DYNAMICS OF LAKE TROUT RECRUITMENT IN MICHIGAN WATERS OF LAKE SUPERIOR by Jessica M. Doemel A Thesis submitted in partial fulfillment of the requirements of the degree MASTER OF SCIENCE IN NATURAL RESOURCES (FISHERIES) College of Natural Resources UNIVERSITY OF WISCONSIN Stevens Point, Wisconsin December 2000 APPROVED BY THE GRADUATE COMMITTEE OF: Dr. Michael J. Hansen, Committee Chair Associate Professor of Fisheries College of Natural Resources . Szczytko Professor of Water Resources College of Natural Resources Dr. Eric Anderson Professor of Wildlife College of Natural Resources Mr. hades R. Bronte Gr t Lakes Fishery Analyst Green Bay Fishery Resources Office United States Fish and Wildlife Service ii ABSTRACT Lake Superior has been the site of an extensive restoration plan focusing on the lake trout (Salvelinus namaycush). Historically, the lake trout was a dominant top predator in the lake and during 1913-50 supported a commercial fishery with an average annual harvest of 2-million kg. In the 1950s, the lake trout population crashed due to overfishing and sea lamprey (Petromyzon marinus) predation. The restoration process ·· began in the 1950s with the stocking of juvenile, hatchery-reared lake trout and controls on sea lampreys and fisheries. In March 1996, after 3 5 years of effort, fishery managers declared victory in their pursuit of lake trout restoration and decided to cease stocking. The objectives were to quantify the contribution of wild and stocked lake trout to contemporary recruitment of wild lake trout and to quantify the effects of large-mesh (114-mm stretch measure) gill net effort on wild lake trout recruitment in Michigan waters of Lake Superior. For the first objective, Ricker stock-recruitment models were used to quantify the contribution of wild and stocked lake trout to recruitment of wild fish. Density of wild lake trout generally increased, whereas density of stocked lake trout generally decreased in all Michigan managemept units of Lake Superior during 1970-1998. The density of wild and stocked parents best described recruitment of wild lake trout during 1970-1990, but stocked parents were only 52.2% as effective as wild parents. Recruitment rates of wild and stocked parents declined significantly with density in all Michigan management units. Peak recruitment was similar for wild and stocked fish, but the stocked fish parental density needed to reach peak recruitment was twice as high for stocked parents as for wild parents. I conclude that both wild and stocked fish have contributed to iii recruitment of lake trout in Michigan waters of Lake Superior, but that wild fish were twice as reproductively effective as stocked fish. For the second objective, I used a Ricker stock-recruitment model to quantify the effect of large-mesh (114-mm stretch measure) gill net effort on lake trout recruitment. Large-mesh gill net fishing effort varied differently in all Michigan management units, but did not account for significant variation in wild lake trout recruitment. I conclude 'that current levels of large-mesh gill net fishing effort are consistent with lake trout restoration goals in Michigan waters of Lake Superior. iv ACKNOWLEDGMENTS This thesis would not have come without the help of many people. First, I want to thank my major advisor, Mike Hansen, who has been an endless source of support and direction. He has seen me through my ups and downs as a graduate student and as a person. It was an honor to be taught by him as an undergraduate and to work with him as a graduate student. He has opened many doors for me and for that I am forever grateful. 'special thanks also to my graduate committee, Dr. Eric Anderson, Dr. Stanley Szczytko, and Chuck Bronte, for guidance with my research. I owe tremendous thanks to all those who made this project possible. There were countless hours spent collecting, organizing, and maintaining the data used for this project. I feel honored to have worked on a project of this magnitude. Wisconsin Sea Grant provided funding for this research. Joan Bratley provided database support. Shawn Sitar provided the lake trout assessment data, as well as many hours of valuable discussion. Bill Mattes and Mark Ebener provided commercial gill net data. Wayne R MacCallum provided the map of Lake Superior management units. Thanks to the Lake Superior Technical Committee for allowing me to present my findings for review and discussion at their meetings. I also want to express my gratitude to the Marquette Biological Station. The station provided me with time, space, and a computer so that I could work on my thesis. Dr. Gary Klar, John Heinrich and Mike Fodale offered encouragement and advice down the final stretch. Furthermore, the station provided funding so that I could attend the National American Fisheries Society meeting to present my thesis results. V I also wanted to thank my friends and fellow graduate students for their support. To Jennifer Durst, Becki Klotz, Ruth Lee, Shannon Tibbetts, Abbe Wendland, Ivy Wendland, and Jill Gerkhe - thanks for understanding when my priorities were turned around. To Pat Schmalz., Pat Short, Kris Stepenuck, Tracy Stephens, Todd Johnson, Joel Ernst, and several other graduate students for support and assistance when the light at the end of the tunnel wasn't always so bright. I want to personally thank Mike Wilberg for 'bearing with my frantic phone calls when I was experiencing computer problems, listening to my hour-long stories (and laughing), and for helping along the way. Finally, I want to thank my family. From the very beginning, my parents have given me unconditional love and support. Your love has been a priceless gift. Your faith in me gave me the courage to pursue my dreams so confidently. I hope that I have made you proud. To my sister, Angela, thanks for the ''therapeutic" arguments. To my brother, Jason, thanks for always being there for me. I also want to thank my future in-laws, Jay and Janice Richards for their support and encouragement. Caryn and Marne, thanks for the much needed tennis and piu.a nights. Ryan, thanks for being so laid back when I was so strung out. And last, but not least, I thank my fiance, Jason. You are my inspiration and my future. Thanks for always believing in me. I can't express in words how much you mean to me. I love all of you so much. My cup runneth over. vi TABLE OF CONTENTS ABSTRACT ..................................................................................... .iii ACKNOWLEDGMENTS ......................................................................v LIST OF TABLES .............................................................................viii LIST OF FIGURES .............................................................................. x INTRODUCTION ................................................................................ 1 Historical Background................................................ _ .................. 1 Restoration Efforts .......................................................................4 Research Objectives .................................................................... 13 METHODS ....................................................................................... 15 Study Site ................................................................................ 15 Relative Abundance ..................................................................... 16 Stock-Recruitinent....................................................................... 20 Large-Mesh Gill Net Effort ...........................................................23 RESULTS ..........................................................................................24 Relative Abundance .....................................................................24 Stock-Recruitinent ......................................................................24 Large-Mesh Gill Net Effort ............................................................ 32 DISCUSSION .................................................................................... 37 Relative Abundance .....................................................................37 Stock-Recruitinent ......................................................................37 Large-Mesh Gill Net Effort ............................................................ 39 MANAGEMENT IMPLICATIONS ...........................................................41 APPENDIX A-ADDITIONAL TABLES ...................................................43 REFERENCES ...................................................................................56 vii LIST OF TABLES Table 1. Ricker stock-recruitment models .....................................................21 Table 2. Comparison of nested Ricker models for significance (P~ 0.05) using extra swn of squares analysis. See Table 1 for model definitions ............. .30 Table 3. Parameter estimates for wild and stocked lake trout in Michigan waters of Lake Superior (Model 3). Wild estimates were determined by the model and stocked estimates were derived from the function (wild estimate*k). Asymptotic standard errors (ASE) are reported for the model estimates (k, wild) ................................................................................. 31 Table 4. Recruitment estimates for wild and stocked lake trout in Michigan waters of Lake Superior. Rm designates the level of peak recruitment CPE (a/l}e) and Pm designates the level of parental CPE that produces peak recruitment (1/1}). CPE was measured as the number offish caught