Do albacore exert top-down pressure on northern anchovy? Estimating anchovy mortality as a result of predation by juvenile north pacific albacore in the California current system

Abstract

Quantifying the mortality of marine fishes is important for understanding spawner–recruit relationships, predicting year-class strength, and improving fishery stock assessment models. There is increasing evidence that pelagic predators can exert a top-down influence on prey, especially during critical early life-history stages. The objective of this study was to quantify predation by North Pacific albacore on Northern anchovy in the California current system (CCS). I estimated the abundance of juvenile albacore in the CCS from 1966–2005 using stock assessment models and spatially explicit catch-per-unit-effort time series. Anchovy abundance (1966–93), both recruitment and total biomass, was obtained from a stock assessment model. Annual rates of anchovy consumption by albacore were calculated using diet studies of albacore in the CCS, an age-structured bioenergetics model, and regional estimates of albacore abundance. The range of estimates was large: albacore may remove from less than 1% to over 17% of anchovy pre-recruitment biomass annually. Relationships between predation and recruitment biomass were consistent with expectations from top-down effects, but further study is required. This is the first attempt to quantify a specific source of mortality on anchovy recruits and to demonstrate potential top-down effects of predation on anchovy.