We critically review four potential causes of a drastic decline in juvenile American eels, Anguilla rostrata, recruiting to Lake Ontario (81-fold decline from 1985 to 1992) and in juvenile eel densities in tributaries to the Gulf of St. Lawrence. Silver eels from the St. Lawrence River were much more contaminated and had a high prevalence of deformities and lesions than a reference stock although they were on average less contaminated in 1990 than in 1982 (1.12 versus 4.54 μgg−1 for PCB; 0.025 versus 0.07 μg∙g−1 for mirex). Lethal toxicity from chemical contamination has been known to occur in St. Lawrence River eels for the past 25–30 yr. Major habitat modifications in the St. Lawrence took place in the 1950's (St. Lawrence Seaway and hydroelectric dams), about 30 yr before recruitment started declining; this long delay argues against these perturbations being primary causes of the decline. There is little evidence that commercial fishing and oceanographic changes caused the decline. Overall, we conclude that we do not know what caused the pronounced recruitment decline. We propose research avenues and hypotheses that may advance understanding and emphasize that because of panmixia, the recruitment decline could be species wide.

Newly hatched mummichog (Fundulus heteroclitus) were exposed in a 96-h static renewal assay to water-accommodated fractions of dispersed crude oil (DWAF) or crude oil (WAF) to evaluate if the dispersant-induced changes in aqueous concentrations of polycyclic aromatic hydrocarbons (PAH) affected larval survival, body length, or ethoxyresorufin-O-deethylase (EROD) activity. Weathered Mesa light crude oil (0.05-1 g/L) and filtered seawater with or without the addition of Corexit 9500 were used to prepare DWAF and WAE At 0.2 g/L, the addition of dispersant caused a two- and fivefold increase in the concentrations of total PAH (sigmaPAH) and high-molecular-weight PAH (HMWPAH) with three or more benzene rings. Highest mortality rates (89%) were observed in larvae exposed to DWAF (0.5 g/L; sigmaPAH, 479 ng/ml). A reduction in body length was correlated with increased levels of sigmaPAH (r2 = 0.65, p = 0.02) and not with HMWPAH. The EROD activity increased linearly with HMWPAH (r2 = 0.99, p = 0.001) and not with sigmaPAH. Thus, chemical dispersion increased both the sigmaPAH concentrations and the proportion of HMWPAH in WAF. Dispersed HMWPAH were bioavailable, as indicated by a significantly increased EROD activity in exposed mummichog larvae, and this may represent a significant hazard for larval fish.

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Abstract-An epidemiological study was conducted to evaluate whether the density of pigmented macrophage aggregates (PMAs) and the prevalences of preneoplastic and neoplastic lesions increased in fish captured downstream from a bleached-kraft pulp mill, causing induction of cytochrome P4501A enzymes (CYP1A). White suckers (Catostomus commersoni) sampled 10 and 95 km downstream from a bleached-kraft pulp mill in the St. Maurice River, Quebec, Canada, exhibited higher densities of PMAs relative to age in liver, spleen, and kidney than fish sampled 10 km upstream. White suckers were also sampled at three sites in the Gatineau River, Quebec, Canada, a reference river with a similar upstream/downstream ecological gradient but no pulp mill. In the Gatineau, density of PMAs was lower in the liver of fish sampled at the most downstream site, and there was no difference among sites in spleen and kidney. Higher growth rates have been documented at downstream sites of both rivers and thus cannot explain the increased density of PMAs observed at downstream sites of the St. Maurice River only. Density of PMAs did not differ between sexes or between fish with or without grossly visible parasites or granulomatous histological lesions. Although we cannot exclude the involvement of unknown infectious agents, density of PMAs appears to be a useful marker of bleached-kraft mill effluent (BKME) toxicity. Several known toxic effects of BKME could contribute to this response, including increased rates of lipid peroxidation associated with induced CYP1A and toxic hemolysis. Further field and laboratory studies are needed to evaluate whether the association between BKME exposure, induction of CYP1A, and increased density of PMA is consistent. Preneoplastic lesions were not observed in fish captured downstream of the pulp mill in the St. Maurice River, and only one case of biliary carcinoma was observed, at the most contaminated site.

An epidemiological study was conducted to evaluate whether the density of pigmented macrophage aggregates (PMAs) and the prevalences of preneoplastic and neoplastic lesions increased in fish captured downstream from a bleached‐kraft pulp mill, causing induction of cytochrome P4501A enzymes (CYP1A). White suckers (Catostomus commersoni) sampled 10 and 95 km downstream from a bleached‐kraft pulp mill in the St. Maurice River, Quebec, Canada, exhibited higher densities of PMAs relative to age in liver, spleen, and kidney than fish sampled 10 km upstream. White suckers were also sampled at three sites in the Gatineau River, Quebec, Canada, a reference river with a similar upstream/downstream ecological gradient but no pulp mill. In the Gatineau, density of PMAs was lower in the liver of fish sampled at the most downstream site, and there was no difference among sites in spleen and kidney. Higher growth rates have been documented at downstream sites of both rivers and thus cannot explain the increased density of PMAs observed at downstream sites of the St. Maurice River only. Density of PMAs did not differ between sexes or between fish with or without grossly visible parasites or granulomatous histological lesions. Although we cannot exclude the involvement of unknown infectious agents, density of PMAs appears to be a useful marker of bleached‐kraft mill effluent (BKME) toxicity. Several known toxic effects of BKME could contribute to this response, including increased rates of lipid peroxidation associated with induced CYP1A and toxic hemolysis. Further field and laboratory studies are needed to evaluate whether the association between BKME exposure, induction of CYP1A, and increased density of PMA is consistent. Preneoplastic lesions were not observed in fish captured downstream of the pulp mill in the St. Maurice River, and only one case of biliary carcinoma was observed, at the most contaminated site.

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American eel (Anguilla rostrata) from the St. Lawrence River are heavily contaminated with chemicals that may be associated with increased incidence of diseases and reproductive impairment. The relationship between tissue mirex concentration and body mass was used to separate eels into two groups: the proportion of eels migrating from contaminated areas (Lake Ontario and upper St. Lawrence River) increased as the migration season progressed. Vertebral malformations and basophilic foci in the liver (preneoplastic lesions) were more frequent at the end of the migratory season, when the eels were more heavily contaminated with organochlorine compounds. In contrast, mesenteric nematodes were more common in the first week of the season, when eels were less contaminated. Diameters and percentages of different stages of oocytes, and density and surface area of pigmented macrophage aggregates in the spleen, did not vary among weeks. While basophilic foci are specific biomarkers of exposure to environmental contaminants, vertebral malformations may be caused by a variety of other anthropogenic or natural factors.\b Further studies are needed to confirm the observed associations between chemical contamination and pathological changes.