Tropical storm Bertha aftermath: juvenile stock survey and mainstem stocking monitoring

Trying to assess the impact of extreme spates on our juvenile stocks is a long held ambition and something I promised to do at the Spey Fshery Board AGM two years ago. Big Bertha provided the ideal oportunity with pre event electrofishing survey data available and the low water in September providing ideal conditions for repeat surveys.

The Spey up in amongst the trees on the 11th Aug 2014

The Spey up in the trees on the 11th Aug 2014

For the monitoring strategy we decided to replicate some of the timed surveys in the Spey mainstem and River Fiddich. Eleven timed sites were repeated on the Spey and four in the lower Fiddich. The results were interesting with a statistically significant reduction in the fry counts but an increase (although not significant) in the parr counts. The three Spey mainstem sites with the largest reductions in fry counts were all in areas where the riverbed was liable to become mobilised during the peak of the spate. There was less of a change in the pre and post spate counts in more stable parts of the river.

We found evidence of downstream movement of juvenile fish, something that was particularly evident in the Fiddich where the thermal uplift from the distillery cooling water has unique effects.

The salmon fry numbers overall in the Spey survey sites after the spate were close to the average for the whole Spey in 2014 summer surveys. In the Fiddich the decline was greater although from a higher base level. The study raised as many questons as it answered. For example how many fry would we normally expect to find in September compared to the usual monitoring time of July? I would probably expect to find similar numbers in September as the bulk of the overwinter mortality probably occurs in the autumn onwards.

The perceived wisdom is that although fry numbers are lower now than they would have been if there had been no damaging spate the survival of those remaining should be better due to less competition. We hope that this will be the case although we will have to wait until next years surveys to establish that. The 2015 smolt production will almost certainly be impacted by the big spate and the subsequent fish kill but no doubt nature has its ways of compensating.

Following the Bertha spate a decison was made to stock the 50,000 lower Spey origin hatchery fish back into the lower mainstem rather than into lower river tributaries. We were able to monitor the impacts of this stocking as the stocked fish were adipose fin clipped by the Spey ghillies. The fin clipping allowed us to successfully identify the stocked fish so that their contribution could be assessed.

The results from the stocking monitoring and the post spate monitoring can be read by clicking here.

There are 2 comments for this article
  1. Charlie Harman at 12:06 pm

    Brian, I’m struck by the fact that your survey seems to have focussed very much on the lower river, with six of the eleven mainstem sites being at Delfur or below.

    You observed that there was downstream movement of juveniles, so presumably these lower river locations will have been replenished by fish from upriver, masking the severity of the losses.

    Wouldn’t surveys of sites much further upstream, and also in the smaller tributaries, have told you more about the real impact of events like this? Was there any reason why you couldn’t look anywhere upstream of Grantown?

    On a separate point, I would have liked to attend your recent London meeting, but was unable to do so. Do you plan to post your presentations here in due course?

    Regards,

    Charlie.

    • Brian Shaw Author at 1:48 pm

      Hi Charlie,

      The main reports of fish mortality were from the lower river. I looked on the flooded riverbank at Blacksboat the day after the spate and found very few dead fish; four fry/parr only after a lot of searching with the assistance of a dog. There were reports of a fish kill in some of the tributaries but the problem was that we had no pre-event 2014 electrofishing data from many tributaries at that time. Our early season elctrofishing concentrated on the mainstem and the Fiddich catchment hence the report focussed on these areas.

      We subsequently surveyed many tributaries, including those where fish kills were reported. The findings from these surveys will be reported in the full 2014 electrofishing report due shortly. I can say that in some of the smaller tributaries surveyed; some of which experienced the full effects of the Bertha rainfall (4″+ in 12 hours), there were still fish present although often in lower densities than generally recorded. In the week after the spate we surveyed a new site on the middle/upper reaches of the Fiddich recording 151 salmon parr in a stretch 28.5m long and 6.46m wide (184.1m2); a density of over 80 parr/100m2. A very good density of parr with a similar fry density; hard to imagine the parr density could have been much higher before the spate. That particular stretch of the Fiddich is stable and moderate gradient so perhaps less affected. I would not want anyone to think that all the fish had been washed downstream from the upper reaches because that is not the case.

      Another justification for the concentration on the lower river was that upstream of the River Avon confluence the peak river levels in the Spey were not actually exceptionally high. At Castle Grant for example the river has been 2-3ft higher in the past whereas in the Aberlour area it was one of the biggest spates seen in recent years. With the lower gradient of the mainstem upstream of Grantown the susceptibility of the fish stock to losses in extreme spates are likely to be less. The findings of the survey suggest a greater impact in a downstream direction. Three of the four sites monitored upstream of the Fiddich confluence showed increases in fry counts.

      As with all these things we have to make the best use of the resources available and we were still at the time trying to complete our normal electrofishing programme.

      I will post the presentations soon, it has been a busy last two weeks!

      Thanks for the comment and interest as always

      Brian

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