The Ross Sea Shelf Survey (RSSS) has been a critical component of Antarctic toothfish stock assessment since 2012, incorporated as either an index of biomass or abundance with associated age composition data. This study evaluates the potential for age-specific abundance indices to improve monitoring of year class strength (YCS) and enhance assessment performance. We compared traditional abundance index approach with age-specific indices derived from RSSS data for ages from 5 to 20, using an integrated Bayesian stock assessment model framework.
Results show that age-specific indices for ages 7‒8 provide optimal performance, showing the strongest correlations (>0.5) with estimated year class strengths and YCS estimates that are more consistent YCS estimates with the fishery age compositions seen in later years. These ages represent fish that have fully recruited to survey gear while maintaining adequate sample sizes (caught in adequate numbers in the RSSS). Younger ages (5-6) showed poor correlations, possibly due to availability or due to gear selectivity, while older ages (10‒20) exhibited reduced correlations likely due to ontogenetic changes in behaviour and spatial distribution (low numbers caught in the RSSS).
The age-specific approach enables direct tracking of cohorts through the population, providing more robust YCS estimates. However, temporal constraints limit YCS estimation to years where age 7 data are available from the RSSS, restricting recent estimates to year classes through 2016 based on current survey coverage.
We recommend using age-specific RSSS indices for ages 7‒8 in future Antarctic toothfish assessments, while maintaining the existing approach for comparison.