Nested species subsets, gaps, and discrepancy |
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Authors: | Richard A Brualdi James G Sanderson |
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Institution: | (1) Department of Entomology, Comstock Hall, Cornell University, Ithaca, NY 14853, USA, US |
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Abstract: | 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 stream in the Rocky Mountains of Colorado, USA, we tested whether changes in the concentration of chemical
cues from visually feeding predatory fish would alter the drift of mayfly nymphs (Ephemeroptera). Stream water was piped from
stream-side tanks with (odour) and without (control) three brook trout to two locations in the stream 3.5 m upstream of drift
nets at six replicate sites. Five-minute drift samples were collected downstream from odour and control pipes before, during
and after the release of water from the tanks into the stream during both the day and night. Almost all drift occurred at
night and consisted predominantly of Baetis bicaudatus nymphs. The odour 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 during the odour addition compared to control drift. In contrast, the drift of small nymphs
(≤0.65 mm HCW) increased both during and 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 (presumably 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 in the stream. Furthermore, we were able to
detect the effect within 5 min. of odour addition, indicating that mayfly behavioural response to trout odour 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 small Baetis.
Received: 12 May 1998 / Accepted: 23 October 1998 |
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Keywords: | Chemical cues Brook trout Prey size Odour concentration Stream predator-prey interactions |
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