Mesopelagic fish have a wide distribution in the ocean twilight zone across the globe. However, their abundance decreases towards the pole, why?

Langbehn et al. set out to find the answers, employing a mechanistic, state-dependent life-history model to characterize vertical migrating mesopelagic fish. This model was run along a latitudinal gradient to make predictions about population consequences.

What they found was that when daylight lasts for 24 hours during the Arctic summer, the fish are trapped in deep waters- not able to migrate upwards to feed. Their only option is to either starve at depth, or being eaten by predators while trying to forage. None of these options result in a growing and thriving population.

The midnight sun plays a huge role in the DVM behaviour of mesopelagic fish. (Picture from Adobe Stock, Menno Schaefer)

Their model predicts that the annual survival of mesopelagic fish drops by 2/3 over a narrow range of 10° of latitude around the Arctic Circle. This is a clear explanation for why the mesopelagic fishes have a limited poleward distribution, and highlights the importance of light in determining the behaviour of these fish.

See the full paper by Langbehn et al. (2021) right here!