Sweepstakes reproductive success is absent in a New Zealand snapper (Chrysophrus auratus) population protected from fishing despite 'tiny' Ne/N ratios elsewhere.

A landmark study published in 2002 estimated a very small N-e/N ratio (around 10(-5)) in a population of pink snapper (Chrysophrys auratus, Forster, 1801) in the Hauraki Gulf in New Zealand. It epitomized the tiny N-e/N ratios (<10(-3)) reported in marine species due to the hypothesized operation of sweepstakes reproductive success (SRS). Here we re-evaluate the occurrence of SRS in marine species and the potential effect of fishing on the N-e/N ratio by studying the same species in the same region, but in a population that has been protected from fishing since 1975. We combine empirical, simulation and model-based approaches to estimate N-e (and N-b) from genotypes of 1,044 adult fish and estimate N using recapture-probabilities. The estimated N-e/N ratio was much larger (0.33, SE: 0.14) than expected. The magnitude of estimates of population-wide variance in individual lifetime reproductive success (10-18) suggested that the sweepstakes effect was negligible in the study population. After evaluating factors that could explain the contrast between studies - experimental design, life history differences, environmental effects and the influence of exploitation on the N-e/N ratio - we conclude that the low N-e of the Hauraki Gulf population is associated with demographic instability in the harvested compared to the protected population despite circumstantial evidence that the 2002 study may have underestimated N-e. This study has broad implications for the prevailing view that reproductive success in the sea is largely driven by chance, and for genetic monitoring of populations using the N-e/N ratio and N-b.