Closing the life-history loop: Density effects on fecundity and egg size of an exploited, amphidromous fish (Galaxias maculatus) in freshwater protected areas

Abstract

Understanding how fecundity and egg size respond to density-dependent and density-independent processes is essential for fisheries assessments because it shows managers how exploited populations respond to fishery closures. Similarly, the extent to which density-dependent and density-independent processes influence fecundity and egg size in freshwater protected areas (FPAs) is important for determining their conservation potential and effectiveness as a fisheries enhancement strategy. The fecundity and egg size of 164 sexually mature Galaxias maculatus females collected from coastal waterways on the West Coast of New Zealand's South Island were determined from a combination of techniques. Specifically, the additive and interactive effects of density, temperature, and body size on fecundity and egg size were examined in 10 G. maculatus populations across a density gradient mediated by fishery closures and tested for an egg size/fecundity trade-off. There was no evidence of an egg size/fecundity trade-off, or of negative density-dependent effects on fecundity or egg size within FPAs. Egg size, gonadosomatic index, and water temperature were correlated with latitude. Fecundity increased with body size, but egg size did not. This highlights the importance of available forage (terrestrial- and stream-based prey) for G. maculatus to maintain individual growth to increase egg production and larval export derived from no-take FPAs. For the current network of no-take FPAs to be more effective as a management tool, efforts should focus on the remediation of in-stream and requisite riparian spawning habitats, the degradation of which has caused population bottlenecks for this culturally and economically important amphidromous species. The major subsidy from FPAs for the G. maculatus metapopulation is sustainable egg production if migration to sea of the greatest possible number of larvae can be ensured.