Keiichi Kawabata

Stomach contents of the landlocked dwarf ayu in Lake Biwa, Japan

Abstract The landlocked dwarf ayu (koayu) Plecoglossus altivelis in Lake Biwa was collected in June and July of 1995–1997, and its stomach contents were analyzed. On average, the standard length of koayu was 73 mm, its dry mass was 1.5 g, and the dry mass of the organisms in its stomach was 6.9 mg. Planktonic crustacea dominated the diet, and littoral benthic animals were minor prey. In particular, Daphnia galeata comprised a major fraction of the prey mass. Two copepods, Eodiaptomus japonicus and Mesocyclops dissimilis, were subdominant prey. About 80% of prey individuals had a dry mass of 2–10 μg, and individuals in this size range consisted mainly of D. galeata, with carapace lengths of 0.6 to 1.0 mm. Koayu occasionally ingested large organisms, such as D. galeata 1.5 mm in length, larval shrimps, aquatic insects, and even larval gobiid fishes. The stomachs of koayu were empty at night and were filled within 2 h after sunrise. The total prey mass then fluctuated until sunset, with a tendency toward reduction in the morning. The total prey mass and the size of ingested D. galeata tended to increase with predator size.

Food Web Structure and Biodiversity in Lake Ecosystems

Aquatic ecosystems are different from terrestrial ones in many aspects, and their existence increases the biodiversity of the earth. Littoral ecosystems are structured around solid substrata, particularly for aquatic plants and algae, and consequently resemble terrestrial ones. In contrast, pelagic ecosystems are unique in being suspended in the water. In addition, aquatic ecosystems involve complex interactions (Carpenter 1988) that may promote biodiversity within the systems.

Mesocyclops dissimilis n. sp. from Lake Biwa, Japan (Copepoda, Cyclopoida)

The status of the Mesocyclops from lake Biwa is reexamined and a new species, Mesocyclops dissimilis n. sp., distinct from M. thermocyclopoides Harada, 1931, is described.

Predator-prey relationship between the landlocked dwarf ayu and planktonic Crustacea in Lake Biwa, Japan

The predator-prey relationship was investigated between the landlocked dwarf ayu (koayu; Plecoglossus altivelis) and three planktonic crustaceans, Daphnia galeata, Eodiaptomus japonicus, and Mesocyclops dissimilis, in Lake Biwa. The abundance of each prey species and daphniid size composition in the lake was compared with those in the stomachs of koayu collected at the same time and place. D. galeata was the main item of diet of koayu in spite of their low density, and the larger individuals were selectively preyed upon by the predator. In the stepwise multiple regression of the mean number of each prey species in the stomachs on prey densities in the lake and predator mass, the density of the prey itself was always selected as the predictor variable. The density of E. japonicus was also selected as the regressor of daphniid number in the stomachs with a negative slope. Multiple regression was also performed each for two size-classes of D. galeata, and a negative effect of large daphniids was detected for the consumption of small ones. The present results and additional information enable estimation of the collective predation rate by the koayu population on the prey populations.