Robust estimates of a high <i>N</i> _e / <i>N</i> ratio in a top marine predator, southern bluefin tuna

Abstract

Genetic studies of several marine species with high fecundity have produced "tiny" estimates (≤10^-3) of the ratio of effective population size (<i>N</i>_e) to adult census size (<i>N</i>), suggesting that even very large populations might be at genetic risk. A recent study using close-kin mark-recapture methods estimated adult abundance at <i>N</i> ≈ 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 <i>N</i>_e per generation and <i>N</i>_b (effective number of breeders) per year and the <i>N</i>_e/<i>N</i> 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 <i>N</i>_e/<i>N</i> is >0.1 and perhaps about 0.5. The genetic estimates supported this conclusion. Simulations using true <i>N</i>_e = 5 × 10⁵ (<i>N</i>_e/<i>N</i> = 0.25) produced results statistically consistent with the empirical genetic estimates, whereas simulations using <i>N</i>_e = 2 × 10⁴ (<i>N</i>_e/<i>N</i> = 0.01) did not. Our results show that robust estimates of <i>N</i>_e and <i>N</i>_e/<i>N</i> 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 <i>N</i>_e/<i>N</i> 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.