INTERACTIONS BETWEEN FISH, GRAZING INVERTEBRATES AND ALGAE IN A NEW-ZEALAND STREAM - A TROPHIC CASCADE MEDIATED BY FISH INDUCED CHANGES TO GRAZER BEHAVIOR

Experiments in laboratory stream channels compared the behaviour of De leatidium mayfly nymphs in the absence of fish with that in the presen ce of either native common river galaxias (Galaxias vulgaris Stokell) or introduced brown trout (Salmo trutta L.). Galaxias present similar predation risks to prey during day and night but are more active at ni ght. Whereas, trout present a higher predation risk during the day. De leatidium maintained a fixed nocturnal drift periodicity that is chara cteristic of streams containing visually feeding fish regardless of th e nature of the predation regime presented in the laboratory. However, the number on the substratum surface, and therefore able to graze alg ae, was lower when fish were present than when they were absent. The n umber was lower during the day in the presence of trout, when they pre sent the highest predation risk, and lower during the night compared t o the day in trials with galaxias when galaxias activity disturbs Dele atidium from the substratum. Increases in the probability of Deleatidi um leaving a patch, reductions in the proportion of mayflies on high q uality patches and reductions in the distance travelled from refuge al so reflected variations in the predation regime. Similar differences i n positioning were observed under the same predation regimes in in sit u channels in the Shag River and these were associated with difference s in algal biomass. Algal ash-free dry mass (AFDM) and chlorophyll a ( chl a) were higher on the tops of cobbles when fish were present. Fish also affected the biomass and the distribution of algae on cobbles as AFDM and chl a were higher on the sides of cobbles from channels with trout compared to those with galaxias. Changes in grazing behaviour, caused by predator avoidance, are likely to have been responsible for differences in algal biomass because no significant differences were d etected between treatments in the biomass of Deleatidium or of total i nvertebrates.