中部北太平洋におけるベニザケの内臓癒着症 <特集 魚病に関する国際セミナー>

1981～83年の春から初夏に中部北太平洋の49水域で漁獲したベニザケ計3935尾を調査したところ, 206尾(5.2%)に内臓癒着が認められた。病魚は尾叉長35～49cmの中型魚に多く, 30cm以下, 65cm以上の魚にはみられなかった。病魚の割合は淡水生活年数が多い程確実に増え, また, 海洋で2冬過ごした魚が最も高かった。病魚は雄の方が高率であった。病魚はまた未成熟魚に最も多く, その割合は成熟にともなって減少した。病魚の割合は一般に南から北へ向う程低下したが, これは緯度によって漁獲される魚の大きさや成熟度組成が異なるためである。A shipboard survey was undertaken to determine the prevalence of visceral adhesions in sockeye salmon, Oncorhynchus nerka, in the central North Pacific Ocean in the spring and early summer of 1981-1983. Of 3,935 sockeye from 49 locations examined, 206 (5.2%) were affected. Although no adhesions were observed in fish less than 30cm in fork length, prevalence increased in medium-sized fish. Visceral adhesions were most common among fish between 35 and 49 cm. In fish above 50 cm, however, the proportion of affected fish decreased with increasing fish size. No adhesions occurred in fish of 65 cm and larger. A close relationship between prevalence and fish age was noted. Prevalence increased steadily with increasing freshwater age (0-winter, 0%; 1-winter, 4.1%; 2-winter, 6.3%; 3-winter, 11.0%). As to ocean age, prevalence was highest (6.9%) in fish which had spent two winters at sea, but was at a low level in fish of each of other ages (1-winter, 1.4%; 3-winter, 1.2%; 4-winter, 0%). The percentage of affected fish was higher in males (7.9%) than in females (2.7%). This was stable among years and most fishing locations. Adhesions also occurred most often in immature fish (male, 9.4%; female, 4.2%). With developing fish maturation, prevalence decreased and reached null in both sexes when fish were mature. Prevalence varied latitudinally, declining from south to north in general. This was reflected by latitudinal changes in the size and maturity compositions of fish. In the areas between 42° and 47°N, most catches consisted of immature medium-sized fish, in which adhesions were often observed. In the areas between 48° and 51°N, however, fish were large and maturing, and adhesions were rarely found in them