Demographic and harvest analysis for blue shark (Prionace glauca) in the Indian Ocean

An estimate of the population growth rate (γ ) is an essential biological information that can be
input into stock assessment models to estimate management quantities for commercial fisheries.
Demographic analysis conducted using the Leslie matrix method was used to estimate γ for Indian
Ocean blue shark (Prionace glauca), with Monte Carlo simulation used to quantify uncertainty. A harvest
analysis was conducted with various selectivity patterns. The results indicated that the productivity
of the blue shark was high, with γ = 0.26–0.32 y−1 under the assumption of a one-year reproductive
cycle. The steepness of the Beverton–Holt stock-recruitment model was estimated to be 0.72 (0.24–
0.87) when the Castro and Mejuto fecundity formular was assumed, and 0.80 (0.65–0.88) when the
Fujinami et al. fecundity formular was assumed. Harvest analysis with tuna longline selectivity led
to lower sustainable harvest rates, but a higher proportion of biomass removed compared to the
selectivity patterns for other fisheries. This study provides crucial prior information on the population
growth rate and steepness parameters, which can be incorporated into stock assessments for blue
shark. Besides, harvest analysis could be a supplement for data-poor stock assessment and risk analysis,
to evaluate a tradeoff among different fisheries when considering a management strategy.