Vulnerability index of taxa: sensitivity and exposure to fishing
To quantify a taxon’s sensitivity to fishing, we selected six traits widely used to characterize the life-history strategies of marine taxa: longevity, maximum length, reproductive guild, fecundity, age at maturity and size of the offspring (Winemiller & Rose 1992; Jenningset al. 1998; Le Quesne & Jennings 2012). Most of these traits came from PANGAEA database (Beukhof et al. 2019a), but were completed by literature. We applied a Hill-Smith analysis (Hill & Smith 1976), a multivariate analysis that enables to use both qualitative and quantitative traits, and gives the same weight to quantitative and qualitative traits (independently of the number of levels for a qualitative trait).
We used the proportion of biomass of a population that is exploited by fishing, to reflect the exposure to fishing. We expressed the exposure of a taxon i as the ratio between its removal by fisheries and its stock biomass in the study-area:
\begin{equation} \text{Exposure}_{i}=\ \frac{C_{i}}{B_{\text{tot},i}}\nonumber \\ \end{equation}
With \(C_{i}\) the biomass of the taxon i landed (i.e. catches) and discarded and \(B_{\text{tot},i}\) the total biomass present in the Celtic Sea (area 7e-j) in 2016. This ratio was directly available from the International Council for the Exploration of the Sea (ICES) working groups’ reports for species whose stocks are assessed and have a spatial distribution relevant with our study area (ICES 2020). For species without stock assessment, exposure was computed using the fishing mortality rates estimated for the corresponding functional groups by the Ecopath with Ecosim (EwE) Celtic Sea model (Hernvann et al.2020). These rates are issued from the ratio of onshelf catches in ICES divisions 7e-j (official landings from STATLANT, (ICES 2019) elevated by discard rates from the DISCARDLESS project, http://www.discardless.eu/) and biomass estimated for the corresponding area from the EwE mass balance equations. As EwE functional groups can gather several species with similar biological characteristics and trophic ecology, the same exposure was attributed to our network’s taxa when matching the EwE functional group.
Vulnerability of a taxon to fishing was defined as sensitivity added to exposure, following the simplified definition of the IPCC (IPCC 2001). Finally, the proportion of each taxon relatively to the total biomass of all taxa in our dataset was computed to provide an order of magnitude of the proportion of the biomass that is sensitive or vulnerable to fishing pressure. To compute proportion of the total biomass represented by each taxon, we used EVHOE data in 2016 (Evaluation des ressources Halieutiques de l’Ouest de l’Europe,(Garren et al. 2019)). The biomass of each taxon was elevated to the depth-sediment strata, to account for the irregular sampling within a stratum, before computing the proportion.