Input data for the 2021 South Pacific blue shark (Prionace glauca) stock assessment
Blue shark (Prionace glauca) are targeted and caught as bycatch in tuna and billfish fisheries in the WCPFC and globally. The present report details data inputs for the South Pacific stock assessment for blue shark, including length frequency information from regional observer programmes, reconstructed catch histories, and a number of alternative catch-rate (CPUE) series.
Length frequencies showed a substantial difference between high latitude (south of 35◦South) and low latitude fisheries, with high latitude fisheries catching both juveniles (approx. 50 cm+) and mature animals (_ 200 cm), whereas lower latitude fisheries encounter mostly large animals (around 200 cm). The EU longline fleet appear to also catch the largest size animals (250 cm+), which do not appear in any other fishery. Based on the length frequency analysis, subsequent analyses were structured latitudinally, or included appropriate standardisation variables to account for spatial and temporal differences in trends among these fisheries.
Catch was reconstructed from observer data using a Bayesian implementation of a spatial GLMM, including a term for non-linear effects of total effect by latitude. The model produced good diagnostics, and led to trend estimates that were comparable to previous analyses, albeit at lower median estimated total catches than previous analysis. Nevertheless, the Bayesian model also produced high uncertainties in catches between the mid 1990s and early 2000s, with overall catches ranging between 100 000 and 1 million sharks per year (90% confidence). CPUE trends from observer models were found to be dominated by observer effort in a restricted area before the mid-2000s, leading us to question their usefulness as indices of abundance for the larger area. Logsheet
CPUE series were attempted, using delta-lognormal GLM models, for a number of areas and fleets, including New Zealand, Australian, Japanese and Chinese-Taipei fleets. Grooming for vessels with consistent reporting attempted to remove vessels with poor reporting rates, and we retained only series from positive observations, discarding presence absence components as potentially biased due to changes in reporting. Although there was some variability among series, there were also consistent trends: all series showed some level of increase in CPUE in the recent decade. In addition, when accounting for ontogeny, high latitude trends align well with low-latitude trends for larger individuals. Disagreements among indices arose mainly for early CPUE (late 1990s), where both the Australian and Japanese indices showed declines between the mid-1990s and early- 2000s, while the high latitude indices did not show corresponding declines.
Our results show a reasonable amount of consistency among datasets in recent trends, suggesting that blue shark may have been increasing aer fishing mortality dropped in the early 2000s. Trends in the 1990s are less certain, due to poor observer coverage, and poor reporting of sharks in logsheet data. Nevertheless, there is some evidence that the adult spawning stock declined during the late 1990s and early 2000s in low latitudes, but catch rates may have been less affected in high latitudes.