Are tag-based integrated stock assessments robust to IUU fishing?

Integrated stock assessments depend on reliable catch and observational data to produce unbiased estimates of fish stock biomass and productivity. However, biases in catch data due to unreported catch are common among many of the world's fisheries, especially those that are high value and therefore more vulnerable to illegal fishing. Some of these fisheries rely on tag-recapture data to support the estimation of biomass in integrated assessments and set precautionary catch limits intended to safeguard the fish stocks from overexploitation. Tag-recapture data is generally considered to be a more powerful indicator of stock abundance than survey data if the underlying assumptions are met. The effect of unreported catches that include tagged fish on biomass estimates in tag-based integrated assessments is unclear. We used a simulation analysis to determine the impact of under-reported catch on a hypothetical Patagonian toothfish (Dissostichus eleginoides) stock. Several catch scenarios are presented that cover different magnitudes and trend types of under-reported catch. We used the CASAL model (commonly employed to assess toothfish stocks) as both the operating model and the estimation model to evaluate how the various scenarios affected estimation performance of biomass depletion. Biomass depletion was increasingly underestimated as the magnitude of under-reporting increased, regardless of trend in catch. When unreported catch exceeds twice the amount reported combined with unreported tag returns, the estimation model will fail to detect biomass has been depleted below a target reference point of 50 % within 20 years of fishing. The lack of detection will falsely indicate current catch limits are sustainable when in reality they are not. Importantly, these estimates remained largely unchanged among different trends in under-reported catch. For assessments reliant on tag-recapture data, this suggests that it is more critical to estimate the overall quantity of unreported catch and the number of unreported tagged fish than the trend.