Effects of Artificial Feeding and Copper Sulfate on Nutrients and Dissolved Oxygen in Fish Ponds

Artificial feed is commonly added to aquaculture ponds to enhance growth and survival of juvenile fish. Thus, feed is usually considered for its role as a direct nutritional subsidy, and is often added in large quantities to maximize fish production.  However, feed may also have complex indirect effects on lower trophic level biota and habitat quality. For example, artificial feeding introduces excessive nitrogen (N) and phosphorus (P) into ponds which may stimulate growth of nuisance algae, and reduce dissolved oxygen (DO). Copper sulfate, which is often added to ponds to control phytoplankton growth and treat parasitic infections, may modify these interactions by reducing algal and microbial productivity. To test the effects of artificial feeding and copper sulfate addition on nutrients and DO in ponds, we measured in situ N and P release by feed, feed biochemical oxygen demand (BOD), and morning DO in ponds over a gradient of feeding rates. We found that total N and P release by feed was excessive compared to source water contributions; N and P were released at a molar ratio of 10:1. BOD increased rapidly with increasing feeding rate, and ponds that received artificial feed had consistently reduced morning DO compared to control ponds. Copper sulfate additions further reduced morning DO in ponds, and reduced feed BOD.  We also observed substantial spatial heterogeneity for DO in all ponds, such that the lowest concentrations were localized near the site of feed addition. Cumulatively, our results suggest that artificial feeding stimulates growth of nuisance algae and severely reduces DO, whereas copper sulfate addition further reduces DO by reducing algal primary production. To optimize fish production in juvenile fish ponds, we suggest that managers tailor feeding regimens by balancing beneficial nutritional effects with adverse habitat effects.