Functional response and switching behavior of young-of-the-year piscivorous bluefish

Young-of-the-year (YOY) bluefish, Pomatomus saltatrix (L.) impose significa nt mortality on recently metamorphosed striped bass, Morone saxatilis (W.) in the Hudson River estuary. Field observations indicate that bluefish sele ctivity on striped bass increases with increasing striped bass density sugg esting a density-dependent feeding response. Functional response and switch ing experiments were conducted to aid in determining the mechanism for this response. The type of functional response exhibited by bluefish under seve ral prey treatments was examined. These were striped bass alone, Atlantic s ilversides, Menidia menidia (L.) alone, striped bass with a background dens ity of alternative prey, and striped bass in the presence of eelgrass Zoste ra marina (L.). The functional response of bluefish to increasing prey dens ity was more similar to a type I or II model than a type III model under al l treatments. Switching was examined by providing bluefish both striped bas s and silversides at five different ratios with and without a prey refuge, eelgrass. Bluefish did not exhibit a type III functional response or switch ing behavior suggesting that these mechanisms do not explain the observed d ensity-dependent selectivity pattern in the field. Switching experiments di d suggest that the presence of prey refuge can determine selectivity by YOY bluefish in the field. The abundance of submerged aquatic vegetation and o ther prey refuge may be important in controlling the predatory impact of bl uefish on striped bass in the Hudson River. (C) 2000 Elsevier Science B.V. All rights reserved.