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.