Fluctuations in food supply drive recruitment variation in a marine fish

Abstract

Reproductive rates and survival of young in animal populations figure centrally in generating management and conservation strategies. Model systems suggest that food supply can drive these often highly variable properties, yet for many wild species, quantifying such effects and assessing their implications have been challenging. We used spatially explicit time series of a well-studied marine reef fish (black surfperch Embiotoca jacksoni ) and its known prey resources to evaluate the extent to which fluctuations in food supply influenced production of young by adults and survival of young to subadulthood. Our analyses reveal: (i) variable food available to both adults and to their offspring directly produced an order of magnitude variation in the number of young-of-year (YOY) produced per adult and (ii) food available to YOY produced a similar magnitude of variation in their subsequent survival. We also show that such large natural variation in vital rates can significantly alter decision thresholds (biological reference points) important for precautionary management. These findings reveal how knowledge of food resources can improve understanding of population dynamics and reduce risk of overharvest by more accurately identifying periods of low recruitment.

Can autocorrelated recruitment be estimated using integrated assessment models and how does it affect population forecasts?

“…These periods could be driven by environmental factors acting on recruit survival (Wilderbuer et al, 2002), adult reproductive output (Jørgensen et al, 2006), or both simultaneously (Okamoto et al, 2012;Wooster and Bailey, 1989), or changes in the abundance of predators (Bailey, 2000). Ideally, researchers can identify measureable environmental factors that are correlated with recruitment deviations or regime shifts, and which can be forecast into the future (Haltuch and Punt, 2011).…”

Stochastic density effects on adult fish survival and implications for population fluctuations

“…The surface represents the prediction from the posterior mean from the Shepherd Model and the previously estimated recruitment models (Okamoto et al . ). The equilibrium density was solved numerically for each intersecting line, assuming a constant environment therein.…”

“…Prior to the period covered by this study, a decrease in foraging habitat availability corresponded to a decline in black surfperch abundance (Okamoto et al . ). Since then, foraging habitat availability varied little compared to variability in food density within those habitats.…”

“…For black surfperch, reducing prey availability leads to decreases in the number of recruits per adult (Okamoto et al . ) and lowers the number of reproductive adults. Such extrinsic influences can be buffered or amplified by intrinsic nonlinearities and cohort effects (Lindström & Kokko ) and recent studies suggest decreases in overall survival via fishing or other persistent changes may increase population fluctuations (Anderson et al .…”

“…Recruitment dynamics are correlated with variation in food supply (Okamoto et al . ) with an average observed juvenile:adult ratio around 1 : 4. We used multi‐generational time series data for black surfperch from Santa Cruz Island, CA to evaluate the evidence for, and expected implications of, food limited and density‐dependent adult survival.…”