Introduction

The ingestion of contaminated food is considered a primary route of metal intoxication in terrestrial organisms. Consequently, the mammalian literature contains a wealth of information on the ingestion, absorption and oral toxicity of metals. In contrast, the effects of oral doses of metals in fish are mostly unknown, except in the context of nutrition (Ogino & Yang, 1978, 1980; Ketola, 1979; Knox, Cowey & Adron, 1982, 1984; Hardy & Shearer, 1985; Poston, 1991). The general belief that the gills are the main route of toxicant absorption in fish has also diverted attention from oral toxicity studies. Early laboratory investigations confirmed that fish can absorb metal poisons across the gut (Aoyama, Inoue & Inoue, 1978a,b; Patrick & Loutit, 1978), but the toxicological significance of these observations remains unclear despite field data on contamination levels in food species (Krantzberg & Stokes, 1989; Yevtushenko, Bren & Sytnik, 1990; Wren & Stephenson, 1991; Miller, Munkittrick & Dixon, 1992). However, at least one research team has confirmed the diet as an important route of contamination in wild fish (Dallinger & Kautzky, 1985; Dallinger et al, 1987). Aquatic microcosms have also been used successfully in trophic studies (Rodgers et al., 1987), while more recently radiotracer studies in whole ecosystems have yielded information on the trophic transfer of toxic metals (Cope, Wiener & Rada, 1990; Harrison, Klaverkamp & Hesslien, 1990). The available information on the occurrence, intestinal absorption, excretion, accumulation, and oral toxicity of Hg, Cd, Al, Cu and Zn are summarized here. References to the mammalian literature are made for comparison and where data on fish are lacking.