated.   The health of an estuary is  displayed in many ways, including declines  in
  fishery productivity, changes in distribution  patterns of important  aquatic  organisms, algal blooms
  hypoxia and anoxia, numbers and kinds of pollutant  discharges, and effluent toxicity.  Specialization has'
  led to creation of discrete disciplines that follow each of these  topics with  little reference to others
  The relevance of other disciplines such as hydrography and sedimentary geology goes unexplored   The
  result is perpetuation of mystery rather than elucidation.  Yet understanding the suitability of an estuary
  for continued population success of one representative  fish species is  shown here  to depend  on a
  blending of  data gathered by all of the disciplines-water quality (temperature, oxygen), hydrography
  (seasonal water flow), sedimentary geology (coastal sediment import and sill development), fish physiology
  (temperature preferences), fisheries biology (seasonal behavior of the fish  in the field)  and so forth   It
 is not sufficient for pollution control  agencies to concentrate on  the traditional physical and  chemical
 tools of the water quality trade.

 Third,  water quality measurements can be used to define  the changing extent of suitable habitat in an
 estuary, not simply to  show values on  a tolerance scale. The volume of water having  suitable  qualities
 and its spatial and temporal distribution  (and  the reasons for changes in volume) constitute especially
 important information for resource management.

 Fourth, many old data sets can be very useful  for  establishing patterns when there is  a hypothesis
 against which to compare them.  Elaborate computerized data management systems may not be the most
 useful  for  recognizing habitat trends,  especially when their organization is  not,  or only poorly
 consistent with natural estuary subdivisions.

 Fifth, prioritization implies having  a management  objective.  "Cleaning up the  bay is not a sufficiently
 clear objective to guide meaningful monitoring  or control efforts when  financial and human resources to
 accomplish  it are necessarily limited.  Priority  areas can be defined when  the analysis becomes specific
 enough to specify the requirements of the most valued components  of the estuarine system, usually the
 Irving resources.                                                                              *

Sixth, the importance of temperature or, more correctly, the seasonal thermal structure of an estuary for
controlling important biological responses  is emphasized as a feature worthy of general attention  The
seasonal distribution of organisms in relation to thermal structure appears to set the background against
which maintenance of  other habitat  requirements  (e.g., needs for dissolved oxygen, toxicity tolerance)

    i                                           35                         ;
Last,' this analysis has tentatively identified critical zones in two major estuaries and offers promise that
m the critical zones in Chesapeake Bay and Albemarle Sound may be further refined for benefit of all
cool^ater Secies and (2) a similar evaluation in other estuaries using the temperature and  oxygen
requirements of indigenous species and broad knowledge of the estuary's geography and hydrography may
be similarly productive.


  Boreman J  and H. M. Austin.  1985.  Production and harvest of anadromous striped bass stocks along
       the Atlantic coast.  Trans. Am. Fish. Soc. 114:3-7.                                               &
       197lon       pM.       data rep0rt' TemPerature>  ^*y,  dissolved oxygen.
       1971-1980.  Data Report No. 19. Virginia Institute of Marine Science, Gloucester Point, Virginia  38
       pp + appendices.                                                                     &
          +a  endes
                                            Water data reprt TemP^ure, salinity, dissolved oxygen.
                                             ^^ f ^^ Sdene' Gloucester Pomt> Virginia  40
  Brooks, T. J.   1983c.  Mattaponi River slack water data report.  Temperature, salinity, and dissolved
       oxygen.  70-1980.  Data Report No. 21.  Virginia Institute of Marine Science, Sou^Ler Point,
       Virginia. 36 pp + appendices.                                                                 '

  Brooks, T J  and C. S  Fang  1983.  James  River slack water data report.  Temperature, salinity, and
       dissolved oxygen  1971-1980.  Data Report No. 12, Virginia Institute of Marine Science, Gloucelr
       roint, Virginia. 38 pp + appendices.

  Brown, R  T  3383  Modeling the effects of aquatic weed loads on Cherokee  Reservoir dissolved oxygen
       Rept. No. WR28-1-12-102, Tennessee Valley Authority, Knoxville, Tennessee.

  Brown, R  T., G. E. Mauser, M. K. McKinnon, and M. C.  Shiao. 1985.  Two dimensional water quality
      modeling   of Fort Loudoun Reservoir.   Rept.  No. WR28-1-10-100, Tennessee Valley Authority
      JfcnoxviUe, Tennessee.                                                                       "
 Carter, H H., R. J  Regier, E. W.  Schiemer,  and J.  A. Michael.  1978.  The  summertime vertical

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                                  *U.S. Government Printing Office : 1988 - 516-002/80235



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