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Chapter 6 - Diadromous FishJames (Virginia), and Edisto (South Carolina) rivers.Low and stable stock abundance was indicated forMassachusetts, Connecticut, Delaware, the ChesapeakeBay, the Rappahannock River (Virginia), and some SouthCarolina and Florida stocks. Stocks in the Potomac andYork Rivers (Virginia) have shown some signs of recoveryin recent years.Other important Atlantic coast diadromous species likestriped bass and sea lamprey play an important role inthe ecosystem of the northwest Atlantic, but populationsappear to be stable, therefore were not included in thisAssessment. They are also likely to benefit from efforts toprotect target species with similar life histories. Moredetail on the individual life histories of target species maybe found in the species accounts in Appendix XX.Ecosystem Interactions andEcological DependenciesRiverine habitats and communities may be strongly influencedby migratory fauna that provide a significant sourceof energy input. Pacific salmon have been recognized askey elements of riparian (streamside) and terrestrial aswell as freshwater systems (Gende et al. 2002); Atlanticcoast species like alewife appear to play an equally importantrole in their freshwater spawning habitats, providingnutrients that assist microbes in the breakdown of leaf litterand the resulting release of that stored energy to consumers(Durbin et al. 1979). Specific associations betweendiadromous fish and other species also exist. For example,many freshwater mussels are dependent upon migratoryfishes as hosts for their parasitic larvae (Neves et al. 1997;Vaughn and Taylor 1999), such that loss of upstream migratoryfish habitat is a major cause of mussel populationdeclines (Williams et al. 1992; Watters 1996).These historically abundant species serve as prey in riversand estuaries for larger predatory fish such as bluefishand striped bass, gulls, osprey, cormorants, river otter, andmink, and at sea for seals, sea birds, and a wide range ofpiscivorous (fish-eating) marine fish. In one study tomcodaccounted for 59% of the diet by weight of young-of-yearbluefish, along with juvenile shad, blueback herring, andsimilar species (Juanes et al. 1993). Clupeids (shad andriver herring) are an important food source for stripedbass, making up a majority of their diet in late spring andearly summer (Dovel 1968).The 2005 Recovery Plan for Gulf of Maine Atlanticsalmon identified diminished runs of clupeids and sealamprey as factors impacting recovery of salmon. Theauthors suggest that an abundance of other diadromousspecies provided three categories of ecosystem services tosalmon: prey buffering (providing an alternative foragebase such that no individual prey species becomes overlydepleted), marine derived nutrient cycling (all sea lampreyand 20% or more of clupeids die after spawning, enrichingfreshwater habitats), and habitat modification andenhancement (sea lamprey build nests that are usedpreferentially as spawning sites for salmon).Northwest Atlantic Distributionand Important AreasMethodsMarine DistributionSee methods overview in Chapter 5.Data Limitations of Marine DataIt is important to note that using trawl data from bottomsurveys to determine distributions of pelagic fish, e.g. riverherring, could underestimate numbers or biomass of fishthat are expected to be distributed throughout the watercolumn. However, the National Marine Fisheries Service(NMFS) trawl data is the only long term fishery-independentdata set available for examining marine distributionsof these species. This information is necessary, but it mustbe interpreted with caution and results must be comparedwith other sources. In order to address this issue with thedata set, marine distributions were mapped and analyzedonly for species that occurred in at least 5% of trawls: alewife,American shad, and blueback herring. More than3,000 individuals of each of these species were recordedin the database.Northwest Atlantic Marine Ecoregional Assessment • Phase 1 Report 6-