ANCHORAGE, Alaska - 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 41.6 million litres 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 favour 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 300-metre 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 per cent 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 23 centimetres, 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 15-metre 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.
Humpbacks are baleen whales. Their throats expand to ingest large volumes of water, which the whales force out across baleen, the flat, flexible plates with frayed edges, arranged in parallel rows, 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 per cent to 7 per cent per year in the North Pacific, with 3,000 to 5,000 using the northern Gulf of Alaska.
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.
John Moran, a National Marine Fisheries Service research biologist in Juneau, evaluated whale abundance, photographing whales and identifying individuals by the distinctive patterns on the underside or their flukes.
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 kilometres long and hundreds of meters deep.
The research indicated 199 humpback whales were using Prince William Sound at some point from September through March, with up to 129 there at one time.
Researchers learned whales don't migrate like caribou: they don't all leave at once. When summer whales left, others that had been feeding on krill moved in, perhaps from deeper water on the continental shelf.
Heintz's model gave a range of how much herring the whales might be eating - 1.5 to 4 gigajoules per day - the caloric equivalent of 600 to 2,200 Big Macs. That translates to a lot of herring, somewhere between 2,200 metric tons and 13,000 metric tons over the winter and a significant portion of the estimated total.
"The whales were able to consume somewhere between 10 per cent and 66 per cent of that prespawning 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 their work is just a snapshot and more research is needed. They want to find out if whales are feeding at night and whether humpbacks have reached juvenile herring, which do not mix with adults but spend winters in schools in shallower water near the heads of fjords.
"It's thought it's the juvenile mortality that seems to be the point of time when they're not recovering," Straley said. "I don't know this, but if it is, then whale predation on that age class could really hurt the recovery of herring."
The presence of humpbacks in Prince William Sound is like a bell curve, with a peak in August, Straley said.
"As the population increases, we're just going to see more and more whales at the tail end of that bell curve," Straley said. "This is what I think is going to happen."