Absolute abundance of southern bluefin tuna estimated by close-kin mark-recapture

Abstract

Southern bluefin tuna is a highly valuable, severely depleted species, whose abundance and productivity have been difficult to assess with conventional fishery data. Here we use large-scale genotyping to look for parent-offspring pairs among 14,000 tissue samples of juvenile and adult tuna collected from the fisheries, finding 45 pairs in total. Using a modified mark-recapture framework where 'recaptures' are kin rather than individuals, we can estimate adult abundance and other demographic parameters such as survival, without needing to use contentious fishery catch or effort data. Our abundance estimates are substantially higher and more precise than previously thought, indicating a somewhat less-depleted and more productive stock. More broadly, this technique of 'close-kin mark-recapture' has widespread utility in fisheries and wildlife conservation. It estimates a key parameter for management-the absolute abundance of adults-while avoiding the expense of independent surveys or tag-release programmes, and the interpretational problems of fishery catch rates.

Figure 1. Two cartoons of parent–offspring-based close-kin mark-recapture.

Reproductive variability is low in a but high in b. Each small juvenile ‘marks' its two parents among the big adults. The dark fish and lines in a show a sample of six adults and four juveniles, including three parent–offspring ‘recaptures'.

Figure 2. Reproductive output per kg of female bodyweight as a function of age.

Shown by solid line for average female of that age. Inferred from estimated length-at-age distributions and estimated relative reproductive output by length. Dotted stair-step line corresponds to previous ‘knife-edge' assumption.

Figure 3. Estimated trends in adult biomass and spawning potential.

Time series of adult biomass estimated by CKMR (‘adult' taken here to be age 10+ to match previous usage in stock assessments), shown as the dark solid line with crosses; 90% confidence intervals are shown as narrow solid lines. Comparable point estimates of adult biomass in three scenarios from a pre-CKMR stock assessment are shown by circles. The dashed line shows total female spawning potential as estimated by CKMR (in units of ‘equivalent 1,000 age-16 females').