Watching these giant bears chase down and pounce on agile salmon in a river has also amazed me. When I am wading in the river, I move slowly. For me to move, my skinny legs have to displace a lot of water. The amount of water I displace when wading is minuscule compared to that of an 800-pound bear chasing salmon down. So, how can these big bears chase and catch fish?
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For an 800-pound bear to float, an amount of water equal to the weight of the bear needs to be displaced. The physics of floatation is explained by Archimedes Principle, which states for a body to float an amount of fluid equal in weight to the body suspended must be displaced. Since the density of water is 8.3 pounds/gallon, an 800-pound bear swimming displaces 96 gallons of water. When the bear is running on the riverbed, not all of its weight is supported by water (i.e., it is not floating), so less water is displaced. To quickly get to a fish, before the fish swims away, the bear has to move its massive body through the water.
When I watch bears chasing salmon, I notice that they use their large front paws as “paddles” to pull them forward through the water. Their strong arm and shoulder muscles, coupled with their enormous paws, enable them to charge through the water rapidly. Their technique combines a swimming action using their front paws with an explosive push off the river bottom with their rear legs.
As the bear lunges through the water towards a school of salmon, it is churning up a lot of water. The turbulent water and air bubbles can confuse the salmon, trying to escape the bear. In shallower water, less water needs to be displaced for the bear to move quickly through the water. Shallower water also reduces the escape routes for salmon as they have to move sideways, close to the surface, rather than diving into deep holes.
At the start of the salmon run, the fish are active and robust. Bears have a harder time catching them during this time. It is the main reason why I like to photograph the bears and salmon at this time — lots of action! Later in the season, when the spawning salmon are near the ends of their exhaustive journey and ready to die, they are much easier for the bears to catch.
The bear’s drive for valuable calories needed to double their weight between spring and when they hibernate in late fall motivates them to pursue fast salmon early in the season. This drive is so innate that I have observed a young bear with front leg injury dine and dash on salmon in this manner, even though it could only use three legs!
Salmon calories are so crucial to the bear’s wellbeing that I am conscientious about minimizing my presence on the river and how it might affect a bear fishing. I use a long focal length lens that allows me to stay very far (100 meters or more) from the bear and, most importantly, the salmon the bear is after.
I can tell a lot about the bear’s concerns and behavior by looking at their eyes using my lens. If I am in the river and considered to be a threat or a hindrance to the bear as it fishes, then I am negatively impacting the bear. It is subtle; a couple of glances over at me by a bear, usually means the bear is concerned about my location. When this happens, it is time for me to move. Conversely, when I am positioned well away from the bear and salmon, the bears completely ignore me and can focus on the difficult task at hand — catching salmon.
