Castle Grant invertebrates……….staring into the caddis abyss

Continuing my upstream look at the invertebrate population in the Spey a sample was collected  from outside the hut at Castle Grant beat 3 last week. Once again it was obvious looking into the tray that this sample was quite different from those collected downstream. We could see quite a lot of baetid mayflies, caddis etc although there was a huge amount of detritus that needed sifting through.

Castle Grant Beat 3 invertebrate site

Castle Grant Beat 3 invertebrate site. Water depths were less than 250mm, flow was generally glide with a cobble and finer substrate.

There was no mistaking that the sample was taken near Grantown as a high proportion of the detritus in the sample consisted of flakes of Scots Pine bark. Simon Crozier the beat ghillie mentioned that previous samples had found a lot of caddis and he wasn’t joking. Once most of the detritus was removed just about all that could be seen were caddis larvae, mainly small cased caddis but also many caseless.

Castle Grant 3 invertebrates sorted. Clockwise from top left: Mayflies; diptera, snails, shrimps worms etc; stoneflies, and bottom left caddis

Castle Grant 3 invertebrates sorted. Clockwise from top left: Mayflies; diptera, snails, shrimps worms etc; stoneflies, and bottom left caddis.


Castle Grant 3 invertebrate population structure

Castle Grant 3 invertebrate population structure

In total 1468 individual invertebrates were identified and counted, the most of any sample collected so far. 45% of the sample comprised caddis. With my developing identification skills there appeared to be 6 types of cased caddis and 3 caseless. The most common cased caddis genus was Lepistomatidae which are commonly known as Little Brown sedges, followed by the Grannom. There were a lot less mayflies in this sample than further downstream samples with baetids the most common. Only 3 of the mayfly nymphs looked like March Browns although I know from personal experience that there is a normally a fantastic hatch in this part of the river. March brown nymphs are normally found in faster flowing water so maybe more would have been found in the riffles.

There were 55 shrimps (the orange coloured ones in the top right dish in the photo above) and 62 snails and 5 freshwater limpets. That was by far the most shrimps found in any of the recent samples but the instream habitat is quite different in this part of the river than found in the Brae Water for example. In the Castle Grant area the river is wide and stable with extensive moss growth on the rocks.

From the limited number of samples collected from the mainstem there doesn’t appear to be any major problem with the invertebrate population in the Spey in fact it looks very healthy and abundant, although some direct comparisons with previous samples would be informative. The samples collected so far have varied but then they were collected from sites 30 miles apart in the mainstem and from habitat typical of each area.

It was interesting to read the article in the most recent Trout and salmon about the trout fishing in the upper river Spey and the fantastic olive hatch. I will take my last sample for the time being from the mainstem in that part of the river for comparison. Discussing these results with Simon Crozier he was able to confirm that there are tremendous hatches of caddis on his beat in the summer. Simon has a theory that the rise of the trout in the spring depends on the severity of the previous winter; with better rises following hard winters. As the winter of 2012/13 shows little sign of loosening its grip there should be a good rise to the fly when spring finally arrives this year.

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