Are Fisheries And Dams Changing Salmon?

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From hatching to spawning, present day salmon run a gauntlet of marginal situations: disturbed spawning and nursery habitats, different predators, dams, and changing ocean conditions – each of which challenges both their adaptability and vigor. These are amazing species, and the more I learn about them the more in awe I am of both their importance in, and their adaptability to, their ecological niches. Yet of all the things we think about that the salmon have to contend with in order to perservere in the human landscape, it may come as a surprise that the single most dramatic effect we have on their population may be that we are changing their evolutionary pathway, chiefly due to fishery practices.

Not that the fisheries are in and of themselves at fault – they are required to follow the guidelines given them by government officials. The government in turn get its information from many sources, chief among which are dedicated scientists who are desperately trying to turn the tide on the troubling fishery decline.

Yet the government doesn’t always listen to the scientists, nor does it operate with full public disclosure on these issues – seemingly strangely so, because these are not national security issues nor are they anything of proprietary interest when we talk about salmon restoration. (Future posts will go into that in more detail.). The government can be very slow to react to even the most dire warnings from the scientific community.

Scientists intuitively tend to follow the ‘Precautionary Principle‘, which states; “When an activity raises threats of harm to the environment or human health, precautionary measures should be taken even if some cause and effect relationships are not fully established scientifically.”

Most, if not all, scientists are trained to look for trends in their data when even only the most tenuous connections can be made; and in cases where welfare is concerned a good scientist will usually step up and sound an alarm before all the data is even in.

Government, on the other hand, tends to balk when the cost of acting on tentative data is high – whether that cost is in dollars, jobs, energy, or security – because it is also our government’s job to protect us and to oversee the growth of our nation in the long term. So our government tends to favor a cost/benefit analysis, and this can be detrimental when swift action is called for with respect to environmental issues, such as in taking decisive action to restore our salmon fishery. In this case though, they have been dawdling for over a century over facing this issue, and are running out of time.

In Toward Darwinian Fisheries Management, the authors state:

When Charles Darwin made his argument that life was evolving he began by showing the potency of artificial selection to modify domesticated species, and how quickly animal breeders were able to create new varieties – he cited examples of talented farmers who created new races of livestock within their own lifetime (Darwin 1859). That fishing could act similarly was, to our knowledge, first mentioned in the scientific literature in 1902, when Cloudsley Rutter wrote: ‘A large fish is worth more on the markets than a small fish; but so are large cattle worth more on the market than small cattle, yet a stock-raiser would never think of selling his fine cattle and keeping only the runts to breed from… The salmon will certainly deteriorate in size if the medium and larger sizes are taken for the markets and only the smaller with a few of the medium allowed to breed’ (Rutter 1902).

Thereafter, it took almost a century before these patterns were clearly identified in data, sparked by Ricker’s (1981) study of declining sizes of Pacific salmon Oncorhynchus spp. returning to spawn; patterns he could not explain by any concurrent environmental trend but that were consistent with evolutionary change driven by the size-selective fishery.

The evolution toward earlier maturation when fishing inflicts elevated mortality is driven by reduced longevity – future reproduction becomes uncertain and, instead of investing in growth to acquire a larger body size, evolution favors individuals that invest resources in offspring earlier in life.

They go on to say:

For each year we fish, it may thus take more than 1 year of no fishing for the inherited traits to recover; this has been termed a ‘Darwinian debt’ because we harvest now in a manner that may entail costs for future generations (the term was coined by Ulf Dieckmann in an interview with the Financial Times, August 28, 2004). With due respect to the precautionary principle, this perspective alone is sufficient, in our opinion, for managers to be concerned about the evolutionary impacts of fishing.

But fishing is not the only way that humans are driving the evolutionary biology of salmon – we have added evolutionary pressures by altering the environment as well.

For instance, dams not only impede the salmon’s journey, but dramatically alter stream and river ecology. Salmon must navigate challenging hatching conditions, endure siltification of their natal streams and rivers, and withstand marginal water temperatures – then survive being flung through the dam turbines on their seaward journey. They must escape predators that gather at the dams, and the young salmon must cope with the lake conditions present in the manmade reservoirs created by the dams (which gives a huge advantage to some of the predatory fish species and is not natural to the salmon). Finally the young fish encounter estuaries that are often inadequate in both food and places to hide.

On their return trip, the salmon must battle the same conditions, and again must make it past those dams, increasing the time they must spend in getting to their natal streams.

It would appear that we are selecting for salmon that can manage all of those barriers and survive years at sea, only to allow fisheries to take up to 65% of the biggest survivors.

Who knows what genetic viability is lost in all of this? The consequences are staggering, and need to be addressed in the near future if we are to save the wild salmon, and provide adequate food for the endangered southern resident orcas.

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