Salmon are an enigma.
No species has been studied more than the king of fish. We know much, but scientific research highlights our ignorance too.
A Current Example
The letter pages of game fishing magazines assert all would be well for salmon across the British Isles if it was not for predation by seals or fish-eating ducks.
Scientists know different.
In 2010 the river Tweed celebrated one of its best-ever annual runs of autumn grilse. The rod catch was tremendous. This ‘tribe’, probably the most prodigious U.K. salmon run, entered the river throughout September and October.
By 2014, only four years later, this population was functionally extinct.
Since then, everyone on the Tweed knows no fresh salmon are seen after the end of August. No one can explain this catastrophic collapse, but it was not caused by Farne Islands’ seals or the river’s mergansers.
Salmon are an enigma!
Another Question
Why are there five species of Pacific salmon (all of genus ‘Oncorhynchus’) and only one Atlantic salmon (Salmo salar)?
Over millions of years, all salmon have evolved from a freshwater ancestor that adopted a marine migration as part of their life cycle. All must return to freshwater streams to breed. All are genetically ‘programmed’ to return to their natal stream rather than to any convenient river. Therefore the ‘fittest’ fish, that have survived as in-river juveniles and enjoyed a successful migration, mate with each other and reinforce these strengths.
The downside to this evolutionary strategy is the isolation of in-river tribes. If one population is wiped out, replacement depends on ‘lost’ salmon filling the ‘empty’ slot. This can take a long time. Records from the great houses in the Tweed valley show that when an autumn or spring run collapses it seems to take about fifty years for a new run to get going. Perhaps we’ll have to wait until the 2060’s to see a new autumn run?
The isolation of in-river tribes of salmon does increase the chance that genetic changes are reinforced, inherited, and eventually lead to the evolution of new species. So, scientists are not surprised that there are different species of Pacific salmon with different life cycles. Some favour particular rivers, and the run times and breeding behaviour of each species is different.
Atlantic salmon vary too.
Grilse spend only one winter at sea. Russian ‘osenka’ salmon run into Kola Peninsula rivers in August and spend the winter under the ice that covers them. They do not breed until the following autumn. There must be an advantage to spend a winter in freshwater rather than under an ice-bound Barents Sea. However, all Atlantic salmon, whether from Southern England or Siberia, are a single species.
One explanation is that the Atlantic Ocean has always been a much tougher place for salmon to thrive than the Pacific. The endless march, over millions of years, of climate and physical changes may mean Atlantic salmon have always had a ‘boom-or-bust’ history. Perhaps regular extinctions of in-river populations followed by slow re-population from other river systems has always happened.
Our own historical record, covering but a flash of geological time, remembers periods of plenty (‘apprentices demand no more salmon’) followed by concerns over vanishing runs. The population dynamics of Atlantic salmon may never have been stable enough to allow the evolution of different species. The Pacific may always have been a ‘kinder’ salmon habitat? No one knows.
Salmon are an enigma!
A Key to Success
Pink Salmon are the most prolific, and wide ranging, Pacific species. They are as beautifully silver as other salmon but after entering fresh water they quickly darken. Their flesh has a distinctive pink colour. Males rapidly develop a ‘kype’ and a distinctive dorsal hump before spawning. American anglers call them ‘humpies’.
‘Pinks’ are successful because they have evolved to enjoy the ‘holy grail’ of salmon adaptations. Every other salmon species lay their eggs in freshwater streams where the hatching fry have a struggle to feed and thrive. Many rivers are poor in nutrients, and prey species are in short supply. Most juveniles fail to establish territories and die. Later, survivors are faced with a downstream migration and difficult physiological transition into salt water. More die.
Pink salmon are different.
Adult pinks enter their natal streams in late summer to breed. Females lay about 2,000 eggs, usually in several ‘redds’, to ensure some survive. In early spring the eggs hatch. Alevins, with a food rich yolk sac, emerge from the gravel and simply drift downstream to the sea. There, they begin to feed on plankton. They stay in coastal waters for the summer before making their way offshore to find bigger prey. A year later, they return as mature adults weighing around 5lbs. to the river of their birth. All die after spawning.
The huge advantage of this life cycle is there is no very challenging juvenile stage in freshwater. Pinks have no nutritional need for, nor biological impact on, their natal river. For them, it is simply a breeding site. Pink salmon can establish strong runs in rivers that would not support other salmon.
If juvenile pink salmon are lucky, and the annual spring plankton bloom is healthy, at-sea survival and growth is excellent. Over only a few years new pink salmon runs, whether from introductions or natural distribution, build quickly. Numbers can be spectacular. No wonder pinks are the most prolific Pacific salmon.
Odd and Even Year Runs
All pink salmon have a precise life cycle. A successful September run of breeding adults means their offspring will return exactly two years later. As a result, right across their range, many rivers have either ‘odd year’ or ‘even year’ runs. Not many rivers have a run of pinks every year.
DNA analysis has revealed that these two ‘tribes’ are genetically distinct. Amazingly, they have not interbred, probably for hundreds of thousands of years or longer. They might as well be separate species.
The ‘odd year’ tribe has a wider range and bigger runs.
Stocking and Hatcheries
Unsurprisingly, fishery scientists have tried hard to introduce hatchery-bred ‘even year’ fish into rivers where ‘odd year’ runs dominate. They have had little success.
Efforts to stock pink salmon from hatcheries into suitable, fishless rivers have also had mixed results.
In the 1960’s, the Canadian government thought it would be an economic boost for local, isolated communities to stock pink salmon in some Hudson Bay rivers in arctic Canada. These rivers were like northern Alaskan rivers where pinks thrive. Millions of dollars were spent over a decade. Every spring fry were flown north from a government hatchery and a huge effort was made to establish runs. It was a complete failure.
Eventually the project was abandoned as money down the drain!
At the hatchery the remaining unwanted fry (no one is sure how many, but it was a very small number!) were, quite literally, flushed down the drain. Guess what happened! The sewer discharged into a stream which eventually ran into Lake Superior, the biggest of North America’s great freshwater lakes. Within a few years this tiny injection of pink salmon fry established healthy runs of ‘odd year’ adults to the river.
Pinks are not the only Pacific salmon that has adapted to a freshwater migration to feed and grow in the Great Lakes. Intentional, successful stocking of Silver (Coho) Salmon has created some well-established runs.
During the same period, Russian scientists enjoyed more success stocking rivers in western Siberia with pink salmon from eastern Russia’s Pacific rivers. Sustainable stocks were established. It is understood no further stocking took place after the 1970’s.
Why are some stockings of pinks successful and some are not? Why are ‘odd year’ fish more successful? Why do odd and even year fish struggle to co-exist? No one knows.
Salmon are an enigma!
Moving West
Over recent years pink salmon originating from Siberia have moved steadily westwards. There are increasing populations in Finland. Norway is treating pinks as an unwelcome ‘invasive species’ and looking to exterminate them. It is probably too late.
If the advancement of pink salmon into the North Sea and the Atlantic cannot be stopped, the good news is that the ‘invasion’ will have little, or no, impact on the rivers they colonise. And they probably will not travel far up-river to lay their eggs. Pinks only travel far enough up most Pacific rivers to find suitable spawning sites. This is often not far above the tide.
From an angling point of view, pinks are good news. Their substantial runs are predictable. They take a fly eagerly and, unlike most other salmon species, are often caught in estuaries and tidal waters as a run builds up.
Latest News
In the last few years, a small number of pink salmon have turned up in Scotland in rivers such as the Ness. It is possible some managed to breed there in 2021.
We should all be watching what happens there, and in other northern rivers, this autumn. We might get a surprise! New pink salmon runs develop very quickly.
Even though salmon are an enigma, it may soon be wise putting in an early bid for a salmon fishing tenancy in September 2025.