With the end of February fast approaching and the start of the salmonid fly fishing season almost upon us, I was thinking about species that signal the start of the season and the March Brown was on the list. The March Brown fly has been a pattern of choice for early season fly fishing since the 1600’s in England and likely used to imitate brown/tan mayflies, including the March Brown (Rhithrogena germanica), the False (or Late) March brown (Ecdyonurus venosus) and possibly even the Large Brook Dun (Ecdyonurus torrentis).
Rhithrogena germanica prefers clean oxygenated flowing waters and typically occurs in the riffle section of larger rivers or their tributaries. The nymphs (larvae) can be found clinging to rocks or plants where they feed on periphyton or fine particulate organic detritus. The march brown is a univoltine species with the nymphs overwintering and emerging in late March through early May, much earlier than other mayfly species. The nymph is brown in colour and approximately 13 – 16 mm in length. It is only really during the hatching process that trout would actively encounter and feed on this life stage. Synchronicity of the hatch is typical, with them hatching en masse around midday when it is warmest. The nymphs drift in the water and moult once they reach the surface in less than 30 seconds to minimise predation. Key elements of the fly patterns chosen to match the nymph are the size and colour, although the hatch can be quite frenetic occurring in less than half an hour so an exact match is not generally required to tempt a trout or two.
The subimago (or ‘dun’) rests on bank side vegetation for 3 to 5 days before moulting its skin again to become an imago (or ‘spinner’). As such, the March Brown has the longest subimago stage of any mayfly. The adult life stages have a body length of 11 – 16 mm and a wing length of 13 – 17 mm. Key elements of identification are a dark brown to blackish brown thorax, transparent wings with brown tints and veination, two tails and a dark spot on the femur. Once mated, the female imago flies upstream and lays eggs by dipping her abdomen onto the surface of the water. She does this either by landing onto the water for short periods or flying just above the water surface. Once the egg supply is finished the spent female spinner falls on to the surface. This egg laying process usually occurs at night so it would be unusual to need to match this life stage, although early risers may sometimes encounter the spinners on the water at dawn. This hatching and egg laying process en masse represents a spring bonanza for hungry trout.
The Late March Brown (Ecdyonurus venosus) is similar to Rhithrogena germanica in many ways, inhabiting similar riverine environments although they can also occur on the rocky shoreline of nutrient poor lakes. From a trout fishing perspective they are also similar in appearance, especially colour (imago: reddish brown to pitch brown) and size (larvae: 10 – 17 mm; imago fore wing length 10 – 16 mm). A key difference is in the species biology with the False March Brown having been found to be bivoltine in certain locations where conditions allow multiple generations per year. The first hatches of over-wintered larvae occur in May and June with later hatches occurring in July through October. Early summer eggs may hatch and emerge as flies within two months, whereas the autumn ones grow much more slowly and emerge the following spring/summer. Later or second generation larvae and flies tend to be smaller than those from earlier in the year.
Flies to buy:
- Larvae – size 12 or 14
- Spinner – size 12 or 14
Flies to tie:
Tight Lines
Greg
