Fish starved to death by Mediterranean water-column stratification and high salinity in pre-evaporitic Messinian

The restriction of the Mediterranean–Atlantic marine connection over the Messinian stage, which led to a salinity crisis and the deposition of the youngest salt giant on Earth, impacted the composition and structure of marine biota in unprecedented ways, but its effects on the biological functions of marine organisms remains unchartered territory. By analyzing the stable oxygen and carbon isotopic composition of the otoliths of two carefully selected fish species, a pelagic (surface) and a benthic (bottom-dwelling) one, we were able to infer not only the sea surface and bottom salinity and oxygenation conditions, but also the fishes’ metabolic response to the paleoceanographic changes in the eastern Mediterranean from 7.2 to 6.5 Ma. The high salinity and stratification of the Mediterranean water column during this interval hampered the ability of the marine fishes to grow, particularly those dwelling in the sea bottom. A first event (6.82–6.81 Ma) of high temperature and salinity induced an increase in the metabolism of fishes across the water column, meaning that they consumed and respired more. Subsequent warming, increased salinity and stratification at 6.70–6.69 Ma further intensified the environmental stress for bottom-water fishes. To cope with these extreme conditions at the sea floor, benthic fishes show increased metabolic carbon despite low food availability, which implies that fishes metabolized (i.e. consumed) their own tissue to survive. The disappearance of benthic fishes in the study area after 6.8 Ma further reinforces this conclusion. Our study promotes the importance of otoliths stable isotopic analyses as tools for reconstructing complex paleoenvironmental histories.