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. 1991) and 1991.
20. Cumulative number of prey species per fecal sample 112 collected during summer (May -July) 1991 in Elkhorn Slough, California.
xi Figure Page
21. Seasonal variation in diversity (mean number of species per 113 scat (S) and mean diversity index (H'), index of specialization (R), and dominance (D) of prey found in harbor seal feces collected during 1991 from Elkhorn Slough, CA.
22. Comparison among seasons using percent number of species 114 identified from harbor sealfecal samples collected in Elkhorn Slough, CA during 1991.
23. Frequency histograms of the estimated prey length of spotted 115 cusk eel and Night smelt recovered from harbor seal feces collected in Elkhorn Slough, CA during 1991.
24. Frequency histograms of the estimated prey length of 116 staghorn sculpin and plainfin midshipman recovered from harbor seal feces collected in Elkhorn Slough, CA during 1991.
25. Frequency histograms of the estimated prey length of rockfish 117 sp. and white croaker recovered from harbor seal feces collected in Elkhorn Slough, CA during 1991.
26. Frequency histograms of the estimated prey length of Dover 118 " sole and English sole recovered from harbor seal feces collected in Elkhorn Slough, CA during 1991.
27. Frequency histograms of the estimated prey length of 119 Pacific sanddab recovered from harbor seal feces collected in Elkhorn Slough, CA during 1991.
28. Frequency histograms of the estimated prey length of 120 Octopus sp. and market squid recovered from harbor seal feces collected in Elkhorn Slough, CA during 1991.
29. Comparison of species caught in Elkhorn Slough during 121 winter to those found in harbor seal fecal samples collected during winter from Elkhorn Slough haul-out sites in 1991.
30. Comparison of species caught in Elkhorn Slough during 122 spring to those found in harbor seal fecal samples collected during spring from Elkhorn Slough haul-out sites in 1991.
xii Figure Page
31. Comparison of species caught in Elkhorn Slough during 123 summer to those found in harbor seal fecal samples collected during summer from Elkhorn Slough haul-out sites in 1991. . 32. Comparison of species caught in Elkhorn Slough during 124 autumn to those found in harbor seal fecal samples collected during autumn from Elkhorn Slough haul-out sites in 1991.
33. Mean, standard deviation, and range of the standard length 125 of fish consumed by harbor seals in Elkhorn Slough in 1991, and those of individuals caught in otter trawls conducted in Elkhorn Slough in 1991.
34. Species comparison, using percent number, between prey 126 identified in harbor seal feces collected from Elkhorn Slough during 1975-1977 (Harvey et al. in press) and 1991.
xiii INTRODUCTION
The Pacific harbor seal (Phoca vitulina richardsi) is an abundant resident off the Pacific coast of North America, found from Cedros Island, Baja California to the Gulf of Alaska, and along the Alaskan Peninsula and Aleutian Islands (King 1983). Harbor seals usually occur in close proximity to the coast, commonly found in rivers and estuaries (Pitcher 1977, Brown and Mate 1983}. At low tide, they often come ashore on mud and sand bars, nearshore rocky islets, tidal reefs, and sand and gravel beaches (Pitcher 1977). At least 40,580 harbor seals live along the west coast of the continental U.S. (Boveng 1988, Hanan et al. 1993). Approximately 7,970 harbor seals inhabit the coastal waters and 6,060 populate the inland waters of Washington (Boveng 1988). Off Oregon, there are 3,420 harbor seals (Harvey et al. 1990). Mainland California has approximately 18,700 harbor seals, and an additional4,433 reside around the Channel Islands (Hanan et al. 1993). Because of their efficiency as predators and the recovery of depleted populations, harbor seals and other pinnipeds often are regarded as competitors with man for marine resources. These conflicts are categorized as operational or ecological interactions (Mate and Harvey 1987, Olesiuk 1993). Operational interactions occur when pinnipeds and fisheries come into direct contact. Entanglement of harbor seals in fishery operations has resulted in damage to gear and pinniped mortality or injury (Miller et al. 1983, Geiger 1985, Harvey 1987, Boveng 1988). Harbor seals also damage and eat catch, such as salmon caught in hook-and-line and gillnet fisheries (Pitcher 1977, Jeffries 1984, Roffe and Mate 1984, Beach et al. 1985).
1 2
Ecological interactions encompass the indirect effects pinnipeds and fisheries have on each other, such as the reduction in prey abundance for pinniped populations or pinniped predation on commercially exploited fish stocks. Whether pinniped predation can effectively reduce an entire prey population remains to be determined. it is difficult to obtain the information necessary for estimating the impacts of a pinniped on its food resources, such as size, composition, and distribution of the pinniped population, length and age class of fish consumed, and daily food requirements of each sex and age class with respect to time of year (Bonner 1982, Harwood and Croxall1988, Reidman 1990, Olesuik 1993}. Information on the distribution, movements, abundance, and natural mortality of pinniped prey also is required (Harwood and Croxall 1988}. Researchers, however, have determined the influence of marine mammal predation on local concentrations and assemblages of prey. Sea
otters (Enhydrus Iutris} reduced and effectively limited populations of urchins and abalone, which influenced the structure of nearshore kelp forest communities (Estes and Palmisano 1974). Predation by California sea lions (Zalophus californianus) was the principal factor affecting numbers of returning
adult wild winter-run steelhead (Oncorhyncl!us mykiss) at Lake Washington, Washington (Fraker 1994). Sea lions consumed more than half the entire steelhead run between 1986 and 1989, reducing the number of returning fish by as much as 70% (Scardino and Pfeifer 1993, Fraker 1994}. Continued predation on returning adult spawners probably will result in unrecoverable damage to this run (Scardino and Pfeifer 1993}. 3
One predator that has the potential to affect abundance and distribution of fish in estuaries is the Pacific harbor seal. Harbor seals are voracious predators, consuming approximately 2.5-5.0% of their body weight per day to meet energetic needs (Nordoy and Blix 1988). Although the harbor seal population in Lower Seal Lake, Quebec was small, it consumed the lake's potential yield of fish annually (Power and Gregoire 1978). Lake trout (Salvelinus namaycush) were smaller and their growth rate greater where seals occurred than in neighboring lakes. Fish longevity was reduced, and age at sexual maturity was halved, primarily because of harbor seal predation (Power and Gregoire 1978). Because harbor seals are abundant year-round residents in many estuaries (Bigg 1969), and require a great amount of biomass for food, they may be a primary source of natural mortality for some estuarine fishes. Harbor seals throughout the Pacific Northwest forage extensively in bays, rivers, and estuaries. Using fish hard parts (otoliths) recovered from feces to identify prey items, Jeffries (1986) found harbor seals in the Columbia River consumed eulachon (Thaleichthys pacificus) and northern anchovy (Engraulis mordax) almost exclusively when these fish schools entered the river to spawn. Eulachon also were eaten exclusively during runs in the Copper River Delta, Alaska (Pitcher 1977). Greatest harbor seal abundance in both of these rivers corresponded with eulachon runs. Most harbor seals exited the Columbia River after the run and moved to adjacent estuaries in response to prey availability and breeding activities (Jeffries 1986). Roffe and Mate (1984) estimated the impact of pinniped predation on salmonid stocks in the Rogue River, Oregon. Steelhead (Oncorhynchus 4
gairdneri) and chinook salmon (0. tslmWIJtscJza) were seasonally important food items of harbor seals. Increased abundance of harbor seals in the river during August and September coincided with the upriver migration of summer steelhead and fall chinook. Steelhead composed 4.4% of their diet, whereas chinook made up 5.6%. Less than 1.0% of the river's salmonid population were eaten by harbor seals and sea lions, indicating little Impact on that fishery. Predation on salmonids, however, may have been underestimated because seals frequently tear apart large prey and discard the head along with the otoliths (Scheffer and Slipp 1944). Brown and Mate (1983) reported harbor seals in Netarts Bay, Oregon fed in the bay and nearshore ocean. Estimated size of the most frequently occurring prey, Pacific sandlance (Ammodytes hexapterus) and English sole (Pleuronectes vetulus), indicated they were consumed in the bay. Harbor seal abundance peaked in October and November, which coincided with a return of chum salmon (Oncorhynchus keta). Harbor seals ate 6.1% of adult chum salmon returning in 1978, 7.2%, in 1979, and 1.5% in 1980. Tagged harbor seals moved between Netarts Bay and Tillamook Bay, probably in response to seasonal prey availability {Brown and Mate 1983). Harvey (1987) found harbor seals off Oregon fed primarily on fishes that inhabited estuaries and the nearshore ocean. Flatfishes comprised most of the annual biomass (1,626 metric tons), followed by Cottidae (764 metric tons), Clupeiformes (713 metric tons), Salmonidae (605 metric tons), Embiotocidae (487 metric tons), and Osmeridae (227 metric tons). Estimated annual biomass of fish taken by seals in Oregon during 1980 was 5,585 metric tons. Seals consumed approximately 22.3% of the commercial catch of 5
flatfishes, and 16.5% of salmonids, having a moderate impact on those fisheries. The five most important fish species, ranked according to greatest estimated biomass consumed, were Pacific staghorn sculpin (Leptocottus annatus; 721 metric tons), Pacific herring (Clupea pallasi; 452 metric tons), shiner surfperch (Cymatogaster aggregata; 441 metric tons) English sole (428 metric tons) and rex sole (Errex zacltirus; 333 metric tons). Radio-tagged seals spent most of their time in the bay in which they were caught, sometimes moving 9km up the estuary. Harvey (1987) also estimated that harbor seals were responsible for 5.5% of the natural mortality of English sole one to four years old. Although 74.3% of English Sole consumed were young-of-year individuals, the proportion of mortality attributed to seal predation was minimal because natural mortality was greatest during the first year. Despite the fact that several workshops indicated potential problems between increasing pinniped populations and fisheries in California (Mate 1980, ICUN 1981, Contos 1982, Beddington et al. 1985), few studies of harbor seal movements and food habits have been conducted off California (Bonnot 1951, Antonelis and Fiscus 1980, Bowlby 1981, Jones 1981, Allen et al. 1987, and Harvey et al. in press-a). Only two studies were conducted on harbor seals off central California (Jones 1981, Harvey et al. in press-a). Jones reported stomach contents of 12 seals stranded along the central California coast. Harvey et al. (in press-a) examined hard parts of prey found in mud and 30 fecal samples collected from haul-out sites at Elkhorn Slough in Monterey
Bay. Also in Elkhorn Slough, Osborne (1985) examined population dynamics of harbor seals, but few observations were made about their food habits. 6
Harbor seal abundance in many estuaries has increased since the enactment of the Marine Mammal Protection Act of 1972 (Harvey eta!. 1990, Hanan eta!. 1993). Although estuaries serve as spawning and nursery grounds for many commercially and recreationally important fish species (Krygier and Pearcy 1986), the impact of increased predation pressure on estuarine fish communities has not been studied in California. The objective of this study was to document abundance, movements, activity patterns, and food habits of harbor seals in Elkhorn Slough to determine how harbor seals interact with Elkhorn Slough and Monterey Bay prey populations. Another goal was to assess the effects of increased harbor seal predation on the fish assemblages of Elkhorn Slough. Preliminary reports of harbor seal food habits off central California conducted 15 years ago (Harvey et al. in press-a), along with previous descriptions of Elkhorn Slough ichthyofauna (Cailliet et a!. 1977, Yoklavich eta!. 1991) and harbor seal abundance (Osborne 1985), provided baseline information for comparison with data collected during this study.
OBJECTIVES The main objectives of this study were to; 1. determine abundance, activity and movement patterns of Pacific harbor seals in Elkhorn Slough,
2. determine the relative importance of prey items seasonally consumed by Pacific harbor seals in Elkhorn Slough, 3. estimate number, size, and biomass of each prey species consumed by harbor seals seasonally in Elkhorn Slough, and to compare these food habits to those described by Harvey et al. (in press-a) fifteen years ago, 7
4. describe the ichthyofauna of Elkhorn Slough and establish seasonal trends in prey availability and distribution to a) determine the proportion of the harbor seal diet which is comprised of prey taken in Elkhorn Slough, b) determine if seasonal food habits of harbor seals are correlated with seasonal prey availability, c) compare with previous descriptions of slough ichthyofauna (Cailliet et al. 1977, Yoklavich et al. 1991) to assess the potential impact of increased harbor seal predation on the fish community, and 5. determine how harbor seals utilize the Elkhorn Slough and Monterey Bay environments.
HYPOTHESES 1. Numbers of harbor seals in Elkhorn Slough should decrease during pupping season (May-June) as adults move to breeding areas along the coast of Carmel, California, and increase during July when seals move into the slough to use its protected haul-out locations for molting. 2. Abundance of harbor seals in Elkhorn Slough should be associated with presence of seasonally abundant prey. 3. Individual harbor seals should exhibit a great degree of site fidelity, and remain in Elkhorn Slough throughout the year because of their proximity to year-round prey resources in Elkhorn Slough and Monterey Bay. 4. Interchange of individual harbor seals between Elkhorn Slough and Monterey Bay in response to prey availability and breeding activity should occur. 8
5. Harbor seals are opportunistic carnivores, feeding upon seasonally abundant prey. Dominant prey items should be benthic fishes (staghorn sculpin and flatfishes) and schooiing fishes (osmerids and embiotodds). 6. Harbor seals should forage frequently within Elkhorn Slough and consume juvenile and adult fishes that use it seasonally as a nursery and/or spawning ground, such as English sole and embiotocids. 7. Number, size, and species composition of Elkhorn Slough ichthyofauna should change concurrently with an increase in the number of harbor seals in Elkhorn Slough so that fishes should be less abundant and less diverse currently than what inhabited Elkhorn Slough in the past, when fewer harbor seals inhabited Elkhorn Slough. 8. Food habits of harbor seals in Elkhorn Slough should have changed in the last 15 years, concurrent with the change in the slough's ichthyofauna, and their diet should reflect these faunal changes.
MATERIALS AND METHODS
STUDY AREA
Monterey Bay is an open embayment, approximately 37 km across (north to south) with an axial length (east to west) of 16 km. It is primarily shallow, 80% of the area is less than 180m depth (Dorfman 1991). The Monterey submarine canyon, the dominate feature of the bay, is more than
4,000 m deep. Elkhorn Slough is a shallow, tidal embayment and seasonal estuary at the head of the Monterey submarine canyon (Yoklavich et al. 1991; Fig. 1). It has an axial length of 10 km, with an average width of 100 m near 9
the entrance, narrowing to 15m furthest inland (Broenkow 1977). Depth of the main channel below mean lower low water is about 5 m at the slough entrance. Further inland, depth decreases to less than 2 m (Yoklavich et al. 1991). Extensive mudflats, exposed during lower tides, border the main channel. There are three major harbor seal haul-out sites in the slough located at Seal Bend, at the mudflats across from a dairy, and in north Moss Landing Harbor (Fig. 1).
SEASONAL ABUNDANCE OF HARBOR SEALS Harbor seal abundance in Elkhorn Slough was mocitored at all three haul-out sites by recording the number of individuals ashore on mud flats exposed at low tides. Surveys were a minimum estimate of harbor seal abundance because they did not include unobserved individuals in the water. Counts were made from shoreline or boat using binoculars. Number of seals ashore was recorded a minimum of twice per month. Differences in mean number observed among seasons was tested using an analyis of variance (ANOVA). Seasons were defined as autumn (August, September, October), winter (November, December, January), spring (February, March, April) and summer (May, June, July; Yoklavich et al. 1991). These seasons were used because they approximate the climatic seasonality of the area in terms of rainfall, air and water temperatures, and salinity (Yoklavich et al. 1991). Previous counts of harbor seals in Elkhorn Slough were obtained from Morejohn et al. (1978), Osborne (1985), and Hanan et al. (1993) and compared with my surveys. 10
HARBOR SEAL MOVEMENTS AND ACTIVITY P AITERNS Harbor seals were captured near their haul-out sites in Elkhorn Slough using methods described by Jeffries et al. (1993). Two outboard-powered boats were used to set a net approximately 120 min length and 8 min depth in the water adjacent to a haul-out site. Each end of the net was pulled ashore, entangling and capturing harbor seals in the process. Seals were removed from the net, placed headfirst into hoop nets, and physically restrained. Once secured, standard length (to the nearest 1 em), girth (to the nearest 1 ern), weight (to the nearest 1 kg), sex, and age class (adult, subadult) were determined. Female harbor seals weighing at least 45 kg, or measuring at least 136 ern, were classified as adults. Male seals weighing a minimum 64 kg, or measuring at least 144 ern, were classified as adults (Bigg 1969) Plastic cattle ear tags with black numerals were placed in the webbing of each hind flipper. Radio tags were attached to the dorsal pelage on the posterior portion of the head using an epoxy adhesive (Fedak et al. 1982, Harvey 1987, Jeffries et al. 1993). Each radio tag operated on a unique frequency, which allowed for individual identification. Tags emitted pulsed signals approximately every second at 164 to 165 MHz. Attachment and radio tags operated for eight to nine months. Harbor seals shed the tags during their annual molt in June. Placement of the radio tag on the head provided a strong and reliable signal each time the animal surfaced. Radio signals had a range of 3 to 7 km when monitored using land-based receivers. Day and night surveys of Elkhorn Slough and Monterey Bay were conducted monthly to locate radio-tagged harbor seals. Day surveys were 11
conducted betw·een 1000 and 1400 h when seals were usually ashore, whereas night surveys were conducted bel:\.veen 1800 and 2100 h. To determine activity patterns and movements of seals among haul-out sites in Monterey Bay, location, time, position of seal (i.e. on land or in water), and weather conditions were noted when each tagged individual was located. Movement and activity patterns of randomly selected radio-tagged seals were monitored twice a month for 24-hours (in two 12-hour intervals on consecutive days) from land. Locations were noted every 30 minutes and recorded on a map. When possible, the presence or absence of other tagged individuals in Elkhorn Slough also was noted. Duration of each emergence, dive, and time ashore was recorded to the nearest 1 s using a digital stop watch. Dives were defined as any cessation in signal reception for greater than five seconds. This eliminated brief artificial dives that are actually interruptions in the signal while the animal was at the surface (e.g. water breaking over the antenna, head tilted backwards with the antenna in water; Harvey 1987). When the tagged animal was not visible, and surface duration was greater than 15 minutes, the animal was considered to be on the haul-out site. Month, location, position, time of day, and tidal height also were noted during each activity phase so that daily and seasonal movements and activity patterns could be determined. For each individual, a Mann-Whitney U-test was used to determine if there was a significant difference in dive durations between bay and slough activities. A Cochran's Q test was used to determine
if the proportion of tagged seals ashore was different significantly for high and low tides. 12
ELKHORN SLOUGH ICHTHXOFAUNA: SEASONAL PREY AVAILABILITY Trawls were conducted monthly during day and night in Elkhorn Slough for one year to collect fishes to establish seasonal trends in prey availability and distribution. A 5-m otter trawl was towed behind a Boston Whaler at three locations in the main channel of the slough, approximately 0.6 km (Bridge station), 3.1 km (Dairies station), and 6.0 km (Kirby Park station) inland (Fig 1). Speed of tows was. approximately three to four knots and the distance towed between 0.4 and 0.6 km. Catch data were standardized to number of fish per ten-minute tow for seasonal and spatial comparisons. Fishes were identified, counted, standard length (SL) measured to the nearest mm, and either released or sacrificed for use in otolith length/fish length and fish length/ fish weight regression analyses. To determine minimum number of tows necessary to adequately describe species composition, cumulative number of species was plotted against randomly pooled number of tows. For day trawls, the Kirby Park station was chosen to best represent the slough environment, and May through October used because they had the greatest number of species, which would give a conservative estimate of sufficient sample number. The bridge station was chosen to represent night trawls for the same reasons. Abundance (mean number of fishes per species per 10-min tow), species richness (mean number of species per tow), and dominance (D =I Pi 2, where Pi is the proportional abundance of species i per tow) were evaluated for each location and season. 13
Species composition and abundance were compared among stations, seasons, day and night, and years (from Yoklavich et al. 1991) using the percent similarity index {PSI; Silver 1975): PSI = :E minimum PH P2i, where P!i and P2i are the relative abundances of species i from seasons "1" and "2", respectively. This index ranges from zero (no similarity) to 1.00 (identical species arrays). Percent similarity indices greater than 0.65 were considered a significant similarity. Abundance of individual species was compared among stations and seasons using a Kruskal-Wallis test.
FOOD HABIT ANALYSIS Food items of harbor seals using Elkhorn Slough were identified using prey hard parts recovered from scats and regurgitated material. Fecal samples were collected weekly for one year at haul-out sites used exclusively by harbor seals. Samples were placed in plastic bags and stored frozen. To recover hard parts, samples were thawed, placed in an emulsification mixture of detergent
and water, and washed with water through a series of three sieves (2 mm, 1 mm, and 0.5 mm; Murie and Lavigne, 1985). Fish otoliths and teeth were sorted, stored dry, and identified to the lowest taxon possible using reference specimens at Moss Landing Marine Laboratories (MLML). Cephalopod beaks were preserved in 50% isopropyl alcohol. Beaks were identified by Steve Osborne (MLML) and hagfish teeth were identified by Eric Johnson (MLML). To determine the minimum number of feces needed to adequately describe seasonal food habits of harbor seals, the cumulative number of species was plotted against the randomly pooled number of samples. 14
Summer months were used to assess adequacy of sample size because potential prey species increased in abundance and diversity at this time (Yoklavich et al. 1991, Oxman unpubl. data). This resulted in a conservative assessment of sufficient sample number. Prey species were enumerated using the greatest number of left or right otoliths or upper or lower cephalopod beaks. Otoliths recovered from scats were measured, using hand-held calipers, to the nearest 0.1 mm, parallel to the sulcus from the anterior tip of the rostrum to the posterior edge. Lower rostral and hood lengths of beaks were measured to the nearest 0.1 mm using a slide micrometer (Clarke 1962). For hagfish teeth, the width at the base of bicuspid and tricuspid teeth was measured with calipers. To compensate for degradation of otoliths during digestion, the length of each otolith was increased by a species-specific correction factor formulated by Harvey (1989). A correction factor of 27.5% was applied to those species not reported in the literature (Harvey 1989). Cephalopod beaks are not reduced in size during digestion (Harvey 1989). Because beaks and hagfish teeth are composed of similar material, it was assumed teeth also were not reduced during digestion. Estimated length of fish and cephalopods eaten by harbor seals was determined from measurements of otoliths and beaks collected from specimens caught in Elkhorn Slough. Fishes caught in trawls were identified, and standard length (SL) measured to the nearest mm. Otoliths were retrieved by cutting away the gill arches and removing the sagitta through the capsular otic bulla at the base of the skull (Cailliet et al. 1986). A least-squares linear regression was used to describe the relationship between otolith length 15
and fish length, for both left and right otoliths. Otolith length/ fish length regressions also were obtained from Harvey eta! (in press-b). To estimate prey length, the length of otoliths found in scats were used in the regression equations. This method was also applied to measurements of lower cephalopod beaks (Wolff 1982) and hagfish teeth. Lower beak length/dorsal mantle length regression equations for squid were obtained from Wolff (1982). Equations for Octopus were provided by Steve Osborne (MLML), and those for hagfish were obtained from Eric Johnson (MLML). Weights of prey species were obtained by using regression equations of length and weight of individuals collected from the slough and from Harvey eta!. (in press-b). Cephalopod dorsal mantle length/weight regressions were obtained from Steve Osborne (MLML), and hagfish from Eric Johnson (MLML). Because rockfish (Sebastes spp.) otoliths were difficult to identify to species, length and weight regressions for the most abundant rockfish species in the area (shortbelly; Sebastes jordani) were used. These regressions were obtained from Phillips (1964) and Echeverria (1987). Shortbelly rockfish are among the smallest rockfish species, providing conservative estimates of length and weight. Prey species collected from feces throughout the year were pooled according to season. Prey composition was compared among season and year using a PSI. Abundance of individual prey species was compared among seasons using a Kruskal-Wallis test. Data on seasonal prey species were compared with seasonal trawl data using a PSI. Prey length distributions were compared among seasons using a Kolmogorov-Smirnov test. 16
The following indices were used to describe the array of prey items seasonally consumed by harbor seals:
Species richness (S) = #of prey species
Shannon-Weaver diversity index: H' = I (Lpi lnpi) I Prey Eveness: J = H' I H'mall:, where H'max =InS Index of Specialization: R = 1 - J Prey Dominance: D = Lpi2 Indices were calculated for each fecal sample so that a mean could be generated and statistically compared among seasons using the Kruskal-Wallis test.
Importance of individual prey items was evaluated for each season using an individual index of relative importance (llU; Pinkas et al. 1971) given as:
IRI =(Mean %Number+ Mean %Mass) x %FO Although the original equation employs volume, mass is an acceptable substitute (Hyslop 1980). To determine the level of competition between fisheries and harbor seals for fish resources, estimates of annual commercial fishery catches were obtained from the California Department of Fish and Game and compared with estimates of annual fish consumption by harbor seals. The following equation was used to estimate annual consumption of a prey species;
Mean biomass (kg) of X Mean# seals in X it days = Biomass of sp. A sp. A/scat/season Elkhorn Slough per season eaten per season per season
L Biomass of sp. A eaten for each season = Annual biomass of sp. A consumed. 17
RESULTS
SEASONAL ABUNDANCE OF HARBOR SEALS Thirty-eight counts of harbor seals on haul-out sites in Elkhorn Slough conducted from January 1991 through Elecember 1991 indicated the number of seals ashore was greatest in July (mean=155.8, SE=12.9) and December (mean= 150.0, SE=2.9). Fewer harbor seals were observed in April (mean= 101.7, SE=14.3), October (mean=83.5, SE=7.9), and November (mean=97.0, SE=3.0; Fig. 2). There appeared to be a seasonal variation in abundance of harbor seals in Elkhorn Slough (Fig. 2), but because counts were variable, there was no significant difference in mean counts among months (Kruskal-Wallis;
H=14.87, P > 0.10) or seasons (ANOVA; F"'1.68, P = 0.19). Maximum counts of harbor seals in Elkhorn Slough during 1991 were five times greater than maximum counts from 1983 to 1987 {Fig. 3). Maximum counts increased 20% through one year, from 150 individuals during 1990 to 180 in 1991.
HARBOR SEAL MOVEMENTS AND ACTIVITY PATTERNS Ten subadult harbor seals were captured from 4 to 5 September, 1991 in Elkhorn Slough. Eight of these, five females and three males, were fitted with radio transmitters and released (Table 1). All tagged seals were observed on haul-out sites in Elkhorn Slough within one day of tagging, indicating effects of capture, handling, and tag attachment on behavior were probably minimal. Each tagged seal was successfully located each month until February. Most seals also were located in March, except #090 and #530. These 18
two individuals were not located during weekly surveys of Carmel, Monterey Bay, or Elkhorn Slough during March. Because tag life was an expected eight months and molting season had not begun, it was assumed that batteries were not operational, and that the animals had not emigrated. No tags were operating by May. During bi-weekly day and night monitoring of the Monterey Bay area, tagged seals were found in Elkhorn Slough during the day an average 94% of the time (Fig. 4), where they were observed ashore or resting in the water. They were not observed east of the dairy haul-out site. Harbor seals were usually found swimming and diving in Monterey Bay at night (mean of 90%; Fig. 4). Harbor seals used Elkhorn Slough diurnally during all months monitored (Table 2). During 24-hr monitoring, no radio-tagged seals were located in the slough between 2100 and 0315 hrs. While in the slough, individuals were ashore for a mean 3.6 to 7.9 hours per day. Harbor seals occasionally went ashore more than once per day when disturbed. The duration of a single haul-out bout averaged 3.6 to 6.6 hrs. Individuals were observed hauled-out on all monitored days. Due to the close proximity of haul-out areas to each other and the strength of the radio signal, it was difficult to discern at which site an individual had come ashore. Sixty-five percent of all haul-out bouts began between 1000 and 1330 hrs, and 59% of bouts ended between 1600 and 1830 hrs. No seals were found ashore between 2015 and 0330 hrs during 24-hr tracks. The proportion of tagged seals ashore was not different significantly for high and low tides (Cochran's Q test; Q=O, P
> 0.999); i.e. seals came ashore regardiess of tidal height. 19
Radio-tagged individuals spent an average 4.6 to 8.0 hours per day swimming and diving in Elkhorn Slough (Table 2). Seals visually observed diving in the slough were bottling (a resting behavior in which most of the seal's body remairu; submerged, except for the nose which pokes above the surface allowing the animal to breathe); moving between haul-out sites, or moving into or out of the slough. Harbor seals spent an average 11.8 to 15.7 hrs per day in Elkhorn Slough (Table 2). Tagged seals spent an average 8.3 to 12.2 hrs swimming and diving in Monterey Bay per night (Table 2). Individuals monitored for two or more 24- hour periods each spent significantly more time diving in Monterey Bay than Elkhorn Slough (Student's t-test; P's < 0.04), with one exception. There was no difference in amount of time spent diving in the bay and slough for seal #530 (Student's t; P = 0.12). This seal was visually observed bottling in Elkhorn Slough for most of the day during one of its observation periods. Although statistical comparisons could not be made among individuals tracked for one 24-hour period, they exhibited similar activity patterns (Table 2). Eighty-two percent of the time raclio-tagged harbor seals departed Elkhorn Slough between 1700 and 2000 hrs. All harbor seals monitored for 24-hr periods spent their nights exclusively in nearshore bay waters, within 0.5 to 7 km of shore (Fig. 5). All individuals initially moved 1.25 km south of the slough entrance, off Moss Landing. Harbor seals remained in this area the entire evening to conduct localized clives 88% of the time, whereas 12% of the time individuals moved north and dove off the Pajaro River mouth (4 km north of Elkhorn Slough) and Suru;et Beach, Santa Cruz (7.25 km north of 20
slough; Fig 5). Similarly, during bi-weekly day and night monitoring of the Monterey Bay area, three seals were occasionally found to the north at night. Harbor seals #090, #250, and #300 were found off Sunset Beach 14%, 44%, and 27% of the time, respectively. Signals received from these individuals were frequently weak and intermittent. They were probably at the limit of the receiver's range and thus could not always be monitored continuously. Harbor seals did not haul-out while in the bay. Harbor seals monitored for 24-hrs always returned to Elkhorn Slough the following morning to haul out or rest in the water. Seventy-seven percent of the time harbor seals returned to the slough between 0515 and 0915 h. None were absent from the slough for more than 16.2 hours. This daily activity pattern of nighttime activity in Monterey Bay and daytime resting in Elkhorn Slough remained unchanged among individuals throughout the monitoring period (Sept.-April; Fig. 6a-b). There were no significant changes among seasons studied in mean time spent hauled-out (ANOVA; F=1.17, P = 0.382), mean time swimming and diving in Elkhorn Slough (ANOVA; F=0.945, P = 0.449), and mean time in Monterey Bay (ANOVA; F=1.47, P = 0.315; Fig. 7). Mean duration of dives for individual harbor seals was 1.0 min (SE=0.1) to 3.7 min (SE=0.4) for dives conducted in Elkhorn Slough, and from 2.8 min (SE=0.6) to 8.2 min (SE=0.7) for dives in Monterey Bay (Table 3). The dive of greatest duration was 16.6 min in the bay. Every month, each tagged harbor seal conducted dives in Monterey Bay that were of significantly greater duration than those performed in Elkhorn Slough (Mann-Whitney U-test, P < 0.005), with two exceptions. Mean duration of dives in the slough was 21
the same as those conducted in the bay for seal #090 in September, and seal #222 in April (Mann-Whitney U test, P > 0.05). These animals, however, were bottling when haul-out sites were unavailable due to extremely high tides (September) and disturbance (April). Average duration at surface for different tagged harbor seals was 0.4 min (SE=0.03) to 0.6 min (SE=0.05) in Elkhorn Slough, and from 0.2 min (SE=0.06) to 0.6 min (SE=0.02) in Monterey Bay (Table 3).
PREY AVAILABILITY Eighty-three otter trawls conducted during daytime in 1991 at three stations in the main channel of Elkhorn Slough resulted in the capture of 1,955 fish representing 41 species (Table 4). Ten species accounted for more than 90% of individual fishes taken in trawls throughout the study period. Relative abundance of these 10 species ranged from 1.6% for lingcod (Ophiodon elongatus; found only at bridge and dairy stations) and bay pipefish (Syngnathus leptorhynclms; found at all stations) to 31.3% for shiner surfperch (Cymatogaster aggregata), which were abundant at all stations, especially Kirby Park. Cumulative species curves indicated approximately 10 tows were needed to adequately describe the ichthyofauna of Elkhorn Slough, and 7 to 10 tows were required to assess at least the dominant species of fishes within the slough (Fig. 8). Because six to nine tows were conducted at each station per season, a comparison of the overall fish species composition could be made to detect temporal differences among stations with regard to dominant species. 22
Greatest abundance and species diversity occurred at Kirby Park, the station furthest from the slough entrance, whereas the Dairy station had the lowest abundance and species diversity (Table 5). There were no significant differences in abundance among stations (Kxuskal-Wallis, H=4.99, P=0.082) because of variation in catch rate. Species composition at the dairy station was somewhat similar to that at the bridge {PSI=0.56) and Kirby Park (PSI=0.59) stations, due largely to the presence of numerically dominant shiner surfperch, speckled sanddab (Citharichthys stigmaeus), and English sole (Pleuronectes vetulus; Fig. 9). Fish assemblages at bridge and Kirby Park were not similar (PSI=0.31; Fig. 9). Relative abundance of the dominant species varied with distance inland (Table 6). Black surfperch (Embiotoca jacksoni) and cabezon (Scorpaenichthys marmoratus), among the dominant species at bridge station, were significantly less abundant at eastern stations (Kxuskal-Wallis; H=l4.77 ·" and H= 30.28, P=O.OOl and P=O.OOO, respectively). Lingcod, caught with equal frequency at both bridge and dairy stations (Mann-Whitney U-test, P=l.OO), were not found at Kirby Park. Although staghorn sculpin (Leptocottus armatus) and California tonguefish (Symphurus atricauda) were caught at all stations, they were significantly more abundant at Kirby Park station (Kruskal-Wallis; H=l2 and H=l7.77, P=0.002 and P=O.OOO, respectively). Abundance of speckled sanddab and white surfperch (Phanerodon furcatus), present at all stations, did not differ significantly among locations (Kruskal Wallis; H-=1.068 and H=0.166, P=0.586 and P=0.920, respectively). Shiner surfperch and English sole, the two most numerically dominant species in Elkhorn Slough, were found in similar abundance at all stations (Kruskal- 23
Wallis; H=5.792 and H=0.5.173, P=0.055 and P=0.075, respectively), although both tended to be more numerous east of the bridge station. Seasonal variation in fish abundance was great. Fewer fish were caught at all stations during winter and spring. Greatest abundance occurred in summer (Fig. 10). This summer peak was significantly greater than other seasons (Kruskal-Wallis, H=24.419, P stations. Lingcod were only taken during summer at bridge and dairy stations (Fig. 11). The remainder of the slough's dominant species were present throughout the year. Staghorn sculpin and English sole (Fig. 11), were most abundant during summer (Kruskal-Wallis; H=8.667 and H=27.841, P=0.034 and P=O.OOO, respectively), especially at Kirby Park. Black surfperch, cabezon, and speckled sanddab (Fig. 11) exhibited no significant difference in seasonal abundance (Kruskal-Wallis; all P's > 0.09), though all tended to be more abundant at the bridge station during autumn. Twenty-nine otter trawls conducted at night during 1991 at two stations (bridge and dairy) in Elkhorn Slough resulted in the capture of 1,461 fish representing 39 species. Ten species accounted for more than 90% of all fishes taken in nighttime trawls (Table 8). Relative abundance of these ten species ranged from 1.6% for California tonguefish to 23.2% for speckled sanddab. The cumulative species curve for night trawls conducted at bridge station indicated more than 10 tows were needed to adequately describe temporal variation in species composition, abundance, and distribution at night (Fig. 12). Because no trawls were conducted during winter, and as few as 3 tows were conducted at the dairy station in subsequent seasons, comparisons could not be made among seasons to detect temporal changes in species composition. Greatest abundance and species diversity at night occurred at the bridge station (Table 5). There was no significant difference in abundance between stations (Mann-Whitney U-test, P=0.55). Fish assemblages at the bridge station were marginally similar to those at dairy (PS1=0.61; Fig. 13). This was 25 due primarily to species of gobies, embiotocids, hexagrammids, liparids, and pleuronectids that were not found east of the bridge station. Most dominant species were found in equal abundance at both stations. Abundance of speckled sanddab, shiner surfperch, staghorn sculpin, English sole, white surfperch, cabezon1 and California tonguefish did not differ significantly between locations (Mann-Whitney U-test, all P > 0.08). Black surfperch, however, were significantly more abundant at the bridge station than dairy (Mann-Whitney U-test, P=0.046). Nocturnal fish assemblages at the bridge station were similar to those caught there during daytime (PSI=0.83; Fig. 14), whereas species caught during night trawls at the dairy station were marginally different than those present there diurnally (PSI=0.63; Fig. 15). Diurnal differences were due to the presence of several species, including yellowfin goby (Acanthogobius flavimanus), tubesnout (Aulorhynchus flavidus), spotted cusk-eel (Chilara taylori) 1 Pacific herring (Clupea pallasi), Kelp greenling (Hexagrammos decagrammos), rainbow surfperch (Hypsurus caryi), starry flounder (Platichthys stellatus), C-0 turbot (Pleuronichthys coenosus), plainfin midshipman (Porichthys notatus), rubberlip surfperch (Rhacochilis toxotes), rockfish (Sebastes spp.), and queenfish (Seriplms politus), which were absent or caught in minimal numbers during day trawls. Significantly greater densities of fish were caught at night at both stations (Mann-Whitney U-test, P's<0.013; Table 5), probably because fish could not avoid the net as well in darkness. The overall diurnal fish assemblage in Elkhorn Slough during 1991 was slightly different from that found in the slough during 1974- 1976 as reported by Yoklavich et al. (1991: P51=0.61; Fig. 16). Mean number of fish per 26 tow decreased 45 to 92% at all stations between years (Table 5). Species diversity decreased by 32.4% at the bridge station and by 40.5% at the dairy station, whereas diversity increased by 13% at Kirby Park. Species previously caught in daytime trawls in Elkhorn Slough in 1974 - 1980, but absent from daytime trawls in 1991 included; topsmelt (Atlterinops affinis), jacksmelt (Atherinopsis californiensis), Pacific herring, threadfin shad (Dorosoma petenense), sand sole (Psettichthys melanosticus), blue rockfish (Sebastes mystinus), queenfish, and night smelt (Spirinchus starksi). Several species, such as shiner surfperch, black surfperch, white surfperch, staghorn sculpin, northern anchovy (Engraulis mordax), and starry flounder, were less abundant in 1991 than 1974- 1980. Four species increased in relative abundance and density: English sole, cabezon, lingcod, and California tonguefish (Fig. 16). Fish assemblages at bridge and dairy station have changed significantly during the past 11 years (PSI=0.52 and 0.46, respectively, Figs. 17-18). Northern anchovy, Pacific herring, plainfin midshipman, rubberlip surfperch, night smelt, and several species of atherinids, gobiids, rockfish, elasmobranchs, and pleuronectids captured in 1974-1980 tows at bridge and dairy stations were absent from 1991 trawls. Considerably fewer shiner surfperch, black surfperch, white surfperch, and starry flounder were caught at these stations during 1991, whereas speckled sanddab, staghorn sculpin, English sole, lingcod, and California tonguefish increased in relative abundance and density (Table 6; Figs. 17-18). Species composition at Kirby Park has remained relatively unchanged (PSI=0.72; Fig. 19). 27 FOOD HABITS Of 325 fecal samples collected in Elkhorn Slough during 1991, 94.2% (306) contained identifiable prey hard parts. Feces contained two to five prey species (mean=3.41, SE=0.13), but occasionally had as many as twelve. Forty taxa were identified to species, and seven to genus. Only 0.9% of otoliths were not identifiable. Of 4,904 individual prey occurrences, 60.7% (2,979) were cephalopods and 39.3% (1,925) were fishes. Octopus (Octopus sp.) was the most abundant cephalopod (79.4%), and market squid (Loligo opalescens) was the only other cephalopod eaten (20.6%). Fishes consumed by harbor seals were predominantly flatfishes (Pleuronectidiae and Bothidae; 26.2%), cusk-eels (Ophidiidae; 19%), and rockfishes (Scorpaenidae; 16%). Cumulative species curves indicated approximately 40 samples were required to adequately assess number of species eaten (Fig. 20). Because 64 scats were collected during winter, 70 in spring, 86 during summer, and 86 in autumn, enough samples were collected for comparing prey species composition among seasons to ascertain temporal variations in diet. Although mean number of species (S) found per scat was greatest during spring (mean=3.614, SE=0.284), and least in winter (mean=3.25, SE=0.201; Fig. 21), there was no significant difference in mean number of species found per scat among seasons (Kruskal-Wallis; P=0.652). The most diverse array of prey species was consumed during summer (H: mean = 0.82, SE = 0.068; Fig. 21), therefore, summer had the least specialization index (R: mean=0.355, SE=0.044) and dominance index (D: mean=0.559, SE = 0.032; Fig. 21). These differences, however, were not significant among seasons (Kruskal-Wallis; P's > 0.115). 28 Twenty-seven taxa of prey were identified in feces collected during winter (Table 9). Octopus sp., the most abundant and frequent prey item, had a considerably greater mean IRI value than that of market squid, the second most abundant prey species. Although octopus beaks could not be positively identified to species, they were believed to be 0. rubesce:ns (S. Osborne, pers comm.) Spotted cusk-eel was the most important fish species consumed, ranking third overalL Night smelt, staghorn sculpin, and plainfin midshipman also contributed significantly to harbor seal diet during winter. Twenty-six taxa were identified in feces collected during spring (Table 10). Octopus sp. remained the most important prey item, whereas market squid ranked fourth. Spotted cusk-eel and Pacific sanddab (Citharichthys sordidus) were the principal fishes consumed, with IRI rankings of second and third, respectively. Other important prey included Dover sole (Microstomus pacificus), hagfish (Eptatretus sp.), rex sole (Errex zacltirus), plainfm midshipman, and English sole (Table 10). More species were identified in feces collected during summer than in any other season (38 taxa; Table 11). Rockfishes had the greatest mean IRI rank, followed by white croaker (Ge:nyonemus lineatus), Dover sole, spotted cusk-eet plainfin midshipman, and English sole. Cephalopods were considerably less important in summer than in other seasons. Octopus sp. ranked seventh and market squid ranked ninth. Several taxa were observed only in feces collected during summer. These included queenfish (Seriphus polit11s), striped bass (Roccus saxatilis), unidentified surfperch (Embiotocidae), pile surfperch (Damalichthys vacca), walleye surfperch (Hyperprosopon 29 argenteum), pink surfperch (Zalembius rosaceous), Pacific argentine (Argentina sialis), bay goby (Lepidogobius lepidus), and Pacific butterfish (Peprilis simillimus}. Diversity of prey items recovered from feces was less in autumn than summer (29 taxa; Table 12}. The greatest mean IRI value was for Octopus sp., and fish continued to be relatively important prey. Spotted cusk-eel ranked second in relative importance, with an IRI value considerably greater than that of third ranked white croaker. Other important fishes included plainfin midshipman, rockfish, English sole, and staghorn sculpin. Market squid ranked eighth. Percent similarity indices indicated harbor seals consumed similar species during autumn, winter, and spring (PSI's > 0.71; Table 13}. Prey composition, however, changed significantly during summer (PSI's < 0.38; Fig. 22, Table 13) because of the increased importance of pelagic species, and decreased importance of cephalopods in seal diet at that time. The abundance of 13 prey species varied significantly in the harbor seal diet among seasons. Several prey species were significantly more abundant during summer; rockfishes (Kruskal-Wallis; P=O.OOO}, Pacific herring (Kruskal-Wallis; P=O.OOO), and shiner surfperch (Kruskal-Wallis; P=0.007). Number of octopus and spotted cusk-eel in the diet decreased significantly during summer (Kruskal-Wallis; P=O.OOO and 0.004, respectively). Rex sole and Pacific sanddab were eaten more during spring (Kruskal-Wallis; P's=O.OOl). Although Pacific hake (Merluccius productus} was found in relatively equal abundance during spring, summer, and autumn (Kruskal Wallis; P=0.126}, no hake otoliths were recovered during winter, whereas number of squid consumed in winter increased significantly (Kruskal-Wallis; 30 P<0.001). Similar numbers of night smelt were found during winter, spring, and summer (Kruskal-Wallis; P=0.187), but none were present in feces collected during autumn. There were significant differences in the number of staghorn sculpin, Dover sole, and white croaker otoliths recovered among seasons (Kruskal Wallis tests). Nonparametric Tukey-type multiple comparisons, however, indicated no significant differences; therefore, the following differences were marginal. More staghorn sculpin were recovered in autumn and winter (Kruskal-Wallis; P=0.03), whereas the abundance of Dover sole in feces increased during spring and summer (Kruskal-Wallis; P=0.012). Slightly more white croaker were eaten during summer and autumn (Kruskal-Wallis; P=0.017). Mean estimated length of fish consumed by harbor seals in Elkhorn Slough was 17.99 em (SD=6.49). Only the following species were sufficently abundant in samples throughout the year to compare temporal changes in length of fish eaten. Harbor seals ate spotted cusk-eel with a mean standard length of 21.32 em (SD=4.08). Size distribution during spring was significantly greater than during winter (Kolmogorov-Srnirnov (KS) test: P=0.039; Fig. 23). Night smelt (mean=11.87 em, SD=1.55) consumed in summer were significantly larger than those eaten during other seasons (KS test: P larger in winter than autumn (KS test: P northern anchovy and Pacific herring, occurred so infrequently in trawls (overall relative abundances < 0.2%) that it was unlikely seals caught them in the slough. Although white surfperch, pile surfperch, walleye surfperch, speckled sanddab, and California tonguefish were abundant in the slough, they were uncommon in feces (overall relative abundances < 0.2%). Only seven species were abundant in both feces and trawls (Fig. 33). The estimated standard lengths of plainfin midshipman, rockfish, English sole, Pacific staghorn sculpin, lingcod, and shiner surfperch eaten by harbor seals were significantly larger than those caught in trawls in Elkhorn Slough (Mann-Whitney U-test; P's<0.001; Fig. 33). Only sizes of spotted cusk-eel consumed by seals were similar to those found in Elkhorn slough (t-test; P=0.23). Prey species consumed by harbor seals during 1991 were significantly different from species reported by Harvey et al. (in press-a; PSI= 0.36; Fig. 34). Thls was due primarily to increased relative abundances of octopus and market squid in 1991, decreased relative abundances of white croaker, Dover sole, and shiner surfperch, and the absence of topsmelt. Harvey et al. (in press-a) identified 33 prey species between 1975 and 1977, whereas 47 species were identified during 1991. Prey not previously described for harbor seals using Elkhorn Slough included Market squid, hagfish, lamprey (Lampetra sp.), basketweave cusk-eel (Otophidium scrippsi), Pacific argentine, bay goby, Pacific butterfish, queenfish, jack mackerel (Trachurus S1J11lmetricus), striped bass, lingcod, shortspine thornyhead (Sebastolobus alascanus), redtail surfperch (Amphistichus rhodoterus ), silver surfperch, pile surfperch, slender sole (Eopsetta exilis), sand sole (Psettichtlrys melanostictus), and California tongueflsh. 33 DISCUSSION SEASONAL ABUNDANCE Harbor seals along North America's Pacific coast have been increasing since the Marine Mammal Protection Act (MMPA} was passed in 1972. California's harbor seal population has an estimated annual growth rate of 14.7% (Boveng 1988). During 1992, the annual survey by the California Department of Fish and Game recorded more harbor seals per site than the previous year, as well as 86 new haul-out sites (Hanan et al. 1993). The Elkhorn Slough harbor seal population exhibited similar trends. Harbor seals, which occasionally occupied a single haul-out site in 1975 (Harvey et al. in press-a), were found consistently in considerably greater numbers at three new locations in Elkhorn Slough during 1991. This increased abundance and expansion of seals may be the result of recruitment, redistribution, immigration, or a combination of these factors. The preponderance of immature seals and lack of pupping in Elkhorn Slough (Osborne 1985, Osborne 1992) indicated that individuals were recruited from outside. Several studies have documented increases in local harbor seal populations due to the dispersal of juvenile harbor seals from pupping areas (Bonner and Whitthames 1974, Boulva and Maclaren 1979, Reijnders 1983). Approximately 80% of seals in Elkhorn Slough are immatures (Osborne 1985, Osborne 1992). Tar on 22% of weaners in the slough indicated they came from the coast, probably Seal Rock or Cypress Point, Carmel (Osborne 1992). A pup tagged and released at Cypress Point during 1991 arrived in Elkhorn Slough 20 days later (Osborne 1992). Before 34 1989, there were no records of births in the slough. Between 1989 and 1991, only seven pups were recorded (Oxman and Trumble, unpubl. data). The MMP A of 1972, which significantly reduced harassment and killing of harbor seals, has probably resulted in increased abundance and redistribution of seals into bays and estuaries Qeffries 1986, Harvey et al. 1990). Rapid population growth of seals after passage of protective legislation has been observed previously (Hewer 1974, Bonner 1975, Payne and Schneider 1984). Greater numbers of harbor seals at primary haul-out sites would increase competition among seals for space, forcing some individuals to move elsewhere and establish new haul-out and pupping locations. A survey conducted off Oregon from 1975 to 1983 indicated abundance of harbor seals and haul-out sites within bays and estuaries had increased after the enactment of the MMPA (Harvey et al. 1990). Jeffries (1986) observed similar trends in Washington. Elkhorn Slough probably provided a good alternative • to crowded coastal haul-out sites because of its numerous mudflats and minimal disturbance. Newby (1973) suggested harbor seal abundance remained low before 1972 because they were harassed and driven from bays and estuaries. Seals often use these protected regions to give birth and care for pups (Brown and Mate 1983, Beach et al. 1985). Frequent disturbance in such areas vital to seal reproduction would have effectively lowered reproductive rate and pup survival by causing mother/pup separation and site abandonment. Conversely, decreased harassment could result in movement of individuals back into these systems and increased reproductive success (Harvey et al. 35 1990). Irrunigration and decreased harassment could thus account for much of the growth observed in the Elkhorn Slough population. Seasonal fluctuations in harbor seal abundance has been associated with changes in site use, reproductive status, molt, and prey availability (Allen et al. 1984, Allen et al. 1987, Thompson 1989). Although the number of seals at haul-out sites in Elkhorn Slough fluctuated dramatically day to day, trends in abundance were observed. Decreased abundance of seals during April and May was probably due to the onset of the pupping/breeding season (April to June), which peaks in May along the central California coast. Because harbor seals move between feeding grounds and seasonal breeding areas (Vaughan 1978, Brown and Mate 1983, Jeffries 1985), absence of many adults and subadults, especially females, from Elkhorn Slough at this time (Osborne 1985) indicated harbor seals had moved to pupping and breeding areas 25 km to the south at Cypress Point, Carmel, and 60 km north to Afio Nuevo Island. Osborne (1992) noted the departure of females, including 14 pregnant individuals, from Elkhorn Slough just before pupping. One of these females was subsequently observed in May at Cypress Point with a pup. Because radio transmitters were not operating by May, movements to these primary pupping locations could not be confirmed. Peak abundance of harbor seals in July in Elkhorn Slough corresponds with their annual molt. This peak was probably caused in part by an influx of seals from nearby pupping areas. Earlier researchers, however, found that numbers peaked in March and were at a minimum during July (Morejohn et al. 1978, Osborne 1985). Harbor seals congregate at haul-out sites during molt, 36 especially those with little disturbance (Morejohn et aL 1978, Everitt et al. 1981, Thompson 1989). Osborne (1985) reported frequent disturbance of harbor seals in Elkhorn Slough by recreational boaters and fishermen, which may have kept seal abundance at a minimum. Consequently, the California Department of Fish and Game minimized disturbance by placing barriers and signs around the Seal Bend haul-out, advertising the presence of harbor seals. The resulting decrease in harassment has probably encouraged more seals to use Elkhorn Slough for molt and pupping. Counts during molt may indicate the degree of disturbance at a haul-out. An increase in seal abundance during summer also coincides with a marked increase in fish diversity and abundance in Elkhorn Slough and Monterey Bay. Brown and Mate (1983) observed that peak abundance of harbor seals in Netarts Bay, Oregon in autumn coincided with seasonal abundance of chum salmon. Number of seals in Bolinas Lagoon, California also increased during summer concurrent with an increase in fish abundance (Allen et al. 1984). Movement into Elkhorn Slough during a period of great prey availability is probably a consequence of the harbor seal's opportunistic foraging strategy. MOVEMENT AND ACTIVITY Elkhorn Slough's relatively isolated mudflats provided excellent habitat for resting harbor seals. Diurnal haul-out patterns exhibited by harbor seals in Elkhorn Slough were consistent with previous studies (Boulva and MacLaren 1979, Stewart 1984, Allen et al. 1987, Yochem eta!. 1987). Unlike these reports, however, harbor seals were not observed ashore nocturnally in 37 Elkhorn Slough. Thompson et al. (1989) suggested such patterns were due to the increased availability of prey at night. Localized dives of great duration were conducted by harbor seals at night in Monterey Bay which indicated seals were foraging nocturnally. Dives in Monterey Bay were of greater "duration than dives in Elkhorn Slough probably because waters are deeper in Monterey Bay, and seals were foraging. Prey items were predominantly species active at night, which did not appear in day trawls conducted in Elkhorn Slough. Harbor seals diving in the slough during the day were observed bottling, moving between haul-out sites, or moving into or out of the slough. Nighttime activity of harbor seals has been well documented (Spalding 1964, Allen et al. 1987, Thompson 1989), and several researchers believe it is indicative of nocturnal feeding (Boulva and MacLaren 1979, Antonelis and Fiscus 1980, Thompson et al. 1989). Influence of tidal cycle on haul-out behavior differs significantly among geographic locations. Haul-out behavior is strongly affected by tide in areas where site availability is tide related (Calambokidis et al. 1978, Sullivan 1980, Schneider and Payne 1983, Allen et al. 1984). In estuaries, such as Elkhorn Slough, where haul-out space is accessible at all but the highest tides (> 6.0 ft), tidal rhythm does not affect behavior (Stewart 1984). Wilson (1978) and Thompson et al. (1989) hypothesized that where supralittoral haul-out space Is available, diurnal cycles may have greater influence than tidal cycles on haul-out patterns. The consistent reduction in numbers during late afternoon at slough haul-outs, and at similar sites in eastern Canada and San Miguel Island, California (Boulva and MacLaren 1979, Stewart 1984), tend to support their hypothesis. 38 The hlgh degree of site fidelity of radio-tagged harbor seals for Elkhorn Slough also has been observed elsewhere (Divinyi 1971, Knudtson 1974, Reijnders 1976, Boulva and MacLaren 1979, Stewart and Yochem 1983). Resident harbor seals at Drakes Estero, California spent two to 10 days foraging at sea, but always returned to the estuary for rest (Allen et al. 1987). Harbor seals from Eynhallow Sound, Scotland also returned to the same haul-out sites after spending extended periods at sea (Thompson et al. 1989). In Elkhorn Slough, however, seals were never absent for more than 17 hr, and always came ashore after an evening in the bay. Because Elkhorn Slough provides short-range access(< 1 km) to productive bay waters, long foraging trips probably were not necessary. Returning inshore between each feeding bout to haul-out was energetically feasible. Coming ashore is important for sleep and rest (Schnieder et al. 1980), predator avoidance (Terhune 1985), and skin growth and maintenance (Feltz and Fay 1966). Time ashore also could be important because immersion in seawater is energetically costly (Watts 1992). Seals rely on blubber and increased metabolism instead of pelage for insulation (Harrison and Kooyman 1968). Reduced metabolism out of water may increase the ability of small endotherms to survive with minimum amounts of blubber (Irving and Hart 1957, Schnieder and Payne 1983). Because tag frequencies were not monitored continuously, it is unknown if seals actually came ashore everyday. Thls observation may have been an artifact of the sampling method, because tagged individuals were not always found in Elkhorn Slough during day surveys. Harbor seals in Elkhorn Slough also spent less time ashore per day (approximately 25%) than seals in northern California (39 to 44%; Sullivan 39 1979, Stewart and Yochem 1983) and at San Miguel Island (37%; Yochem et al. 1987). These differences may be the result of habitat influences on behavior, age and sex composition of sample, or season of data collection. At Drakes Estero, California, harbor seals spent approximately the same amount of time ashore per day as seals in Elkhorn Slough (Allen et aL 1987). Researchers have attributed differences in time ashore to different tidal influences and disturbances. In Elkhorn Slough, however, site availability is not tidally mitigated and disturbance was infrequent. Thus, the lesser time on haul-out sites was either real or a consequence of sampling bias. Because movement and activity patterns may differ among age groups, radio-telemetry data may have been biased because adult seals were not tagged. Although harbor seals are generally considered to be site-specific with limited local movements (Scheffer and Slipp 1944, Stoel1981, Bigg 1981, Stewart and Yochem 1983, Brown and Mate 1983, Jeffries 1986, Harvey 1987), juveniles generally move greater distances compared with adults. All subadult males and 90% of subadult females radio-tagged in the Columbia River, Washington were resighted in other areas, whereas adults exhibited considerably less movement and greater site fidelity (Jeffries 1986). Dispersal of juveniles up to 250 km from large pupping areas was documented by Bonner and Whitthames (1974} and Boulva and MacLaren (1979}. Loss of marked seals due to dispersal was noted in Eynhallow Sound, Scotland (Thompson 1989). In Elkhorn Slough, however, movements of juveniles were highly localized. The large proportion of juveniles in the slough indicated a lack of dispersion. Young harbor seals may emigrate to Elkhorn 40 Slough from pupping areas and haul-out sites that have reached carrying capacity. Minor seasonal fluctuations of harbor seal abundance in Elkhorn Slough may have been the result of movements by the smaller, adult portion of the population. Changes in reproductive status in adult seals may lead to changes in site use. In the Columbia River, seal abundance decreased during pupping season, whereas numbers increased at known pupping locations in adjacent Oregon and Washington estuaries (Brown 1986, Jeffries 1986). Allen et al. (1987) found that most long range movements (> 25 km) of harbor seals at Drakes Estero, California occurred before the breeding season to established pupping locations. At Cypress Point, the pupping area nearest Elkhorn Slough, harbor seal abundance increased significantly during breeding season (Trumble unpubl. data}. If only adult harbor seals in Elkhorn Slough exhibit seasonal changes in site use, it would help explain the lack of significant temporal variation in seal abundance in Elkhorn Slough. Information on movements and activity is incomplete because transmitters did not operate during summer, when changes in activity patterns of harbor seals would have most likely occurred. A combination of factors, including breeding season, annual molt, and increased prey availability during summer, may result in less time spent foraging in Monterey Bay and more time spent resting and hauled-out in Elkhorn Slough. During the breeding season, females stay ashore for pupping and lactation (Thompson 1989}, and males may attempt to remain near females for copulation (Thompson eta!. 1989). During molt, seals spend greater periods ashore (Stewart 1984, Allen et al. 1984, Thompson 1989} because the 41 elevated skin temperature may allow seals to molt more rapidly (Ling and Bryden 1981). Such behavior would restrict individuals to shorter feeding trips. Consequently, breeding and molt coincide with periods of increased prey availability, thus min.imi.zing the physical impact of any reduction in time spent foraging. Changes in harbor seal diet during summer also suggests changes in activity patterns. Dive behavior of seals depends partially on prey abundance, depth of prey, and differences in foraging strategies among age groups. These factors probably vary temporally and individually. Increased consumption of pelagic prey species during summer could result in decreased dive durations. Although molting coincided with the movement of schooling fishes into Elkhorn Slough, relatively few slough fishes were consumed at that time. Harbor seals were apparently attracted to Elkhorn Slough more for its isolated haul-out sites and proximity to increased prey concentrations offshore, rather than for its fish resources. Because harbor seal movements were localized, changes in haul-out behavior probably accounted for much of the fluctuation in seal numbers observed in Elkhorn Slough. Increased time spent ashore during molt would result in elevated counts. Seasonal peaks in onshore abundance of seals during molt has been described previously (Yochem eta!. 1987, Boveng 1988, Hanan and Beeson 1993). Reduced hauling frequency may have accounted for the decline in counts of harbor seals during winter. Decreased abundance of seals at other haul-out sites along California in autumn and winter has been partially attributed to a reduction in time spent ashore (Stewart and Yochem 1983, Allen eta!. 1987, Harvey 1987). In winter, prey availability is 42 probably greatly reduced, especially in estuaries, and seals may require more time to forage. Harbor seals at Drakes Estero, California spent less time hauled-out between November and February, than between August and October when prey abundance was greatest (Allen et al. 1987). Although movements of harbor seals among haul-out sites in response to seasonal changes in prey availability has been responsible for fluctuations in seal abundance elsewhere (Brown and Mate 1983, Herder 1986, Jeffries 1986), there was no evidence harbor seals in Elkhorn Slough switched haul-out locations to be closer to feeding areas, or that seasonal changes in abundance resulted from individual changes in site use. FOOD HABITS The limitations involved with using prey hard parts recovered from feces to investigate pinniped food habits have been well described (Pitcher 1980, Hawes 1983, DaSilva and Neilson 1985, Jobling 1987, Dellinger and Trillrnich 1988, Harvey 1989). In the context of this study, the greatest source of bias probably resulted from effects of digestion on otolith recovery rates. Several captive feeding experiments have indicated that, because the probability of complete otolith dissolution is dependent on otolith size and thickness, prey species with small, fragile otoliths that are easily dissolved are probably under-represented in feces relative to fishes with larger, more robust otoliths (Prime 1979, Hawes 1983, Murie and Lavigne 1986, Jobling 1987, Delinger and Trillrnich 1988, Harvey 1989). Increased digestion of otoliths, however, may occur in captive pinnipeds because confined, less active individuals may have reduced passage rates of material through the digestive 43 system, increasing time during which otoliths would be exposed to digestive fluids (Grunewald and Tucker 1985, Dellinger and Trillmich 1988, Harvey 1989). Therefore, recovery of otoliths in the field may be more representative of diversity and number of fish eaten than predicted from studies in captivity (Harkonnen and Heide-Jorgensen 1991): In addition, all prey items eaten by harbor seals may not have been represented in fecal samples. Evidence of cartilaginous fishes and some species and age-classes of bony fishes may not appear in feces because their statoconia or otoliths are minute and may be completely digested before excretion (Everitt and Gearin 1981, Harvey 1987). Otoliths of large prey are not always recovered from feces because harbor seals frequently tear such prey apart and discard the head (Pitcher 1980, Brown and Mate 1983). Although differential recoverey rates of otoliths from feces probably caused an underestimation of the number, biomass, and diversity of prey eaten, the relative proportions of prey consumed may not have changed significantly. After a study with captive otariids, Dellinger and Trillmich (1988) concluded relative proportions of prey could be assessed accurately using fecal samples, with the above exceptions, because ratios of large to small otoliths in feces corresponded closely to those in the diet. Assuming the same is true for phocids, and considering the potential biases of captive feeding experiments, conclusions regarding the relative importance of different species in the diet of harbor seals were probably valid. Natural variation in otolith size/fish length/fish weight relationships, along with the effect of partial digestion on otolith length, makes it difficult to estimate prey biomass accurately. Otoliths passed through the alimentary 44 canal are reduced significantly in length, which can cause underestimations of prey length and, consequently, biomass (Hawes 1983, DaSilva and Neilson 1985, Harvey 1989, Pierce and Boyle 1991). Harvey (1989), however, demonstrated that, because the amount of otolith dissolution was not related to species of fish, a non-specific correction factor could be used to compensate somewhat for this reduction. Estimates of population food consumption require not only accurate biomass assessments, but other data as well. Information on population structure and size, and diet composition of all age classes is needed, because energy requirements vary with sex, age, and reproductive status (Harwood and Croxall1988, Pierce and Boyle 1991). Multiplying the estimated annual biomass of prey eaten per seal by the number of seals in a population provides only a general and conservative estimate of annual biomass consumed. When quantifying food consumption, movements, daily feeding patterns, and passage rates must be known to determine the time frame and feeding locale represented by each fecal sample. Movements of harbor seals radio-tagged in Elkhorn Slough indicated seals foraged for approximately 12 hrs in Monterey Bay before returning to the slough for rest. Because passage rates of meals in captive harbor seals typically averaged 13 to 17 hrs (Havinga 1933, Prime 1979, Harvey 1989), rates which are possibly greater than those of free-ranging individuals, it is likely seals passed much of their daily meals at Elkhorn Slough haul-out sites and that most otoliths recovered from each sample represented prey consumed in Monterey Bay and Elkhorn Slough during the same 24 hr period. It is also probable that some material was passed in the water. Passage rates may vary with changes in diet, activity, and meal size (Prime and Hammond 1987, Harvey 1989) and initial defecation in harbor seals can occur within 5 hrs of ingestion (Helm 1984). If some feces were excreted before going ashore, and if different prey types pass through the digestive system at different rates, species which pass rapidly would be under represented in feces collected at haul-out sites (Pierce and Boyle 1991). Dellinger and Trillmich (1988), however, found that smaller otoliths were not passed more quickly than larger ones. Consequently, the proportion of prey species in the diet were accurately assessed, although loss of otoliths probably resulted in further underestimations of prey number and biomass. Cephalopod beaks are often retained in the stomach and regurgitated en masse after accumulation, rather than passed through the intestinal tract (Pitcher 1980, Bigg and Fawcett 1985). Therefore, octopus and squid can be substantially under-represented in feces, biasing estimates of number, biomass, and relative importance. Even so, estimates of importance were probably reliable. During seasons when cephalopods dominated the diet, relative importance would not have been affected significantly by the loss of beaks to regurgitation, although they possibly dominated the diet to a greater extent than indicated by fecal samples. Although the relative importance of octopus and squid were likely underestimated during seasons in which they were less dominant, seasonal changes in their rankings mirrored their availability, indicating bias was minimal. Despite its limitations, analysis of fecal samples is generally considered to be the most useful technique for investigating food habits (Pitcher 1980, Payne and Selzer 1989, Dellinger and Trillmich 1988). Samples are easily collected in large numbers with minimal disturbance to individuals. They 46 provide a biased but consistent sample of prey ingested, as well as information on temporal changes in diet and habitat utilization. Previous researchers have used nocturnal dive patterns, such as those displayed by harbor seals in Elkhorn Slough, to infer nocturnal foraging activity (Spalding 1964, Boulva and Mcl1tren 1979, Roffe 1980, Brown and Mate 1983, Stewart 1984, Osborne 1985, Yochem et al. 1987, Thompson et al. 1989). Further evidence for nighttime feeding can be provided by an analysis of the activity patterns of their dominant prey species (Brown and Mate 1983, Thompson et al. 1989, Harvey et al. in press-a). Many of the species frequently consumed by harbor seals in Elkhorn Slough either increase their activity or change their behavior nocturnally making them more vulnerable to predation. Octopus, spotted cusk-eel, and plainfin midshipman, which are active nocturnally, den or burrow during the day (Fitch and Lavenberg 1970, Dorsey 1976, Eschmeyer 1983), making " them inaccessible to seal predation diurnally. Trawls conducted in nearshore Monterey Bay revealed cusk-eel were only caught between 2100 hrs and 0300 hrs (Harvey et al. in press-a). Pacific sanddab use cryptic coloration to blend with the benthic environment and avoid predation during the day, but they come off the bottom at night to feed (Love 1991). Several schooling prey, such as white croaker, night smelt, Pacific hake, and some rockfish, form tight groups during the day for protection but may form looser, less protective aggregations at night while feeding (Fitch and Lavenberg 1970, Hobson et al. 1981, Emmett et al. 1991). Identification and estimated size of prey also indicated feeding was generally restricted to Monterey Bay. Of the 47 species consumed by harbor 47 seals in Elkhorn Slough, only plainfin midshipman, English sole, shiner surfperch, lingcod, spotted cusk-eel, rockfish, and staghorn sculpin were abundant in slough waters and seal diet. Only 0-age and small juvenile life stages of these fishes, however, were observed in trawls, with the exception of cusk-eel and shiner surfperch, whereas 1he estimated length of those recovered from feces indicated harbor seals consumed mostly larger, adult individuals. Because these species enter estuaries as young-of-year and juveniles and move to deeper waters offshore as they mature (Wang 1986, Emmett et al. 1991), seals from Elkhorn Slough probably fed on them in Monterey Bay. Although similarities in length between spotted cusk-eel from Elkhorn Slough and those estimated from otoliths recovered from feces indicated harbor seals could have fed on them in the slough, it is more likely they caught them offshore. Adult spotted cusk-eels were an abundant prey item of harbor seals throughout the year, but few were caught during trawls conducted in Elkhorn Slough. Furthermore, cusk-eels were found in the slough only at night, when seals were in the bay. Spotted cusk-eels are probably not slough residents and likely enter Elkhorn Slough only on occasion to forage. They are presumably much more abundant, and thus easier to catch, in Monterey Bay. Shiner surfperch, however, were possibly consumed in Elkhorn Slough. Most shiners appeared in trawls and feces during spring and summer when they move into estuaries to spawn (Wang 1986, Emmett et al. 1991). Although seals ate significantly larger shiner surfperch than were caught in trawls, the average size of shiners from trawls may not have been 48 representative of those available. Shiner surfperch are viviparous and the stress of capture frequently induced females to give birth. Because all fish were measured, these young-of-year effectively decreased the mean size of surfperch caught in Elkhorn Slough. Harbor seals also may have fed on shiner surfperch in Monterey Bay because they are abundant in coastal waters, often schooling in the shallows during the day and moving into deeper water at night (Emmett et al. 1991). As in previous studies, harbor seals foraged in the nearshore environment on benthic assemblages (Wahl1977, Brown and Mate 1983, Harvey 1987, Harkonnen and Heide-Jorgensen 1991, Thompson et al. 1993). Seals were generally found within 7km of shore, often just beyond the surf zone, over sand and mud substrates in water 8 to 180m depth. All species consumed, with the exception of squid, northern anchovy, night smelt, Pacific herring, Pacific hake, rockfish, and surfperch, are associated with or inhabit soft, flat bottomed seabeds in shallow to moderate depths. Although harbor seals foraged nearshore, feeding locations were associated with changes in undersea topography. The Monterey Submarine Canyon reaches relatively close to shore near feeding areas off Moss Landing and Zmudowski beaches. Seals also foraged near Soquel Canyon, off Sunset Beach. Such topographical features can cause upwelling of nutrient rich waters, resulting in increased concentrations of plankton and fish in surrounding waters (Pingree et a!. 1978, Evans 1987). These concentrations of prey form focal points for predators, such as seabirds, cetaceans, and pinnipeds (Evans 1971, Evans 1974, Brown 1980, Dunstan et al. 1981, Evans 1987). 49 Similar to observations of harbor seals elsewhere (Brown and Mate 1983, Harvey 1987, Harkonnen and Heide-Jorgensen 1991, Olesuik 1993, Torok 1994), seals in Elkhorn Slough appeared to be opportunistic carnivores, feeding primarily on seasonally abundant cephalopods and fish. Although composition of seal diet remained relatively unchanged between autumn and spring, it changed significantly during summer, and the importance of several individual prey items varied considerably among seasons. Such temporal variation in food habits indicated seasonal fluctuations in prey availability and abundance has a strong influence on the composition of seal diet. Indeed, most changes in diet correlated with changes in availability. Octopus sp., which dominated harbor seal diet autumn through spring, was significantly less important during summer, despite being abundant in Monterey Bay throughout the year. Common offshore on sandy mud, 0. rubescens migrates to deep water (approx. 200m) in late winter and early spring to mate (Morriss et al. 1980). By summer, the population moves inshore to spawn in rocky areas of the intertidal and shallow subtidal zones. Females lay their eggs in dens, where they remain for 6 to 8 weeks to brood their young. Thus, during summer 0. rubescens was either inaccessible to predation or inhabiting areas not usually frequented by foraging harbor seals. The importance of octopus to harbor seals along the Pacific coast is well documented (Scheffer and Sperry 1931, Imler and Sarber 1947, Spalding 1964, Kenyon 1965, Bishop 1967, Pitcher 1980). However, contrary to data collected during 1991 and 1992 in Elkhorn Slough and along the coast of Monterey Bay {Trumble, MLML, pers comm.), previous researchers did not find octopus to be an important prey item of harbor seals in the Monterey Bay area Oanes 50 1981, Harvey et al. in press-a). 1his discrepancy may be due, in part, to an insufficient number of samples collected by both Harvey et al. (n=30) and Jones (n=12). It may also have been the result of mild El Nino conditions which persisted along California during most of the study period. Under such conditions, increased water temperatures and decreased upwelling cause reductions in local food resources and changes the availability of various prey species (Reidman 1990). Opportunistic predators are thus forced to exploit alternative prey resources. Lowry et al. (1990) found California sea lions (Zalophus califomianus) at San Clemente Island switched to octopus following a reduction in squid availability during the 1982-1983 El Niflo. Diet of harbor seals in the Channel Inlands also changed during the 1982-1983 event. Diversity of prey consumed per seal decreased and the proportion of prey eaten changed. Harbor seals ate more octopus and fewer rockfish during the El Niflo than during more typical climatic conditions (Stewart and Yochem 1985). Increased consumption of market squid by harbor seals during winter and spring coincided with increased abundance of squid in nearshore Monterey Bay. Market squid usually occur offshore over the continental shelf and beyond, but move inshore to spawn in large dense schools 30 to 40 m thick and 70 to 200m in diameter (Mais 1974, Recksiek and Frey 1978). Spawning generally occurs in well defined portions of the bay between April and July, with an additional peak between November and January (Fields 1965, Recksiek and Frey 1978), providing a predictable resource for harbor seals and other predators. Although El Niflo conditions in Monterey Bay 51 during 1991 and 1992 resulted in below average yields of squid to the fishery (CDF&G 1992), it apparently did not affect their availability to harbor seals. Many studies have indicated market squld is an important prey of harbor seals (Scheffer and Sperry 1931, Pitcher 1977, Pitcher 1980, Selzer et al. 1986, Olesuik 1993, Beeson 1994). Although squid is a vital resource for many marine birds and mammals in Monterey Bay (Morejohn et al. 1978), its importance to harbor seals was not previously known (Jones 1981, Morejohn et al. 1978, Harvey et al. in press-a). This disparity was probably due to the small number of samples collected during these studies, which likely provided an incomplete description of seal diet. Although the estimated dorsal mantle length of squid and octopus eaten by seals differed among seasons, these changes were probably not biologically significant. Variations in length were small and great differences in number of beaks recovered each season could have biased results. Pacific staghorn sculpin were eaten throughout the year, but were more important to seals during autumn and winter, when they form spawning aggregations in shallow coastal waters and estuaries (Wang 1986). After mating, adults move offshore, usually to waters less than 18m depth, whereas young-of-year (YOY) and juveniles remain on their nursery grounds (Emmett et al. 1991). Despite the consumption of smaller sculpin during summer, seals ate individuals that were significantly larger than those inhabiting the slough, indicating seals probably fed on them offshore. The small number of intact otoliths recovered winter through spring, however, may not have provided an adequate sample of sizes eaten. Although harbor seal predation on staghorn sculpin has been well documented in Oregon and Washington 52 estuaries (Calambokidis et al. 1978, Graybill1981, Brown and Mate 1983, Beach et al. 1985, Harvey 1987), it has seldom been reported in California (Harvey et al. in press-a). The greater diversity of prey eaten by harbor seals in Elkhorn Slough during summer was associated with increased abundance and diversity of fishes in Monterey Bay. Numerous species of fish move into productive nearshore environments during spring and summer to feed, grow, and reproduce (Fitch and Lavenberg 1970, Love 1991). The resulting increase in abundance and diversity has been observed in both Monterey Bay (Cailliet et al. 1979) and Elkhorn Slough (Cailliet et al. 1977, Yoklavich et al. 1991, this study). The consumption of several species by harbor seals coincided with this seasonal inshore migration. Predation on juvenile and adult flatfish often corresponded with their peak in nearshore abundance. Increased utilization of Pacific sanddab and rex sole in spring was associated with their annual movement inshore from their spawning grounds (Fitch and Lavenberg 1970, Love 1991). Dover sole were also mostly eaten after they had moved into the coastal environment during late spring and summer. Because only adult sole generally make this migration (Fitch and Lavenberg 1970, Love 1991), larger individuals tended to be represented in feces collected during summer. The relative absence of rex and Dover sole from the diet during winter, and of Pacific sanddab in autumn, was probably related to their emigration to waters as deep as 600 m for spawning (Fitch and Lavenberg 1970, Matarese et al. 1989), which may be beyond the diving capabilities of harbor seals. 53 All species of flatfish identified in this study, with the exception of California tonguefish, have also been identified in fecal samples collected in Elkhorn Slough by Harvey et al. (in press-a). However, their small sample size precluded any temporal comparisons. Flatfish have frequently been reported as important prey of harbor seals elsewhere, though species vary with location (Bowlby 1981, Brown and Mate 1983, Harvey 1987, Harkonnen 1987). Geographic variation in diet is expected as faunal components change and species substitutions occur. It is a reflection of their opportunistic foraging strategy. Adult white croaker, eaten throughout the year, were marginally more abundant in fecal samples collected during summer and autumn. This coincides with their inshore migration to shallow waters less than 30m depth during summer, and their subsequent peak in nearshore spawning between September and May (Wang 1986). Although an offshore migration occurs in winter, croaker are generally found in waters less than 110m (Eschmeyer 1983), and thus remain accessible to seals. Slightly larger individuals were eaten in winter, but the small number of unbroken otoliths recovered during winter made the results questionable. White croaker has been previously reported as important prey of harbor seals in Elkhorn Slough (Harvey et al. in press-a). Although harbor seals usually ate benthic species, pelagic schooling fishes were consumed when they became abundant during summer. Their near absence from the diet during subsequent seasons implied they were otherwise not available in sufficient quantities for seal predation. Several pelagic species, including Pacific butterfish, striped bass, pile surfperch, 54 walleye surfperch, and pink surfperch were eaten only during summer. Adult shiner surfperch were mostly eaten after large schools had moved into coastal and estuarine spawning grounds during summer (Wang 1986, Emmett et al. 1991). The preponderance of rockfishes·in the diet during summer coincided with their increased abundance nearshore as juvenile aggregations moved into shallow waters (P. Adams NMFS, pers. comm.). In Monterey Bay, Cailliet et al. (1979) reported significantly more rockfishes (as many as 10,000 in a single haul) in shallow and rnidwater trawls during summer than during winter, when yields declined dramatically. Increased predation on seasonally abundant rockfishes has also been observed with California sea lions in southern California (Antonelis et al. 1984, Lowry et al. 1990) and harbor seals of British Columbia (Spalding 1964). Rockfish have been previously reported as prey of harbor seals in Elkhorn Slough, though seasonal comparisons were not made (Harvey et al. in press-a). Estimated size of rockfish recovered from samples indicated seals ate large juveniles throughout the year, consuming slightly larger individuals in winter. However, because rockfish otoliths could not be identified to species, the assumption they were all shortbelly rockfish may have biased temporal comparisons. Harbor seals ate Pacific herring only during winter and summer when schools move from the open coast and continental shelf to the shallow waters of bays and estuaries for spawning (Eschmeyer 1983). Similar patterns of use have been reported in Alaska (Pitcher 1980), British Columbia (Spalding 1964, Olesuik 1993) and Washington (Everitt and Gearin 1981). Although only one herring was caught during trawls conducted in Elkhorn Slough, they have 55 been known to occur there in greater numbers (Yoklavich et al. 1991). If herring were present in the slough, they would not have been caught during the survey because they usually inhabit areas that were not sampled, such as tributaries and shallow eelgrass beds northeast of Kirby Park at Hudson's Landing (Yoklavich et al. 1991). Regard[ess, it was unlikely seals fed on them within the slough because seals were not observed east of the Dairies or in tributaries. Only two pelagic schooling species, night smelt and Pacific hake, were eaten frequently most of the year. Adult night smelt probably were consumed as they formed dense spawning aggregations near the surf zone winter through summer (Fitch and Lavenberg 1970). Their absence from the diet in autumn was associated with their movement offshore to deep water (Fitch and Lavenberg 1970). Although significantly larger smelt were recovered during summer, differences in estimated size were so small that it was probably not biologically significant. Pacific hake were eaten in all seasons except winter, which coincides with their peak spawning period. Spawning occurs up to 200 miles offshore, at depths greater than 350m (Lave 1991), well beyond the range of harbor seals. Trawls in Monterey Bay also revealed hake were absent during winter (Cailliet et al. 1979). Pacific herring, night smelt, and Pacific hake have all been previously identified as prey of harbor seals in Elkhorn Slaugh (Harvey et al. in press-a). Pacific herring and Pacific hake are important prey of harbor seals throughout much of the Pacific coast (Bowlby 1981, Everitt and Gearin 1981, Beach et al. 1981, Graybill1981, Brown and Mate 1983, Harvey 1987, Olesuik 1993, Beesean 1994). Although night smelt have seldom been identified (Bowlby 1981, 56 Brown and Mate 1983), other osmerids have frequently been reported (Pitcher 1977, Pitcher 1980, Jeffries 1984, Roffe and Mate 1984, Harvey 1987). Predation on several species did not vary seasonally because they were available throughout the year. Juvenile and adult English sole, which were most important to seals during summer and autumn when they inhabited shallow nearshore waters (Wang 1986), also were consumed abundantly during winter and spring after moving offshore to spawn (Emmett et aL 1991). Spawning usually occurs at depths between 50 and 70 m (Emmett et al. 1991, Love 1991), well within the foraging range of harbor seals. Plainfin midshipman also were eaten in similar abundances each season. Midshipman occupy shallow coastal waters while spawning in late spring and summer, moving into deeper waters by autumn (Fitch and Lavenberg 1970, Wang 1986). Although they have been caught in waters 370 m deep (Fitch and Lavenberg 1970), little is known about their offshore migration. Because they were still important prey while residing in deeper waters, midshipman must occur abundantly at depths that are within the diving capabilities of harbor seals. Significantly larger English sole and plainfin midshipman were eaten during summer than autumn, which coincides with the movement of adults into nearshore bay waters. Although there was no difference among other seasons, the small number of intact otoliths collected between November and April may not have provided an adequate sample of sizes eaten. Although significantly fewer spotted cusk-eel otoliths were recovered from samples during summer, they remained one of the most important prey species consumed. Cusk-eel is not known to make extensive inshore/ 57 offshore migrations (Wang 1986), and is probably available throughout the year. Decreased consumption of cusk-eel in summer was probably not related to decreased abundance, but to increased availability of other prey, such as rockfish and Dover sole. The estimated lengths of cusk-eel recovered from feces collected during spring were significantly larger than those found in autumn. However, the difference was only 2.17 em; too small to be biologically significant. English sole and plainfin midshipman were important prey of harbor seals throughout much of the Pacific coast (Antonelis and Fiscus 1980, Beach et al. 1981, Bowlby 1981, Brown and Mate 1983, Harvey 1987, Beeson 1994, Harvey et al. in press-a). In contrast, cusk-eels, although they are common from Baja, California to northern Oregon (Miller and Lea 1972), have only been reported as prey of harbor seals in Monterey Bay /Elkhorn Slough (Harvey et al. in press-a, Trumble MLML, pers comm.) and the Southern California Bight (Beeson 1994). Although harbor seals are generally considered non-migratory, movements have been associated with changes in prey availability. Peak abundance of harbor seals and their con.sumption of salmonids in the Rogue River and Netarts Bay, Oregon coincided with the upriver spawning migrations of chum salmon, chinook salmon, and steelhead (Brown and Mate 1983, Roffe and Mate 1984). Jeffries (1986) reported the number of harbor seals in the Columbia River increased three fold when eulachon entered the river to spawn, whereas populations in the adjacent estuaries of Gray's Harbor and Willapa Bay decreased significantly. During the run, seals ate eulachon almost exclusively. The population shifted back into adjacent 58 estuaries after the run. Seasonal prey resources in Elkhorn Slough, however, did not appear to have any influence on harbor seal movement or abundance patterns. Although peak abundance of harbor seals in Elkhorn Slough during late spring and early summer coincided with movement of fishes into the slough, seals did not consume them. Feeding activity was concentrated in the same relatively small region of Monterey Bay throughout the year. Such fidelity to feeding grounds indicate that, although ichthyofauna changed seasonally, prey was available in the bay all year. Because harbor seals apparently had little need to leave the vicinity of their Elkhorn Slough haul out sites to locate prey, there was no significant variation in seal abundance in the slough. The peak in harbor seal abundance during summer was probably related to molt activities. The combination of population growth and localized feeding activity can have a considerable impact on local prey populations (Power and Gregoire 1978, Riedman 1990, Scardino and Pfeifer 1993). In Elkhorn Slough, a four fold increase in harbor seal abundance during the last 10 years was accompanied by a significant decrease in fish diversity and abundance. However, Harvey et al. (in press-a) and this study found that seals fed almost exlusively in Monterey Bay. This is in contrast to harbor seals in other estuarine systems along the Pacific coast, which forage extensively on 0-age, juvenile, and adult fishes that use these areas as spawning and nursery grounds (Pitcher 1977, Brown and Mate 1983, Roffe and Mate 1984, Jeffries 1986, Harvey 1987, Torok 1994). Instead, harbor seals used Elkhorn Slough more because of its relatively isolated haul-out sites and its proximity to great concentrations of prey in Monterey Bay. 59 Harbor seals were probably the only predator capable of having an impact on fish assemblages in Elkhorn Slough. Although numerous birds, including pelicans, herons, egrets, terns, and a variety of shorebirds inhabit the slough, most only feed on invertebrates or shallow water fishes. Also, few piscivorous fish, such as leopard shark (Triakis semifasciata), striped bass, California halibut (Paralichthys californicus), Pacific staghorn sculpin, and cabezon (Scorpaeniclzthys mannoratus), occur abundantly in Elkhorn Slough. Changing environmental conditions, however, could possibly have affected the abundance and distribution of fish in Elkhorn Slough. Until recently, there has been little fresh water input into the slough because of drought conditions that have persisted in California for several years. In addition, erosion of the banks has increased the turbidity of water in Elkhorn Slough considerably (Malzone MLML, pers comrn.) The resulting increase in salinity and light attenuation could have adversely affected primary production, and the distribution, abundance, and diversity of fishes. It is also possible assemblages in Elkhorn Slough have not changed. Because Yoklavich et al. (1991) conducted their survey in 1977, tidal flushing has eroded the main channel considerably, resulting in an increased rate of flow into and out of the slough (Malzone MLML, pers comrn.). Fishes which normally congregate in the channel may have been forced to move to shallower waters to avoid these increased current velocities. Thus, trawls conducted in 1991 may have missed these aggregations. In addition, sample size may not have been large enough to have adequately described ichthyofauna because fewer trawls were performed during this study than were conducted by Yoklavich et al. {1991). 60 There are several potential explanations why harbor seals restricted feeding activities to Monterey Bay. Elkhorn Slough may be too small, only 10 km long and 100 m wide at its mouth, to accommodate numerous foraging seals. Harbor seals are solitary feeders (Reidman 1990), and a great number of individuals foraging in such a small area would likely result in frequent intraspecific interactions. By feeding in the relatively open space of Monterey Bay, seals probably minimize intraspecific competition. Because several species of fish use Elkhorn Slough as a nursery, it may not be energetically efficient to prey on them when greater concentrations of larger prey are accessible nearby in the bay. In addition, visibility in Elkhorn Slough is poor. Because harbor seals are believed to be visual predarors, it may be more efficient for them to forage in the relatively clear waters offshore. Although harbor seals apparently do not affect Elkhorn Slough ichthyofauna, their predation can possibly influence nearshore fish communities. Because seals ate predominantly large juveniles and adults, their impact is two fold; they reduce the number of fish that reach sexual maturity and the abundance of reproductively mature adults, thereby causing decreases in recruitment and future production. However, the effects of a predator on a population or fishery are difficult to assess because fish recruitment and mortality can vary dramatically among years (Harvey 1987). Harbor seals in Elkhorn Slough did not feed extensively on commercially important species. Of the 47 species eaten by seals, nine are commercially important in Monterey Bay (Table 15). Commercial landings for these species never dropped below 48,000 kg during 1991 (CDF&G 1992), whereas annual consumption by harbor seals from Elkhorn Slough did not 61 exceed 6,900 kg for any given species. However, competition between seals and fisheries was apparent when biomass estimates included the entire bay population (Table 15). I assumed the diet of harbor seals in Monterey Bay was similar to the diet of those in Elkhorn Slough, and preliminary comparisons indicated a similarity (Trumble MLML;pers comm.). Seals consumed approximately 143% of the commercial catch of white croaker, and 86% of sanddab landings. There was also moderate competition for lingcod (14.9% of landings) and English sole (11.0% of landings). Predation on these species represent a loss of approximately $113,200 to commercial fisheries. Because the harbor seal population throughout California and the Pacific Northwest in increasing (Harvey et al. 1990, Hanan et al. 1993), the frequency of harbor seal/fishery interactions will likely intensify. Considering the potential impact increased harbor seal predation could have on fish populations and fisheries, information regarding pinniped food habits is essential if fishery management practices are to be effective. 62 SUMMARY I CONCLUSIONS Estuaries along the Pacific coast provide excellent habitat for harbor seals: isolated locations for resting ashore, great quantities of prey, and protected waters for pupping. Elkhorn"Slough should provide such an environment, but use of this embayment by harbor seals differs from their use of other estuarine systems. Such differences in habitat utilization were probably related to the relatively small size of the slough and the young age of the seals. Harbor seals used Elkhorn Slough primarily as a diurnal rest area. Limited pupping was probably due to the lack of suitable habitat. Estuaries in which seals give birth are usually much larger, with extensive networks of tributaries. These channels are often in remote and secluded areas of the estuary, and so offer ideal habitat for giving birth. Elkhorn Slough has no tributary system large enough to accommodate harbor seals. All major mudflats and haul-out sites face the main channel, and are subjected to relatively frequent disturbance from boaters and fishermen. Such sites probably do not provide sufficient isolation for pupping. In addition, because Elkhorn Slough is essentially a catch basin for juvenile harbor seals emigrating from crowded coastal haul-out sites, its population has few reproductively active individuals. Harbor seals in Elkhorn Slough are opportunistic carnivores, feeding primarily on seasonally abundant benthic cephalopods and fish. Although harbor seals frequently exploit seasonally abundant prey in other estuaries, the nocturnal foraging activities of seals in Elkhorn Slough are apparently 63 restricted to nearshore Monterey Bay throughout the year. Because Elkhorn Slough is a small estuary, its spatial and prey resources are probably not adequate to support the feeding activities of a large harbor seal population. Elkhorn Slough's proximity to the year-round resources of Monterey Bay precluded the need for long-range movl?ments related to prey availability, and resulted in a great degree of fidelity to rest and feeding areas throughout the year. Because no adults were radio-tagged, and no seals were monitored during sununer, information regarding movements and activity patterns of harbor seals in Elkhorn Slough was incomplete. Additional radio-tracking is needed to provide a more complete understanding of how and when seals use the Elkhorn Slough and Monterey Bay environment. Although harbor seals had only a minimal impact on fishes in Elkhorn Slough, continued monitoring of the food habits of harbor seals along the central California coast is needed to determine how their increasing population affects Monterey Bay's nearshore fish assemblages and commercial fisheries. A significant change in diet could indicate that the population has reached carrying capacity and had seriously affected prey abundance. 64 LITERATURE CITED Allen, S.G., D.G. Ainley, G.W. Page, and C.A. Ribic. 1984. The effects of disturbance on harbor seal haul out patterns a Bolinas Lagoon, California. Fish. Bull. 82:493-500. Allen, S G., D.G. Ainley, L. Fancher and D. Shufford. 1987. Movement and activity patterns of harbor seals (Phoca vitulina) from the Drakes Estero population, California, 1985-1986. NOAA Technical Memorandum. NOS MEMO 6. 35pp. Antonelis, G.A. and C.H. Fiscus. 1980. The pinnipeds of the California current. Cal. COFI Rep. 21:68-78. Antonelis, G.A., Jr., C.H. Fiscus, and R.L. DeLong. 1984. Spring and summer prey of California sea lions, Zaloplms californianus, at San Miguel Island, California, 1978-79. Fishery Bulletin 82:67-76. Beach, R.J., A.C. Geiger, S.J. Jeffries, and S.D. Treacy. 1981. Marine mammal fisheries interactions on the Columbia River and adacent waters, 1981. NWAFC processed rept. 82-04. Seattle: NMFS. Beach, R.J., A. C. Geiger, S.J. Jeffries, S.D. Treacy, and B.L. Troutman. 1985. Marine mammals and their interactions with fisheries of the Columbia River and adjacent waters, 1980-1982. NMFS, Northwest Alaska Fish. Center, Seattle, WA Processed Rept., 85-04: 1-316. Beeson, M.J. 1994. Assessing Pacific harbor seal, Phoca vitulina ric/mrdsi, feeding ecology in the Southern California Bight. Abstracts of the Tenth Biennial Conference on the Biology of Marine Mammals. Beddington, J.R., R.J.H. Beverton, and D.M. Lavigne (eds.). 1985. Marine Mammals and Fisheries. Geo. Allen & Unwin, London. 544pp. Bigg, M.A. 1969. Clines in the pupping season of the harbour seal, Phoca vitulina. J. Fishery Reserves Board Canada, 26:449-455. Bigg, M.A. 1981. Harbour seal, Phoca vitulina Unnaeus 1758 and Plwca Iargha Pallus 1811. pp 1-27. In: Handbook of Marine Mammals. Vol. 2: Seals Academic Press, London, 359pp. 65 Bigg, M.A. and I. Fawcett. 1985. Two biases in diet determination of northern fur seals (Callorhi11us ursi11us), p. 153-169. In: J.R. Beddington, R.J.H. Beverton, and D.M. Lavigne (eds.), Marine Mammals and Fisheries. George Allen & Unwin, London, U.K., 354pp. Bishop, R.H. 1967. Reproduction, Age Determination, and Behavior of the Harbor Seal, Phoca vituli11a L. in the Gulf of Alaska. Unpubl. M.S. Thesis, Univ. Alaska, Anchorage: 121pp. Bonner, W.N. 1975. Population increase of grey seals at the Farne Islands. Rapp. P. V. Reun. Cons. Int. Explor. Mer. 169: 366-370. Bonner, W.N. 1982. Seals and Man: A Study of Interactions. WA. Sea Grant Pub. Seattle: Univ. of Washington Press. Bonner, W.N. and S.R. Witthames. 1974. Dispersal of common seals (P!wca vituli11a), tagged in the Wash, East Anglia. J. Zool., 174: 528-531. Bonnot, P. 1951. The sea lions, seals and sea otters of the California coast. Calif. Fish Game, 37: 371-389. Boulva, J. and I.A. McLaren. 1979. Biology of the harbor seal, Phoca vituli11a, in eastern Canada. J. Fish. Res. Board. Can., Bull. No. 200, 24p. Boveng, P. 1988. Status of the Pacific harbor seal population on the U.S. west coast. Southwest Fisheries Center. NMMS. Report LJ-88-06 43pp. Bowlby, C.E. 1981. Feeding Behavior of Pinnipeds in the Klamath River, Northern California. M.A. Thesis, Humboldt State Univ., Arcata, CA. 74pp. Broenkow, W. 1977. Water chemistry of Elkhorn Slough and Moss Landing Harbor, p. 387-464. In: J. Nybakken, G. Cailliet, and W. Broenkow (eds.), Ecologic and Hydrographic Studies of Elkhorn Slough, Moss Landing Harbor, and Nearshore Coastal Waters. Moss Landing Marine Laboratories Technical Publication, Moss Landing, CA. Brown, R.G.B. 1980. Seabirds as marine animals. In: J. Burger, B.L. Olla, and H.E. Winn (eds.), Behavior of Marine Animals. Vol. 4: Marine Birds, Plenum Press, N.Y., N.Y. 1-39. 66 Brown, R.F. and B.R. Mate. 1983. Abundance, movements, and feeding habits of harbor seals, Plloca vitulina, at Netarts and Tillamook Bays, Oregon. Fish. BulL, 81: 291-301. Brown, R. F. 1986. Assessment of Pinniped Populations in Oregon. NatL Mar. Fish. Serv., Northwest Alaska Fish. Center, Seattle, Washington, Final Rept., 44 pp. Cailliet, G .M., M.B. Antrim, D. Ambrose, S. Pace, and M. Stevenson. 1977. Species composition, abundance, and ecological studies of fishes, larval fishes, and zooplankton in Elkhorn Slough, p. 216-385. In: J. Nybakken, G. Cailliet, and W. Broenkow (eds.), Ecologic and Hydrographic Studies of Elkhorn Slough, Moss Landing Harbor, and Nearshore Coastal Waters. Moss Landing Marine Laboratories Technical Publication, Moss Landing, CA. Cailliet, G., K.A. Kaprov, and D.A. Ambrose. 1979. Pelagic assemblages as determined from purse seine and large midwater trawl catches in Monterey Bay and their affinities with market squid (Loligo opalescens}. CALCOFI Rep. VoL XX. 1979: 21-30 Cailliet, G.M., M. Love, A. Ebling. 1986. Fishes: A Field and Laboratory Manual on their Structure, Identification, and Natural History. Wadsworth Pub. Co, CA.125pp. Calambokidls, J.K., K Bowman, S. Carter, J. Cubbage, P. Dawson, T. Fleischner, J. Schuett-Hames, J. Skidmore, and B. Taylor. 1978. Chlorinated Hydrocarbon Concentrations and the Ecology and Behavior of Harbor Seals in Washington State Waters. NatL Sci. Found., Evergreen State College, Olympia, WA, Final Rept. 121pp. California Department of Fish and Game. 1992. Monterey Bay commercial fisheries report, 1991-1992. CDF&G Newsletter #4. Clarke, M.R. 1962. The identification of cephalopod 'beaks' and the relationship between beak size and total body weight. Bull. Br. Mus. Nat. Hist. (Zool.) 8: 419-480. Contos, E. 1982. Marine mammal fisheries interactions coordination workshop, 26 October 1981. Research Summaries, Vancouver, Washington. 64pp. 67 DaSilva, J. and J.D. Neilson. 1985. Limitations of using otoliths recovered in scats to estimate prey consumption in seals. Can. J. Aquat. Sci. 42: 1439- 1442. Dellinger, T. and F. Trillmich. 1988. Estimating diet composition from scat analyses in otariid seals (Otariidae): Is it reliable? Can. J. Zool. 66: 1865- 1870. Divinyi, C.A. 1971. Growth and movements of a known-age harbor seaL Journal of Mammalogy 52(4):824. Dorfman, E. 1991. Distribution, Behavior, and Food Habits of Harbor Porpoise, Phocena phocena, in Monterey Bay, California. MA thesis, Moss Landing Marine Laboratories., Moss Landing , CA 48pp. Dorsey, E.M. 1976. Natural and Social Behavior of Octopus rubescens. M.S. thesis, University of Washington Dunstan, P., R.C. Smith, and K.S. Baker. 1981. Mesoscale patterns of oceanic chlorophyll and the distribution of marine mammals. Marine Mammal Information, June 1981, pg 4. Echeverria, T.W. 1987. Relationship of otolith length to total length in rockfishes from northern and central California. Fish. Bull. 85(2):383- ''-' 387~ Emmett, R.L., F.L. Stone, S.A. Hinton, and M.E. Monaco. 1991. Distribution and Abundance of Fishes and Invertebrates in West Coast Estuaries. Vol. 2; Species Life Histories Summaries. ELMR REPORT#8 NOAA/ NOS. Strategic Environmental Assoc. Div. Rockville, MD. 329 pp. Eschmeyer, W.N. 1983. A Field Guide to Pacific Coast Fishes of North America. Dept. Fish Game, Fish. Bull. 157. 320pp. Estes, J. A. and J. F. Palmisano. 1974. Sea otters: their role in structuring nearshore communities. Science, 185: 1058-1560. Evans, P.G.H. 1987. The Natural History of Whales and Dolpins. Facts on File Pub. N.Y. 343pp. Evans, W.E. 1971. Orientation behavior of delphinids: radio telemetric studies. Ann. N.Y. Acad. Sci.,188, 142-160 68 Evans, W.E. 1974. Radio telemetric studies of two species of small odontocete cetaceans. In; Schevill, W.E. (ed.) The Whale Problem: a Status Report, Harvard University Press, Cambridge, Mass.,385-394pp. Everitt, RD., P. J. Gearin, J. S. Skidmore and R. L. DeLong. 1981. Prey items of harbour seals and California sea lions in the Puget Sound, Washington. The Murrelet. Win~er, 1981; 83-96. Everitt, RD. and J. Gearin. 1981. Prey items of harbor seals and California sea lions in Puget Sound, Washington. General Notes, the Murrelet. (4): 83-86. Fedak, M.A., S.S. Anderson, and M.G. Curry. 1982. Attachment of a radio tag to the fur of seals. Notes Mamm. Soc., 49:298-300. Feltz, E.T. and F.H. Fay. 1966. Thermal requirements in vitro of epidermal cells from seals. Cryobiology, 3:261-264. Fields, W.G. 1965. Structure, development, food relations, reproduction, and life history of the squid Loligo opalescens. CDF&G Fish. Bull. 131. Fitch, J. E. and R J. Lavenberg. 1970. Marine Food and Game Fishes of California. Univ. Ca. Press. Berkley. 145pp. Fraker, M.A. 1994. California sea lions and steelhead trout at the Chittenden Locks, Seattle, Washington. Marine Mammal Commission; 1825 Connecticut Ave. NW, Washington DC 20009. May 1994. 42pp. Graybill, M.R. 1981. Haul-out Patterns and Diet of Harbor Seals, Phoca vitulina, in Coos County, Oregon. M.S. Thesis, Univ. Oregon, Eugene. 55pp. Geiger, A C. 1985. Documentation of marine mammal interactions with coastal salmon gill net and other fisheries of the Columbia River and adjacent waters, 1980-1982. Processed Report 85-04. NMFS, Northwest and Alaska Fisheries Center, Seattle, WA. Grunewald, K.K. and T.J. Tucker. 1985. Gastric emptying in exercised mice. Comp. Biochem. Physiol. BOA; 173-175. Hanan, D.A., L.M. Jones, M.J. Beeson. 1993. Harbor seat Phoca vitulina, census in California, May-June, 1992. Final Report. NOAA fisheries/NMFS Southwest Region. 53pp. 69 Harkonen, T. 1987. Seasonal and regional variations in the feeding habits of the harbor seal Phoca vitulina, in the Skagerrak and Kattegat. J. Zoo!. Lond. 213:535-543. Harkonen, T. and M.P. Heide-Jorgensen. 1991. The harbour seal, P/toca vitulina, as a predator in the Skagerrak. Ophelia 34(3): 191-207. Harrison R.J. and G.L Kooyman. 1968. General physiology of the pinnipedia. In: The Behavior and Physiology of Pinnipeds. R.J. Harrison ed. Appleton-Century-Crofs, N.Y., 41lpp. Harvey, J.T., R.C. Helm, and G. V. Morejohn. (in press-a). Food habits of harbor seals, Phoca vitulina, inhabiting Elkhorn Slough, Monterey Bay, California. Calif. California Dept. of Fish and Game. Harvey, J.T., T.R. Loughlin, M.A. Perez, and D.S. Oxman. (in press-b). Relationship between fish size and otolith length for 62 species of fishes from the eastern north Pacific Ocean. NOAA Tech. Rept. Harvey, J.T., R.F. Brown, B.R. Mate. 1990. Abundance and distribution of harbor seals (Phoca vitulina) in Oregon, 1975-1983. Northwestern Naturalist, 71:65-71. Harvey, J.T. 1987. Population Dynamics, Annual Food Consumption, Movements, and Dive Behaviors of Harbor Seals, P/wca vitulina, in Oregon. Ph.D dissertation, Oregon State Univ.177pp. Harvey, J.T. 1989. Assessment of errors associated with harbor seals Phoca vitulina faecal samples. J. Zoo!. Lond. 219:101-111. Harwood, J. and J.P. Croxall. 1988. The assessment of competition between seals and commercial fisheries in the North Sea and the Antartic. Mar. Mamm. Sci. 4(1}: 13-33. Havinga, B. 1933. Der seehund (Phoca vitulina) in den hollaendischen gewaessern. Ned. Dierkd. Ver.,Tijdschr., 3: 79-111. Hawes, S.D. 1983. An Evaluation of California Sea Lion Scat Samples as Indicators of Prey Importance. M.S. Thesis, San Francisco State Univ., California, 50pp. Helm, R. C. 1984. Rate of digestion in three species of pinnipeds. Can. J. Zoo!. 62: 1751-1756. 70 Herder, M.J. 1986. Seasonal movements and hauling site fidelity of harbor seals, Phoca vitulina richardsi, tagged at the Klamath River, California. M.S. Thesis, Hwnbolt State Univ., Arcata, CA. 52pp. Hewer, H. R. 1974. British Seals. Williams Collins Press, London. 256pp. Hobson, E. S., W. N. McFarland and J. R Chess. 1981. Crepuscular and nocturnal activities of Californian nearshore fishes, with consideration of their scotopic visual pigments and the photic environment. Fish. Bull. 79(1): 1-30. Hyslop, E.J. 1980. Stomach content analysis- a review of methods and their applications. J.Fish Biol. 17: 411-429. Imler, R.H. and H.R. Sarber. 1947. Harbor seals and sea lions in Alaska. U.S. Fish Wildl. Serv., Spec. Sci. Rept., 28: 1-22. Irving, L. and J.S. Hart. 1957. The metabolism and insulation of seals as bare skinned mammals in cold water. Can. J. Zool. 35:497-511. IUCN. 1981. Report of IUCN Workshop in Marine Mammal/Fishery Interactions, La Jolla, CA 30 March- 2 April 1981. International Union for the Conservation of Nature and Natural Resources. Morges, Switzerland. 57pp. + appendices. Jeffries, S.J. 1984. Marine mammals at the Coltunbia River estuary. Final report on the marine mammal work unit of the Colwnbia River Estuary Data Development Program. 80pp. Jeffries, S.J. 1986. Seasonal movements and population trends of harbor seals, P/wca vitulina richardsi in the Coltunbia River and adjacent waters of Washington and Oregon: 1976-1982. Report to the U.S. Marine Mammal Comm, Contract No. MM2079357-5. Jeffries, S.J., R.F. Brown, and J.T. Harvey. 1993. Techniques for capturing, handling, and marking harbour seals. Aquat. Mamm. 19(1): 21-25. Jobling, M. 1987. Marine mammal faeces as indicators of prey importance- a source of error in bioenergetic studies. Sarsia 72:255-260. Jones, R.E. 1981. Food habits of smaller marine mammals from Northern California. Proceedings of the CA Acad. Sci. 42:409-433. 71 Kenyon, K. W. 1965. Food habits of harbor seals at Amchitka Island, Alaska. J. Mamm. 46(1): 103-104. King, J. E. 1983. Seals of the World. British Museum Nat. His. (2nd Ed.). Cornell Univ. Press. 204pp. Knudtson, P.M. 1974. Mother-pup behavior within a pupping colony of harbor seals (Phoca vitulina) in Humbolt Bay, California. M.S. Thesis, CSU Humbolt, CA. 42pp. Krygier, E.E. and W.G. Pearcy. 1986. The role of estuarine and offshore nursery areas for young English sole, Parophn;s vetulus, of Oregon. Fish. Bull., 84: 119-132. Ling, J.K. and M.M. Bryden. 1981. Southern elephant seal - Mirounga leonina. In: Handbook of Marine Mammals. Vol. 2, Seals. S.H. Ridgeway and R.J. Harrison eds.). Academic Press, London. 297-327pp. Love, R. M. 1991. Probably More Than You Wanted to Know About the Fishes of the Pacific Coast. Really Big Press, CA. 215pp. Lowry, M. S., C. W. Oliver, and C. Macky. 1990. Food habits of California sea lions, Zalophus californianus, at San Clemente Island, California, 1981- 1986. Fish. Bull. 88: 509-521. Mais, K.F. 1974. Pelagic fish surveys in the California current. Cal. Dept. Fish and Game, Fish Bull. 162: 1-79. Matarese, A. C., A. W. Kendall jr., D. M. Blood, and B. M. Vinter. 1989. Laboratory guide to early life history stages of NE Pacific fishes. U. S. Dept. Commerce.NOAA Tech Rept. NMFS 80. Mate, B.R. 1980. Workshop Report on Marine Mammal Fisheries Interactions in the Eastern Pacific. Natl. Tech. Inf. Serv., 80pp. Mate, B.R. and J.T. Harvey. 1987. Acoustical deterrents in marine mammals conficts with fisheries. Workshop, 17-18 February, 1986, Newport Oregon. Oregon State University, Publ. No. ORESU-W-86·001, 116pp. Miller, D. J. and R. N. Lea. 1972. Guide to the Coastal Marine Fishes of California. Calif. DepT. Fish Game. Fish Bull. 157, 235p. 72 Miller, D. J., M. J. Herder and J.P. Scholl. 1983. California marine mammal fishery interaction study, 1979-1981. NMFS, Southwest Fisheries Center, Admin. Rep. JL-83-13c. Morejohn, G. V, Harvey, J. T., and L. T. Krasnow. 1978. The importance of Loligo opalescens in the food web of marine vertebrates in Monterey Bay, California. Cal. Fish and Ga!fie, Fish Bull. 169: 67-97. Morris, RH., D.P. Abbott, and E. C. Haderlie. 1980. Intertidal Invertebrates of California. Stanford University Press, Stanford, California. Murie, D. J. and D. M. Lavigne. 1985. A technique for the recovery of otoliths from stomach contents of piscivorous pinnipeds. J. Wildl. Mgmt. 49: 910-912. Murie, D.J. and D.M. Lavigne. 1986. interpretation of otoliths in stomach content analyses of phocid seals: quantifying fish consumption. Can. J. Zool. 64: 1152-1157. Newby, T.C. 1973. Changes in the Washington State harbor seal population. Murrelet, 54: 4-6. Nordoy, E. S. and A. S. Blix. 1988. Sources of error in estimating food requirements of seals. Polar Record, 24: 62-64. Olesiuk, P. F. 1993. Annual prey consumption by harbor seals (Phoca vitulina) in the Strait of Georgia, British Columbia. Fish. Bull. 91: 491-515. Osborne, L. S. 1985. Population Dynamics, Behavior, and the Effect of Disturbance on Haul-out Patterns of the Harbor Seal, Phoca vitulina richardsi, in Elkhorn Slough, Monterey Bay, California. Senior thesis, ucsc. 75pp. Osborne, L. S. 1992. A report of preliminary findings: Harbor seal studies in Elkhorn Slough, 1989 to 1991. CDF&G, Monterey. Payne, P. M. and D. C. Schneider. 1984. Yearly changes in abundance of harbor seals, Phoca vitulina, at a winter site in Massachusetts. Fish. Bull., 82:440-442. Payne, P.M. and L.A. Selzer. 1989. The distribution, abundance and selected prey of the harbor seal, Phoca vitulina concolor, in southern New England. Mar. Mamm. Sci., 5: 173-192. 73 Phillips, J.B. 1964. Life history studies on ten species of rockfish (genus sebastodes). CDF&G Fish Bull. 126. 70pp. Pierce, G.J. and P.R Boyle. 1991. A review of methods for diet analysis in piscivorous marine mammals. Oceanogr. Mar. Bioi. Annu. Rev. 29: 409-486). Pingree, RD., P.M. Holligan, and G.T. Mardell. 1978. The effects of vertical stability on phytoplankton distributions in the summer on the northwest European Shelf. Deep Sea Research. 25: 10011·28. Pinkas, L., M.S. Oliphant, and I.L.K. Iverson. 1971. Food habits of albacore, bluefin tuna, and bonito in California waters. Calif. Fish. and Game. Fish. Bull. 152: 1-105. Pitcher, KW. 1977. Population productivity and food habits of harbor seals in the Prince William Sound Copper River Delta area Alaska. Report to U.S. Marine Mammal Comm. Report No. MMC-75/03 36pp. Pitcher, KW. 1980. Stomach contents and feces as indicators of harbor seal, Plwca vitulina, foods in the Gulf of Alaska. Fish Bull. 78(3): 797-798. Power, G. and J. Gregoire. 1978. Predation of freshwater seals on the fish community of Lower Seal Lake, Quebec. J. Fish. Res. Board Canada, 35: 844-850. Prime, J.H. 1979. Observations on the digestion of some gadiod fish otoliths by a young common seal. Int. Counc. Explor. Sea CM 1979/N No. 14. Prime, J.H. and P.S. Hammond. 1987. Quantitative assessment of grey seal diet from fecal analysis, pp. 165-181. In: A.C. Huntley, D.P. Costa, G.A.J. Worthy, and M.A. Castellini (eds.), Approaches to Marine Mammal Energetics. Society for Marine Mammalogy, Lawrence, Kansas. Recksiek, C.W. and H.W. Frey. 1978. Background of the market squid research program, basic life history and the California fishery. In: Biological Oceanographic, and Acoustic Aspects of the Market Squid (Loligo opalescens) C.W. Recksiek and H.W. Frey (eds.). CDF&G, Fish Bull. 169. 74 Reijnders, P.J.H. 1976. The harbour seal (Phoca vitulina) population in the Dutch Wadden Sea: size and composition. Netherlands Journal of Sea Reasearch 10: 223-235. Reijnders, P.J.H. 1983. The effect of seal hunting in Germany on the further existence of a harbour seal population in the Dutch Wadden Sea. Z. Saeugetierkd. 48: 50-54. Riedman, M. 1990. The Pinnipeds; Seals, Sea lions, and Walruses. Univ. Calif. Press. Berkely. 439pp. Roffe, T.J. 1980. Populations, Food habits, and Behavior on Pinnipeds in the Rogue River and their Relationship to Salmonid Runs. Ph.D. Thesis, Oregon State University. 155pp. Roffe, T.J., and B.R. Mate. 1984. Abundance and feeding habits of pinnipeds in the Rogue River, Oregon. J Wildl. Manage., 48: 1262-1274. Scheffer, T.H., and J.W. Slipp. 1944. The harbor seal in Washington state. Amer. Midland Nat. 32: 372-416. Scheffer, T.H., and C.C. Sperry. 1931. Food habits of the Pacific harbor seal, Phoca richardsi. J. Mamm., 12:214-226. ··• Schneider, D. C. and P.M. Payne. 1983. Factors affecting haul-out at a site in southeastern Massachusetts. J. Mamm., 64: 518-5520. Scardino, J. and B. Pfeifer. 1993. Sea lion/steelhead conflict at the Ballard Locks: A history of control efforts to date and a bibliography of technical reports. NMFS, northwest region, Seattle. Nov. 1993. Selzer, L.A., G. Early, P.M. Fiorelli, P.M. Payne, and R. Prescott. 1986. Stranded animals as indicators of prey utilization by harbor seal Phoca vitulina concolor, in southern New England. Fish. Bull. 84(1): 217-221. Silver, M. W. 1975. The habitat of Salpa fusiformis in the California current as defined by indicator assemblages. Urnnol. and Oceanogr. 20(2): 230-237. Spalding, D. J. 1964. Comparative feeding habits of the fur seal., sea lion and harbor seal on the British Columbia coast. Fish. Res. Bd. Can. Bull. No. 146. 52pp. 75 Stewart, B. S. 1984. Diurnal hauling patterns of harbor seals at San Miguel Island, California. J. Wildl. Mgmt., 48: 1459-1461. Stewart, B. S., and P. K. Yochem. 1983. Radiotelemetry studies of hauling patterns, movements, and site fidelity of harbor seals (P/Joca vitulina richardsi) at San Nicholas and San Miguel Islands, California, 1982. Hubbs-Sea World Research Inst.,_ Tech. Rept., 83-152:1-56. Stewart, B. S., and P. K. Yochem. 1985. Feeding habits of harbor seals (Piwca vitulina) at San Nicholas Island, California, 1980-1985. In: Proceedings of the Sixth Biennial Conference on the Biology of Marine Mammals, Nov. 22-26, Vancouver, B.C. 130pp. Stoel, P. 1981. Site fidelity and movements of harbor seals (Piwca vitulina) on the central Oregon coast. Abstract, Fourth Biennial Marine Mammal Conference, San Francisco, December 14-18. Sullivan, R.M. 1979. Behavior and ecology of harbor seals, Phoca vitulina, along the open coast of northern California. J. Mamm., 62: 825-831. Sullivan, R.M. 1980. Seasonal occurrences and haul-out use in pinnipeds along Humboldt County, California. J. Mamm. 61(4): 754-760. Terhune, J.M. 1985. Scanning behavior of harbor seals on haul-out sites. J. Mamm., 66: 392-395. Thompson, P. M. 1989. Seasonal changes in the distribution and composition of the common seal (Phoca vitulina) haul-out groups. J. Zool. Land. 217:281-294. Thompson, P. M., M.A. Fedak, B.J. McConnell, and K.S. Nicholas. 1989. Seasonal and sex-related variation in the activity patterns of common seals (P/Joca vitulina). J. Applied Ecology. 26: 521-535. Thompson, P.M., D. Wood, D. Tollit, H. Corpe, and P. Racey. 1993. The behavior and ecology of common and grey seals in the Moray Firth. Report to SOAFD. Jan. 1993. 29pp. Torok, M. L. 1994. Movements, Daily Activity Patterns, Dive Behavior, and Food Habits of Harbor Seals (P/Joca vitulina riclmrdsi) in San Francisco Bay, California. M.S. Thesis Moss Landing Marine Laboratories/CSU Stanislaus. 88pp. 76 Vaughan, R.W. 1978. A study of common seals on the Wash. Mammal Review,8: 25-34. Wahl, T.R. 1977. Sight records of some marine mammals offshore from Westport, Washington. Murrelet 58: 21-23. Wang, J. C. S. 1986. Fishes of the Sacramento-San Joaquin Estuary and adjacent waters, California. A guide to the early life histories. Interagency ecological study program for the Sacramento-San Joaquin Estuary. Tech. Rep., 9. 323 pp. Watts, P. 1992. Thermal constraints on hauling out by harbour seals (Phoca vitulina). Can. J. Zool. 70:553-560. Wilson, S.C. 1978. social organization and behavior of harbor seals, Plwca vitulina cancolor, in Maine, Final Rep. Mar. Mammal Comm. MM6AC013 NTIS PB-280 188. 102PP. Wolff, G. A 1982. A beak key for eight eastern tropical Pacific cephalopod species with relationships between beak dimensions and size. Fish. Bull., 80:357-370. Yochem, P. K., B.S. Stewart, R. L. DeLong, and D.P. Demaster. 1987. Diel haul out patterns and site fidelity of harbor seals (Phoca vitulina richardsi) on San Miguel Island, California in Autumn. Mar. Mamm. Sci., 3:323-332. Yoklavich, M.M., G.M. Cailliet, J.P. Barry, D.A. Ambrose, and B. S. Antrim. 1991. Temporal and spatial patterns in abundance and diversity of fish assemblages in Elkhorn Slough, California. Estuaries, 14(4): 465-480. 77 Table 1: Data on seals captured and radio-tagged in Elkhorn Slough, CAin September. 1991. B/W indicates black spots on white pelage, whereas W /B indicates white spots on black pelage. • Seals 170,222, and 530were captured at Seal Bend on 9/4/91. All remaining seals were caEtured at Dairies on 9I 5I 91. Freq. (164) 020 070 090 170 222 250 300 530 Sex M M F F M F F F FLIPPER TAG# Left 217 224 209 205 202 220 222 207 Right 216 225 210 206 203 221 223 208 Wt (kg) 41 41 30 37 48 53 57 36 SL (em) 108 117 103 121 126 115 125 111 Girth (an) 84 86 72.5 84 89 91 97 86 Cranial wid!:h(mm) 104 93 100 95 98 91 108 84 ULTRASOUND (mm) Rtaxial 15 14 12 20 18 Mid-abdominal 13 13 12 19 18 PINCH TEST (mm) Rt axial 16 24 25 21 23 25 43 30 Mid-abdominal 16 23 25 22 24 29 49 26 Mid-dorsal 25 31 Pelage description B/W B/W W/B W/B W/B B/W W/B W/B 78 Table 2. Mean, standard error (SE) and range of hrs/ day ashore in Elkhorn Slough (ES), duration of haul-out bout, hrs/ day diving in ES, Total time spent in ESper 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. An 'n' of 1 indicates only one complete 24-h.r track. (Note: Harbor seals did not go ashore while in Monterey Bay) SEALI.D. 020 070 090 "170 222 250 300 530 Hrs/day ashore in ES mean 6.6 5.4 5.8 5.9 7.9 3.9 5.8 3.6 SE 0.1 1.3 1.5 0.4 1.3 0.9 Range 6.5-6.7 1.4-8.0 3.0-7.6 7.3-8.8 4.5-7.0 2.0-7.0 n 2 5 1 3 3 1 2 5 Duration of bout mean 6.6 5.4 5.8 4.5 5.9 3.9 5.8 3.6 SE 0.1 1.3 1.7 1.9 1.3 0.9 Range 6.5-6.7 1.4-8.0 0.4-7.2 0.3-8.8 4.5-7.0 2.0-7.0 n 2 5 1 4 4 1 2 5 Hrs/day Diving inES mean 4.9 5.2 6.5 4.6 5.5 8.0 4.8 6.0 SE 0.8 0.8 0.5 2.6 Range 4.1-5.7 4.3-7.5 4.1-5.1 3.0-11.3 n 2 4 1 2 1 1 1 3 "). Total time spent in ES mean 11.9 11.8 15.7 13.0 13.5 12.3 11.8 11.9 SE 0.2 1.3 1.2 2.0 Range 11.8-12.2 8.4-14.5 11.8-14.3 7.8-14.3 n 2 4 1 2 1 1 1 3 Hrslday Diving inMB mean 11.9 12.2 8.3 10.9 10.5 11.8 12.2 12.2 SE 0.2 1.3 1.2 2.0 Range 11.8-12.2 9.5-15.6 9.8-12.2 9.8-16.2 n 2 4 1 2 1 1 1 3 79 Table 3. Mean and standard error (SE) of dives and surface activity for eight harbor seals radio-tagged in Elkhorn Slough, CA, September 1991 to Apri11992. All maximum dive durations were recorded in Monterey Bay. Mean Mean Seal surface: surface: Mean dive: Mean dive: Max. dive I. D. ES (SE) MB (SE) • ES (SE) MB (SE) (min) (min) (min) (min) (min) 020 0.4 0.6 1.7 3.9 12.1 (0.03) (0.02) (0.1) (0.2) 070 0.4 0.4 1.6 4.6 15.4 (0.03) (0.01) (0.1) (0.1) 090 0.4 0.2 1.0 2.8 7.4 (0.07) (0.06) (0.1) (0.6) 170 0.4 0.6 2.0 4.8 16.4 (0.02) (0.02) (0.2) (0.2) 222 0.4 0.4 2.6 3.9 16.6 (0.04) (0.02) (0.3) (0.2) .,, 250 0.6 0.5 3.3 5.1 7.9 (0.05) (0.05) (0.2) (0.4) 300 0.4 0.4 3.7 8.2 15.5 (0.03) (0.04) (0.4) (0.7) 530 0.6 0.6 1.6 3.6 9.2 (0.13) (0.01) (0.1) (0.1) 80 Table 4. Mean number per 10-min tow (n), standard error (in parentheses), and relative abundance(%) of fish species caught in 83 day-time otter trawls at three locations in Elkhorn Slough. Overall relative abundance and rank are indicated. Dash(--) indicates species not collected. Brid!le Dairy Kirbz.: Overall Taxon Mean (S.E.) % Mean(S.E.) % Mean (S.E.) % % Rank Citl!aricl!tlrys sti"""""'s 3.07 (1.21) 18.4 1.6l. (0.44) 14.4 0.89 (0.27) 2.1 7.83 5 Clevelandia ios 0.04 (0.04) .03 0.14 (0.10) 0.3 0.26 18 Cym.atof(aster af(;.rr;.watn 1.78 (0.76) 10.6 1.82 (0.85) 16.3 18.29 (6.75) 43.0 31.25 1 Damalicllthys 'IJacca 0.15 (0.07) 0.9 0.14(0.08) 1.3 0.07 (0.07) 0.2 0.51 15 Embiafaco jacksoni 2.56 (0.94) 15.3 0.04 (0.04) 0.3 0.04(0.04) 0.08 3.63 8 En;rraulis morda.x 0.11 (0.06) 0.3 0.15 22 Gibbsonsia sp. 0.19 (0.08) 1.1 0.26 18 Hexa~ammos sp. 0.04 (0.04) 0.2 0.05 29 Hexagrammos superciliosus 0.04 (0.04) 0.3 0.05 29 Hyperprosopon arRenteum 0.11 (0.08) 0.7 0.04 (O.o4) 0.3 0.25 (0.11) 0.6 0.56 14 Hypsopsella ;;uthl/ala 0.18 (0.18) 0.4 026 18 Lepidagobius tepid us 0.04 (0.04) 0.08 0.05 29 Lcplacaltus amtlllus 0.74 (0.25) 4.4 1.11 (0.38) 9.9 5.68 (1.61) 13.4 10.74 3 Loliga opa]escens 0.04(0.04) 0.08 0.05 29 Micronzetrus mittimus 0.56 (0.29) 3.3 0.07 (0.07) 0.6 0.04 (0.04) 0.08 0.92 12 Mustelus lwnlei 0.04 (0.04) 0.3 0.71 (0.31) 1.7 1.07 11 Myliolmtis coli{nmico 0.04 (0.04) 0.3 0.07 (0.05) 0.2 0.15 22 Neoclinus uninotatus 0.07 (0.05) 0.4 0.10 25 Ophiodon elongalus 0.48 (0.28) 2.9 0.64 (0.38) 5.8 1.59 9 Paralichtl!ys coli{arnicus 0.07 (0.05) 0.4 0.18 (O.D9) 0.4 0.36 16 Pleuronec.tes vetulus 1.52 (1.01) 9.1 2.61 (1.15) 22.4 9.00 (3.64) 21.2 18.72 2 Plumerodon furcatus 1.19 (0.71) 7.1 1.57 (0.81) 14.1 1.00 (0.53) 2.4 5.32 6 Piatichthys stellatus 0.04 (0.04) 0.2 0.04 (0.04) 0.08 0.10 25 Plalyrhinaidis lriseriala 0.04(0.04) 0.3 0.04 (0.04) 0.08 0.10 25 Pleuronicltthys decurretts 0.04 (0.04) 0.2 0.07 (0.07) 0.6 0.15 22 Pleuronichthys ritteri 0.04 (0.04) 0.2 0.05 29 Poricltlhys nota/us 0.04 (0.04) 0.08 0.05 29 Rhacochilus toxates 0.04(0.04) 0.08 0.05 29 Scorpaem'cltt1Jys nwrmora.tus 2.89 (0.76) 17.3 0.07 (0.05) 0.6 0.11 (0.06) 0.3 4.25 7 Sebastes sp. 0.19 (0.08) 1.1 0.21 (0.15) 1.9 0.04 (0.04) 0.08 0.61 13 S. auricu/atus 0.07 {0.05) 0.6 0.10 25 S. camatus 0.04 (O.o4) 0.2 0.05 29 S. f/avidus 0.04 (O.D4) 0.2 0.05 29 5. melanops 0.04 (0.04) 0.08 0.05 29 S. paucLr::pinis 0.04 (0,04) 0.08 0.05 29 S. rastrellif(er 0.22 (0.12) 1.3 0.04 {0.04) 0.08 0.36 16 Syn;;nalhus sp. 0.63 (0.26) 3.9 0.32 (0.19) 2.9 0.18 (0.09) 0.4 1.59 9 Symphurus alricauda 0.07 (0.07) 0.4 0.54 (0.37) 4.8 5.11 (1.43) 12.0 8.18 4 Triakis semi[asciata 0.04 {0.04) 0.3 0.11 (0.06) 0.3 0.20 21 T riden!iger lri!{onocephalus 0.04 (0.04) 0.08 0.05 29 Urolophus halleri 0.04(0.04) 0.2 0.05 29 81 Table 5. Surrunary of diversity, dominance index, and abundance of fishes captured with otter trawls at three stations in Elkhorn Slough from 1974 -1976 (from Yoklavich et al. 1991) and 1991 -1992. 1974-1976 (day) 1991-1992 (day) 1991-1992 (nite) Bridge Dairy Kirby Bridge Dairy Kirby Bridge Dairy #of species 37 37 26 25 22 30 33 24 Dominance 0.26 0.19 0.32 Q.12 0.14 0.26 0.13 0.17 Total# 7,326 3,244 3,807 452 312 1,191 1,018 443 Mean#/ tow 209.3 66.2 77.7 16.74 11.14 42.54 59.88 36.92 (S.E.) (82.4) (2.0) (15.3) (5.11) (2.91) {10.71) (23.94) (10.27) #of tows 35 49 49 27 28 28 17 12 82 Table 6. Relative abundance(%) of dominant species totaling 80% or greater of fishes collected by otter trawl at three stations in Elkhorn Slough from 1974 to 1976 (from Yoklavich et al. 1991) and 1991- 1992. 1974-1976 (da:J::) 1991-1992 (da:J::) 1991-1992 (nite) SEecies Brid!le Dai!J: Kirbv Brid!le Dairy Kirb:z: Bridge Dai!_L Citharichthys stiJ<~rmeus 10.3 4.4 18.4 14.4 21.9 26.2 Cmrmtoxaster a:zy:re>(ala 43.4 28.8 53J 10.6 16.3 42.9 13.6 21.2 Embiotom ;acksoni 14.6 13.4 15.3 14.7 Leptocattus ammtus 13.1 4.4 9.9 13.4 21.0 Ophiodon elott>(atus 5.8 Pleuronectes vetulus 4.0 10.7 9.1 23.4 21.2 7.7 8.8 Phanerodon furmtus 16.6 28.4 7.1 14.1 11.0 Platichthys stellatus 3.9 5.9 Scorpaenicltlh)IS mannorntus 17.3 12.4 Smnphurus atricauda 12.0 4.1 Total(%) 84.9 82.9 83.4 82.2 83.9 89.5 81.3 81.3 No. of dominant sEecies 4 6 4 7 6 4 6 5 83 Table 7. Seasonal comparison of species composition at each station from daytime otter traw Is conducted during 1991 in Elkhorn Slough based on percent similarity index (PSI). Dashes(-) indicate redundant comparisons. BRIDGE Spring Summer Autumn Winter 0.53 0.45 0.58 Spring 0.31 0.34 Summer 0.63 DAIRY Spring Summer Autumn Winter 0.49 0.07 0.07 Spring 0.20 0.22 Summer 0.75 KIRBY PARK Spring Summer Autumn Winter 0.58 0.13 0.15 Spring 0.44 0.32 Summer 0.68 84 Table 8. Mean number per 10-min tow (n), standard error (in parentheses), and relative abundance(%) of fish species caught in night-time otter trawls at two locations in Elkhorn Slough. Overall relative abundance and rank are indicated. Dash (--) indicates species not collected. Bridge Dai!'l Overall Taxon Mean (S.E.) % Mean(S.E.) % % Rank AcanthnRobius flavinmnus 0.18 (0.13) 0.29 0.21 21 Aulorlryncltus flavidus 0.12 (0.12) 0.20 0.14 24 Chilara taylori 0.41 (0.26) 0.69 0.17 (0.11) 0.45 0.62 13 CWwricltf!tys sii![n!I11?Us 13.12 (5.90) 21.91 9.67 (5.56) 26.19 23.20 1 Cleve/andia ios 0.17 (0.17) 0.45 0.14 24 Oupea pallasi 0.08 (0.08) 0.23 0.07 32 Cymt!IO!Iaster •K!/Te)?ala 8.12 (3.67) 13.56 7.83 (3.18) 21.22 15.88 2 Damaliclttltys vacca 0.24 (0.18) 0.39 0.25 (0.25) 0.68 0.48 16 Embiotoca jacksoni 8.76 (6.53) 14.64 0.17 (0.17) 0.45 10.34 4 Gibbsonsia sp. 0.24 (0.14) 0.39 0.25 (0.25) 0.68 0.48 16 He:raxrammos deca~ammus 0.41 (0.30) 0.69 0.48 16 Hyperprosopon nr~enteum 0.12 (0.08) 0.20 0.14 24 Hypsurus caryi 0.18 (0.10) 0.29 0.21 21 Lcpido)lobiuslepidus 0.12 (0.08) 0.20 0.50 (0.36) 1.35 0.55 14 Lcptacottus armatus 3.53 (0.99) 5.89 7.75 (3.50) 20.99 10.47 3 Liparis sp. 0.12 (0.12) 0.20 0.14 24 Lo!i)IO upalescens 0.12 (0.08) 0.20 0.14 24 Micrometros minimus 1.29 (0.84) 2.16 1.51 11 Muslelus henlei 0.08 (0.08) 0.23 0.07 32 M.vliobahts califrJmica 0.12 (0.12) 0.20 0.14 24 Neoc.linu.s unitwtatus 0.24 (0.18) 0.39 0.27 20 Ophiodon elon)latus 1.35 (0.64) 2.26 1.17 (0.66) 3.16 2.53 8 "'t Paralichthys californicus 0.17 (0.11) 0.45 0.14 24 Pleuronectes vetuJus 4.59 (2.72) 7.66 3.25 (1.63) 8.80 8.01 7 Phanerodon furcatus 6.59 (3.62) 11.00 0.92 (0.67) 2.48 8.42 6 P1alichthys stellatus 0.06 (0.06) 0.10 0.07 32 Pleuronicltthys coenosus 0.18 (0.13) 0.29 0.21 21 Pleuronichthys decurrens 0.06 (0.06) 0.10 0.08 (0.08) 0.23 0.14 24 Pleuronic1zthys ritteri 0.06 (0.06) 0.10 0.07 32 Porichthys 1wtatus 0.06 (0.06) 0.10 0.58 (0.36) 1.58 0.55 14 RJmcochilus toxofes 0.06 (0.06) 0.10 0.07 32 Scnrpaenichtltys mammrafn.s 7.41 (5.69) 12.38 0.67 (0.36) 1.81 9.17 5 Sebastes sp. 0.08 (0.08) 0.23 0.07 32 5. auriculatus 0.76 (0.42) 1.28 1.00 (0.56) 2.71 1.71 9 S. rastrelli}ier 0.35 (0.21) 0.59 0.08 (0.08) 0.23 0.48 16 Seripirus po1ifus 0.06 (0.06) 0.10 0.07 32 Sy11:?11athus sp. 0.59 (0.36) 0.98 0.42 (0.29) 1.13 1.03 12 Symphurus atricauda 0.29 (0.14) 0.49 1.50 (0.75) 4.06 1.57 10 Synadus lucioceps 0.08 (0.08) 0.23 0.07 32 85 Table 9: Mean percentage number ('Yon), mean percentage mass {%M), percent frequency of occurrence (%FO),and mean index of relative importance (IRI) of prey items found in harbor seal scats collected in Elkhorn Slough, CA during winter (Nov.-Jan.; n=64) 1991. Standard error in parentheses. Listed in order of decreasing IRI. Prey species Mean%n Mean%M %FO MeaniRI (SE) (SE) (SE) Octopussp. 39.69 (4.50) 31.87 (4.34) 75.00 5367.77 (663.45) Loligo opalescens 19.54 (3.85) 18.81 (3.86) 54.69 2097.81 (421.65) Chilara taylori 7.44 (2.12) 7.10 (2.17) 43.75 635.81 (188.20) Spirinchus starksi 5.97 (2.65) 4.51 (2.39) 12.50 131.05 (63.07) Leptocottus armatus 1.81 (0.82) 4.04 (1.58) 18.75 109.86 (44.96) Poriclztlrys notatztS 2.85 (1.63) 3.92 (1.99) 15.63 105.81 (56.48) Sebastes sp. 2.66 (1.09) 3.44 (1.21) 14.06 85.76 (32.32) Genyonemus lineatus 1.36 (0.61) 4.35 (1.65) 14.06 80.24 (31.70) Citl!arichthys sordidttS 4.21 (2.00) 5.07 (2.41) 7.81 72.47 (34.43) Pleuronedes vetulus 2.30 (1.23) 259 (1.39) 9.38 45.97 (24.60) Eptatretus sp. 3.31 (2.19) 3.72 (225) 6.25 43.94 (27.77) Microstomus pacijicus 1.54 (0.66) 0.54 (0.23) 14.06 29.29 (12.53) Opltiodon elongatus 0.58 (0.35) 3.18 (1.62) 6.25 23.53 (12.30) Anoplopoma fimbria 1.82 (1.29) 2.53 (1.78) 3.13 13.59 (9.62) Errex zachirus 0.95 (0.65) 0.69 (0.49) 4.69 7.73 (5.36) Clupea pallasi 1.08 (1.08) 1.01 (1.01) 1.56 3.26 (3.26) ">\ Citlmrichthys sHgmaeztS 0.83 (0.60) 0.18 (0.12) 3.13 3.15 (2.27) Engraulis mordax 0.74 (0.56) 0.16 (0.12) 3.13 284 (2.14) PlaHchthys stellatus 0.14 (0.11) 0.37 (0.27) 3.13 1.59 (1.19) Atl!erinapsis californiensis 0.26 (0.26) 0.72 (0.72) 1.56 1.53 (1.53) Zoarcidae 0.17 (0.12) 0.26 (0.19) 3.13 1.35 (0.97) Ampl!isHchus rhodoterus 0.52 (0.52) 0.28 (0.28) 1.56 1.25 (1.25) Pleuronichthys sp. 0.31 (0.31) 0.36 (0.36) 1.56 1.04 (1.04) Cymatogaster aggregata 0.11 (0.11) 0.22 (0.22) 1.56 0.51 (0.51) Eopsetta exilis 0.03 (0.03) 0.04 (0.04) 1.56 0.11 (0.11) Hyperprosopon ellipHcum 0.04 (0.04) 0.03 (0.03) 1.56 0.10 (0.10) Symphurus atricauda 0.03 (0.03) 0.001 (0.001) 1.56 0.05 (0.05) 86 Table 10: Mean percent number(%n), mean percent mass (%M), percent frequency of occurrence (%FO),and mean index of relative importance (IRI) of prey items found in harbor seal scats collected in Elkhorn Slough, CA during spring (Feb.-April; n=70) 1991. Standard error in parenthesis. Usted in order of decreasing IRI. Prey species Mean%n Mean%M %FO MeaniRI (SE) • (SE) (SE) Octopus sp. 28.85 (4.06) 21.37 (3.68) 62.86 3156.51 (486.52) Chilara taylori 10.67 (2.70) 10.68 (2.77) 48.57 1036.93 (265.03) Citharichthys sordiaus 10.70 (2.74) 14.80 (3.38) 28.57 728.53 (174.78) Loligo apalescens 13.29 (3.50) 12.56 (3.40) 27.14 701.57 (187.47) Microstomus pacificus 6.78 (1.91) 5.41 (1.71) 31.43 383.29 (113.95) Eptatretus sp. 5.00 (1.96) 5.55 (2.23) 14.29 150.65 (59.88) Errex Zllcltirus 3.23 (0.85) 2.06 (0.59) 22.86 120.95 (32.77) Porichthys notatus 2.06 (0.82) 3.39 (1.28) 17.14 93.58 (36.11) Pleuronectes vetu!us 1.35 (0.43) 3.91 (1.25) 15.71 82.66 (26.37) Spirinchus starski 3.13 (1.30) 0.98 (0.55) 14.29 58.71 (26.44) Genyonemus lineatus 1.39 (0.60) 3.70 (1.44) 11.43 58.16 (23.32) Sebastes sp. 3.12 (2.01) 3.16 (2.01) 7.14 44.86 (28.70) Leptocottus armatus 1.45 (0.83) 2.36 (1.46) 8.57 32.69 (19.56) Cymatogaster aggregata 1.07 (0.44) 0.54 (0.29) 12.86 20.63 (9.43) Merluccius productus 1.03 (0.44) 0.55 (0.22) 10.00 15.80 (6.61) Engraulis mordax 1.61 (1.43) 1.52 (1.43) 4.29 13.40 (12.26) Ophiodon elongatus 0.49 (0.32) 2.36 (1.47) 4.29 12.21 (7.68) Zoarcidae 1.38 (1.01) 1.12 (0.97) 4.29 10.73 (8.47) Lampetra sp. 1.43 (1.43) 1.43 (1.43) 1.43 4.08 (4.08) Phanerodon furcatus 0.71 (0.71) 1.32 (1.32) 1.43 2.91 (2.91) Citharicl!thys stigmaeus 0.74 (0.71) 0.17 (0.16) 2.86 2.59 (2.49) Pleuronichthys sp. 0.17 (0.13) 0.32 (0.23) 2.86 1.40 (1.03) Eapsetta jordoni 0.12 (0.12) 0.34 (0.34) 1.43 0.66 (0.66) Amphistichus rhodoteniS 0.07 (0.07) 0.19 (0.19) 1.43 0.38 (0.38) Eapsetta exilis 0.08 (0.06) 0.05 (0.04) 2.86 0.36 (0.28) Hippoglossoides elassodon 0.08 (0.08) 0.06 (0.06) 1.43 0.21 (0.21) 87 Table 11: Mean percent number(%n), mean percent mass (%M), percent frequency of occurrence (%FO),and mean index of relative importance (IRI) of prey items found in harbor seal scats collected in Elkhorn Slough, CA during summer (May.-July; n=86) 1991. Standard error in parenthesis. Listed in order of decreasing IRI. Prm species Mean %n(SE) Mean %M(SE) %FO Mean IRI (SE) SebllStes sp. 16.16 (3.29) • 17.96 (3.53) 33.72 1150.55 (230.27) Genyonemus lineatus 7.43 (1.79) 11.74 (2.64) 25.58 490.39 (113.33) Microstomus paci{iCIIs 9.03 (2.25) 7.89 (2.24) 24.42 413.12 (109.76} Chilara taylori 8.73 (2.31) 6.39 (2.14) 25.58 386.73 (113.75) Porichthys notatus 7.53 (2.47) 8.36 (2.67} 18.61 295.77 (95.66) Pleuronectes vehtlus 6.07 (1.95) 8.40 (2.39) 19.76 286.01 (85.68) Octopus sp 5.39 (1.41) 3.14 (1.24) 25.58 218.26 (67.74) Merlucdus productus 3.92 (1.25) 6.84 (2.14) 17.44 187.56 (59.16) Loli)1o opalescens 5.86 (2.09) 4.12 (1.79) 16.28 162.55 (63.36) Ctpnato!(llSter awe!(ata 2.87 (0.93) 2.43 (1.15) 15.11 80.13 (31.39) Clupea pallllSii 325 (1.45) 3.98 (1.76) 10.47 75.68 (33.63) Eptatretus sp. 3.89 (1.66) 2.69 (1.42) 10.47 68.92 (32.22} En)1raulis mordax 4.00 (1.62) 2.91 (1.64) 9.30 64.25 (30.34) CitharichthtfS sordidus 2.65 (1.02) 2.33 (1.02) 10.47 52.09 (21.38) Embiotocidrte 3.21 (153) 1.21 (1.16) 5.81 25.76 (15.66) Atherinopsis californiensis 1.07 (0.70) 2.07 (1.29) 3.49 10.97 (6.97) Spirinchus starski 1.26 (0.71) 0.33 (0.23) 5.81 9.25 (5.51) Leptocottus annatus 1.32 (0.72) 0.53 (0.29) 4.65 8.62 (4.69) Sciaenidae sp. 0.77(059) 0.67 (0.57) 3.49 5.03 (4.05) Citharichthys sti!(lnaeus 0.79 (0.52) 0.13 (0.10) 4.65 4.32 (2.89) Seriphus politus 052 (0.37) 1.07 (0.77} 2.33 3.71 (2.65) Errex zachirus 0.45 (0.27) 0.32 (0.19) 4.65 355 (2.16) Peprilus simillimus 0.87 (0.65) 0.64 (0.57) 2.33 352 (2.83) Eopsetta iordrtni 0.23 (0.16) 0.60 (0.49) 3.49 2.89 (2.24) Zoarcidrte 0.48 (0.29) 0.15 (0.10) 3.49 2.21 (1.34) Damalichthys vacca 0.31 (0.22) 0.46 (0.32) 2.33 1.79 (1.26) Roccus saxatilis 0.20 (0.17) 0.53 (0.39} 2.33 1.70 (1.29) Anoplopoma fimbria 0.23 (0.23) 0.92 (0.92) 1.16 1.34 (1.34) Amphistichus rhodotems 0.18 (0.17) 0.23 (0.19) 2.33 0.97 (0.83) Hyperprosopon ar)1en teum 0.06(0.05) 0.25 (0.25) 2.33 0.72 (0.69) Phanerodon furcatus 0.25 (0.20) 0.003 (0.002) 2.33 0.59 (0.47) Lepido!(Obius lepidus 0.21 (0.14) 0.02 (0.02) 2.33 053 (0.38) Ar!(entina sial is 0.19 (0.19) 0.04 (0.04) 1.16 0.28 (0.28) SebllStolobus alllScanus 0.04 (0.04) 0.10 (0.10) 1.16 0.17 (0.17) Lornpetra sp. 0.09 (0.09) 0.05 (0.05) 1.16 0.16 (0.16) Symphurus atricauda 0.11 (0.11) 0.01 (0.01) 1.16 0.13 (0.13) Eopsetta exi!is 0.05 (0.05) 0.04 (0.04) 1.16 0.11 (0.11) Zalembius rosaceus 0.06(0.06) 0.02 (0.02) 1.16 0.08 (0.08) 88 Table 12. Mean percent number (%n), mean percent mass (%M), percent frequency of occurrence (%FO),and mean index of relative importance (IRl) of prey items found in harbor seal scats collected in Elkhorn Slough, CA during autumn (Aug.-Oct.; n=86) 1991. Standard error in parenthesis. Listed in order of decreasing IRl. Prey species Mean%n Mean%M %FO MeaniRI (SE) (SE) (SE) Octopus sp 35.42 (4.29) 27.27 (3.96) 59.30 3718.26 (489.89) Cl!ilnra lmJlori 14.61 (2.43) 10.35 (2.01) 54.65 1363.99 (242.25) Genyonemus lineatus 6.99 (2.06) 12.02 (2.80) 26.74 508.24 (130.02) Poriclzthys no latus 4.56 (1.15) 6.17 (1.57) 27.91 299.38 (76.11) Sebastes sp. 6.47 (1.98) 6.99 (2.13) 22.09 297.36 (90.82) Ple11ronectes vetulus 4.66 (1.56) 6.32 (1.79) 18.61 204.24 (62.38) Leptocottus armatus 3.64 (1.35) 5.17 (1.81) 16.28 143.45 (51.40) Loligo apalescens 4.29 (1.55) 2.38 (0.95) 18.61 124.11 (46.48) Microstomus pacificus 3.37 (1.14) 2.92 (1.24) 12.79 80.46 (30.34) Cit71llrichtl!ys sordidus 2.38 (1.31) 2.87 (1.42) 11.63 61.03 (31.75) Merluccius productus 2.39 (1.34) 3.40 (1.66) 8.14 47.15 (24.43) Pleuronichth:ijs sp. 1.74 (0.79) 3.10 (1.23) 9.30 45.04 (18.82) Errex zachirus 1.84 (0.61) 1.31 (0.59) 13.95 43.94 (16.75) Eptatretus sp. 2.79 (1.38) 2.17 (1.23) 8.14 40.34 (21.22) Ophiodon elongatus 0.89 (0.44) 2.96 (1.47) 5.81 22.41 (11.14) Eapsetta e:rilis 1.02 (0.65) 1.15 (0.95) 4.65 10.10 (7.46) Cit11llrichthys stigmaeus 0.85 (0.46) 0.19 (0.10) 5.81 6.06 (3.26) Cymatogaster aggregata 0.21 (0.13) 0.34 (0.18) 4.65 2.53 (1.42) Sebastolobus alascanus 0.23 (0.18) 0.32 (0.25) 2.33 1.28 (0.99) Platichthys stelliltus 0.19 (0.15) 0.25 (0.18) 2.33 1.04 (0.75) Atherinapsis californiensis 0.03 (0.02) 0.41 (0.33) 2.33 1.03 (0.83) Pleuroniclzthys verticalis 0.23 (0.23) 0.58 (0.58) 1.16 0.95 (0.95) Zoarcidae 0.18 (0.13) 0.20 (0.15) 2.33 0.90 (0.66) Psettichthys melanostictus 0.17 (0.17) 0.46 (0.46) 1.16 0.72 (0.72) Sciaenidae sp. 0.23 (0.23) 0.16 (0.16) 1.16 0.45 (0.45) Engraulis mordax 0.29 (0.29) 0.05 (0.05) 1.16 0.39 (0.39) Otophidium scrippsi 0.17 (0.17) 0.14 (0.14) 1.16 0.36 (0.36) Symphurus atricauda 0.14 (0.14) 0.13 (0.13) 1.16 0.31 (0.31) Eopsetta jordani 0.01 (0.01) 0.09 (0.09) 1.16 0.11 (0.11) 89 Table 13. Seasonal comparison of prey species composition from harbor seal scats collected during 1991 in Elkhorn Slough based on percent similarity index (PSI). Dashes(--) indicate redundant comparisons. Summer Autumn Winter Spring 0.364 0.713 0.743 Summer 0.383 0.255 Autumn 0.733 90 Table 14. Percent similarity indices (PSI) comparing species composition 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. N I A indicates no trawl data was available for a comparison. DAY Trawl station Winter Spring Summer Autumn Bridge 0.00 0.01 0.13 0.05 Dairy 0.01 0.02 0.13 0.11 Kirby Park 0.03 0.03 0.10 0.06 NIGHT Bridge N/A 0.07 0.11 0.07 Dairy 0.05 0.07 0.13 0.10 91 Table 15. Conunercial fishery landings and estimates of annual fish consumption by Elkhorn Slough harbor seals, in kilograms, and by the entire harbor seal population of Monterey Bay, CA, during 1991. Conunercial Estimated annual Estimated annual Species fishery consumption by consumption by landings (kg) harbor seals in harbor seals in Elkhorn Slough (k~) Monterey Bay (kg) En:;;raulis mordax 608,814 192 3,031 Ophiodon elon:{atus 123,034 2,108 18,415 Sebastes spp. 1,418,296 6,828 31,898 Citharichthys spp. 48,124 2,678 41,421 Microstomus pacificus 645,588 1,427 7,669 Pleuronectes vetu/us 115,770 2,832 13,083 Errex zachirus 75,364 500 2,434 Loli:{o opalescens 6,116,742 3,980 28,371 Genyonemus lineatus 48,124 4,507 69,064 Figure 1. Three haul-out sites (e) for harbor seals, and trawling stations (t'YLi) in Elkhorn Slough, CA. Modified from Yoklavich et aL (1991). 93 180 160 e 140 ~ 120 ~ «: (!) .: 100 8· 8$ 0 .... e (!) s 80 i 60 ~ 40 20 0 .n ...... ;:.... (!) 00 .... > u z.. 0.. tl 0 180 160 ::r: 140 CJ ::;J 0 ...J Lfl 120 12 100 ;,.;~ ...J 3"' 80 ;:J < "',. 60 X < ::;s 40 20 0 1975 1983 1984 1987 1990 1991 YEAR Figure 3. Maximum number of seals observed in Elkhorn Slough during the past 16 years. Counts from 1975 from Harvey et aL (in press-a), and 1983- 1990 from Osborne (1985) and Hanan et al. (1993). 100 90 80 ib;:; 0 Cil 70 k.... ] 60 i5 ~ .s 50 >. u k 40 ~ 30 ~ !'I; ""c 20 10 0 20 70 90 170 222 250 300 530 20 70 90 170 222 250 300 530 Day Night Seal I. D.# Figure 4. Proportion of times radio-tagged harbor seals were found in Elkhorn Slough during surveys of the Monterey Bay area conducted during day (1000 ·1400 h) and night (1800- 2100 h). 96 Figure 5. Foraging locations of harbor seals radio-tagged in Elkhorn Slough, California. Seals were monitored between Sept. 1991 and April1992. Percent time Percent time Percent time ~ ab~~6gjgj~~~g 0 :S::E 06-08 06-08 06-08 0 ()'q !li ' & -g"' ;::J Ei OB-10 08-10 0""' OB-10 a ""'0 "' 1il" ro "' 0 nl 10-12 10-12 10-12 '"' "<: "'!" 12-14 12-14 12-14 tJ:l '"0 ro ::l 14-16 14-16 14-16 "<: .., "' n ro ~ 1&-18 1&-18 16-18 ;::J 0 "' "~ :s: ~ 18-20 18-20 18-20 tJ:l 0 :::r. ~ - '< 20-22 20-22 20-22 "';::J ~ 22-24 22-24 22-24 0.. :::r trl 24-02 24-02 24-02 "'& ~ 0 02-04 02-04 02-04 ... 04-06 ()4..06 04-06 3 "'ro "'0 "' "'OJ §I ~ 0..c;· ~ ~ ~ ~ trl ~' :c oe::s:~g;~as~;:;gs(3g oo~gs~~s~~~8 oO~~c!;~g)~~t§g - ()'q "" r:;'()'q"' "' 0 z 06-08 06-08 0.., 06-08 "' ro ~ ~ 0.. N 0 N :l 08-10 jj OB-10 OB-10 en ro s· Cl 0 10-12 10-!2 10-!2 "';::J trl II 12-14 12-14 12-14 "" ro - ' '1:l"' ":!!.0 ~ til 14-16 14-16 14-16 Iii 3 0 :;· 16-18 1&-18 1&-18 so" i£l ro 0 m 18-20 18-20 18-20 ... c til -o:::r~()'q 20-22 20-22 20-22 \0>-''1:) "' 22-24 22-24 22-24 (!) § ;::J 24-02 24-02 24-02 ~ II 0.. 02-04 02-04 02-04 '5i' "'~ ()4..06 til 04-06 04-06 e ~· ""0 El "<: t:;· 5' >-'()'q s: "''- (D t::lo. ~ ~ oO~~~g)~~~~g oC~~~~g;~~~8 oo~gs~~g;~~~8- ir g: ';;;' z ~()'q 06-08 :> 06-08 ..,0 06-0B ~ ~- ~ 0 0 p.U1 ~ OB-10 Cl 08-10 OB-10 0 0 "' ~ .9 10-12 ...... " ~ 10-12 (.n 10-12 s; § 12-14 12-14 0 12-14 n 0.. "' "'::1: :::r 14-16 14-16 14-16 0 g ;::J 1&-18 16-18 16-18 "'E. 0 (() "~ ~- 18-20 18-20 18-20 "' 0.. 1,7 e:.. 0' 20-22 20-22 20-22 tn c '< 0 ~ ()'q 22-24 22-24 22-24 ::;· s: 24-02 24-02 24-02 ro 0 p - 02-04 02-04 02-04 s· ()4..06 04-06 04-06 L6 ,, Jan 222 Jan 170/300 Feb 250 100 100 100 90 90 90 m 80 80 80 70 70 Jj ~ /'J 60 60 ~ 50 %J 50 50 !i 40 40 40 !') 30 30 30 "' 20 20 20 10 /: 10 10 0 M tjj tf1 VGI Y-" 1 i 1 I I I I 0 0 1 I I QN-:fi<0\:0 ~ s ~ ~ ~ ~ ~ ~ ~ s ~ ~ :8 s :::1 "' 0 "' ;<:898 ,....., ...... ,...., - roO ?l ~ ;;; !'! ~ ;g ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ,, ' ~ "'~ "'J: J; d: 6 ~ '6~~..0 ·~ •.;]. ·~ ~ 0 rl ...... ~ N Fj N Q o a ~ ~ ...... ~ ~ N ~ N o 0 a i.'l ~ ~ ~ '"' ~;!;~:~::l: gs ..... 8 Feb 020 Mar070 Mar 300 100 100 90 90 • 80 80 h 70 70 il 60 60 - so 50 ffi 40 " 40 ~ 30 30 ~ 20 20 1 10 8i I I !?! I v~ I V1 I V1 I l7l I 0 ~\.0~0 i8 sa ~ ~ ?l~;<;8~~ 1'3 s :::1 i'l ;c'j 8 9 ;g 0-"' 0 "'~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ,;, ~ ~ "':!: "'J; ~ ' ~ ~ ~ ~ ;b ~ ' ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 0 0 ,....., ,....., ~ ~ ~ ~ ~ ~ ~ 0 i.'l ~ ~!:i~st 0 i.'l ~ ,...... - rl - N N N 0 s -- ..... ,....., 0 Apr 170 Apr222 Apr070 100 90 :/J • 80 Jj ~ ~ 50 e 40 ~ 30 &: 20 !O:::j0 1?;1 1?1 1?1 !?l !fJ . I ! l l I I I ~ 0 N • ~ ~ 0 ~ ~. B ~ ;g ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ s ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 0 ~ ~ ' ~ ~ d ~ ~ ~ ~ 6 ~ ~ ~ ~ ~ 6 ~ ~ ~ ~ ~ ~ ~ ~ ~ 0 0 ,..... ~ ~ - - N ~ N ~ g 0 0 ...... -< ..-< rl ..... N N N 0 0 0 - rl - - rl N N N 0 0 Time of Day Time of Day I lz;) %HO !II %SD in ES Ill %SD i~Ma I Figure 6b. Percent time harbor seals radio-tagged in Elkhorn Slough spent swimming/diving {SD) and hauled-out {HO) in Monterey Bay (MB) and Elkhorn Slough (ES) from Jan. to April, 1992. Seals not tracked May-Aug. Sealidentification is also given. 1§5 99 12 -ill!- Haul-out (E5) 10 -,jjj1- 5/DE5 8 >, --&-- 5/DMB "0"' -...... 6 !:i"' ! f ~ 4 2 0 Autumn Winter Spring Figure 7. Mean amount of time harbor seals spent ashore in Elkhorn Slough (ES), swimming and diving (S/D) in ES, and S/D in Monterey Bay (MB). Vertical bars represent standard deviatioiL 100 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 #tows Figure 8. Cumulative species curves for trawls conducted during the day at the Kirby Park station in Elkhorn Slough. ~~0 .... \'l ~ \!3g Q ""~;:r0. "'.... a r;; ~· g ~_. s 0 "'':-' 0 """';; :;r. 0 0 !!' cs· "".,- IV t.J W 7:; 0. 0 U1 0 U1 0 ~'-'' ~0 v• ::;· percent ]'JUI11ber 40 120 40 Bridge Dairy Kirby Park c 35 35 100 ·g 30 30 9 s 25 25 80 '~ 20 20 60 ~ 15 6 15 40 ~ 9 6 10 10 :li 20 ~ 5 5 u 0 0 0 w s su A w s su A w s su A 10 10 10 8 8 8 Ill 1il 6 6 6 fr.' ~ 4 4 4 ,.0 2 2 2 0 0 0 w s su A w s su A w s su A 1 1 1 0.9 0.9 0.9 0.8 0.8 0.8 0.7 0.7 "v 0.7 ~ 0.6 0.6 0.6 0.5 0.5 0.5 -~ 0.4 0.4 0.4 0 Q 0.3 0.3 0.3 0.2 0.2 0.2 0.1 0.1 0.1 0 0 0 w s su A w s su A w s su A Figure 10. Seasonal variation in mean catch, mean number of species, and mean dominance index per 10-:min tow of fish from otter trawls at each station in Elkhorn Slough, CA. Vertical bars indicate standard error. Number of samples is indicated by data points. Y-axis labels apply to the entire row. Shiner Surfperch 14 Tonguefish 8 White surfperch 40 ~ 12 B 6 30 J, 10 -~ 8 I I --L 0 ' 4 ~ 20 ' 6 ~ I I ' 4 I " 10 2 § 2 :a .. jJ··· 0 0 0 w s su A w 5 su A w 5 su A 4 Lingcod 14 Staghom sculpin 35 English sole ~ 12 30 .9 10 25 -~ B 20 0 2 ~ J,, 6 15 I \ ~ ,,f' "'c 4 -, 't 10 I \ ~ ' :a0 2 1...... ··+··· ...... 'I 5 0 0 0 w s su A w s su A w s su A 8 Black surfperch 10 Cabezon 10 Speckled sanddab ~ B B 6 a -~ 6 6 0 4 ~ ' 4 4 ""§ 2 2 2 ::;; - -1- " 0 0 0 - - w s su A w s su A w s su A -e- Bridge ···'!>·· Dairy -.lr Kirby Park Figure 11. Seasonal variation in mean abundance per 10-min tow of the most dominant fish species collected during 1991 in Elkhorn Slough, CA. 1--' 0 (;) 104 30 28 00 ·c;"' 26 "'eo. "' 24 C) ""> ~ 22 "3 E ;j u 20 18 16 1 2 3 4 5 6 7 8 9 10 #tows Figure 12. Cumulative species curves for trawls conducted at night at the bridge station in Elkhorn Slough. %N f'l 0 ~·""' ::>a. "2..., Pi ro Acttttiltogobius fortim:mus Fir ~ A ularhyrtdnJSJlnvidus a. !-" n C!tt1ttrn fll!J!ari ::r 0 Cif!:michl!rys sfigrnaeus td !3 CJeveb:md/11 ios ~'"0 OJ Clupr:l: J.Mflasi :::r0 ..., ..., Cymofagasier :1.ggregata ;:l -·0 "' Dam:J/idt!lrys wcm (f) ;:l 0"' E!T'./JWtocn jad:smti 0 /1) .:: 'i Gibbsmts/:1 sp. '!} ro Hr:mgrmrtmos demgntntrmJS a. .:: ;:l"' H~rzrgr:nh>ttm 5· "'~ Hypsurus a:ryi OQ ::c. U:pid'gobi"' lcpMus ,.... '"0 ::> LeplfXXIIfiiS d11tJtl/115 "" Liparis sp. "":--' "'c: s·"' Lnligu opakscms OQ Mii;mmdrus mittimus '"0 Mustdus Ju:nld rl"' My!ichalus ca.lfformi:n ::> Nwdinus uufnoiahls "'~ Opln"mitm elatJgatus :J Pantfic/tl!:ys ctJlifamicJJs ~ Plt:llromrle; vdu!IIS 1\) ..., Pl~iJJtC'rodcn fitr'rnitts 0 PWidtllrys slel/afiiS - Pleurr.nridrilrys c:oenosus '"0 "'ro n Pleurculchlbys decum:n..'i ro· Plcuror:idt{}tys riileti "'n Poridtilrys ttoialus 0 RJ~~JCodJil!IS loxoies ii?f'l Scnrpaettidithys m.1rmamtus Fir SeWttessp. a. S. n:m'C11/.atus .... -0 S, rostrdiiger 8 Setipltus polilus a. Syngnnlhus sp. g. Symphun1s nlricflfld:r ;r Syn.orfus lucioa:ps ~ 00 SOI Mean#/10-rnin tow ,_. N 0 N 0 0 .Aamtlwgabiu.sflavimrmus i~~~~=~:::=~=~==:::~==~==~==~~ Auiarl IiyperprosopmJ argeute~mt Hypsurus mryi Lepidogabiu.s lepidus teptarottllS annntu.s Uparissp. Lo!igo opalesccns Micrometms mbtim: 901 Mean#/10-min tow 0 N Clewlandiil ios Clupea paiiasi Cymatogaster aggregata Dam4liclstltys uaca1 Embi{Jtaca jacksmi Gibbsousin sp. He::r:agrammos superciliosus Hyperprosopo11 dlipticum Lepidvgobiuslepidus Leplocottus amllltus Micrometrns minimus Musteluslumlei Myliubaitts mlifomic:a. Ophindott elOH.gatus Parulicltthys califomic.us ~ Pleu.n:medes TJetulus Ci P!umerodon fim:atus '-:';"' Plntyrhinoidis tn'scriflta Ci >--· Pleuronichthys decurrens Smrpaenichtltys marnwratus Sebast(S sp, 5. •mriculatus 5. mstn:Uiger Syng11at1w.s sp. Sympluuus atdamda Synodus lucfm:eps Triaki$ semifosciata LO! Mean #/10-min tow N 0 0 Atlw.rinops affinis Allu:rirwps californiensis Citlum'clttltys slignum.IS Clevelandia ios Ctupea pal!asi Cymatoga.stcr aggreg.1ia Dam.aliclrtJrys vacCll DoroSOTtw petertetJ:>-e Embioloca jacksoni Engrnulis nwrda.:r Gibbsonsia sp. Hcxagrunmws sp. He:r.agrrmmws supercr1iasrt.S Hyperprosopon argeuiettm Hypsopsetta gutttifaia l..epidogabiu.slepidus Leptocottus amwtus -t'v:C:::T Loiigo opafesccns Micrometrus minim us Mastdll.S i:enlei Myliobatw; califomica Neru.:linus tminotaitts Opltiadmt elrmgatus Pnralicllt1tys cal~ 801 Mean #/10-min tow ..... C)'j 0 0 0 0 Atl:erinops affinis. ii:;~=~==~~==::::==::=~~~::::~~::==~ Atherinops caiifomleusis Citl14ricltihys stigmaeu.s Cievelandia ios Cymntogastcr aggregata Dam.alicl1t11ys tJacca EmbinJoca jacksoni Eugraulis mordru: Gibbsonsia sp. Herngnmmws sp. Hyperprosopon argenieurn Lepidogobius lepidus Leptocatfus arnmtus Micrometrus min£mus Myliobatu.s caiifomica Nevclitms wtirwtatus Ophindnn elongatus Paralicilfhys rolifomicus Pleuronedes retulus Plumerodon furcatus Plalicltfilys steUatus Pletuonichtbys decumms Plcurrmicblitys riiteri Parichthys twlafus RJwcoeltilu.s to:mtes Srorpaenichtltys marmorotus Sebastes sp. S. auricuJatus S. camatus 5. flar:idus S. mystinus 5. paucispbds S, mstrelli~ Spirinchu.r;. Stilrksi Syngnatlms sp. Symplwros atric;Jiida T rinkis scm:fasciat,, Uroloplru.s halferi 601 Mean #/10-min tow N 0 0 Atlterinops affittls 1~~~:::::=~~~=::::::::::::::::::~~~==~~~=~ Atlterinops califomiensis CitJwriclttltys stigmaeus Ocvcl:mdia ius Clupea pallasi Cymatogaster oggrcgata Dam:llicl!tkys oacca Embiotoca jackscmi Engraulis nwrd.ax Hexagrammos superdliosu.s Hyperprosopon a.rgenteum Hypsapsella gullulata Lepidogabiuslepidus Leptacoftus amu:tus Micromdrus minimus Musteluslumlci My!iobatus califomial Ophindau e!angntus Pleurrmedes ve:tulus Plumerolkm jura1tus Pfatichlhys stellatus Platyrltiuoidis triseriata Pleunmichtllys decurrms PariciztltysnoiltftiS Rllilcocltilu.s tomtes Srorpaeniclltltys marmnmtus Sebasles sp. S. rmriculatus S. mystiuus S. paucispittis Syttgnnthus sp, Symphurus atricauda Triakis semifosdata Urolophu.s Judleri on Mean#/10-rnin tow N Hypaprosopott argctltL"ll1rt Hypsopsdta guttu1ala Lepidogoblus JepidtLS Lt:ptrx:ottus m71Ifllus -f'\);:1::j§ Loligo opa!escens Micromefm'> ntinirlius Mustclnslumlci MyfiDbntus mlifamica Parolichtlrys rniifonticus Plett.ronedes treWlus Piumerodon fi~.ro~tus Piatichtltys sJeilatus PlatyrhiJtOidis tdseriata PoriciltJrys rrotatus Rliacoc1Jilus lo:mtes Sro:rpaenicJttiJys marmomtus Sebnstes sp. S.auriculatus S.nu>lanops S. paucispiuis S. rastreUiger Seriphus poiitus Syngm1thus sp. Symphurus atricau.da Trlakis semifo.scitlla Tritimtiger trigrmoceph.alus Uroloplnts Jwlkri HI 112 40 Cumulative prey species: summer 35 30 :;,.., Q) 15.. 25 '*g; :!:l 20 "3"' § 15 u 10 5 0 1 6 ' 11 16 21 26 31 36 41 46 51 #samples Figure 20. Cumulative number of prey species per fecal sample collected during summer (May- July) 1991 in Elkhorn Slough, California. 113 4 3.5 3 -Ill- s 2.5 ··0·· D 2 -A- H' _,.. R 1.5 1 - - - - - ~ ------t------! I. - - lit ...... ·IV-... '"' ...... IV-...... (!) 0.5 f------.x.. --- ::J: r -----+--·---·-'% Winter Spring Summer Autunm Figure 21. Seasonal variation in diversity {mean number of species per scat (S) and mean diversity index (H'), index of specialization (R), and dominance (D) of prey found in harbor seal feces collected during 1991 from Elkhorn Slough, CA. Vertical bars indicate standard error. %N tn 115 Spotted cusk eel Night smelt 25 25 spring (n=53) spring (n=53) 20 mean 22.24 20 mean= 11.88 15 (3.90) 15 (1.38) 10 10 5 5 O;-rrrr~~~~~~~~~~ 0 -1-r-r---r-...... --r 0 2 4 6 8 10 12 14 16 18 25 25 summer (n=76) summer (n=13) 20 mean = 20.88 20 mean= 13.67 15 (4.04) 15 (0.73) 10 10 5 5 o;,,.,TT~~~~rn><~ 0 -1-r-J--r-r--r-,....-T 0 2 4 6 8 10 12 14 16 18 25 25 autumn (n=114) autumn 20 mean = 21.74 20 15 (3.97) 15 (none found) 10 10 5 5 o~...... ~~~~~ .. ~~ 0-!-,-.,-.-r-r-~~~ 0 2 4 6 8 10 12 14 16 18 25 25 winter (n=50) winter (n=34) 20 mean 20.07 20 mean= 11.17 15 (4.82) 15 (1.51) 10 10 5 5 O~n Standard length (em) Figure 23. Frequency histograms of the estimated prey length of Spotted cusk eel and Night smelt recovered from harbor seal feces collected in Elkhorn Slough, CA during 1991. Standard deviation in parentheses. 116 Staghom sculpin Plainfin midshipman 25 5 spring (n=S) 4.5 spring (n=12} 20 4 mean= 15.63 3.5 mean=20.16 15 (4.28) 3 (5.46) 10 5 04,~~~~~~~~ 25 5 summer (n=10) 4.5 summer (n=19) 20 mean= 10.39 4 mean= 21.88 3.5 15 (1.74) 3 (3.73) 2.5 10 2 1.5 5 1 0.5 04,~rr~~~~~~ Q~~TTrr~~yy~~ 25 autumn (n=55) mean= 18.11 20 mean= 16.39 (5.49} 15 (2.78) 10 5 o~~rr~~~~~~~ 25 winter (n=ll) 20 mean= 18.77 15 (1.50) 10 5 D~rrrr~~Ar~~~• Standard length (em} Figure 24. Frequency histograms of the estimated prey length of staghom sculpin and plainfin midshipman recovered from harbor seal feces collected in Elkhorn Slough, CA during 1991. Standard deviation given in parentheses. 117 Rockfish sp. White croaker 40 35 spring (n=2) 14 spring (n=B) 30 mean= 12.24 12 mean= 23.74 10 25 (1.79) (2.54) 20 8 15 6 10 4 5 2 o;.,,,.,,~~~ 40 summer (!1=116 35 14 summer (n=39) 30 mean= 15.52 12 mean=23.25 10 25 (2.83) (2.66) 20 8 15 6 ;;., 10 4 2 l ~ +-r-r-r-r--r-,.tll,l!IYI,llll,.tJII,.IIII,.tlll,--, o+r...... ~~~~rn ~ ON~~~~~~~~~~~~ p;. 40 autumn (n=25) 35 14 autumn (n=26) 30 mean =11.70 12 mean= 23.18 10 25 (4.22) (2.07) 20 8 15 6 10 4 5 2 o;-r~.-~~~~~T;, o;,,.rrrrrr~~~rn 40 35 winter (n=ll) 14 winter (n=lO) 30 mean= 15.38 12 mean=25.24 25 10 (2.29) (1.34) 20 8 15 6 10 4 5 2 o;-.,,-r~~~~~~· o+r,,.,,.,.~~rn Standard length (em) Figure 25. Frequency histograms of the estimated prey length of rockfish sp. and white croaker recovered from harbor seal feces collected in Elkhorn Slough, CA during 1991. Standard deviation given in parentheses. 118 Dover sole English sole 14 spring mean= 14.57 14 spring (n=10) 12 (n=43) (4.27) 12 mean =25.91 10 10 8 8 (7.60) 6 6 4 4 2 o~nT~~~~~~ ~ +r,..,..,..T"'!Tlw,l~,.,.,..lll,ll,.,.-, 14 summer (n:40) 14 summer (n=35) 12 mean= 18.39 12 mean= 25.49 10 10 (4.89) 8 (6.44) 8 14 autumn (n=20) 14 autumn (n=42) 12 mean= 15.04 12 mean = 20.59 10 (4.06) 10 8 8 6 6 4 4 2 2 0~~~~~~~~~~ 0 ~,.,..,..,..,..., 14 winter (n=6) 14 winter (n=5) 12 mean = 11.65 12 mean= 19.56 10 10 8 (2.69) (4.92) 8 6 6 4 4 2 2 o~~~~~~~~~n o~,.,.., Standard length (em) Figure 26. Frequency histograms of the estimated prey length of Dover sole and English sole recovered from harbor seal feces collected in Elkhorn Slough, CA during 1991. Standard deviation given in parentheses. 119 Pacific sand dab 20 18 spring (n:61) 16 mean 21.16 (2.75) 14 12 10 8 6 4 2 0~;-.-.-~~~.-r-T o 2 1 6 s w u u 1& 1s w ~ a ~ m w a 20 18 summer (n=17) 16 mean 18.81 (3.45) 14 l2 10 8 6 4 2 o~-.~~~r-r-T-,-~mr. o 2 4 6 s w u u u 1s w ~ a ~ m w a 20 18 autumn (n=9) 16 mean= 19.29 (4.89) 14 12 10 8 6 4 2 0~.-~~~~~~r-~~~~~~~-, o 2 ' & s w u u u 18 w ~ H ~ m w a 20 18 winter (n=19) 16 14 mean= 22.16 (6.43) 12 10 8 6 4 2 0 +-r--r-,--,.--,r--r.llllll.r o 2 1 1 8 w u u 11 u w ~ a u m w a Standard length (em) Figure 27. Frequency histograms of the estimated prey length of Pacific sanddab recovered from harbor seal feces collected in Elkhorn Slough, CA during 1991. Standard deviation given in parentheses. 120 Octopus sp. Market squid 200 180 180 spring (n=300) 160 spring (n=76) i:~ mean= 4.71 (052) 140 mean= 12.12 (0.89) 120 120 100 100 80 80 60 60 40 40 20 20 o~~~~~~~~~ 0 -!---.--.--.--.--.--,. I 0 2 4 6 8 10 12 14 16 200 180 180 summer (n=32) 160 summer (n=ll) 160 mean= 4.84 (054) 140 mean =11.30 (0.48) 140 120 120 100 100 80 80 60 60 40 40 20 20 o~~~~~~~~~ 0-1---.--.--.--.--.-~~-.-~ o~o~o~c~=~=~=~=~ oo~~~N~m~~~~~~~~ 0 2 4 6 8 WllUM 200 180 180 autumn n=458) 160 autumn (n=55) 160 mean=4.29 140 mean= 10.76 (L33) 140 120 120 (0.67) 100 100 80 80 60 60 40 40 20 20 0-l--~rr~~~~~~ 0 +-,-,-,--,--,Jill.., =~=~=~o~o~=~=~o~ cid~~~~~~~~~~~~~~ 0 2 4 6 8 W12U M 200 180 180 wintEr (n=533) 160 winter (n=251) 160 140 mean=4.87 140 mean= 11.53 (1.03) 120 120 (0.56) 100 100 80 80 60 60 40 40 20 20 0-l--rrrrTT~~~~~ 0 -1---r-..,..--r--r-,-J!IIl, =~=~=~=~=~=~=~=~ d~~~~N~~~~~~~~~~ 0 2 4 6 8 W12U 16 Dorsal mantle length (em) Figure 28. Frequency histograms of the estimated prey length of Octopus sp. and market squid recovered from harbor seal feces collected in Elkhorn Slough, CA during 199L Standard deviation given in parentheses. 6() 5() 4() 30 z'# 3() 2() 1() () t-H-~-M+++\trrrrmurrrrv ~ '?! ~ :U ~ '!! ii- ill- fl. \ '% \ l1 -li 'i .~ ,l t ~ 1 \-% l! ll ~<- ..!i ii- ~ l! ll \ -" i t ttaila\1\\ !el\l\ttla lt\tll~lt•tt, ~-~''"""'~"-l~a••\•••a~ ~~l»••l;ao•l••~~~~••·,lt""'-""'~ ~"~ ·••·lll\-,:~.""'m"• 0 1t\t t~ • \ill··,a~il'll1° 0 1tila\ at.J~~~:§ ~,t~:.··~ .~~l.~ 41 ~~~ .s-ti',·~~tl tlj~ fi t"'C ~it~ ;::. Ill;>., :::>... ~ ".S; "(, ':1:\ c u % <; ~ Winter prey species >-',.., Figu'' 29 C~P•- ol •!""'' ooughl in E\l day and night tows. %N 0 I 0 (centimeters) SPECIES RANK I II II I II II '" II '" II I I I II ,, II I '" I! ,, I II '" !! I I I II I 0 5 10 15 20 25 30 35 40 45 50 CuskEel #3 (seal) ~~ #19 (trawl) Midshipman #6 #19 Rockfish #6 #11 English sole #8 #3 Staghorn #11 sculpin #4 Shiner #17 ~ surfperch #1 ~ Lingcod #9 #16 Figure 33. Mean(:',), standard deviation (delineated by box), and range of the standard length of fish consumed by harbor seals in Elkhorn Slough in 1991, and those of individuals caught in otter trawls conducted in Elkhorn Slough in 1991. %N )ooo..Jjoo.w.INNWW.P.II;:>.U1 OOlO(JlOCJlOtnOUlC I OU1~.'-4NNWW~~U1 OU10U10U10U10 %N 9Zl