THE EFFECTS OF FLOW REDUCTION RATES ON FISH STRANDING IN BRITISH COLUMBIA, CANADA ROBYN L. IRVINE, a * TREVOR OUSSOREN, ay JAMES S. BAXTER b and DANA C. SCHMIDT a a Golder Associates Ltd., 201 Columbia Avenue, Castlegar, BC, V1N 1A8 Canada b Fish and Wildlife Compensation Program, 103-333 Victoria Street, Nelson, BC, V1L 4K3 Canada ABSTRACT Juvenile fish can strand in pools or in interstitial spaces when the water elevation drops in regulated rivers due to flow reductions. Three years of summer and winter experiments on the Columbia and Kootenay Rivers (Canada) assessed the effect of the rate of change in water level (ramping rate) on the probability of pool and interstitial stranding for juvenile (<100 mm) fish. The factors of wetted history of the site, time of day, natural fish density and the occurrence of a conditioning reduction prior to the operational reduction were also examined for their effect on stranding. Experimental net pens were constructed to test these factors in situ in the varial zones of the two rivers. Linear models with plausible additive combinations of the potential explanatory factors and a null model were fitted to the logistically transformed data and ranked using the second-order Akaike Information Criterion (AIC c ). The null model was the top ranked model for the interstitial stranding analyses, highlighting that none of the factors tested were significant variables in predicting the probability of stranding. Natural fish density, wetted history of the site, ramping rate and the presence of a conditioning reduction were variables included in the top three ranking models for the pool stranding analyses. Probability of pool stranding in summer was reduced by the occurrence of a conditioning reduction prior to the operational reduction. Higher natural fish density, longer periods of wetted history and higher ramping rates all led to higher probabilities of pool stranding. Copyright # 2008 John Wiley & Sons, Ltd. key words: hydro-peaking; fish stranding; hydro-electric dam; Columbia River; Kootenay River; AIC c ; conditioning reduction Received 2 November 2007; Revised 10 April 2008; Accepted 15 April 2008 INTRODUCTION Although rivers and streams are naturally highly heterogeneous environments, the biota below dams are often poorly adapted to the short-term, recurring disturbances that can result from hydro-electric dam operation (Berland et al., 2004). The stranding of juvenile fish due to rapid reductions in river height is one of several ecological problems associated with dam operations (Cushman, 1985; Bragg et al., 2005). Fish stranding can occur in either the pools that remain when the water level drops or in the interstices on cobble or gravel banks or bars (Bradford, 1997). Factors shown to influence the rate of stranding include ramping rate (the rate of stage change in the river) (Bradford et al., 1995), time of day (Bradford et al., 1995), water temperature and season (Saltveit et al., 2001). Numerous previous studies have explored the impact of these various factors on the stranding of juvenile fish, but have generally only addressed the effects on salmonid species (Woodin, 1984; Hunter, 1992; Bradford et al., 1995; Bradford, 1997; Harby et al., 1999; Halleraker et al., 1999; Saltveit et al., 2001). Exceptions include studies on the Hanford Reach of the Columbia River and a study on the Mississippi River. Hoffarth et al. (1997) assessed stranding rates of all species in the Hanford reach, but primarily focussed on the response of Chinook salmon (Oncorhynchus tshawytscha). Experimental studies in the Mississippi River system addressed warm water species such as shovelnose sturgeon (Scaphirhynchus platorynchus), blue catfish (Ictalurus furcatus) and largemouth bass (Micropterus salmoides) (Adams et al., 1999). These authors found large variation in stranding rates among species but no significant effect of ramping rate. However, the Mississippi River study RIVER RESEARCH AND APPLICATIONS River. Res. Applic. 25: 405–415 (2009) Published online 12 June 2008 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/rra.1172 *Correspondence to: Robyn L. Irvine, Golder Associates Ltd., 201 Columbia Avenue, Castlegar, BC, V1N 1A8, Canada. E-mail: rirvine@golder.com y Present address: BC Hydro, 601 18th Street, Castlegar, BC, V1N 2N1, Canada. Copyright # 2008 John Wiley & Sons, Ltd.