Decadal changes in standing crops of fish and crustaceans in Lake Kasumigaura, Ibaraki Prefecture

Abstract

We evaluated the relationships between decreased commercial catch and the standing crops of fish and crustaceans in Lake Kasumigaura, Ibaraki Prefecture. We analyzed the changes in weight and individual numbers for 38 fish and 2 crustacean species. The organisms were collected in set nets from the littoral zone at two sites (north and south) in the lake between 1986 and 2005. The mean weight of all 40 species captured at each sampling event increased gradually during the study period. Similarly, we observed an increase in the mean weight of individuals collected at both the north and south sites over time. The change in mean catch weight per sampling event varied among species. For example, at the northern site the mean catch weight per collection increased signif cantly for 11 species, including Plecoglossus altivelis altivelis, Hemibarbus barbus, Cyprinus carpio, Carassius spp., and Ictalurus punctatus, but decreased for 6 species; Hypomesus nipponensis, Hyporhamphus intermedius, and 4 Gobiidae species (e.g., Tridentiger brevispinis). Our results strongly suggest that there is a large standing crop of fish and crustaceans that has increased in the lake over the last 20 years, although the size of the standing crop differs among the various species, and the commercial catch has decreased. They also highlight the need to monitor the standing crops of fish and crustaceans as opposed to relying on analysis on fishery statistics. Furthermore, the increase in the standing crops of all 40 species was caused by an increase in the standing crops of mid- and large-sized individuals but not of small individuals.