Trophic ecology drives spatial variability in growth among subpopulations of an exploited temperate reef fish

Blue cod (Parapercis colias) support important fisheries around New Zealand, but limited mixing over small spatial scales renders subpopulations vulnerable to depletion and highlights the importance of resolving fine-scale population structure. Fiordland represents a rare opportunity to achieve this in a relatively pristine environment and intact ecosystem. Between 2000 and 2004, spatial population structure in three fjords was used to test how spatial variability in growth between inner and outer fjord subpopulations can be statistically explained by diet, demography and site-specific factors. Sex ratios and size distributions vary between habitats, with a female bias and a higher frequency of individuals in the larger size classes within the inner fjords. Stomach content analysis shows that in the inner fjords the diet of blue cod includes a diverse array of benthic species while in the outer fjords P. colias is able to utilise a less diverse array of pelagic prey that may be of a higher nutritional quality. Stable isotope analysis reveals that in two of the three outer fjord habitat sites the Laminarian Ecklonia radiata accounts for a large component of basal organic matter to the local population (6695%). Consequently, fish condition and individual growth rates are high in these kelp-bed fuelled outer fjord habitats. Our findings demonstrate that baseline variability in the nutritional landscape can result in spatially structured populations, highlighting the importance of spatially-explicit management measures in the sustainable exploitation of reef fishes.