Benefits of Aquatic Vegetation for Fish in an Ecosystem Dominated by an Invasive Piscivore

Abstract

Aquatic vegetation plays vital roles throughout aquatic ecosystems through increasing habitat complexity, providing forage and refuge for a variety of organisms, and also through improving water quality. For fish, a host of studies have found a strong correlation between aquatic vegetation abundance, fish density, and species richness, and postulate that this positive effect is due primarily to aquatic vegetation providing refuge from predation. However, detailed behavioural examination of how different fish species use aquatic vegetation and the mechanisms behind the resulting general positive impact are lacking. To address this knowledge gap, detailed real-time video analysis of behaviours of fish using aquatic vegetation was conducted along with seasonal line census surveys of aquatic vegetation and fish in Lake Nojiri, Japan, which is currently dominated by an invasive piscivore smallmouth bass. The aquatic plant community comprised five native species, which generally increased in density in late summer and autumn, and the fish community was comprised of three native and four invasive species. Overall, aquatic plant density was found to have a positive effect on total fish density. Fish behaviour analysis showed that aquatic plant density was negatively associated with total foraging effort across all fish species. However, a notable behavioural change was observed in young-of-the-year individuals (goby and bluegill) in the presence of a potential predator (primarily smallmouth bass), as both species shifted from frequent water column foraging with body positions suspended above the aquatic vegetation to infrequent aquatic plant stalk foraging with body positions near the aquatic vegetation. These results indicate that the positive effects of aquatic vegetation refuge likely outweigh the negative effects of aquatic plant density on fish foraging, leading to the observed overall positive effect of aquatic plant density on fish density. Consequently, patchy aquatic vegetation habitats with varying plant densities in close proximity are likely valuable foraging grounds for juvenile fish as they are able to forage in the water column at high rates when predators are absent but can quickly retreat to adjacent vegetation cover when predators arrive.