Striped Bass and Atlantic Menhaden Predator–Prey Dynamics: Model Choice Makes the Difference

Abstract

AbstractA version of multispecies virtual population analysis (MSVPA‐X) had been the sole means of evaluating predation mortality by Striped Bass Morone saxatilis, Bluefish Pomatomus saltatrix, and Weakfish Cynoscion regalis on Atlantic Menhaden Brevoortia tyrannus during 1982–2008. Simplified models may offer a different view of predation, so we developed alternative estimates of predation mortality on Atlantic Menhaden with a simpler biomass dynamic predator–prey (Steele–Henderson or S–H) model. Strongly contrasting predictions of the importance of predation on age‐1+ Atlantic Menhaden, particularly Striped Bass predation, emerged between the MSVPA‐X and the S–H model. Striped Bass selected mostly age‐0 and age‐1 Atlantic Menhaden when modeled by MSVPA‐X, even though large Striped Bass (>40% of biomass after the mid‐1990s) were capable of consuming all ages. Consumption of Atlantic Menhaden by Striped Bass in the S–H model was drawn from aggregated age‐1+ biomass. Combined predation mortality from Striped Bass, Bluefish, and Weakfish was well below fishing mortality for all but the earliest years in the MSVPA‐X but was greater than or equivalent to fishing mortality throughout most of the S–H model time series. Striped Bass were a relatively minor source of Atlantic Menhaden predation mortality with the MSVPA‐X but were the major source since the late 1990s in the S–H model. Annual estimates of Atlantic Menhaden biomass consumed per unit of Striped Bass biomass from the S–H model were roughly five times higher than MSVPA‐X estimates and more consistent with estimates from bioenergetics models and trends of Striped Bass well‐being indicators (tag‐based natural mortality rates and weight at age). Use of an alternative S–H model provided a more complete picture of the potential range of predation pressure.