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A progressive fall in serum thyroid hormone levels was found in Lake Ontario coho salmon, Oncorhynchus kisutch, between May 1978 and January 1979 concomitant with an increase in goitre frequency and thyroid hyperplasia. Despite marked differences in overt goitre frequency in sexually mature adult salmon collected from Lakes Ontario (43-7%), Erie (93,5Oh), or Michigan (O%), serum thyroid hormone levels and thyroid histology were similar in fish from the three lakes. Two gonadotrop-like basophil cells (' globular ' and ' vacuolar ') were found in the pituitary, particularly in sexually maturing and mature adults and 'jacks '.Whereas in salmon from Lakes Ontario or Michigan, the globular cells were vacuolated in Lake Erie salmon they were not; this observation may be correlated with the absence of obvious secondary sexual characteristics in Lake Erie salmon. Serum free fatty acids serum, triglycerides and cholesterol concentrations in Lake Ontario salmon fell progressively from the condition in fish collected from the Lake in May 1978, to that of fish collected during their potamodrous migration

Yearling coho salmon, Oncorhynchus kisutch, were fed diets contaminated with PCBs (500 μg or 50 μg/g dry feed), Mirex (50 μg or 5 μg/g dry feed), or a mixture of Mirex and PCBs (5 μg Mirex + 50 μg PCB/g dry feed) daily for 2 or 3 mo. Serum T3 and T4 levels were significantly reduced in fish fed the high Mirex diet for 2 or 3 mo. Serum T3 levels were reduced in fish fed the high PCB diet for 3 mo. Serum T3 and T4 levels were significantly reduced in fish fed the mixed Mirex–PCB diet for 3 mo. The significant increase in serum T4/T3 ratios in fish given the high Mirex or high PCB diet was not evident in fish given the mixed Mirex–PCB diet, suggesting a synergistic effect that is different from the effect of individual contaminants. Fish given the high PCB or high Mirex diet had significantly lower body weights than the control fish. Key words: organochlorines, teleost fish, thyroid gland, hormones, endocrines, synergism, pituitary gland