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480 bear blog

How many salmon will a bear eat?

By Mike Fitz

On June 30, 2018, I watched one of Brooks River’s largest bears, 747, catch and eat eighteen salmon in a little over three hours. Earlier in the day, bearcam viewers watched him catch and consume many others. Conservatively, his catch that day can be reasonably be estimated at 30 salmon. If each salmon weighed five pounds (2.3 kg) then 747 ate 150 pounds (68 kg) of fish in just one day.  Is this normal though? How many salmon do they eat over an entire summer?

We often observe bears like 747 or 480 Otis partake in marathon fishing sessions at Brooks Falls. They sit for hours in the water, catching fish after fish after fish. During these moments, their hunger appears insatiable. Adult bears at Brooks Falls can catch and eat dozens of salmon per day when fishing conditions are optimal. While no study has tried to quantify a Brooks River bear’s annual salmon consumption, a study from Kodiak Island can offer some insight.

Kodiak bears, although currently considered a subspecies of brown bear (Ursus arctos middendorffi), are very closely related to bears on the Alaska Peninsula, including Katmai. Like all brown bears, they shed their fur once per year in early to mid summer. Since new fur grows during the bears’ active season, it contains a record of what the bears ate during that time. Studies of captive bears had previously determined the relationship between mercury intake and hair mercury content. To apply this to bears on Kodiak, researchers first determined how much mercury is found in the Pacific salmon that spawn on Kodiak. They then analyzed the mercury content found in the bears’ fur to gain an estimate of salmon consumption. Not surprisingly, the results indicate bears eat a lot of salmon.

Adult male bears on Kodiak ate the most salmon on average, consuming an incredible 6,146 pounds (2,788 kg) per bear per year! Adult females ate 3,007 pounds (1,364 kg). Salmon consumption varied among subadult bears, independent juvenile bears between 2.5 and 5.5 years old. Subadult females ate 1,248 pounds (566 kg) while subadult males ate 1,305 pounds (592 kg) of salmon per bear per year.

Bears at Brooks Falls don’t often eat 150 pounds of fish per day, like 747 demonstrated earlier this summer. A specific set of circumstances—very hungry bears, high densities of salmon at Brooks Falls, and little competition from other bears for the most productive fishing spots—must coincide to produce a consumption rate that high and most of the time the catch rate is less. Based on the results from Kodiak, however, Katmai’s bears will easily consume thousands of pounds of fish over the course of summer and fall. In Katmai as well as on Kodiak Island, salmon are the brown bears’ most important food.

Bears must eat a year’s worth of food in six months or less to survive. This drives their nearly insatiable urge to catch and eat vulnerable prey. The incredible number of salmon they will eat illustrates their profound hunger.

  • Jillbru

    I was close to the female bears’ salmon intake! I guessed 3,640 lbs a year (based on 20 5lb salmon per day, 182 days per year.)

  • Birgitt

    Thank you for some actual numbers Mike. As camera viewers have been bemoaning starvation for the subadults, I was wondering how much salmon they actually NEED for survival. Based on the Kodiak numbers, it appears that subadults consume about 21/22% (F/M) of what an adult male does. And since most of our amazement focuses on what the big boars are eating, I think that is what most of us are using for a comparison. (Although, this year, I believe that a sow like 409 Beadnose is going to be above the 3,000 pound average for a sow.)

    Looking at the average for a boar (which I think 747 probably also exceeds), 747 would need to eat 150 pounds of salmon on 41 days to reach 6,146 pounds. He may or may not reach that at Brook River and at some of the smaller streams where the salmon go to spawn, but he will also have lesser days and days when he is eating dead and dying salmon with their lower caloric content.

    So if an average subadult needs to eat an average of 21.5% of what an average boar needs to eat (and 747 is above average at 150 lbs), guessing that a more average boar eats perhaps 125 lbs of salmon a day, a sub would need about 27 lbs of salmon a day or perhaps 5 – 6 fish. This obviously varies by a) the age and size of the sub and b) whether the sub is eating entire salmon or previously high graded scraps.

    So it is unrealistic to think that a subadult will be seen eating more than a couple of salmon in each feeding session…and we can stop worrying!

    • https://fitznaturalist.com/ Mike Fitz

      I edited the consumption numbers in in the blog post to remove +/- for clarity. The researchers found quite a bit of variability among bears. All numbers below are salmon consumed per bear per year.

      Adult male: 6,146 pounds (2,788 kg) +/- 4,253 (1,929 kg)
      Adult female: 3,007 pounds (1,364 kg) +/- 2,780 pounds (1261 kg)
      Subadult female: 1,248 pounds (566 kg) +/- 794 pounds (360 kg)
      Subadult male: 1,305 pounds (592 kg) +/- 717.5 pounds (325 kg)

      So, yes some bears will eat far more than average and some will eat far less. This holds among and between age/sex classes. Every bear will have good and poor feeding days as well. 747 doesn’t need to eat 150 pounds of fish per day to make a living (at that rate, he’ll eat 4,500 pounds of fish in a month) but he’ll certainly take it when that many salmon are available. Their world is one of feast and famine, a place where they can’t ignore food.

      • Birgitt

        Thanks for the clarification on the numbers. I assumed that 747 would be consuming more than, say 151 Walker. And 503 would have severely skewed numbers in any survey last year.

        So my next (unanswered) question involves teeth and lifespan. If a bears teeth wear out as a result of feeding and that can be a cause of death, as it possibly was with 1 Diver, then do bears with smaller physiques, who require less food overall to survive, have the potential to live longer? (And yes, I realize that Diver does not support that hypothesis at all.)

        • https://fitznaturalist.com/ Mike Fitz

          I’m not sure that smaller bears experience less tooth wear/damage in adulthood compared to larger bears. All bears seem to have significant tooth wear and damage in late adulthood, and I’m not aware of any study that suggests smaller body size is correlated with longer life span.

  • Sophia Latham

    Hi Mike, I watched Otis last night eat at least 15 fish over a period of hours. He was still catching and eating when I shut down at 10:30 pm. He just sat in the office and caught fish after fish. He even chased Scare D Bear out. It was interesting to watch.

    • https://fitznaturalist.com/ Mike Fitz

      Bears like 747 and 480 Otis set the curve for salmon consumption by bears. Both are very patient and skilled. They are also big enough that they aren’t displaced from prime fishing locations very often. In 2015, we watched Otis catch and eat over 40 fish in a day at Brooks Falls. That’s the record as far as I know.

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