European hake (Merluccius merluccius) assessment based on size frequencies and basic biological parameters in the SW Mediterranean

Assessments of small-scale, coastal fisheries are often hindered by the lack of complete data sets fulfilling the requirements of conventional stock assessment methods In contrast, existing information from the regular monitoring of fisheries coupled with data-limited assessment methods can help provide the information needed for effective management of fisheries. We investigated the year-to-year length-based estimates of the spawning potential ratio (SPR) of European hake, a species with moderate data availability (i.e., reasonable knowledge of the life-history traits and temporal length structures of the population). We explore the effects of considering fast- vs. slow-growth hypotheses and the uncertainties of input parameters on the annual estimates of the SPR (1998-2010) in a bottom trawl fishery in the SW Mediterranean (Murcia region). The SPR estimates under four life-history scenarios were sensitive to the quality (sample size) of the length frequencies. Consequently, comparable and more accurate SPR estimates were often found for annual samples larger than 2000 individuals. The F/M ratio varied less when size compositions were grouped by period, suggesting there are transitory-population size structures. The SPR estimates were insensitive to the underlying growth hypothesis since similar M/k ratios >2.3 were derived from the four sets of von Bertalanffy growth parameters. The assessment indicated overexploitation of hake (F/M > 2, SPR < 10%) that is comparable with that derived from conventional assessments for this species elsewhere in the Mediterranean Sea. According to our findings, the LB-SPR method can provide reliable stock assessments and allows population trends to be calculated for data-limited species. However, this method requires a good understanding of the input data and their possible sources of bias.