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Chapter 2 - Coastal Ecosystems2007). A similar, albeit less drastic, pattern of regionalresource exploitation is evident for hard clams (Stanleyand DeWitt 1983), softshell clams (Abraham and Dillon1986), and bay scallops (Fay et al. 1983). Overharvest istypically of most concern for repeat spawners like oystersand clams; scallops die after they spawn and therefore maybe less susceptible to damaging impacts from late-seasonharvest.Direct removal of shellfish brood stock has most certainlydiminished populations, but indirect impacts from fishingactivities, including dragging, dredging, and boatwakes, also threaten shellfish beds by damaging habitat.Fishing activities can scour benthic habitats, destroy hardsubstrates and seagrass beds critical for spawning, andsuspend sediments that deposit silt on intertidal beds andcloud seagrasses. Destruction and removal of shell substrateduring oyster harvesting eliminates the foundationon which future generations of oysters will settle.PollutionPollution inputs from nutrient and sediment sources are along-standing and accelerating problem for estuarine andcoastal waters along the entire Atlantic coast. The mostrecent EPA National Coastal Condition report (2004)ranked the Maine-to-Virginia section of coast with itslowest national ratings for sediment quality and benthicindices, and its second-to-worst rating for water quality.In particular, nutrient pollution is extensive in the heavilypopulated region. High or moderate eutrophic conditions(i.e. elevated chlorophyll, low dissolved oxygen, extensivemacroalgae, and diminished seagrasses) were detectedin two thirds of the region’s estuaries, with conditions inmost expected to worsen by 2020 (Bricker et al. 1999).Shellfish suffer from pollution from a number of sources,but direct and indirect effects of algae blooms are amongthe worst, as nutrient-mediated phytoplankton blooms(i.e., green, brown, and red tides) inhibit growth and causerecruitment failures (Summerson and Peterson 1990;Kraeuter and Castagna 2001). Dense beds of macroalgae,such as Ulva, disrupt filter feeding and eliminate suitablesettling areas (Galtsoff 1964).Sediment pollution is also major threat, as resuspendedsediments and siltation events harm shellfish gills, interruptfeeding, and lower recruitment success (Kennedy etal. 1996). Marine shellfish ingest, retain, and bio-accumulatetoxic metals and organic compounds from filteredseawater. Elevated levels of organic contaminants andmetals found in shellfish tissue have been shown to inhibitgrowth and disrupt reproductive functions (Kennedy etal. 1996; Kraeuter and Castagna 2001).Parasites, Diseases, and Invasive SpeciesHarmful parasites are prevalent in filter-feeding bivalves,especially oysters and hard clams. In particular, oysterpopulations in the region suffer from high infection ratesby the protozoans Dermo (Perkinsus marinus) and MSX(Haplosporidium nelsoni) (Kennedy et al. 1996). These diseasesmay be limiting factors in the re-establishment ofhealthy oyster populations in many parts of the region,from Chesapeake Bay to New Hampshire. Likewise, hardclams suffer from a parasite known as Quahog ParasiteUnknown (X) or QPX that causes wide-spread but lesscatastrophic mortality in beds from Canada to Virginia(Lyons et al. 2007).The invasive European green crab (Carcinus maenas) isconsidered omnivorous and known to be an importantpredator of many shellfish species. In current areas ofabundance from Gulf of Maine to Delaware Bay, this speciescan cause significant losses of shellfish populations,especially for clams and mussels (Kraeuter and Castagna2001).Altered Freshwater RegimeHuman activities that result in freshwater diversions(e.g., dams, impoundments, freshwater withdrawals) canproduce stressful conditions and higher mortality in estuarineshellfish populations. Lethal disease outbreaks inoysters are linked to higher salinity conditions (Kennedyet al. 1996), and several common shellfish predators suchas the oyster drill (Thais haemastoma), starfish (Asterias forbesi),and whelk (Fasciolaria hunteria) are limited in distributionto higher salinity areas (Kennedy et al. 1996).Northwest Atlantic Marine Ecoregional Assessment • Phase 1 Report 2-43