Protecting old fish through spatial management: is there a benefit for sustainable exploitation

1. Spatially defined restrictions on fishing activity are considered to be important for biodiversity conservation in marine ecosystems. However, it is uncertain whether such restrictions also benefit wider populations of exploited fish species, in terms of a reduced risk of overexploitation. Since fishing leads to contraction of the age structure, one potential benefit of protection is recovery of the highly fecund older age classes, potentially leading to higher recruitment levels. This investigation explores the benefits of protecting older fish and how spatial management can be used to adjust the age structure and improve the sustainability of the catch (productivity). 2. We use a non-spatial equilibrium model accounting for biomass growth, mortality and recruitment to describe the relationship between mean age of the population and productivity for the South African deepwater hake Merluccius paradoxus trawl fishery. Our results indicate that management measures capable of increasing the mean age at which fish are caught may be of benefit. Furthermore, although the contribution of older fish to recruitment is important, the biomass growth of individuals before they are caught is responsible for the most significant productivity benefit of an older population age structure. 3. Older M. paradoxus are found in deeper water, so that distinct age classes can be defined spatially using empirical data. We describe a non-equilibirum model to examine spatial management alternatives for adjusting the age structure by targetting these different age classes. We investigate the benefits of protecting either older or younger fish, with results suggesting that it is more important to limit fishing on the younger (shallow) sections of the population if productivity benefits are to be realized. 4. Synthesis and applications. We conclude that the age structure of a population can be modified through spatially managed targeting of different age classes, and that, contrary to previous assumptions for M. paradoxus, protecting older fish has a negative consequence for the age structure of the resource. Instead, younger sections of the population should be protected through limitations on fishing in shallower waters, if older age classes are to recover.