Predation on fishes and shrimps by channel catfish inside a large fyke-net in Lake Kasumigaura, Japan in spring and summer

Abstract

　In order to clarify predatory impacts on fyke-net catches by invasive channel catfish, the stomach contents of 176 specimens (4.6-57.6 cm in standard length, SL), collected with a large fyke-net in Lake Kasumigaura, Ibaraki Prefecture, Japan between afternoon and the next morning once a month from June to August 2016 and from March to August 2017, were examined. Major food items of the catfish comprised undigested important fisheries species, such as Hypomesus nipponensis, Salangichthys microdon, Gymnogobius urotaenia and Macrobrachium nipponense, frequently occurring also in the fyke-net catches. Mean stomach fullness index in each size class larger than 20 cm SL was extremely high (6.7-8.8) compared to those in the natural environment. It is therefore highly possible that individuals larger than 20 cm SL ate small fishes and shrimp in the fyke-net. The estimated proportion of predation on each species by the catfish in the fyke-net throughout the study period was 73.7%, 80.1%, 36.6% and 33.3% for H. nipponensis, S. microdon, G. urotaenia and M. nipponense entering the net, respectively. Thus, predation by the catfish on important fisheries species inside the fyke-net was considered to be a major problem in local lake fisheries.