Atlantic tomcod Microgadus tomcod is an abundant anadromous species in the HR and other estuaries north to southern Labrador. The HR estuary supports their southernmost spawning population and summertime temperatures may present thermal stress. Although tomcod move seasonally within estuaries, they do not coastally migrate; their entire life cycles are spent within natal estuaries making them effective sentinels of environmental degradation. Tomcod are benthic, have lipid-rich livers, and tend to bioaccumulate high levels of lipophilic contaminants. Tomcod are the only wintertime spawners in the Estuary making their young life stages important prey to HR resource and other species (R. C. Chambers, National Marine Fisheries Service, personal communication). Historical data from the HR utilities have demonstrated an overall downward trend in the abundance of adult tomcod in the HR over the past decades suggesting recruitment impairment (Anonymous 1999).
Tomcod collected during spawning in the HR from the mid 1970s to the early 1980s exhibitedhigh prevalences of gross liver tumors that were later histologically defined as hepatocellular carcinomas (Smith et al., 1979). More than 50 percent of one-year-old and 90 percent of two-year-old HR tomcod exhibited hepatic tumors compared to prevalences of 0-10 percent in tomcod from cleaner estuaries in New England (Cormier and Racine, 1990). Concurrently, tomcod from the HR exhibited a truncated age structure compared to tomcodfrom elsewhere (Deyetal., 1993). More than 97 percent of the HR spawners were one-year-olds, less than 3 percent were two-year-olds and three-year-olds were almost absent. In comparison, spawning aggregations in Canadian rivers were comprised primarily of three andfour-year-olds and tomcod up to seven years of age were regularly observed.
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