The evidence regarding flood damage

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:

  1. Salmon/trout fry densities/numbers in the year after the spate were good
  2. Salmon/trout parr densities/numbers were depressed in the year following the spate event
  3. Smolt production was reduced in year following spate
  4. 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.

River Spey 2014

River Spey 2014 salmon fry monitoring before (late July) and after (early Sept). The fry counts were significantly lower although there were salmon fry present at all sites. There was a hint of a greater impact further downstream as counts at some of the upper sites had increased.

 

River Fiddich 2014

River Fiddich 2014 salmon fry monitoring. Here the impact was greater although the before spate counts were amongst the highest we have recorded. Note that there were still moderate to good fry counts at all sites.

What about the parr? I mentioned that the Spey post spate surveys found the parr counts had increased; although not so in the Fiddich.

Spey before and after parr counts

Spey before and after parr counts. This table shows the total fry and parr counts during all eleven pre and post spate monitoring surveys. Fry counts had decreased whilst parr had increased? The result of displacement perhaps?

 

River Fiddich parr counts

River Fiddich before and after counts at four repeat survey sites. Both fry and parr had decreased in the Fiddich; did some subsequently establish themselves in the Spey mainstem?

Fiddich monitoring site TSF19 which was located just above Dufftown. Before the spate thsi site supported an exceptional number of fish, we captured more that 500 during the three minute survey. Numbers were considerably reduced after the spate but not wiped out.

Fiddich monitoring site TSF19 which was located just above Dufftown. Before the spate this site supported an exceptional number of fish; we captured more than 500 during the three minute survey. Numbers were considerably reduced after the spate but there was no not wipe out.

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.

Lovely spawning gravels in the lower Fiddich in Nov 2014

Lovely spawning gravels in the lower Fiddich in Nov 2014. Note the large redd(s) and fish.

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.

Salmon fry counts from a sequence of timed survey sites in the Fiddich 2014 and 2015

Salmon fry counts from a sequence of timed survey sites in the Fiddich 2014 and 2015…..

Same graph showing trout fry

….and the same graph showing trout fry. There were, on average higher salmon and trout fry counts in 2015. Note that in the salmon graph fry were absent in site FWF2/3, whereas they had been present the previous year – a reflection on the low spawning stock perhaps, or maybe just river levels at spawning time?

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.

River Fiddich salmon

River Fiddich salmon counts per minute 2014/15

River Fiddich trout

River Fiddich trout counts per minute 2014/15. Compare these two graphs with the fry graphs above. At all but one site the salmon and trout parr counts were lower in 2015 than the previous year. Of course this is only a snapshot over two years but we found the same in other areas monitored over the last two years.

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.

Dulnain and Fiddich

This table shows how the proportions of presmolt size parr has varied at routine monitoring sites in the Dulnain and Fiddich. The Dulnain site is at Dalnahaitnach. In 2013 there was a great parr density but they were largely small 1+ with only a few larger presmolt size. In 2015 the density was a lot lower but the number of presmolt size fish was the highest in the sequence (site not surveyed in 2014). In the Fiddich sites (results from two sites combined) the parr counts in 2015 were only 27% of that recorded in 2014 but the number of larger parr was 79% of the previous years. Both good examples of how the juvenile salmon population rapidly adjusts following a disturbance to the equilibrium.

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?

Salmon from three different age classes dead on the riverbank after the Aug 2014 spate.

Salmon from three different age classes dead on the riverbank after the Aug 2014 spate. The larger presmolt parr are irreplaceable at this stage.

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!

 

There are 3 comments for this article
  1. Jon G at 9:55 pm

    Interesting read. While it does seem that a one in 50? year flood event may only impact on smolt production for one year following…what happens if these events become one in 2 or 3 years due to climate change? Natures normal mechanisms to compensate would surely not work.

    • Brian Shaw Author at 9:41 am

      Hi Jon, The Spey 2014 spate was a one in ten year event, a relatively frequent occurance, much lower in magnitude than the recent spates on the east coast. My main point, that the population has a innate capcity to recover, would apply whatever the magnitude of the spate, although granted the degree of damage in this case may be greater. The results posted were observations rather than proof of anything, but as a fishery biologist the response observed is what I would expect. But you are right, if events turn out as you describe then we will be in trouble; restoring catchment resiliance to extreme flows should, in my opinion, be our highest priority.

      My secondary, and equally important, point is what I can only describe as a general air of malaise amongst salmon angling. Tenants are dropping out, even in prime time and are not being replaced. Poor catches, conservation limits, dry summers, extreme spates… its all happening. What the sector needs is promotion but where is that going to come from? Existing fishery organisations are looking inwards, Wild Fishery Reform is consuming a lot of energy and that process that will take 3 to 5 years to settle down. The Gov seem to think that the National Wild Fisheries Strategy will transform things but that will take more that a redecoration of the office. The new organisations would need radical new powers for that to happen – will the process deliver?. Promotion of angling is one of the strategies key objectives – can it wait that long?
      Brian

      • Charlie Herd at 11:20 am

        Brian,

        Whilst I share your concern about the gloom in certain fishing circles, it is worth noting that are still many rivers in Scotland where tenants are not dropping out and where catches have not deviated significantly from long term averages; perhaps we should be making greater efforts to find out why these river have not been affected by this ‘malaise’. Even some of these ‘self centred’ articles you mention may not be entirely without merit !

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