There are five species of Pacific salmon, plus the closely related cutthroats and steelhead, and an interesting story could be told about any of them. Here I focus on sockeye (red) salmon, because they illustrate some important points very well. These are things to think about wherever one watches salmon coming into our rivers to spawn.
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All adult salmon tend to return to spawn in the streams where they were born, finding their way by recognizing the smell of the home stream. The strong homing tendency means that the populations of many streams may be composed of the same family lineages year after year, with little genetic input from other populations. However, a certain, usually small, proportion of the population strays to other streams. Straying individuals from one population to the next is most common between streams that are close together, as might be expected. As the distance between streams increases, straying decreases. The effect of distance on the genetic separation of populations is stronger in sockeyes than in pinks and chums, because sockeye are usually very good at homing.
There are 110 to 230 distinct sockeye populations in Southeast, depending on who is counting. Some of the differences among populations may arise because the strays that colonized those streams happened to be genetically different from the original population. Once a new population is established, the homing tendency will tend to keep the new population separate from the parent one. These largely separate populations then have the potential to become more differentiated from each other, chiefly because they adapt to different conditions.
Over the geographic range of sockeye, most populations have juveniles that rear in lakes, and these populations are often well differentiated from each other. There are also some populations with stream-rearing juveniles and others whose juveniles go to sea soon after hatching; these populations tend to be less well differentiated from each other than lake populations. Curiously, the Situk River system near Yakutat has all
three life-history types.
In addition, a number of sockeye populations have given rise to land-locked derivative populations called kokanee. This can happen when, for instance, juveniles are prevented from migrating to sea by some new barrier such as a landslide or glacier that blocks the channel, and find decent rearing conditions where they are. This apparently is the origin of the kokanee in Lake Kathleen near Haines Junction.
Differentiation of local populations can involve a wide array of characteristics. Here in Southeast, there are several distinctive populations of sockeye. Many of the differences are in body size, sex ratio, or degree of sex difference in time spent in fresh water or in salt water. For example, several systems have a high proportion of 'jacks'- males that spend little time in salt water and return as spawners at a relatively young age and generally smaller size. The Hasselborg River on Admiralty Island is the only island stock that is characterized by juveniles that rear in a salt chuck. Adults in the Karta River on Prince of Wales are unusually large, but those associated with Benzeman Lake on Baranof Island are unusually small. These differences are likely to be adaptive to local conditions, although they may have arisen by happenstance.
Studies in southwest Alaska, in streams supporting the famous Bristol Bay sockeye harvest, have shown that body size and shape, and other features, can be quite fine-tuned to very local differences in ecology. Sockeye in these systems spawn in both small and large streams and on lake beaches where ground water wells up through the gravel. These different spawning areas are often very close together, but each has its own type of sockeye. Both males and females that spawn on beaches are deeper-bodied and less streamlined than those in large streams, which in turn are deeper-bodied and less streamlined than those in small, shallow streams. Streamlining is favored in shallower waters because sleek, slim fish are better able to negotiate the shallows and current and to escape from bear predation. In addition, females that spawn in large gravels produce larger eggs than those spawning in fine substrates. These kinds of adaptive differences among salmon that spawn in adjacent waters under different ecological conditions can arise in just a few generations.
The existence of local adaptations to particular ecological conditions has meant that attempts to transplant salmon to new locations commonly fails, because the transplants are not suited to the new area. There are notable exceptions to this trend, but the trend is strong. Local adaptations also may be disrupted by an influx of fish released from hatcheries, because hatchery fish commonly derive from other, usually distant, populations and, in addition, have their own peculiar adaptations to hatchery life.
The bottom line is that, for sockeyes as well as other salmon, when you've seen one you have NOT seen them all. Many stream systems have populations that are adapted to local conditions. Even different tributaries of the same system may have fish stocks that exhibit different adaptations. This genetic diversity is very important ecologically, because local, well-adapted populations reproduce and survive better (by definition) than poorly adapted ones-and lots of wildlife depends on good salmon populations.
Genetic diversity is also important commercially. Such diversity increases the resilience of regional aggregates of populations to environmental changes, for example, climate changes, because local populations that may do relatively poorly under some environmental conditions do better when conditions change. This stabilizes the regional numbers of fish and permits long-term, more reliable harvests, such as have occurred in Bristol Bay. The same principles apply to other systems, such as the Taku, whose several tributaries host different salmon stocks.