Intense exploitation over millennia has depleted Mediterranean Sea species from the large to the small, including the Mediterranean monk seal, sea turtles, bluefin tuna, groupers, red coral, lobsters, and limpets. Although history suggests that these impacts have been significant, it is difficult to evaluate their magnitude because there is no rigorous historical baseline for the abundance of marine species. However, an international group of researchers have now established the first current comparable baseline of ecosystem structure at the Mediterranean scale, focusing on nearshore rocky reefs.
What would a ‘healthy’ Mediterranean rocky bottom look like? There are no pristine sites (i.e. undisturbed by humans) left in the Mediterranean against which to compare the health of current ecosystems. However, research on pristine, historically unfished sites in the central Pacific show that intact, complex reef ecosystems harbour large biomass (ie total weight of all fish in an area) of fishes and high coral cover. It seems reasonable to suppose that large fish biomass equals healthy reef therefore.
The scientists therefore conducted underwater surveys using SCUBA divers across rocky reefs throughout the Mediterranean Sea, including in Spain, Italy, Greece, Morocco and Turkey. They did this both in marine protected areas and open access sites. The surveys showed remarkable variation in the life on these reefs.
The results suggest that the healthiest shallow rocky reef ecosystems in the Mediterranean have both large fish and algal biomass.
Fish biomass was significantly larger in well-enforced no-take marine reserves. However, there were no significant differences between marine protected areas which allow some fishing and open access areas at the regional scale.
Fish data were collected using standard underwater visual census techniques. Sampling stations within sites were spaced at least 1 km apart from the next, except in very small marine reserves. The data revealed three groups of sites mainly constituted by: (1) well-enforced no-take marine reserves with high fish biomass, (2) partial marine protected areas and weakly enforced no-take marine reserves with lower fish biomass, and (3) non-enforced marine protected areas and areas open to fishing.
The sites with the least amount of fish were in Turkey and Morocco. These had among the lowest reported ever in scientific literature for shallow reefs, even lower than the most overfished coral reefs in the Caribbean.
The Scandola Natural Reserve in Corsica is probably the best example of a ‘healthier’ rocky reef, without fishing and with good water quality.
There was an absence of sharks at the study sites. Sharks were much more abundant historically in the Mediterranean and they used to be an important component of nearshore food webs. The largest predators now are male dusky groupers (Epinephelus marginatus).
A major insight is that, at the Mediterranean scale, partially protected Marine Protected Areas which allow some fishing are not effective in restoring fish populations. For this you need well enforced no-take marine reserves.
The researchers hope that their findings will be used to assess the health of any similar habitat in the Mediterranean, and to evaluate the efficacy of marine protected areas.
Sala E, Ballesteros E, Dendrinos P, Di Franco A, Ferretti F, et al. (2012) The Structure of Mediterranean Rocky Reef Ecosystems across Environmental and Human Gradients, and Conservation Implications. PLoS ONE 7(2): e32742. doi:10.1371/journal.pone.0032742