This is going to upset a lot of people, but here goes.
We are obsessed with predators and predation. TV nature programmes are mostly animals eating each other. Big cats, birds of prey and whales command our attention. I suppose we can add marlin, pike, and salmon to that list of apex predators. Does it come from deep inside us? We know we are, and have been, the most damaging and the most unthinking mass killer of wildlife the world has ever seen.
Salmon numbers are declining.
Many salmon anglers passionately believe fish-eating birds, grey seals and mythical human predators using invisible Atlantic trawlers are to blame.
I beg to differ. Let’s start with the Soapbox Article on the goosander ‘problem’ by Lord Carlisle K.C. in the November 2023 edition of Trout and Salmon. I greatly respect the noble lord’s political and legal expertise but question his ecological credentials.
I also question articles on predation by grey seals in another magazine by a respected, retired academic. In my opinion these were misleading because highly respected, published scientific papers on the subject were ignored.
Gamekeepers who release naïve pheasants and those of us that keep a few chickens know that foxes eat birds! Lord Carlisle provided evidence that goosanders eat small fish. I have seen a grey seal eight miles from the sea, upstream of two large waterfalls in my favourite salmon river. It was not there to enjoy the scenery!
We know goosanders and seals eat salmon. The question is what is their impact on the numbers, and health of, in-river populations of returning adults.
Biologists have always been fascinated by the predator prey relationship. Laboratory and field work on this topic is notoriously, especially in wild, natural habitats.
A famous predator prey ‘experiment’ in a pristine environment was not an experiment at all but it had all the discipline, accuracy and monitoring that science demands.
Throughout the 19th and early 20th centuries there was an insatiable demand for fur in Europe. Much of it was supplied by the Hudson Bay Company, based in London, which operated animal trapping stations in Northern Canada. Thousands of ‘trap-lines’ and areas were allocated to hardy trappers who inspected the lines constantly, re-set the traps and prepared the furs. This carried on unchanged for many decades. The number of each species of animal trapped, and their source, was meticulously recorded. After all, trappers were paid for every valuable pelt they delivered to the trading stations. The Hudson Bay Company’s London records, in neat copperplate handwriting in leather-bound books, provided an unquestionably accurate picture of the changes in animal numbers in a true wilderness over many decades.
The numbers of apex predators which provided the most valuable pelts, such as Arctic Foxes, Lynx and Wolverine were cyclical. There were regular peaks followed by a rapid crash and then a slow build up to another peak.
Further research revealed the reason.
Lemmings are a small, mouse-like mammal that is native to the Arctic tundra and northern forests. All predatory mammals prey on them. The lemming population is dramatically cyclical, although no one is exactly sure why, and numbers can be enormous during a ‘lemming year’. There are even apocryphal tales of mass migrations of lemmings and suicidal falls over cliffs.
There was complete correlation between the rise and fall in lemming numbers and the numbers of mammalian predators. When prey is abundant predators prosper and their numbers increase. When prey numbers fall, predators starve, and their numbers fall.
So, when biologists look at predator/prey relationships in wild populations their starting point is that predator numbers are likely to be controlled by available food and that, overall, predation has little impact on prey numbers.
The likely reason fish-eating ducks are thriving on healthy Welsh streams is that there are plenty of small fish, such as trout, minnows, and coarse fish for them to eat. Sure, they’ll eat young salmon too, but it makes no sense to believe there is a disproportionate impact on this one species. The numbers below put this into perspective.
Let’s look at an example of real and very serious salmon predation from the Emerald Isle.
Ireland has a strong record of recording salmon and grilse runs and now has over thirty monitoring stations that accurately monitor the numbers of fish running each year on different rivers. Numbers have followed a downward trend for more than three decades.
In 2007 the country closed the salmon fishery that had always operated off its northwest coast. Many believed this ‘predation’ was the cause of the decline in adult salmon numbers.
The effect was immediate. The following year there was an increase in numbers running through the counters on Irish rivers.
Unfortunately, this change was short-lived, and the downward trend has continued ever since. Ending drift netting, which probably killed more salmon and grilse than any natural predation ever could, temporarily increased the numbers reaching their natal streams, but it was not the cause of the downward trend.
Discussion on salmon population dynamics makes more sense if it is tempered by real numbers and informed guesstimates.
For example, the Tyne, a river I love, is blessed with an accurate Environment Agency counter which has recorded the adult salmon run since 1996. The recorded numbers demonstrate both the risks, and potential benefits, of long-distance migration because the run of adult salmon into the Tyne varies enormously each year. The timing of runs has changed too. The spring salmon and autumn grilse runs have collapsed. The summer run of salmon has increased.
The average annual number over more than twenty years of recording is around 30,000. Some of the best years were in the last decade which surprises the doomsters. Tyne salmon are in no danger of extinction!
Let’s assume half the 30,000 salmon that make it into the upper reaches and tributary streams of the South and North Tyne in a typical autumn are female. If each hen fish lays around 4,000 eggs, then around 60 million eggs are laid. If 95% of these are fertilized and survive the winter in their redds this means 57,000,000 baby salmon (or alevins) emerge from the Tyne gravels in the spring.
For a few days they survive on a nutrient rich yolk-sac but then they are forced to join an intense scramble for food.
The major tributaries of the Tyne emerge from the North Pennines. The peaty water is too acidic to support a lot of invertebrates, such as shrimps and molluscs, and is classified as ‘unproductive’. Food for baby fish is limited. Such rivers are tough places. Disastrous floods, extreme droughts, hard frosts, and summer heat in unshaded streams can impact severely on food supply. Salmon parr need to establish a ‘territory’ that provides a place in the stream with a food supply and a refuge from the regular attention of predators such as brown trout, fish-eating ducks, herons, and otters. Most fail and starve.
The population numbers at the adult end of the Tyne salmon life cycle are just as revealing. Current estimates from several rivers suggest that under 5% (much less on the west coast of Scotland due to sea lice pollution) of the 2-year-old salmon smolts that make it downstream to the estuary return as adult fish to breed. This would mean that on the Tyne system at least 600,000 smolts leave the river to start their marine migration each spring to maintain current number of adult returners.
This means the 57,000,000-baby salmon that emerge from their redds each spring have been reduced to less than 1,000.000 healthy smolts leaving the estuary two year later. Anyone who thinks the death of 56,000,000 fish is due to predation from a relatively small number of avian predators and a few big trout cannot do maths! This reduction is almost certainly due to starvation.
Perhaps the number of juvenile survivors could be significantly improved by river engineering and tree planting to turn sterile, straight ditches into serpentine streams, pools and runs and fry-friendly habitat. Or overall, the lack of available food may be the critical factor and the river system simply could not feed a bigger juvenile population than it does today.
The obvious headline figure is that if the annual number of returning Tyne salmon increased to 30% of departing smolts, as was assessed the last time British salmon numbers were at the top of their cycle in the 1960’s and 70’s, the number of returning adults each year would be around 180,000. If those days returned, the Tyne would be thick with salmon by October!
No one knows why there is currently such high marine mortality of salmon in the adult phase of their life cycle.
The catastrophic decline in grilse numbers that occurred between 2010 and 2014 in many British and Irish rivers is a recent memory. This was a time when I, and many other salmon fishers, caught emaciated little grilse of only a pound or two that had clearly starved at sea. The obvious hypothesis is that the marine migration route and destination taken by these grilse was, and is, a disaster. Grilse numbers are not increasing
What about grey seals, are they to blame?
Over the past 50 years, there has been a remarkable increase in grey seal numbers around the Scottish coast and as far south as the Wash in the North Sea.
We all know that any seal will eat a salmon if it catches one. We know that they do sometimes take up temporary residence in tidal pools and harry salmon shoals. But the important question is, ‘does predation by seals significantly impact on the numbers of returning adult salmon to British rivers?’
The recent magazine article by a retired scientist assumed predation by seals was impacting hugely on fish stocks, including salmon, and demanded that ‘spineless and gutless politicians should take grey seals seriously……and remove them from areas where they do not belong.’
The article failed to refer to three considerable pieces of research. This was carried out in 1985, 2002 and in 2010/11 on behalf of Marine Scotland (the department of the Scottish Government responsible for fishery policy) by the Sea Mammal Research Unit at the University of St. Andrews. This Unit has global reputation for the quality of its research.
The reason Marine Scotland commissioned and repeated the research was because of their concern that the huge increase (by 2010 numbers were already as they are today) in grey seal numbers was impacting on Scottish commercial fish stocks.
The detailed work required was considerable. It followed a sophisticated, tested model. Through 2010 and 2011 over 2,000 grey seal scats (poo!) were collected. Sampling was carefully managed to include both a wide range of haul out sites and regions including Western Scotland and different areas of the North Sea from Shetland to the Wash. Sampling took place throughout the year to ensure account was taken of prey fish movements and abundance. Many were from beaches and estuary sites in major east coast salmon rivers.
Seal diet was estimated by counting the hard remains of prey (fish otoliths and cephalopod beaks) which are not digested. Otoliths are ‘ear-stones’ which identify the species of fish eaten. Again, well-tested modelling means diet composition can be estimated as the percentage by weight of each prey species for each region and throughout the year. It is assumed all seals met their annual energy requirements i.e., despite their high numbers all were finding enough food.
In total, the 2,205 grey seal scats processed yielded 68,465 otoliths and beaks. Almost every species of fish and cephalopod (cuttlefish and squid) that lives around Scotland and in the North Sea was identified.
Amazingly, not one of all the Atlantic or North Sea samples from the Scottish or English mainland contained a single Salmonid otolith. Not one seal had eaten a salmon or a sea trout. From all the samples taken Salmonid otoliths were only identified in one or two scats collected in Shetland. They were found only in the first quarter of the year and amounted to less than 1% of the prey species eaten by Shetland seals.
Interestingly, Shetland is not known for its wild Salmonid populations, but it is home to many salmon farms.
In the Western Isles the seal diet was dominated by sand eel and gadid prey, particularly cod and ling. In the Northern Isles, the diet was also dominated by sand eel but also cod and saithe. In the central North Sea, the diet was heavily dominated by sand eel but was more varied in the southern North Sea.
Overall, it is estimated that the British Isle’s grey seals consume a grand total of 200,000 tons of food each year. Their diet appears to have changed little in the Western isles over the three studies between 1985 and 2011. Around the Northern Isles the contribution of sand eel has gradually declined over the same period while gadid consumption has increased. The reverse is true in the central North Sea where sand eel consumption has steadily increased since 1985.
Perhaps this should not surprise us. Defensive ‘bait balls’ of sand eels are easier prey than free swimming fish. Bottom-feeding cod are an easier option than salmon.
It is part of the human condition to arrogantly seek control. When salmon numbers are not as we would like we blame ourselves (global warming, over-exploitation, open-cage salmon farming, poor predator control, in-river habitat destruction) and believe we can do something about it (catch-and-release, ban commercial fishing, kill predators, habitat restoration/improvement). Of course, if there is scientific backing, we should act.
However, we may also have to accept the real (rod-caught/commercial catch records) and anecdotal (‘apprentices demand no more salmon’) evidence that adult salmon numbers and seasonal runs have always fluctuated wildly. The most likely explanation is that numbers depend on the success of their marine migration. Predators are not to blame.
Lucky salmon fishers are those that live in a era when numbers peak and most salmon follow a fruitful migration to rich marine feeding grounds.
That’s something none of us can do anything about!