Exploring Salmon's Global Adventures: An Odyssey

Salmon are anadromous fish, meaning they migrate from the ocean to freshwater rivers to spawn. Salmon are born in freshwater streams and spend their juvenile years in rivers, lakes, and freshwater wetlands before migrating to the ocean as adults. Once they reach sexual maturity, they return to their natal rivers to reproduce. This can involve travelling hundreds of miles and swimming against strong currents and rapids. Salmon are able to do this by using the Earth's magnetic field as a compass and their sense of smell to find their way back to their birthplace. This behaviour is known as homing and has been shown to depend on olfactory memory.

How do salmon know where to go?

Salmon are known to migrate thousands of miles from their home streams to the ocean and back again to reproduce. But how do they know where to go?

Scientists believe that salmon use a combination of techniques to navigate back to their home streams. Firstly, they use the Earth's magnetic field as a compass to navigate back to the general region of their birthplace. They then use their sense of smell to find their way back to their home stream. Young salmon are particularly sensitive to the unique chemical odours of their locale when they begin migrating to the sea. These odours are stored in their brains and become important direction-finding cues when they return as adults to spawn.

Salmon also appear to use some form of "map and compass" navigation based on information about their position and direction of travel. This information likely comes from a range of environmental cues, including day length, the sun's position, the polarization of light, the Earth's magnetic field, and water salinity and temperature gradients.

Salmon have also been known to imprint on the chemical nature of their surroundings. They imprint not only on the smells of their freshwater environment but also on the smells of their ocean environment in the vicinity of their home stream. This allows them to migrate thousands of miles and return to the exact location where they were spawned.

Salmon are able to hear using low-frequency sound waves that vibrate through the water to a row of sensory pores called lateral lines on their sides. They can also detect minute variations in the Earth's magnetic field using their lateral line, which acts as an ultra-sensitive sensory organ.

How far do they travel?

Salmon are anadromous, meaning they reproduce in freshwater streams and rivers, but spend a portion of their lives in the ocean, where they gain most of their adult weight. They are born in freshwater streams and migrate to the ocean, travelling hundreds of miles. Once they reach the ocean, they might travel an additional 1,000 miles to reach their feeding grounds. In the ocean, they travel in large, loose schools and feed on small fish, krill, and crustaceans. They remain in the ocean for 2 to 8 years, travelling hundreds, even thousands of miles.

When they reach sexual maturity, the adults return to the upstream rivers to reproduce. They return to the same streams that their parents used, and even to the very spawning ground of their birth. This is known as homing. It is thought that they use magnetoreception to locate the general position of their natal river, and once close, they use their sense of smell to home in on the river entrance and their natal spawning ground. They can smell chemicals down to one part per million.

Salmon can migrate more than 3,000 kilometres upstream through freshwater to spawn. They often travel 50 kilometres per day on their spawning journeys, which is equivalent to running more than a marathon every day. They can jump up to two metres to cross obstacles in rivers.

Once salmon begin the trip, they will not stop to feed, deriving all their energy from stored fats. They travel an average of 150 miles from the sea to reach their spawning grounds. Each trip contains its own set of obstacles: waterfalls, man-made dams, and hungry predators.

Why do they travel?

Salmon travel from their freshwater birthplace to the ocean, and then back again to reproduce. This is known as a salmon run.

Salmon are anadromous, meaning they migrate from the sea to freshwater rivers to spawn. They typically hatch in the shallow gravel beds of freshwater headstreams and spend their juvenile years in rivers, lakes, and freshwater wetlands. They then migrate to the ocean as adults, where they live like sea fish and gain most of their body mass. When they reach sexual maturity, they return to their freshwater birthplace to reproduce.

Salmon travel these huge distances because the ocean provides them with the energy they need to grow big and strong, making them less likely to be eaten. The ocean also provides females with a higher number of eggs. A lot of eggs are needed to produce enough juveniles to sustain the population.

Salmon travel hundreds of miles from their home stream to the ocean, and then a further 1,000 miles to their feeding grounds. Once they have spent a few years in the ocean, they return to their freshwater birthplace to spawn. This can involve travelling hundreds of miles upstream against strong currents and rapids. For example, Chinook and sockeye salmon from central Idaho travel over 1,400 km and climb nearly 2,100 m from the Pacific Ocean as they return to spawn.

Salmon are able to navigate these huge distances by using the Earth's magnetic field as a compass. Once they find the river they were born in, they start using their sense of smell to find their way back to their home stream.

What are the dangers?

Salmon face many dangers during their migrations, both in freshwater and saltwater. One of the primary dangers is human activity, which has led to a decline in salmon populations. Logging around streams can reduce the shade and nutrients available to the stream and increase the amount of dirt in the water, which can choke salmon eggs. Dams can also cause salmon to die from the shock of going through turbines and make them more vulnerable to predators as they emerge from the dam. Overfishing is another factor contributing to the decline in salmon populations.

Salmon are also threatened by natural predators such as bears, eagles, and other fish. The post-spawning death of salmon is important for the ecosystem as their nutrient-rich carcasses provide food for other animals and add nutrients to the water and surrounding environment. However, this also means that salmon populations need to be healthy and abundant to support these other species.

In addition, climate change poses a significant threat to salmon. Warmer waters can make it difficult for salmon to migrate and reproduce successfully. Urbanisation, including the building of dams and the removal of riparian vegetation, also affects water temperatures and flow velocities, creating more challenging conditions for salmon migrations. As salmon are sensitive to water temperatures, these changes can have detrimental effects on their physiology and reproductive success.

Salmon also face dangers specific to their life stage and migration route. For example, when returning to their natal streams to spawn, they may encounter obstacles such as waterfalls and rapids, which require them to jump or swim against strong currents. The journey can be exhausting, and some salmon die en route or soon after spawning due to physical deterioration and disease.

How do they benefit the ecosystem?

Salmon are a keystone species, meaning they are integral to the ecosystem, and their removal would result in drastic changes. Salmon play a critical role in the health of the ocean, mountain streams, and forests. They are also important to the economies of coastal communities, especially in the Pacific Northwest.

Salmon are anadromous fish, meaning they are born in freshwater, migrate to the ocean, and then migrate back to freshwater to spawn and die. This unique spawning behaviour supports the diverse ecosystems that lie along the coast of the Pacific Northwest. Salmon typically choose their nesting sites in riffles, where the water is shallow and the flow is fast and turbulent. Their nests, called redds, are small pockets in the gravel riverbed, created by the female salmon using her tail. By fanning her tail over the gravel, she creates the hydraulic conditions to lift gravel out of the riverbed, leaving a shallow hole where she can lay her eggs. The female then swims upstream of the nest and fans more gravel to cover the eggs, shielding them from predators. This behaviour requires gravel with a diameter of up to 4 inches, which provides protection for the eggs and allows water to flow through, delivering oxygen for their development.

Salmon play a significant role in the survival of certain ocean species during their time in salt water. For example, Chinook salmon are the primary prey for southern resident killer whales. In addition, salmon support populations of eagles, gulls, seabirds, and more by providing them with nutrients essential for overwinter survival and migrations. The amount of salmon in a stream has been shown to be an indicator of the density and diversity of bird species in the surrounding ecosystem.

Salmon are also an important source of nutrients for bears in coastal watersheds. The population density of bears can be up to 20 times greater in areas where salmon are abundant compared to areas where they are not. In areas with abundant salmon, bears consume an average of 15 salmon per day, obtaining 33-94% of their annual protein from these fish.

When salmon die at the end of their life cycle, their carcasses provide valuable nutrients to streams and rivers, enhancing the productivity of the surrounding ecosystem. These nutrients are transferred to all levels of the food chain, and some species adjust their survival strategies to benefit from these additional resources.

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