Effect of springtime water temperature on the time of emergence and size of Pteronarcys californica in the Henry's Fork catchment, Idaho, USA

1. The emergence time of Pteronarcys californica in streams in the Henry's Fork catchment, Idaho, U.S.A. was negatively correlated with mean April wat er temperature. Emergence was in mid- to late May at sites influenced by gr oundwater, where April water temperature averaged 7.9 degrees C. Adults eme rged in mid-June in streams receiving run-off from snowmelt (mean April wat er temperature 5.4 degrees C). Intermediate emergence times were observed i n a regulated section of river where water temperature was influenced, on o ne bank, by dam release (mean April water temperature 4.5 degrees C) and, o n the other, by a spring-fed tributary stream (mean April water temperature 6.3 degrees C). 2. During each of the three study years, emergence was earlier on the bank of the regulated section that was warmer during April and May. The mean bod y length of P. californica exuviae, collected from the warm side of the riv er, averaged 1.2 mm longer than those collected from the cold side. 3. We tested the effect on emergence of altering spring time water temperat ure by translocating P. californica in cages from one location to another d uring April. Individuals moved to sites with higher April water temperature emerged earlier than individuals that remained at the site from which they were collected.