WATER QUALITY IN CATFISH PONDS AFTER PARTIAL FISH HARVEST

Intensification of United States catfish aquaculture involves hybrid catfish (♀channel catfish Ictalurus punctatus x ♂ blue catfish I. furcatus) grown in ponds with abundant aeration and high feeding rates. High feeding rates cause water quality deterioration because most of the nitrogen, phosphorus, and organic matter in feed is not converted to harvestable crop but rather is lost to the water as waste. High feeding rates may therefore indirectly lead to accumulation of potentially toxic nitrogen compounds, excessive phytoplankton biomass consisting of undesirable cyanobacteria, and persistent dissolved oxygen deficits. This study was conducted to determine whether water quality is improved when feeding rates are reduced in midsummer after partial harvest to remove faster-growing fish.

Twelve, 1.0-acre ponds were stocked with 10,000 hybrid catfish fingerlings in March 2015. Six ponds were partially harvested in August to remove fish larger than ~ 0.57 kg. Fish in the six control ponds were not partially harvested. All remaining fish were removed in October and November.

Mid-summer partial fish harvest removed about 25% of the fish crop and reduced feed additions by a corresponding amount, but had no meaningful effects on water quality, phytoplankton biomass, phytoplankton and zooplankton community structure, or need for supplemental aeration. Lack of water quality improvement was caused by persistent internal recycling of nutrients derived from feeds added to ponds before and after partial fish harvest. Internal recycling of N, P, and other nutrients caused a lag (hysteresis) in the response of water quality to reduced external nutrient loading that lasted until final fish harvest in late autumn.

Size-selective partial fish harvest may have economic implications by removing faster-growing fish that could exceed the maximum size (and therefore subject to discounts) preferred by fish processor. Also, periodic fish biomass reduction can lessen fish loss from infectious diseases, bird predation, and when catastrophic power outages cause loss of aeration capabilities. Decisions to use partial fish harvest to manage single-batch catfish ponds should therefore be based on economics and risk reduction rather than the expectation that biomass reduction will markedly improve water quality.