Genetic studies of several marine species with high fecundity have produced "tiny" estimates (≤10^-3) of the ratio of effective population size (N_e) to adult census size (N), suggesting that even very large populations might be at genetic risk. A recent study using close-kin mark-recapture methods estimated adult abundance at N ≈ 2 × 10⁶ for southern bluefin tuna (SBT), a highly fecund top predator that supports a lucrative (~$1 billion/year) fishery. We used the same genetic and life history data (almost 13,000 fish collected over 5 years) to generate genetic and demographic estimates of N_e per generation and N_b (effective number of breeders) per year and the N_e/N ratio. Demographic estimates, which accounted for age-specific vital rates, skip breeding, variation in fecundity at age, and persistent individual differences in reproductive success, suggest that N_e/N is >0.1 and perhaps about 0.5. The genetic estimates supported this conclusion. Simulations using true N_e = 5 × 10⁵ (N_e/N = 0.25) produced results statistically consistent with the empirical genetic estimates, whereas simulations using N_e = 2 × 10⁴ (N_e/N = 0.01) did not. Our results show that robust estimates of N_e and N_e/N can be obtained for large populations, provided sufficiently large numbers of individuals and genetic markers are used and temporal replication (here, 5 years of adult and juvenile samples) is sufficient to provide a distribution of estimates. The high estimated N_e/N ratio in SBT is encouraging and suggests that the species will not be compromised by a lack of genetic diversity in responding to environmental change and harvest.

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