The literature pertaining to the prevalence and histopathology of endocrine cell and tissue tumours and related lesions (TRL) in bony and cartilaginous fishes is catalogued and reviewed in the context of the embryological origin of the various components of the endocrine system. Reports of TRLs of endocrine cells or tissues of fishes are relatively rare, and of these, most describe thyroid neoplasms that appear to be simple hyperplasia. The vast majority of published reports of endocrine TRLs in fishes are case‐studies, and thus, with a few notable exceptions, the aetiology of the lesions is unknown. The exceptions include the epizootics of thyroid neoplasms in introduced Pacific salmon in the Great Lakes, which appear to have an environmental aetiology, and epizootics of gonadal stromal cell neoplasms. The latter were all found in either hybrid species or highly inbred captive strains of fishes, suggesting a genetic involvement in the tumourigenesis. The possible aetiologies for the reported TRLs in wild and captive fish species are discussed, and various hypotheses proposing infectious agents, genetic factors and environmental contaminants are considered.

Rainbow trout, Oncorhynchus mykiss (Walbaum), with a mean initial weight of 196 g were fed the β-agonist ractopamine incorporated into a commercial rainbow trout diet at 0, 5, 10, 20 or 40 p.p.m. for 12 weeks. The experiment was conducted as a completely randomized design, with five treatments and four replicate tanks. At the start of the trial and every 4 weeks subsequently, fish were euthanized, and blood serum and liver samples were collected. Serum glucose concentrations were significantly higher in fish fed 40 p.p.m. ractopamine for 8 weeks, while all treated groups demonstrated significantly elevated glucose levels relative to controls at the 12 weeks sampling interval. Nonesterified fatty acid (NEFA) levels were significantly higher in fish fed 5 and 20 p.p.m. ractopamine for 4 weeks, but those fed 10 p.p.m. displayed depressed NEFA concentrations compared to controls. At 4 weeks, plasma growth hormone (GH) levels were significantly lower (P Ͻ 0.05) in the 5, 20 and 40 p.p.m. groups compared to controls. The results suggest that dietary administration of ractopamine to rainbow trout modestly alters the concentrations of circulating GH, glucose and NEFA.