The injurious effects of neoplasms are not always obvious. Typical effects of neoplasms include mechanical impediments to locomotion, interference with protective coloration, and increased susceptibility to predation (Carlisle and Roberts, 1977). External tumours can result in the fish being affected by secondary infections or osmotic imbalance, and some species of wild fish would be more susceptible to capture by gill nets. For both cultured and wild fish, neoplasms on the jaws or lips can physically interfere with feeding. Plasmacytoid leukaemia of chinook salmon (Oncorhynchus tshawytscha) grown in netpens can cause a high rate of mortality (Kent et al., 1990), which is unusual for a neoplastic disease.
Brown bullheads (Ameiurus [=Ictalurus] nebulosus) older than 4 years were scarce in the polluted Black River, Ohio, compared with populations at a reference site and in previous studies of brown bullheads (Baumann et al., 1990). There are several possible explanations for this abnormal age distribution, but it could be caused by the early death of older fish, which had a high prevalence of hepatic carcinoma.
Because of concern about adverse effects on humans, considerable emphasis has been placed on the use of fish neoplasms as sentinels for the presence of chemical carcinogens (Sonstegard and Leatherland, 1980; Dawe, 1990; Grizzle, 1990). However, a fish population exposed to chemical carcinogens could also be adversely affected by the toxicity of environmental pollutants; therefore, neoplasms can also be considered as sentinels for less conspicuous impacts of pollutants on the fish themselves. Effects on behaviour (Ostrander et al., 1988) and the immune system (Faisal et al., 1991; Seeley and Weeks-Perkins, 1991; Weeks et al., 1992) of fish have been documented. Because of complex effects of pollutants on food chains, growth rates of fish in polluted environments can increase or may not change, but reduced growth rates of fish have occurred in some polluted environments (Grizzle et al., 1988a). Decreased reproduction could be caused by several mechanisms, including toxicity to fish larvae (Weis and Weis, 1987; Walker et al., 1991) and decreased serum levels of vitellogenin (Chen et al., 1986). Genotoxic carcinogens could also cause germ-cell mutations, which would be of greater concern than somatic changes in populations with surplus reproduction (Wurgler and Kramers, 1992).
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