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Bears and even wolves benefit from the yearly bounty of salmon

Posted: Sunday, August 19, 2007

Watching black or brown bears at salmon spawning streams is a popular activity in coastal Alaska.

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Both residents and tourists enjoy visits to such spots as McNeil River falls, Anan Creek, Pack Creek and our own Steep Creek near the Mendenhall Glacier Visitor Center. In addition to the fun of seeing large, charismatic mammals in the act of hunting, one never knows what else might be seen - maybe a tussle between bears for a particularly succulent fish, or cubs sent scuttling up a tree by a protective mama, or a yearling in a tree, trying to defend its prize from a pair of opportunistic ravens. The 'extras' are worth the investment of patience.

We have it good here, because bears are extinct or rare in much of the Lower 48. Our coastal bears are more numerous, bigger, and have more cubs than bears in the Interior, because they have access to a yearly banquet of fish. In addition, more of those fish are in good condition than the tired long-distance travelers that spawn in the Interior.

But even here, the biggest, most dominant bears have the best pickings. Dominant bears forage for longer periods, more often, and catch more fish than subordinates; they range over more of the stream and don't carry their catch far from the stream. Subordinates spend more time watching out for dominants and avoiding confrontations with them; when they catch a fish, they usually carry it well back into the forest. Social status is likely to have especially important effects in years or sites where few salmon return. Then the big dominants can claim an even greater share and the subordinates get the leavings, with the probably result that they survive or reproduce less well. With bears, fat mamas are better mamas, because the fat accumulated in fall is used to create milk for the cubs born during the winter.

Some bears in Southeast do not visit salmon streams but remain in alpine areas. By avoiding the bearish congestion near salmon streams, they reduce the risk of encountering aggressive, potentially murderous, dominants. Thus, they make a trade-off between safety and rich foraging, and they may produce fewer cubs as a result. It is not entirely clear, however, why these particular individuals make this choice.

One study showed that, in the right conditions, bears can capture an average of 20 or so fish per hour (with a maximum of over 30; catch rates were higher at night than during the day), with an average capture efficiency of 27 percent. The smaller the stream, the more the bears can catch. And the more fish present in a stream, the more the bears can harvest, but only up to a point. So the larger the run, the smaller the fraction generally taken by bears. Thus, the more abundant the returning fish, the higher the proportion that remains to spawn. As a result, if human harvesting reduces the returning run too much, bears may take most of the remainder, leaving few fish to spawn.

Bears can be very selective foragers. Given a choice, they usually prefer fresh fish to spawned-out fish, large fish to smaller fish, and often females to males. Furthermore, they commonly consume only the eggs from a captured female (and sometimes the brain), leaving the rest for scavengers. Captured male salmon, in contrast, often lose their hump and their brains. These body parts contain the most fat, which provides more energy per bite than protein or carbohydrate. Brain tissue also provides relatively high amounts of certain essential fatty acids. Such 'high-grading" is most prevalent when there are lots of fish or the stream is so small that they are easy to catch; when foraging is poor, more of the fish's body is consumed. Dominant bears seem to 'high-grade' more than subordinates.

Wounded salmon sometimes escape from hungry bears and attempt to carry on their reproductive activities. Pacific salmon only have one chance in a lifetime to reproduce, so the spawning drive is really strong. One can occasionally find male salmon, whose brain has been chomped away by a bear, still alive and still trying to mate. Hmmmm....

Less is known about wolves foraging on spawning salmon. Adult wolves in coastal British Columbia averaged 27 fish per hour, mostly pink salmon. Pups had lower catch rates, and a much lower efficiency of capture. Adults were successful in 49 percent of attempts, but pups succeeded in only 13 percent of capture attempts. Wolves 'high-graded' the salmon even more than bears; they ate only the head area of almost all captured fish. In addition to obtaining calories and important fatty acids, these wolves might also have avoided certain parasites in the body of the fish that are lethal to wolves and other canids. Apparently we don't have information on the importance of salmon foraging for wolf survival and reproduction. Wolves also are known to capitalize on the salmon cafeteria on Prince of Wales and probably do so elsewhere in Southeast. But little information is available for our area, and I have found nothing in the published literature about possible interactions with bears.

• Mary F. Willson is a retired professor of ecology and a Trail Mix board member.



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