Possible causes of annual change in the year class strength of flathead flounder Hippoglossoides dubius (Pisces, Pleuronectiformes) in Funka Bay, Hokkaido

Hippoglossoides dubius

水産海洋学会

水産海洋研究

To elucidate the mechanism for the annual fluctuation of year class strength of flathead flounder Hippoglossoides dubius in Funka Bay, Hokkaido, annual changes in age composition, spawning period, egg abundance, food availability at the first feeding stage, and hydrographic conditions were investigated from 1988 to 1999. Strong year classes occurred in 1989, 1991, and 1995. Comparison between annual fluctuations of commercial catch of flathead flounder and the abundance of eggs suggests that there is no relationship between the spawning size and the resultant year class strength of flathead flounder in Funka Bay. The density of copepod nauplii, the primary food for larval flathead flounder, was 7.8 individuals・l^[-1] in January and then increased in February (18.1 individuals・l^[-1]) and March (12.9 individuals・l^[-1]) in 1991. High densities of copepod nauplii in January were also found in 1994 (13.1 individuals・l^[-1]), 1995 (12.3 individuals・l^[-1]), and 1996 (13.8 individuals・l^[-1]). However, the strong year class of flathead flounder did not occur in 1994 and 1996. Therefore, the high density of copepod nauplii does not determine the magnitude of the year class strength of flathead flounder in this area. During the study period the spawning period varied widely among years from January to March and early spawnings were found in the years when the high water temperatures were observed from November to December. The high water temperatures at the surface layer in January were observed in 1991. Relatively warm water were also observed from January to February in 1995, while temperatures during January and February were low in 1989. From the results, it is considered that the early spawning and relatively high water temperature affect the magnitude of the year class of flathead flounder in this area. For the 1989-yaer-calss, high water temperature in mid December 1988 might reduce the mortality during the early life period of flathead flounder in Funka Bay in spite of low temperature after the intrusion the Oyashio Coastal Water (cold water) in January.