EPA's Vessel General Permit and Small Vessel General

alteration, ecosystem alteration, disease transmission, and habitat modification. We determine
whether these responses are likely to reduce an individual’s fitness (i.e., survival and
reproductive).
Fishes
Several ESA-listed fish species are likely to be adversely affected by the introduction of ANS
and the resulting stressors of competition, predations, food web alterations, changes in ecosystem
structure and function, hybridization, transfer of diseases and parasites, and adverse modification
or destruction of critical habitat. Though there are significant differences among fish species,
their responses to ANS invasions are likely to be similar. Rather that discussing each species’
responses individually, we group them taxonomically.
Salmonids. Here we evaluate the possible responses of listed salmonids (including Pacific
salmon, Atlantic salmon, and steelhead trout) to the stressors of ANS invasions. The following
discussion is extracted and summarized from a review of ANS risks to Pacific salmonids written
by Sanderson et al. (2009), whose results indicate that the effect of nonindigenous species on
salmon could equal or exceed that of four commonly addressed causes of adverse impacts
(habitat alteration, harvest, hatcheries, and the hydrosystem). They suggest that managing
nonindigenous species may be imperative for salmon recovery.
During their life cycle, salmonids traverse large geographic areas spanning freshwater, estuarine,
and ocean habitats where they encounter numerous nonnative species, including introduced
sportfish, invertebrate, and plant species. For Pacific salmonids in the Columbia River system
alone, at least eight introduced fish species prey upon and compete with juvenile salmonids.
Introduced channel catfish, large and smallmouth bass, and walleye prey upon juvenile salmon.
In Columbia River reservoirs, large channel catfish (> 67 centimeters) consume thousands of
juvenile salmon, which comprise 50% to 100% of their diets (Vigg et al. 1991). Although both
smallmouth and largemouth bass prey on juvenile salmon, the impact is better documented for
smallmouth bass, which consume 35% or more of juvenile salmon outmigrants in some regions
(Fritts and Pearsons 2004). Walleye consume an estimated 250,000 to 2,000,000 Pacific salmon
smolts annually in the Columbia River (Rieman et al. 1991, Tinus and Beamesderfer 1994).
Juvenile American shad (Alosa sapidissma) compete with juvenile Chinook salmon for
zooplankton prey (Haskell et al. 2006). Brook trout are associated with a 12% reduction of
juvenile salmon in Snake River basin streams, though the mechanism for this reduction remains
unknown (Levin et al. 2002). Many of the above-described invasive species were intentionally
introduced for the purposes of recreational fishing; however, ballast water, hullfouling, and other
vessel-related vectors may transport these species (as gametes, fry, or in their adult phase) into
novel environments. For example, at least 40 introductions of 32 non-native fish species have
been linked to ballast water transport (Wonham et al. 2000). As described above, the
introduction of these species is highly likely to reduce the survival of individual salmonids
through predation and competition for prey.
Invasive invertebrates species are also likely to adversely affect salmonids in the Pacific
Northwest. The New Zealand mud snail (Potamopyrgus antipodarum), which represents more
than 95% of the invertebrate biomass in some areas, has been reported to consume 75% of
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