Stock-recruitment relationships in elasmobranchs: Application to the North Pacific blue shark
Stock-recruitment relationships in elasmobranchs are uncertain due to a lack of data, especially regarding pre-recruit survivorship. A pre-recruit survival model for the early life history of elasmobranchs was developed and parameterized using biological data. Then, an existing age-structured model for the reproductive ecology of teleost fishes was modified for elasmobranchs and combined with the pre-recruit model. The combined model was then applied to the North Pacific blue shark (Prionace glauca). The stock-recruitment relationships of this species were clarified using the biological data collected from wide areas of the western North Pacific. The model provides a point estimate for steepness, which represents a fraction of the unfished recruitment when spawning stock biomass is 20% of the unfished spawning stock biomass. Numerical simulations were conducted to incorporate uncertainties in the biological parameters and produce the variance of steepness. The mean value and its standard deviation for steepness with the Beverton-Holt model were 0.584 (standard deviation=0.099). The results suggest that the stock-recruitment relationship in North Pacific blue shark remains highly density-dependent and that its productivity is higher than that of other viviparous elasmobranchs.