ABSTRACT:
Over the last several decades, people living in the Pacific Northwest have seen dwindling numbers of salmonid fishes (which include five species of salmon and two of trout) in local rivers and streams. Much of the loss can be attributed to the destruction of habitat as the region becomes more developed. Some of the blame can also be placed on the inadequate regulation of fishing. The resulting problems go beyond the obvious decline in numbers: The genetic diversity in these fish is also being eroded. Strategies to restore Pacific salmonids to a healthy state will need to ensure that adequate biodiversity is maintained.
The number of Pacific salmon has declined dramatically. But the loss of genetic diversity may be a bigger problem.
QUOTES:
But more than a century ago, the number of salmon returning from the sea began to fall dramatically in the Pacific Northwest. The decline accelerated in the 1970s, and by the 1990s the U.S. Endangered Species Act listed 26 kinds of salmon as endangered or threatened.
Pacific salmon have now been extirpated from nearly 40 percent of their historical habitat in the Pacific Northwest. Investigators are just now beginning to understand that human activities have selectively eliminated some populations of salmon while favoring others, resulting in the loss of much of the genetic heritage in these amazing animals.
Because salmon typically return to reproduce in the river where they were spawned, individual streams are home to local breeding populations that can have a unique genetic signature.
The general needs of these fish are really quite simple: They must have plenty of gravel in which to spawn; they need enough clean, cool water to swim, escape predators and find food; and they require sufficient vegetation along the river banks to protect the stream bed from excessive erosion or sedimentation, to add nutrients to the water and to provide woody debris as shelter from strong currents.
Over the past 150 years, mining, livestock grazing, timber harvesting, agriculture as well as recreational and urban development have eliminated or substantially disturbed salmon habitat. The numerical effect is obvious-there are fewer salmon in degraded regions than in pristine ones. As with fishing, however, habitat loss and destruction have the potential to reduce genetic diversity.
Although the homing instinct of salmon to their natal stream is strong, a fraction of the fish returning from the sea (rarely more than 15 percent) stray and spawn in nearby streams. Low levels of straying are crucial: The process provides a source of novel genes and ensures that a location can be repopulated should the fish there disappear. Yet high rates of straying can be problematic, because the misdirected fish may interbreed with the existing stock, diluting any local adaptations that may be present.
Hatcheries:
Each year, facilities situated around the Pacific rim release more than six billion juvenile salmon; on the Columbia River alone, hatcheries produce about 200 million fish. But the benefits of all this work have yet to be demonstrated. Indeed, this program may well be a contributing factor in the long-term decline of salmon.
The risk of changing the salmonid gene pool as a result of such practices was once thought to be minimal, because these fish typically experience natural conditions for most of their lives. Recent research, however, suggests that the artificial propagation of salmon can permanently alter genetic makeup and ultimately reduce the viability of wild populations. (emphasis added)
The survival of hatchery-raised steelhead released into a river in Oregon was about 20 percent lower than that of their wild counterparts. In a separate study, Ian Fleming and Mart Gross of the University of Toronto determined that coho bred in hatcheries tend to be less aggressive than wild coho and thus have less success in spawning. The ability to avoid predators, the timing of reproduction and their degree of territoriality also vary between hatchery and wild salmon. When hatchery-raised fish stray, they can pass on their genes to others. And although the hypothesis is still a subject of much research, many studies suggest that such interbreeding between hatchery and wild fish results in offspring that are less fit to survive.
Clearly, conservation managers need to rethink the traditional role of hatcheries. People who run hatcheries must focus their attention on the production of salmon that are more like their wild counterparts, and they must find ways to minimize adverse effects on wild populations.
How Important Is Biodiversity?:
Because much of the diversity within and among Pacific salmon has at least some genetic component and because there is little gene flow among these populations, one expects to see some local differences in homing ability, disease resistance and response to stream flow· The failure of most attempts to transplant stocks to a new habitat also suggests that salmonids have evolved specializations suited to particular local environments.
Hendry, suggested that reproductive isolation and evolutionary divergence can happen in as little as 13 generations.
Too much remains unknown about the importance of biodiversity to discount it at present. So we favor a precautionary approach. That is, people should consider the consequences of being wrong about their assumptions. If biodiversity proves less important than anticipated, so be it. But if biodiversity turns out to be more critical than is now believed, we want to ensure that salmon will not have been sent down the path to extinction without the genetic resources to escape.
Conservation biologists have generally argued that biodiversity is valuable for two reasons. First, it leads to a greater abundance of the species in question, because different populations can exploit different habitats and resources in unique ways. Second, it fosters enhanced long-term stability by spreading the risk and providing redundancy in the face of unpredictable catastrophes. Both arguments apply well to salmon-and strongly support the precautionary approach to conserving their biodiversity. Genetic diversity offers insurance against the vagaries of climatic fluctuations that can affect salmon populations dramatically.
The current quest for a precise definition of how much habitat salmon need, how many can be safely harvested, or how little biological diversity needs to be preserved is not consistent with the needs of the species. For millennia the salmon have hedged their bets against major catastrophes, such as ice ages, continental uplifts, and volcanic eruptions. They have done so by maintaining a diversity of populations and habitats - in short, they have developed a rich and varied set of genes. Salmon should be allowed to continue pursuing their survival strategy ö a strategy that worked before humans arrived on the continent and, if these fish manage to survive, will work long after we are gone.