River fish populations

What are Pacific salmon?

• Genus: Oncorhynchus

• A group of migratory fishes that are very important to people and rivers throughout the Pacific Rim

• More salmon in the ocean now than there’s ever been in the last 100 years

• A lot of that abundance is made up of pink and chum salmon

• Some populations aren’t doing well: chinook, sockeye, coho, steelhead

What is the Pacific salmon life cycle?

• Eggs are buried in gravel redds and fertilized

  • Redds are 2m2 and 40cm deep

  • Chinook dig redds 20m2 and 30cm deep

• Eggs subsist off nutrients over winter and emerge into alevins

• Freshwater phase can last 1-2 days or several years

• Head out to ocean as smolts (undergo osmoregulation and other adaptations for freshwater → saltwater)

• Grow to adults in ocean and return to stream via chemical cues

• Can achieve high abundances in small freshwaters where they spawn

• Males have a dorsal hump to impress females/compete with other males

• Productivity from the ocean jammed into small rivers that otherwise couldn’t support that many fish

• Spawn in different habitats

  • Locally-adapted sockeye populations that spawn in lakes have larger dorsal humps because of deep water and competition

  • Shorter dorsal humps in shallow streams to avoid predation from grizzlies

• Anadramous: go to ocean and back again, spawn in freshwater

• Semelparous: spawn once then die

  • Provides nutrients to offspring

  • Strenuous return journey, can invest more energy in offspring than survival

  • Steelhead invest less energy → smaller eggs

What is compensatory capacity?

• High reproductive capacity + density dependence

Is salmon harvest sustainable?

• Indigenous peoples have harvested salmon for millennia

• As glaciers receded from coastline and created new habitats, salmon populations and Indigenous peoples co-migrated

• Last 100 or so years, relationship has changed with commercial fisheries

• Goal: not take too much, but enough to support economics and fishery

• Salmon are very productive

What is a stock-recruit relationship?

The relationship between the number of parents and the number of offspring they produce to the next generation

What is a Beverton-Holt relationship?

• Stock-recruit relationship

• Saturating curve

• High per capita production at low abundance (lots of offspring per adult)

• Produce more at higher abundance but saturates eventually

• Competition for spawning grounds, rearing habitat, carrying capacity

What is a Ricker curve?

• Stock-recruit relationship

• Hyper-abundances → less salmon

• Medium numbers → highest recruits

• Too many young salmon eat all the zooplankton, which take a while to recover

• Lots of fish depletes oxygen

• Poor evidence

• Helps justify fishing

• Controversial

How does density dependence relate to harvest?

• Low abundance of spawners → high per capita reproductive output

• High spawner abundance → low per capita reproductive output

• Equilbrium point: same # recruits as spawners

  • If above, population decreases

  • If below, population increases

• MSY: largest difference between 1:1 line and actual line, often halfway between a and K

• K: carrying capacity

What is MSY?

• Maximum sustainable yield

• The most fish that could be harvested indefinitely

How does ocean climate control salmon abundance?

• Increasingly important

• El Niño Southern Oscillation

• Areas of the ocean that are warmer or cooler than usual will favour certain stocks

• Climate change continuously rewriting the rules, models need to evolve

• Large events like the Blob reshuffle the food web

How do river conditions control salmon abundance?

• Water temperature

• Salmon are cool-water species

• Mortality is temperature-dependent

• 17C+ is often lethal

• rivers in BC are starting to hit 17C, large scale mortalities

• Hydrology: how much water is in the river

• Salmon need more water than was thought

• Years with low water → population decreasing

• Years with high water → population generally increasing

• Salmon are migratory and controlled by both freshwater and ocean conditions

How does competition with pink salmon control salmon abundance?

• High numbers of pink salmon, many from hatcheries, associated with low survival, smaller sizes, older ages

• Pink salmon doing well but quite aggressive

• Ocean reaching salmon carrying capacity?

What is biocomplexity?

• Different salmon populations have different dynamics through time

  • Different life histories and adaptations, genes, sizes, age structures

  • Different exposure to stressors

  • Locally-adapted populations

• Fisheries can be more stable if they integrate across this diversity (portfolio effects)

• Lots of different salmon stocks → more stability and resilience to climate change

• hidden biodiversity within a single species

• Rainbow trout eat salmon eggs, juvenile salmon

• Bears and other wildlife can go to different places and feed on different stocks for consumption stability

What are hatcheries?

• Rear fish and let them go

• Put out more young fish into the ocean

• Can sustain/increase fishing opportunities

• Can increase populations that are low

• Risk of increased competition with wild stocks

• Risk of increased fishing pressure on wild stocks

• Genetic effects: remove selective pressures → decrease the productivity of wild stocks