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LIFE HISTORY ASPECTS and POPULATION DYNAMICS of a COMMERCIALLY EXPLOITED STINGRAY, Dasyatis Dipterura

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LIFE HISTORY ASPECTS and POPULATION DYNAMICS of a COMMERCIALLY EXPLOITED STINGRAY, Dasyatis Dipterura LIFE HISTORY ASPECTS AND POPULATION DYNAMICS OF A COMMERCIALLY EXPLOITED STINGRAY, Dasyatis dipterura A thesis submitted to the faculty of Moss Landing Marine Laboratories and San Francisco State University In partial fulfillment of The requirements for The degree Master of Science In Marine Science by Wade Daniel Smith San Francisco, California May, 2005 Copyright by Wade Daniel Smith 2005 LIFE HISTORY ASPECTS AND POPULATION DYNAMICS OF A COMMERCIALLY EXPLOITED STINGRAY, Dasyatis dipterura Wade Daniel Smith San Francisco State University 2005 The diamond stingray, Dasyatis dipterura (Jordan and Gilbert, 1880) inhabits shallow coastal waters from southern California to Chile, including the Galapagos and Hawaiian Islands. The directed batoid fishery at Puerto Viejo in the Bahia Magdalena Bay lagoon complex, Baja California Sur, Mexico was surveyed during the course of this study to assess species- and sex-specific catch composition during peak summer landings (June, August). Rhinobatos pro ductus, D. dipterura, Narcine entemedor, and Gynmura marmorata were the four most commonly landed rays. Median disc width (DW) at maturity was determined to be 57.3 em for females and 46.5 em for males, respectively. Growth characteristics were estimated by a detailed examination of banding patterns present in thin-sectioned vertebral centra. Age estimates were obtained from 304 fishery- derived specimens (169 female, 135 male). An annual nature of band deposition was confirmed through centrum edge and marginal increment analyses. Gompertz and von Bertalanffy growth models were fit to the age-at-DW/weight data. Growth characteristics differed significantly between females and males. The maximum estimated age was 28 years for females and 16.5 years for males. Traditional three parameter von Bertalanffy growth models indicated relatively low growth rates: females (DWco = 92.4 em, k = 0.055, t0 = -7.61); males (DWco = 62.2 em, k= 0.10, t0 = -6.80). Demographic analyses were developed from the resulting maturity, longevity, and growth information to estimate population growth characteristics and potential responses to fishing pressure. Monte Carlo simulation was incorporated to introduce demographic variability and uncertainty in age at maturity, fecundity, survivorship, and longevity. Ten models were developed from deterministic (4) and probabilistic (6) approaches based on maximum observed age (28 years), theoretical maximum age (63 years), and variable longevity (25- 39 years) under unexploited and exploited (F=0.05) conditions. Elasticity analyses detennined the relative contribution and impact of changes in fertility, juvenile, and adult survival to population growth rates (/c). Annual survivorship probabilities ranged between 0.71-0.94. Projections generated through the incorporation ofMonte Carlo simulations produced mean A. of 1.05-1.06 (5-6% increase) per year, net reproductive rates per generation (R 0 ) of2.4-3.0, and generation times (A) from 14.4-15.2 years. Introducing relatively low fishing mortality into population models produced a maximum A. value under an optimal, static scenario of 1.09 and a minimum value of 1.01 annually using probabilistic models. Deterministic and probabilistic demographic analyses indicate that the Bahia Magdalena lagoon complex population of D. dipterura has a low intrinsic growth potential and possesses a limited resilience to fishing pressure. I certify that the Abstract is a correct representation of the content of this thesis. " ' I Date ACKNOWLEDGMENTS This study was made possible, in part, by funding provided from the General Service Foundation, Homeland Foundation, Earl H. and Ethel M. Myers Oceanographic and Mruine Biological Trust, National Marine Fisheries Service via the National Shark Research Consortium m1d Pacific Shark Research Center, P ADI Foundation, P ADI Project AWARE, SFSU Student Project Fund, and the Packard Foundation Scholarship. Additional support was received through the Sally Casanova Pre-Doctoral Program, Martha Jolmston Memorial Scholarship, and the Kim Peppard Memmial Scholarship. This study was conducted in accordance with and under the approval of the SFSU Committee for the Protection ofHuman and Animal Subjects Protocol #01-038. Joe Bizzano offered umnatched, unwavering support under demanding conditions in Baja. His assistance through tropical stonns, car accidents, scorpion stings, food poisoning, blazing heat, ru1d countless sampling complications made this study possible to a large extent. I am deeply grateful for his professionalism and friendship. I sincerely thank my committee members for their help with this process; Dr. Gregor Cailliet, my friend and advisor, for his guidance and support throughout my extended and often complicated time at MLML, Dr. Ralph Larson for his editing and helpful comments, and Dr. Enric Cmies for his invaluable technical input on demographic analyses. I hold enom1ous gratitude for Dr. Cailliet, who lent me his personal vehicle for use in the rather extreme conditions of Baja. Further orientation and field assistance was provided by Carolina Downton-Hoffinan. Carlos Villavicencio-Garayzar and Everardo Mariano-Melendez VI offered archived vertebral samples for use in the study. Jason Cope challenged me with many topics of inquiry and freely lent both quantitative and creative ideas throughout the course ofthis research. Mindy Hall proved to be an excellent intern. Dr. Dave Ebe1i ensured that there was considerable flexibility with my work schedule at the Pacific Shark Research Center which allowed me the time to focus and make substantial progress over the past year in particular. With much appreciation, I acknowledge Aldo DeRose, Joan Parker and the library staff, Dr. Keru1eth Coale, and Dmma Kline for their assistance, patience, and reliability. My father, Gary Smith, encouraged exploration and fostered within me a sense of wonder for the natural enviromnent from an early age. For that spark and his support, I will always be fmiunate and thankful. I also thank the rest of my family, Rosemary Smith (mother), Rose Murphy (sister), Sara Smith (sister), Helen Sterba (grandmother), and June Bosarge (grandmother), for their encouragement, interest, and love. Debbie Feichtinger offered immeasurable support prior to and during my lengthy graduate career at MLML. My rock climbing, bad movie-watching, soccer and foosball playing companions and colleagues (Rhea Sanders, Jason Cope, Joe Bizzarro, Andrew Thurber, Lisa KelT, Matt Levey, JeffField, Aaron Carlisle, Tonatiuh Trejo, Chris Rinewalt, Dawn Taru1er, and John Haskins) worked hard to provide much needed distractions and balance in my life. The encouragement and fiiendship of many at Moss Landing Marine Laboratories will always be remembered. Vll TABLE OF CONTENTS List ofTables ....................... ·......................... x List of Figures ................................................ xiii List of Appendices ............................................ xvii GENERAL INTRODUCTION ................................... 1 Literature cited ............................................ 8 CHAPTER 1 - Catch composition and the contribution of Dasyatis dipterura to an artisanal batoid fishery in the Bahia Magdalena lagoon complex, Baja California Sur, Mexico ............................. 12 Introduction . 13 Study area . 16 Methods ................................................. 20 Results .................................................. 22 Discussion ............................................... 27 Literature cited ............................................ 3 7 CHAPTER 2 - Life history characte1istics of Dasyatis dipterura: Maturity, age, growth, and longevity ..................................... 53 Introduction .............................................. 54 Methods ................................................. 57 Field sampling.......................................... 57 Matmity assessment. ..................................... 58 Age and Growth ........................................ 60 Results .................................................. 73 Maturity and reproductive observations ...................... 73 Age, growth, and longevity................................ 7 5 Discussion ............................................... 83 Vlll Maturity and reproduction. 83 Age, growth, and longevity................................ 93 Literature cited . ... 109 CHAPTER 3 -Population dynamics and potential responses to fisheries exploitation of Dasyatis dipterura in the Bahia Magdalena lagoon complex, B.C.S. Mexico ............................................... 143 Introduction . 144 Methods ................................................. 149 Demo graphic models ..................................... 149 Monte Carlo simulation . 156 Life history parameters ................................... 157 Natural mortality scenarios ................................ 161 Fishery exploitation scenarios .............................. 162 Results . 164 Natural mortality and survivorship .......................... 164 Demographic analysis .................................... 165 Elasticity analysis. .................................. 169 Fisheries impacts. 170 Discussion . 172 Mortality and survivorship. 172 Demo graphic analysis . 177 Elasticity analysis. .................................. 182 Fisheries impacts and conservation implications ................ 184 Model limitations, potential improvements, conclusions ......... 187 Literature cited ...........................................
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