Mass immigration of fishes, especially uncommon species, to a floodplain marsh through a river main channel caused by an extreme high-flow event.

Abstract

In a middle reach of the Shigenobu River (Ehime Prefecture, southwestern Japan), a floodplain marsh has been created as a part of a habitat rehabilitation project. On 1 July 2023, an extreme high-flow event occurred in this river during our fyke-net survey on fishes immigrating to the marsh from the river main channel. In this report, we described a mass immigration of fishes caused by this high flow event, by comparing with monitoring data on fish immigration to the marsh from the river during 2015-2016 (152 days through 18 months). During the high-flow event, the total number of immigrating fishes caught by the fyke net was 503 individuals (per two days), which was 10-100 times higher than the usual values recorded by the 18-month monitoring. The dominant immigrating fishes were fat minnow (Rhynchocypris oxycephala: 383 individuals), crucian carp (Carassius spp.: 44 indiv), and two gudgeons (Gnathopogon elongatus: 49 indiv; Pseudorasbora parva: 12 indiv), the latter three of which were uncommon species in the river main channel adjacent to the marsh. In particular, the two gudgeons are species that had never been caught by the 18-month monitoring during 2015-2016, and had rarely been found both in the marsh and adjacent river reach. Therefore, the two gudgeons and crucian carp caught by the fyke net during the high-flow event are likely to have immigrated not from but through the river main channel from other lateral aquatic habitats. This mass immigration shows that an extreme high-flow event can facilitate colonization of a distant habitat by freshwater fishes.