An exploratory longline fishery for Antarctic toothfish (Dissostichus mawsoni) has been carried out in the Ross Sea region since the late 1990s. No fishery-independent methods of monitoring stock abundance were plausible, and hence a multi-year mark-recapture tagging program was initiated by New Zealand fishing vessels in 2001, and tagging has been an ongoing requirement for all vessels fishing in the fishery since 2004. Although tagging experiments are commonly used for assessing fish stocks, most rely on estimates of abundance from a single release and subsequent recapture events. An integrated Bayesian stock assessment model was developed for Antarctic toothfish in the Ross Sea region that incorporates multi-year release of tagged fish and subsequent multi-year data of recapture of tagged fish in conjunction with fishery catch-at-age data.

This is the first published stock assessment of toothfish in the Ross Sea region. It demonstrates the value of multi-year mark-recapture programs to estimate fish stocks even where no fishery-independent estimates of adult biomass are available.

Monte-Carlo Markov Chain (MCMC) estimates of initial (equilibrium) spawning stock abundance (B₀) were 68 790 tonnes (95% credible intervals 59 540–78 470), and current (B₂₀₁₃) biomass was estimated as 74.8% B₀ (95% CIs 71–78). Sensitivity analyses carried out showed the data selection provided a precautionary estimate of biomass, and that these estimates were insensitive to selectivity assumptions regarding cryptic biomass of older fish. Results also suggested that a sub-adult survey series that started in 2012 will provide a useful signal indicative of recruitment fluctuations as the time series develops, a signal which is not present in the fishery-dependent data.