River Carron Catchment Scale Restoration Programme Phase IIc: Origin of 2016-2018 Carron Smolts

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Background

Starting in 1994, the River Carron Improvement/ Conservation Association (RCCA) has been undertaking a programme of supplementary stocking with the aim of supporting the Atlantic salmon population in the river. To monitor the impacts of this programme, the RCCA has collected a rich dataset that includes detailed cross and stocking records, results of electrofishing surveys, smolt trapping, adult rod catches and coded wire tagging of stocked juveniles, and thousands of genetic samples taken from broodstock fish, juveniles, smolts and returning adults. This dataset provides the opportunity to understand of how stocking aimed to support and restore a small population of native Scottish salmon has actually impacted that population, and in turn to shape future conservation stocking efforts.

Explainer Q&A with Bob Kindness MBE

 What’s new in this phase of the research?

We’ve answered one of the last outstanding questions about stocking, whether fish reared through conservation stocking survive at sea and successfully return to their river of origin. Having already shown how to design the programme to preserve the genetic health of the native population, we have now demonstrated that fish returned to the river at the correct early life stage survive their marine migration and return to spawn at rates equivalent to wild fish.

Why is that so important?

Because without either piece of evidence, conservation stocking could not confidently be relied upon as a recovery tool. This study provides some of the most detailed long-term field evidence available anywhere for this method of stocking, and we hope it will inform future policy discussions.

Are we running out of time?

On some rivers, yes. Populations, especially in smaller rivers, have dropped very quickly. If intervention comes too late, unique native populations can disappear entirely.

Why does that matter?

Each river has its own genetically distinct salmon, adapted by nature for the unique challenges of their native river. If that stock disappears, you can’t simply replace it, you’ve lost something unique.

Is stocking needed everywhere?

No. But where populations are close to collapse, it may be the only realistic way to maintain numbers while longer-term fixes take effect.

What happens if we don’t act?

You move from decline to local extinction. And once that happens, recovery becomes virtually impossible.

This differs from other evidence on stocking outcomes.

I can’t comment on other people’s work, but I am always happy to demonstrate how we operate the RCCA programme. With any youngster, you want to give them the best possible start as that is reflected in the life they go on to lead. We have spent 30 years evaluating different approaches to understand how to give salmon that are struggling the best beginning so they can go on to thrive in the wild. If you are looking at other research, it is therefore important that you are comparing like for like, rather than programmes that may differ significantly.

How might your work differ from other stocking programmes?

Interestingly, most of the research on marine survival of stocked fish has focused on fish released as smolts, and our result indicates that fish released at a younger stage can have better survival. We find this gives them the best start as they are protected from predators and spates (flash floods) for long enough to dramatically increase survival, but not for so long that they suffer the drawbacks of being domesticated. They then get at least 6 months in the wild in the river before going out to sea. We also showed, in the previous phase of research, how to design a stocking programme to ensure the genetic health of the native population is optimised.

What major questions remain about conservation stocking?

The impact of stocked fish on wild juveniles is not yet fully understood. The key concern is whether stocked fish competitively displace wild-spawned fish, which is why we are currently recommending stocking as a tool to prevent extinction in populations already at critical risk. More research is needed to establish the optimal time to intervene, as it does need to be early enough to ensure a genetically viable founding population still exists to work with. The relative reproductive success of returning stocked adults compared to wild fish also remains to be fully quantified, and this is something that will be better understood as the approach is applied to rivers where full, self-sustaining recovery is the goal.

What now?

We hope our findings will help inform and accelerate the development of evidence-based guidance for conservation stocking, so that the methods most likely to succeed can be deployed on rivers facing imminent extinction, before it is too late.