Updated CPUE of blue shark (Prionace glauca) in the Indian Ocean estimated from Japanese observer data between 1992 and 2019

We updated the Japanese observer data until 2019 and standardized nominal catch-per-unit-effort
(CPUE) of blue shark caught by Japanese tuna longline fisheries in the Indian Ocean from 1992 to
2019. We used generalized linear model (GLM) with negative binomial error distribution to
standardize the nominal CPUEs. The most parsimonious model was selected by Akaike Information
Criterion (AIC) as the best model for the estimation of annual CPUEs. The goodness-of-fits were
diagnosed by residual plots. The 95% confidence intervals were estimated from the bootstrapping
method. The annual CPUEs had a similar trend to those shown in the previous analysis except in 2000.
The annual CPUE increased in 1990s and reached to the peak in 2000, and then gradually decreased
with a large fluctuation until 2013. Since 2014, the annual CPUE showed an increasing trend. We
suggest that the estimated annual CPUE should be utilized as one of the candidates of primary
abundance indices in the next stock assessment of blue shark in the Indian Ocean scheduled in 2021
because the Japanese observer data covers a wide range of the main distribution area (temperate water)
of blue shark in the Indian Ocean and a longer time period compared to the other fleets’ CPUE data