Around and Around They Go
By Bill Hauser
What happens to salmon smolts after they enter the ocean?
Let’s begin with a brief overview of a generalized salmon life cycle beginning with the egg.
Spawning is during summer and eggs incubate until mid-winter when the fish hatch and in spring, become free swimming fry. After some development and growth, they become juveniles called parr or fry until they become smolts that migrate to the ocean. In the ocean, they grow and mature until they migrate back to freshwater to spawn. This sounds easy enough but it is really not that simple.
Most salmon smolts enter the ocean as small fish, about three inches in length. What does that tell you? They are in danger of being eaten. And they better start eating and growing. Bigger is better. Larger smolts can eat bigger food items and survive better. But there is more. The ocean, at first glance, seems like a big well blended pot of soup but it is not. It is more like a thin stew in which the eater never knows if they will get a spoonful of weak broth or a vegetable or a chunk of protein. So it is important that the young salmon enter the ocean when the right kind of food is abundant. For example, survival of young salmon varies greatly depending on the water temperature at the time of ocean entry because the seasonal water temperature, oceanic conditions and currents govern the abundance of preferred food.
And the rest of the story. Who is eating the salmon? Just about everyone. Fish such as dogfish sharks, bigger salmon and other fishes, all manner of birds, marine mammals, and more. Coho salmon smolts especially like to eat pink salmon.
Where do they go in the ocean? Fishery scientists are still working on this question but salmon exiting most Alaska streams and rivers south of the Alaska Peninsula begin their journey by making a right turn and heading in a generally northerly direction; then westerly. From north of the Alaska Peninsula, most young salmon move more southwesterly and through the passes of the Aleutian Islands. Most juvenile salmon end up in the North Pacific and they just keep moving west, then south, east, and north again in a large circular pattern that follows the North Pacific gyre and fills an entire year.
At this point, different things start to happen. Pink salmon and coho salmon spend only one summer and winter in the ocean feeding and growing. Sockeye and chum salmon may spend one to three winters at sea and Chinook salmon may spend one to four winters at sea - rarely one, usually three, and occasionally five or six winters. Each year in the ocean means one more circle around the North Pacific and one more year of eating and growing. But also, of course, they need to avoid predators such as killer whales, sharks, bigger salmon, etc.
What do they eat? Chinook salmon eat more fish than other salmon species. (They especially like sand lance.) Coho salmon eat less fish but more squid and shrimplike organisms called krill or euphausids. Pink salmon eat even less fish and a more general diet. Sockeye salmon eat large amounts of shrimp and shrimplike organisms. (This is the reason their flesh is so red. They derive a dye, carotene, from a diet of more shrimp and krill. The abundance of carotene is stored in their muscles and fat) Chum salmon feed lowest on the food chain; lots of jellyfish and their relatives. As salmon grow bigger and older, there is relatively more fish in their diet.
What is the trigger or cue that tells them that it is time to leave the ocean? I don’t know. But I can speculate that it has something to do with size. And hormones related to sexual development.
“Jacks” are male salmon that become mature after only one year at sea. (Sometimes, Chinook salmon returning from the ocean after two years are referred to as “two ocean jacks.”) Jacks are functionally mature but usually do not participate in the spawning.
Okay. The salmon have survived hatching, rearing, the smolt transformation, and predator avoidance one to several years at sea and are heading for home. What is next? Anything missing? Think about this. These fish are roaming around the ocean. How do they know where they are? How do they know which direction to head to get to their home stream? Stay tuned. We will talk about this next month.
Of course, here is the usual disclaimer. Most fish have not read the textbooks so they do not all exactly follow all the “rules” we have listed.
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