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Marine Ecology Progress Series

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MEPS 464:245-256 (2012)  -  DOI:

Continuous acoustic studies of overwintering sprat Sprattus sprattus reveal flexible behavior

Ingrid Solberg1,*, Thor A. Klevjer1, Stein Kaartvedt1,2

1Red Sea Research Center, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia
2University of Oslo, Department of Biology, PO Box 1066 Blindern, 0316 Oslo, Norway

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 high-resolution 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 mid-water 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

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Cite this article as: Solberg I, Klevjer TA, Kaartvedt S (2012) Continuous acoustic studies of overwintering sprat Sprattus sprattus reveal flexible behavior. Mar Ecol Prog Ser 464:245-256.

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