Signatures of fishing on marine foodweb structure

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The Marine Strategy Framework Directive requires EU states to develop indicators characterizing marine environmental status. As acknowledged by the EU Commission in 2010, there is a need to develop indicators that capture the structure of foodwebs. To allow informed management, it is essential that chosen indicators are sensitive to anthropogenic pressures. We consider three species‐resolved foodweb indicators and assess their sensitivity to an increasing fishing mortality rate F, using a multi‐trophic species‐resolved model parameterized for the Northeast Atlantic. This model uses a special algorithm mimicking community assembly, which generates communities with hundreds to thousands of dynamically coexisting model species, reflecting the species richness of real marine foodwebs. Thus, this modelling approach is a highly appropriate method for projecting future states of fished foodwebs to assess the sensitivity of species‐resolved foodweb indicators. The three indicators we consider are the Shannon–Wiener Index (SWI), average degree of satiation (related to average productivity) and average number of prey species, as computed using all fish species. We found that with increasing F, the SWI and average prey diversity decreased whereas the average degree of satiation increased. However, for all three indicators, the percentage change was < 25% for F ≤ 0.5 year–1. Thus, these indicators are not particularly sensitive to fishing. We show that sensitivity can be substantially improved when computing these indicators using a subset of fish species in a specified range of trophic levels or species size classes.