A population of humpback whales off northern 91原创 Island is taking advantage of diving seabirds to exploit herring stocks, new research shows.
Christie McMillan, a fisheries science master鈥檚 student at Simon Fraser University, says that seabirds such as rhinoceros auklets and common murres dive into the ocean and swim beneath the herring so the prey forms tight schools known as 鈥渂ait balls鈥�.
鈥淚t鈥檚 a response of all herring,鈥� she said in an interview at a weekend marine mammal symposium at the University of B.C. 鈥淲hen they鈥檙e startled, their response is to group up tight. It鈥檚 instinctual: 鈥楾here鈥檚 a predator, get close to my neighbour, and hopefully he gets eaten instead of me.鈥欌��
The strategy works better on a one-kilogram bird than a 25-tonne whale.
鈥淎 bird eats one herring at a time, but whales just gobble them all,鈥� said McMillan, who is also president of the Marine Education and Research Society.
Humpbacks in Queen Charlotte Strait have learned to let the seabirds do the initial work before lunging in and swallowing the bait balls, with individual whales consuming hundreds of kilograms per day. Gulls also benefit as the bait ball rises to the surface.
McMillan said she observed the practice during two years of field research.
鈥淚鈥檇 look at a school of herring in the distance and think,鈥� OK, I鈥檒l go sample that,鈥� and often the humpback would beat me to it. They鈥檇 go over and engulf it. They seem to be really benefiting from the birds in that area.鈥�
McMillan estimates up to two dozen humpbacks a day use Queen Charlotte Strait from August to November. During that time, juvenile herring represent about 84 per cent of the annual nutritional requirements of an adult male humpback and 65 per cent of a pregnant female.
鈥淚t鈥檚 pretty important they meet their energy requirements because it will lead to successful reproduction,鈥� she noted.
The findings may have management implications for humpbacks, which are officially listed as threatened by the 91原创 government, although it鈥檚 unclear whether these particular whales have alternative prey sources in the area if herring numbers decline.
McMillan said more than 2,000 humpback whales are thought to feed in the rich waters of B.C. before winter migrations to Hawaii or Mexico, where they give birth.
The next step in the research is to compare the feeding habits of this population of humpbacks with other humpbacks on the B.C. coast to determine whether they, too, specialize on herring or other species.
