The diversity of recent trends for chondrichthyans in the Mediterranean reflects fishing exploitation and a potential evolutionary pressure towards early maturation

Abstract

Chondrichthyans are a vulnerable group that has been overexploited for almost half a century in the Mediterranean. Since in this area most chondrichthyans are rarely incorporated into international statistics, the impact of fishing on their populations is difficult to assess. Here, we evaluate temporal trends in order to understand the recent history of chondrichthyans in the western Mediterranean. Fishery-independent data were obtained from scientific surveys carried out from 1994 to 2015 in three geographical sub-areas. Our results reflect fairly stable populations in terms of diversity, with some increase in density and standardized biomass of some species dwelling on the continental shelf, and even for some species dwelling on the slope. In contrast, decreasing trends were observed in some deep-water species. This can be explained by the reduction of the trawling effort on the continental shelf over the last few decades, and the shift of the fleet towards deep waters, along with the greater resilience displayed by some species. Furthermore, a decreasing trend in maturity of Scyliorhinus canicula was detected, suggesting an evolutionary response to overfishing. These results improve scientific knowledge for developing true adaptive management in the Mediterranean that will implement measures to strengthen or initiate the recovery of chondrichthyans. Marine ecosystems are under pressure from overfishing, habitat degradation and climate change which have altered the populations of most of their species 1,2 , especially chondrichthyans (sharks, rays and chimaeras), due to their low population resilience 3-5. Global chondrichthyan landings, reported to the Food and Agriculture Organization of the United Nations (FAO), increased over 200% from 1950 to the peak year in 2003 and subsequently dropped almost 15% by 2011 6. In the Mediterranean, the temporal evolution of chondrichthyan landings has gone through three stages: a development phase at the beginning of the 1950s; followed by intense fishing pressure from the mid 1950s to 1970, corresponding to the exploited phase; and a decreasing trend from 1970 to recent years, indicating an overexploited status (Fig. 1), such as has been reported for most demersal and pelagic bony fish and shellfish stocks a decade later during the eighties 7-10. This earlier overexploitation in chondrichthyans is to be expected due to their low resilience. Studies of long time series of chondrichthyan catches conducted in several areas of the Mediterranean have also shown decreasing trends over the last few decades 11-14. Furthermore, there is evidence that Mediterranean chondrichthyans have declined in diversity. At present, several species such as Dipturus batis (recently split into two cryptic species, D. cf. flossada and D. cf. intermedia 15 , Pristis

Sharks, rays and skates (Chondrichthyes, Elasmobranchii) from the Upper Marine Molasse (middle Burdigalian, early Miocene) of the Simssee area (Bavaria, Germany), with comments on palaeogeographic and ecological patterns

“…Aetobatus and Chaenogaleus ) (Last et al 2016 ; Froese and Pauly 2019 ). This is consistent with the current distribution of cartilaginous species in the Mediterranean Sea, which are mostly distributed on the continental shelf (Froese and Pauly 2019 ; Ramírez-Amaro et al 2020 ). However, purported deep-water sharks are also present in the Simssee area (i.e.…”

The use of fishers’ Local Ecological Knowledge to reconstruct fish behavioural traits and fishers’ perception of conservation relevance of elasmobranchs in the Mediterranean Sea

“…Similarly, pelagic species such as Alopias spp., Lamna nasus, P. glauca and I. oxyrinchus have recently experienced drastic decreases, and the LEK results indicated similar trends (Ferretti et al, 2008). Conversely, other species have remained at stable levels or are less affected by fishery exploitation, as is the case for G. melastomus, S. canicula and R. clavata in the western Mediterranean Sea (Abella et al, 2017;Ramírez -Amaro et al, 2020) and in the northern Ionian Sea (Serena, 2014;Ricci et al, 2021). If the correspondence between LEK and scientific data is important to evaluate LEK reliability, fishers provided new insights for other species, both commercial, such as D. oxyrinchus in GSA 6, and even noncommercial, such as A. bovinus, in GSA 19.…”

“…Interviewed fishers used different fishing gear and were therefore able to provide information on different species. Their ability to provide a reliable picture is supported by the high correspondence of the species distribution and changes over time reconstructed by fishers' LEK with those evaluated with scientific and fishery surveys in the Mediterranean Sea (Ferretti et al, 2013;Barausse et al, 2014;Colloca et al, 2017;Follesa et al, 2020;Ramírez -Amaro et al, 2020). For instance, according to LEK, Mustelus spp., S. acanthias and S. squatina were widely caught in several GSAs in the past.…”

Physiology: An Important Tool to Assess the Welfare of Aquatic Animals

“…These are primitive vertebrates that are of great scientific interest for the study of evolution of certain physiological strategies, such as the specialization of steroid hormones [ 95 ]. Moreover, as key elements of marine (and some freshwater) ecosystems, it is important to maintain their biodiversity at healthy levels [ 96 , 97 ]. Some countries have signed binding agreements to release sharks captured as bycatch in certain fisheries [ 98 , 99 ], and efforts are being made to minimize the damage suffered by the fishing process.…”