Effect of environmental variations on sharks and other top predators in the deep Mediterranean Sea over the last 60 years

Abstract

The deep Mediterranean, which is characterized by high thermohaline stability, has suffered important changes in the last century, but these have seldom been analyzed with regard to megafauna. Long-term changes in fish and decapod assemblages in the Balearic Basin (western Mediterranean) were analyzed after compiling (since 1952) the species compositions of hauls taken at depths of ca. 400 to 600 m. Multidimensional scaling analysis (MDS) showed significant segregation between hauls performed in the period 1952−1964 and hauls taken later. MDS segre- gated hauls taken off Catalonia from those off the Balearic Islands. The most general change recorded was a drop in the occurrence of most of the dominant deep-sea chondrichthyans, espe- cially the shark Etmopterus spinax. Environmental analyses (canonical correspondence analysis and BIOENV) suggested a direct relationship between increases in temperature and salinity in intermediate waters and the decline of deep-water sharks. Abundance of E. spinax also decreased with decreasing O2 in the Levantine Intermediate Water (LIW) that may be related to changes in LIW in the western Mediterranean since the 1950s. This is in addition to increased fishery effort as a factor explaining the drop in shark abundance. In other regions of the western Mediterranean subjected to fishery pressure, E. spinax is still dominant. We suggest that small changes, both anthropogenic and climatic, occurring in the temperature and salinity of deep-water masses since the 1950s in the Balearic Basin have changed the composition of fish (especially sharks due to their high vulnerability) and crustacean assemblages.