Rapid size-specific changes in the drift of Baetis bicaudatus (Ephemeroptera) caused by alterations in fish odour concentration

Chemical cues from fish can alter the behaviour of stream invertebrates in experimental tanks but their effect in natural streams has received little attention. By adding brook trout (Salvelinus fontinalis) odour to a trout s tream in the Rocky Mountains of Colorado, USA, we tested whether changes in the concentration of chemical cues from visually feeding predatory fish wo uld alter the drift of mayfly nymphs (Ephemeroptera). Stream water was pipe d from streamside tanks with (odour) and without (control) three brook trou t to two locations in the stream 3.5 m upstream of drift nets at six replic ate sites. Five-minute drift samples were collected downstream from odour a nd control pipes before, during and after the release of water from the tan ks into the stream during both the day and night. Almost all drift occurred at night and consisted predominantly of Baetis bicaudatus nymphs. The odou r manipulation had no measurable effect on Baetis drift during the day but statistical. power was low. During the night, however, the drift of large ( > 0.65 mm head capsule width, HCW) Baetis nymphs decreased significantly du ring the odour addition compared to control drift. In contrast, the drift o f small nymphs (less than or equal to 0.65 mm HCW) increased both during an d after the odour addition in comparison to control drift. Since the stream contains brook trout (0.04 0.18 m(-2)), and water from the stream (presuma bly containing fish odour) altered the behaviour of fishless-stream Baetis nymphs in another experiment, we conclude that the changes in Baetis drift density were a response to an increase in the concentration of fish odour i n the stream. Furthermore, we were able to detect the effect within 5 min. of odour addition, indicating that mayfly behavioural response to trout odo ur was rapid. These results suggest that mayflies can distinguish different concentrations of trout odour in natural streams and that the response is size-specific. according to the relative risk of predation of large and sma ll Baetis.