Harvest-Induced Size Structure Shifts Alter Nutrient Release By a Population of Omnivorous Fish

Body size can have important consequences for physiological factors, such as feeding, respiration, and nutrient excretion.  Thus, the size structure of a fish population can greatly impact its role in lake nutrient cycles.  We examined how shifts in body size impacted phosphorus cycling by gizzard shad in Lake Dora, Florida, which underwent a size selective fish harvest during 2005 and 2006.  We combined a size structured stock assessment model with excretion prediction coefficients to determine how shifts in population biomass and size distribution impacted nutrient cycling.  The harvest selectively removed larger gizzard shad, reducing the population biomass by 27% during the two years after removal, and shifting the population toward dominance by smaller size classes.  However, phosphorus excretion decreased by only 18% during this time period because smaller fish excreted more P per gram.  Three years following the harvest, biomass averaged 8% below the pre-harvest average, but phosphorus release returned to near the baseline average in approximately 1 year.  Our results indicate that size shifts that accompany many biomanipulation efforts may offset some of the reductions in internal nutrient cycling.