vvEPA
United States
Environmental Protection
Agency
Environmental Research
Laboratory
Corvallis OR 97330
EPA 600 3 78 083
August 1978
Research and Development
Use of Small
Otter Trawls in
Coastal Biological
Surveys
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ECOLOGICAL RESEARCH series. This series
describes research on the effects of pollution on humans, plant and animal spe-
cies, and materials. Problems are assessed for their long- and short-term influ-
ences. Investigations include formation, transport, and pathway studies to deter-
mine the fate of pollutanfe and their effects. This work provides the technical basis
for setting standards to minimize undesirable changes in living organisms in the
aquatic, terrestrial, and atmospheric environments.
This document is available to the public through the National Technical Informa-
tion Service. Springfield, Virginia 22161.
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EPA-600/3-78-083
August 1978
USE OF SMALL OTTER TRAWLS
IN COASTAL BIOLOGICAL SURVEYS1
by
Alan J. Mearns
and
M. James Allen
Southern California coastal Water Research Project
1500 East Imperial Highway
El Segundo, California 90245
Grant No. R801152
Project Officer
Richard C. Swartz
Marine and Freshwater Ecology Branch
Newport Field Station
Corvallis Environmental Research Laboratory
Newport, Oregon 97365
Corvallis Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Corvallis, Oregon 97330
Contribution No. 66, Southern California Coastal Water
Research Project.
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DISCLAIMER
This report has been reviewed by the Corvallis
Environmental Research Laboratory, U.S. Environmental
Agency, and approved for publication. Approval does not
signify that the contents necessarily reflect the views
and policies of the U.S. Environmental Protection Agency,
nor does mention of trade names or commercial products
constitute endorsement or recommendation for use.
11
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FOREWORD
Effective regulatory and enforcement actions by the
Environmental Protection Agency would be virtually im-
possible without sound scientific data on pollutants and
their impact on environmental stability and human health.
Responsibility for building this data base has been
assigned to EPA's Office of Research and Development and
its 15 major field installations, one of which is the
Corvallis Environmental Research Laboratory.
The primary mission of the Corvallis laboratory is re-
search on the effects of environmental pollutants on
terrestrial, freshwater, and marine ecosystems; the be-
havior, effects, and control of pollutants in lake systems;
and the development of predictive models on the movement
of pollutants in the biosphere.
This report presents a review of techniques for using
otter trawls to survey demersal fish and invertebrate
populations. The health and abundance of these populations
is often used as an indicator of the biological effects
of marine pollution.
A.F. Bartsch, Director
Corvallis Environmental
Research Laboratory
111
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ABSTRACT
Ecological surveys using small otter trawls provide useful and
informative data on demersal fish and epibenthic macroinverte-
brates of coastal soft bottom areas.
This report presents recommendations for selecting and using
small otter trawls in coastal biological surveys and suggests
methods for handling catches and processing data.
Use of small trawls in monitoring surveys is an adaptive use
of their original purpose in commercial fishing. Many inves-
tigators have made effective use of small trawls in ecological
surveys and some of this work is reviewed.
Nets ranging in headrope length from ten to sixteen feet are
recommended for shallow waters, estuaries, lagoons and aboard
small boats; twenty-five foot nets are recommended for open
coastal areas.
Use of the gear, accessory gear and care and trouble-shooting
procedures are described. Some aspects of survey design, data
summarization and data analysis are reviewed. The report covers
a period from March 1975 to March 1976; work was completed May 1978
This report was submitted in partial fulfillment of EPA Grant
No. R801152 by the Southern California Coastal Water Research
Project, El Segundo, California, June 1975.
xv
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CONTENTS
Page
FOREWORD iii
ABSTRACT iv
LIST OF FIGURES vi
ACKNOWLEDGEMENTS vii
SECTIONS
I CONCLUSIONS 1
II RECOMMENDATIONS 2
III INTRODUCTION 3
Applications of trawl surveys 3
Limitations of gear and procedures 4
Criteria for recommending gear and 4
procedures
IV SAMPLING 6
Sampling objectives 5
Trawl sampling gear 5
Types of nets 6
Accessory rigging 9
Running gear 10
Shooting and retrieval 10
Care and troubleshooting 12
Handling, sorting and processing the catch 14
Abiotic data 17
Sampling stations and grids 18
V DATA SUMMARY AND ANALYSIS 22
Data summarization 23
Sampling variables 23
Biological variables taken from individual
organisms 23
Biological variables at the sample level 23
Physical variables 23
Diversity 26
Summary statistics 27
Analysis and reporting 28
BIBLIOGRAPHY 31
v
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FIGURES
NO. Pac?e
1. Basic features of a small otter trawl 8
under tow
2. View of an otter board 13
3. Examples of histogram-form field data 16
sheets for recording size of a species
4. Station grids for trawl surveys in three 20
coastal areas
VI
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ACKNOWLEDGMENTS
Biologists, field personnel and skippers from many local
public and private agencies contributed useful comments and
suggestions toward this report and the recommended procedures.
We particularly appreciate the assistance and collaboration
of field staff from the Sanitation Districts of Los Angeles
County, Los Angeles City, and Orange County. Mr. Jim Willis,
netmaker, Morro Bay, California, contributed important sug-
gestions and gear modifications. Messrs. Harold Stubbs and
Michael Moore, of the Coastal Water Research Project, are due
particular thanks.
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SECTION I
CONCLUSIONS
One of the first questions asked by the public about marine
pollution studies is "how are the fish?" Thus, ecologists sur-
veying coastal marine communities should become familiar with
methods for surveying epibenthic fish and invertebrate popu-
lations .
Small otter trawls have been used by a number of investigators
to survey abundance of bottom fish, crabs, shrimp, and other
larger invertebrates. While there may be considerable quanti-
tative variations among catches, statistics from entire surveys
do show general trends and can be useful contributions to an
overall ecological assessment.
Diseased and abnormal fishes occur in some coastal areas. Sur-
veys using small otter trawls are a useful way of making a
quantitative assessment of disease types and frequencies.
There are no published standard procedures for using small otter
trawls in coastal survey assessment and the effect of variations
in gear, gear use procedures and gear trouble shooting are not
well understood. This report summarizes how to use small otter^
trawls, how to identify when gear is not working properly and
how to make adjustments frequently needed aboard ship.
There are also no standard procedures for summarizing data from
small otter trawl surveys. Therefore, this report
recommends some methods that have proved useful in southern
California coastal surveys.
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SECTION II
RECOMMENDATIONS
The objective of otter trawl sampling should be to assess in a
standardized fashion the relative abundance, diversity, and
health of all available fishes and larger invertebrates living
on or near the bottom. The objective does not include catching
as many fish as possible; generally 200 to 1000 animals repre-
senting 20 to 30 species are more than adequate to assess domi-
nant biological characteristics of samples.
Single-warp otter trawls ranging in headrope length from ten
to sixteen feet are recommended for shallow waters, estuaries,
lagoons and aboard small boats; twenty-five foot nets are
recommended for deeper open coastal waters. Body mesh of the
nets should not exceed 1.5 to 2.0 inches, stretch mesh and the
cod-end should be fitted with fine-mesh liners (0.25 or 0.5 inch,
stretch measurement) to retain juvenile fishes. Trawls should
be fitted with chain or lead weights on the foot rope, attached
to a pair of doors or otter boards fitted with steel shoes and
towed by a pair of swivel-mounted bridles that are three times
the headrope length,
Towing time and rate should be standardized, e.g., 10 minutes
on bottom time at 2.5 knots along isobaths. Trawl scope ratios
should range from 5:1 in very shallow water (5 to 20 meters) to
2:1 in very deep water (700 to 900 meters).
Recommendations for setting gear, hauling, retrieval, and
trouble-shooting are given in the text together with methods
for quickly sorting and measuring catches and summarizing
data.
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SECTION III
INTRODUCTION
The use of small shrimp or otter trawls for assessing
demersal fish and invertebrate populations is an important
and informative addition to other kinds of water quality and
faunal surveys. This report summarizes criteria for the use
of otter trawls for sampling coastal areas and suggests guide-
lines for collecting and interpreting data from otter trawl
surveys.
APPLICATIONS OF TRAWL SURVEYS
While benthic infaunal surveys (e.g., Holme and Mclntyre
1971) provide a quantitative assessment of local ecological
effects, trawl surveys have the potential for relating local
patterns of the health, diversity and abundance of bottom
fish to larger-scale regional trends. Trawl surveys also
deal more directly with organisms of economic importance
(e.g., shrimp, flatfish) and with a properly experienced
staff,organisms captured by trawl can be readily identified,
examined and returned overboard in the field, thus minimizing
laboratory effort and maximizing analysis of data in a short
period of time. Trawl surveys do not replace other kinds of
biological surveys, but they do add to the total ecological
description of a coastal fauna.
Trawls were developed empirically through many years of
fishing experience. They have been designed to select,
capture and retain various kinds of organisms in commercial
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quantities. Thus their use in ecological monitoring is
relatively new and is developing rapidly so that choice
of gear and gear use procedures remain an empirical problem
based on each new experience. The papers selected for the
bibliography of this report reflect the variety of gear,
use procedures and objectives, and findings resulting from
recent coastal otter trawl surveys. The latter include
assessment of pollution-related diseases, assessing background
data on communities and describing general effects of pollution
on fish populations.
LIMITATIONS OF GEAR AND PROCEDURES
As quantitative sampling devices, otter trawls are not free
of insufficiencies and biases. Efficiencies of capturing
and retaining animals are very low, probably on the order of
10 to 50 percent, depending on the abilities of various
organisms to avoid gear or pass through the webbing, or on
towing conditions, sea state and weather. And, with the
exception of major fishery resource surveys, trawl gear and
gear use procedures for monitoring have never been standardized;
agencies involved in monitoring surveys use different sized
nets, with different mesh sizes, different towing speeds,
towing times and retrieval procedures. This does affect the
regional value of data if gear and gear use procedures are
not adequately defined in reports from surveys.
CRITERIA FOR RECOMMENDING GEAR AND PROCEDURES
The guidelines suggested below are based on our own five
year experience in sampling the open coastal shelf of southern
California with a variety of otter trawls, agencies and ships.
Additional literature was consulted to evaluate experiences
in other environments.
Use of the techniques recommended should provide a general
informative semi-quantitative data base for describing the
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health, diversity and abundance of benthic fishes and inver-
tebrates not readily quantified by other methods. Experience
with the methods will show where gear and gear use modifications
need to be made to conform to local conditions, terrain and
weather.
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SECTION IV
SAMPLING
SAMPLING OBJECTIVES
The objective of otter trawl sampling is to assess in a
standardized fashion the relative abundance, diversity and
health of all available fishes and invertebrates living on
or near the bottom. This objective can only be met if
variations in gear and gear use procedures (towing time or
distance covered, speed) are minimized by adopting stan-
dardized gear and ocean survey procedures.
The objective does not include attempting to catch as many
organisms as possible; experience indicates that individual
catches on the order of 200 to 1000 animals (fishes and
invertebrates) representing 20 to 30 species are more thar.
adequate to assess the dominant biological characteristics
of the samples.
TRAWL SAMPLING GEAR
A trawl is a cone-shaped bag of netting towed in the water
column or on the bottom by one or two vessels. The term
otter trawl, or otter board trawl, is generic and separates
nets with variable mouth openings from fixed opening gear,
such as beam trawls.
Types of Nets
Although there are a variety of commercial otter trawls
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available, those used most frequently for monitoring surveys
are small (10 to 40 foot openings) and adapted from commercial
gear; shrimp trawls and trynets are the most frequently used
and are the main subject of this discussion. Beam trawls may
be valuable in some shallow water situations and for accurate
quantification of juvenile flatfishes (Holme and Mclntyre
1971) but are generally less efficient for larger fishes,
shrimp and other animals active just off the bottom. The
advantage of beam trawls is that their opening remains con-
stant.
Important features of a rigged otter trawl are shown in
Figure 1 and are described in more detail by Hodson (1967)
and Garner (1962). Most available otter trawls include a
set of otter boards or doors which when attached to net-
wings by each pair of leg-lines, serve to spread the net
during towing. The top leading edge of the net (headrope)
is held up by a row of four to eight floats, and the lower
leading edge held down by a foot-rope chain or lead weights
which are closely attached to the net. The cod-end, which
retains captured fish and invertebrates, should be fitted
with a 1/2 or 1/4 inch stretch mesh liner. Garner (1962)
presents a useful detailed glossary of terms and gear.
Single warp (i.e., towed by a single cable, Figure 1) otter
trawls ranging in headrope length from eight to sixteen feet
(about 3 to 5 meters) are recommended for use in shallow
coastal waters (e.g., to 50 ft. deep), estuaries, tidal
lagoons and bays. Nets of this size range are also suitable
for use in small boats (14 to 25 feet).
For general use in open coastal waters, 16-, 25- or 30-foot
single warp otter trawls are recommended. Body mesh of the
nets should not exceed 1.5 to 2 in. and the cod end (Figure 1)
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otter board*
(etc
>pe faith floats)
footrope
1 Mith
00
(with liner)
bridles
Figure 1. Basic features of a small otter trawl under tow.
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should be fitted with fine mesh (0.25 or 0.5 in.) liners to
retain juvenile fishes.
Accessory Rigging
Most otter trawls are fitted with a heavy chain or l->ad W3ights
on the foot rope. The function of this added weight is to
keep the bottom leading edge of the net on or near the bottom
during towing. Nets with weights on the foot rope tend to
sift through bottom sediments thus capturing large quantities
of starfish, sand dollars and sea urchins when they are
present. Chain-rigged nets are more versatile and can be
adjusted to avoid this "epibenthic" fauna or to capture and
retain it. The investigator should be aware of these con-
siderations in planning his sampling program and objectives.
Otter boards also have special attributes and require exper-
ience for proper adjustment and rigging. The boards should
be fitted with a "shoe" (i.e., a heavy iron strip covering
the leading and bottom edge). Boards fitted with four rather
than one or two towing chains are also recommended since these
can be individually slackened or tightened to adjust the angle
of attack of the boards. These adjustments are described in a
section below.
The trawl should be attached by a pair of bridles to a three-
way swivel on the end of the towing cable. The length of
the bridles should be about three times the headrope length (and
not less than two) for proper spread of the otter boards.
Steel or stainless steel towing cable (1/8 to 3/8 in.) on a
hydraulic winch is recommended over nylon or polypropylene
line for towing and retrieving the trawl. The maximum length
of towing cable required aboard the boat should be at least
four and preferably five times the maximum depth anticipated.
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The "scope ratio" (ratio of cable plus bridle length to depth)
during towing decreases with depth. For very shallow water (5 to
20 meters) the ratio should be 4 or 5:1; at coastal shelf depths
(20 to 50 meters) a scope of 4:1 may be the maximum needed.
Beyond 50 meters scope ratios may decrease as follows: 3 to 3.5:1
at 100 - 200 meters; 2.5 to 3.0 at 200 to 350 meters and 1.8 to
2.1:1 at 700 to 900 meters.
Running Gear
In addition to a hydraulic gasoline or electrical winch with
sufficient cable, other shipboard equipment should include a
davit and pulley for assisting retrieval of heavy catches, a
live box with flowing water for holding and sorting fishes,
a sorting table and a recording fathometer for monitoring
depth changes and obstructions. Some accessory gear for a
small boat is described by Baldwin (1961).
Shooting and Retrieval
Methods of releasing ("shooting") and retrieving a trawl are
important to proper functioning of the gear. Prior to shoot-
ing, the ship skipper should make one pass over the desired
trawl site, watching for depth changes, obstructions, and
fish targets. If the site is appropriate, the vessel should
move a sufficient distance from the starting point so that
the net will touch bottom at the desired location after shoot-
ing and lowering the gear.
The net is laid out on the stern deck in the form it will be
shot. It should be checked first for twists, snags, or broken
twine/floats. The boards, placed in normal fishing position,
should be shackled to the leglines, then to the coiled bridles.
Care need be given to avoid crossing any portion of the star-
board leglines and bridle with the port-side lines. A 4 to 5-
foot fine twine rope can be used to secure the cod end. A
quick release knot is best, with at least three bindings.
10
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The net is placed in the water as the vessel approaches sta-
tion start location. Two deck hands, standing to the outside
of the bridles, launch the boards on notice from the skipper
while the vessel is in motion. By carefully metering out the
bridles, the boards can be encouraged to spread on the surface.
The net should show the configuration of the rig and possible
torquing or alignment problems while on the surface before
cable is let out. If excessive torque or twists are observed,
retrieve the net and adjust or replace it.
Scope (length of cable out T depth) should be calculated
beforehand and called out to the winch operator as the trawl
rig descends. If available, an angle-measuring device can be
used to check the wire angle and to verify scope.
On deck, while trawling, the winch operator should report any
unusual tension or slack in the cable while the skipper should
be asked to record time and date and station number, prefer-
ably on a recording fathometer read-out. The skipper signals
the winch operator when the appropriate on-the-bottom time is
up and records the time. The winch operator should haul in
the net while the boat is advancing at trawl speed.
The otter boards should break the water together but not be
twisted together. Retrieving the trawl gear will vary with
vessel equipment. If more hauls are to be made, then care
should be given to stacking the bridles, boards, and net neatly
for the next station.
Consult Hodson (1967) for additional information on shooting
trawl nets.
11
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Care and Troubleshooting
After hauling out the last catch of the work period, the
boards can be removed with the bridles. Wear on the board
shoes should be observed; i.e., the shoe wear pattern should
verify that each door was tilted forward and "toeing in" when
riding over the bottom (Figure 2). Bridles should be tied
together on a cleat to check that their lengths are equal.
Unequal stretching can cause torquing or tangling of the net
and impair fishing configuration.
The net can be tied off on the stern to wash during return to
port. At dockside, it should be rinsed off with fresh water
and laid out in a safe, secure place to dry.
Leadline, floatline and meshes on the net should be checked
occasionally for slippage of knots and tears. Repairing rips
and holes should be done when observed by cutting rough edges
off a tear and sewing in a square patch of appropriate webbing
over the hole. Drawing tears together without using patch-
webbing can tighten the net excessively in one spot causing
distortion (see Hodson 1967).
Broken floats should be replaced with new plastic floats sewn
onto the headrope by an additional loop of rope and not directly
onto the headrope itself. Floatation varies with headrope
length; a 25-foot net is generally equipped with five to six
floats concentrated toward the center of the headrope.
If the boards are not spreading properly or not attacking the
bottom at the correct angle, they need adjustment. The adjust-
ment could be in changing legline length between the legs and
doors or by adjusting chain lengths on the boards. The follow-
ing chain adjustments have been found useful for 25-foot nets
(Figure 2) :
12
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Starboard Bridle
Front Top Chain-*
Back
- Bottom
\ Chain
Front
«- Bottom
Chain
Back Top Chain
Shoe
Shoe Wear Pattern
Figure 2. View of an otter board.
on shoe.
Note wear pattern
13
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At normal trawling speeds (1.5 to 3 knots), the tendency should
be for the doors to "toe" or tilt forward. If the door is not
digging enough (i.e./ shoe bottom is wearing equally from front
to back), take up one link on the rear bottom chain. If still
not steep enough, slack off one chain on the top front chain.
If the board is "riding-on-toe" (tilting too far forward, thus
raising the legline and footrope off bottom), take up on the
top legline rope about two inches per adjustment. If the board
is angling too sharply backward (heeling, e.g., shoe wear is
all in the back of the shoe), slack off on the top legline or
take up slightly on the bottom legline. Wear on the shoe can
be readily observed by spraying the dry shoe with flat-black
fast-drying paint and examining wear pattern after retrieving
the doors.
The spread of the net can often be observed as it leaves or
approaches the surface. An estimate of actual door-to-door
spread can be made during surface towing by calculation from
measuring the angle of bridle spread, or the distance between
bridles at a known distance from their juncture (Mearns and
Stubbs 1974). Ketchen (1951) provides additional methods.
HANDLING, SORTING AND PROCESSING THE CATCH
The cod-end, containing most of the catch, is brought aboard
last. It may be weighed using a spring balance on a davit to
obtain a rough estimate of total weight and then opened and
the contents shaken carefully into a large sorting box or a
live tank. Care should be taken to remove all organisms that
are stuck or gilled in the net before dropping the trawl on
the next station.
Animals should be roughly sorted by obvious species or higher
taxons into large buckets partially filled with seawater prior
to identification, counting and measuring. This helps to keep
14
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the organisms alive and also aids in sorting as they do not
stick to each other. Fine sorting into species can then be
done prior to measuring.
A minimum of two technicians is required to identify, count
and measure fishes and large invertebrates, if this is to be
done in the field. Generally, two teams of two to three
people each (one or two identifying, measuring and counting
and one recording) are desirable, one group for fish and one
for invertebrates.
Where rapid identification is possible, technicians can simply
and consecutively measure individual fish (to board standard
length, B.S.L. or total length, T.L.), shouting out measure-
ments to a recorder. Either the actual length of a fish to mm
or a mark in a histogram data sheet (Davenport and Harling
1964 or Figure 3) can be entered and the total count for a
species tabulated later (Anderson 1964 describes methods for
selecting size intervals). During measuring, fishes should
be examined for ectoparasites (including those in the gill
cavity), deformities, tumors, frayed or apparently diseased
fins, color variations and gross structural deformities (Fig-
ure 3). If specimens are to be saved for histopathological
examination, they should be preserved in 10 percent phosphate
buffered (pH 7.0) formalin together with a few apparently nor-
mal specimens from the same catch. Often, fish will show some
external signs of bearing eggs or larvae or may have regurgi-
tated food. Notes should be made from these observations.
Again, specimens may be quick frozen or preserved in formalin
(as above) for future examination of reproductive state or
stomach contents. If this is to be done, it is advisable to
preserve a range of sizes of a species; fishes larger than
three to four inches should be slit along the lower right side
of the abdominal cavity (by convention, symmetrical fishes are
usually photographed on their left side) to allow preservation
of the viscera.
15
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Figure 3. Examples of histogram-form field data sheets
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16
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Fishes and invertebrates to be used for analysis of trace ele-
ments or other pollutants should be gently washed in clean
seawater and frozen in labeled packages (aluminum foil is used
for specimens intended for hydrocarbon analysis; plastic bags
for those used in metal analyses). Location, date, depth,
species, time of capture and the recorder's initials should be
on the package labels. Contamination is an important problem
with many trace material measurements and experts on this sub-
ject should be consulted.
Invertebrates such as shrimp can be measured in the field
(e.g., carapace length), but they may consume shiptime if abun-
dant. In such cases, the animals are returned to the labora-
tory for identification and analysis. Often a variety of small
invertebrates attached to rocks, cans and bottles, wood debris,
and kelp holdfasts are encountered and should be preserved for
later examination; the occurrence of such materials should be
recorded since they may explain anomalous or apparently rare
invertebrate species.
In the absence of a qualified taxonomist, appropriate taxo-
nomic keys and field guides for marine organisms found in the
survey area should be acquired before the survey. Species
lists from previous surveys in the area can aid considerably
in learning to identify the organisms.
ABIOTIC DATA
Information on trawl conditions (success of tow, sea state,
weather and wind, occurrence of jellyfish, flotsam, etc.)
should be noted together with date, time of tow, direction of
tow, station location, number, and depth, trawl speed, and net
type on a master log for each cruise.
17
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At a minimum, a measurement of bottom and surface water temper-
atures should be made at each trawl location during the tow
(using a calibrated bathythermograph, BT) or after the tow
using a BT or a series of reversing thermometers. Surface and
bottom water dissolved oxygen is also an important parameter
to measure at each trawl site. Oceanographic accuracies to
four or more significant figures are not necessary for these
parameters unless needed for interpretation of current flow
and structure. In open coastal areas, salinity is not impor-
tant, except as an aid in interpreting identification of water
masses (in which case oceanographic accuracy is required).
However, in estuarine situations, bottom and surface water
salinities (or through the water column) may be vital in inter-
pretation of species distributions and patchiness.
SAMPLING STATIONS AND GRIDS
Several factors must be considered prior to determining the
location and number of stations to be surveyed:
1. Size and topography of the survey area, especially
knowledge of obstructions, reefs and other hazardous
objects.
2. Depth contours and maximum and minimum depths of
the survey area with particular attention to submarine
canyons and peninsulas.
3. The location of "control" sites that are not expected
to be influenced by existing or proposed discharges or
structures.
4. The occurrence of similar surveys in adjacent areas
or regions, past surveys, and the sampling methods employed.
5. Human and material resources for the survey.
Generally, eight to ten otter trawl samples can be taken on the
open coastal shelf in a normal field working day with a crew
of three to four technicians, one or two deck hands, and a
18
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skipper. Limiting factors include maximum depth desired,
gear wear, and sea state conditions.
Depth (and depth-related hydrographic features) is probably
the single most important factor affecting the distribution
of demersal fish in open coastal areas that have relatively
steep slopes. Thus, coastal survey grids should be designed
so that sampling effort is divided equally among three or
more depth intervals in both control and test areas. In
shallower estuaries, distance from river or discharge sources
and tidal fluctuations may be more important as well as day-
night differences in the distribution of animals (e.g. Dahlberg
and Odum 1970; Roessler 1965). Thus, the survey grid should
include a transect of three or more stations located at inter-
vals between freshwater sources and the mouth of the estuary.
Sampling the same sites at night as well as during the day
will be required in these habitats.
For a good survey grid at a coastal outfall or ocean dumping
site (e.g. Carlisle 1969), locate stations according to at
least three depth intervals (for example, 30, 60, and 120 feet)
and along at least three transects laid perpendicular to shore
or to depth contours (e.g. Carlisle 1969). An example is
shown in Figure 4. All tows should be made along, rather than
across, depth contours to avoid integrating animals from dif-
ferent depth zones.
Annual and seasonal changes are very important to document in
any kind of fish survey work (e.g. Dahlberg and Odum 1970;
Gallaway and Strawn 1974; McErlean et al. 1972). Thus, a sur-
vey site should be resampled for at least a year at maximum of
three-month (quarterly) intervals; sampling more frequently
will aid in refining seasonal trends in abundance, diversity
and disease incidence.
19
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LOS
ANGELES
COUNTY
SOTS'
Santa Monica Bay
f/f'30'W
Angeles County)
Mites
~
X Overflight
Station
Orange Countu
t
/
Treatment Vfent
/-<&•; Newport Beach
Figure 4. Station grids for trawl surveys in three coastal
areas.
20
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For coastal surveys, a single sample per station will gener-
ally suffice to document the common and dominant species and
species associations. Replicate samples may show great vari-
ability with respect to total numbers of fishes or inverte-
brates, but will generally confirm the presence or absence of
the common resident species (Roessler 1965; Clark 1974). For
trawl surveys, then, the focus is generally not on estimating
absolute abundances but on documenting presence and absence
of species, relative abundances of species, size distributions
and frequencies of anomalous organisms. In addition, repeated
trawls over the same site within a few hours or days may itself
affect the distribution of organisms. Rather than replication,
we recommend additional effort be devoted to sampling additional
stations until general aspects of the fauna are known.
Completion of a preliminary trawl survey prior to establishing
a long-term monitoring program is well worth the effort; the
kinds of organisms to be expected can be documented and their
identification learned, the suitability of sites for trawling
can be assessed and staff can become familiar with gear and
gear use procedures.
21
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SECTION v
DATA SUMMARY AND ANALYSIS
Data on a number of sampling, biological and abiotic (chemical
and physical) variables will have been collected for each sam-
ple at the end of a trawl survey. The methods for summarizing,
analyzing and reporting these data vary with the objectives of
the surveys. Some recommended procedures are described in this
section. Papers cited in the Bibliography should be consulted
for additional suggestions on describing and reporting data.
22
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DATA SUMMARIZATION
It is extremely important to summarize data in tabular forms
which will allow ready access to biological and sampling vari-
ables. The variables of importance include:
Sampling Variables
a. sample location characteristics: longitude, lati-
tude , depth, and time
b. tow characteristics: duration, direction, speed of
tow, scope and length of bottom area covered
c. gear characteristics: size of net mouth (headrope
length or door spread), mesh size, and bridle length
Biological variables taken from individual organisms
a. taxonomic identification
b. length
c. condition (disease or health state)
Biological variables at the sample level
a. species (number, taxonomic identification)
b. abundance (sample, species)
c. biomass (sample, species)
d. diversity (sample)
Physical variables (include water and sediment characteristics)
a. temperature (surface, bottom)
b. dissolved oxygen (surface, bottom)
c. clarity (Secchi depth or transmissometer readout)
d. salinity
Once the data have been collected, it should be summar-
ized prior to statistical analyses to answer more specific
questions. Such a summary should present the following infor-
mation :
23
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1. Station map - Labeled stations should be accurately
plotted on a map that includes three depth contours, and
a longitude and latitude scale on the boundaries.
2. Sample information table - This should include sample
numbers on the right margin and sampling variables (e.g.
locational, tow and gear) in columns.
3. Catch summarization table - This includes sample numbers
in columns and biological and physical variables (at the
sample level) in rows on the right margin. The following
biological variables should be included for each sample:
the total catch, total biomass, total number of species,
and sample diversity (described below). In addition,
survey characteristics should be summarized at the bottom
including: total catch, average catch/haul, total biomass,
average biomass/haul, total species (not additive), and
average diversity/haul.
4. Catch information table - Each species is
tabulated with corresponding abundance and number of indi-
viduals obtained in each sample noted. In addition, aver-
age abundance per sample for each species and a coefficient
of dispersion (to measure degree of clumping) can be given.
A similar table of biomass can also be presented.
5. Species lists - A list of the species, arranged taxonomi-
cally should be given. If common names exist (such as
Bailey et al., 1970, for fishes), these can be included
here. In addition, lists of the most abundant and most
common species can also be included.
6. Disease frequency tables - These present a species column
and total number of individuals, number of diseased indi-
viduals and percent diseased, for each disease.
24
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7. Variable maps - All sample variables can be plotted on
maps of the stations. Hopps et al. (1969) gives method-
ology of plotting contours, shading maps, and symbol maps.
In general, contour mapping should be done when variables
are thought to form a continuous gradation from one place
to another, whereas shading maps are more appropriate for
variables that are relatively discontinuous (i.e., schools
of fishes make abundances somewhat discontinuous). If
shading maps are used, three or four divisions should be
made, each division represented by a different shade. The
range of values, divided by 4 (or 3) allows an unequal num-
ber of values to occur in each division, thus giving the
reader some indication of the skewness of variable fre-
quency distributions. Shading should grade from dark to
light and high to low partition values (or low to high).
"Symbol" maps have different sized symbols indicating
different partition values (partitions can be determined
in the above manner). These maps avoid estimating values
for areas intermediate between the stations, although pat-
terns are sometimes more difficult to visualize. Maps
showing disease incidence should include the distribution
of a given species in the survey in addition to a distri-
bution of diseased individuals of that species.
8. Length frequency histograms or tables - Length frequen-
cies of all species can be given for the survey as a
whole and/or for each sample. If the data are presented
graphically, the species are indicated on the vertical
axis and size class (i.e. 10 mm intervals) on the hori-
zontal. Otherwise, the number of individuals of each
size class is tabulated for each species. The value of
these graphs or tables increases when plotted against a
line representing the known size range of each species,
thus giving a perspective as to what portion of the popu -
lation of each species is being captured. It should be
25
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noted that the statistics giving average sizes are less
meaningful here as size distribution histograms are often
multimodal. The frequency of individuals in different
size classes that are affected by disease can be plotted
relative to the frequency of all individuals of that spe-
cies to determine if the disease is size specific.
9. Physical, biological, and sample variables can be plotted
against each other to describe correlations. Comparison
of catch data with bottom water temperature and dissolved
oxygen values may reveal seasonal effects or short-term
anomalous conditions.
DIVERSITY
Most of the above variables can be directly measured or enumer-
ated in the field. Measures of diversity, which may be indi-
cative of environmental gradients, are generally calculated
from formulas requiring two or more of the above variables,
including: species, total abundance or biomass, and in some
cases, abundance (or biomass} per species. The simplest measure
of diversity (representing a variable) is merely the number of
species in the sample (D = S). In many instances, the number
of species in the sample is influenced by the number of indi-
viduals in the sample, and use of the Gleason Index (D = f^ ,
InN
a measure of species richness) gives a better representation
of the diversity of the sample. However, since most of the
catch in a given sample is usually distributed among a few of
the species, information theory diversity analyses (i.e. Brillouin,
H = if ln n 'n *?'—JTT'* Shannon-We aver, R' = - Z £i. In g^, etc.)
IN u^.n2. .. .ns. N N
give a better description of the sample because the number of
individuals per species is included in the calculation. By
scaling these values to take into account differences in sample
H -H H1 -H1
size (i.e. g ^ ; H,max _R ) , a better description of the
max min max min
26
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evenness of the distribution of individuals per species can
be attained. Another measure of this evenness is the standard
deviation diversity. Each of these indices essentially func-
tions in analyses as a variable (although not measured directly
in the field) and can be summarized in the same manner as a
variable.
SUMMARY STATISTICS
Statistics used in the data summarization tables generally
describe locational and dispersion characteristics of the fre-
quency distributions of the variables. This generally includes
measures of central tendency (i.e. mean, median, mode) or dis-
persion (i.e. ranges, standard deviations, standard errors).
The manner in which these measures are expressed (i.e. mean vs.
median) is determined by whether the criteria necessary in the
assumptions for the use of each parameter are met. Parametric
statistics (i.e. means, standard errors), which are those that
are based upon a set of assumptions that include a normal fre-
quency distribution, are most descriptive when used with vari-
ables that have frequency distributions which follow a normal
curve. It has been our experience that the number of species
per trawl sample approximates a normal curve and is adequately
represented by parametric statistics without additional trans-
formations .
Skewed unimodal curves frequently appear for biomass and number
of organisms per sample and are not always adequately repre-
sented by parametric statistics without transformation. Various
transformations have been used to account for Poisson, negative
binomial and logarithmic distributions prior to applying para-
metric statistics and should be consulted (see Roessler 1965).
Medians and non-parametric statistics were used by Mearns and
Greene (1974) and appeared adequate to assess data from a
synoptic trawl survey. However, transformation and use of para-
metric statistics can give a more detailed description of these
27
-------
kinds of data and should be used when possible. Other assump-
tions for parametric statistics and general statistical for-
mulae can be found in most general statistical textbooks.
The Coefficient of Dispersion (variance to mean ratio) is a
useful index of the distribution pattern of the species (i.e.
CD = 1, pattern is random; CD > 1, pattern is clumped; CD < 1,
pattern is even). This index can be applied to individual
species over the whole survey. By including all samples (even
those in which the species is not found) highly clumped species
may be interpreted as occupying discrete habitat patches or ag-
gregations. By only including samples in which the species is
present, the interpretation is shifted toward aggregated or
schooling species.
ANALYSIS AND REPORTING
Analyzing and reporting the results of a trawl survey should
address the objectives of the survey as well as additional
questions of basic importance. The investigator should direct
some attention to the following:
1. Was the performance of the surveys affected by weather,
sea state conditions, gear changes or other physical
problems?
2. Do the data show any obvious geographical or depth-related
gradients for the catch variables? What species account
for these gradients? What physical factors relate to them?
3. Over time (seasons, etc.), what obvious changes or trends
occur in the catch variables and what species account for
them? Do disease and parasite infestation show seasonality?
28
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4. When and where do juveniles of dominant species appear in
the catches? Do these species appear to reside in the
survey area or migrate in and out? Do they show obvious
growth patterns? Are diseases species-specific or speci-
fic to certain size groups?
5. How do catch variables compare with data taken by other
methods in past years or with trawl data from adjacent
areas?
6. Was sampling sufficient to meet the objectives?
Variable values can be compared from two or more points in
space or time (i.e. outfall vs. control stations, predischarge
surveys vs. postdischarge surveys), by using t-tests or analy-
sis of variance (or their nonparametric equivalents such as
Mann-Whitney-U tests, Krushal-Wallis) to determine where sig-
nificant differences exist.
In addition to the above analyses, a variety of clustering
techniques can be used. These may be based on clustering simi-
larity coefficients such as Sorenson's (1948). Clustering
techniques fall into two basic types: site clustering and
species clustering. Site clustering techniques form clusters
of similar sites, based upon species composition and abundances
(Stephenson ejt al. 1972) . Species clustering techniques form
clusters of species based upon frequency of joint occurrences
(Fager 1957, 1963) or correlation (or some other similarity
measure) of abundances. Both techniques are useful and have
been combined by some authors (Stephenson e_t al. 1972) . Both
types generally begin by calculating an index of similarity
between pairs of sites or species (e.g. Sorenson 1948) and
applying a grouping technique (Stephenson et al. 1972).
Groups can be presented in boxes (species or sites clustered
29
-------
above a given value of a similarity index) or in dendograms.
Species groups often represent natural communities in nature,
particularly if sampling has been over a very large area (e.g.
Fager and Longhurst 1968; SCCWRP 1973), and changes in their
structure or composition may be relatively more important to
the ecology of an area than changes in other variables.
30
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BIBLIOGRAPHY
Allen, G.H., A.C. DeLacy and D.W. Gotshall. 1960. Quantita-
tive sampling of marine fishes—a problem in fish behavior
and fishing gear. Iri Waste Disposal in the Marine Environment.
E.A. Pearson (ed.) Pergamon Press. 448-511.
Anderson, K.P. 1964. Some notes on the effect of grouping
data with special reference to length measurements. Meddelelser
fra Danmarks Fiskeri-og Havundersugelser. 4(4):79-92. Copenhagen.
Aron, W., and S. Collard. 1969. A study of the influence of
net speed on catch. Limnol. Ocean. 14(2):42-49.
Bailey, R.M. , J.E. Fitch, E.S. Herald et. a_l. 1970. A list of
common and scientific names of fishes from the United States
and Canada. Amer. Fish. Soc., Special Publ. No. 6. 150 pp.
New York.
Baldwin, W.J. 1961. Construction and operation of a small
boat trawling apparatus. Calif. Fish and Game. 47(l):97-95.
Bechtel, T.J., and B.J. Copeland. 1970. Fish species diversity
as indicators of pollution in Galveston Bay, Texas. Contr. Mar.
Sci. Univ. Texas. 15:103-32.
Carlisle, J.G. 1969. Results of a six-year trawl study in an
area of heavy waste discharge, Santa Monica Bay, California.
Calif. Fish and Game. 55:26-46.
Clark, S.H. 1974. A study of variation in trawl data collected
in Everglades National Park, Florida. Trans. Amer. Fish. Soc.
103(4) :777-85.
Dahlberg, M.D. 1972. An ecological study of Georgia coastal
fishes. Fish. Bull. 70(2) :323-53.
Dahlberg, M.D. and E.P. Odum. 1970. Annual cycles of species occur
rence, abundance and diversity in Georgia estuarine fish popu-
lations. Amer. Midland Naturalist. 83(2) :383-92.
Davenport, D. and W.R. Harling. 1965. Method of rapid measure-
ment for large samples of fish. J. Fish. Res. Bd. Canada.
22(5):1309-10.
Ebert, T.A. 1973. Estimating growth and mortality rates from
size data. Oecologia (Berl.). 11:281-96.
Fager, E.W. 1957. Determination and analysis of recurrent
groups. Ecology. 38:586-95.
31
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1963. Communities of organisms. In The Sea, vol. 2.
M.N. Hill (ed.) New York: Interscience. 115-37.
Fager, E.W. and A.R. Longhurst. 1968. Recurrent group analysis
of species assemblages of demersal fish in the Gulf of Guinea.
J. Fish. Res. Bd. Canada. 25:1405-21.
Gallaway, B.J. and K. Strawn. 1974. Seasonal abundance and
distribution of marine fishes at a hot-water discharge in Gal-
veston Bay, Texas. Contr. Mar. Sci. 18:71-137.
Garner, J. 1962. How to make and set nets. Fishing News
(Books) Ltd. London. 95 pp.
Holme, N.A. and A.D. Mclntyre. 1971. Methods for the study of
marine benthos, IBP Handbook 16. Blackwell Scientific Publications,
Oxford, Eng. 334 p.
Hodson, A. 1967. Introduction to trawling. Fishing News
(Books) Ltd. London. 77 pp.
Hoese, H.D. 1973. A trawl study of the nearshore fishes and
invertebrates of the Georgia coast. Contr. Mar. Sci. 17:63-98.
Hopps, H.C., R.J. Cuffy, J. Morenoff, et al. 1969. MOD compu-
terized mapping of disease and environmental data. In Mapping
of Disease (MOD) Project, Report. 3-29 to 3-49.
Ketchen, K.S. 1951. Preliminary experiments to determine the
working gape of trawling gear. Prog. Rpt., Pac. Coast Sta.
Fish. Res. Bd. Canada. 88:62-65.
Lagler, K.F. 1968. Capture, sampling and examination of fishes.
In Methods for Assessment of Fish Production in Fresh Waters.
w7E. Ricker (ed.) Int'l Biol. Programme Handbook No. 3.
Oxford, Eng.: Blackwell Scientific Publ. 2:7-45.
Lowe-McConnell, R.H. 1968. Identification. In Methods for
Assessment of Fish Production in Fresh Waters. W.E. Ricker (ed.)
Int'l Biol. Programme Handbook No. 3. Oxford, Eng.: Blackwell
Scientific Publ. 3:46-77.
McErlean, A.J., S.G. O'Connor, J.A. Mihursky, and C.I. Gibson.
1972. Abundance, diversity and seasonal patterns of estuarine
fish populations. Estuarine and Coastal Mar. Sci. 1:19-36.
Mearns, A.J. and C.S. Greene (eds.) 1974. A comparative trawl
survey of three areas of heavy waste discharge. So. Calit.
Coastal Wat. Res. Proj. El Segundo. Tech. Mem. 215, 76 pp.
Mearns, A.J. and M.J. Sherwood. 1974. Environmental aspects
of fin erosion and tumors in southern California Dover sole.
Trans. Amer. Fish. Soc. 103(4):799-810.
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Mearns, A.J. and H.H. Stubbs. 1974. Comparison of otter
trawls used in southern California coastal surveys. So. Calif.
Coastal Wat. Res. Proj. El Segundo. Tech. Mem. 213, 15 pp.
Moore, D., H.A. Brusher and L.Trent. 1970. Relative abundance,
seasonal distribution and species composition of demersal
fishes off Louisiana and Texas, 1962-1964. Contr. Mar. Sci.
15:45-70.
Oviatt, C.A. and S.W. Nixon. 1973. The demersal fish of
Narragansett Bay: an analysis of community structure, distri-
bution and abundance. Estuarine and Coastal Mar. Sci. 1:361-78.
Peryera, W.T. 1963. Scope ratio-depth relationships for beam
trawl, shrimp trawl and otter trawl.
Comm. Fish. Rev. 25(12):7-10.
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96(3):363-4.
Roessler, M. 1965. An analysis of the variability of fish
populations taken by otter trawl in Biscayne Bay, Florida.
Trans. Amer. Fish. Soc. 94 (4) :311-18.
SCCWRP. 1973. The ecology of the Southern California Bight:
implications for water quality management. So. Calif. Coastal
Wat. Res. Proj. El Segundo. Tech. Rept. 104, 531 pp.
Stephenson, W., W.T. Williams, and S.D. Cook. 1972. Computer
analyses of Petersen's original data on bottom communities.
Ecol. Monogr. 42:387-415.
Sorensen, T. 1948. A method of establishing groups of equal
amplitude in plant sociology based on similarity of species
content and its application to analyses of the vegetation on
Danish commons. Biol. Skr. (K. danske vidensk. Selsk. N.S.)
5:1-34.
Stickney, R.R. and D. Miller. 1974. Chemistry and biology of
the lower Savannah River. J. Wat. Poll. Cont. Fed. 46(10) :
2316-26.
Wiebe, P.H. 1972. A field investigation of the relationship
between length of tow, size of net and sampling error. J_._
Cons. Int. Explor. Mer. 34(2):268-75.
.33
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
REPORT NO.
EPA-600/3-78-083
3. RECIPIENT'S ACCESSION NO.
TITLE AND SUBTITLE
Use of Small Otter Trawls
In Coastal Biological Surveys
5. REPORT DATE
August 1978
6. PERFORMING ORGANIZATION CODE
AUTHOR(S)
Alan J. Mearns and M. James Allen
8. PERFORMING ORGANIZATION REPORT NO.
PERFORMING ORGANIZATION NAME AND ADDRESS
Southern California Coastal Water Research Project
1500 East Imperial Highway
El Segundo, California 90245
10. PROGRAM ELEMENT NO.
1BA608
11. CONTRACT/GRANT NO.
Grant No. R801152
2. SPONSORING AGENCY NAME AND ADDRESS
Environmental Research Laboratory-Corvallis
Office of Research and Development
U.S. Environmental Protection Agency
r Ay-wall ic nv-pgnn
5. SUPPLEMENTARY NOTES
13. TYPE OF REPORT AND PERIOD COVERED
final- March
14. SPONSORING AGENCY CODE
EPA/600/02
6. ABSTRACT
Ecological surveys using small otter trawls provide useful and informative data on
demersal fish and epibenthic macroinvertebrates of coastal soft bottom areas.
This report presents recommendations for selecting and using small otter trawls in
coastal biological surveys and suggests methods for handling catches and processing
data.
Use of small trawls in monitoring surveys is an adaptive use of their original purpose
in commercial fishing. Many investigators have made effective use of small trawls in
ecological surveys and some of this work is reviewed.
Nets ranging in headrope length from 10 to 16 feet are recommended for shallow waters,
estuaries, lagoons and aboard small boats; 25-foot nets are recommended for open
coastal areas.
Use of the gear, accessory gear and care and trouble-shooting procedures are described
Some aspects of survey design, data summarization and data analysis are reviewed.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COSATI Field/Group
closed ecological system environments
otter trawls
ecological surveys
biological surveys
macroi nvertebrates
08/A
06/F,K
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Release to public
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Unclassified
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Unclassified
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EPA Form 2220-1 (Rev. 4-77)
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