Physical structure of habitat network differently affects migration patterns of native and invasive fishes in Lake Biwa and its tributary lagoons: stable isotope approach

A good understanding of how migratory animals use their habitat network is expected to provide important insights for the prediction of population dynamics at both local and regional scales. We focused on how the physical structure of a habitat network could affect fish migration between Lake Biwa and its tributary lagoons. Although the lagoons provide suitable breeding and nursery grounds for native fishes, it is a matter of concern that they can also be a hotbed of invasive exotic fishes. Here, we assessed the migration patterns of native crucian carps ( Carassius spp.) and exotic largemouth bass ( Micropterus salmoides ) and bluegill sunfish ( Lepomis macrochirus ) using their carbon and nitrogen stable isotopes as migration tracers. As there were marked differences in the carbon isotope ratios of basal food webs between the main lake and its tributary lagoons, stable isotopic signatures of individual fishes collected from each lagoon enabled us to judge whether they were residents of the lagoon or recent immigrants from the main lake. The analysis revealed that native and invasive fishes showed different migration patterns across a variety of lagoons. Exotic fishes frequently immigrated from the main lake to the lagoon as the distance of the channel connecting these two habitats was short. For native crucian carps, in contrast, their migrations were unaffected by the channel distance but were promoted by narrow channels. Physical barriers of weirs and dense vegetation within the channel obstructed their migrations. Such ecological information on migration behavior will be vital to plan designs for habitat restoration to conserve native fishes.