The Lifecycle of Salmon
The Significance of Salmon
Salmon are the bedrock and foundation of the ecosystems in which they live. They feed those who eat them directly—humans, coastal brown bears, birds of prey (to name a few). But more elementally, the many thousands of salmon who successfully make it to their spawning grounds to reproduce inevitably die not long after. Through their death they provide life and nutrients to everything else in the river: to plants, even more animals, and the surrounding earth. Their lifecycle of 6 distinct phases is a unique odyssey of physical adaptation, journeys of thousands of miles, and a final return to their roots.
1. Eggs. The salmon's lifecycle begins and ends in freshwater. Not knowing what dangers and grueling travels their parents have undertaken, eggs are fertilized in high mountain streams and river beds in cold water amidst polished gravel deposits. Water temperature and ph levels are instrumental in the eggs' development. They need cold, clean water. Studies have shown that if the water temperature is 59°F (15°C) or more, the warmth causes salmon embryo development to accelerate and results in birth defects and death. Colder water can prolong the incubation period. Salmon eggs are also susceptible to the level of acidity in the water with eggs being damaged at a pH level of 5, burned at a pH level of 4.5-4, and unable to survive at any stage of life at a pH level of 4.0.
There are differing incubation periods for each species of salmon. Among the five species that live in Katmai, the longest incubation period is pinks (aka "humpy") at 18 weeks, then chum (dog) at 16 weeks, then chinook (king) at 12 weeks, then sockeye (red) at 6-9 weeks, and finally coho (silvers) at 6-7 weeks.
2. Alevins. After incubation, the eggs hatch and tiny alevins (about an inch long) emerge. Interestingly, alevins hatch before their mouth or digestive system have fully developed and they rely on the remaining yoke sac that is still attached to their bellies for nutrients. Most species stay hidden in the gravel where they were deposited as eggs for 2-5 weeks. Pinks and Chum seem a bit more adventurous in letting current carry them downstream towards the ocean while they finish developing, essentially skipping the next phase of the salmon lifecycle: fry.
3. Fry. Once their digestive system and mouths are fully developed, the salmon enter the next phase of their life as fry. Fry swim to the surface of the water, fill up their swim bladders with oxygen (this allows them to hold their position underwater), and begin to feed on aquatic plants, plankton, and whatever else the river brings them that they can manage. They develop vertical strips on their bodies called "parr marks" which helps camouflage them in what is the most perilous stage of their life. Larger fish (rainbow trout are a chief predator in the Moraine Creek drainage, which is why this is a dream location for sport fishermen and well as bear viewers) and birds are responsible for a high mortality rate among fry.
Sockeye, Coho, and Chinook spend 1-2 years as fry, in what is referred to as Parr-Smolt Transformation (PST). During this time, the fry have the scent of their natal stream imprinted on their memory so they can find it again as spawners. Current studies seem to support the hypothesis that Chum and Pinks imprint earlier as alvins.
4. Smolt. Fry make their way downstream and in the brackish estuaries they undertake a fascinating and critical change as they adapt for life in salt water. When immersed in salt water, a typical fresh water fish would have the water syphoned out of their cells by the salt (through osmosis) and they would die of cellular dehydration. For salmon to survive in the ocean, they have to get their osmoregulatory organs (organs that are responsible for maintaining a balance between salt and water, such as their gills and kidneys) working differently than they would in fresh water. They spend 2-5 months (depending on the species) acclimating to higher and higher concentrations of salt. They begin drinking more water to make up for the amounts their cells are losing. Their kidneys extract salt and the salmon expel it from their bodies in their urine.
Salmon take on their ocean colors, losing their parr marks. And gradually, they find themselves in the ocean—adults.
5. Adults. The ocean phase is the bulk of a salmon's adult life. Most salmon spend a minimum of 1.5 years here, some can spend up to 5 years in the ocean. During this phase, salmon are believed to swim an average of 2,000 miles as they relentlessly hunt. Part of being cold water fish is gaining high levels of fat, and these fats not only make them delicious and good for us (omega-3 fatty acids are linked to protecting agains heart disease, strokes, rheumatoid arthritis, etc.), the salmon will rely on their fat reserves to carry them through their spawning migration.
Sockeye spend 1-4 years in the ocean and weigh 4-15lbs. Coho spend 2-3 years in the ocean and typically weigh between 8-12lbs, but have been known to exceed 30lbs. Chinook spend 1-5 years in the ocean and weigh above 30lbs in their adulthood, making them the largest pacific salmon. The largest recorded chinook was 126lbs. Pinks are the smallest salmon, weighing in at 3.5-5lbs. They stay in the ocean for 2 years. In Alaska, this 2 year schedule has resulted in a larger number of returning spawner pinks in odd-years than even years. Chum spend 2-5 years in the ocean and weigh 12lbs or more.
6. Spawner. After whatever amount of time they spend in the ocean maturing and building up their fat reserves, salmon begin their last migration from the salty ocean back to freshwater. Once salmon reach freshwater they undergo their final transformation as they begin dying. They stop feeding and drinking (continuing to drink as they do in salt water would completely overwhelm their osmoregulatory system), and rely entirely on their fat reserves for the energy to continue moving upstream, compelled by their indomitable, unyielding instinct that drives them up the streams and rivers from which they spawned (thank you, scent/taste imprint).
Their bodies and markings transform. Sockeye salmon, who are very common in the Moraine Creek drainage, lose their silver and develop ruby red bodies and emerald green heads that glisten in the streams. When reaching their natal streams, females carve out nests, also known as "redds," with their tails, resulting in dimples in the river and stream beds. Females lay up to 5,000 eggs in a single one of these gentle, shallow depressions. Males develop savage-looking hooked noses with snaggled teeth in order to fight off other males as they compete for fertilizing the most redds.
The salmon who manage to reach their natal streams and successfully spawn are absolutely spent and die soon after.
7. Decomposition. Many accounts of the lifecycle of salmon end with spawning. But Mother Nature is not done with these salmon, not yet, so this article will go one step further and point out some ecological impacts of this "7th phase" of death and decomposition.
One impact we closely observe with our guests at Emerald Air Service is that the spawned out salmon are a life-saving feast for the inexperienced fisher-bear. These adolescent bears live off of the less-desired, mushy flesh of decomposing salmon and off of the proficient bears' leftovers (the best bears become picky, smelling out the female salmon, eating delicious eggs, and leaving the rest of the carcass). Mother Nature never wastes and even with the seeming excess of the 1,200 lb. male bear who indolently picks away at roe (eggs) and turns up his nose at the rest of his kill, the trickle down effect means a young, inexperienced bear lives another winter and will gain more wisdom in the following year to better earn his keep.
By the time the next generation of salmon develop into their fry phase and start feeding on small particles in the water, their spawned out parents have decomposed enough to be that food. This guarantees rich sources of food close to the redds.
Fascinatingly, and further illustrating the cyclical effects of nature, the carcasses that simply rot on the rivers and streams (there are a countless number) enrich the soil surrounding the stream. This causes shrubs, such as alder and willow, to thrive on the banks. In return, the shrubs cast their shadows over the water, keeping the streams' temperatures cool for the incubating salmon and preserving this cycle of life.
Summary: 5 Phases of Katmai's 5 salmon species
graph data gathered from South Puget Sound Salmon Enhancement Group