Assessing the Seasonal Effects of Bull Trout Predation on Pacific Salmon in a Riverine Food
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Trophic relations and seasonal effects of predation on Pacific salmon by fluvial bull trout in a riverine food web Erin Douglas Lowery A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science University of Washington 2009 Program Authorized to Offer Degree: School of Aquatic and Fishery Sciences University of Washington Graduate School This is to certify that I have examined this copy of a master’s thesis by Erin Douglas Lowery and have found that it is complete and satisfactory in all respects, and that any and all revisions required by the final examining committee have been made. Committee Members: _____________________________________________________________ David A. Beauchamp _____________________________________________________________ Timothy E. Essington _____________________________________________________________ Edward Connor Date:______________ In presenting this thesis in partial fulfillment of the requirements for a Master’s degree at the University of Washington, I agree that the Library shall make its copies freely available for inspection. I further agree that extensive copying of this thesis is allowable only for scholarly purposes consistent with “fair use” as prescribed by the U.S. Copyright Law. Any other reproduction for any purposes or by any means shall not be allowed without my written permission. Signature________________________________________ Date________________________________________ University of Washington Abstract Trophic relations and seasonal effects of predation on Pacific salmon by fluvial bull trout in a riverine food web Erin Douglas Lowery Chair of the Supervisory Committee: Professor David A. Beauchamp School of Aquatic and Fishery Sciences Bull trout Salvelinus confluentus occupy upper trophic positions in most ecosystems where they occur. Since federal listing, natural resource managers are frequently challenged to manage bull trout and their prey which can often include federally listed Pacific salmon Oncorhynchus spp. The Skagit River in Northwest Washington State contains one of the largest populations of bull trout Salvelinus confluentus and Chinook salmon O. tshawytscha in the Puget Sound region, and a regionally large population of steelhead O. mykiss; all three species are listed as threatened under the Endangered Species Act (ESA). The objective of this investigation was to determine the trophic ecology of bull trout, especially their role as predators and consumers in the river food web. I sampled distribution, diets, and growth of bull trout in mainstem and tributary habitats during 2007 and winter and spring 2008. Consumption rates were estimated with a bioenergetics model to determine annual and seasonal energy budgets of bull trout and to estimate their potential predation impacts on juvenile Pacific salmon populations. Salmon carcasses and eggs contributed approximately 50% of the annual energy budget for large bull trout in mainstem habitats but were largely inaccessible in tributary habitats. The remaining 50% was acquired from juvenile salmon, resident fishes, and immature aquatic insects. Predation on listed Chinook salmon and steelhead/rainbow trout was highest during winter and spring (January-June). Predation on juvenile salmon differed between 2007 and 2008, and was likely due to the dominant odd-year spawning cycle for pink salmon O. gorbuscha. The population impact on ocean- and stream-type Chinook salmon was negligible while the impact on steelhead/rainbow trout was potentially very high. Due to the ESA-listed status of bull trout, steelhead, and Chinook salmon, the complex trophic interactions in this drainage create both challenges and opportunities for creative adaptive management strategies. Table of Contents Page List of Figures.................................................................................................... iii List of Tables ..................................................................................................... iv Introduction..........................................................................................................1 Trophic relations and seasonal effects of predation on Pacific salmon...............7 Introduction..................................................................................................7 Study Area .................................................................................................10 Methods......................................................................................................10 Sample Collection......................................................................................11 Growth and Spawning Losses....................................................................12 Stable Isotope Analysis..............................................................................13 Diet Analysis..............................................................................................14 Population and Size Structure Assessment................................................16 Bioenergetics Modeling.............................................................................18 Predation Impact Scenarios........................................................................21 Results........................................................................................................22 Stable Isotope Analysis..............................................................................22 Diet.............................................................................................................23 Energy Density...........................................................................................24 Population Survey and Size Structure........................................................25 Modeling Consumption .............................................................................25 Variability in Consumption Estimates.......................................................27 Estimated Energy Budgets.........................................................................27 Predation Estimates....................................................................................28 Predation Impact Scenarios........................................................................29 Discussion..................................................................................................29 i References..........................................................................................................36 Appendix............................................................................................................63 ii List of Figures Figure Number Page Number 1. Map of the study area.…………………………………………………………..43 2. Mean Length at age of fluvial bull trout………………………………………...44 3. Carbon and nitrogen stable isotope values for food web members……………..45 4. Ontogeny of carbon and nitrogen values in fluvial bull trout…………………...46 5. Size at piscivory for fluvial bull trout…………………………………………...47 6. Comparison of fish prey length and bull trout length…………………………...48 7. Estimated biomass consumed…………………………………………………...49 8. Annual energy budget for fluvial bull trout in 2007……………………............50 9. Winter and spring energy budgets in 2007 and 2008……………………………51 A1. Biomass consumed using WMi………………………………………………...71 A2. Biomass consumed using Wi…………………………………………………..72 A3. Annual bull trout energy budget using WMi…………………………………...73 A4. Winter Spring 2007 and 2008 bull trout energy budget using WMi…………...74 A5. Annual bull trout energy budget using Wi……………………………………..75 A6. Winter Spring 2007 and 2008 bull trout energy budget using Wi……………..76 iii List of Tables Table Number Page Number 1. Bioenergetics model inputs and outputs……………………………….52 2. Bioenergetics model parameters……………………………………….53 3. Mean monthly Skagit River water temperature………………………..54 4. Observed diet proportions……………………………………………..55 5. Energy densities of fluvial bull trout and major prey…………………56 6. Length of prey fish from bull trout diets………………………………57 7. Estimated size and age structure of the bull trout population…………58 8. Variability in estimated diet proportions………………………………59 9. Individual prey consumed……………………………………………..60 10. Consumption scenarios………………………………………………62 A1. Diet proportions using MWi…………………………………………77 A2. Diet proportions using Wi……………………………………………78 A3. Bioenergetics model inputs and outputs…………………………….79 A4. Individual prey consumed using WMi……………………………….80 A5. Individual prey consumed using Wi…………………………………82 A6. Consumption Scenarios using WMi………………………………….84 A7. Consumption scenarios using Wi…………………………………….85 iv Acknowledgements This project was fully funded by Seattle City Light and the Puget Sound Anglers Fidalgo Chapter and I thank both organizations very much. I would like to extend a very special thank you to Matt Smith, Jamie Thompson (and his raft), Mike Mazur, Dave Pflug, Ed Connor, and Steve Stout. All of whom had a strong hand in the technical development of this project and provided invaluable logistical and scientific support. Anna Buettner, Mike Shepard, Nathanael Overman, Cathy Ekblad, and Jim Matilla gave great effort in the field or laboratory during different phases of this project, and I thank them. I would also like to acknowledge Eric Schoen, Liz Duffy, Jen McIntyre, and Verna Blackhurst in the Washington Cooperrative Fish and Wildlife Research Unit who helped out in a variety of ways, usually giving sound advice and help beyond my expertise. I especially want to extend a huge thanks