Examining the effects of changes in the minimum legal sizes used in the Hauraki Gulf snapper ( Pagrus auratus) fishery in New Zealand

Abstract

A minimum legal size (MLS) is seen as a fisheries management tool with the ability to protect juvenile fish, maintain spawning stocks, and control the sizes of fish caught. Described here is a simulation study examining the effects of different MLS strategies in the Hauraki Gulf snapper ( Pagrus auratus) fishery. This fishery is currently managed under two separate MLSs, 270 mm for recreational fishers and 250 mm for commercial fishers. Results showed that the current MLSs were larger than those which would assist in short term stock rebuilding, but that they were closer to those that would maximize rebuilding over the longer term. It is shown that increases in the current MLS for longline and recreational fishing methods would reduce population rebuild in the short term due to the impact of the increased fishing effort required to catch the ‘quota’. Within the snapper fishery the different fishing gears were separated into two groups based on their fishing patterns, one for mesh methods and one for hook and line methods. It was found that due to the higher selectivity (of smaller fish) and higher rates of discard mortality for the mesh methods, very low or no MLSs were optimal. Overall, it was found that selectivity, catch allowances, and population size were the important determinants of the optimal MLS. The importance of these indicated that the effects of different MLS strategies may be examined both with a population dynamics model and within a yield-per-recruit framework.