Herring that survived Alaskan oil spill may be endangered by humpback whales

ANCHORAGE -- Something is holding down the herring population of Alaska's Prince William Sound, and marine scientists are tailing some rather large suspects: humpback whales.

Humpbacks, once hunted to near extinction, are thriving in waters fouled 21 years ago by the Exxon Valdez, the supertanker that ran aground and leaked nearly 11 million gallons of crude oil.

The herring population crashed after the spill but should have rebounded by now. One hypothesis is that humpbacks, traditionally summer residents in the sound, are taking a big bite out of vast herring schools that form in the deep water of the sound's fjords each autumn.

Jan Straley, a marine biology professor at the University of Alaska Southeast, and other researchers have studied whales the last two winters with surprising results. Humpbacks are showing up in significant numbers, even in winter.

This research "did show that whales were exerting predation pressure on Prince William Sound herring, which is potentially impeding the recovery," Straley said.

The gash in the 987-foot-long Exxon Valdez on March 23, 1989, oozed oil into the sound about the time adult herring were laying eggs. By 1993, just 25 percent of the expected adults were returning to spawn. State regulators closed commercial fishing in 1993, and it has stayed closed most of the time since then.

Herring play a vital role in the food chain. The silvery fish with blue-green upper bodies, considered large when they reach nine inches, are food for eagles and other seabirds, halibut, cod and -- most important to humans -- five varieties of Pacific salmon.

The Exxon Valdez Oil Spill Trustee Council, formed to oversee restoration of the injured ecosystem, says the reasons for the poor recovery remain largely unknown. It sees no indication that herring spawning areas overlap with remaining oil. Other suspects include disease, ocean changes, contaminants and competition from other fish. Straley and others funded by the trustee council are looking at humpbacks.

Humpbacks are baleen whales. Their throats expand to ingest large volumes of water, which the whales force out across baleen, which are flat, flexible plates that filter out and catch herring, zooplankton or krill, tiny floating crustaceans.

Though still listed as endangered, humpbacks have made a promising comeback, increasing 5 to 7 percent per year in the North Pacific.

Anecdotal evidence from fishermen and other boaters, Straley said, indicated that humpbacks were increasingly using Prince William Sound in winter. Straley's research confirmed that whales were feeding mostly on herring. Ron Heintz, another research biologist, set up a model to estimate the proportion of spawning biomass that could be consumed by whales in winter, when herring bunch in schools that can be miles long and hundreds of feet deep.

Heintz's model gave a range of how much herring the whales might be eating: between 2,200 and 13,000 metric tons over the winter, a significant portion of the estimated total.

"The whales were able to consume somewhere between 10 and 66 percent of that pre-spawning biomass," Heintz said. "Another way to look at that is that the last commercial fishery in Prince William Sound was about 3,500 metric tons, so the whales are clearly capable of consuming a biomass that would be in the ballpark of a commercial fishery in Prince William Sound." The biologists say that their work is just a snapshot and that more research is needed. They want to find out if whales are feeding at night and whether humpbacks have reached juvenile herring.

-- Associated Press