Spey mainstem analysis: parr size distribution

The Spey mainstem is too big for quantitiative electrofishing surveys in the normal manner but last year we re-introduced some of the quadrat surveys with defined areas of the riverbed electrofished semi-quantitatively i.e. single run survey protocol.

This blog post reports on the results from two of the sites, one in Aberlour and the other upstream at Blacksboat.

Site S7 in Aberlour

Site S7 in Aberlour. The area fished is 22m long by 5m. In 2015 ranunculus had made a reappearance, providing cover for fish but making them difficult to extract; as a result we missed quite a few fish, although not enough to significantly alter the results.


Site S9, just upstream of Blasksboat. The dimensions of this site were similar to S7.

Site S9, just upstream of Blacksboat. The dimensions of this site were similar to S7, although just under 20m long. This was a new site a short distance downstream of the site surveyed in 2014 but will be a more suitable site going forward.

The river at the Aberlour site is very stable and there were no significant physical changes since last year other than the reappearance of ranunculus. The surveys were completed within a day of each other, in early Oct in 2014 and 2015.

In 2014 we caught 151 salmon fry and 70 salmon parr at the site in Aberlour whilst in exactly the same area yesterday we caught 110 fry and 28 parr.  This is a sigificant reduction in fish densities, certainly for parr especially when considering that the 2014 survey was after the damaging August 2014 Hurricane Bertha spate.

An analysis of the size distribution of the fish shows some interesting differences.

Aberlour site, S7 1st OPct 2014.

Aberlour site, S7 1st Oct 2014. Of the 70 salmon parr captured 63% were over 90mm and therefore likely to smolt in 2015, the remainder would probably spend another year in the river before smolting in 2016 (if they survive that long).

Aberlours site, S7, 2nd Oct 2015

Aberlour site, S7 2nd Oct 2015. 28 salmon parr but all well above the 90mm smolting threshold.

In 2014 the mean size of the fry was 54mm, whilst on practically the same day in 2015 it was 63mm.  The mean size of the 1+ parr in 2014 was 91mm whilst in 2015 it was 114mm ( note that the scales have still to be read for the 2015 samples but aging by size distribution won’t be far out). So there was a much lower density of parr at that site in 2015 but they are all above the smolting size threshold.

The large size of the fry has been a feature of the 2015 surveys even though the summer temperatures were never high and the mean water temperature must have been lower than in 2014. A factor contributing to the bigger fry is probably the lower parr densities. High parr densities would suppress the fry population through predation, harrassment and displacement from the best feeding lies.

The findings at the Blacksboat sites were similar regarding the average sizes of each age class and parr densities, although fry densities were higher here in 2015.

Blacksboat 2014

Blacksboat site 1st Oct 2014. Of the 29 parr captured 72% were above the 90mm threshold. (Not sure why the colour coding for age classes didn’t work for this graph when exported from the database)

Blacksboat 2015

Blacksboat site 2nd Oct 2015; all the salmon parr were above the 90mm smolting threshold.

The  salmon fry at the Blacksboat site were smaller in 2014 than in 2015 (53mm in 2014 and 57mm in 2015), with the 1+ parr 89m in 2014 compared to 103mm in 2015.

All of this highlights the dynamic situation that occurs within any territorial species when population densities or environmental conditions change. Parr densities are down in 2015 but those present are larger with a higher proportion likely to smolt in 2016; partially offsetting any loss in production, and possibly improving subsequent survival.

This could result in a situation whereby in 2016 there are few 2+ parr but a large class of 1+ parr from which the 2017 smolt run will be derived. We intend to maintain these selected semi quantitative mainstem surveys on an annual basis providng the opportunity to monitor these changes in the parr population structure over a longer period.



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Salmon parr and freshwater pearl mussels

The Spey is a Special Area of Conservation for a number of different species including salmon and freshwater pearl mussels. The decline in the Spey salmon catch has been well documented but the pearl mussel population has also declined with a 50% reduction in population size recorded in the last decade. One feature of the pearl mussel lifecycle that is not well known is their reliance on salmon and/or trout parr for the first overwinter stage. In summer the adult mussels “spat” or release fertile larvae mussels into the water column. These larvae are short lived and they have to attach to the gills of a salmon or trout almost immediately. If the larvae are fortunate enough to pass through a fishes gills  and attach they become encysted on the gills and in the spring/early summer of the following year can be seen by the naked eye. In the summer of that year the surviving glochidia drop of the fishes gills and settle into the river substrate for the next stage of their life. See here for an explanation of the life cycle of freshwater pearl mussel.

It appears from research that the mussel larvae are not always found equally on salmon and trout, even where both species are present; in some rivers they are more common on trout parr even when salmon parr are more abundant.

Some work on encystment in the Spey was carried out a number of years ago but given the concern about the status of the population Elizabeth Clements, a Masters Student from the Scottish Centre for Ecology and the Natural Environment, Loch Lomond (mussel boffin!), visited the river to assess the situation. We were delighted to assist by electrofishing at a few sites along the river from Grantown downstream.  I joined the team this morning at Aberlour to see the technique in action.

The electrofishing site in Aberlour normally supports a lot of parr so it was not difficult to catch the 50 or so required. Most of the parr at this time of year, and in this part of the river, are 1+ years old and generally below 100mm, most of the larger parr having emigrated as smolts.

Elizabeth and Steve checking the gills for glochidia at Aberlour

Elizabeth and Steve checking the gills for glochidia at Aberlour.

My eyesight is not what it used to be but I could make out the encysted glochidia on fish where they were present. I didn’t stay till the end nor take a note of the results but about 20% of the salmon parr appeared to  support glochidia, a few with 100+.

This salmon parr supported a high number of glochidia which can be seen as white spots on the gill.

This salmon parr supported a high number of glochidia which can be seen as white spots on the gill.

The assessment technique qwas not high tech! The key component was a darnign needle - not too sharp but pointy enough to lift each gill arch in turn.

The assessment technique was not high tech! The key component was a darning needle – not too sharp but pointy enough to lift each gill arch in turn.

It was very interesting to see the survey technique in action and good to see that there appeared to be glochidia on the gills of many fish. Scaled up across the whole river the number of infected fish must be large. It is considered that the role of carrying glochidia overwinter doesn’t harm the parr, indeed they were all in superb condition today.

No trout were caught at the Aberlour site but further upstream some were but they supported no glochidia. Hitching a ride on the gills of salmonids also provides a mechanism by which the pearl mussels can overcome that perennial problem for all river dwelling life – how do you counteract the continual downstream direction of travel? Some of the infected parr will migrate upstream before shedding the glochidia, thereby providing a mechanism for the species to at least maintain its range in the river.


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