Recent decline in native bitterling populations with reference to invasion of a non-native species, Acheilognathus macropterus , in Lake Kasumigaura, Ibaraki

Four bitterling species, Tanakia lanceolata, Acheilognathus typus, A. melanogaster and A. tabira erythropterus, are native to Lake Kasumigaura, Japan. Although their local extinction is a matter of concern, the reasons for recent population declines and their underlying mechanisms have been poorly understood. In this study, we conducted field research on the population dynamics of these species from 1999 to 2011, to determine the cause of the population decline. Special attention was paid to the ecological impact of non-native bitterlings and the availability of unionid mussels as their spawning hosts. During the study period, T. lanceolata and A. typus were not collected at all around the lake basin. Populations of A. tabira erythropterus and A. melanogaster gradually disappeared by the year 2010. In contrast, a non-native bitterling, A. macropterus, which newly invaded around the year 2000, showed steady increase in its population after 2005. Of the other non-native bitterlings, Rhodeus ocellatus ocellatus showed a population decline, while A. rhombeus was rarely found throughout the study period. Generalized additive mixed model (GAMM) analysis revealed that the non-native bitterling, A. macropterus, has no impact on native bitterling populations. Since there was a remarkable decrease in the abundance of unionids by 2006, the population decline of the native bitterlings can be attributed to the decreased availability of their spawning hosts. In spite of the unionid mussels absence, the non-native A. macropterus population expanded. It is likely that A. macropterus can utilize as its spawning host a cultivated freshwater pearl mussel, which is a hybrid between a Japanese Hyriopsis schlegeli and a Chinese H. cumingii, suggesting the possibility that its population is supported by pearl culture.