ESTIMATING PISCINE PREY SIZE FROM PARTIAL REMAINS - TESTING FOR SHIFTS IN FORAGING MODE BY JUVENILE BLUEFISH

Knowledge of prey sizes consumed by a predator aids in the estimation of predation impact. Young-of-the-year bluefish, Pomatomus salatrix, a ttack their prey tail-first and often bite their prey in half; this po ses a unique problem in determining prey sizes from stomach content an alysis. We developed a series of linear regressions to estimate origin al prey lengths from measurements of eye diameter and caudal peduncle depth for striped bass, Morone saxatilis, bay anchovy, Anchoa mitchill i, American shad, Alosa sapidissima, blueback herring, Alosa aestivali s, Atlantic silverside, Menidia menidia, and white perch, Morone ameri cana. We then used these regressions to estimate original prey sizes f rom pieces of prey found in stomachs of bluefish collected in the Huds on River estuary from 1990-1993. Lengths of prey that were swallowed w hole were compared to estimated lengths of prey that were consumed in pieces. Lengths of prey that were consumed in pieces were larger than prey that were consumed whole. We determined the prey length/predator length ratio at which bluefish began shifting from swallowing their pr ey whole to partial consumption. Shifting occurred at a ratio of appro ximately 0.35 irrespective of prey species, suggesting that prey lengt h plays an important role in predator foraging decisions and may contr ibute to gape limitations. Shifts in foraging mode effectively reduce gape limitation and allow bluefish to consume larger prey sizes which may increase their effect on prey populations.