Jon Gibb. The rationale behind the agreed 3-year stocking plan
It may be helpful to recap the advantages of various stocking strategies and why (even when marine survival rates are at all-time lows) it is still worth removing a small amount of the overall spawning adults from the river to maximise the future benefit the progeny of those fish bring to the population.
The table below uses accepted wild salmon survival rates (Mills and Shelton, AST, 1986) at all life cycle stages, compared to the survival rates of reared hatchery smolts. Clearly it makes sense to take a small proportion of your spawning fish and utilise them in this way as the eggs generate proportionally far more returning adults. However, any such activity must be done within the bounds of what is an acceptable risk to genetic variation (this is why we stock only 50,000 smolts as opposed to a million!)
In terms of which life stage to stock at, it makes the most sense to stock smolts…. if you can afford to do so. In our case it makes perfect sense to do so as, due to funding, it costs the RLA a fraction of the normal price of running a smolt hatchery (normal cost for 50,000 smolts = £100,000pa @£2 per smolt)
This next table shows a comparison of 2 different stocking strategies – smolt stocking and fry stocking. It demonstrates why the most productive strategy is smolt stocking, even when marine survival rates have collapsed (as they almost certainly did these last 2 years). It is also why the 3-year hatchery plan (agreed in February 2017) is to stock both smolts and fry if enough broodstock can be obtained.
Fundamentally though, until marine survival rates of both stocked/wild smolts return to their recent range (circa 4% for wild, 1.5% for stocked) then the hatchery will not be likely to be benefiting fishermen in the short-term, but the long-term benefits of extra egg deposition remains. Importantly, it takes a very small shift upwards in survival rates to have a very large impact on returning numbers (including those to the rod). Meantime, using the hatchery, we can create significantly more eggs deposits than if we had left the fish in the river, and hope for a turnaround very soon in marine survival.