Smolt Production in the Dee 2016-2019

October 03, 2019

Since 2016 the River Dee Trust has been engaged in a major piece of work to find out more about smolt production on the lower Dee. The report, Smolt Production in the Dee 2016-2019, is an important step in our understanding and will allow us to establish a baseline, monitor change over time and plan mitigation. The work we undertake as a result of these studies is geared to one purpose: to help maximise smolt production on the Dee.

RDT Biologist Dr Bas Buddendorf said, ‘Historically, we have an idea of adult numbers based on the rod catch, while electrofishing helps us estimate fry and parr numbers. Smolt trapping has been going on at the MSS traps on the Girnock and Baddoch for 60 and 30 years and has helped us to understand smolt production in the upper catchment. The missing piece has been an estimate of the numbers of smolts which leave the lower river each year to begin their ocean migration. The commencement of trapping on the lower catchment in 2016 helps us to develop a fuller picture of smolt production overall. The upper and lower catchments are very different; they have different characteristics and are subject to different pressures.’


Productivity

About productivity on the two burns Bas said, ‘The results show that the Beltie is not producing smolts to its full potential. The trapping data we collected shows the Beltie is producing far fewer fry and parr than what was expected based on a benchmark from MSS. Additionally, the fish counter on the Beltie suggests that the numbers of spawning females returning to the Beltie should have produced almost 25% more juvenile fish, which may be linked to a degraded environment in what is a heavily agricultural catchment. The production of juvenile fish on the Sheeoch is more in keeping with expectations, although the population did appear to have experienced a decline linked to Storm Frank.


The Limited Impact of Storm Frank

Most of us could be forgiven for thinking that Storm Frank would have had a serious impact across all age classes of juvenile salmon and sea trout. The picture which has emerged on the Beltie and Sheeoch makes for interesting reading.

Salmon smolt numbers on the Beltie were relatively stable over the period, while sea trout were more variable, said Bas. ‘In contrast the Sheeoch shows a decline in salmon numbers, but an increase in sea trout. It is interesting to note the uneven and limited impact of Storm Frank on the two catchments. The data suggests the impact of Storm Frank on Beltie salmon smolts was minimal, with the 2016 cohort not much reduced in comparison to other years. In contrast the Sheeoch shows a reduction; the 2018 smolt numbers are the lowest over the period.

Beltie sea trout appear to have affected by Storm Frank, with 2018 producing the lowest number of smolts over the period. We see another contrast with the Sheeoch sea trout doing well and increasing substantially across the monitoring period, which suggests they were less affected than salmon by the Storm.’


Looking Forward

Commenting on the work over the 2016-2019 period Bas said, ‘Overall we are happy with the performance of both the rotary screw traps and fyke nets, by the generally high recapture rates of marked fish on both the Beltie and Sheeoch. We will continue to run the traps each spring to monitor production on the lower Dee. We had some challenges, mostly relating to low flows, but where such issues occurred, we have taken an approach that avoids overestimating the number of migrants and gives us robust estimates of fish.

Looking ahead we can use this information to help us in our main goal as a Trust and Fishery Board- putting as many smolts to sea as possible. We have a big task in ensuring the salmon nursery streams throughout the catchment provide the best possible spawning and rearing habitats for salmon and sea trout. We have some major habitat restoration work underway and I am hopeful this work will produce positive results over the next few years.

To find about more about Smolt Production on the Dee, please see click here for the full report.

Bas can be reached on email at bas@riverdee.org or on 013398 80411