Diel and seasonal variations in brown trout (Salmo trutta) feeding patterns and relationship with invertebrate drift under natural and hydropeaking conditions in a mountain stream |
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| Institution: | 1. Cabinet d’études IMAG’O (Ichtyologie, Milieux aquatiques et Gestion des Eaux), 26 rue Dels-Pibouls, Labastide Falgarde, 31120 Lacroix Falgarde, France;2. Centre dˈEcologie des Systèmes aquatiques continentaux, UMR C5576, 118 route de Narbonne, 31062 Toulouse cedex 4, France;3. Laboratoire Environnement aquatique, ENSAT, 1 avenue de l’Agrobiopôle, BP 107 Auzeville-Tolosane, 31326 Castanet-Tolosan, France;4. Cabinet d’études EMA (Etudes milieux aquatiques), La Croix Cadet, 63460 Joserand, France;5. UPRES Biodiversité, 1 place Victor Hugo, 13331 Marseille cedex 3, France;1. Michigan Department of Natural Resources, Marquette Fisheries Research Station, 484 Cherry Creek Rd., Marquette, MI 49855, United States;2. University of Wisconsin – Milwaukee, School of Freshwater Sciences, 600 East Greenfield Ave., Milwaukee, WI 53204, United States;3. University of Washington, School of Aquatic and Fishery Science, Box 355020, Seattle, WA 98195-5020, United States;4. U.S. Fish & Wildlife Service, Ashland Fish & Wildlife Conservation Office, 2800 Lake Shore Drive East, Ashland, WI 54806, United States;1. Quantitative Fisheries Center, Department of Fisheries and Wildlife, Michigan State University, East Lansing, MI 48824, United States;2. Biological Services – Great Lakes Section, Great Lakes Indian Fish and Wildlife Commission, Odanah, WI 54861, United States;1. Environment and Climate Change Canada, National Wildlife Research Centre, Ottawa, ON, K1A 0H3, Canada;2. Geography and Environmental Studies, Carleton University, Ottawa, ON, K1S 5B6, Canada;3. Centre D’études Nordiques (CEN), Département de Sciences Biologiques, Université de Montréal, Montréal, QC, H2V 2S9, Canada;4. Alberta Environment and Parks, Government of Alberta, Calgary, AB, T2E 7J2, Canada;5. Institute for Interdisciplinary Mountain Research, Austrian Academy of Sciences, A-6020 Innsbruck, Austria;6. Environment and Climate Change Canada, Canada Centre for Inland Waters, Burlington, ON, L7S 1A1, Canada |
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| Abstract: | The abundance of prey ingested by brown trout (Salmo trutta) were compared with the abundance of drifting invertebrates in a mountain stream managed by hydropeaking, upstream (site A) and downstream (site B) of a hydroelectric power plant. During power generation, flow and temperature were the two main environmental factors modified. The natural flow in the river below the outlet may be enhanced several times a day from 1 to 11 m3.s–1 in summer and winter, and from 5 to 15 m3.s–1 during spring spates. During hydropeaking, the water was cooled by an average of 6 °C in summer and warmed by an average of 2 °C in winter. Overall drift density was higher at site B than at site A. There was a clear diel pattern of drift at site A, with low drift density during the day and high drift density at night, whereas no clear diel pattern was observed at site B. Below the plant, at site B, drift pattern appeared to be influenced most by hydropeaking. The flushing action of peaking flows caused a catastrophic drift, which was highest in autumn when the difference between natural and peak flows was greatest. Juvenile trout were adversely affected by hydropeaking conditions and subsequently their density and biomass were reduced by 30% from site A to site B, whereas no significant difference was noticed for adults. Gut contents analysis showed that brown trout chiefly fed on the most available prey items at both sites. Fish did not seem to feed in response to diel drift patterns above the plant, whereas they chiefly used drift pulses generated by peaking flows below the outlet. Under natural conditions, fullness indices increased from autumn to summer, suggesting they may be related to prey availability and changes in water temperature. In the regulated section, fullness indices were the lowest in spring, i.e. the season when peak flows added to snowmelt floods, suggesting a prominent role of high current velocities through habitat suitability, position maintenance, and ability to capture preys. Although hydropeaking is known to disturb trout population dynamics in this and other rivers, this kind of river regulation (natural discharge except during periods of power generation, and intermittent hydropeaking from a separate reservoir) allowed the maintain of brown trout below the outlet, probably because the river returned to natural conditions when the plant was inoperative, and because daily artificial fluctuations in flow and temperature remained within the limits of natural seasonal variations. |
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