Selection of indicators for assessing and managing the impacts of bottom trawling on seabed habitats

1. Bottom-trawl fisheries are the most-widespread source of anthropogenic
physical disturbance to seabed habitats. Development of fisheries-,
conservation- and ecosystem-based management strategies requires the
selection of indicators of the impact of bottom trawling on the state of
benthic biota. Many indicators have been proposed, but no rigorous test of
a range of candidate indicators against 9 commonly-agreed criteria
(concreteness, theoretical basis, public awareness, cost, measurement,
historical data, sensitivity, responsiveness, specificity) has been
performed. 2. Here, we collated data from 41 studies that compared the
benthic biota in trawled areas with those in control locations (that were
either not trawled or trawled infrequently), examining 7 potential
indicators (numbers and biomass for individual taxa and whole communities,
evenness, Shannon-Wiener diversity and species richness) to assess their
performance against the set of 9 criteria. 3. The effects of trawling were
stronger on whole-community numbers and biomass than for individual taxa.
Species richness was also negatively affected by trawling but other
measures of diversity were not. Community numbers and biomass met all
criteria, taxa numbers and biomass and species richness satisfied a
majority of criteria, but evenness and Shannon-Wiener diversity did not
respond to trawling and only met few criteria, and hence are not suitable
state indicators of the effect of bottom trawling. 4. Synthesis and
application. An evaluation of each candidate indicator against a commonly
agreed suite of desirable properties coupled with the outputs of our
meta-analysis showed that whole-community numbers of individuals and
biomass are the most suitable indicators of trawling impacts as they
performed well on all criteria. Particular strengths of these indicators
are that they respond strongly to trawling, relate directly to ecosystem
functioning, and are straightforward to measure. Evenness and
Shannon-Wiener diversity are not responsive to trawling and unsuitable for
the monitoring and assessment of bottom trawl impacts.05-Mar-2020