One of the highlights of the UK trout fishing season, and coming soon, is the mayfly hatch, which generally evokes images of idyllic days on southern chalkstreams. Less well-known, is that we experience a similar hatch here in the Scottish Highlands, particularly in Sutherland & Caithness.

For the most part it is confined to our stillwaters, and it starts later, typically in early-mid June, extending to mid July, with a few mayfly coming off in some locations as late as August.

This blog accompanies a map of known Highland mayfly distribution, that we’ve published in the ‘Special Maps’ section.

What is a Mayfly?
For North American readers, it needs pointing out, that scientists here use the term ‘mayfly‘ as the common name for all species of the order Ephemeroptera (as you do), and in the UK we have different 51 species. UK anglers, however, refer to most of these as ‘olives’ and the term ‘mayfly’ is only used for the 2 largest species: Ephemera vulgata and Ephemera danica. Ephemera danica, also known as the ‘Green Drake’, is the most common and widespread in the Highlands.

The life cycle of Ephemeroptera species is unique: egg → nymph → sub-imago (dun) → imago (spinner). While the 2 adult stages (which together can span just 24- 48 hours) are of most interest to anglers, it’s the larval/nymph stage (typically 2 years), that is most influenced by environmental factors and is therefore a major dynamic in its distribution.

Distribution
Mayflies need clean, unpolluted, well oxygenated water, ideally slightly alkaline, as provided in areas with limestone, chalk or marl outcrops. The nymphs burrow into mud and silt substrate on the loch’s bottom to feed on algae and organic matter, so do not thrive in extremely stony or rocky habitats.

It seems the combination of these factors is key to mayfly distribution. Two of the best known areas for mayfly hatches are the Assynt/Inchnadamph area and Caithness. The water sources in Assynt spring from outcropping Durness Limestone, and in Caithness the underlying sedimentary rocks include layers of limestones and marls (lime-rich mudstones). The most famous limestone lochs in the Highlands are those in Durness, but to my knowledge they do not experience a mayfly hatch, possibly due to an unsuitable habitat on the loch bottom.

Tactics
Visiting anglers looking to fish during the mayfly hatch should have flies for all life-cycle stages from nymph to spinner, but most importantly the emerging dun. Nymphs can be productive before or early in a hatch, and better quality trout are often selective.

During the hatch, it’s useful to have flies to imitate both emergers and fully emerged duns, in wets and dries to cover different wind/wave conditions.

It’s also worth having some mayfly spinner patterns. I have not seen the evening mating and spinner fall fished in the Highlands as it is in Ireland, but there must be locations where falls of spinners occur on a similar scale, and fishing the spent gnat would be successful.

Check out our Highland Mayfly Distribution map, where you can also submit your own sightings.

 

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Here's a short video of fishing a mayfly hatch on Loch Craggie: