Concrete protects the oceans

by Rebecca Nesbit

Fishing trawlers are an important industry along the north coast of Spain but it is one of the most damaging human activities affecting the shallow waters. The inner coastal shelf (at depths of up to 100m) is a nursery ground for several commercial fish species and is home to a great diversity of fish and other marine organisms.

Bottom trawling is banned in these shallow regions, but sadly continues illegally. To prevent this artificial reefs (OK, if you read further it emerges these are concrete blocks) have been built, making the area inaccessible to trawlers.

These reefs provide an ideal opportunity to study the effects of trawling – how has the ecosystem recovered since reefs put an end to trawling?

The first finding was that overall fish and invertebrate biomass increased when trawling stopped, which is what you would expect. However, not all species showed a steady increase.

The likes of red mullet, john dory and skate kept increasing as time went on. These species are the ones who are most reliant on the shallow waters and don’t normally move beyond them, so it’s not surprising protection of these waters has such a big effect on them.

But the populations of some species quickly stabilised after an initial recovery phase of about 4 years. Why the numbers stop increasing is different for different species. The conger eel, for example, needs holes and crevices to hide in and the availability of suitable hideouts could limit their numbers. Once all the crevices are taken the population doesn’t increase any more.

Some species didn’t increase at all, and in fact some of the main fishery species continued to decline. These fish are caught by longlines and other fishing techniques too, so the reefs provide no protection from these. Many of them actually spend time further out to sea as well, so the areas protected by reefs are less important to them.

The invertebrate community showed a massive increase in biomass, most notably the sea urchins, and also the starfish and common octopus (good food apparently). In the case of the octopus it’s not just the lack of trawling that is likely to have caused its increase but the addition of the ‘reefs’ themselves because they like to hide under hard objects and need cracks and crevices to spawn in.

Overall, it wasn’t just biomass that increased, but also the species richness and the size of the largest fish. Species richness, i.e. the number of species found, can be used as a measure of the sensitivity of the ecosystem to disturbance.

The moral of the story is that artificial reefs which prevent bottom trawling are good for the marine community. An unusual situation whereby concrete is good for the environment…

This research was published this month in:

Serrano, A., Rodríguez-Cabello, C., Sánchez, F., Velasco, F., Olaso, I., & Punzón, A. (2010). Effects of anti-trawling artificial reefs on ecological indicators of inner shelf fish and invertebrate communities in the Cantabrian Sea (southern Bay of Biscay) Journal of the Marine Biological Association of the United Kingdom, 91 (03), 623-633 DOI: 10.1017/S0025315410000329