1. LENTIC SESTON ENHANCEMENT OF TAILWATER FISHERIES John S. Sproul 1, Kent A. Hatch 2, Dennis K. Shiozawa 3 1 Biology Program, Brigham Young University, Provo, Utah 84602 USA; 2 Long Island University, C.W. Post Campus, Brookville, NY 11548 USA; 3 Department of Biology, Brigham Young University, Provo, Utah 84602 USA

2. What makes tailwater fisheries so productive? Thermal Hypothesis - High productivity seen in tailwaters may be largely attributed to the thermal effects of the impoundment. Impoundments act as temperature buffers to tailwaters Hypolimnetic releases create cooler summer highs and warmer winter lows in tailwaters These buffered temperature conditions provide more degree- days for invertebrate growth and optimal conditions for growth and development of fish

3. Thermal Hypothesis Green River prior to FG Dam 1958-1959 (USGS Greendale Gage 09234500) Green River below FG Dam 2004-2005 (USGS Greendale Gage 09234500)

4. Thermal Hypothesis Is temperature the only factor affecting productivity? The sources of the energy generating the increase in productivity are not fully defined in the thermal hypothesis. The emphasis on moderated temperatures alone implies that autochthonous primary production is also enhanced during the extended growing season and that it results in the increased secondary production in the tailwater system. Yet many investigators have measured significant densities of zooplankton rich reservoir seston in tailwaters. We attempted to quantify the subsidy of reservoir seston (zooplankton) on tailwater food-webs.

5. Alloctonous Carbon Source Autocthonous Carbon Source Additioinal Allocthonous Carbon Source (animal protein) *Organisms expanding their diet to include reservoir zooplankton are likely to show an upward shift in trophic position.

6. MethodsMethods -We collected invertebrates below impoundments of three reservoirs -We compared their stable C and N isotope ratios to invertebrates above the respective reservoirs. -We also analyzed stable isotope ratios in drift, as well as zooplankton netted from the reservoirs. -We performed MANOVA’s to test for significant differences between sites.

8. ResultsResults

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16. ResultsResults Nitrogen LocationSpeciesMean BelowMean AboveMean Diff.DFt-Valuep-Value Electric lakeDrift4.1063.2110.89632-3.1570.0035 Baetis7.3755.8081.55623-11.326<0.0001 Cynigmula6.745.65481.0859-4.4840.0015 Drunella g.7.7095.6272.03621-5.705<0.0001 Rhyacophila9.3147.5031.88218-4.2080.0005 Simuliidae7.5615.3272.23410-3.2470.0088 Strawberry DamDrift7.5193.0844.43531-3.8850.0005 Baetis11.2266.0845.14124-5.646<0.0001 Epeorus10.8085.8454.9639-1.7170.1201 Simuliidae10.3156.0054.30917-3.7830.0015 Strawberry TunnelDrift7.6451.266.38532-13.802<0.0001 Baetis8.6466.0972.54919-4.560.0002 Brachycentrus9.9745.5974.3769-7.205<0.0001 Drunella g.11.5226.3345.18710-9.098<0.0001 Hydropsyche11.1397.3223.81818-10.948<0.0001 Pteronarcella9.7534.8164.93712-6.551<0.0001 Rhyacophila11.666.9384.72212-4.4360.0008 Simuliidae10.744.6236.11810-28.999<0.0001 ScofieldDrift5.251.0973.33432-3.480.0015 Asellus7.4097.679-0.27150.4560.655 Leech10.45611.09-0.63490.80.4444 Simuliidae7.565.6851.87518-3.370.0034

17. ConclusionsConclusions Lentic seston can provide a significant subsidy to the food base in tailwater food-webs. Lentic seston can provide a significant subsidy to the food base in tailwater food-webs. -The importance of this subsidy likely varies with factors like reservoir trophy, volume of discharge, etc. Future studies could investigate the distance downstream that the isotopic signal of the reservoir persists. Future studies could investigate the distance downstream that the isotopic signal of the reservoir persists. An understanding of these dynamics can influence management decisions as agencies consider the impact of restoring natural flow/temperature regimes to impounded rivers. An understanding of these dynamics can influence management decisions as agencies consider the impact of restoring natural flow/temperature regimes to impounded rivers.

18. AcknowledgementsAcknowledgements -Keith Tanner, Heather Stutz, Michelle Barney, and Chris Hansen for field and laboratory work. -Funding came from a grant from the Roger and Victoria Sant Educational Endowment for a Sustainable Environment, to Dennis K. Shiozawa.