A big brown bear with a big salmon. There is more to the fishing behavior of Ursus arctos than meets the eye! Photo by Scott W. Michael. “In small backwaters where the fish collect in shoals and where the fins of the fish are visible while they move against the current. The bears either enter the water attempting to drive the fish into the shallows, or stand in the water, lowering the head to the surface while waiting for the fish and catching it rapidly with the paws.... The catching of fish is easier in spawning places with calm water, where the fish are less cautious and strong. When the fish are numerous the bear eats mainly the head and part of the back, when fish are scarce, they are eaten completely. The stomach of a large bear may contain 20 kg of fish. However, plants and berries are always found together with fish.” Yu. V Averin (1948) describing the feeding behavior of the Russian brown bears in the Kronoki reserve (in Stroganov 1969)
As far back as 1889, bear-watchers reported that bears selectively ate parts but not all the salmon they caught. A. M. Nikol’skii (in Stroganov 1969) stated that when fish are abundant, Russian brown bears eat only the head and roe of the salmon, leaving the body. When the fish are scarce, they consume their entire catch. Yu Averin, in the 1948 quote above, also reported on this phenomenon.
Most salmonids are anadromous. That is, they hatch in freshwater, move into the sea, and return to the freshwater streams where they hatched to spawn themselves. While it varies from one species to the next, in many parts of Alaska, three different species of salmon spawn during the months of July through early October. The salmon enter their spawning grounds and typically live in freshwater for 5 to 25 days (depending on the species and location) until they die of senescence. Many die before this time as a result of bear predation. In one study, of 1,933 salmon tagged by researchers as they entered three different Alaskan streams, 64 % of the tagged fish were killed by bears, 33 % died of senescence, and 2.6 % died from other causes (like stranding on the beach or were killed by other predators).
When a salmon makes its way into freshwater, it stops feeding and expends incredible amounts of energy on reproduction and the act of swimming upstream – as a result, from this point forward the nutrient value of a fish will drop as they burn stored fat and protein. Researchers Hendry and Berg (1999) found that the total energy content of a salmon drops from 40 to 50 % and the lipid content plummets as much as 80 to 95 % from the time the fish enter a stream and the time when they die of senescence. It would make sense then that brown bears would attempt to capture and consume those fish that have spent less time in Alaska’s streams and that the fish that the bears find least attractive would be those that are reproductively spent and in the process of dying.
But how could a bear determine the nutrient value of a fish (e.g., how long it had been in freshwater)? In some salmon species there are very overt signs of senescence. For example, male red or sock-eye salmon (Oncorhynchus nerka) develop hooked jaws during the spawning period. Salmonid energy stores are depleted and the immune system begins to suffer, as a result of stress and malnutrition, the skin becomes discolored and patches of fungus appear. The longer a salmon is in freshwater engaging in the spawning process, it picks-up more wounds on the body from fighting and frayed fins as a result of nest digging. So, it maybe that these outward signs of a salmon’s physical condition (potential energy content) could be used by grizzlies to help select the most valuable prey.
Researchers have examined if brown bears target those fish that have been in freshwater for a shorter period of time (“younger” fish) and thus have greater nutrient content. They found that in streams where bears can easily capture salmon that they do select younger salmon. In one such stream, the bruins fed most heavily on fish that have been in the stream for three days or less, while they totally ignored fish that had been in freshwater for over 12 days. However, in streams that were wider, had deep pools and/or lots of structure (e.g., sunken limbs and overhanging banks) brown bears fed more on “older” fish (individuals that have been in the stream for over 12 days). In these streams, the salmon that have recently entered freshwater are more energetic and better able to elude the bears, while “older” fish that have expended lots of energy and are “on their last fin” are easier to catch. While the “younger” fish may have more nutrients, the bears will expend more time and energy to catch them. Therefore, in these streams, going after the less elusive, but less nutrient-rich fish, is a better strategy.
Other factors that will impact brown bear foraging strategies are the density of fishes in a particular stream and the weather. When the streams are chock-a-block with amorous fish, the bears can be more selective, while if the fish are few and far between, they cannot afford to be as particular. Likewise, if the streams are breaking their banks and the water is turbid as a result of excessive rain, bears are not as selective. It is get what you can, when you can! There are also individual bears that apparently develop a taste for rotting salmon carcasses. Bledsoe (1987) tells of a bear named “Zubin” at McNeil Falls that would leave a good fishing spot to go scavenge for dead fish.
Food handling in brown bears sometimes changes as the fishing season goes on. At first, the bears will consume the entire salmon, but as the season goes on, on if it has been a good salmon run, the bruins become more selective, restricting their feeding efforts to the more nutritious bits. They peel off the skin, they nibble out the brains, and tear open the abdomen to gain access to the roe. It is not uncommon to see a grizzly step on a fish, causing a stream of orange, nutritious eggs to spew forth. The bears use their tongue to lap-up the eggs.
References:
Bledsoe, T. 1987. Brown Bear Summer: Life Among Alaska's Giants. New York, New York, E P Dutton. 249 Pp.
Hendry, A. P. and O.K. Berg. 1999. Secondary sexual characters, energy use, senescence, and the cost of reproduction in sockeye salmon. Can. J. Zool. 77:1663-1675.
Quinn, T.P., S.M. Gende, G.T. Ruggerone, and D.E. Rogers. 2003. Density-dependent predation by brown bears (Ursus arctos) on sockeye salmon (Oncorhynchus nerka). Can. J. Aquat. Sci. 60:553-562.
©2008 Scott W. Michael
No comments:
Post a Comment