Analysis of seamount-specific catch and tagging data in the Amundsen Sea, SSRU 88.2H

Catch, effort, and mark-recapture data for Antarctic toothfish from individual seamount features in SSRU 88.2H were analysed to compare multiple raw indicators of biomass trends in isolated local areas. These metrics were also calculated for SSRU 88.2H as a whole. The analyses indicate that:

• Fish seldom move among seamounts within the complex, and residence time on particular seamounts is typically 1-2 years before fish depart from these seamounts, presumably back to the slope SSRUs. Therefore, it is recommended that the stock assessment include only recaptures of tagged fish that have been at liberty for less than 2 years, and that data from the Amundsen Sea slope SSRUs (88.2C–G) are included in the stock assessment along with 88.2H.
• Fishing has occurred on almost every seamount in every year, and no evidence for serial depletion or spatial patterns that would produce a current bias in recapture probability among seamounts was found.
• Single-season tag recapture rates exceeding 20% on some seamounts suggest relatively high local exploitation rates and the potential for localized depletion on some seamounts.
• Results from opportunistic post-hoc Leslie depletion analyses conflicted with other biomass indicators and suggested that opportunistic CPUE-derived abundance measures derived from Olympic fishery catch data are unlikely to index abundance at small temporal and spatial scales.
• Although indicative biomass as estimated by the CPUE by seabed analogy method produces values that vary markedly depending on the choice of reference area, the method produced lower biomass estimates than mark-recapture estimates when based on adequate sample sizes for this seamount fishery.