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

There is increasing evidence that territorial stream fish populations exhib it 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) protecti on of spawners; (3) decreasing maximum-size limits; and (4) slot limits (pr escribe lower and upper size limits of fish that must be released) were ana lysed in terms of population size and mean body size in the population afte r harvest. Biomass and numbers harvested, mean size of fish taken and propo rtions of different sizes in the population after harvesting were also anal ysed. Combinations of high exploitation rates and high minimum-size limits maximized both the number and biomass harvested while it favoured post-harv est abundance and the proportion of larger sizes in the population. When ha rvest rates and minimum-sizes were increased, the combinations of these tha t maintained or increased yield were successively narrowed. Protection of s pawners and slot limits did not come close to reaching the levels of post-h arvest abundance, yield, positive size structure and endpoints of harvest r ates that were obtained with a high minimum size applied to the fishery. Ma ximum-size limits favoured the abundance of smaller size-classes. The resul ts emphasize the advantages of setting the largest sizes of fish in the pop ulation as a minimum size that can legally be retained.