Bottom trawl impacts on Mediterranean demersal fish diversity: Not so obvious or are we too late?

Abstract

Measures of biodiversity change may be useful as indicators if they are responsive to manageable drivers of biodiversity loss. However, there are many candidate indicators that are considered to be robust to survey artifacts and sensitive to manageable impacts. Using extensive survey data on demersal fish assemblages around the Balearic Islands (western Mediterranean) we analyze relationships among 'traditional', taxonomic and functional diversity indices, to identify a minimum set of indices that provide a good representation of the different aspects of diversity. Secondly we model the responses of the demersal fish community diversity to bottom trawl fishing pressure. To do so, we used two different approaches: i) considering fishing effort and depth as continuous explanatory variables; and ii) grouping samples according to bathymetric sampling strata and contrasting levels of fishing effort. The results show that diversity can be described using different complementary aspects such as species richness, evenness, and the taxonomic and functional breadth of the species present in a given community, displaying different responses to fishing pressure. However, the changes in diversity in response to fishing may only be detectable in those communities where the levels of fishing pressure have remained relatively low. When communities have been exposed to high levels of fishing pressure for a long period, the relevant changes in diversity may have happened long before the onset of monitoring of the fishery, and hence it may be too late to detect differences between levels of fishing effort. This seems to be the case on the middle slope of the Balearic Islands, where vulnerable species have disappeared or are very infrequent, and have been replaced by species better-adapted to fishing impacts.

Spatio-temporal trends in diversity of demersal fish assemblages in the Mediterranean

“…The increasing trend in N 90 with decreasing fishing effort reinforces the previous results that confirm the usefulness of this index for detecting the effects of fishing on demersal fish diversity. The increase of evenness with increasing fishing effort has been suggested by some authors (Murawski 2000, Zhou et al 2010 due to the reduction of dominant species by fishing (Cury et al 2000, Rice 2000 and has been confirmed by the study of the effects of fishing on evenness indices (D'Onghia et al 2003, Farriols et al 2017. However, the expected increase in S and decrease in J' with decreasing fishing effort are not always observed in our results.…”

“…The spatial distribution of the bottom trawl fishing effort by GSA shows that the number of vessels per km 2 is low on the continental shelf of the Balearic Islands, Sardinia and the Aegean Sea (Colloca et al 2017). The coincidence of areas with a low fishing effort with areas with a high diversity is in accordance with previous works, in which higher values of N 90 and S and lower values of J' were associated with areas with a low fishing effort (Farriols et al 2017). The lower fishing effort exerted by the relatively smaller bottom trawl fleets in these areas could have preserved, at least to some extent, their fish diversity along with a better conservation of their sensitive and essential habitats, such as maerl and Peyssonnelia beds.…”

“…In contrast, S and J' in a group are calculated from their mean values and consequently species identity is lost. This may explain why extreme values of fishing effort were needed to detect the effects of fishing in S and J' in previous works (Farriols et al 2017).…”

“…Diversity indices, such as species richness (S) and Pielou evenness (J'), which have shown some kind of response to fishing impact for demersal fish assemblages in the Mediterranean (Farriols et al 2017), were also included in this work. These traditional indices are also helpful for comparison with previous works.…”

“…Assessment at a community level is therefore crucial, particularly due to the multispecies nature of the bottom trawl fishery and also because a decline in the diversity of demersal assemblages has been reported due to fishing exploitation (e.g. Ungaro et al 1998, Sabatini et al 2013, Farriols et al 2017. Assessment at a community level is also a requirement for the implementation of an ecosystem-based management of fisheries (Browman and Stergiou 2004).…”