The diel vertical migration patterns and individual swimming behavior of overwintering sprat Sprattus sprattus

Abstract

We addressed the behavioral patterns and DVM dynamics of sprat overwintering in a Norwegian fjord (150m) with increasing hypoxia by depth. An upward-facing echosounder deployed at the bottom and cabled to shore provided 4months of continuous acoustic data. This enabled detailed studies of individual behavior, specifically allowing assessment of individual vertical migrations at dusk and dawn in relation to light, analysis of so-called rise-and-sink swimming, and investigation of the sprat’ swimming activity and behavior in severely hypoxic waters. Field campaigns supplemented the acoustic studies. The acoustic records showed that the main habitat for sprat was the upper ∼65m where oxygen concentrations were ⩾0.7mLO2L−1. The sprat schooled at ∼50m during daytime and initiated an upward migration about 1h prior to sunset. While some sprat migrated to surface waters, other individuals interrupted the ascent when at ∼20–30m, and returned to deeper waters ∼20–50min after sunset. Sprat at depth was on average larger, yet individuals made excursions to- and from upper layers. Sprat were swimming in a “rise and sink” pattern at depth, likely related to negative buoyancy. Short-term dives into waters with less than 0.45mLO2L−1 were interpreted as feeding forays for abundant overwintering Calanus spp. The deep group of sprat initiated a dawn ascent less than 1h before sunrise, ending at 20–30m where they formed schools. They subsequently returned to deeper waters about ∼20min prior to sunrise. Measurements of surface light intensities indicated that the sprat experienced lower light levels in upper waters at dawn than at dusk. The vertical swimming speed varied significantly between the behavioral tasks. The mixed DVM patterns and dynamic nocturnal behavior of sprat persisted throughout winter, likely shaped by individual strategies involving optimized feeding and predator avoidance, as well as relating to temperature, hypoxia and negative buoyancy.