Extensive gas bubble release in Norwegian spring spawning herring (Clupea harengus) during predator avoidance
Working paper
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http://hdl.handle.net/11250/105853Utgivelsesdato
1998Metadata
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Events of extensive gas bubble releases in overwintering Norwegian spring spawning
herring (Clupea harengus) were repeatedly observed acoustically and visually in
Vestfjorden, northem Norway, during attacks from killer whales (Orcinus orca) and
saithe (Pollachius virens). Gas bubble production was so extensive that large areas of
the sea surface were sometimes covered with white foam after an event. Gas bubbles
were visually observed to come from the swimbladder of individual herring
swimming very close to the surface. Acoustically, gas bubbles could be identified on
the echosounder as strong echoes covering the upper 0-30 m of the water column.
Schools of herring were forced from 30-100 m depth up to the surface by predatory
killer whales and saithe. I suggest that herring expel gas near the surface as a
concequence of the rapid change in depth, and that gas bubble release may confuse
and deflect both visually and acoustically oriented predators due to increased
scattering of light, reduced range of vision, and confusing effects of the reflection
energy of the bubbles and the fish. Such events may have considerable effect on the
target strength and estimated stock sizes during acoustic surveys.