The trophic-level based model: A theoretical approach of fishing effects on marine ecosystems

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We propose the trophic-level based model as a new approach to analyse the functioning of marine ecosystems in both ecological and fisheries contexts. The model considers a virtual ecosystem where all the animal biomass is distributed along a continuum of trophic level classes. Biomass moves from one class to the upper ones according to predation and ontogenic processes. From a given secondary production occurring at trophic level 2, the ecosystem biomass distribution can therefore be expressed as the result of the biomass flow passing through the ecosystem, from low to upper trophic levels. The model is based on two main equations. One is regarding biomass flow, which decreases according to fishing and natural losses occurring during transfers. The other expresses the speed of the flow per trophic level, assuming that high metabolism rates induce fast transfers at the lower trophic levels. Additionally, various hypotheses of ecosystem functioning are considered, dealing with the extent of top-down controls, the intensity of feedback effects on secondary production through biomass recycling and the occurrence of a biomass inaccessible to fisheries. Depending on which trophic levels are targeted, various scenarios are simulated. Results highlight the impact of increasing fishing efforts on the ecosystem, in term of total biomass, biomass distribution and mean trophic level. We notably show that high fishing pressure and low trophic level of first catch may lead to severe biomass depletions, even if no overfishing is generally observed. Transfer efficiencies as well as flow kinetics appear as key characteristics of the ecosystems functioning, determining its response to fishing pressure. Feedback effects on secondary production amplify the fishing effects. Conversely, the top-down control may be a major feature of the ecosystem resistance to fishing. It implies that any catch may have effects for all trophic levels and can induce cascade effects in the ecosystem. More generally, we show that the trophic-level based model, built from a small number of very simple and rather unquestionable assumptions, leads to a relevant representation of ecosystem impacts of fishing. It might be regarded as a theoretical basis contributing to our understanding of such impacts. We finally discuss on the use of the model in real cases and we address its usefulness to build a general theory on marine ecosystem functioning.