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

Events of extensive gas bubble release in over-wintering Norwegian spring-s pawning herring (Clupea harengus) were repeatedly observed acoustically and visually inVestfjorden, northern Norway, during attacks from killer whales (Orcinus orca) and saithe (Pollachius virens). Gas bubbles were visually o bserved to come from the swimbladder of individual herring swimming very cl ose to the surface and gas bubble production was so extensive that large ar eas of the sea surface were sometimes covered with white foam after each ev ent. Acoustically, gas bubbles could be identified on the echosounder as st rong 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 pre datory killer whales and saithe, after which they expelled gas as a consequ ence of the rapid change in depth. Gas release may confuse and deflect both visually and acoustically oriented predators due to increased scattering o f light, reduced range of vision, and confounding effects of the reflection energy of bubbles and fish. Such events may have considerable effect on ta rget strength and estimated stock sizes during acoustic surveys. (C) 1998 I nternational Council for the Exploration of the Sea.