Continuous acoustic studies of overwintering sprat Sprattus sprattus reveal flexible behavior

Abstract

The clupeid fish Sprattus sprattus was studied in a 150 m deep Norwegian fjord throughout an entire overwintering period during which the fjord froze over and a major water renewal occurred. A bottom-mounted (upward-facing) echosounder provided continuous highresolution data and enabled studies of swimming speed and behavior of individual sprat in addition to population behavior. The continuous acoustic studies were supplemented with intermittent field campaigns. The sprat displayed different behavioral modes with changing environmental conditions. During the first part of the winter, the majority of the population occurred in deep waters during both day and night, yet exhibited a shallower night-time distribution. Individual sprat swam alternately up and down, a 'rise and sink' behavior likely a compensation for negative buoyancy because of swim bladder compression. Because feeding was negligible in deep waters, the swimming pattern was not inferred as prey search behavior. Another part of the population schooled at shallower depths during the day and carried out vertical migration to upper waters at night. However, individuals were observed as they switched between these behavioral groups. A sudden change in both swimming behavior and vertical distribution occurred as the fjord became ice covered. Near-bottom 'rise and sink' swimming was replaced by schooling in midwater during the day, and the sprat aggregated in dense layers near the surface at night. We suggest that the ice made the sprat shift their antipredator strategy from hiding at depth to hiding in schools in the darker waters below the ice. This long-term acoustic study has shown that sprat have a flexible behavioral repertoire, displaying different overwintering strategies within a population, depending on environmental conditions. KEY WORDS: Sprat · Overwintering · 'Rise and sink' swimming · Schooling · Ice cover 464: 245-256, 2012 Populations of sprat are also present in environments that may be covered with ice, e.g. the Baltic Sea and some Norwegian fjords (Ojaveer & Kalejs 2005, Casini et al. 2006). Ice will affect not only light conditions, but also the ability for this physostome fish to ascend to the surface for gulping air in order to fill its swimbladder (Blaxter & Batty 1984). There are, however, no studies on how ice covering may affect the overwintering sprat. Resale or republication not permitted without written consent of the publisher OPEN PEN ACCESS CCESSMar Ecol Prog SerThe inner part of the Oslofjord is easily accessible for long-term acoustic measurements. In the present study we took advantage of this to continuously monitor a population of sprat throughout a whole overwintering period. We deployed an echosounder, cabled to shore, at the bottom of the fjord during a winter where waters were hypoxic, yet sufficiently oxygenated for the sprat to inhabit the whole water column. Because the echosounder was situated at the bottom, it provided high-resolution data of acoustic targets in deep waters, enabling studies of ...

Seasonal and diel patterns in sedimentary flux of krill fecal pellets recorded by an echo sounder

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Vertical distribution and diel vertical migration of krill beneath snow-covered ice and in ice-free waters

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Acoustic assessment of the austral sardine, Sprattus fuegensis, in the inner sea of the Chilean Patagonia

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