Summary

Data from the Environment Agency resistivity counter at Carreghofa on the Afon Tanat were analysed and compared to flows and electric fishing survey results. Redd counts and fish survey results were also analysed for the Afon Rhiew and Mochdre Brook. Results for the Afon Tanat show a general decline in salmon numbers.

During 2009 the Vaki counter was running at Ashford fish pass on the River Teme giving some good images of salmon passing through the counter. However, the count recorded from this fish counter is only a partial count as salmon can also get over the weir in high flows.

It had been hoped that the video cameras on Shrewsbury fish pass would have also provided some data, but they are still not fully operational (although some images of fish using the counter were caught in April 2010)

1 Introduction

Adult Atlantic salmon (Salmo salar L.) migrate from the sea and ascend to their natal rivers to spawn. Although the salmon may move up the river throughout the year they do not spawn until the first frosts of autumn. The cock fish generally get to the spawning grounds first where they will compete with other males for territories. Then the hens move in and excavate their redds (nests) in the gravels. When the redd is ready she will settle into it and start shedding her eggs. At this point the male moves in beside the female and then he fertilises the eggs as she ejects them. Straight after spawning is over the hen buries the eggs by shifting the stones from upstream back into the hollow. Most Atlantic salmon die after spawning with the males suffering the most fatalities due to the stresses involved, normally under ten percent of the adult salmon will survive.

Photograph 1. A salmon redd viewed from the river bank

The eggs hatch, and the young salmon (initially referred to as fry, and later as parr) remain in fresh water for up to three years; they then enter the smolt stage and migrate downstream to begin the marine phase of their lifecycle. Most will remain in the North Atlantic for between one and three years before returning to freshwaters again to spawn.

The Environment Agency in West Area, Midlands Region monitors salmon numbers and movements using fish counters, redd counting and juvenile salmon electric fishing surveys.

Three different types of fish counter have been installed to monitor salmon movements in West Area at three different locations (see map). At Carreghofa on the Afon Tanat there is a resistivity counter that measures changes in conductivity within the water column and can tell whether movement of fish is upstream or downstream. The weir at Carreghofa is impassable to salmon even at high flows so all fish moving upstream have to pass through the fish pass. At Ashford on the River Teme there is a Vaki counter. This records fish movement through the counter as silhouettes through infra red beams (see figure 1). A debris screen has been added to prevent blockage of the pass. The weir at Ashford is passable by salmon during high flows.

Figure 1 salmon moving through Vaki counter at Ashford on the River Teme

On the River Severn at Shrewsbury there is being installed a video camera system, which uses four cameras at the edge of the pass. Special motion detection software can be used to detect fish movement through the pass. No images of fish were recorded in 2009 as problems arose with lighting of the pass. The weir at Shrewsbury is also passable by salmon in high flows.

Redds were to be counted on the main Afon Tanat between LLangedwyn and Llangynong, the Afon Rhiew and the Mochdre Brook. This programme was seriously depleted in 2009 due to high flows.

The Afon Tanat is the main the catchment for salmon in West Area and so this has been studied in more detail. From its source in the Berwyns it flows East South East until it flows into the Afon Vyrnwy, a length of approximately 34km. The valley floor geology is dominated by soft mudstones. Although a mainly rural catchment there are a number of small communities spread along its length. Therefore the main impacts in this catchment are agricultural. These include sheep dip pollution, poaching by livestock and most recently an increase in the planting of maize on the valley floor.

2 Results

Afon Tanat

Fish counter

Figure 2 Upcounts from the resistivity fish counter at Carreghofa, Afon Tanat for 2009

A small number of male and female salmon (usually 20 of each) are removed from Carreghofa during the autumn run to transport to our hatchery at Clywedog. In 2009 the trap went in on the 30th October and was removed on the 9th November. Most of the fish were caught between the 4th and 9th November relatively easily with no reports of red vent syndrome. Upcounts through the year are shown in Figure 2 showing the main run of fish in October and November. Numbers recorded at the end of the year are lower than average. Figure 3 shows seven years worth of upcounts recorded at the Carreghofa counter. Figures continue to show a decline in salmon movements recorded through the counter. In 2009 there appeared to be a good run of salmon until the exceptionally high flows at the end of the year caused a drop in salmon movements recorded by the counter (see figure 4).

Figure 3 Salmon upcounts for Carreghofa, Afon Tanat for 2003 to 2009

Figure 4 Relationship of salmon upcounts and flow on the Afon Tanat for 2009

Figure 5 Graph of flows and salmon upcounts for the period 1/9/09 to 31/12/09 on the Afon Tanat

Salmon moved up through the counter in reasonable numbers through the early part of the year. However, when the main salmon run picked up in November river flows exceeded 2000Ml/d and remained at this level for the rest of the run period (see figure 5). As with previous years movement of salmon through the counter are generally when river flow is decreasing after a sharp rise.

Redd counting

It was not possible to redd count on the Afon Tanat during the winter of 2009 due to high flows. Redd counts over the previous six years have shown a progressive decrease.

Table 1 Redd counts on Afon Tanat

Electric fishing surveys

Figure 6 Bar chart of 0+ salmon densities caught on electric fishing surveys on the Afon Tanat

The results from electric fishing surveys over the last five years are shown graphically in Figure 6. The 0+ salmon caught on electric fishing surveys have shown a gradual decrease at all sites. The results from 2007 were generally poor as they were directly after the summer floods of that year and so could be discounted.

Afon Rhiew

Redd counting

Table 2 Redds counted on the Afon Rhiew

Table 2 shows that redds counted on the Afon Rhiew since 2002 have been variable and at lower densities than on the Afon Tanat. The main Afon Rhiew and the North Arm have shown a decrease in redds counted over this timescale.

Electric fishing surveys

Figure 7 Bar chart of electric fishing survey results on the Afon Rhiew

Electric fishing surveys on the Afon Rhiew showed an improvement at the majority of sites in 2009. Survey data from 2007 and 2008 were impacted by summer high flows and so not as comparable.

Mochdre Brook

Redd counting

Table 3 Redds counted on Mochdre Brook

A slight increase in numbers, although only one of these redds was above the weir that has been recently taken out.

River Teme

Fish counter

Figure 8 Graph of flows and salmon upcounts from 7/5/09 to 31/12/09 on the River Teme (flow from Tenbury and upcounts from Ashford)

Figure 9 Graph of flows and salmon upcounts from 1/10/09 to 31/12/09 on the River Teme (flow from Tenbury and upcounts from Ashford)

The results from the vaki counter at Ashford can be seen in Figures 8 and 9 in relation to flow taken from Tenbury gauging station. As with the Carreghofa counter there is a good correlation between flow and salmon upcounts. There were two small runs in the summer tied in with increases in flow during early June and mid July. The main run of fish began in early November when river flows began to rise. Just as an addition the largest fish recorded going through the counter was 114cm going up on the 13th November 2009.

Figure 10 Image of largest salmon measured going through Ashford counter in 2009

River Severn, Shrewsbury

No results from the fish counter in Shrewsbury for 2009 due to problems with installation.

3 Discussion

At present it is not possible to say what impact these high flows of autumn 2009 have had on the salmon population within the Severn catchment. Adult salmon will struggle against the high flows to get to the top of the catchment. When suitable gravel areas have been found for spawning these could easily have been washed out or eggs swept away while being fertilised. Also when river levels finally dropped some redds would have been left stranded out of the water leaving the eggs to die. Of course on the other hand high flows would make some obstructions easier to pass.

The results tend to show a general decrease in salmon within the Afon Tanat catchment. However, fish counter data has been compromised. Flows reaching over 2000Ml/d appear to cause a reduction in salmon counted. This figure has been noted in previous studies to be the point where salmon either can not make it through the pass easily, or the counter can not cope with the large amount of water coming through (salmon need to be within 30cm of strips on bottom to be counted).

Whether this perceived decrease is due to a decrease in the returning adult salmon or problems for the juveniles within the catchment is uncertain. There are so many variables to consider that it is impossible within the remit of this report to cover them. The results from the redd counts only look at the main river and has decreased in its magnitude. It also does not account for fish using the smaller tributaries. With the high flows recorded during 2007, 2008 and 2009 it is anticipated that these smaller tributaries were used increasingly.

A few observations from within the catchment that could impact juvenile numbers are that farming practices have been changing within the Tanat valley. More maize is being grown, an increase in predatory birds has been reported and a number of summer flood events that could have washed out juveniles.

HABSCORE surveys for the electric fishing survey sites would give a better indication of the utilisation of the habitats available. Past data from 2005 showed that Pedairfford and Llangynog had very good Habitat Utilisation Index (HUI) scores but with this river system being so dynamic more recent surveys are required.

Regarding the Afon Rhiew. Redd count data was not very robust in 2009. Only one survey was able to be conducted due to the high flows and redds could have easily been washed in by the high flows. This would have given a disproportionably low count and not very reliable. Juvenile fish numbers on the Afon Rhiew in 2009 were generally very encouraging. The main exception to this being Baxters Mill on the North Arm of the Rhiew. Tied in with this is the low number of redds counted on the North Arm over the last few years. One major issue appears to be a loss in spawning gravels on this section of the Rhiew. It is doubtful though that this can be the full explanation for the drop in 0+ salmon at Baxters Mill. Further work is required including the need to conduct HABSCORE surveys for each sampling site.

Mochdre Brook is still not being fully utilised by salmon on the evidence of redd counting. Electric fishing surveys on the Mochdre Brook would clarify this situation to see if the removal of the weir has improved slmon numbers upstream. Like other rivers in this area spawning gravels are limited in the upper reaches with bedrock being the dominant substrate.

The vaki counter at Ashford worked reasonably well in 2009. However, processing the data can take a considerable time when other fish species use the pass and numerous images that do not resemble fish are picked up. It is difficult to put the actual count into context with no previous years data to compare with, particularly as it is only a partial count (fish were getting over weir at high flows instead of using the pass). There was a similar relationship between flow and upcounts as seen at Carreghofa on the Afon Tanat with a drop off of counts once the river flow rose too high.

4 Conclusion

The high flows encountered towards the end of 2009 will have had an impact on the spawning success of salmon in the Severn catchment. Summer movement in salmon was assisted by reasonable river flows, but the main run of fish were then held back until mid October on the Tanat and the beginning of November on the Teme. The flows then rose to such a level that salmon movements through the fish passes might have been impeded, and the fish counters could not operate effectively.

Results from redd counting were very poor during 2009 and showed that this type of survey can not be fully relied upon as only partial counts were achieved on the Mochdre Brook and Afon Rhiew. No redd counting was carried out on the Afon Tanat in 2009. Mochdre Brook requires more monitoring and not just relying on redd counting. Electric fishing surveys and HABSCORE would give a better indication of the salmonid population within this brook.

Juvenile salmon numbers on electric fishing surveys were good on the Afon Rhiew at the majority of sites (except the North Arm) showing that 2008 had been a good year for salmon recruitment. Unfortunately this was not the same on the Afon Tanat where juvenile salmon numbers have continued to decrease. HABSCORE surveys must be prioritised for 2010 to quantify these changes on the Rhiew and the Tanat.

The main finding of this report is the continued perceived decline of salmon in the Afon Tanat catchment. Further investigation and monitoring is required to determine the causes of this decline and maybe look into competition with other species e.g. brown trout.