The role of sex change, growth and mortality in Pandalus population dynamics and management

Abstract

Sex change, growth, and mortality of Pandalus populations are important to stock management. To investigate the importance of these life history traits in setting management strategies, we performed a variety of simulatoins using a length-based model. We compared three populations: a Kachemak Bay, Alaska type (K_pop); a hypothetical one with the same growth but without sex change (H_pop); and a northern Barents Sea type (B_pop). The main difference between K_pop and B_pop is growth rate; K_pop has faster growth and a shorter live span, whereas B_pop lives farther north, and therefore has slower growth and greater longevity. We found that populations with sex change were more sensitive to fishing pressure. Continuous fishing was detrimental to K_pop when instantaneous fishing mortality was above 0.3, but threshold management, i.e. closing the fishery at a low population level, greatly improved the population performance. B_pop, the slower-growing, longer-lived population required larger mesh sizes than K_pop, but increasing mesh size could not prevent collapse under high fishing mortality, and fishery closure under threshold management was necessary to sustain the population. Abrupt decrease of age one and two males caused populations with sex change to fluctuate in spawning biomass. Seasonality in growth and natural mortality is also pertinent to shrimp management. Given faster growth in summer, higher natural mortality in summer means fishing after spring hatching tends to result in higher cumulative yield than fishing after fall mating, and vice versa. In addition, fishing after hatching is in general more robust to overfishing. From the simulation analyses, we concluded that both sex change and seasonality of growth and mortality of Pandalus populations should be taken into account in shrimp management.