SEASONAL ABUNDANCE, MOVEMENTS, AND FOOD HABITS OF HARBOR SEALS (PHOCA VITULINA RICHARDS!) INELKHORNSLOUGH,CAL~ORNIA A Thesis Presented to the Faculty of California State University, Stanislaus and Moss Landing Marine Laboratories In Partial Fulfillment of the Requirements for the Degree of Master of Science in Marine Science By Dion Seth Oxman May, 1995 Copyright © 1995 by Dion Seth Oxman 11 ABSTRACT Harbor seals were counted at least twice per month at haul-out sites in Elkhorn Slough during 1991. Although there was no seasonal variation in abundance, numbers had increased three fold since 1984. Eight juvenile harbor seals (5 females and 3 males) were caught in Elkhorn Slough and fitted with radio transmitters to study movements and activity patterns. Radio­ tagged individuals were usually found resting ashore inside the slough during the day (mean = 94% of the time), and diving in Monterey Bay during nighttime (mean = 90% of the time). At night, tagged harbor seals moved as far north as Sunset Beach, but usually were found off Moss Landing. Dives in Monterey Bay (mean = 4.32 min, S.D. 2.35 min) were significantly greater in duration than those performed in Elkhorn Slough (mean = 1.80 min, S.D. = 1.50 min). Prey hard parts found in harbor seal feces collected from Elkhorn Slough indicated harbor seals consumed mostly benthic species, including octopus {Octopus sp. ), spotted cusk-eel (Chilara taylor£), and flatfishes, throughout the year. Rockfishes (Sebastes sp.), and other pelagic species were eaten when they became abundant during summer. There was no significant similarity (PSI's < 0.13) in species composition between seal diet and otter trawls conducted in Elkhorn Slough, indicating seals fed mostly in the bay at night. \lV ACKNOWLEDGMENTS Special thanks to my advisory committee Dr. Jim Harvey, Dr. Gregor Cailliet, and Dr. Pamela Roe for their guidance, encouragement, and wisdom. Thanks also to the Moss Landing Marine Laboratories staff, especially the librarian Sheila Baldridge and assistant director Gail Johnston. This study could not have been conducted without all the help I received from fellow students at Moss Landing. Thanks to Rob Suryan, Kim Raum-Suryan, Patience Browne, John Mason, Meg Lamont, Michele Hester, Mike Torok, and Tony Bennett who donated their time and energy, at considerable risk to life and limb, to capture harbor seals. My deepest appreciation to Steve Trumble for his encouragement, enthusiasm, and friendship. He made this an extremely enjoyable experience. Thanks also to Tomoharu Eguchi for spending countless hours helping me monitor radio­ tagged seals. This study was generously supported by the Earl & Ethyl Meyers Oceanographic Trust, the Packard Foundation, the California Department of Fish & Game, and the Elkhorn Slough Foundation. Special thanks, of course, to my wife Jennifer and my family for their love, support, and patience. v TABLE OF CONTENTS Page COPYRIGHT ...... ii CERTIFICATION OF APPROVAL iii ABSTRACT . !V ACKNOWLEDGMENTS v LIST OF TABLES . viii LIST OF FIGURES X INTRODUCTION 1 OBJECTIVES 6 HYPOTHESES 7 MATERIALS AND METHODS 8 STUDY AREA. 8 SEASONAL ABUNDANCE OF HARBOR SEALS . 9 HARBOR SEAL MOVEMENTS AND ACTIVITY PATTERNS 10 ELKHORN SLOUGH ICHTHYOFAUNA: PREY AVAILABILITY. 12 FOOD HABIT ANALYSIS 13 RESULTS . · · · · · · · · 17 SEASONAL ABUNDANCE OF HARBOR SEALS . 17 HARBOR SEAL MOVEMENTS AND ACTIVITY PATTER.l\JS 17 PREY AVAILABILITY. 21 FOOD HABITS. 27 DISCUSSION . 33 SEASONAL ABUNDANCE . 33 MOVEMENT AND ACTIVITY 36 FOOD HABITS. 42 . SUMMARY AND CONCLUSIONS 62 vi LITERATURE CITED. 64 TABLES. 77 FIGURES 92 vii LIST OF TABLES Table Page 1. Data on seals captured and radio-tagged in Elkhorn Slough 77 2. Mean, standard error (SE) and range of hrs/day ashore in 78 Elkhorn Slough (ES), duration of haul-out bout, hrs/day diving in ES, Total time spent in ES per day, hrs/ day diving in Monterey Bay (MB), and number of complete activity phases monitored (n) for eight harbor seals radio-tagged in Elkhorn Slough, CA, during September 1991. 3. Mean and standard error (SE) of dives and surface activity 79 (in sec) for eight harbor seals radio-tagged in Elkhorn Slough, CA, September 1991 to April 1992. 4. Mean number per 10-min tow (n), standard error (in 80 parentheses), and relative abundance (%)of fish species caught in 83 day-time otter trawls at three locations in Elkhorn Slough. 5. Summary of diversity, dominance, and abundance of fishes 81 captured with otter trawls at three stations in Elkhorn Slough from 1974- 1976 (from Yoklavich et al. 1991) and 1991- 1992. 6. Relative abundance (%)of dominant species totaling 80% or 82 greater of fishes collected by otter trawl at three stations in Elkhorn Slough from 1974 to 1976 (from Yoklavich et aL 1991) and 1991 to 1992. 7. Seasonal comparison of species composition at each station 83 from daytime otter trawls conducted during 1991 in Elkhorn Slough based on percent similarity index (PSI). 8. Mean number per 10-min tow (n), standard error (in 84 parentheses), and relative abundance (%)of fish species caught in night-time otter trawls at two locations in Elkhorn Slough. viii Table Page 9. Mean percent number (%n}, mean percent mass (%M), percent 85 frequency of occurrence (%FO}, and mean index of relative importance (llU} of prey items found in harbor seal feces collected in Elkhorn Slough, CA during winter (Nov. to Jan.; n=64) 1991. 10. Mean percent number (%n), mean percent mass (%M), percent 86 frequency of occurrence (%FO), and mean index of relative importance (llU} of prey items found in harbor seal feces collected in Elkhorn Slough, CA during spring (Feb. to April; n=70) 1991. 11. Mean percent number (%n}, mean percent mass (%M), percent 87 frequency of occurrence (%FO), and mean index of relative importance (llU) of prey items found in harbor seal feces collected in Elkhorn Slough, CA during sununer (May to July; n=86) 1991. 12. Mean percent number (%n), mean percent mass (%M), percent 88 frequency of occurrence (%FO), and mean index of relative importance (llU) of prey items found in harbor seal feces collected in Elkhorn Slough, CA during autumn (Aug. to Oct.; n=86) 1991. 13. Seasonal comparison of prey species composition from harbor 89 seal scats collected during 1991 in Elkhorn Slough based on percent similarity index (PSI). 14. Percent similarity indices (PSI) comparing species composition 90 of daytime and nighttime otter trawls conducted in Elkhorn Slough during 1991 to prey species identified from harbor seal feces collected from Elkhorn Slough during 1991. 15. Commercial fishery landings and estimates of annual fish 91 consumption by Elkhorn Slough harbor seals, in kilograms, and by the entire harbor seal population of Monterey Bay, CA, during 1991. ix LIST OF FIGURES Figure Page 1. Location of three haul-out sites for harbor seals, and trawling 92 stations in Elkhorn Slough, CA 2. Monthly mean, standard error (delineated by box), and range 93 (vertical lines) of number of harbor seals at three haul-out sites in Elkhorn Slough from 38 counts made January 1991 through December 1991. 3. Maximum number of seals observed in Elkhorn Slough 94 during the past 16 years. 4. Proportion of times radio-tagged harbor seals were found in 95 Elkhorn Slough during surveys of the Monterey Bay area conducted during day (1000- 1400 h) and night (1800- 2100 h). 5. Foraging locations of harbor seals radio-tagged in Elkhorn 96 Slough, California between Sept. 1991 and Apri11992. 6a. Percent time harbor seals radio-tagged in Elkhorn Slough 97 spent swimming/diving (SD) and hauled-out (HO) in Monterey Bay (MB) and Elkhorn Slough (ES) between Sept. 1991 and January 1992. 6b. Percent time harbor seals radio-tagged in Elkhorn Slough 98 spent swimming/diving (SD) and hauled-out (HO) in Monterey Bay (MB) and Elkhorn Slough (ES) between Jan. and April, 1992. 7. Mean amount of time harbor seals spent ashore in Elkhorn 99 Slough (ES), swimming and diving (S/D) in ES, and S/D in Monterey Bay (MB). 8. Cumulative species curves for trawls conducted during the 100 day at the Kirby Park station in Elkhorn Slough. 9. Comparison among stations, using% number of species 101 collected from daytime trawls conducted in Elkhorn Slough during 1991. X Figure Page 10. Seasonal variation in mean catch, mean number of species, 102 and mean dominance index per 10-min tow of fish from otter trawls at each station in Elkhorn Slough, CA. 11. Seasonal variation in mean abundance per 10-min tow of 103 the most dominant fish species collected during 1991 in Elkhorn Slough, CA. 12. Cumulative species curves for trawls conducted at night at 104 the bridge station in Elkhorn Slough. 13. Comparison between stations using percent number of 105 species collected from night trawls conducted in Elkhorn Slough during 1991. 14. Species comparison using mean number per 10-min tow 106 between day and night trawls conducted at the bridge station in Elkhorn Slough during 1991. 15. Species comparison, using mean number per 10-min tow, 107 between day and night trawls conducted at the Dairy station in Elkhorn Slough during 1991. 16. Species comparison, using mean number per 10-min tow, 108 between trawls conducted in Elkhorn Slough during 1975 (from Yoklavich et al. 1991) and 1991. 17. Species comparison, using mean number per 10-min tow, 109 between trawls conducted at the bridge station in Elkhorn Slough during 1975 (from Yoklavich et al.) and 1991. 18. Species comparison, using mean number per 10-min tow, 110 between trawls conducted at the Dairy station in Elkhorn Slough during 1975 (from Yoklavich et al. 1991) and 1991. 19. Species comparison, using mean number per 10-min tow, 111 between trawls conducted at Kirby Park in Elkhorn Slough during 1975 (from Yoklavich et al.