ABSTRACT Seasonal migrations of diadromous fish transport marine‐derived nutrients into freshwater ecosystems, enriching communities and restructuring food‐web dynamics. By diminishing these migrations, fishing pressure could disrupt cross‐ecosystem connectivity, thereby altering predator–prey interactions and community size structure. Despite this potential for fishing‐induced disruption, the functional role of migratory species remains poorly understood. We examined whether fishing‐induced declines in amphidromous post‐larvae, or ‘whitebait’, from three galaxiid species ( Galaxias fasciatus , G. argenteus and G. postvectis ) influenced freshwater fish community size structure and enrichment by reducing a potential marine‐derived prey subsidy. We compared fish community diets (using stable isotope analyses) and size structure within five fished and three unfished streams on the West Coast of New Zealand's South Island before and during peak whitebait migration. Whitebait prey were primarily consumed by large (> 100 g) piscivorous fish, particularly longfin eels ( Anguilla dieffenbachii ) and, to a lesser extent, G. argenteus . Despite substantially greater whitebait influxes in unfished streams, there was no difference in the abundance of large fish between stream types or migration periods. However, abundances of medium‐sized (10–100 g) fish increased during peak whitebait influxes, especially in unfished streams. Although no size‐specific differences in whitebait consumption were observed between stream types at the individual level within any species, the greater whitebait influx in unfished streams likely supported a greater number of piscivorous fish, explaining the increase in medium‐sized fish abundance. Moreover, despite generally not appearing to consume whitebait, G. postvectis diets were dominated by freshwater prey in unfished streams and by a combination of terrestrial and freshwater prey in fished streams, suggesting indirect effects of whitebait influxes on non‐piscivorous fish. Overall, although predominantly consumed by large piscivorous fish, influxes of marine‐enriched whitebait influenced recipient ecosystems by directly and indirectly shifting predator–prey interactions and alleviating food limitation for subordinate fishes. Incorporating these measures within an ecosystem‐based framework that accounts for predator–prey interactions and intraspecific and interspecific competition will help safeguard aquatic ecosystem integrity and support both migratory and resident fish populations over the long term.
Crichton et al. (Thu,) studied this question.