Impacts of an introduced predatory fish on mayfly grazing in New Zealand streams

The replacement of native common river galaxias (Galaxias vulgaris Stokell) by introduced brown trout (Salmo trutta L.) in many streams of the Taieri River system, New Zealand, has altered the predation regime for Deleatidium mayfly nymphs. Brown trout present a higher predation risk to mayfly prey during the day than at night but common river galaxias present similar risks both day and night. To determine whether this alteration has changed mayfly grazing behavior, we investigated the diel gut Illness of Deleatidium nymphs in five galaxias streams and five trout streams. Gut fullness was measured by the fluorescence of algal pigments extracted from Deleatidium guts. Nymphs from trout streams generally had fuller guts at night, whereas nymphs from galaxias streams tended to have more in their guts during the day or similar amounts both day and night. The difference reflects the high predation risk presented by brown trout during the day, restricting mayfly access to the algae-rich upper surfaces of stones. These results indicate that the influence of predatory fish on mayfly grazing is detectable at a large spatial scale in the Taieri River system.