Background Image: Alaskan sockeye salmon (Oncorhynchus nerka) in spawning colors. Here, the changes in male physiology in this species associated with spawning (large hooked jaws and a large dorsal hump) are clearly visible.
What Is A Salmon?
The term "salmon" is a general term used to describe two genera of anadromous (an animal that spawns and hatches in freshwater, then migrates to saltwater to mature) fish in the family Salmonidae, which are distributed in both the Atlantic and Pacific Ocean basins.
The Atlantic genus, Salmo, has only one representative species referred to as a salmon (other members of the genus are referred to as trout), the so-called Atlantic salmon, Salmo salar. The Pacific genus, Oncorhynchus, is made up of five or six species (depending on whose phylogeny of the genus is being referenced) recognized as salmon, and several other species referred to as trout.
Classically, salmon of all species are viewed as highly-migratory animals who, after spending one or more years maturing and growing in the open ocean, return to their natal river upon adulthood to spawn. However, there are populations of several salmon species from both Oncorhynchus [1] and Salmo [2] that do not migrate, and remain in freshwater for their entire lives.
The Atlantic genus, Salmo, has only one representative species referred to as a salmon (other members of the genus are referred to as trout), the so-called Atlantic salmon, Salmo salar. The Pacific genus, Oncorhynchus, is made up of five or six species (depending on whose phylogeny of the genus is being referenced) recognized as salmon, and several other species referred to as trout.
Classically, salmon of all species are viewed as highly-migratory animals who, after spending one or more years maturing and growing in the open ocean, return to their natal river upon adulthood to spawn. However, there are populations of several salmon species from both Oncorhynchus [1] and Salmo [2] that do not migrate, and remain in freshwater for their entire lives.
Salmon Life Cycle
The typical salmon life cycle begins when adult fish migrate back to the approximate river they were born in to spawn. Upon reaching freshwater, pigments within the animals' bodies begin to migrate outwards to their skin as they burn through muscle and fat on the struggle to swim upriver; this is why salmon turn bright red before they spawn.
Salmon lay their eggs in shallow, cold, fast-moving rivers and streams. The female scoops out rocks and silt with her tail to form a small pit (called a redd). Depending on the size of the female, and the species, each female can make one or several redds. Fertilization occurrs externally.
All species of Pacific salmon die after spawning. The bodies of the adults provide vital nutrients to the surrounding ecosystem, and will also provide nutrients to the next generation of salmon after hatching.
After an incubation of several months, salmon eggs hatch into alevin, mobile baby salmon still with large yolk sacs. Once these yolk sacs are depleted, the salmon are known as fry; this is the stage in which the salmon begin to forage for themselves.
After another several months, when the young salmon have approximately doubled in size, they are known as parr. Parr make their way down to the estuary of the river they were born in. This is where their bodies make the hormonal and physical adjustments to living in saltwater. At this stage, the salmon are known as smolts.
Smolts will leave the river after another few weeks to months, depending on species, and head out to sea to begin their adult lives. Depending on species, salmon remain in the ocean for one to eight years, growing and feeding on small fish (although one extant species, Oncorhynchus nerka, or the sockeye salmon, feeds exclusively on plankton). Once the adult fish are mature, they swim back to the approximate river they were born in to start the cycle all over again. [17]
Salmon lay their eggs in shallow, cold, fast-moving rivers and streams. The female scoops out rocks and silt with her tail to form a small pit (called a redd). Depending on the size of the female, and the species, each female can make one or several redds. Fertilization occurrs externally.
All species of Pacific salmon die after spawning. The bodies of the adults provide vital nutrients to the surrounding ecosystem, and will also provide nutrients to the next generation of salmon after hatching.
After an incubation of several months, salmon eggs hatch into alevin, mobile baby salmon still with large yolk sacs. Once these yolk sacs are depleted, the salmon are known as fry; this is the stage in which the salmon begin to forage for themselves.
After another several months, when the young salmon have approximately doubled in size, they are known as parr. Parr make their way down to the estuary of the river they were born in. This is where their bodies make the hormonal and physical adjustments to living in saltwater. At this stage, the salmon are known as smolts.
Smolts will leave the river after another few weeks to months, depending on species, and head out to sea to begin their adult lives. Depending on species, salmon remain in the ocean for one to eight years, growing and feeding on small fish (although one extant species, Oncorhynchus nerka, or the sockeye salmon, feeds exclusively on plankton). Once the adult fish are mature, they swim back to the approximate river they were born in to start the cycle all over again. [17]
Tinbergen's Four Questions
In 1963, Niko Tinbergen described a paradigm for understanding and asking questions about animal behavior [18]. This seminal paper, "On aims and methods of Ethology", was a foundational document for the field and is still highly influential today. While it is not the only or maybe even best way to discuss animal behavior, it is certainly the most common. Tinbergen established two fundamental binary attributes for the classificiation of these questions. These categories can be described validly in many different ways - one way they can be reframed is as "Proximate" vs. "Ultimate" and "Snapshot" vs. "Story".
- Mechanism - How is this behavior generated? What are the mechanistic bases that facilitate this behavior? (Proximate, Snapshot)
- Ontogeny - How does this behavior develop within an organism's life span? (Proximate, Story)
- Adaptive Value - Why does this behavior exist the way it does? What is its utility? (Ultimate, Snapshot)
- Phylogeny - Why is this behavior present in this species? What is the evolutionary utility of this behavior and in what form does it exist in related animals? (Ultimate, Story)