Palaeontological evidence for community-level decrease in mesopelagic fish size during Pleistocene climate warming in the eastern Mediterranean

Mesopelagic fishes are an important element of marine food webs, a huge,
still mostly untapped food resource, and great contributors to the
biological carbon pump, whose future under climate change scenarios is
unknown. The shrinking of commercial fishes within decades has been an
alarming observation, but its causes remain contended. Here, we
investigate the effect of warming climate on mesopelagic fish size in the
eastern Mediterranean Sea during a glacial-interglacial-glacial transition
of the Middle Pleistocene (marine isotope stages 20–18; 814–712
 Kyr B.P.), which included a 4ºC increase of global seawater
temperature. Our results based on fossil otoliths show that the median
size of lanternfishes, one of the most abundant groups of mesopelagic
fishes in fossil and modern assemblages, declined by ~35% with climate
warming at the community level. However, individual mesopelagic species
showed different and often opposing trends in size across the studied time
interval, suggesting that climate warming in the interglacial resulted in
an ecological shift toward increased relative abundance of smaller-sized
mesopelagic fishes due to geographic and/or bathymetric distribution range
shifts, and the size-dependent effects of warming.