Epizootiology of Tumors on Flathead Sole (Hippoglossoides elassodon) in East Sound, Orcas Island, Washington

Abstract

Tumors on flathead sole in East Sound, Orcas Island, Washington, showed a very orderly sequence of tumorigenesis both in the field and laboratory. Only fish age 12 months or less had angioepithelial nodules (AEN), fish 8–13 months had transitional stages, and the earliest epidermal papillomas (EP) were on 10-month-old fish. Nearly all tumor-bearing fish in age-groups I III had EP. The greatest number of tumors (4–5 AEN per fish) occurred on the youngest fish and steadily declined with age until by about 14 months the average number of EP was 1–2. There were significantly more AEN on the eyed than on the blind side of the fish, suggesting that the tumorigenic event(s) occurred at, or shortly after, metamorphosis. The incidence of tumor-bearing flathead sole was the same for both sexes. Incidence in samples varied with gear and subarea, but significant variation was not seen by month or year. Incidence of tumor-bearing fish was three times higher in age 0 fish than age I, and about five times higher than in age II fish. Tumor-bearing fish rarely survived past age II, while about 25% of the normal fish did. A comparison of length and weight revealed significant differences between tumor-bearing fish and normal fish beginning with age I, after which both length and weight of tumor-bearing fish became substantially less than that of normal fish. There was no reason to suspect a causal relationship between pollution and tumor-bearing fish.