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Abstract

In the context of stock assessment and fishery conservation, determining catch per unit effort (CPUE) accurately and reliably is essential. This study estimated trends in the relative abundance of blue sharks (Prionace glauca) in the Indian Ocean from 2005 to 2022. Blue sharks constitute a resilient bycatch species in pelagic tuna and swordfish longline fisheries. We employed a multimodel approach that included delta-lognormal, zero-inflated negative binomial, and vector autoregressive spatiotemporal (VAST) models to standardize catch rates recorded by observers in the Taiwanese large-scale longline fishery. Our analysis concentrated on the CPUE of blue sharks, including standardizing the number of fish caught per 1,000 hooks. Model residual analysis indicated that the VAST model was the most favorable of the models considered. Although the overall pattern of the standardized CPUE series indicated neither overexploitation nor a consistent downward trend, this pattern revealed fluctuations in the relative abundance of blue sharks rather than a clear and consistent upward trend from 2005 to 2022. The observed stability in blue shark catches by the Taiwanese large-scale longline fishery suggests that blue shark stocks in the Indian Ocean are currently at an optimal utilization level. Future research enhancements could involve incorporating environmental factors into the proposed model and utilizing longer-term observations to enrich the depth and scope of the present research findings.

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