Salmon, Bears, and Trees

Tlingit Totem Pole depicting salmon, Sitka Historical Park

Salmon! Last week in southeast Alaska we hiked and kayaked beside streams crowded with salmon swimming upstream to spawn and die. We saw salmon jumping in the waterways near our ship, the Sea Bird. We watched bears snatching salmon from the streams and eating their favorite parts. We heard lectures on the natural history of salmon and on its importance to the local native people, the Tlingit. We visited a fishing village, Petersburg. And several times, we ate local smoked, cured, or fresh salmon.

Salmon swimming up a stream (Len's photo)

Dead salmon in the stream

The salmon, we learned, are not just good for human food. They play an incredible role in forest ecology, bringing the nutrients they ate while at sea back to the forest where they were born: "When they return to spawn, salmon become a veritable conveyor belt for nutrients. For example, an adult chum salmon returning to spawn contains an average of 130 grams of nitrogen, 20 grams of phosphorus and more than 20,000 kilojoules of energy in the form of protein and fat; a 250-meter reach of salmon stream in southeast Alaska receives more than 80 kilograms of nitrogen and 11 kilograms of phosphorous in the form of chum salmon tissue in just over one month."

Trees by the salmon stream

Using all these nutrients, trees near salmon streams grow faster and larger than trees further away: beside the streams, "Sitka spruce take 86 years, rather the usual 300 years, to reach 50 cm thick." Growth rings in the trees are larger in years with good salmon runs.

"And just as trees need salmon, salmon depend on trees. Every part of a tree participates in enriching a stream for aquatic life, from its tiny needles to its strong twisted roots. Streamside vegetation shades spawning streams, keeping developing eggs cool." -- from "Why Fish Need Trees and Trees Need Fish" by Anne Post.

Distribution of the nutrients brought back from the sea by salmon is the job of the bears who fish in the streams and then spread the leftovers and their droppings nearby: "each adult female grizzly bear on the Kenai Peninsula in Alaska eats about 3000 lbs of salmon per year". The bears are much better nourished by salmon than by the berries and other plants or small animals that they feed on: "Grizzly bears that eat a lot of salmon are 80% larger, produce 25% more cubs, and live in populations that are up to 50 times denser than grizzly bears that fatten primarily on fall berries." -- Washington State University Bear Center.

The entire quality of the stream is created by nearby trees, birds, animals, insects, and other plants. All are nourished by salmon that died after spawning or were partially eaten by bears; the salmons' bodies particularly feed the insect life that later feeds the growing salmon larvae. A tight food chain!

From a kayak we saw this bear with a salmon in  his mouth: gulls were waiting for leftovers.

Note: for a quantitative study of the transfer of nutrients, see this: "Fertilization of Riparian Vegetation by Spawning Salmon: Effects on Tree Growth and Implications for Long-Term Productivity" by James M. Helfield and Robert J. Naiman