Planktivorous fish in a future Arctic Ocean of changing ice and unchanged photoperiod

Climate change drives fish and plankton species ranges toward the poles, often related to warmer waters mediating geographic distributions via changes in vital rates. Yet, the distribution of fish may also be governed by less acknowledged mechanisms. Ice limits access to air for physostomous fish filling their swimbladders at the surface. We hypothesize that release of ice constraints may facilitate northward expansion of physostomes, with implied impact on their zooplankton prey. On the other hand, even in a changing Arctic, the extreme high-latitude photoperiod will persist. The abundance of mesopelagic fish is low in the Arctic Ocean. Feeding conditions may be inferior during the darkness of winter and in light summer nights. If the photoperiod is constraining distributions, biogeographic boundaries of mesopelagic fish may be relatively unaffected by climate change. Alternatively, if low temperatures are their main constraint, we hypothesize that northward extensions in a warmer ocean may be detrimental to key Arctic copepods as we argue that their current success relates to low mortality during overwintering in the absence of mesopelagic fish. It is therefore essential to discriminate the role of the light climate at high latitudes from those related to temperatures for assessing future biogeographic boundaries.