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

Events of extensive gas bubble releases in overwintering Norwegian spring spawningherring (Clupea harengus) were repeatedly observed acoustically and visually inVestfjorden, northem Norway, during attacks from killer whales (Orcinus orca) andsaithe (Pollachius virens). Gas bubble production was so extensive that large areas ofthe sea surface were sometimes covered with white foam after an event. Gas bubbleswere visually observed to come from the swimbladder of individual herringswimming very close to the surface. Acoustically, gas bubbles could be identified onthe 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 predatorykiller whales and saithe. I suggest that herring expel gas near the surface as aconcequence of the rapid change in depth, and that gas bubble release may confuseand deflect both visually and acoustically oriented predators due to increasedscattering of light, reduced range of vision, and confusing effects of the reflectionenergy of the bubbles and the fish. Such events may have considerable effect on thetarget strength and estimated stock sizes during acoustic surveys