Data from: Robust estimates of a high Ne/N ratio in a top marine predator, southern bluefin tuna

Genetic studies of several marine species with high fecundity have produced “tiny” estimates (≤10−3) of the ratio of effective population size (Ne) 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 Ne per generation and Nb (effective number of breeders) per year and the Ne/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 Ne/N is >0.1 and perhaps about 0.5. The genetic estimates supported this conclusion. Simulations using true Ne = 5 × 105 (Ne/N = 0.25) produced results statistically consistent with the empirical genetic estimates, whereas simulations using Ne = 2 × 104 (Ne/N = 0.01) did not. Our results show that robust estimates of Ne and Ne/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 Ne/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.

针对多种具有高繁殖力的海洋物种开展的遗传学研究，得出了有效种群大小（effective population size, Ne）与成年个体普查数量（adult census size, N）之比的「极低」估值（≤10⁻³），这表明即便种群规模极大，仍可能面临遗传风险。近期一项采用近亲标记-重捕法（close-kin mark-recapture）的研究估算得出，作为高繁殖力顶级捕食者且支撑着年产值约10亿美元盈利性渔业的南方蓝鳍金枪鱼（southern bluefin tuna, SBT），其成年个体丰度N≈2×10⁶。我们借助同一套遗传与生活史数据——即5年间采集的近13000尾样本——分别估算了每世代的有效种群大小（Ne）、每年的有效繁殖种群数量（effective number of breeders, Nb）以及Ne/N比值。考虑到年龄特异性生命率（age-specific vital rates）、繁殖跳过（skip breeding）行为、年龄相关繁殖力差异以及繁殖成功率（reproductive success）的持久个体差异的人口统计学估算结果显示，Ne/N比值大于0.1，甚至可能达到0.5左右。遗传估算结果也印证了这一结论。以真实Ne=5×10⁵（对应Ne/N=0.25）开展的模拟实验所得结果，与经验遗传估算值在统计学上相符；而以Ne=2×10⁴（对应Ne/N=0.01）开展的模拟则未得到一致结果。我们的研究结果表明，只要采集足够多的个体样本、使用足量的遗传标记（genetic markers），且时间重复采样（temporal replication，本研究中为5年的成年与幼体样本）能够为估算值提供分布依据，即可针对大型种群获得稳健的Ne与Ne/N估算值。本次研究中南方蓝鳍金枪鱼的高Ne/N估值令人振奋，这意味着该物种在应对环境变化与捕捞压力时，不会因遗传多样性不足而受到负面影响。