Growth and reproduction in mesopelagic fishes: a literature synthesis

The mesopelagic zone covers a vast expanse of the World’s oceans and contains some of the most abundant vertebrates on the planet. This midwater region is central to the transfer of energy and carbon between the atmosphere and the deep, yet there are large knowledge gaps in our understanding of the life history of its animals. Here we synthesize the current state of knowledge of research on age, growth, and reproduction of mesopelagic fishes, the basic biological information fundamental to understanding the population dynamics of species in this ecosystem. Collectively, two-thirds of life history research on mesopelagic fishes has been undertaken on myctophids, yet many other abundant and important groups are lacking research. There are generally hotspots of mesopelagic fish research mostly centred in the northern hemisphere, with little to no coverage in the Indo-Pacific region nor the poles. Furthermore, the effects of some anthropogenic stressors—chiefly climate change and resource extraction—on the life history of the animals in this zone is uncertain and needs to be considered. Knowledge of growth and reproduction are key traits required for a holistic assessment and understanding of this ecosystem, and hopefully this synthesis will provide a springboard for greater focus in this area.