THER­MAL STRATIFICATION

Trout Fisherman (UK) - - Advice -

We need to be aware that ther­mal stratification can oc­cur in sum­mer. This hap­pens in ex­treme heat and calm con­di­tions when the wa­ter has lit­tle chance to be suf­fi­ciently mixed. Ther­mal stratification refers to a dif­fer­ence in wa­ter tem­per­a­tures at var­i­ous depths. As colder wa­ter is denser than warm wa­ter, a band of warmer wa­ter (epil­imnion) sits on top of deeper ly­ing colder wa­ter (hy­polimnion). These are sep­a­rated by the ther­mo­cline (di­a­gram 2). When ther­mal stratification is present, trout tend to be­come wide­spread through­out the epil­imnion zone, of­ten push­ing away from the mar­gins and into the tarn cen­tre where they feed on daph­nia and the like. Ob­vi­ously, this takes them well out of our cast­ing range when bank fish­ing. Re­mem­ber though that most hill tarns are ex­posed where wind and wave ac­tion churn up wa­ter to mix both warm sur­face cur­rents and un­der­ly­ing colder parts to­gether, re­sult­ing in ac­cept­able tem­per­a­tures for trout to func­tion, even in high sum­mer.

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