Neoplasms typically become more common in older fish (Ozato and Wakamatsu, 1981; Etoh et al., 1983). This relationship between age of fish and tumour frequency also occurs in wild fish exposed to chemical carcinogens (Baumann et al., 1987, 1990; Becker et al., 1987; Rhodes et al., 1987 ).

The stage of development at which fish are exposed to carcinogens can also affect carcinogenicity. The percentage of rainbow trout (Oncorhynchus mykiss) with neoplasms 10-12 months after a prehatching exposure to aflatoxin B1 (AFB1) is higher if embryos are exposed after, rather than before, they have reached the stage when the liver is present as a discrete organ (Wales et al., 1978). Incidence is even greater if yolk-sac larvae are exposed (Hendricks et al., 1980d).

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