Page 105 - Environment: The Science Behind the Stories

thE SCIENCE BEhINd thE StOry

determining Zebra that larvae and juveniles of 10 littoral
fish species would increase in number,
Mussels’ Impacts on increase in growth rate, and shift
Fish Communities upriver to regions of greatest zebra
mussel density.
When zebra mussels appeared in To test their predictions, the
the Great Lakes, people feared for researchers analyzed data from fish
sport fisheries and estimated that fish surveys carried out by DEC scientists
population declines could cost billions over 26 years, spanning periods before
of dollars. The mussels would deplete dr. david Strayer samples aquatic and after the zebra mussel’s arrival.
the phytoplankton and zooplankton invertebrates Strayer’s team compared data on
that fish depended on for food, people abundance, growth, and distribution of
reasoned. invertebrates in shallow water (espe- young fish before and after 1991.
However, food webs are compli- cially in the nearshore, or littoral, zone) The results supported their predic-
cated systems, and disentangling them had increased, because the mussels’ tions. Larvae and juveniles of open-
to infer the impacts of any one species shells provide habitat structure and water fish, such as American shad,
is difficult. Thus, even 15 years after the their feces provide nutrients. blueback herring, and alewife, tended
arrival of zebra mussels, there was no These contrasting trends in the to decline in abundance in the years
solid evidence of widespread harm to benthic shallows and the open deep after zebra mussels were introduced
fish populations. water led Strayer’s team to hypoth- (Figure 1a). Those of littoral fish, such
So, aquatic ecologist David Strayer esize that zebra mussels would harm as tessellated darter, bluegill, and
of the Institute of Ecosystem Studies open-water fish that ate plankton but largemouth bass, tended to increase
in Millbrook, New York, joined Kathryn would help littoral-feeding fish. They (Figure 1b).
Hattala and Andrew Kahnle of New predicted that following the zebra Growth rates showed the same
York State’s Department of Environ- mussel invasion, larvae and juveniles trend: Open-water fish grew more
mental Conservation (DEC). They of six common open-water fish spe- slowly after zebra mussels invaded,
mined data sets on fish populations in cies would decline in number, decline whereas littoral fish grew more quickly.
the Hudson River, which zebra mussels in growth rate, and shift downriver In terms of distribution in the
had invaded in 1991. toward saltier water, where mussels 248-km (154-mi) stretch of river studied,
Strayer and other scientists had are absent. Conversely, they predicted open-water fish shifted downstream
been studying this community for years.
Their data showed that after zebra
mussels invaded the Hudson: 1000 1000
• Biomass of phytoplankton fell by
80%. 100
• Biomass of small zooplankton fell Millions of larvae Thousands of juveniles 100
by 76%.
• Biomass of large zooplankton fell
by 52%. 10 10
197519801985199019952000 197519801985199019952000
Zebra mussels increased filter- Year Year
feeding in the community 30-fold, (a) American shad (b) Tessellated darter
depleting phytoplankton and small zoo- Figure 1 Zebra mussels harm open-water fish but help littoral fish. Larvae of American
plankton and leaving larger zooplankton shad (a), an open-water fish, were increasing before zebra mussels invaded (red points
with less phytoplankton to eat. Overall, and trend line). After zebra mussels invaded, shad larvae decreased (orange points). In
zooplankton and invertebrate animals contrast, juveniles of the tessellated darter (b), a littoral fish, were decreasing (red points
of the open water (which are eaten by and trend line) but increased after zebra mussels invaded (orange points). Source:
open-water fish) declined by 70%. Strayer, D., et al., 2004. Effects of an invasive bivalve (Dreissena polymorpha) on fish in the Hudson River
However, Strayer had also found estuary. Canadian Journal of Fisheries and Aquatic Sciences 61: 924–941. © 2004. Reprinted by permission
that benthic, or bottom-dwelling, of NRC Research Press.

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