We used moored upward-facing echosounders in combination with field campaigns to address the overwintering ecology of the clupeid sprat (Sprattus sprattus) throughout four separate winters in a Norwegian fjord. The stationary echosounders were cabled to shore and provided continuous measurements at a temporal resolution of seconds. The long-term coverage of several winters enabled study of the sprat behavior in relation to different biotic parameters like abundance, vertical distribution and taxonomic composition of potential prey and predators, as well as environmental conditions like ice-free vs. ice-covered waters and hypoxic- vs. normoxic conditions. Also the size distribution of the sprat differed significantly between years. The majority of the large-size classes had empty stomachs, particularly prominent in one winter. Otherwise, the diet of the sprat seemed to vary according to the fluctuating mesozooplankton community, yet with calanoid copepods being the most common prey in the sprat stomachs all winters. Krill were not common prey apart for the largest sprat in one winter, but particularly large concentrations of krill appeared to mitigate predation pressure from gadoids, which then preferred krill as prey. During daytime, sprat distribution and swimming behavior varied according to the oxygen conditions. Solitary swimming in near-bottom-waters (~150 m) prevailed in moderate hypoxia (30% O2 saturation) as opposed to schooling in mid-waters when the deep waters were oxygen depleted (0–7% O2 saturation). Nevertheless, a bimodal vertical distribution with an additional part of the sprat population distributed in upper waters was common in all years. The sprat carried out diel vertical migration (DVM) in all winters, but the patterns varied, and included both normal and asynchronous DVM, including fish with a somewhat deeper nocturnal than daytime distribution. Moreover, individual sprat carried out short and rapid excursions to the surface during the night in all years, likely for gulping atmospheric air. Ice conditions imposed a behavioral response with the sprat moving to shallower depths after the ice covering. The varied ecology and behavior observed throughout the course of four consecutive years underlines the importance of conducting long-term studies for the understanding of overwintering strategies. Overall, this study provided unique insight into the dynamic conditions that a population of fish may encounter while overwintering, providing novel information on a scarcely described phase in the life history of fish at high latitudes.
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