Anadromous fish as a component of coastal stream restoration
A very basic question that needs to be answered in the Bay watershed is the role of
migratory fishes, particularly the anadromous clupeids, in structuring and driving
ecosystem processes during freshwater residence. That role has been the focus of
relatively extensive and recent research on anadromous salmonids in the Pacific
Northwest. There, it has been recognized that both conservation of aquatic habitats, as
well as stream restoration activities, involve more than just restoring hydrology and bank
stability etc. Conserving and restoring Pacific salmon runs is a fundamental component
in conserving/restoring the aquatic community because migratory salmonids "provide a
substantial contribution of [marine] nutrients to the stream where they spawn, to the
vegetation bordering the channel and to various species of [terrestrial] wildlife" (Bilby, et
al. 2001). Streams with anadromous salmonids have enhanced primary production,
greater invertebrate density and biomass, and increased fish production and juvenile
growth rates, compared to streams without	 Stable isotope studies have demonstrated
that much of this increase in stream productivity is due to direct uptake of marine carbon
and nutrients supplied by the postreproductive, adult salmon (Cederholm et al. 1999).
It is hypothesized that a similar phenomenon occurs in the east, involving American shad
and other anadromous clupeids as the transport vector marine-derived organic matter.
Though not well documented, alosid spawning migrations may stimulate blooms of
phytoplankton, which in turn fuel zooplankton and macroinvertebrate densities, which in
turn provides food for juvenile fishes, etc. There are at least three main pathways for
assimilation of marine nutrients provided by anadromous fish. First, large schools of
migrants, concentrated both temporally and spatially excrete marine-derived nitrogen in
the form of ammonia (Browder and Garmam 1994). Second, decomposition of carcasses
through bacterial and fungal activity releases nitrates and phosphate making these
allochthonous nutrients available to primary producers. Third, alosids can contribute to
freshwater energy budgets through direct consumption of adults by freshwater predators.
Recent stable isotope studies in Virginia involving migrating blueback herring and
predatory blue catfish (Garman and Macko 1998, MacAvoy, et al. 2001) have shown that
between 40% and 60% of the predators' energy budgets are derived from anadromous
clupeid fishes .
Although a limited number of studies have documented the importance of anadromous
clupeids as vectors of marine derived nutrients to freshwater habitats in the east, there
exists some evidence to support this hypothesis Menhaden, a non-migratory alosid
closely related to shad, have been shown to stimulate phytoplankton blooms in the
vicinity of their schools, presumably due to increased ammonia levels (Atlantic States
Marine Fisheries Commission, 2000). A microcosm study in New England showed that
addition of alewife stimulated a large phytoplankton bloom, which resulted in increased
litter respiration, which is an indication of microbial and invertebrate activity. According
to the authors of that study 'this increased production in the litter community would
support a long lasting supply of insect and benthic invertebrate food for young fish'. The
same study documented that litter respiration was significantly higher in streams with
alewife migration compared to streams without (Durbin et al., 1979). Preliminary

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research in Chesapeake Bay tributaries has documented significant spikes in stream
ammonia levels corresponding to anadromous fish densities within fresh tidal creeks.
Researchers believe that the ammonia levels recorded are sufficient to stimulate primary
production, but monitoring is needed to confirm preliminary findings (G. Garman, VCU ,
pers.com.).
In addition to phytoplankton, 'fertilization' by migratory clupeid fishes could enhance
growth in emergent macrophytes and submerged aquatic vegetation (SAV), which are
critical components to fresh tidal nursery areas, providing both feeding areas and refuge
to juvenile fishes and macroinvertebrates (nekton). Stable isotope analysis of emergent
macrophytes sampled from a fresh tidal creek in the Rappahannock during alosid
residency is currently being conducted, and may reveal if marine-derived nitrogen is
being assimilated during the spring growing period. This may be of particular
conservation interest because a study in the tidal fresh Hudson provided evidence that
invasives such as Phragmites and purple loosestrife are more efficient than native marsh
species at sequestering available nitrogen. Thus, if nitrogen is limiting in the spring, as
the Hudson study suggested, these non-indigenous species may have a competitive
advantage. Perhaps restoration of anadromous clupeid fishes could enhance the ability
of wetlands in spawning areas to resist invasions of these non-native species by providing
nitrogen subsidies at critical early growth stages
Finally, adults, eggs, fry and carcasses provide carbon and nutrients for predators and
scavengers. A stable isotope study in Ward's Creek, a tidal tributary of the James River,
offers strong evidence that migrant clupeids supply a significant amount of carbon to
predators during the alosid spawning period (Garman and Macko 1998). A similar study
is now underway to document the contribution of marine derived nutrients (from
anadromous fishes) to piscivorous birds nesting along Chesapeake Bay tributaries.
If speculations about the importance of anadromous fish to the annual energy budgets of
freshwater ecosystems are correct, then the timing, duration, and magnitude of spawning
runs each spring may in fact be an important component for stream restoration. Without
anadromous clupeids entering freshwater systems in large numbers each spring,
conserving, maintaining, and restoring the biotic integrity of these ecosystems may be
more difficult, (the back to the future angle.. .Historical importance of Alosa ecological
roles... .in the process of restoring not just these species, but these ecological functions...)
Bilby, R.E., B.R. Fransen, J.K. Walter, C.J. Cederholm, W.J. Scarlett. 2001. Preliminary
evaluation of the use of nitrogen stable isotope ratios to establish escapement levels for
pacific salmon. Fisheries 26: 6-14.
Browder, R.G., and G.C. Garman. 1994. Increased ammonium concentrations in a tidal
freshwater stream during residence of migratory clupeid fishes. Transactions of the
American Fisheries Society 123: 993-996.

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Cederholm, C.J., M.D. Kunze, T. Murota, and A. Sibatani. 1999. Pacific salmon
carcasses: essential contributions of nutrients and energy fro aquatic and terrestrial
ecosystems. Fisheries 24: 6-15.
Durbin, A.G., S.W. Nixon, and C.A. Oviatt. 1979. Effects of the spawning migration of
the alewife, Alosapseudoharengus, on freshwater ecosystems. Ecology 60: 8-17.
Garman, G.C., and S.A. Macko. 1998. Contribution of marine-derived organic matter to
an Atlantic coast, freshwater tidal stream by anadromous clupeid fishes. The North
American Benthological Society 17:277-285.
MacAvoy, S., S. Macko, and G. Garman, 2001. Isotopic turnover in aquatic predators:
quantifying the exploitation of migratory prey. Can. J. Fish. Aquatic Sci. 923-932.

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