Interactions between herring fishery and killer whales in northern Norway

No abstracts are to be cited without prior reference to the author.

The main prey of killer whales (Orcinus orca) occurring in the coastal waters of northern Norway is Norwegian spring-spawning herring (Clupea harengus). A long term study (1990-2004) has been conducted in the wintering grounds of herring to study interactions between killer whales and their prey. Killer whales have been studied using behavioural observations, underwater video, high-frequency sonar and satellite-tracking. During daytime herring is present in deep waters (150-300 m) and killer whales can chase schools from this deep layer towards surface (dives down to 312 m recorded). However, the whales prefer to search out small patches of herring, which occur in shallow waters in early morning. Feeding occurs mainly in shallow waters and in areas with underwater seamounts (habitat covering less than 10 % of the study area), which aid the whales in herding their prey. Killer whales are not capable of catching herring unless they have stunned them first with tail slaps. As the herring catch quotas have increased from 70 000 tn in 1990 to 470 000 in 2004, the whales have learned to follow the fishing fleet and feed on herring that falls from the nets when the catch is being pumped in. The herring fishery has thus become a major factor influencing the movement pattern, habitat use and behaviour of killer whales.