THE IMPACT OF PREDATION ON FISH STOCKS – ENTERING THE LIONS DEN!
Why am I writing this?
Once upon a time, in a different life, I helped make government policy.
I learned to listen politely to zealots but not to engage. Passionate, unswerving believers do not like to be confused by facts. This is why scientists don’t challenge the stream of letters and articles in fishing magazines telling us that low salmon numbers are caused by seal predation.
So why am I getting involved in this fruitless debate?
In April, I was delighted to see a scientist, Professor Eric McVicar, was writing a two-part series of articles about Grey Seals in Fly Fishing and Fly Tying Magazine. Now we’ll get some facts, I thought. More on this later.
Does predation control prey numbers?
Since the days of Charles Darwin, biologists have been fascinated by the predator prey relationship and its link to the evolution of new species. Many ecologists have studied the impact, the absence, or the introduction of predators to isolated populations such as those on islands.
Laboratory and field work on this topic is notoriously difficult. One famous predator prey ‘experiment’ in a pristine, wild 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 pristine wilderness over many decades.
The numbers of the apex predators which provided the most valuable pelts, such as Arctic Foxes, Lynx and Wolverine were cyclical. There were regular peaks every ten years or 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 Irish example
Back to salmon numbers. A good example of the impact of predation comes 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 at least the last two decades.
In 2007 the country closed the drift net fishery that had always operated off its northwest coast. The ending of this significant human predation on returning adult salmon was immediately apparent. The following year there was in the increase in numbers running through the counters on most Irish rivers. Unfortunately, this change was short-lived, and the downward trend has continued. Ending drift netting, which was significant predation (albeit by humans and probably much more than any natural predation on adult salmon) temporarily changed the numbers reaching their natal streams but did not halt the downward trend.
The Tyne – some real numbers
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 emphasise 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 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 and is classified as ‘unproductive’. Food for baby fish is limited. Spate rivers are tough places to live. Disastrous floods, extreme droughts, hard frosts, and summer heat in unshaded streams can impact severely on food supply. Predatory fish such as brown trout, some ducks and herons and otters all eat little fish, but the likelihood of starvation drives the number that survive.
The population numbers at the other end of the Tyne salmon life cycle are just as revealing. Some current estimates from several rivers suggest that only 5% or less 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 around 600,000 smolts leave the river to start their marine migration each spring.
Grey seal predation
The inspiration for this blog was the recent two-part article by Professor Eric McVicar on predation of salmon by grey seals.
The first half of the article gave lots of details of the remarkable increase, over the past 50 years, in grey seal numbers around the Scottish coast and as far south as the Wash in the North Sea. As I read, I was asking, “yes, everyone knows this, but do grey seals eat salmon?”
We all know that any seal would eat a salmon if it could catch one. We know that they do sometimes take up temporary residence in tidal pools and harry salmon shoals. But the important question, which was not addressed in the article is, ‘does predation by seals significantly impact on the numbers of returning adult salmon to British rivers?’
The second part of the professor’s article simply assumed predation by seals was impacting hugely on fish stocks, including salmon, and 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. Large shoals of small fish such as sand eels form defensive ‘bait balls’ when they come under attack from dolphins, seals, large fish and birds. They are easier prey than free swimming fish. Bottom-feeding fish such as cod are an easier option than salmon.
What does all this mean?
The Tyne example suggests that, in a typical year, the 57,000,000-baby salmon that emerge from their redds have been reduced to less than 1,000.000 healthy smolts leaving the estuary two year later. Anyone who thinks this 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 available food throughout this part of the juvenile life cycle which is, of course, highly dependant on big floods, extreme drought, and other factors.
Perhaps the number of survivors could be significantly improved by intensive river engineering and tree planting to turn sterile, straight ditches into serpentine streams with pools and runs that create juvenile-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.
What is clear is that there is an intense scramble for food, and most do not make it. Stocking with more juveniles would be stupid. All it would do would be to make the competition worse and reduce the chances of individual survival.
The obvious headline figure is that if the annual number of returning Tyne salmon increased to 30% of departing smolts, as assessed the last time British salmon numbers were at the top of their cycle, the number running 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 disastrous and few survived.
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) and believe we can do something about it (catch-and-release, ban commercial fishing, kill predators). Of course, if there is scientific backing, we should act. However we may also have to accept the real (estate/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.
Lucky salmon fishers are those that live in a era when numbers peak and most salmon follow a fruitful migration pathway to rich marine feeding grounds.
That’s something none of us can do anything about!