SUPPORTING NATURE RECOVERING
COMPARING DIFFERENT SALMON STOCKING STRATEGIES
At the present time the Board operates a salmon egg hatchery which has a capacity to incubate up to 1.5million ova.
Until recently it has been our policy to stock these out either as eyed ova or what are termed "unfed" fry, that is fry which have just absorbed their yolk sacs and are ready to start feeding. We have not traditionally started the fry feeding in the hatchery.
While there are other hatcheries which operate along the same lines, some other hatcheries do feed the fish for some time, ranging from weeks to months, before they are released into the wild. The logic being that by giving the fish a growth advantage they may survive better in the wild. However, others have contended that this advantage may be offset by the fact that the fish become progressively "tamed" in the hatchery and may become progressively less suited to release into the wild with increasing time in the hatchery.
As far as we can see, much of the advice being given amounts to opinion rather than hard tested fact and if we are to optimise our hatchery operation (feeding-on comes at greater cost) we must be sure that whatever strategy we are using is the most effective one. As the Board took over the former Marine Scotland Science Fish Rearing Unit at Almondbank in 2011, we now had the facilities to also rear on fry and parr for ourselves. Therefore in 2011 we commenced controlled trials between eyed ova / unfed fry and fed fry / parr to determine which is the most successful.
We have been running such trials now for several years. The results so far have been summarised in our recent Annual Reports.
Broadly speaking, although we are continuing the trials, the results so far indicate that in streams with relatively good chemical fertility, the survival of fed on parr is relatively good and appears higher than that of eyed ova or unfed fry in some cases. However, in really infertile upland streams the initial growth advantage does not seem to translate into good survival thereafter. In some places the fish appear to struggle after release.
The trials continue. However, it is already clear that where eyed ova or unfed fry are stocked into under-populated streams they can and do survive well enough to saturate the available habitat, more here.