Monday, February 8, 2010

They may be charismatic, but megafauna often get wrongly blamed for poor fishing

Hawai'i's humpback whales are being blamed for the low herring count in Prince William Sound, just as the depleted Atlantic bluefin tuna are being blamed for low sardine counts off Europe.

Even Hawaiian monk seals are blamed for declines in human nearshore fishing success.

It's a bizarre phenomenon. Among the folks pointing fingers are anglers, chafing under fishing restrictions, are finding culprits to blame, and doing it largely on purely anecdotal evidence.

In the case of the Alaskan herring, researchers concede that the Exxon Valdez oil spill, which dumped 11 million gallons into the sound 21 years ago, crashed the herring population. The spill occurred just during the spawning season, and the next year's herring take was down 75 percent.

Since then, the little fish haven't recovered. Meanwhile, as Hawai'i residents know, Humpback whale populations have roughly doubled. The whales winter in tropical waters, like those in Hawai'i, and summer in the resource-rich feeding grounds off Alaska and other northern waters.

Anglers and others say that just like Hawai'i, they're seeing more Humpbacks around Alaska. And some of the big whales seem to be staying the entire winter, passing up their winter migration.

Researchers concede they are considering other factors, like other fish having moved into the traditional herring spot on the food chain, climate change, toxins, and a rampant disease, Icthyophonus, which is attacking young herring. Oh, and maybe residual impacts from the oil spill have some role.

But clearly humpback whales do eat herring, and always did. So do seals, porpoise, birds, and other fish. Other species of fish also feed on the herring eggs.

The research is concluding that, with regard to humpbacks, yep, whales are big animals and they eat a lot of herring.

It seems inescapable, though, that generally, if you crash a prey population and the feeding pressure from all kinds of predators remains strong, you'll have a delayed recovery.

Here's a page on the biology of Atlantic herring.

And while we're in the Atlantic, there's a related story about the prime sashimi fish, bluefin tuna. It eats sardines. Also herring, mackerel, anchovy, flying fish (malolo), squid, eels and various crustaceans.

The bluefin has been overfished to the point of near collapse, according to the best fisheries scientists. They're arguing for a complete ban on fishing for bluefin, to save the species.

Many fishing industries understandably don't want a ban—even a temporary ban to let the species recover.

Among the arguments: even this depleted bluefin population is responsible for herring decline.

Seems to be a pattern.

In Hawai'i, endangered Hawaiian monk seals have sometimes been blamed by anglers for low fish numbers—but when someone actually went and did the science, they found that humans and seals generally eat different fish

The aggressive digging and rock-flipping behavior of seals tends to favor seafloor species that are not preferred by human fish-hunters. And the feeding often occurs in deeper water than most coastal anglers use for fishing.

© Jan TenBruggencate 2010

1 comment:

Carol Bain said...

If lower fish count around Hawaiian Islands has been documented, that lack of food source would have a clear negative impact on Kaua'i seabirds. The last mammal I would blame is the whale.