A simulation-approach to assess the size structure of commercially exploited fish populations within the European Marine Strategy Framework Directive

Assessing the size structure and composition of fish stocks, as demanded by the Marine Strategy Framework Directive (MSFD), is considered to be critical for the evaluation of the health status of exploited populations. The MSFD explicitly contains the concept that many large individuals within a population are indicative of a healthy stock. To reflect the abundance of large individuals, the EU-Commission suggested several size-based indicators (SBI). Only few of these SBI have been tested within a pressure-state relationship, in which a state indicator is sensitive, responsive and specific to a given pressure. Sensitivity and responsiveness of pressure-state relationship can be validated by cross-correlating time-series of pressure and state indicators. In the real world, however, time-series of ecological indicators are not only affected by the influence of a pressure, but by natural variability, changes in the sampling method and stochasticity. Hence observed cross-correlations between pressure and state indicator time-series may not be based on a true causal link, especially if time-series are short (less than 30 years). To overcome these limitations, the performance of eight SBI was tested with a population model in which pressures (fishing mortality) and states of ecosystem components (SBI) were precisely known. We distinguished between relative SBI reflecting proportions of size-classes and absolute SBI reflecting absolute entities of the size-distribution such as the observed maximum size in a given survey year. Relative SBI were more sensitive to recruitment than absolute SBI, but not to fishing pressure, which makes relative SBI unsuitable for the assessment of the abundance of large individuals within a population. The outcomes of the model simulation were confirmed by a case study on North Sea cod, where the mean length of the largest ten individuals caught in a given survey year (Lmax10) emerged as a promising indicator for the assessment of size structure. Exemplary, we demonstrate how the Lmax10 may be incorporated into the existing International Council for the Exploration of the Sea (ICES) assessment framework by defining target and limit values of the good environmental status.