Extensive gas bubble release in Norwegian spring-spawning herring (Clupea harengus) during predator avoidance

Abstract

Events of extensive gas bubble release in over-wintering Norwegian spring-spawning herring (Clupea harengus) were repeatedly observed acoustically and visually in Vestfjorden, northern Norway, during attacks from killer whales (Orcinus orca) and saithe (Pollachius virens). Gas bubbles were visually observed to come from the swimbladder of individual herring swimming very close to the surface and gas bubble production was so extensive that large areas of the sea surface were sometimes covered with white foam after each event. Acoustically, gas bubbles could be identified on the echosounder as strong echoes covering the upper 0-30 m of the water column. It is suggested that herring schools were forced from large depths up to the surface by predatory killer whales and saithe, after which they expelled gas as a consequence of the rapid change in depth. Gas release may confuse and deflect both visually and acoustically oriented predators due to increased scattering of light, reduced range of vision, and confounding effects of the reflection energy of bubbles and fish. Such events may have considerable effect on target strength and estimated stock sizes during acoustic surveys.