River Carron Stocking Programme – Supporting Data for the DNA Report
The DNA Report relating to the River Carron salmon stock and produced by the Rivers and Lochs Institute of the University of the Highlands and Islands, concluded that “fish stocked in the Carron clearly survive to adulthood and make a substantial contribution to the fishery” and “there is no indication that the 2014 stocking presented a risk to the genetic diversity of Carron Atlantic salmon as a whole.” The report identifies both smolts migrating to sea in 2016 and 2017 and adults appearing in the rod-catch of 2017, 2018 and 2019 that were stocked into the river as well-fed fry in 2015, having been produced from eggs stripped in the winter of 2014/15. By using this information and referring to the detailed records of hatchery fry production and the stocking programme, it is possible to evaluate various aspects of the stocking process in terms of its success or otherwise.
Broodstocks
The eggs produced in the winter of 2014/15 and subsequently providing the fry for stocking in 2015, came from three distinct broodstocks:
Wild broodstock: These were hens and cocks caught on rod-and-line towards the end of the angling season, September and October, and then kept in tanks until ripe. The eggs from each hen were fertilised using milt from a single cock with clips being taken from each parent for identification by DNA analysis. Each set of eggs was incubated so that appropriate sets could be combined at the first feeding stage to form batches for stocking out (Table 1).
Attadale broodstock: This was a captive broodstock produced by retaining some wild Carron eggs, incubating and hatching the eggs and then rearing the young through to maturity entirely in freshwater. The maturing fish were held in large ponds. Eggs from up to 7 hens were combined before being fertilised by milt from a single wild cock to avoid the risk of crossing siblings. As with the wild broodstock, clips were taken for parent identification and distinct batches were formed for stocking out (Table 1).
Uist Broodstock: As with the Attadale broodstock, this stock was produced from Carron wild eggs. In this case eggs from several pairs of MSW hens and MSW cocks were retained and transferred to North Uist by the Scottish Salmon Company. This stock was reared through to the S1 smolt stage in freshwater tanks and then in sea water cages to maturity. A proportion of the stock matured as grilse and, from these grilse, eggs were produced and brought to Lochcarron as eyed ova. Clips were again taken for parent identification. The offspring were stocked out as one batch (Table 1).
Stocking strategy
When considering the stocking strategy to adopt, three questions need to be answered:
Where within the catchment should the stocking take place?
At what developmental stage should the fish be stocked?
When should the fish be stocked out?
At a time when marine survival is poor and, as a consequence, spawning escapement is less than in the past, the aim of a stocking programme is to maximise smolt production from the river using appropriate native stock. This is the goal of the Carron stocking strategy and was sought by addressing the 3 questions in the following way:
To ensure that as much of the river system was utilised as possible, every part of the mainstem, where there was suitable habitat and spare capacity in terms of wild fish, was stocked with fry. This included sections devoid of suitable spawning gravel but where young fish could do well. In addition to the mainstem, fry were also stocked into the 3 biggest burns entering the Carron, Allt Coire Crubaidh at the top of the river at Glencarron, Fionn Abhainn that joins the river at the top of the Kelso beat and the River Taodail that mets the river at he top of the tidal water.
Table 1 matches the stocking batch number with the stocking location. From the top of the river down to the sea, the stocking locations were as follows:
Loch Sgamhain (Batch 8) – mid-way down the loch on the north bank.
Glencarron (Batch 3) – from below Loch Sgamhain down to the Black Pool upstream of Glencarron Lodge and including Allt Coire Crubaidh from the road bridge to the river.
Arineckaig (Batch 7 and half of the Uist fry) – from where the narrows of Loch Dughaill drop down into the river below down to the railway bridge.
Fionn Abhainn (Batches 5 and 6) – from the falls (pre-hydro) down to the road bridge.
Kelso (Batches 1 and 2 and half of the Uist fry) – from the mouth of Fionn Abhainn down to the Strathcarron road bridge.
Attadale (Batch 9) – from the Strathcarron road bridge down to the top of the tidal water.
River Taodail (Batch 4) – from the falls down to the road bridge.
For each of the stocking locations, the fry were spread out as much as possible. This has 2 benefits, to keep densities low thereby enabling fish to establish territories and thrive and reduce predation pressure.
Regarding the most appropriate stage of development for stocking , opinions are split. There is a school of thought that fry held in tanks through the summer become “domesticated” making them unable upon release to cope with conditions in the wild in terms of finding food and avoiding predators. To avoid this, the recommendation is to stock eyed ova or un-fed fry. However, in the case of the Carron, which is typical of all the west coast rivers, this approach would lead to disappointing results. Almost every winter the Carron suffers from at least one spate big enough to wash eggs out of the gravel (SEPA data since 1979). Alevins in the gravel would also be prone to wash-out and un-fed fry could suffer high losses from predation. To stock out at such a vulnerable stage when there is the risk of high losses makes no sense, especially when the number of eggs available from a struggling salmon population would be very limited.
The approach on the Carron has followed a totally different path. Fry are started on feed in shallow GRP tanks between mid-April and mid-May (timing depends on the ambient temperatures through the winter and early spring). The tanks have solid lids and disturbance is kept to a minimum. This undoubtedly helps to maintain the natural instinct of seeking cover when a potential predator appears. In terms of feeding, in tanks the fry grab anything moving in the water. This will normally, but not exclusively, be dry fish food. When released into the wild they will continue to grab anything moving in the current and therefore have no problem feeding. At first-feeding, fry coming off the yolk-sac weigh approximately 0.2 gms. For the 2015 stocking, the smallest fry stocked out had an average weight of 1.7 gms and the biggest of 4.8 gms. All stocked fry were well established.
The timing of stocking is largely dependent on the size of the fish. Although all the eggs that created each batch of fry were stripped on the same day and the resultant fry started feeding at the same time, some fry grow faster than others. This means that as the fry progress in a particular batch, a range of sizes become evident. If nothing is done, the faster growing fry will dominate the food and out-compete the smaller fry resulting in ever greater differences in size. Small fry are held back but perhaps, more importantly, the larger fry can get to a size beyond the ideal for stocking out in the late summer or early autumn. The trigger for a parr to become a smolt in the spring , occurs towards the end of September of the previous year. If it reaches a length of approximately 8 cms (about 6 gms) it will become a smolt the next spring even though it makes little growth through the winter. Since it is highly likely that such small smolts will survive less well at sea than larger smolts, steps should be taken to minimise the number in the stocking programme. This can be achieved by size grading the fry as they grow to keep the populations as uniform in size as possible. The fry graded out as large can then be stocked out ahead of the rest and before they reach the trigger size in the autumn.
In Table 1, stocking dates are provided for each batch of fry. Small batches were stocked out on single days while larger batches were over several days and over a longer period of time. This would either have been linked to size grading or simply thinning down populations to avoid tank stocking densities becoming too high. It is also the case that to stock effectively takes time involving the moving of fish to various parts of the river on any one day. Therefore there is a limit to how many can go out on each occasion.
Stocking outcomes
Atlantic salmon are anadromous, spending the early part of the life-cycle in freshwater before migrating down to the sea as smolts, travelling to the North Atlantic to feed and grow before returning, mainly to their natal river, as maturing adults. The return journey takes place after 1, 2 or 3 sea winters at sea. In view of this life-cycle, the success of a salmon population and by implication a salmon fishery is dependent on how many smolts successfully leave the river and how many adults return. In the absence of a method of counting returning adults, this figure is normally estimated from the rod catch. Monitoring smolt numbers and returning adults is also essential in assessing a hatchery programme providing the stocked fish can be identified. This is not always an easy task given that once a stocked fish has been in the river for a few months it will be indistinguishable in appearance from a fish from natural spawning. In this study, DNA analysis of broodstock used to produce fry for stocking, smolts sampled from a rotary screw trap and returning adults in the rod catch has been used to monitor the success of salmon fry stocked out in 2015.
DNA analysis of fish leaving the river as S2 smolts, as evidenced from screw trap smolts, suggested that approximately a third of them came from the 2015 stocking. This corroborates data collected from some tagging work carried out in 2008 when 40,000 fry at an average weight of 4 gms were released into various parts of the catchment in the month of September. When these fish reached the smolt stage they were monitored through the rotary screw trap positioned towards the lower end of the Kelso beat. These smolts could be easily identified from an adipose fin-clip although in every other respect they looked identical to non-clipped smolts. 137 of these smolts went through the trap in 2009 as small S1 smolts while 822 went through in 2010 as S2 smolts. The trap efficiency was calculated from the release of almost 1,000 recognisable smolts over the two years 2011 and 2013. In both years between 16 and 17% of these smolts went through the trap. Applying a figure of 17%, it meant that 805 S1 smolts and 4,835 S2 smolts were estimated as being produced from the 40,000 autumn fry. When adding the estimated small number of 47 S3 smolts from 2011, the overall total of 5,687 smolts represents 14.2% of the stocked autumn fry. From the DNA analysis and the previous tagging work, it is clear that stocked autumn fry produce significant numbers of smolts.
While it is essential that stocked fry generate healthy numbers of smolts, it is important for both conservation and the fishery that this increased smolt output produces a greater return of adult fish to the river. To judge the overall success of stocking, it is essential to monitor returning fish in the rod catch whereby stocked fish can be identified. In the case of the Carron this is achieved through the DNA analysis of all the broodstock used to produce the fry for stocking and of a large proportion of fish in the rod catch. Not only can the number of stocked fish returning to the river be estimated but grilse can be distinguished from 2SW salmon and each fish can be matched with the batch of fry from which it came. This links each rod caught fish with the number of fry in the batch, where they were stocked, at what size and when the stocking took place.
Rod caught fish identified as coming from stocked fry were predominantly grilse. For the 3 different broodstocks, the Attadale broodstock produced 73% grilse, the Uist broodstock 61% and the Wild broodstock 90% grilse. This is perhaps not a surprise for two reasons. Firstly, stocked fry get a much faster start in life than wild fry with most of them leaving the river as S2 smolts. Fish getting an early advantage will tend to maintain that advantage through the marine cycle and are therefore likely to come back as grilse to complete their cycle. The longer a fish stays at sea the less chance it has of surviving to successfully spawn the next generation. Secondly, fry batches 1 and 3 came from grilse eggs so, if grilse tend to produce grilse, then this may account for the high number of grilse from the Wild broodstock. In the case of the Attadale broodstock, some of these fish had matured as grilse and some of salmon, being captive stock, and this may reflect the slightly higher proportion of salmon in the rod catch. However, there was an interesting anomaly relating to the Uist broodstock. Although grilse still dominated the rod catch, the proportion was lower than for the other two broodstocks, and within the salmon catch there were 5 unexpectedly large fish at 23, 20, 20, 18 and 17 pounds. This was surprising since the eggs producing the Uist fry were stripped from grilse. An explanation for this may come from the history of the broodstock. Eggs were selected out from a few batches of Carron MSW eggs fertilised by MSW cocks. These eggs were taken to North Uist, where they were incubated, hatched and then reared through to the S1 smolt stage in a hatchery before being transferred into sea cages. After one sea winter, the majority of the fish identified as being grilse were harvested out with the exception of 34 hens and a few cocks. These fish were brought through to maturity to produce some eggs for the Carron. Although the fry produced from these eggs had grilse parents, they has 2SW salmon for grandparents which may explain the appearance of these big fish in the rod catch.
Despite the fact that the capture location of most of the fish of stocked origin is known in addition to the area where the fry were stocked, it is not possible to determine whether these fish were returning to spawn in the areas where they were stocked. This is because fry and parr can migrate within the river and rod caught fish may not be at their final destination. However, the relative success of fry stocked into different parts of the river can be assessed by matching returning stocked fish with the fry batches. This can be expressed as the number of stocked fry required to produce a fish in the rod catch (Table 2). From the table, it appears that the fry stocked into the Kelso beat performed best along with those stocked into the Fionn Abhainn burn. This burn discharges into the top end of the Kelso beat so there is a strong possibility that fry and parr could have migrated down into the river and therefore have a similar success to fry stocked directly into the Kelso beat. Fry from Batch 7 stocked into the Arineckaig beat produced less rod caught fish than the Kelso beat but these fish came from the Attadale broodstock and seem to be slightly less successful than the Wild or Uist broodstocks (Table 3). The Uist fry were split between the Kelso and Arineckaig beats and had a success rate very similar to fry from the Wild broodstock (Table 3). Both of these broodstocks had a life-cycle which had a freshwater followed by a marine phase before being back in freshwater for artificial spawning. The comparable success rates demonstrate that using a captive broodstock of native origin is a perfectly acceptable approach when the number of wild fish available from the river is limited. It enables substantial numbers of eggs to be produced which is essential for a stocking programme to be effective.
In contrast to the Kelso and Arineckaig beats, the return rate from fry stocked at the top of the river, Glencarron and Loch Sgamhain, was measurably lower. This may be as a result of less available habitat but may also be due to a lack of angling effort resulting in lower catch returns. This would be the case if fish coming from the upper part of the river headed back there for spawning. Once these fish reached Loch Dughaill, about mid-point in the catchment, there would be little chance of capture before spawning time. It is also the case that Loch Sgamhain has a healthy brown trout population that would predate on any stocked fish.
The Arineckaig and Kelso beats cover the section of the river from the Narrows of Loch Dughaill down to the road bridge at Strathcarron. In addition to having extensive good habitat for young fish, these beats account for a high proportion of the fish caught on the river. It is therefore not surprising that a higher percentage of the fry stocked into these beats appear in the rod catch. Predation pressure also influences the survival of stocked fry. There are a few pairs of goosanders that arrive in the spring to coincide with the smolt run. These birds target smolts descending the river but then broods of young feed on fry and parr throughout the summer. This could result in different predation pressure for parts of the river depending on where these broods are being reared.
The success of stocked fry from different stocking locations is not entirely clear based on the 2015 stocking alone. Clips from broodstock for DNA analysis have been collected since 2011 and from a proportion of migrating smolts and of returning adults in the rod catch since 2013. Analysis of these clips would clarify the differences in success of the various locations. It would also reinforce conclusions to be drawn on the benefits or otherwise of stocking as opposed to basing them on the results from only one year.
Stocked fish as broodstock
Because the DNA Report features the analysis of returning adults in the rod catch, the conclusions relate to catches and therefore the river as a fishery. However, of greater importance, at a time of decreasing Atlantic salmon populations throughout much of their range, is how stocking relates to the conservation of the species particularly in smaller rivers. To have a positive impact on conserving salmon stocks, stocked fish returning to the river need to be able to spawn successfully and to produce viable offspring. In the Carron, hen kelts of stocked origin have been caught that obviously went through the spawning process in the river. Although it is impossible to determine how successful the offspring were from these fish, it is reasonable to assume that stocked fish are perfectly capable of spawning naturally.
It has been suggested that stocked fish are less fit than naturally spawned fish and should therefore be deterred from spawning in the river. Fitness is judged by the quality of the offspring that are produced. Within the group of returning adults in the rod catch of 2019, 6 hens, retained as broodstock, had come from the fry stocked out in 2015. Until the DNA was read it was impossible to distinguish these fish from wild hens. The details of these fish and the results from their stripped eggs are shown in Table 4. Three of the hens were from the Uist broodstock, two from the Attadale broodstock and one from the Wild broodstock. All the brood hens produced high quality eggs on a par with what would be produced from spawned wild hens. Eggs with such a high percentage reaching the eyed stage go on to produce high quality fish throughout the freshwater part of the life-cycle. Five of the hens produced fry that were combined with other fry for feeding but the eggs and subsequent fry from the 22lb hen (ICO 062) were kept separate until they were well established on feed with a mean weight of 3 gms. At this stage the tank held a total of 11,200 fry, an exceptional number from 11,450 stripped eggs.
These brood hens had been stocked out as fry but an equally excellent result was obtained from brood hens that had been stocked out as S1 smolts in an earlier period of the stocking programme. In 2003, a total of 6,000 S1 smolts were released into the river. These fish were tagged with coded wire tags and adipose fin-clipped before release. 21 of them appeared back in the rod catch as grilse in 2004 and in 2005, two returned as 2SW hens. These hens were about 9lbs in weight and were used as broodstock before being killed for tag removal. The two hens produced a combined total of 16,945 eyed eggs at 99.6% survival to the eyed stage. The hatch rate was 99.9% and the fry were subsequently reared on with minimal losses.
In terms of conservation, it is clear that stocking can play an important role in saving and maintaining salmon populations that are in danger of extinction. In the case of the Carron, assuming a rod capture rate of no more than 15%, since for the river as a whole angling pressure is light, the significant numbers of stocked fish in the rod catch translate into a very meaningful spawning stock putting many extra eggs into the river. However, it is important that action is taken before stock levels so far that it may be difficult for intervention to be successful.
Table 1: Salmon Batches stocked out in 2015
Broodstock Batch number Hens Size range (gms) Fry number Stocking dates Stocking location
Wild 1 13 G 2.3 to 3.17 28,000 11/09 and 15/09 Kelso beat
Wild 2 6 S 2.7 to 2.9 22,000 12,17,27/08 Kelso beat
21/09
Wild 3 15 G 1.7 to 3.0 33,500 31/08, 08,28/09 Glencarron beat
01/10
Wild 4 1 S 2.12 6,000 23/08 River Taodail
Wild 5,6 3 G, 1 S 4.1 9,000 22/09 Fionn Abhainn
Attadale 7 36 2.25 to 3.5 31,600 12,15,18,22,26/08 Arineckaig beat
23/09
Attadale 8 10 4.8 5,100 13/10 and 14/10 Loch Sgamhain
Attadale 9 8 2.6 to 3.5 18,550 20,29/08, 25/09 Attadale beat
04,05/10
Uist 34 1.75 to 2.6 94,000 30/06, 02,07,08,12 Arineckaig beat
14,16,17,20,31/07 Kelso beat
03/08
Table 2: Adult Returns in the Rod Catch per Batch
Batch number Adults in rod catch Stocked beat Stocked fry Stocked fry per adult return in rod catch
1 14 Kelso 28,000 2,000
2 11 Kelso 22,000 2,000
3 4 Glencarron 33,500 8,375
4 1 River Taodail 6,000 6,000
5,6 5 Fionn Abhainn 9,000 1,800
7 10 Arineckaig 31,600 3,160
8 0 Loch Sgamhain 5,100 -
9 5 Attadale 18,550 3,710
Uist 34 Arineckaig, Kelso 94,000 2,765
Table 3: Adult returns in the Rod Catch per Broodstock
Broodstock Stocked fry Stocked fry per adult return in rod catch
Wild 98,500 2,814
Attadale 55,250 3,683
Uist 94,000 2,765
Table 4: Stocked Hens retained as Broodstock
Broodstock Clip number Hen weight Stripped eggs Eyed eggs Percentage eyed
Uist ICO 066 8 lbs 4,383 4,350 99.2%
Uist ICO 078 9 lbs 7,394 7,360 99.5%
Uist ICO 085 10 lbs 6,100 6,000 98.4%
Attadale ICO 062 22 lbs 11,450 11,400 99.6%
Attadale ICO 067 10 lbs 5,816 5,800 99.7%
Wild ICO 051 6 lbs 3,528 3,500 99.2%
These 6 salmon hens stripped in November/December 2019 produced 38,671 stripped eggs, 38,410 eyed eggs at an overall survival rate of 99.3% to the eyed stage.