Zooplankton Selectivity by Lacustrine Sockeye Salmon Oncorhynchus nerka in Lake Numazawa, Fukushima Prefecture, Japan

Abstract

We investigated the food selectivity of lacustrine sockeye salmon by analyzing their stomach contents and the composition of the zooplankton community in Lake Numazawa. Seven zooplankton species were found in the ambient water of the lake. Salmon (mean total length±SD: June, 206±35 mm, n = 12; September, 228±7 mm, n = 17) fed mainly on Daphnia longispina and Cyclops strenuus. By number, D. longispina accounted for >94% of the stomach contents in June and September, although their proportion in the water column was <31%. Therefore, the salmon positively selected D. longispina, with a high Chesson’s selectivity index (mean: 0.95), which are the second largest zooplankton (up to 1.05 mm) and are slow-moving. In contrast, the salmon did not prefer C. strenuus, even though it was the largest zooplankton. Since the salmon selectively preyed on specific zooplankton species, excess release of lacustrine sockeye salmon fry might cause a decline of their preferred zooplankton species, and thus negatively affect salmon stocks in following years.