What’s hatch­ing?

The ap­pear­ance of large dark olives mean tro­phy trout aren’t far be­hind

Trout Fisherman (UK) - - Contents -

OF all the up­wing species found on rivers, large dark olives ( Baetis­rho­dani ) are per­haps the com­mon­est and most pre­dictable when it comes to pin­ning down a hatch. Of­ten re­ferred to as a ‘foul weather f ly’ they’re hap­pi­est emerg­ing in mis­er­able con­di­tions too. Although March, April and early May see large dark olives (LDOs) at their most ac­tive, a sec­ond brood can be ex­pected in au­tumn with Septem­ber and Oc­to­ber con­sid­ered prime months. Life starts when fe­male LDOs de­posit their eggs, which de­velop into tiny nymphs weeks later. As they grow, these nymphs un­dergo sev­eral moult stages (in­stars). Ma­tur­ing nymphs spend their time amongst stones, or aquatic weed and fran­ti­cally dash about when dis­turbed. It’s with good rea­son then they’re known as ‘ag ile dar ters’. Come the fol­low­ing year, nymphs as­cend to the sur­face in open wa­ter. In­evitably they’re ex­tremely sus­cep­ti­ble to pre­da­tion from trout and grayling now. Once at the sur­face, nymphs lever them­selves free of their shuck be­fore pump­ing f luid around their bod­ies to un­fold crum­pled wings. Wait­ing for their wings to har­den (not dr y) so they can be­come air­borne, the duns (sub imago) per­ilously ride the sur­face for sev­eral min­utes. Again, trout are quick to take ad­van­tage. As duns (sub ima­gos) are in­ca­pable of mat­ing, they un­dergo a fi­nal trans­for­ma­tion into a sex­u­ally ma­ture adult, known as the ‘spin­ner’ (imago). A cu­ri­ous dark ma­hogany colour now, male and fe­male spin­ners don’t waste time in mat­ing. This lit­er­ally takes place in days as – un­able to feed or take on liq­uids – these adults t y pi­cally live for 24-36 hours. Fol­low­ing mat­ing, fe­male spin­ners de­posit their eggs along the mar­gins. Un­like may f lies or blue-winged olives

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