Coho salmon, often called silver salmon due to their beautiful bright chrome color, are the most aggressive of the Pacific Salmon species. There aren’t as many cohos as there are pink salmon, chum salmon, or sockeye salmon, but coho salmon manage to spawn in tributaries and small headwaters inaccessible to other salmon species. A Coho can leap vertically six feet (1.8 m) to overcome obstacles such as rapids or waterfalls on its return journey to its spawning stream, and cohos usually migrate to their spawning streams later than other salmon species at a time when fall floods allow them to access areas not available during lower water levels. Even young cohos are aggressive, and they often eat the young of other salmon species. This aggressive nature of coho salmon makes them excellent fighters on a rod and reel and an important species for the sportfishing industry in Alaska.
Range
The natural range of coho salmon is from Monterey Bay, California north to Point Hope, Alaska, west across the Bering Sea to the Anadyr River in Siberia, and south along the coast of Asia to Japan. Cohos have been introduced to the Great Lakes and other lakes in the Continental U.S. Cohos spawn in ponds, lakes, and pools within streams and rivers.
Description
Coho adults usually weigh between 8 and 12 lbs. (3.6-5.4 kg) and range from 24 to 30 inches (61-72 cm) in length, but fish weighing up to 31 lbs. (14.1 kg) have been caught. During their marine phase, adults are bright silver and have small, black dots on the back and the upper lobe of the tail fin. In their marine phase, cohos look very similar to king (Chinook) salmon, but king salmon have spots on both lobes of the tail, while cohos only have spots on the upper lobe. Another way to distinguish the two species is cohos have white gums, while king salmon have black gums.
When cohos return to freshwater to spawn, males turn dark to bright green on the head and back, bright red on the sides, and dark on the belly. Breeding females are not as brightly colored as males. Spawning males develop a hooked jaw called a kype.
Life Cycle
In Alaska, coho salmon return to their spawning streams from July to November, usually during periods of heavy rain and high runoff. On Kodiak, most cohos return from late August through October. A female coho selects a suitable spot and then begins to dig a nest, or redd. She turns on her side and gives several powerful flips of her tail to remove silt and other debris and then continues to dig for two to five minutes until she has a shallow nest. While she is digging her nest, she aggressively drives away other females. Meanwhile, a male coho stays near the female while she digs the nest, occasionally swimming close to her. He may also stop beside her and quiver, swim over her and touch her dorsal fin, or nudge her side with his snout. When the female has finished excavating the nest, she drops into the deepest part of the depression, and the male immediately joins her. The two fish remain side by side in the nest while they open their mouths, quiver, and release eggs and sperm. The female then moves upstream and begins digging a new nest, covering the eggs in the first nest with the gravel she dislodges from the second nest. Nest digging and spawning may continue at intervals over the next several days, until the female deposits all her eggs. The male might then leave and seek another female, but the female will continue the digging process until she grows weak and dies.
A female coho lays between 2,400 and 4,500 eggs. The eggs develop over the winter and hatch in the early spring. The alevin, with the yolk sac attached, remains in the gravel until May or June before emerging. Young silver salmon fry usually spend between one and three winters in fresh water, but some spend as many as five years in a lake before migrating to the ocean. Young silver salmon in Karluk Lake on Kodiak Island often stay two, three, or even four years in fresh water before migrating to sea. The length of time spent at sea also varies for coho salmon. Some males, called jacks, mature and return after only six months at sea, but most cohos remain in the ocean for 18 months to three years and return as full-size adults.
Prey and Predators
Cohos are aggressive feeders. In freshwater, they eat a wide range of aquatic insects and plankton. They also eat salmon eggs, and as they grow, the fry also consume smaller fry. In the ocean, cohos eat herring, sand lance, other fish, and squid.
Young coho are eaten by birds, larger fish, and a variety of other predators. Killer whales, sharks, sea lions, seals, bears, humans, and other land mammals prey on adult cohos.
Next week, I will cover commercial and sport fisheries for cohos as well as threats to coho salmon populations.
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