More salmon, less grilse, but uncertain times ahead

Just about all of the fisheries management world is now agreed that we are currently entering a MSW salmon cycle and are fast coming out of a 1SW grilse cycle. If history repeats itself, it may last up to 60 years! We are at a pinch-point in that transition and all it takes in any given year is for a poor MSW salmon run (as happens frequently, even in normal times) and the whole season collapses, as the salmon component of any annual run in a river is always far less numerous than grilse. I suggest this is probably exactly what we saw in rivers such as the Awe and the Lochy in 2017.

We are still fortunate though that the Lochy, compared to most west coast rivers, supports a strong genetic strain of large MSW salmon. Their time to shine might well be about to happen! It can be certainly no coincidence that only last season (2016) we had the best MSW spring salmon run for decades but the poorest grilse run in recent memory…

The reason for this grilse to salmon shift appears to be a warming sea (especially in the western edge of the NE Atlantic) and the complex changes it is having on (parts) of the ocean. Some areas of the NE Atlantic are still fairly stable, probably due to lesser impacts from shifting warmer currents (there is a known ‘cold blob’ just west of Norway for instance) but in other areas the lack of ‘grilse sized’ food is catastrophic: West of UK and South of Iceland being 2 examples. This is probably the reason for the northern Scottish rivers holding up particularly well and the southern and western rivers on the point of collapse in some cases. In times of grilse abundance (1960’s to 1990’s) the much cooler ocean would have held food uniformly all over – nowadays the food supply (and temperature) is highly patchy.

Sand-eel numbers have spiralled downwards due in part to the explosion in numbers and the unprecedented geographic spread of mackerel and other pelagic fish as waters have warmed. Grilse require abundant sand-seels to grow strong enough to survive their first winter at sea. A post-smolt salmon is a relatively rare creature in the vast ocean and the competition from any super-abundance of any marine species such as mackerel will completely swamp the vulnerable post smolt’s ability to compete for the same food source.

Conversely, MSW salmon travel further north for food where the problem is less pronounced. MSW salmon may also benefit if they can survive the first winter at sea and grow big enough to predate on the mackerel themselves. Possibly why, in amongst the current declines of most salmon rivers, some MSW salmon are returning at huge sizes (you grow fat quickly if you become big enough to eat mackerel!)

Anyway, all of the above is having an equal impact of sand-eel eating seabirds such as kittiwakes (doing very badly) but is favouring deep diving fish eaters such as gannets (doing very well). I cannot recommend highly enough the following programme which explains all of these oceanic changes on seabirds (and salmon) far better than I can. The mechanisms at play for both species are exactly the same.

Jon Gibb, Hatchery Manager.