If we begin by saying Autumn fish spend much less time in the river then their Spring and Summer brothers and sisters.
Spring and Summer salmon spend more time in fresh water before spawning, the main reason for this, we know, is because as adults, most [certainly not all] threats, “natural” predators are found in the ocean. This means the longer they remain in fresh water, fasting prior to spawning, the greater the chance of survival of that cohort. Spring and summer salmon “trickle” into the river, entering in smaller numbers over a longer period [Feb – July] whilst Autumn running salmon arrive in greater numbers over a shorter period [Sept/Oct].
My own feeling is that “safety in numbers,” although this is a great safety mechanism, it has a downside. Both in the ocean and freshwater, all those fish are together, in one place for more of the time than those other cohorts. This means that logically speaking, a healthy Autumn Salmon Run, in all probability, will be a by-product of "lower than normal” loss due to predation, overfishing, in both freshwater and the ocean. Another factor to be taken into consideration is a combination of greater in-river predator exploitation of this cohort and loss of juveniles through gravel shift and flooding in those lower parts of the river. Seals, and Avian predators have also increased after years of a failure to manage.
People speak of Cycles. However, my guess is this is not cycler. It is not. For example, Haley’s comet, Lunar and Sunspot activity can be measures exactly, this is cycle and suggest “will” come around again. With regard salmon, there are no regular cycles, only fluctuations based on how different cohorts of the species adjust and adapt to environmental factors which create abundance of food in various parts of the ocean, and during the freshwater phase too.
Whilst there is no doubt that run times fluctuate and they will change in the future, what we have done, or not, to both our rivers and the ocean in the last 40 years has done the species a massive injustice. We knew so much about salmon 100years ago. What predator species we needed to control for them to prosper but for many reasons, the balance between “managing” and “researching” tipped in favour of the latter around 40 years ago. Whilst I am a big believer in research, this “must” be done as part of a wider management strategy. What we now have is “Feeble”!
For this Autumn cohort to do better requires low ocean predation and massive lower river production; obviously, I add man as a predator here too. I mention Lower River production because this is where those autumn fish spawn. Unlike Winter, Spring and Summer Salmon, which utilise the whole river, for spawning, true Autumn salmon spawn in the lower river where gravel banks are unstable and vulnerable to flooding, which we had much of between 2005 and 2020.
Grilse, or young salmon are similar, albeit the have early, mid, and late running cohorts too. Like Autumn salmon they arrive in fresh weather in much bigger shoals in the summertime.
The most prolific and “normal” run of salmon are spring and summer salmon. Those are the cohorts that stand the test of time because, although they shoal in the ocean, those shoals are smaller. As adults, do not rely quite as much in safety in numbers for as long in their homeward migration as Autumn salmon or Grilse, instead a more solitary instinct, different from, but like when they were in the Parr, or river stage returns. This defence mechanism is much more robust over prolonged periods of time where defence mechanisms adopted by Autumn fish and Grilse a specialised to more specific conditions at sea. It is a fascinating subject
I see salmon as both a sentinel and a teacher. Studying their journeys both in marine and freshwater environments provides us a wealth knowledge and insight into the health of our ecosystems and, relative to this piece, potentially why some cohorts fare better than others.
From my experience at sea off Northumberland, its rare to see multiple nos of salmon caught at one shoot of a Drift- Net, even off a salmon river but outside the playground. When it occurs, the fish are usually spread out along a c 610-yd net. This concurs with research off West Greenland when occasionally up to 3 were caught. like this but using a c 300-yd net but by less experienced crews ( researchers ). Turell and Shelton found shoals of up to c 27 smolts heading North up the Slope current. I once saw 96 salmon taken by one crew in one shift ( c 12 hours ) when a gale restricted effort to only the brave or…