Laggan invertebrates

Last week I collected a sample of invertebrates from the Laggan beat of the Spey below Carron Bridge. The habitat here was mixed with cobbles and large embedded boulders in finer substrate, generally a dark riverbed with a coating of algae. It was immediately obvious that there were a lot of caddis in comparison to samples from further downstream.

Invertebrates from Laggan sample sorted. Mayflies top left, worms diptera and snails top right, stoneflies bottom right and caddis (caseless and cased) bottom left.

Invertebrates from Laggan sample sorted. Mayflies top left, worms diptera and snails top right, stoneflies bottom right and caddis (caseless and cased) bottom left.

One of the mayfly nymphs had what loooked like luminous spots on the legs and thorax. I have asked several experts in the field for an explanation of these spots with a “mutation” offered in most cases. I saw a lot of these in Ayrshire and a few from the Spey. These nymphs are highly visible and would surely be at higher risk of predation? I would be interested to know if the spots are permanent on individual specimens or is it a transitionary phase?

Heptagnid nymphs with luminous spots

Flattened mayfly (Ecdyonurus) nymphs with luminous spots

I counted 758 invertebrates in the sample with a much larger diversity of species than recorded in the Brae Water sites.

Invertebrates recorded at Laggan site, categories into main groups

Invertebrates recorded at Laggan site, categorised into main groups

Mayflies were the most abundant followed by stoneflies and cased caddis. There were 7 different species of cased caddis. As they largely hatch in the summer months many were very small but the new microscope we have purchased makes identification a more realistic prospect. Included in the sample were 12 gammarus shrimps and 30 snails. Again this sample scored A1, the top score, using the inhouse scoring system. It would also score very highly in other index systems. The diversity present will ensure that there are hatches of flies over a long season.

It is probably worth pointing that to an entomologist (someone who studies insects) the term mayflies means something quite different to an angler, although with the increasing numbers of anglers interested in fly life in recent years, this is changing. To many anglers “mayfly” means the large upwinged fly species that hatches on southern chalkstreams and Irish loughs in May and June. They are also found in many Scottish lochs and a few rivers. Entomologists use the term “mayfly” for a whole range of upwinged fly species, of which there are about 50 found in the UK. In the Laggan sample most of the mayflies were of the Rhithrogena genus split between Marchs Browns and Olive uprights. A small number of baetids were present (Large Dark Olives and others) with a few Heptaginids (Yellow May Dun) and 15 of the Ecdyonurus genus, none of which were present in the Brae Water samples. There are 4 species of Ecdyonurus found in the UK the common names of which are Autumn Dun, Large Green Dun, Large Brook Dun and Late or False March Brown. Identifying these to species level is difficult . I would certainly expect to find Large Brook Duns and False March Browns in the Spey.

So another very healthy invertebrate sample from the Spey. I intend to collect a couple more from the middle and upper reaches for full analysis and from a few key tributaries.





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