One of the objectives linked to sustainable fisheries is for European fish stocks to reach sustainable levels by 2015. Classically, the reference sustainable levels are assessed analytically by single-species approach estimating the Maximum Sustainable Yield (MSY), usually with constant natural mortality term. However, while moving towards ecosystem-based fisheries management (EBFM), it is necessary to consider predator-prey relationships when assessing the dynamics of fish stocks. Recently, in the Baltic Sea, the variability of the natural mortality due to predator-prey relationship has been integrated to the assessment of stocks. Here, using an ecosystem model we aim at evaluating the variability of mortality sources under various fishing pressures for several exploited stocks of the eastern English Channel.
For a first set of species, when increasing the fishing mortality, the predation mortality decreases but slightly, resulting in increasing total mortality. This illustrates the existence of competition between fisheries and predators, which can switch their diet for more prey.
For a second set of species, increasing fishing mortality leads predation mortality and total mortality both increasing. This pattern has to be linked to the importance of the species in trophic functioning, as the amount of food eaten is not reducing, even under strong fishing pressure.
Lastly, for some species, there is no linear response between changes in fishing mortality and predation mortality. For those species, still to be investigated, an increase in fishing mortality can lead to a switch of ecosystem functioning and thus a different trophic position for the species of interest.
These results are of importance when aiming in short term to reach sustainable levels for all exploited stocks, as they will interact and single-species MSY may not be the reference target to reach if predators and/or competitors are to vary consequently.