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The Effects of Sea Star and Walrus Predation on Bivalves in Norton Sound, Alaska

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The Effects of Sea Star and Walrus Predation on Bivalves in Norton Sound, Alaska THE EFFECTS OF SEA STAR AND WALRUS PREDATION ON BIVALVES IN NORTON SOUND, ALASKA Allan K. Fukuyama San Francisco State University 1985 groenlandicus, and Yoldia hyperborea are abundant in sub- tidal, soft sediments in Norton Sound, Alaska. All species exhibit distinct size distributions with many small, recent- ly settled animals, few of intermediate size, and a dis- tinct adult population. Predation by sea stars, especially Asterias amurensis, is the most likely explanation for this size distribution. Gut content examinations of Asterias showed it to be an important predator on bivalves <10 mm. There were several refuges from sea star predation. Burrowing and leaping are used by Yoldia and Serripes, respectively, while Macorna and Mya use a depth refuge, and a related size refuge, to avoid predation by sea stars. When bivalves reach approximately 40 mm, they become sub- jected to seasonal predation by walruses. The preferred prey is Serripes since it is a shallow burrower. Populations of this species have been reduced; walruses now app.ear to be feeding mostly on populations of Mya and Macoma. ACKNOV.'LEDG11ENTS A study in remote areas could only be accomplished with the aid of many organizations and people. Foremost I thank the Benthic Bubs, M. Silberstein, P. Slattery, E. O'Connor, and J. Oliver for three fun-filled field seasons. Others who helped with field work included J. Oakden, J. Beine, G. Van Dykhuizen, D. Canestro, B. Stewart, R. Kvitek, A. Baldridge, R. Clevenger, and B. Matsen. The Alaska Department of Fish and Game in Nome, Alaska kindly provided use of a boat and facilities and I especially thank R. Nelson. The University of Alaska Institute of Marine Science, Seward, Alaska, under the directorship of D. Dieter provided laboratory space and accommodations. The crew of the R/V Alpha Helix, captained by S. Bailey, provided us with an atmosphere condusive for scientific exploration and I thank the crew for their patience and help. R. Kvitek, M. Silberstein, V. Hironaka, T. Herrlinger, J. Oliver and J. Nybakken revised early drafts. I am grateful for the editorial help and friendship of R. Gill and c. Handel, who provided different perspectives. K. Lohman gave advice with statistics and I give special thanks to S. Baldridge for procuring many obscure and sometimes hard to get references. I also thank the members of my thesis committee, J. Nybakken, B. Wursig, R. Larson, and a special thanks to "Oli" Oliver, who obtained funding ill for this project, organized the expeditions, provided many hours of ear-bending discussions and showed me the science of banditry. Financial support was provided by the National Science Foundation Grant #DPP 8121722 and the San Jose State University Foundation. Thanks also go to the many friends at Hoss Landing Marine Laboratories and to my parents and family. v. TABLE OF CONTENTS INTRODUCTION • .. .. .. .. .. .. .. .. .. .. .. .. '"" .. 1 METHODS • • • . 3 Study Area 3 Field Methods • • . • . • • . • • • • • • . 6 Laboratory Methods . • 9 RESULTS . • • .12 Bivalve Densities and Population Structure • 12 Predators and Their Effects on Bivalves ••• 25 Ophi uroids.. .. .. .. .. .. .. .. .. .. .. 25 Asteroids .. .. .. .. .. .. .. .. .. .. .. .. 25 Behavioral Observations • • • • • • • • 27 Walrus Predation •••.••••••••••.• 30 Behavioral Refuges •••.••••••••••.• 33 Depth Refuges .. .. .. .. .. .. .. .. .. .. .. .. .. .. • 34 Size Refuges ........................ 36 DISCUSSION • • • • • . • • . • • • • • • • • •• 38 Population Structure • • • • • • • • • • • • • • .38 Spawning and Settlement • • • • • • • 43 Spatial Refuges • • • • • • • • • • • • • • • • • 44 Behavioral Escape Responses • • • • • • • • • • • 48 Size Refuges • • • • • • • • • • • • • 49 Walrus Predation ...................... • SO CONCLUSION • • • • 56 LITERATURE CITED • .. .. .. .. 58 APPENDIX • .. .. .. .. .. .. • • .. .. 66 vi LIST OF TABLES TABLE Page 1. Densities (mean nurnber/m2 + 1 SE) of the bivalves Mya truncata, Macorna calcarea, Serripes groenlandicus, and Yoldia hyperborea from three sites. • • • • 21 2. Mean abundances (+ 1 SE) of the asteroids Asterias amurensis and Lethasterias nanimensis from transects at three sites in Norton Sound, June-July 1982.. 26 3. Stomach contents of Asterias examined in the laboratory •.• • • . • 28 4. Prey items found on the oral surface of Asterias is examined in the field. 29 5. Numbers of bivalves eaten by Asterias amurensis in aquaria with and without sediment. Bivalves are separated into two size groups -- Macorna calcarea, Yoldia hyperborea, Lyonsia sp., Mya truncata, and Thyasira flexuosa range in size from 1 6 mrn and Macoma balthica range in size from 6-10 mm. • • • . 35 6. Time for Asterias arnurensis of three size classes to capture and eat bivalves of various species and sizes.. • • • •• 37 7. Maximum depths to which nine species of bivalves have been found burrowed •• • 47 8. Depths at which six predators have been found to dig to obtain bivalves. • • • • • • 51 vii LIST OF FIGURES FIGURE Page l. Location of study sites in Norton Sound, Alaska.. .. .. .. .. .. .. .. .. .. 4 2. Sizes of Yoldia hyoerborea collected by cores and scoops at Cape Nome and Sledge Island, Alaska from 1981-1983. • ••• • 13 3. Sizes of Macoma calcarea collected by cores and scoops at Cape Nome and Sledge Island, Alaska from 1981-1983 .. .. .. .. • 15 4. Sizes of Mya truncata collected by cores and scoops at Cape Nome and Sledge Island, Alaska from 1981-1983. • • • . • • • • • • • .17 5. Sizes of Serripes groenlandicus collected by cores and scoops at Cape Nome and Sledge Island, Alaska from 1981-1983. • • • • • • 19 6. Comparison of sizes of Mya truncata from the dead shell record at Cape Nome in 1981 and 1982 with live bivalves collected by digging from 1982 •••. 23 7. Combined zes of three bivalve species consumed by walruses from Cape Nome and Sledge Island, Alaska in 1981 and 1982 •••••••••• 31 vii.i INTRODUCTION Studies of the ecology of Arctic soft-bottom subtidal communities have been limited since these areas are remote and frequently inaccessible due to ice cover or inclement weather conditions. However, epifaunal and infaunal animals of the eastern Bering Sea are known from grab, dredge and trawl samples (McLaughlin 1963;. Fay et al. 1977; Wolotira et al. 1977; Stoker 1978; Feder and Jewett 1978, 1980). These studies have described distribution and abundance patterns of invertebrates, including bivalves, but little is known about factors structuring bivalve communities, particularly the effects of predation on these communities. The bivalves, Mya truncata Linne, Macoma calcarea (Gmelin), Serripes groenlandicus (Bruguiere), and Yoldia hyperborea Loven form an important component of the infauna, especially as food for predators such as walruses (Vibe 1950; Fay 1982), bearded seals (Lowry et al. 1980), fishes (Feder and Jewett 1978, 1980; Jewett and Feder 1980), large invertebrates such as king crabs and snow crabs (Feder and Jewett 1978, 1980, 1981) and sea stars (Kim 1969, Feder and Jewett, 1980). Food analyses of these predators have shown that bivalves of various sizes are preyed upon, but there has been little quantitative information on the effects that such predation may have on bivalve population 1, structure. 2 The Norton Sound region is utilized by walruses during their spring northward migration (Fay 1982). Thus, bivalve populations are subjected to intense seasonal predation by walruses, coupled with predation by sea stars, particularly Asterias amurensis. There is little information on the effects that such predation have had on bivalve populations. This study was initiated on the biva.lves inhabiting soft-bottom areas of Norton Sound, Alaska in order to seek answers to the following questions: [1] what determines the size structure of bivalve populations? [2] what animals prey on these bivalves and what sizes are utilized by these predators? [3] what effects do these predators have on various life stages of the bivalves? and [4] what refuges from predation exist for these bivalves? METHODS STUDY AREA The seafloor of Norton Sound consists of extensive flat ~reas of fine and muddy sand (Sharma 1974). Sedimentary characteristics of the area have been descp_bed by Nelson and Creager (1977) and Larsen al. (198U, who observed distinct layering of the sediment with a soft silty-clay layer extending 10-15 em deep and a consolidated hard-packed clay layer below that. The area is co7ered with sea ice from the fall until late spring, but rD ice gouging of bottom sediments occurs in this area (Lars=n et al. 1979). Current patterns are influenced by runoff from the Yukon River as well as by tides and winds. Tbe p:edominant current flow is .from east to west and is detailed in Muench et al. (1981). Bottom currents of approximat~ly 1-2 knots were occasionally encountered duri~ this study. Water visibility was usually 0.5-3 m and bottom temperatures were l-l0°C. Four study sites were established within Norton Sound: the primary site (Cape Nome) was about 24 km southeast of Nome (64°15'N, 165aOO'W); another site was near Sledge Island (64°30'N, 166°15'W)(Fig. 1). These two sites were feeding grounds for migrating walrus (Oliver et al. 1983). Secondary sites were sampled at Square Rock (64° 30'N, 163"40'W), and Golovin Bay (64°20'N, 163°00'W)(Fig. 1). 3 4 Figure 1. Location of study sites in Norton Sound, Alaska. 5 . - 0 • z :!'l < w "'0 !!;"' ~·- u < <.. <"' ...1"' < ;;; u "'0 u ;: u \ a: ![ "\, < • •• (} ·" •. < w • " <( "z • ... 'q 0 "'m .ll c: • .. :"' o iE ..::> :j- ~ '!> " ""--.
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