EPA's Vessel General Permit and Small Vessel General

Stressors
Established ANS are stressors that have the potential to jeopardize the future existence of
threatened and endangered species through the following mechanisms:
� Predation
� Competition
� Food web alterations
� Alteration of the structure or function of ecosystems
� Introgressive hybidization
� Transfer of disease and parasites
� Destruction or modification of critical habitat
These stressors are the mechanisms by which ANS adversely affect native species; they are often
multifold and interlinked. Here we highlight two examples of the multitude of stressors caused
by ANS included in EPA’s Biological Evaluation. Introduction of the green crab (Carcinus
maenus) has been implicated in the destruction of soft-shell clam (Mya arenaria) fisheries in
New England (Cohen and Carlton 1995) and the reduction of populations of other commercially
important bivalves including the scallop, Argopecten irradians, and the northern quahog,
Mercenaria mercenaria (Grosholz and Ruiz 2002). In Connecticut, weekly rates of crab
predation on scallops were as high as 70% leading Tettlebach (1986) to observe that green crabs
were responsible for most observed mortality in scallops and were a limiting factor in population
size. MacPhail et al. (1955) concluded that the green crab was "one of the worst, if not the
worst, clam predators we know." Their ability to out-compete native species for food resources,
high reproductive capacity, and wide environmental tolerances enable them to fundamentally
alter community structure in coastal ecosystems.
Many ANS are filter feeders, which at high densities can deplete the phytoplankton consumed by
zooplankton, juvenile life stages, and planktivorous fish. The Asian clam (Potamocorbula
amurensis) invasion of the San Francisco Bay is closely correlated to the shutdown of the spring
phytoplankton bloom, which usually fuels much of the pelagic ecosystem (i.e., zooplankton and
larval fish). Most of this primary production is transferred to the benthic food webs, which are
dominated by benthic invertebrates and bottom-feeding fishes (Grosholz 2002). The overbite
clam (Corbula amurensis) spread throughout the waterways surrounding San Francisco Bay
within two years of being detected in 1986. The clam accounts for up to 95% of the biomass in
some shallow portions of the bay floor. It is believed to be a major contributor to the decline of
several pelagic fish species in the Sacramento-San Joaquin River Delta, including the threatened
delta smelt, by reducing the planktonic food base of the ecosystem (Takata et al. 2011).
Response Analysis
Here we consider the probable responses of ESA-listed species to ANS invasions. We group
species by four taxonomic groups (fishes, mollusks, corals, and cetaceans) because similar
species are likely to respond similarly to the stressors described above. We focus on the
individual responses to the following ANS-related stressors: predation, competition, trophic
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