Application of a multi-annual generalized depletion model to the Mediterranean sandeel fishery in Catalonia

Abstract

A multi-annual generalized depletion (MAGD) model was applied to the data-limited Mediterranean sandeel boat seine fishery in Catalonia. The results show that catch and effort data at high temporal frequency (month), complemented with biological information on mean body weight and initial estimates of natural mortality and time of recruitment to the fishery, can be used to produce assessment results of interest to fisheries management. The estimates of fishing mortality obtained for the Mediterranean sandeel in this fishery are very low compared to natural mortality estimates (less than 10 %) and the important fluctuations in recruitment strength suggest that the dynamics of this fishery are mainly driven by density-dependent effects on the population dynamics of the species. Given the high rate of natural mortality estimated (∼ M = 2 y−1), its short life-span (1–2 y) and that exploitation is based on age 0 individuals, it is recommended to keep exploitation rates at current levels (<4%), which can be adapted on a short time basis, subject to the perceived availability of the stock. This low exploitation rate is compatible with a precautionary escapement biomass of 40 %. A parallel stock assessment was carried out with a surplus production model (SPiCT), which led to similar trends in biomass and fishing mortality as the MAGD model application. The parameter estimates obtained with both models have wide confidence intervals, particularly for the quantities related to population size, suggesting that these data-limited models are likely to suffer from low precision. Precision was lower for SPiCT than for MAGD, and the performance diagnostics for SPiCT were poor. The MAGD model can be used for small-scale fisheries that are unlikely to meet the requirements of standard stock assessments based on catch-at-age methods and can provide indicators (fishing mortality, vulnerable biomass) of interest to fisheries managers.