The use of echosounders for long-term studies of the overwintering ecology of sprat (Sprattus sprattus)

Upward-facing echosounders, were used to study the overwintering ecology of sprat during four winters in a Norwegian fjord. The echosounders provided continuous data at a temporal resolution of seconds and enabled studies of individual swimming behavior of sprat in addition to population behavior. The long-term coverage of several winters enabled us to study how the sprat responded to different environmental conditions, like ice-free waters versus ice covered waters and hypoxic conditions versus well-oxygenated waters. The studies unveiled that the overwintering strategies of the sprat are flexible, varying in accordance with environmental conditions.