I haven’t read the latest monthly angling/sporting magazines yet but from what I have heard from others it is all doom and gloom. Apocalyptic tales of flood damage everywhere combined with self-centred, critical and depressing articles foretelling multigenerational recovery times; is it any wonder there is a general malaise amongst salmon anglers?
What actual evidence regarding the impacts of damaging spates on salmon stocks have we found? Our work over the last two years in this field has resulted in four main findings:
- Salmon/trout fry densities/numbers in the year after the spate were good
- Salmon/trout parr densities/numbers were depressed in the year following the spate event
- Smolt production was reduced in year following spate
- Reduced competition in year after spate resulted in enhanced growth of remaining fish, with increased percentages of parr large enough to smolt in year 2 following a damaging spate, mitigating to a high degree the loss in overall parr stocks.
We started looking at this issue in detail in August 2014 in the aftermath of the ex-tropical storm Bertha which brought, what now appears to have been a moderate level of, havoc across the north. Our analysis showed there was an impact on subsequent rod catches following a damaging spate, albeit a relatively modest impact, but it was statistically significant. Regarding the juvenile stock pre and post spate monitoring showed that fry counts had declined significantly in the Spey mainstem and in the Fiddich, although interestingly, parr counts in the Spey mainstem had increased although not significantly.
What about the parr? I mentioned that the Spey post spate surveys found the parr counts had increased; although not so in the Fiddich.
That was a quick summary of our findings in 2014, in the immediate aftermath of the spate. What could we expect to find in 2015?
One very visible impact of the big spate was the widespread appearance of high quality spawning gravels. The Spey is not generally short of spawning gravels although the distribution of this vital resource is definitely patchy.
So despite the 2014 rod catch being the lowest on record (indicative of a low spawning stock?) the abundance of spawning gravels provided hope that the fry numbers in 2015 would be okay. And that is what we found.
Our three year electrofishing strategy means we only visit a limited number of sites annually but the general pattern of good fry counts/densities was apparent at most sites. Not only were there good numbers of fry at some sites they were also large. Granted it was late August before we completed all our mainstem surveys in 2015 but we have never before had fry averaging over 70mm in a mainstem site. At that same site, which was located just upstream of Boat o’Brig, the largest fry were 86mm – a size large enough to suggest that a proportion would smolt as one year olds in 2016.
We had expected to find good fry numbers but the expectation was not the same for parr. The loss of fry in the Bertha spate was sure to have an impact on parr counts in 2015, and that was generally what we found.
One thing I have learnt this year is not to focus too much on parr densities, there is quite a difference between parr densities and smolt production! As we noted with fry, larger parr; the ones destined to smolt in 2016, formed a higher proportion of the total. So even though parr densities were reduced the compensatory growth amongst the remaining fish mean there is not a direct relationship with smolt production.
We had thought that smolt production in 2015 would be down, it is impossible to make up for the loss of large numbers of parr which had been left stranded on the riverbank or even washed out to sea, many of which would have gone on to smolt. The results of the smolt trapping in the River Avon showed that the 2015 smolt output, from what is our largest tributary, was 54% of that recorded in the previous year. On the other hand the smolt traps in the upper Fiddich recorded almost identical combined counts of trout and salmon smolts in 2014 and 2015. This may also be evidence of greater impacts of spates in the lower reaches where stream power is greater. The Avon was battered by the Bertha spate so we had anticipated an impact on smolt production but smolts were still produced and other parts of the catchment were not so badly affected. Perhaps some of the additional parr counted in the mainstem during the post spate monitoring may have gone on to smolt in 2015?
So to summarise these large spates do have an impact on catches but the effect seems to be of limited duration i.e. primarily on parr stocks and smolt outputs in the year following the spate. The flexible growth strategy adopted by juvenile salmonids allows the surviving fish; of all year classes, to grow quicker, and probably with higher survival rates, mitigating to a large extent the loss of smolt production by the second year following a spate event.
Our findings relate to a single large magnitude spate event which occurred in the summer. The impacts of spates which occur at other times of the year or multiple spate events may be different. However, other monitoring opportunities will soon provide more insights e.g. it will be interesting to see what the smolt outputs from the Girnock and Baddoch in the Dee catchment will be in 2016. I know from conversations with Marine Scotland staff that the flows in these tributaries were extreme but the traps have been restored into operational condition. The catch reports from the first week of the new season on the Dee, which was one of the worst affected by the recent winter spates, have been encouraging and reasonable numbers of kelts have been reported, to the surprise of many. The salmon is indeed a resilient fish and clearly with great powers of recovery.
Final thought; variations in smolt output are only one component driving adult returns – marine survival is of at least equal importance. Since 2000 marine survival at Scotland’s monitored river, the North Esk, has varied four fold on a smolt year class basis!