The Role of Resource Subsidies in Ecosystems: Lessons from Artificial Feeding in Fish Ponds

Food subsidies may be critical determinants of consumer population dynamics, particularly in resource-limited ecosystems where they can enhance early life growth and survival via bottom-up effects. In highly productive systems, however, subsidies may increase ecosystem productivity to undesirable levels that can reduce habitat quality and potentially destabilize food webs. In controlled experiments, we manipulated additions of artificial feed subsidies (0%, 1%, or 3% body-mass/day) to age-0 channel catfish ponds to better understand subsidy-ecosystem interactions on fish population dynamics, quantifying both direct (i.e., nutritional) and indirect (e.g., water quality) effects. To identify the mechanisms underlying size and fitness differences among individuals within and among subsidy treatments, we used stable isotope and otolith microstructure approaches to reconstruct the diet of individuals and link it with their growth and survival. Preliminary results suggest that a nonlinear response occurred between feeding rate and fish growth and survival. Although feed subsidized up to 50% of the assimilated carbon and nitrogen in juveniles (through direct consumption; bottom-up effects on live prey were negligible), high food subsidies reduced water quality and did not benefit population-level production.  We discuss the implications of our research for understanding the effects of resource subsidies on consumer recruitment into adult populations and for identifying artificial feeding practices that optimize fish production to harvest in aquaculture ponds.