Migrating organisms can exert strong effects on the ecosystems they traverse, physically altering habitats and delivering subsidies. In Northwestern California great numbers of Pacific Lamprey (Entosphenus tridentatus) annually swim into the freshwaters of the Eel River to spawn. While engineering their redds, lamprey mobilize large amounts and sediment, and disturb the macroinvertebrates living in and on the benthos. We tested whether (1) bioturbation during redd building increased the number and biomass of drifting invertebrates downstream and (2) whether juvenile Steelhead (Oncorhynchus mykiss) foraged more behind active redds.
Results/Conclusions
Greater numbers and higher biomass of invertebrates were collected in drift samples downstream of Lamprey actively building redds, relative to drift in channel adjacent to redd building lamprey. Additionally, different taxa were more abundant in drift samples downstream of lamprey: Simuliids, Chironomids, and Elmids were more numerous. Juvenile Steelhead foraged more frequently while drift feeding downstream of redd building lamprey. These findings suggest Lamprey facilitate juvenile Steelhead by increasing invertebrate drift while digging redds and preparing to spawn. Surveys of recent Lamprey numbers indicate this interaction could contribute substantially to juvenile Steelhead growth. Lamprey returns in recent years are diminished compared to historic accounts. This not only decreases the amount of nutrients and energy Lamprey deliver after spawning and subsequent death, but also lowers the positive effects on Steelhead growth from this facultative interaction.