Craigellachie small, and not so small, fry

I provided some information about the Spey mainstem salmon fry monitoring in the last post along with details of the three sites completed at Craigellachie. I had a further look at the size distribution of the fry at the Craigellachie sites this morning. Remember all three sites were within a few hundred metres of each other.

The size distribution graphs for each site are shown below:

Salmon fry size distribution Craigellachie Hut site

Salmon fry size distribution Craigellachie Telford Bridge site

Salmon fry size distribution Craigellachie Upper Slabs site

The scales are the same for all three graphs so it should be clear that the fry in the Hut site were significantly smaller that at the other two sites. Here the average size was 54mm whilst it was 60mm and 59mm at the Bridge and Upper Slabs sites respectively. All three sites must have experienced similar water temperature regimes and the fry now occupying all three sites would have emerged at, you must assume, similar times.

So why the big difference in size? I think this is purely down to habitat suitability. The fry in the Hut site were occupying slower flowing glide where cover from cobbles and boulders was at a premium compared to the faster flowing water at the two other sites. The lack of cover means the fry are likely to expend more energy fending of neighbouring fry with whom they are in visual contact. They may also have to spend more time sheltering from the risk of predation; be that from larger fish or birds, all of which means that there is less time available for feeding. By comparison the fry in the other two sites exist in habitat where there was a much greater extent of cover and the higher water flow is likely to mean greater availability of food. Another factor may that the smaller, less dominant, fry are forced to occupy the less favourable habitat with the larger fish occupying the better territory.

There is also a hint of a bimodal size distribution in the Telford Bridge fry, although the human brain capacity must be factored in also, more on that below! Late in the summer a bimodal size distribution indicates that the fry may be following different strategies. Fry large enough to become one year old smolts grow much quicker and continue to grow during the winter. We used to see this clearly in the more southern climes of Ayrshire but I’m not sure if there will be many one year old smolts produced this far up the Spey. We will try and visit the site again in October to check the size distribution at the end of the summer growth period. If fry are 90mm by the end of the first summer they are generally considered more likely to become smolts the following year. An alternative explanation is that the larger fish are destined to become precocious parr, this we can also check later in the year.

And the human factor? Well looking at the Telford and Upper Slabs graphs the lack of fish at size 59mm is glaring. I must have something against 59 so that any fish between 58 and 59mm were subconsiously given the size 58mm! I have noticed the tendancy to avoid specific sizes to be common amongst those measuring fish, however, you would have thought that I would be more prone to exaggerating the size of my fish rather than underestimating.

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