Poleward distribution of mesopelagic fishes is constrained by seasonality in light

Abstract

Aim Mesopelagic fishes have a near‐global distribution in the upper 1,000 m from tropical to sub‐Arctic oceans across temperature regimes. Yet, their abundance decreases poleward and viable populations seem excluded from high latitudes. Why? Location North Atlantic between 50–85°N, with implications for high‐latitude oceans globally. Time period Present‐day. Major taxa studied Diel vertically migrating (DVM) mesopelagic fishes. Methods We use a mechanistic, state‐dependent life‐history model to characterize DVM mesopelagic fishes. This model links light‐dependent encounters and temperature‐dependent physiology, allowing optimal DVM strategies to emerge. We run the model along a latitudinal gradient with increasing seasonality in light and track individual fitness‐related measures, that is, survival and surplus energy, through the annual cycle to make predictions about population consequences. Results Mesopelagic fishes thrive in the oceans’ twilight zone, and many are dependent on periods of darkness for safe foraging near the surface, before migrating back to depth during daytime. When daylight lasts for 24 hr during the Arctic summer, these fish are trapped in deep waters void of prey because it is never safe to forage in the shallow waters where zooplankton prey are found. Hence, they are left with two poor options, starvation at depth or depredation while foraging. Our model predicts surplus energy, vital for reproduction and growth, to halve from 50–85°N and annual survival to drop by two‐thirds over a narrow range of 10° of latitude around the Arctic Circle. Thus, low recruitment and high predation mortality during summer make polar waters population sinks for mesopelagic fishes because of the extreme seasonality in light. Main conclusions At high latitudes, foraging mesopelagic fishes are exposed to sunlight in upper waters also at night. This makes them easy prey for visual predators, which limits their poleward distribution. Our findings highlight the importance to think beyond temperature to explain high‐latitude range limits.

Species distribution models describe spatial variability in mesopelagic fish abundance in the Southern Ocean

“…We suggest that the high relative importance of DO 200 for predicting abundance of E. antarctica, P. bolini and K. anderssoni might be a consequence of these species utilizing oxygen minimum zones as a predator avoidance strategy. Alternatively, studies on Benthosema glaciale in the Boreal to Arctic northeast Atlantic show that the latitudinal variation in seasonal light regime may also be an important control of mesopelagic fish depth distribution patterns (Langbehn et al, 2019;Langbehn et al, 2022), highlighting the need for further investigations into the environmental drivers of myctophid vertical migration behavior in the Southern Ocean.…”

“…Myctophid fish concentrate along the periphery of anti-cyclonic eddies where other prey species aggregate (Pakhomov and Froneman, 2000). Additionally, abundance at Antarctic latitudes is driven by oceanic processes, such as high intensity eddies, which facilitate expatriation of mesopelagic fish (Langbehn et al, 2022), for example this has been shown to occur in the Scotia Sea (Saunders et al, 2017;Murphy et al, 2021).…”

“…Discrepancies between our findings and those of other studies may be linked to the spatially and temporally dynamic nature of the DVM habits of fish. DVM can vary with season (Saunders et al, 2015;Langbehn et al, 2019;Langbehn et al, 2022) and latitude (Proud et al, 2017;Langbehn et al, 2019;Langbehn et al, 2022), for example, during the summer months at high latitudes the longer daylight hours confine mesopelagic fish to greater depths to avoid predators, leading to a decline in vertical migration amplitude (Langbehn et al, 2019;Langbehn et al, 2022). Further, DVM behaviors can vary depending on the life stage of some species (Collins et al, 2008;Moteki et al, 2009;Lourenco et al, 2017).…”

Feeding ecology of Benthosema glaciale across the North Atlantic

“…According to the "light exclusion" hypothesis (Kaartvedt, 2008;Langbehn et al, 2022) mesopelagic fish at high latitudes are expected to face adverse feeding conditions, as the absence of periods of sufficient darkness during the polar summer is hypothesized to restrict migration into surface waters at night. Such restricted migration was indeed observed in the Iceland Sea a month before our sampling, i.e., the shallowest nighttime depth of the mesopelagic Cumulative distribution of prey weight in the stomachs relative to body weight.…”

“…Available data suggests that abundances of myctophids declines at high latitudes (Sameoto, 1989;Knutsen et al, 2017) and this has been hypothesized to be an effect of the extreme light environment at these latitudes (Kaartvedt, 2008 "light environment exclusion" (LEE) hypothesis), with continuous daylight during summer "preventing" DVM into the productive upper waters and continuous darkness during winter leaving visual predators less efficient at detecting prey. This mechanism was analysed in a recent model study simulating myctophid feeding, growth and survival along a latitudinal gradient from 50°N to 85°N (Langbehn et al, 2022). Their simulation suggests that because of LEE, myctophids are increasingly unable to accumulate sufficient surplus energy to reproduce, particularly at latitudes higher than ~65°N.…”