Salmon are amazing fish that can migrate long distances and survive both freshwater and saltwater environments during their complex life cycles. If you’re wondering how salmon reproduce, the answer lies in their use of external fertilization, which is fascinating biological process perfectly adapted to their needs.
In this comprehensive article, we’ll examine the full salmon reproduction process, looking at the anatomy of male and female salmon, their spawning behaviors, how eggs are fertilized externally, and the subsequent embryo development stages.
We’ll also overview the salmon life cycle and highlight why external fertilization is so critical for enabling salmon to reproduce successfully.
Anatomy of Male and Female Salmon
Differences in Appearance and Size
Male and female salmon exhibit distinct differences in their anatomy and appearance. Males tend to be smaller than females, with some species having males that are only one-fifth the size of females! Males also develop a pronounced hooked jaw during spawning season, which they use to fight other males when competing for females.
Their bodies become more laterally compressed and take on a humped back shape.
Females undergo changes too, growing much larger abdomens as their eggs develop. A female salmon can lay between 2,000 to 5,000 eggs, so their bellies become very distended and rounded for carrying all those eggs. The eggs are stored in the ovaries prior to spawning.
Females also exhibit a change in head shape and coloration as they prepare to spawn, but do not develop the hooked jaws or humped backs of the males.
Eggs and Milt
The most significant anatomical difference between male and female salmon is in their reproductive systems. Females produce eggs and males produce milt, which is the fish term for sperm.
The eggs are large, spherical orbs that range in color from orange to red depending on the salmon species. Chinook salmon eggs can reach up to 0.2 inches (6 mm) in diameter before water hardening! The eggs contain all the nutrients required for embryo development and are stored within skeins in the ovaries prior to spawning.
Milt refers specifically to the seminal fluid of fish and contains the sperm needed to externally fertilize the eggs. Milt is cloudy or milky in appearance and color. It does not contain the nutrients and yolk needed for embryo growth like the eggs do.
During spawning, the female digs a nest in the gravel riverbed in which to deposit her eggs. At the same time, males release milt over the eggs to achieve external fertilization. The fertilized eggs then incubate in the gravel before hatching as juvenile fish called fry or alevins.
Salmon Spawning Behaviors
Migration back to Natal Stream
Salmon are anadromous fish, meaning they hatch in freshwater streams and rivers, migrate to the ocean to mature, and return to their natal streams to spawn and reproduce (NOAA). This journey can be arduous, with some salmon species traveling over 900 miles upstream against strong currents to reach their original spawning grounds (Alaska Department of Fish and Game).
Scientists believe salmon may use the stream’s unique scent as a homing signal to identify their natal stream.
Nest Building
Once they reach their spawning grounds, female salmon will start to build nests called redds in the gravel of stream beds, using their tails to create small depressions. The females will deposit between 2,000 to 7,000 eggs in these redds, depending on the species and size of the salmon (US Forest Service).
The nests provide protection and oxygenation for the salmon eggs until they hatch, which can range from 40-150 days depending on water temperature.
Mate Selection and Courtship Displays
Male salmon will compete vigorously for access to females and perform elaborate courtship dances to entice spawning partners. Dominant males may spawn with several females, while less dominant salmon may not get a chance to pass on their genes at all.
Once a female selects a mate, she allows him to swim beside her and externally fertilize her eggs as they settle into the gravel of the redd (Arizona State University). The parents provide no further care, often dying soon after spawning from stress and exhaustion after their long journey.
External Fertilization Process
Release of Eggs and Milt
During the spawning season, female salmon release eggs from their ovaries into the water while male salmon release milt, containing sperm, simultaneously (1). This process usually occurs in shallow waters with gravel bottoms and moderate current flow.
According to the Department of Fisheries and Oceans Canada, a female Chinook salmon can release between 3,000 to 17,000 eggs in a single spawning (2). The release of eggs and milt at the same time in the same location is essential for increasing the chance of external fertilization.
Fertilization in Water
Once the eggs and milt join the stream water, fertilization happens externally without direct copulation between salmon. A male salmon’s milt contains seminal fluid and sperm. The seminal fluid protects the eggs when they combine.
Meanwhile, the fast-moving sperm cells compete to penetrate the eggs’ membranes to achieve fertilization (3). Notably, only a small percent of the thousands of eggs spawned will become fertilized, as many fail to intersect with milt or get fertilized by low-quality sperm.
The fertilized eggs, now embryos, contain genetic material from both parents and continue developing if conditions like water temperature and oxygenation levels stay suitable.
Egg Development Stages
The initial stage of embryonic development of fertilized salmon eggs is called the “eyed” stage, occurring around 2-3 weeks after fertilization. Small dark eyespots become visible in the eggs, indicating the optic nerves and eyes are forming.
The next phases involve continued cell division and differentiation into organs, muscle, bone, etc. Hatching occurs several weeks after fertilization when the baby salmon, called “alevins”, emerge with yolk sacs still attached for early nourishment.
Shortly after, when the yolk depletes, the young salmon called “fry” must hunt food and continue maturing until their seaward migration (4). Survival rates depend greatly on environmental conditions and predators at each phase.
Detailed information on salmon’s amazing external fertilization journey available on government sites like NOAA Fisheries.
Salmon Life Cycle Overview
Migration from Freshwater to Ocean
After hatching in freshwater streams and rivers, young salmon called smolts undergo physiological changes to adapt to saltwater. In an amazing feat of navigation, they migrate downstream and out to the ocean to feed and mature.
Some species like sockeye salmon travel hundreds or even thousands of miles to reach productive feeding grounds.
Years at Sea Feeding and Growing
Salmon spend between one to five years roaming the ocean, feeding and growing. The rich cold waters of the North Pacific and North Atlantic Oceans allow salmon to get big and strong. Some king salmon reach sizes over 30 pounds!
Talk about amazing growth from their early days as eggs and frail young fry. Their time at sea prepares them for the ultimate challenge – returning to their natal streams to spawn.
Return Migration to Natal Stream to Spawn
In another remarkable navigation feat, salmon migrate back to the exact river or stream where they hatched. Scientists believe they use their keen sense of smell to identify unique scents in their home waters that guide them back.
After epic journeys, salmon undergo striking transformations to adapt their bodies for spawning. Using external fertilization, females release eggs into a nest called a redd in stream gravels while males fertilize them.
Sadly, most salmon die soon after spawning, but their offspring hatch the following spring, starting the salmon life cycle anew.
Advantages of External Fertilization for Salmon
Large Numbers of Offspring
One major advantage of external fertilization is that salmon can produce enormous numbers of offspring. A single female chinook salmon may lay from 3,000 to 14,000 eggs in a single spawn (ADFG). With a single male able to fertilize multiple females’ eggs, salmon have evolved to produce copious amounts of offspring to overcome the low odds of survival.
Out of the thousands of eggs laid, only a small percentage will hatch and survive to adulthood. Producing such high numbers allows the species to continue successfully despite many offspring falling victim to predators, starvation, and environmental factors.
No Need to Invest Resources in Parental Care
Another major advantage is that salmon do not need to spend energy caring for their young after spawning. The fertilized eggs are left on their own, buried in gravel nests the female constructs. Parental care would require salmon to remain with eggs/fry and protect them, expending precious energy during their strenuous spawning migration.
Instead, the lack of parental duties allows salmon to focus purely on migrating, sexual selection, and reproduction. They can allocate maximal energy to the long journey from ocean to freshwater streams and spawning activities like nest construction.
Flexibility in Spawning Environment
Since external fertilization occurs in open water, salmon have great flexibility in choosing spawning habitat. They are not constrained to specific locations and can spread out to reduce competition for prime sites.
Species like chum and pink salmon often spawn in intertidal zones, taking advantage of stream/ocean water mixing.
Their eggs only require clean gravel, proper water flow, and optimal temperature ranges. Salmon migrate extensively to locate sites meeting these criteria. Compared to internal fertilizers, external fertilization allows salmonids much more adaptability in reproduction without the high energy demands of parental care.
Conclusion
As we’ve explored, salmon are perfectly adapted to make use of external fertilization as their reproductive strategy. The male and female anatomy, spawning behaviors, fertilization process, and salmon life cycle all align to make external fertilization highly effective for salmon reproduction.
This allows salmon to produce enormous numbers of offspring to perpetuate future generations, while investing minimal parental resources. External fertilization gives salmon flexibility to spawn in a diverse range of freshwater environments.
Overall, this specialized fertilization strategy enables salmon to thrive and account for their evolutionary success.
