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.
When summer resident whales leave, others humpbacks move in. Some summer residents are even skipping their annual transoceanic mating and birthing trips to Hawaii, Mexico or other warm waters in favor of icy Alaska waters.
"It did show that whales were exerting predation pressure on Prince William Sound herring, which is potentially impeding the recovery," Straley said.
Many Prince William Sound fishermen still curse Exxon for the absence of herring.
Record commercial harvests were recorded in the late 1980s. The gash in the 987-foot Exxon Valdez on March 23, 1989, oozed oil into the sound about the time adult herring were laying eggs, which adhere to plants and rocks before hatching two weeks later into larva that feed on the spring plankton bloom, and after about 10 weeks grow into juvenile fish.
Herring took a major hit. By 1993, just 25 percent of the expected adults returned to spawn. State regulators closed commercial fishing in 1993, and other than openings in 1997 and 1998, it has stayed closed.
Herring play a vital role in the food chain. The silvery fish with blue-green upper bodies, considered large when they reach 9 inches, are food for eagles and other sea birds, halibut and 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, and backed by a $900 million civil settlement with the petroleum company, says the reasons for the poor recovery remain largely unknown. Exxon Valdez oil remains trapped along miles of gravel beaches, but there's no indication that herring spawning areas overlap with that oil, according to the trustee council.
There are other suspects in the herring mystery: disease, ocean changes, contaminants and competition from other fish. One researcher is studying whether juvenile herring spend so much energy fighting a disease, Icthyophonus, that they don't survive the winter when there's no food. Straley and others funded by the trustee council are looking at humpbacks.
For an angler trolling Alaska ocean waters in a tiny skiff, hoping a salmon will bite the dead herring on the end of his line, few things are as terrifying or thrilling as an interruption by a humpback whale.
Seemingly not a threat from hundreds of yards away, humpbacks can dive and surface a stone's throw away. Fishermen know the leviathans are not going to intentionally ram them, but seeing a 50-foot black hulk undulate out of the gray water, heaving a fountain of spray out a blowhole, can make casting from shore seem like a far better idea.
Anecdotal evidence from fishermen and other boaters, Straley said, indicated more humpbacks were using Prince William Sound in winter. Four studies funded by the trustee council suggest they're having an effect on herring.
Straley's research confirmed whales were feeding mostly on herring. Ron Heintz, another NMFS 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.