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The behavior of the mesopelagic fish Benthosema glaciale was studied at 60°N in mid‐summer. We hypothesized that diel vertical migration (DVM) is constrained by short and dusk nights (surface illumination > 10−2 μmol m−2 s−1) and that individuals are active at depth during the long summer days. Submerged echosounders provided high‐resolution data throughout the water column. During the day, a part of the population ascended toward the increasing daylight. Short vertical relocations were followed by minutes of vertical inactivity. Swimming included horizontal turns and loops associated with the vertical steps. Normal DVM was initiated ~ 4 h before sunset and reflected independent individual decisions. The fish initially ascended stepwise but switched to mostly straight upwards swimming attaining 3–4 cm s−1. Their vertical speed was faster than the slow ascent of isolumes and even the deepest living fish potentially could reach upper layers shortly after sunset. However, many individuals aborted their ascent and returned to depth before the darkest time of the night, while others returned downward closer to sunrise. The daytime swimming and individual variability in diel migration behavior have implications for encounters with prey and predators in the twilight zone and the biological carbon pump. A principal conclusion is that mesopelagic fishes can modify their behavior and migration patterns to suit a wide range of changing conditions. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Variability of mesopelagic scattering layers is often attributed to environmental conditions or multi-species layer composition. Yet, little is known about variation in behaviour among the individuals forming scattering layers. Based on a 10 months high-resolution dataset from stationary echosounders in a Norwegian fjord, we here assess short-term and long-term behaviour of a single mesopelagic fish species, the pearlside Maurolicus muelleri. The daytime vertical extension of the monospecific pearlside scattering layers spanned four orders of magnitude ambient light in the autumn and winter and less than one order of magnitude in summer. While the main layers tracked relatively stable light levels over daytime, some individuals actively crossed light gradients of up to 1.5 orders of magnitude. This included individuals that moved between scattering layers, and apparently bold individuals that made regular upward excursions beyond the main population distribution. During the daytime, M. muelleri mitigated the risk of predation by forming tight groups in the upper scattering layer and, at light levels >10−6 µmol m−2 s−1, by instantly diving into deeper waters upon encounters with predators. Our observations suggest that individual, and probably state-dependent, decisions may extend the pearlsides' vertical distribution, with implications for predator–prey interactions. [ABSTRACT FROM AUTHOR]

While top–down cascades from fish to phytoplankton have been a core topic in limnology for the past four decades, it has attracted far less interest in marine ecology. This is partly for historical reasons, since lake studies have been motivated by the ability to regulate algal blooms induced by cultural eutrophication, while marine studies have been more oriented towards fish yield and thus bottom–up processes. Also freshwaters are closed ecosystems with lower diversity and complexity, making models and predictions on trophic levels and interactions comparatively simpler. Here, we compare some key properties of freshwater and marine top–down cascades and argue that despite some striking differences, the large number of freshwater studies may pose valuable insights also for marine systems. Moreover, we argue that there is an urgent need for more focus on top–down cascades in marine systems that address how top predators or fishing may propagate through the food web and impact autotrophic biomass, production, C-sequestration and thus ultimately the global carbon cycle and climate. [ABSTRACT FROM PUBLISHER]

The article offers the author's insights over the research paper by Xabier Irigoien regarding the concetual model of the role of lipids in the life history of the marine copepod Calanus finmarchicus. It points out the possibility of the absence of metabolic expenditures during diapause. It also explores the assumption that predation and efficiency of protein and fat synthesis play a factor in the copepods's early descent to overwintering habitats