The EU Common Fisheries Policy and similar legislation around the world requires all fish stocks to be managed above maximum sustainable yields (MSY). However, the data needed for full assessments are only available for some stocks. This has renewed interest in simpler methods to estimate stock size from limited data.
This paper focuses on a new length-based Bayesian biomass estimator (LBB) that reduces the data needed for preliminary stock assessments. The reasoning behind the LBB is that the rate of natural mortality in most commercial species has coevolved with growth rate such that maximum age is reached at maximum length. Mortality caused by fishing can also be expressed relative to the growth rate. The combined total mortality can then be estimated from the decline in numbers of longer fish.
LLB takes information on numbers of fish of different lengths and estimates the corresponding gear selectivity and mortality rates. This information can then be used to give preliminary estimates of important fisheries reference points. This, in turn, can be used either directly in the management of data-poor stocks, or to inform other stock assessment methods that use additional data.
This is also relevant in the context of the Marine Strategy Framework Directive (MSFD), which requires that exploited species have a size and age structure indicative of a healthy stock. This ensures the presence of large and highly fertile spawners as a buffer against recruitment failures.
This study shows that optimizing the length at first capture results in a mean length of spawners similar to that without fishing, thus presumably fulfilling this requirement of the MSFD.
Fishers sorting and measuring the catch of a beach-seine in False Bay, South Africa. Undersized fish are collected in a hoop-net for immediate release. Of 170 permits for such operations in the 1970s, only four remain today. Methods such as LBB can help in estimating the stock status of the four main species and in determining minimum landing sizes that reduce the impact of fishing on these stocks. Photographer: Robert Tarr
Paper title: A new approach for estimating stock status from length frequency data
Authors: Rainer Froese, Henning Winker, Gianpaolo Coro, Nazli Demirel, Athanassios C Tsikliras, Donna Dimarchopoulou, Giuseppe Scarcella, Wolfgang Nikolaus Probst, Manuel Dureuil, Daniel Pauly