Comparing size-limit strategies for exploitation of a self-thinned stream fish population

There is increasing evidence that territorial stream fish populations exhibit some degree of self-thinning. Four size-limit strategies were examined, under which a size-structured model population exhibiting self-thinning was exploited. The effects of: (1) increased minimum-size limits; (2) protection of spawners; (3) decreasing maximum-size limits; and (4) slot limits (prescribe lower and upper size limits of fish that must be released) were analysed in terms of population size and mean body size in the population after harvest. Biomass and numbers harvested, mean size of fish taken and proportions of different sizes in the population after harvesting were also analysed. Combinations of high exploitation rates and high minimum-size limits maximized both the number and biomass harvested while it favoured post-harvest abundance and the proportion of larger sizes in the population. When harvest rates and minimum-sizes were increased, the combinations of these that maintained or increased yield were successively narrowed. Protection of spawners and slot limits did not come close to reaching the levels of post-harvest abundance, yield, positive size structure and endpoints of harvest rates that were obtained with a high minimum size applied to the fishery. Maximum-size limits favoured the abundance of smaller size-classes. The results emphasize the advantages of setting the largest sizes of fish in the population as a minimum size that can legally be retained.