The Web of Life (22-1-1) Pacific Salmon

 Pacific Salmon

Of all species living in and beside the river, the salmon is the most beautiful. ~ Canadian author Bruce Hutchison

The temperate coastal rainforest of British Columbia has some of the oldest and largest trees on Earth: up to 90 meters and 1,500 years old. The trees are watered by a network of rivers and tributaries and fertilized by the fish in them.

Pinkish pearls placidly float under gravel in a Pacific Northwest cold-water stream; the peaceful beginning of an otherwise tremendously turbulent life. Eggs are laid in the fall, and incubate over the winter, typically protected under a layer of snow and ice.

In late winter, the eggs hatch into alevins: tiny aquatic beings with enormous eyes, secured to bright orange sacs which are their food supply, which is a perfectly balanced diet. A good flow of pure water is essential to alevin survival.

Having eaten their sacs away, alevins emerge as fry in May and June. Free-swimming fry, some 2.5 cm long, are a tasty treat for larger fish. Mortality is at its highest at this stage.

Fry stay briefly where they hatched. They then travel downstream to the river, or upstream to a lake, depending upon the species; lingering from a few days to a year or more, as fingerlings that grow to 10 cm.

In a process called smolting fingerlings undergo physiological changes that enable them to survive in salt water. During the season of freshets – in the spring, with the rising of the river from snowmelt – smolt head to the sea.

Salmon savor their maturity in the ocean, growing rapidly by eating greedily on plankton and smaller fish, such as krill, herring, and pelagic amphipods (small crustaceans). Some species spend up to 8 years at sea, traveling in schools for thousands of kilometers throughout the north Pacific.

There are 5 known Pacific salmon species: chinook, chum, coho, pink, and sockeye.

Pink are the smallest adults (1–2.5 kg), and the most abundant. Their life cycle is but 2 years.

Coho are bigger than pink (2–4.5 kg average, though up to 9 kg may occur), and much more powerful swimmers. Coho can negotiate their way up waterfalls that pink could not dream of surmounting. Coho stay at sea 2 or 3 years before returning to spawn.

Chum have the widest distribution, and the 2nd-largest size (20 kg). Chum fry migrate directly to the sea soon after winning their fins. Their return to spawn varies from 2 to 7 years.

Sockeye are the most varied species by both look and lifestyle. The small kokanee (35 cm) live their lives in freshwater, never going to sea. Anadromous (migrating) sockeye spend a year in a freshwater lake after emergence, then 1 to 4 years at sea before returning to spawn.

Chinook adults can reach 45 kg. Chinook migrate to sea in their 1st year, typically within 3 months after becoming fry, but then spend much of the adulthood – 2 to 5 years – in coastal waters. They return to their natal river in the autumn to spawn.

Salmon are gregarious, with social ranking roughly corresponding to size. Top-drawer salmon are first to grab choice food morsels and are less likely to be preyed upon. Socially dominant chum salmon tend to have pronounced vertical markings, whereas subordinate chums are predominantly marked by horizontal patterns.

Migrating salmon are particularly susceptible to predation, so, while normally swimming at shallow depths, they prefer having a deep-water retreat in case of attack. The flashy sides of salmon can act as an illusion to confuse a predator, which may mistake a salmon for a much larger predator.

Ultimately, after years in coastal waters, if not out in the open sea, the biological clock goes off with the urge to spawn. Sexual maturity is typically felt at the onset of summer, though the time of year varies by species.

In preparation for their journey home, salmon separate into groups by native stream.

To facilitate their migration, Pacific salmon are born with an inherited magnetic map that allows them to knowledgeably navigate via geomagnetism.

Salmon sight is excellent, but nearsighted: 1 meter max in front of them. Perception of details is sharp. Salmon can see a full spectrum of colors: red, green, blue, and ultraviolet.

Salmon also have a terrific sense of smell. They can detect a drop of their home stream among 760,000 liters of sea water: 1 part in 100 million. In migrating to spawn, salmon hone in on their natal home by its smell.

Salmon also have sensitive tastes; but then, taste is an adjunct to smell.

Salmon hearing – via tiny inner ear bones termed otoliths – is attuned to high frequencies. Salmon also have the lateral line common to fish; especially apt for detection of nearby movement at the side, and for lower frequencies. The 2 perceptions mentally converge to provide senses of balance and depth.

Fish use the lateral line to localize the source of sudden sounds, and, more generally, to detect flows of water like we sense slight air flows. Fishes’ sense of their watery surroundings is analogous to land animals with an atmospheric environment. Evolution has respectively honed perceptiveness for both fluid mediums.

Salmon possess a well-developed sense of touch. They are quite sensitive to tactile sensation.

In the brackish waters before reentering freshwater, salmon go through profound physiological changes once again. The bodies of both sexes metamorphize for the trip home: toughening up, as well as transforming. Salmon skin thickens significantly to survive the upcoming battering on the rocks.

Some changes give females an advantage. Males grow a humped back, particularly noticeable in pink salmon – a handle that makes them more likely to be snatched up by predators. Males also turn a bright red; alluring to females, but also raising the risk of being easily spotted by predators.

Males widen into sumo form, making them more difficult for other males to wrestle with in the upcoming mating contests. Female abdomens swell, while their gums recede to expose their teeth.

Pacific salmon may travel up to 3,000 km to reach the same spot where they spawned. Arduous is an understatement for what the trip takes – “astounding stamina” a restraint of expression.

Salmon must swim upstream against a strong current, and repeatedly jump waterfalls of impressive heights: what to a human would be equivalent to leaping a 4-story building. Salmon may clock up to 32 kilometers per day in upstream travel.

They do this on little food. Many eat nothing on the trip home. Salmon may use up to 96% of their body fat in their marathon swim to their birthplace.

Salmon do take shortcuts, though the term is a laughable euphemism here. Salmon sometimes swim in tight schools upstream, the ones behind drafting on the wake of the chargers ahead. Lingering behind larger fish can reduce water drag by 50%.

Sockeye salmon may struggle upstream for a dozen days before taking a break; slipping into a side stream for a snooze. Many rest before leaping rapids.

The need for salmon to reach their birthplace to spawn is legend. But some salmon will settle for less.

Pink salmon seem less attached to their birth stream than other species. They have been seen spawning some 560 km from their natal nursery.

The way home is treacherous. Hungry bears, subsisting on grasses and greens before the arrival of salmon, wait in the streams to snatch salmon on the fly, or step on them in shallow waters as salmon take a siesta, before grabbing them up for dinner. Salmon are safe from bears in deeper water, as bears don’t like to get their ears wet.

Bears’ demand for salmon is ravenous. A single bear may kill 1,000 salmon or more during spawning season to fatten itself for winter. Bears depend upon the salmon run to survive the winter’s cold.

Other mammals and birds also make a feast of salmon on the run. Bald eagles snatch salmon out of streams from the skies. Salmon are a mainstay of the river otter diet, but run-time is an especial time of easy pickings.

Many species rely upon bear leftovers. Secondary scavenging feeds a host of animals: wolves, raccoons, shrews, and other small mammals, as well as numerous birds, less adept than eagles in their ability to order salmon takeout.

Mottled and tattered, nearly depleted to sinewy swimming skins, the remaining salmon reach their natal nook. The males compete for the haggard females with threat displays, jostles, and fights.

Females require courtship before committing to digging a nest. A male dances in the water nearby, then slides over a female’s back in a caress. A male’s change to bright coloration before the swim upstream, whatever the enhanced risk of predation, now pays off in female appreciation.

Fights between the females for prime nesting sites is common. They will bite at each other: at the gills or tearing at the tail to win an appealing nesting spot.

Salmon are fussy about where they mate. The water must be clear, cool, fast-flowing, and devoid of predators. The gravel must be pebbles rather than smallish stone bits.

A female prepares a nest in the streambed gravel by sweeping digs with her tail. The task may take days. By the time she is done, her tail is typically frayed and torn.

Meanwhile, as a tension reliever, some nerve-racked males may dig nests of their own. These never get used for eggs, but this displacement behavior takes the edge off the wait.

When ready, a female bends her body into a U, dropping her anal fin into the nest as a tempt. This positioning excites the male. He hovers close by. Upon his approach, she dips her back fin in again.

Often both of the mating salmons’ mouths open from the tension release. With a delicate quiver, the female lays her eggs, which a male duly fertilizes with a cloud of sperm (milt). Salmon sex per se may take less than 20 seconds.

Depending upon the species and size, each female has 4–5 nests, laying 500–5,200 eggs per nest. Each male milt is loaded with 50 million sperm.

The female then carefully covers the eggs by fanning nearby gravel with her tail. She then builds her next nest a bit upstream. This routine is repeated until she is exhausted. The set of nests, a female’s redd, are her legacy.

A mother-to-be may have enough energy to guard her redd for a few days, adding gravel and chasing off other females that might disturb her nests, to try to replace them with their own. Eventually she joins others rocking in the wake of the river, awaiting her last breath.

Males that outlast their mates will try for another mate: another ticket in life’s lottery. Only 1 in 2,000 eggs that hatch survive to spawn. That’s after the 85–90% of eggs that die before hatching.

The urge to live is strong, even as the life force is waning. Some exhausted salmon struggle to the center of the stream, where the water flow is strongest, and life-giving oxygen rushing over their failing gills the richest.

All these moments will be lost, in time, like tears in rain. Time to die. ~ Roy Batty in the movie Blade Runner (1982)

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Pacific salmon live a much more dramatic life than their Atlantic cousins, which face a relative jaunt upstream to spawn. Some 6% of Atlantic salmon survive spawning and return to the sea.

In the Pacific, some steelhead salmon may live to return to the sea. Some 6–30% of those that do may make one more spawning trip home to freshwater.

Gulls, bears, and other animals rely upon the short summer buffet of salmon to survive the rest of the year. Salmon predators move from stream to stream, following the slightly warming water temperatures that trigger spawning at different locations.

This hydrological diversity in the network of spawning streams affords a tripling of the time that salmon eaters enjoy: from a few weeks to 3 months or more. This is possible because the streams are but a kilometer or 2 apart.

Courtesy of bears, salmon play an outsized role in growing the gigantic coniferous forests that filter the snowmelt into the pure water that flows past their eggs. Having snagged a snack, bears casually haul their salmon into the woods. Facing overwhelming abundance, bears are sloppy eaters: consuming less than 25% of the biomass they have caught. Large chunks are left uneaten.

Rich in bio-nutrients, caught salmon fertilize the firs and pines after feeding the insects that swarm on the carcasses. Salmon discards by bears are equivalent to 4,000 km per hectare of commercial fertilizer, though much more valuable in readily accessible nitrogen.

20% of the streamside nitrogen in the soil comes directly from the decayed salmon; 20–30% from the insect consumers of the bears’ leftovers.

Kneeling angelica are 1- to 2-meter plants that reside near salmon streams. They produce clusters of tiny white blossoms, but quite late in the season, long after most pollinating insects are gone.

Blow flies lay their eggs on salmon carcasses. Before laying their eggs, the blowflies feast on nectar and spread pollen from the only flower about: kneeling angelica.

Dead salmon left in the stream provide food for emerging alevin. Juvenile salmon grow twice as fast in streams rich with salmon carcasses.

Part of the growth effect is indirect. Solvent salmon are savored by plankton, thus providing a food source for the fry of the next generation.

Salmon are prey in freshwater. They are predators only in the ocean.

In the finale, salmon returning from the sea to spawn inland represents a substantial transfer of nutrients from the ocean to terrestrial life. In life and death, salmon are a keystone species.