An end-to-end model to evaluate the sensitivity of ecosystem indicators to track fishing impacts

Abstract In order to assist fisheries managers, ecological indicatorsare needed to evaluate the effects of fishingactivities on marine ecosystems and to improve communication of these effects in both public and scientific contexts. Finding appropriate indicators is challenging given the complexity of marine food webs as well as the ecosystem response to fishingpressure. In this study, an end-to-end model developed in the Gulf of Gabes ecosystem (Tunisia) was used to compare the performance of a set of ecosystem indicators in assessing the impact of fishing. This end-to-end model aimed to represent the ecosystem functioning by coupling two existing sub-models, the multispecies individual-based model OSMOSE, representing the dynamics of exploited species and the biogeochemical model Eco3M-Med. The aim of the indicator selection method is to evaluate the sensitivity of a set of ecological indicatorsregardless the fishingmanagement plan. This method was performed in two major steps. The first step consisted in simulating three simple contrasted fishingstrategies in the OSMOSE model exploiting target species (i.e. high trophic level, low trophic levelor all species) and then applying a fishingeffort multiplier for each fishingstrategy to the focus target species. In the second step, three paradigms defining the desirable properties of an ecological indicatorhave been specified: i/the indicator decreases with increasing fishingpressure, ii/the indicator responds linearly to an increase in fishingpressure and iii/the indicator responds consistently across different fishingstrategies. Our results highlighted that the majority of indicators have quite similar performance regarding the trend and the linearity of their responses. However, the size-based indicators seem to be the most robust to track ecosystem effects of fishingwhen the fishingstrategy changes. A focus on size-based indicators showed that Large FishIndicators (40 cm) derived from demersal or all surveyed species were the most suitable to reflect a change in the status of the Gulf of Gabes ecosystem due to fishingpressure.