The effectiveness of broad-scale legal minimum lengths for protecting spawning biomass of Haliotis rubra in Tasmania

Abstract

A legal minimum length (LML) aims to protect spawning biomass but the implementation uncertainty associated with broad-scale LMLs is rarely assessed. LML estimates can require both growth and maturity data; however, growth data is often lacking and this can limit the estimation of an appropriate LML. To overcome the shortfall in growth data, a method is introduced for inferring growth parameters from size at maturity data. In this way, fine-scale theoretical LML estimates were obtained in the absence of empirical growth data. A total of 252 populations of blacklip abalone (Haliotis rubra) were examined and, of those, 46 were identified as having a relatively lower level of protection because they matured at larger sizes. These 46 populations are potentially at greater risk of over-fishing as a consequence of implementation uncertainty. Furthermore, the majority of the 46 populations were in an economically valuable region of the fishery. With extensive data on size at maturity, the relative risk to recruitment potential can be assessed and equalised across a fishery.