Mediterranean underwater fish visual census surveys data

The positive effect of fully protected Marine Protected Areas (MPAs) on
marine biodiversity, specifically on fishes, has been widely documented.
In contrast, the potential of MPAs to mitigate the impact of adverse
climatic conditions has seldom been investigated. Here, we assessed the
effectiveness of MPAs, quantified as increasing fish biomass, across wide
geographic and environmental gradients across the Mediterranean Sea. We
performed underwater visual surveys within and outside MPAs to
characterize fish assemblages in 52 rocky reef sites across an extent of
over 3,300 km. We used the steep spatial temperature gradient across the
Mediterranean as a ‘space-for-time’ substitution to infer climate-driven
temporal changes. We found that, as expected, Mediterranean MPAs increased
fish biomass. At the same time, higher seawater temperatures are
associated with decreased fish biomass, changes in species composition,
and shifts towards more thermophilic species. Importantly, we found that
the rate of decrease in fish biomass with temperature was similar between
protected and fished sites. Taken together, these results suggest that the
capacity of MPAs to harbor higher fish biomass, compared to surrounding
areas, is maintained across a broad temperature range. At the same time,
MPAs will not be able to offset larger-scale biotic alterations associated
with climate change. Policy implications: Our results suggest that
sustained warming will likely reduce fish biomass in the Mediterranean Sea
and shift community structure, requiring more conservative targets for
fishery regulations. At the same time, protection from fishing will remain
an important management tool even with future high-water temperatures, and
MPAs are expected to continue to provide local-scale benefits to
conservation and fisheries.