Isotopic turnover in aquatic predators : Quantifying the exploitation of migratory prey

In the tidal freshwaters of Virginia, U.S.A., the blue catfish (Ictalurus furcatus), an introduced piscivore, derives a significant proportion of its nutrition from spawning anadromous fish (genus Alosa, including blueback herring (A. aestivalis), American shad (A. sapidissima), and alewife (A. pseudoharengus)). Because the Alosa are not continually available to I. furcatus, there is an isotopic turnover, defined as change in isotope composition due to growth and metabolic tissue replacement, in I. furcatus tissues associated with the diet switch from freshwater to anadromous fishes. However, isotopic turnover rates for ictalurid fish are unknown. This study determined the maximum isotopic turnover rate of channel catfish (Ictalurus punctatus) tissues and compared this maximum rate with that of I. furcatus captured in the field over the 3-month Alosa spawning run. Maximum turnover rates for δ13C were 0.014 and 0.017‰ per day in muscle and blood. For δ34S, rates were 0.017 and 0.020‰ per day in muscle and blood, respectively. Isotopic turnover of muscle carbon reflected growth rate, but sulfur did not match growth as well. Ictalurus furcatus captured in the field showed no enrichment during the Alosa spawning run owing to slow turnover and variable diet. In aquatic ecosystems that have migrating prey, exploitation by predators may be underestimated using isotopes because of slow tissue turnover.