Rem 475 - Streams And Salmon

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Salmon

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oncorhynchus salmonidae

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Anadromy

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when an individual is born in freshwaters, migrates to the ocean to grow large, and then migrates back to freshwaters to spawn. This life-history strategy is thought to have evolved to take advantage of the relatively predator-free freshwaters for the vulnerable life-stages (eggs/fry/parr) as well as the better growing conditions of the ocean. There are non-anadromous versions of salmonids, such as rainbow trout vs. steelhead and kokanee salmon vs. sockeye.

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Homing

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Salmon almost invariably return to spawn to the site where they were spawned. This has
important consequences for salmon, such as strong local adaptations and population differentiation.
However, there are often some small frequency of straying, individuals that go to different habitat,
perhaps a bet-hedging strategy by the mother.

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Semelparous

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When an individual dies after spawning. Most salmon species are semelparous. This
allows salmon to put everything into spawning (large eggs), and not save anything for the journey back out. Steelhead trout are not semelparous, instead they are iteroparous, having the ability to reproduce in multiple years.

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Life-history diversity

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The number of years spent in each life-stage varies enormously across- and within-species.

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Steelhead

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(Oncorhynchus mykiss) Anadromous version of rainbow trout. Anadromous and iteroparous. This species has incredible flexibility in their life-history.

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Chinook

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(aka King or Tyee or Spring)—(Oncorhynchus tshawytscha). Largest of the Pacific salmon. Least
abundant of the five semelparous salmon. Spawners can either be olive or maroon and black gums.

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Coho

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(aka Silvers)—(Oncorhynchus kisutch). Spawn in small streams with moderate gradient along the
coast. Spawners often have maroon/red sides and the males have exaggerated hooked noses. Spawners
are ~ 2-5 kg.

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Sockeye

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(aka Reds or Kokanee for the land-locked version)—(O. nerka). Bright red with green heads.
Sockeye juveniles generally rear in lakes, so this species is dependent on connections between river and lake ecosystems. Second most abundant salmon species. Farthest migration of this species is 1600 km to Redfish Lake. Mature at 2-4 kg.

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Chum

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m (aka Dog or Keta)—(O. keta). Spawners are greenish, with white tips on fins, and vertical stripes.
Generally spawn in lower reaches of rivers. All are anadromous and semelparous. Spawners are 3-6 kg.

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Pink

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(aka Humpies or Humpback)—(O. gorbuscha). Males have exaggerated hump, often with dark
brown backs and white bellies. Smallest of the Pacific salmon (~2 kg). Often spawn in low gradient
coastal streams with smaller sediment. They mature at 2 years old. Most abundant salmon species. Are semelparous.

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Impacts of salmon on fresh waters

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 Salmon are large and can reach extraordinarily high densities on spawning grounds relative to
the size of the constrained freshwaters where they spawn.
 This is probably a function of their anadromous and semelparous life-history.
 Due to these high densities and sizes, salmon can have important impacts on coastal
ecosystems.
 This has raised concerns that salmon population losses have had substantial ecosystem
consequences.

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Ecosystem engineering

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Salmon, during digging their nests, physically modify stream ecosystems. This
removes fine sediments and alters stream communities.

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Nutrient subsidies

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Salmon act as a “conveyor belt” of nutrients from the ocean to confined
freshwaters. These nutrients can fertilize freshwater and riparian ecosystems.
o Scientists have suggested that salmon declines have caused a “cultural oligotrophication” of
coastal North America.
o Hypothesized feedback loop for salmon populations, where nutrients from the parents
increase future salmon production. To replace these nutrients, there have been many
artificial fertilization attempts.

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How are salmon a fairly resilient group of species.

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 Fast generation times
 Have lots of offspring
 Rapidly evolve
 Phenotypic plasticity
 Diversity of populations

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Threats to salmon are classified as the 4H’s

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1. Hatcheries—can swamp local adaptations of wild fish through genetic effects, competition with
wild fish, increasing harvest rates on wild populations.
2. Habitat—salmon need relatively cold, well oxygenated water. They also need sediments that
don’t have much fine sediment as this can smother eggs.
3. Hydropower—dams can block upstream habitat as well as degrade downstream habitat.
4. Harvest—commercial fisheries often harvest upwards of 60+% of returning fish. Recreational
and First Nations can also contribute to direct harvest of returning salmon.
5. Plus some other emerging threats (that don’t conveniently start with H):
a. Disease
b. Climate change
c. Invasive species

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What is the state of salmon

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In B.C., in an assessment of 5487 salmon populations
o 142 salmon populations gone extinct
o 624 populations at high risk of extirpation
 B.C. salmon populations are estimated to be about 13-50% of historical abundance.
 Status of salmon the United States is much worse, where approximately 50% of populations
were extirpated or at risk of extirpation.

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HOW ARE SALMON POPULATIONS GENERALLY MANAGED?

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Commercial landings $390 million per year (1992 – 2001).
Salmon management often focuses on how many fish can be harvested by large-scale commercial
fisheries. The fishery usually happens at the mouths of the rivers as the salmon are on their way back to spawn.

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Basic population model

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Salmon management demands models to predict how many fish come back.
These models can be complex or simple. The simplest models usually assume that the number of fish that will come back is some function of the number of parents, while taking into consideration density dependence.

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Density dependence

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the number of successful offspring an individual will be influenced by the density.
Usually, offspring produced per spawner will decrease at higher fish densities. For example, competition for food will drive density dependence.

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Beverton-Holt Stock-Recruit Function

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R=aS/(1+bS)
R = Recruits. Recruits to the next generation. This could be the number of spawners produced by the
previous generation of spawners. This is what the model is predicting.
S=Spawners. The number of parents. This is a model input.
a = constant. This parameter can be interpreted as the productivity of the stock (without any density
dependence).
b = constant. This parameter influences the strength of density dependence.
 These a and b parameters will determine the maximum sustainable yield.
 a and b will vary depending on population and species.

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Maximum sustainable yield

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Most salmon management is based on trying to harvest as many fish as
possible, while allowing enough to spawn so that the next generation is big enough.

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What are the challenges to salmon management

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Environmental variability. a and b could vary through time, perhaps ocean variability influences stock productivity. This also makes it difficult to estimate a and b.
Mixed stock fisheries. Fisheries at the mouths of rivers often harvest multiple populations of salmon at the same time. If these populations are differently productive or differently vulnerable to the fishery then stocks will get driven to extirpation.