The role of toxicants in population declines of striped bass

Striped bass abundance coastwide during the late 1970s and early 1980s was at or near record low levels and several studies attempted to identify the causes of these declines. Some of these focused on the possible role of toxicants. Initially, Mehrle et al. (1982) reported that vertebrae of striped bass collected in 1978 from the HR had less strength and stiffness, and ruptured at lower forces than in those from three other estuaries, perhaps due to elevated levels of contaminants. Burdens of contaminants in different life stages of striped bass from the HR were not dramatically higher than those from other estuaries, with the exception of three Aroclor PCB mixtures (Mehrle and Ludke, 1983). Although extensive congener specific analyses were not conducted, these authors cautioned that PCDD/Fs, and non-ortho substituted PCBs levels in striped bass from the HR were sufficiently high to warrant concern.

Striped bass larvae were exposed at different salinities to graded doses of a mixture of inorganic and organic (including PCBs) contaminants that reflected their concentrations and profiles in water and striped bass from several spawning rivers. Survivorship of larvae was significantly reduced by the contaminant mixture, but results were highly dependent on salinity - mortality was more pronounced at lower salinities. Larvae exposed to the mixture were significantly impaired in swimming capacity and predation-induced mortality compared to controls. Tissue residues of contaminants in larvae that had been exposed to the mixture were "within the range of what young-of-the-year striped bass could be exposed to in their natural habitats," suggesting that contaminant exposure might be contributing to decreased recruitment to natural populations.

The effects of PCBs and other contaminants on early life-stage and reproductive success was experimentally addressed. Larvae from striped bass females from the HR were fedArtemia nauplii from two sources; one contaminated with PCBs and the second cleaner (Westin et al., 1983). No effect on survivorship or growth was observed after feeding on PCBs-contaminated diet. However, while embryos and early larvae from untreated mothers had higher concentrations of PCBs than larvae from the same mothers that had been feeding on the contaminated shrimp, there was considerable variation among the mothers in the burdens of PCBs that they off-loaded to their eggs, and it was noted that contaminant burdens in the earliest life stages were diluted by growth during larval development.

In a second study, striped bass eggs were obtained from females from three hatcheries including one from the HR (Westin et al., 1985). Total PCBs levels in the unfertilized eggs of HR females were high and exceeded those from Chesapeake Bay and SC females. A significant inverse relationship was found between the concentrations of four chlorinated hydrocarbons in unfertilized eggs; PCBs, DDTs, hexachlorobenzenes, and chlor-danes, and median mortality time in starved larvae from the three locales. The effect of chlorinated hydrocarbons-contaminated diet on the bioaccumulation of these four xenobiotics and their toxicity in post yolk sac larvae was evaluated. Maternally, rather than dietary-contributed contaminants, had the most impact on survival of larvae even after feeding for twenty days on contaminated prey. These results highlight the importance of maternal transfer of contaminants on early life-stage success in fish.

White perch Morone americana, which is closely related to striped bass, was used as a surrogate in studies in which both genders were treated with environmentally relevant PCB77 (Monosson, Fleming, and Sullivan, 1994). Neither sex exhibited reductions in plasma concentrations of estradiol, testosterone, or vitellogenin. But, the highest dose of PCB77 significantly delayed and suppressed ovarian growth and maturation, reduced oocyte diameter, and decreased survival of larvae. The investigators suggested that PCBs may compromise reproductive function in HR fish populations.

In summary, although striped bass from the HR may exhibit higher levels of PCBs and other toxicants compared to conspecifics from elsewhere, there is little empirical evidence to suggest that their population declines could be attributed to toxicant exposure. In fact, retrospective analysis of PCB concentrations in adult females from 1976-97 and estimates of abundance of eggs, larvae, and juveniles found no relationship between PCB exposure and any measures of abundance (Barnt-house, Glaser, and Young, 2003). The imposition of severe management restrictions in the 1980s coastwide to limit or ban fisheries resulted in a rapid and massive resurgence of abundance of the coastal migratory stock of striped bass suggesting that overfishing was the primary reason for declines.

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