THE ROLES OF FISHING AND CLIMATE IN THE POPULATION DYNAMICS OF BOCACCIO ROCKFISH

Abstract

Climate is an important force influencing the dynamics of populations and communities in many terrestrial, aquatic, and marine systems. The precipitous decline of stocks of bocaccio (Sebastes paucispinis) since the mid‐1970s has been attributed to a combination of a shift in the climate regime in the eastern Pacific Ocean during the winter of 1976–1977 (leading to lower productivity) and overfishing. We used a bocaccio recruitment time series (1959–1997) and indices of climate and ocean condition (e.g., sea surface temperature, Northern Oscillation Index, El Niño–Southern Oscillation Index, and upwelling indices) to examine the relationship between bocaccio productivity (recruits per spawning output, R/S) and both interannual (e.g., ENSO) and interdecadal (regime shifts) scale variation in climate and ocean conditions.We did not detect an effect of the 1976 climate regime shift on bocaccio productivity. Mean log R/S and the frequency of “good” recruitment years (log R/S > 1 sd above the long‐term mean) were the same before and after the 1976 regime shift. Good recruitment years occurred about 13% of the time in both regimes. At an interannual time scale, cooler ocean temperatures during the period spanning egg production to the end of the larval stage correlated with higher log R/S both before and after the regime shift. Prior to the 1976 shift, high log R/S values were more likely when upwelling was low during the settlement period, but there was no relationship after the regime shift. A matrix population model suggested that in the absence of fishing a “good” recruitment year needs to occur 15% of the time to achieve a nonnegative population growth rate of (λ ≥ 1.0), similar to the observed frequency of good recruitment events. Good recruitment years needed to occur >90% of the time under historic levels of fishing mortality to achieve a population growth rate ≥1.0. Taken together, these results suggest that the 1976 regime shift did not contribute to the decline in bocaccio stocks through processes affecting recruitment, and that the degree to which populations of bocaccio can withstand fishing depends on the frequency of climatic conditions that promote strong recruitment.