Evaluation of pre-spawning monitoring of Barents Sea capelin for assessment and advice

Abstract The capelin is a dominant forage fish in the Barents Sea and is also harvested commercially. With its short life cycle, death after one spawning and a fishery targeting only maturing fish close to spawning, it is a challenging stock to manage and assess. The only abundance estimate used for stock assessment is from September—6 months prior to spawning, and a reliable survey closer to the time of the spawning could therefore improve the assessment. Here, we evaluate the results from a five-year series of capelin monitoring in early March during pre-spawning along the Norwegian coast. The confidence range of the survey results and the stock assessment forecast overlapped for 7 of the 8 survey coverages, but in 6 of 8 cases mean biomass from the survey was below the 25% quantile of the forecast. A lower measured than predicted biomass is probably at least partly caused by the lack of survey coverage in the east. The survey abundance estimates were associated with high uncertainty and relative sampling errors above 0.25 for 6 of the 8 estimates, likely due to the patchy distribution of capelin. The survey in its present form is hence more suited as a fallback in case the autumn survey fails than for direct implementation in stock advice. We found weight-at-length of capelin to be lower during pre-spawning than in autumn, which is presently not considered in the assessment. This highlights the importance of a pre-spawning survey as validation of key parameters in the stock forecast.