Geographic variation in demography of black perch (Embiotoca jacksoni): Effects of density, food availability, predation, and fishing

It is well known that demographic rates such as growth and mortality may vary among populations, but the degree of variation in demography is often unknown, especially when considering multiple populations at large spatial scales (e.g., across hundreds of kilometers). Likewise, of the many factors that may affect demography, it is often unknown which are the major factors responsible for spatial variation in demographic rates. In this study, we estimated rates of growth and examined age distributions within nine populations of black perch (Embiotoca jacksoni) distributed across ~350 km of coastline. We tested whether spatial variation in demography was associated with several explanatory variables including population density, predator density, food availability, and fishing activity. Our study revealed a substantial amount of spatial variation in demographic rates. Among-population variation in growth accounted for 55% of the total variation, and the coefficient of variation for mean age within a population was 27%, suggesting substantial variation in mortality. Spatial variation in growth was most sensitive to variation in food availability and mean age was lowest at locations with high levels of spearfishing activity. Although the fishery for black perch is not a major one, fishing mortality rates may be very high in some areas. Overall, our results shed light on the ecological forces that shape demographic variation of this species, and suggest that fishing mortality may have a major influence on the dynamics of perch populations.