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Chapter 6 - Diadromous Fishchanges that enable the animals to move from a freshwaterto a saltwater environment. Adult silver eel are believed tospawn in the Sargasso Sea during winter and early spring(USFWS 2007), although spawning has never been observed.American shad (Alosa sapidissima)The spawning range of American shad is from Floridato the St. Lawrence River. Shad ascend tributaries in thespring when water temperatures reach 16.5oC to 19oCand spawn preferentially in shallow water over gravel orrubble substrates (Collette and Klein-MacPhee 2002).Pelagic shad eggs are carried downstream by the current.Larvae and early juveniles use natal rivers during summerand begin downstream migration to the sea in response todecreasing water temperatures in the fall (Weiss-Glanzet al. 1986). In the northern part of their range, shad mayspawn up to five times. The percentage of adults that liveto be repeat spawners decreases with decreasing latitude;south of Cape Hatteras shad are semelparous (reproduceonly once during their lifetime; Collette and Klein-MacPhee 2002).Shad form seasonal aggregations and undertake extensiveoceanic migrations; fish tagged in the summer in the Bayof Fundy have been recaptured in rivers all along the coast,up to 3,000 km from the tagging location (Dadswell etal. 1987). By late June immature shad are in coastal watersof the inner Bay of Fundy, the Gulf of St. Lawrence, andnorth to Newfoundland while the spawning fish are upstreamin coastal rivers. In late fall and winter shad moveto deeper waters further offshore, up to 175 km from thenearest land. Young of the year are thought to overwinternear the mouths of their natal streams (Collette andKlein-MacPhee 2002). At sea they eat zooplankton, smallbenthic crustaceans, and occasionally, small fish.Atlantic salmon (Salmo salar)Atlantic salmon are found in coastal waters on both sidesof the North Atlantic, from Spain to the Arctic circle,Long Island Sound to Labrador, and a few rivers in westernGreenland (Collette and Klein-McPhee 2002).Atlantic salmon spend their first few years in smallstreams and rivers feeding primarily on aquatic insects.These young, mostly solitary fish are called “parr.” Afterreaching a size of about four inches, the fish become“smolts” in the spring and begin migrating to the ocean. Ittakes two to five years to become a smolt, less in the southernportion of the range and more in the north wheregrowing seasons are short. During their downstream migrationsmolts begin schooling and develop the salinitytolerance needed to survive in the ocean. Fish becomes alarger proportion of their diet as they grow.Feeding while they migrate, the salmon move towardtheir major feeding grounds in the North Atlantic nearGreenland and Iceland. After spending one or two yearsat sea, salmon begin their journey back to their natal rivers.Salmon may reenter fresh water in spring, summer, orfall, but spawning occurs in the fall. Unlike Pacific salmon,Atlantic salmon typically do not die after spawning.Interestingly, a small group of salmon native to NovaScotia, New Brunswick streams in the inner Bay of Fundyregion are thought to utilize Gulf of Maine waters mostof the year and don’t undertake long ocean migrations.These “resident” salmon stocks, like the long-distance migrants,are impacted by degradation of freshwater habitatby flow alteration and acid precipitation. Atlantic salmonare among a small group of diadromous fish that requireaccess into remote upstream tributaries up to hundreds ofmiles from the sea. The extent of habitats required to supporta salmon throughout its life cycle led to the species’extirpation from all but a few of its native rivers in theUnited States by the time of the Industrial Revolution.Atlantic sturgeon (Acipenser oxyrinchus)Historically, Atlantic sturgeon were present in approximately38 rivers in the United States from St. Croix,Maine to the Saint Johns River, FL, 35 of which have beenconfirmed to have had a historical spawning population.Atlantic sturgeon are currently present in the same 35rivers, and spawning occurs in at least 20 of these rivers(Atlantic Sturgeon Status Review Team 2007).6-28Northwest Atlantic Marine Ecoregional Assessment • Phase 1 Report