First signs of recovery of shark and ray populations in Balearic Islands

Two recent studies by scientists from the Balearic Oceanographic Center of the Spanish Institute of Oceanography (IEO, CSIC) suggest that shark and ray populations in the Balearic Islands seem to show signs of stability and in some cases even recovery, although during the last decades their conservation status in the Mediterranean has worsened considerably.

The first study analyzes the trends between 2002 and 2022 of nine indicators developed within the framework of the Marine Strategy Framework Directive in order to assess the conservation status of the elasmobranch community of the Balearic Islands. Specifically, the trends of 19 species of bottom-dwelling elasmobranchs (7 sharks and 12 rays) inhabiting between 50 and 800 meters depth have been analyzed through data recorded in oceanographic surveys of the MEDITS program.

The results reveal that between 30 and 50% of the populations show increasing trends. This is especially true for the range, abundance and biomass of rays. Even some of the species listed by the IUCN as vulnerable or endangered, such as the mussel (Mustelus mustelus) or the white skate (Rostroraja alba), show clearly increasing trends for some of the indicators assessed, attributable to the decrease in fishing intensity in the Balearic Islands.

"Although the shark and ray community of the Balearic Islands shows signs of stability and recovery, it is still difficult to know if it will be able to recover the levels of more than half a century ago, when the industrial development of trawling began," says Alba Serrat, researcher at the Balearic Oceanographic Center and first author of the first study.

The second study analyzes with new production models the data from the MEDITS campaigns together with the historical catch data from the Palma fish market and shows that the biomass of the stingray Raja clavata - the most commercially important ray species in the Mediterranean - has suffered a sharp decline in the Balearic Islands since the industrial development of trawling began. Even so, in the last 10 years there has been a decrease in fishing effort on this species and a recovery of its biomass in the study area, although it is not enough to consider that the species is being exploited sustainably.

In addition, this study uses novel genetic methods to analyze its conservation status, such as genetic diversity in mitochondrial DNA. The results show that it has very low levels of genetic diversity, probably due to the high levels of exploitation it has suffered historically. Although its population size currently shows signs of recovery, the recovery of genetic diversity is a much slower process.

"This loss of genetic diversity may imply a notable decrease in the adaptive potential of this species, further increasing its vulnerability to fishing and climate change," says Francesca Ferragut, researcher at the Oceanographic Center of the Balearic Islands and leader of the second study.

Sharks and rays are slow-growing animals, reach sexual maturity late and have a low reproduction rate, which makes them especially vulnerable to overexploitation. They represent a significant fraction of the bycatch from trawl fisheries in the Mediterranean, but most will not be landed. As a result, more than half of the sharks and rays in the Mediterranean are threatened by fisheries exploitation and 43% of the species are classified as "endangered" or "critically endangered" on the International Union for Conservation of Nature's Red List. Shark and ray populations are collapsing worldwide.

"Thus, although the results of these studies may seem optimistic, it is urgent and necessary to implement mitigation measures for the conservation of Mediterranean sharks and rays," concludes the scientific team involved in both analyses.