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

Abstract

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.