Salmon in migration

Posted: Sunday, August 05, 2007

The sockeye are returning to Steep Creek near the Mendenhall Glacier. Hatchery chum salmon are appearing in Salmon Creek, Sheep Creek and elsewhere. The eagles, gulls, and photographers are gathering! Other species of salmon will appear in Juneau streams throughout the late summer and fall, when they can be observed along several local trails.

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But I suspect that we seldom contemplate the costs and benefits of this annual phenomenon. Here are some things to think about when watching the salmon returning to their spawning streams.

When salmon come back from the sea to enter their spawning rivers, they face a number of challenges. There are tidal currents and river currents to contend with, and physiological changes required by the move from salt to fresh water. In some cases, they spend several weeks or months in fresh water before spawning, typically without feeding.

In addition, there is the sometimes prodigious challenge of the distance between river entrance and spawning grounds, as well as a rise in elevation. Some salmon achieve extraordinary migrations. The longest route for pink salmon that I've found is roughly 430 miles in eastern Asia, although this is far more than most pinks do. Sockeye swim more than 620 miles (and hundreds of feet of elevation gain) to reach spawning areas in the upper reaches of the Fraser River in British Columbia.

The Yukon River system seems to harbor the champion upstream migrators for the three remaining species. Coho go over 1,360 miles and up the Porcupine River and Fishing Branch, chum about 1,740 miles, and still more remarkably, a few chinooks make it 1,980 miles (above Whitehorse and Teslin). In contrast, some salmon, especially pinks and many chums, spawn in intertidal areas that have upwellings of ground water through the gravels, and these have negligible migration costs. Between the intertidal and the record-setting long migrations lies a whole range of migratory distances.

Upriver migrations are commonly accomplished at travel rates up to 30-50 miles per day. The most efficient swimming speeds are insufficient to counter the river currents, so the fish must swim at average speeds close to their maximum. They can save some energy by seeking out the channels with slower currents, but at other times they have to sprint to make headway. Thus, they are swimming faster than what is most efficient, often for long distances, not eating, sometimes jumping up waterfalls and over beaver dams.

Upstream migrations happen at a cost, and the longer the migration and the greater the elevation gain, the higher the travel costs. At the same time, the bodies of the salmon are maturing sexually, and the developing reproductive organs and sexual morphology (e.g., humps and snouts in males) also require energy. Competition among males for females and among females for nest sites adds to the costs. The combined costs of migration and reproduction can be large, and salmon pay these costs from the stores of fat and protein they accumulate during their life at sea. Fish that migrate farther typically store more fat than those with shorter migrations, but they also need more. As much as 99 percent of the stored fat may be used to pay the cost of migration. And much of the body's protein, up to 85 percent or so, may be metabolized to fuel the development of reproductive features including gonads.

The more migration costs, the less there is to spend on reproduction. So fish that migrate long distances (even though they start off with more fat) tend to have smaller humps and snouts - which also makes them more streamlined. They have smaller gonads (relative to body size) than short migrators. Long-distance migrators tend to produce smaller eggs, but more of them, than fish that move shorter distances. Migration distance is sometimes associated with other life-history differences, such as run timing or likelihood of juveniles rearing in rivers instead of lakes or ocean.

Consideration of the large costs of migration raises a question: Why do it? If there are high costs, there must be compensating high benefits.

The prevailing scientific opinion seems to be that salmon or their ancestors originally were freshwater fish that gradually began to exploit marine environments. In the temperate zone, the sea is much more productive than fresh water and sea-going salmon could grow to much larger size than those that remained in fresh water. This gave them an advantage in fecundity (large females can make more eggs) and in sexual competition (large individuals usually outcompete smaller ones). But these large fish remained tied to fresh water for nesting, perhaps because fresh water is a safer environment than the sea (and safety is further increased because females bury their eggs and defend their nests). In this view, the large benefits of going to sea, payable on the spawning grounds, necessitate the high costs of upstream migration.

There are potential consequences of migration costs to salmon harvesters, both wildlife and humans. Predators far in the Interior have access only to fish with depleted fat and protein stores. This is likely to make dependence on salmon more difficult than on the coast and increase the importance of alternative sources of food.

• Mary F. Willson is a retired professor of ecology and a Trail Mix board member.

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