1Graduate School of Fisheries Sciences, Hokkaido University, Hakodate, Hokkaido 041-8611, 2Hokkaido Central Fisheries Experimental Station, Yoichi, Hokkaido 046-8555, 3Hokkaido National Fisheries Research Institute, Fisheries Research Agency, Kushiro, Hokkaido 085-0802, 4The Institute of Cetacean Research, Chuo, Tokyo 104-0055, 5Department of Marine Science and Technology, Hokkaido Tokai University, Sapporo, Hokkaido 005-8601, Japan
The effects of infection with parasitic copepods Haemobaphes diceraus and Clavella perfida on the growth and maturity of walleye pollock Theragra Thalcogramma off the coast of eastern Hokkaido, Japan were examined in the feeding season and spawning season. In the feeding season, the condition factor (KF) and hepatic somatic index (HSI) of walleye pollock decreased as the number of H. diceraus increased, but were not related to the number of C. perfida. In the spawning season, the KF, HSI, and gonadic somatic index (GSI) of both sexes of fish decreased as the number of H. diceraus increased. Reproductively spent fish were more often infected with H. diceraus than those of other maturity levels were. No atretic oocytes were detected in the gonads infected with H. diceraus. Histological observation of fish gonads suggests that H. diceraus infection may cause spawning to occur earlier or to be suspended. The lengths of fish at age did not differ between individuals infected with H. diceraus and uninfected ones. The infection rate of H. diceraus increased steadily with age, but no more than three H. diceraus per host were observed. We conclude that H. diceraus infection significantly in-uences the reproduction of walleye pollock through the nutritional condition.