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Episode 289: Stunned by Salmon

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The Atlantic Salmon (image from Pacific Salmon | Species | WWF (worldwildlife.org)

Salmon is the common name for several species of ray finned fish (Actinopterygii), a clade of bony fishes, so named because their find are webs of skin supported by bony spines (rays). This class is a huge group, comprising almost 99% of the 30,000 species of fish characterised. Although many will be well acquainted with the fish from their appearance on many a menu, salmon are amazing fish with an incredible life history. 

There are seven species of Pacific Salmon, with five occurring in North American waters; the chinook, coho, chum, sockeye and pink. There is also one species of Atlantic salmon, found in the Northern regions of the Atlantic Ocean.

Salmon have long played an important part in human history, with evidence that the Neanderthals ate salmon, with archaeologists finding carvings of salmon in ancient cave dwellings, carbon dated to reveal they were inhabited 25000 years ago, The Romans constructed ponds to preserve salmon caught from the rivers in an early form of the industrialised farming we see today. Native Americans in the Pacific Northwest region revered the salmon, and the fish were a vital part of the lives of coastal settlers. Tribes in these regions understood the importance of protecting spawning rivers and ensuring healthy salmon populations. They used the whole fish, creating toys from the bones, clothing from the skin and glue from their bladders. In Celtic lore from medieval Europe, the salmon was seen as the keeper of wisdom as they could leap away from predators and could travel through salt and fresh water to find their place of birth, seemingly magically to the people of historical Europe. Salmon were so important that even one of the most famous documents in the world, the Magna Carta, the foundation of English laws, had a section placing controls of salmon farming and the protection of vital spawning rivers. Unfortunately, this didn’t protect salmon in the UK, as the advent of the Industrial Revolution eventually drove the salmon in UK rivers almost to the point of extinction. 

A carving of a salmon from the Poisson Shelter, a cave from the Dordogne region of France dating back to the Paleolithic era. (Image from Abri du Poisson: Paleolithic Cave Containing Fish Relief Sculpture (visual-arts-cork.com)

Today, salmon are a popular dinner choice across the world, listed as one of the top 5 species of fish consumed, and the estimated global consumption is put at 3.2 million tonnes of fish per year. Not only is it delicious, but salmon is low in saturated fat, high in omega 3 and fatty acids and is a great source of vitamin B12, iron and potassium, all of which can lower the chances of cardiovascular disease and cognitive diseases like Alzheimers. Most commercially available salmon is farmed, making up 72% of the total salmon harvest, with the two main producers being the Norwegian fjords and Chilean bays as their coastlines and waters have perfect conditions for salmon farming. 

Salmon are listed as least concern by the IUCN, although populations of Atlantic salmon have declined considerably since the 1970s despite measures to reduce fishing. This is largely attributed to the degradation of vital rivers used by salmon to spawn as well as increasing levels of mortality of adult fish at sea. 


Finding a way back home: the amazing life cycle of the salmon

The amazing lifecycle of the salmon (image from Linn Benton Salmon Watch Salmon Watch (lbsw.org))

In November to December each year, rivers across the Pacific Northwest are suddenly bursting with leaping salmon, fighting their way back up raging currents to return to their birthplace to spawn. Females excavate a shallow depression in the gravel bed of the river called a redd, in which they bury their eggs. This protects the eggs from predation and the dynamics of the river, such as low flow or currents that could disperse the eggs away from the redd.  The females release pheromones during this time, which attract the males to her redd to fertilise the eggs. This makes the success of salmon breeding reliant on olfaction, and some research now shows that river pollution, acidification and pesticide run off can reduce salmons ability to detect the important chemical signals attracting males to females redds, thus reducing reproduction. Environmental management methods, like protecting riparian vegetation, the plant communities along river margins, can help prevent sediment or agricultural runoff into rivers and streams, reducing pollution and providing better spawning conditions. 

After spawning, 90-95% of salmon die. This is where the different species of salmon differ in their life cycles. Pacific salmon are semelparous, meaning that they always die immediately after spawning as the process is so draining on their bodies. In contrast, Atlantic salmon are iterparous, meaning they don’t always die immediately after spawning, although they rarely survive more than two.

Salmon embryo, known as alvein, rely on their yolk sac for a lengthy period, only emerging when this food source has run out. Temperature and oxygen levels are the best indicators for the hatching of salmon, and this is well timed to ensure the salmon hatch when there is plenty of food resources in the river. However, as with the spawning, this hatching stage can be massively impacted by environmental conditions that can cause a mismatch between hatching time and the availability of resources. Climate change causing alterations in temperature or increased sedimentation from pollution or urban development can have a serious detrimental impact on salmon hatching and the timing of these important life stages. 

Once hatched, the now salmon fry tend to disperse away from the redd at night, which is suspected to be a predator avoidance technique. There is fierce competition for resources between the tiny fish, and many die at this stage, either through starvation or through predation from larger fish that are canny enough to catch them. 

The next stage of the life cycle are referred to as parr, and they develop rapidly in preparation for the life in the big open ocean. Again, temperature plays a massive role at this developmental stage. Between the optimal temperatures of 16 and 19oc, the fish develop normally and reach the right size quickly, but above this, growth is vastly reduced, and between the higher temperatures of 22 and 24oc, the fish enter into a stress response and may even die if this heat persists. As well as the temperature, the availability and quality of food resources can affect parr growth, and their development at this early stage of life can affect their eventual survival in the ocean. 

Parr spends months developing in their home river, but eventually, once they have reached the sufficient size and have started to undergo changes in behaviour, body shape, colour ad physiology, they are ready to make the journey to the open ocean, up rivers and estuaries. At this stage the salmon are known as smolts. They generally tend to migrate in the spring, when the conditions are the best. Like their dispersal from the redds, they migrate up the estuaries at night in large shoals to confuse predators. Manmade barriers, like dams and other coastal developments can stop this vital migration, and artificial light and noise from boats and human settlements can also throw the salmon off their path, leaving them off course and unable to reach the sea with all the resources needed to support them. 

If they make it all the way to the ocean, salmon are now post smolts and face a host of new challenges in the markedly more hostile marine environment. Adult salmon undertake long migrations at sea, although no one knows exactly where they go or why. In recent years, adult survival at sea has been steadily decreasing, but researchers have not been able to attribute this to any one thing, and it is likely there is a complex interplay of factors affecting the survival of the fish in the open ocean. Scientists are looking to conduct more tagging studies, attaching GPS trackers to fish and releasing them to follow their movements across the ocean to better understand their lives at sea. 

Spawning sockeye salmon in their iconic red and green mating colours. (Image from Sockeye Salmon | NOAA Fisheries)

Food for thought: salmon as a keystone species 

An Alaskan Brown Bear catching a salmon as it leaps up a salmon run. (Image from animal.memozee.com)

Salmon are not just a vital part of human diets and an important part of out economy, but they are also a key link in the food chain. Salmon are a keystone species, a species which has a significant effect on its ecosystem and food chain. In the Pacific Northwest, it is s common sight in the wilds to see Grizzly bears lined up, waiting for the migrating salmon to come charging up rivers to spawning grounds, right into the bears waiting paws. From hours spent watching bears in a key research study, scientists found that in 1km of river, 8 individuals caught up to 3000km across the study period, and that up to half of the salmon foraged were deposited onto the forest floor. Bears will very often carry the salmon away from the river into the treeline, where they can eat in peace, and it is this movement that provides a vital link between the ocean and the forest. The salmon carcasses decompose on the forest floor, depositing vital nutrients from the rivers and oceans that would otherwise never reach the forests. This makes the interactions between bears and salmon an example of ecosystem engineering, when a species or species interaction modifies their surrounding environment, often providing huge benefits to the ecosystem.

Decreasing salmon populations, therefore, wouldn’t just affect the menu at your favourite restaurant, but would have serious consequences for the ecosystems they call home. Although globally salmon are classed as least concern, some populations of Atlantic salmon are falling to critically low numbers. Between 1983 and 1990, salmon numbers fell dramatically, from around seven million to five million fish, and this falling population, while no longer as dramatic, is still ongoing. This is happening despite significant fishery reductions put in place and the formation of the North Atlantic Salmon Conservation Organisation, who created large, protected areas. As mentioned previously, river pollution and coastal development affect important stages of the salmon’s life cycle, alongside the issues caused by warming oceans and rivers. Although salmon farms provide much of the salmon eaten globally, and despite these farms having generally low carbon footprints and providing a good alternative to overfishing of wild populations, there are concerns over the transfer of disease from captive, farmed salmon reaching wild populations. There has also been issues with farmed salmon escaping and interbreeding with wild populations, affecting their genetic integrity. Farmed salmon is still an appealing alternative to purely relying on wild caught salmon and potentially affecting the ecosystems they inhabit, but there still needs to be tight regulations in place to stop the farmed populations harming the wild salmon that are so important in their food chains. 

Conservation optimism

Despite the concerns over the future of the salmon, there are many conservation efforts currently in full swing. 

The NASCO (North Atlantic Salmon Conservation Organisation) is an international organisation established in 1984 in response to falling salmon populations. They work to conserve, restore and carefully manage Atlantic salmon through cooperation with a range of international bodies, from governments to fisheries. They created a large, protected zone in which fishing was prohibited and they stopped the operation of the Northern Norwegian Sea fisheries, which was at the point of collapse in the 1980s. You can check out their work at  North Atlantic Salmon Conservation Organization – NASCO to learn more about how they manage stocks, conserve habitat, monitor fish levels and preserve healthy environments for the spawning salmon. 

Awesome videos!





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North Atlantic Salmon Conservation Organization – NASCO

Salmon Life Cycle – Cefas (Centre for Environment, Fisheries and Aquaculture Science)

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July 27, 2022
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