BEHAVIOR AND ECOLOGY OF PACIFIC WHITE-SIDED DOLPHINS (Lagenorhynchus obliquidens) IN MONTEREY BAY, CALIFORNIA

A thesis submitted to the faculty of San Francisco State University in partial fulfillment of the requirements for the degree

Master of Science in Marine Science

by

Nancy A. Black

Pacific Grove, California

December, 1994 Copyright by Nancy A. Black 1994 BEHAVIOR AND ECOLOGY OF PACIFIC WHITE-SIDED DOLPHINS

(Lagenorhynchus obliquidens) IN MONTEREY BAY, CALIFORNIA

Nancy A. Black San Francisco State University 1994

Between 1987 and 1991, the distribution, relative abundance, behaviors, and food habits of Pacific white-sided dolphins (Lagenorhynchus obliquidens) were investigated in Monterey Bay, California and surrounding waters.

Relative abundance of Pacific white-sided dolphins was greatest near the shelf-break and up to 10 km beyond it, in Carmel Bay, and near the northern rim of Monterey Submarine Canyon. Mean (±SO) group size of dolphins was

203±395.4, 50.6% of groups contained 50 or fewer dolphins. During the upwelling season (Mar-Jul), relative individual and group abundance was low and group sizes were small. During the oceanic season (Aug-Oct), relative group abundance was high, and dolphins often fed and milled in dispersed subgroups. During the Davidson Current season (Nov-Feb), relative individual abundance was high, and group sizes were large.

Pacific white-sided dolphins were observed milling, 33.3% of the time, feeding 23.9%, traveling 21.9%, socializing 17.9%, and resting 3.0%. Pacific white-sided dolphins were observed feeding significantly more in shallower depths, closer to the shelf-break, and in areas with greater bottom relief compared to other behaviors. Dolphins were observed traveling significantly more often in deeper depths, further from the shelf-break, and in the largest cohesive groups. Pacific white-sided dolphins had the greatest coefficient of association values with northern right whale dolphins. Risso's dolphins, and

California sea lions.

Three radio-tagged Pacific whlte-sidecJ dolphins exhibited a mean (±SO) dive duration of 23.5±1 .92 sec, mean (±SO) respiration rate of 2.5±.32, with a mean (±SO) speed of 7.6±2.1 9 km/hr.

Fifteen anomalous-colored Pacific white-sided dolphins were photographically identified, thirteen of which were predominantly white in color.

These dolphins were resighted from one to eight times during particular oceanic seasons, as well as among oceanic seasons in different years which indicated particular dolphins frequented the Monterey area over variable periods of time, rather than new groups continually moving through.

Relative individual and group abundance, and sigrting distance to the shelf-break were positively correlated to temperature and the near-shore fronte.l gradient. When sea surface temperature anomaly was high, dolphins were more abundant and occurred closer to the shelt-·break.

Pacific whiting, plainfin midshipman, northern anchovy, Sebastes sp.,

Gonatidae, Loligo, and Onychoteuthis were the most importullt prey of Pacific white-sided dolphins found dead along the central California coast.

Pacific white-sided dolphins occurred year-round, were seasonally abundant, were not randomly distributed, and were frequently observed feeding. The;efore, certain locations in the Monterey Bay area are important for these dolphins, providing a predictable and abundant toad source. ACKNOWLEDGMENTS

I thank my committee members, Dr. Bernd WOrsig who initially encouraged and advised me during t!;le initial phases of this project, Dr. James

Harvey who greatly assisted me during the final phases, and Dr. Gregor

Cailliet. Tracy Thomas, captain of the AN Ricketts from Moss Landing Marine

Laboratories, spent many days at sea searching for dolphins with me, and I thank Tracy as well as Mike Prince for allowing me extensive boat time on the

Ricketts as well as the Boston Whalers. I especially thank Debra Shearwater of Shearwater Journeys for generously providing me with many days of boat lime on her natural history trips, for her interest in this project, and for often remaining with dolphins long enough for me to photograph them. I also especially thank Richard Ternullo, •' captain of the MN Pt. Sur Clipper, for his exceptional ability to maneuver around dolphins, extensive knowledge and insight of animals inhabiting the

Bay, and for his encouragement throughout. Richard also provided me with additional sighting data and reviewed drafts of this work.

I thank Alan Baldridge for sharing his vast knowledge about marine mammals of Monterey Bay, and for his advise and interest in this project. I greatly appreciate help from Tom Kieckhefer who offered invaluable assistance with all aspects of this project. Tom as well as Pamela Byrnes and

vi Tom Jefferson reviewed and improved drafts of this work.

I also thank Sheila Baldridge, Dr. Randy Wells, Dr. John Hall, Susan

Kruse, Dawn Goley, Carol Keiper, David Lemon, Charlie Denney, Steve

Bailey, Ron Branson, Eric Dorfman, Dave Ekdahl, Peter Pyle, Francisco

Chavez, Steven Ramp, and Dave Husby. I also thank the many people who assisted with surveys and the radio-tracking efforts. I thank my parents for their encouragement and support through this effort.

Robert Jones (U.C. Berkeley), Long Marine Laboratory, Moss Landing

Marine Laboratories, and California Academy of Sciences collected stranded dolphins and provided stomach contents. William Walker, of the National

Marine Mammal Laboratory, Eric Hochberg, of Santa Barbara Museum of

Natural History, and James Harvey assisted with identification of cephalopod beaks and fish otoliths.

This work was supported in part by Earl and Ethyl Meyers

Oceanographic Foundation, Monterey and Los Angeles chapters of the

American Cetacean Society, Cetacean Society International, American

Museum of Natural History, National Geographic Society, and Moss Landing

Marine Laboratories, Sam's Fishing Fleet.

vii TABLE OF CONTENTS

List of Tables...... x List of Figures...... :...... xi Chapter 1 Introduction ...... 2 Methods 7 siui:iy.Area::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::.:::::::::::::::::::::::::::::::::::::.".7 Surveys ...... 9 Distribution and Relative Abundance Analysis ...... 13 Behavior Analysis ...... 14 Radio-Tag and Track...... 17 Environmental Data Analysis and Correlations ...... 18 RMU~ 20 ofstrihutlaii··a.iic:i·FieiaHvei.Ai:iiiiiCiaiice······················································2oBehavior Observations ...... 34 SocialEnvironmental Factors andFactors Behavior ·a.iid.8eiiavfar······················································s4 ······················································ 41 MuIt i-S pecies Associations_.::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: 45 CalvesPhoto-iCie·ri-tiilcatiC!ri···················································································· 5051 Radio-Track ·····················································································53 Environ me ntiil. Carre iaifo ii s:: :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: 55 DiscussionDistrfi:iiitfaii··································································································· 60 SeasonalitY::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::Behavior 6864 Radio-TrackPhoto-ldeiitlflcatfaii·····················································································7s ...... 73 Environmenta"l.ca·riCiftfoiis::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: 75 Chapter 2 lntroduction ...... -88 Methods ...... 91

viii Results...... 93

Discussion...... 1 02

References...... 115

ix LIST OF TABLES

Table Page

1. Mean depth (m), distance to the shelfbreak (km), and contour index

for quadrats with predominant behaviors exhibited by Pacific white-

sided dolphins ...... 39 2. Coefficient of association values for all cetacean multi-species groups

encountered during the study period ...... 46 3. Anomalous colored Pacific white-sided dolphins, including dolphin

number, date, group size, associated species and their group size ..... 54

4. Summary of radio-track data ...... 56 5. General oceanographic characteristics of the three seasons as

related to Pacific white-sided dolphin occurrence and relative

abundance ...... 76 6. Mean (±SO) of eight environmental variables for the three

oceanographic seasons during the study period ...... 77 7. Prey of 16 Pacific white-sided dolphins collected off central

California.----····································································································· 94 8. Percent frequency of occurrence of Pacific white-sided dolphin prey

from southern California, Monterey Bay, northern California, and

Washington ...... 1 04

X LIST OF FIGURES

Figures Page

1. Study area encompassing Monterey Bay, California and surrounding

waters ...... 8 2. (A) Survey effort: number of km traversed in each quadrat during

dedicated Pacific white-sided dolphin surveys and opportunistic

surveys. (B) Additional effort covered by sport fishing vessels...... 12 3. Relative abundance of Pacific white-sided dolphins represented as

number of dolphins per km per quadrat...... 21 4. Relative group abundances of Pacific white-sided dolphins represented

as number of dolphin groups per km per quadrat...... 22 5. Number of occurrences of the distance Pacific white-sided dolphin .. groups were sighted from the shelfbreak (km) ...... 23 6. Observed and expected percent frequency of occurrence of Pacific

white-sided dolphin sightings among five temperature categories ...... 25

7. Observed and expected frequency of occurrence of Pacific white-

sided dolphin sightings among five contour index classes ...... 26

8. Number of occurrences of Pacific white-sided dolphin group sizes. ____ 27

9. Mean group size of Pacific white-sided dolphins per quadrat. 28

xi 10. Pacific white-sided dolphin sightings by season and group size ...... 30

11. Percent frequency of occurrence of Pacific white-sided dolphin

group sizes for each oceanographic season ...... 31

12. Mean Pacific white-sided dolp~in gourp size (±SE) for each month

and oceanographic season ...... 32 13. Relative group abundance of Pacific white-sided dolphins, represented

as the mean (±SD) number of dolphin groups per km each month and

oceanographic season ...... 33 14. Relative abundance of Pacific white-sided dolphins, represented as

the number of dolphins per km for all months and seasons during the

study period ...... 35 15. Mean depth (m), mean distance to the shelfbreak (km), and mean

contour index for Pacific white-sided dolphins engaged in milling,

travellin~, socializing, and feeding ...... 36 16. Quadrats where Pacific white-sided dolphins exhbited predominant

behaviors (mill, social, feed, travel) ...... 38 17. Percent frequency of occurrence of behaviors exhbited by Pacific

white-sided dolphins during morning, mid-morning, and afternoon .... 40

18. Mean Pacific white-sided dolphin group size for each behavior ...... 42 19. Percent frequency of occurrence of Pacific white-sided dolphin group

size categories among behaviors ...... 43

xii 20. Percent frequency of occurrence of group cohesiveness, aerial

behavior, and multi-species associations among behaviors exhibited by

Pacific white-sided dolphins ...... :...... 44 21. Mean group size of Pacific white-.sided dolphins, northern right whale

dolphins, and Risso's dolphins for single species groups and multi-

species groups when associated with one another ...... 47 22. Percent frequency of occurrence for behaviors that Pacific white-sided dolphins were engaged in while associating with other species. ______49

23. Example of anomalous-colored Pacific white-sided dolphins ...... _____ 52

24. Dive duration histogram, mean dive duration, percent pattern type

for radio-tagged Pacific white-sided dolphins ...... 57

25. Mean evironmental variables and dolphin measurements ...... 58

26. Mean estimated standard lengths (em) and estimated weights (g) for " eight fish species ...... 95

27. Mean estimated mantle lengths (em) and estimated weights (g) for

13 cephalopod genus/species ...... ~---····· .. ····------···· 96 28. Frequency histograms of estimated standard lengths (em) for four

fish species found in Pacific white-sided dolphin stomachs ...... 98 29. Frequency histograms of estimated weight (g) for four fish species

found in Pacific white-sided dolphin stomachs ...... 99

xiii 30. Frequency histograms of estimated mantle lengths (em) for eight

cephalopod genus/species found in Pacific white-sided dolphin

stomachs ...... 1 00 31. Frequency histograms of estimatec[ weights (g) for eight cephalopod

genus/species found in Pacific white-sided dolphin stomachs ...... 1 01

xiv 1

CHAPTER 1

DISTRIBUTION, RELATIVE ABUNDANCE, AND BEHAVIOR OF PACIFIC WHITE-SIDED DOLPHINS, IN MONTEREY BAY, CALIFORNIA 2

INTRODUCTION

Distribution, movements, and behavior of many species of terrestrial mammals are correlated with several environmental variables. Food type, availability, and distribution directly influences the ecology of these vertebrates.

Terrestrial social mammals, such as baboons (Papio cynocephalus; Dunbar and Dunbar 1975), chimpanzees {Pan troglodytes; Goodall 1986), lions

{Panthera leo; Schaller 1972), and elephants (Loxodonta africanus; Moss 1988), living in areas with seasonal variations in food supply generally exhibit flexible group structures and associations. However, the core social unit, usually matriarchal linkages, remains intact regardless of environmental conditions. In contrast, social mammals with stable year-round food supplies, such as mountain gorillas (Gorilla gorilla beringei) have relatively fixed social groups with small overlapping home ranges and no territorial defense (Schaller

1963, Fossey 1983). Similar ecological correlations occur with cetaceans, although describing patterns in the marine environment is difficult. Cetacean prey often are unknown, or clumped, and spatially and temporally variable. The occurrence and distribution of baleen whales within feeding areas is non-random and related to oceanographic features (e.g. fronts, eddies, upwellings, and physiography; Gaskin 1982, Brown and Winn 1989), where specific prey species concentrate (Murison and Gaskin 1989, Piatt et al. 1989, Reilly and

Thayer 1990, Schoenherr 1991 ). The characteristics and types of prey also 3

influence behavior and aggregations of whales (Jurasz and Jurasz 1979,

Wursig et al. 1984, WOrsig et aL 1986, Dolphin 1987, Guerrerro 1989,

Kieckhefer 1992).

In contrast to baleen whales, which migrate seasonally to specific feeding areas, most small cetaceans exhibit more subtle seasonal changes in distribution, abundance, and behavior (Wursig 1989). The interpretation of their ecological patterns, however, can be difficult, because dolphins feed on diverse and poorly understood fish and cephalopod prey. The availability and distribution of food resources, predation pressure, physical characteristics of the environment, sex and age class segregation, and reproductive status influence the ecology of small cetaceans (Miyazaki and Nishiwaki 1978, Norris and Doh I

1980b, Wells et aL 1980, Myrick et al. 1986, Wells 1991 ).

Seasonal changes in oceanographic conditions within specific habitats commonly affect prey and their predators. Distribution and seasonal movements of odontocetes are related to temperature (Gaskin 1968, Evans 1975,

Leatherwood et al. 1980, Wursig and Wursig 1980), bathymetric features

(Evans 1971, Hui 1979, Doh! et al. 1986, Kenney and Winn 1 986, Selzer and

Payne 1988), currents and water masses (Gaskin 1968, Miyazaki et al. 1974,

Kasuya and Jones 1984, Au and Perryman 1985, Smith et al. 1986, Reilly

1990), and a combination of environmental factors (Smith and Gaskin 1983,

Watts and Gaskin 1985, Dorfman 1990). In a few cases, the occurrence of odontocetes has been directly correlated with prey type and availability (WOrsig and Wursig 1980, Shane 1984, Kenney and Winn 1986, Selzer and Payne

1988, Scott et aL 1990, Felleman et aL 1991). 4

Behavior and group structure of coastal dolphins is related to depth (WOrsig and WOrsig 1980, Shane 1990, Cipriano 1992), distance to shore

(Cipriano 1992), physiography (Norris et a1. 1994, Heimlich-Baran 1988, Scott et al. 1990, Felleman et al. 1991, Cipriano 1992), time of day (Brager 1993), season (Hui 1979, Saayman and Taylor 1979, Shane 1990, Cipriano 1992,), and prey patterns (WOrsig and Bastida 1986).

The behavioral ecology of pelagic dolphin species is generally poorly­ known because they are usually not visible from shore. Exceptions include common dolphins, Delphinus spp. off southern California (Evans 1971, Evans

197 4, Evans 1975, Hui 1979, Dahl et al. 1986), and in the northwestern Atlantic

(Kenney and Winn 1986, Seizer and Payne 1988), Stene/la spp. in the eastern tropical Pacific (Perrin et al. 1973, Perrin et al. 1979, Au and Perryman 1985,

Polacheck 1987, Reilly 1990), Atlantic white-sided dolphins (Lagenorhynchus acutus) in the northwestern Atlantic (Kenney and Winn 1986, Seizer and Payne

1988), and Risso's dolphin (Grampus griseus) off central California (Kruse

1989). In some cases, methods used to study coastal species, such as photo­ identification, radio-tracking, and stomach content analysis, have been used successfully with a few pelagic species.

The Pacific white-sided dolphin (Lagenorhynchus obliquidens) is one of the most abundant pelagic species of dolphins endemic to the temperate North

Pacific (23 oN to 61 oN; Leatherwood et al. 1984). There is a smaller northern form and a larger southern form that cannot be distinguished at sea, with a distributional overlap in the Southern California Bight (Walker et al. 1986).

These dolphins commonly occur in groups of less than several hundred but can 5

form herds containing thousands of individuals, often in association with northern right whale dolphins (Lissodelphis borealis). They feed opportunistically on a variety of schooling fishes and cephalopods (Stroud et al.

1981). Calving occurs from May through September (Brown and Norris 1956,

Ridgway and Green 1967, Harrison et al. 1969, Dohl et al. 1983) corresponding to a mid to late summer breeding season (Ridgway and Green 1967).

Off California, Pacific white-sided dolphins inhabit productive continental shelf and slope waters (Fiscus and Niggol1965, Dahl et al. 1983, Leatherwood et al. 1984) generally within 185 km of shore (Barlow in press). Although seasonal movements are not well documented, Dohl et al. (1983) found that this species was the most abundant cetacean off central and northern California, with greatest abundance during fall and winter and lowest during spring. Near northwestern Baja, California, southern California and Monterey Bay, dolphins are seen year-round and some may be resident with seasonal changes in abundance (Leatherwood et al. 1984).

Aside from knowledge of Pacific white-sided dolphin general distribution and abundance, factors influencing their behavior, local distribution, and occurrence are not well known. Because Pacific white-sided dolphins were known to frequent the Monterey Bay area year-round in the near-shore, but deep waters of the Monterey Submarine Canyon (A. Baldridge, R. Ternullo, pers. comm.), and were relatively accessible, a detailed study was conducted between 1987 and 1991. This study investigated the importance of Monterey

Bay to Pacific white-sided dolphins, based on the null hypotheses that dolphins are distributed randomly throughout the bay, are year-round residents with daily 6

behavioral patterns, and are not influenced by variations in oceanographic conditions. Therefore, the study objectives were to: (1) determine year-round distribution and relative abundance; {2) identify behavioral patterns; and (3) relate abundance, distribution, and behavior to physiography and environmental variables. 7

METHODS

Study Area

The study area included 1,062 km2 off the central California coast

between 36.2 °N and 37.0 °N (Fig. 1). with water depth of 10m to 2,800 m. The

area east of a line drawn between Santa Cruz and Pt. Pinos was considered

Monterey Bay, west of the line was considered outer Bay waters. The Monterey

Submarine Canyon is the most prominent bathymetric feature in the area. It

begins 100 m off Moss Landing and extends 82 km offshore. The canyon

divides the bay on a roughly east-west axis into two shallow shelves. The shelf­

break occurs at 150 m in most areas. Associated canyons include the

Ascension Canyon complex in the north; Soquel Canyon, a branch of the main

Monterey Canyon, and Carmel Canyon in the south. Carmel Canyon approaches shore within 0.5 km of Pt. Lobos, where the shelf-break occurs at a depth of 100m.

Monterey Bay is influenced by the California Current, an eastern boundary current that transports subarctic water towards the equator (Mar-Sep), and the poleward California Undercurrent, which occurs at depths below 150 m, and is termed the Davidson Current when it surfaces during winter (Nov-Feb). A cyclonic gyre begins north of the bay, flows towards Pt. Pinos in the south, and then curves towards the north inside the Bay (Breaker and Broenkow 1994 ).

During strong northwest winds, coastal upwelling occurs north of the bay at Pt.

Ano Nuevo and south at Pt. Sur, with advection of cold, upwelled waters entering the Bay from the north (Broenkow and Smethie 1978, Rosenfeld et al. 8

. I I I I I I I I I I I I

1500

1 1ooo I ~I 5 km I ~------~-----

Figure 1. Study area (dashed line) encompassing Monterey Bay, California 2 and surrounding waters. Study area was approximately 1 ,062 km • 9

in press). During wind relaxation there is shoreward advection and surtace

warming. Frequent eddy-like features are located west of the Bay, and internal

waves are common in the Canyon (Brdenkow and Smethie 1978, Shea and

Broenkow 1982, Koehler 1990, Breaker and Broenkow 1994). Three distinct

oceanographic seasons occur in Monterey Bay. The upwelling period (Mar-Jul)

is characterized by strong northwest winds, low surtace temperatures, high

surtace salinities, a steep rise in isotherms, and strong coastal fronts between cold upwelled and warm offshore waters. The oceanic period (Aug-Oct) occurs

when winds relax and warmer California Current water approaches the shore,

producing near-shore thermal gradients, increased surface stratification and

deeper isotherms. The Davidson Current period (Nov-Feb) is dominated by

southerly winds, low surtace salinities and temperatures, reduced horizontal

and vertical temperature gradients, and a deep mixed layer (Skogsberg 1936, Bolin and Abbott 1963, Broenkow and Smethie 1978, Chavez et al. 1991,

Breaker and Broenkow 1994). The onset of each season is variable from year to year, usually with an abrupt "spring transition" into the upwelling period (Huyer et al. 1990). These three periods were used for seasonal analyses, with the onset of each period determined by surtace and vertical temperature changes

(Chavez et al. 1991, F. Chavez, pers. comm.) throughout the study period.

Surveys

Vessel surveys were conducted from June 1987 to June 1991, although most surveys occurred from 1987 to 1989. Sixty-six dedicated surveys were conducted approximately twice monthly aboard the 10.7-m RN Ed Ricketts and '( ?:

10

a 4.6-m Boston Whaler. One-hundred fifty opportunistic surveys were conducted

on various natural history/research cruises, mainly aboard the 16.5-m MN Pt.

Sur Clipper. Twenty-three percent of st.Jrveys occurred during the upwelling

season, 30% during the Davidson season, and 47% during the oceanic season.

One to three dedicated observers were present on all cruises, and one (the

author) was consistent through all surveys. Additional sighting data were

obtained from a network of sport-fishing vessels in the area (R. Ternullo, pers.

comm.), and additional effort and sighting data were obtained from individuals

conducting harbor porpoise (Phocoena phocoena) surveys in near-shore Bay

waters (Dorfman 1990}.

During dedicated dolphin surveys, the RN Ed Ricketts departed from

Moss Landing Harbor and usually surveyed areas where water depths were

greater than 1oo·m. The Boston Whaler was limited to surveying the southern

Bay to Pt. Lobos. These vessels headed towards locations of reports of dolphins

from other vessels or, if none, searched waters throughout the Bay. Observers

equipped with binoculars were on constant watch during surveys when sea

states were Beaufort 4 or less and visibility was 3 km or more. The effective

sighting distance was up to 1 km from the vesseL During these surveys; time,

position, course, Beaufort sea state, weather conditions, and sea surface

temperature were recorded every half hour, at course changes, and when

cetaceans were sighted. When Pacific white-sided dolphins were sighted,

additional data recorded included estimate of group size for all cetaceans,

associated species of marine mammals and birds, general behavior state of the

Pacific white-sided dolphins, degree of group cohesiveness, occurrence and 1 1

type of aerial activity, and presence of calves. These data were recorded at the initial sighting and thereafter every 15 min until observations of that group ceased. Surveys were conducted durimJ daylight periods (generally 0700 to

1500 h) for seven to eight hours duration. Dolphins with distinct markings on their dorsal fins and anomalous-colored individuals were photographed for identification (WOrsig and Jefferson 1990).

Opportunistic surveys were conducted in conjunction with bird­ watching/natural history trips (Shearwater Journeys), a study of blue whales

(Balaenopte.ra musculus; Earthwatch, R. Wells and S. Kruse), and gray whale

(Eschrichtius robustus) watching trips. All vessels departed Monterey Harbor and spent seven to eight hours per day (0700 to 1500) at sea. Excluding gray whale watching trips, courses were haphazard, and usually covered 90 to 140 km per day. Depending on trip type, either inner bay shallow waters, waters overlying the Canyon, or offshore waters were traversed. Vessels stopped for periods of up to one hour during marine mammal or bird observations. Data were collected similar to dedicated dolphin surveys.

The study area was divided into a grid containing 256, 4x4 km2 quadrats.

The number of km surveyed in each quadrat was calculated and divided into four categories of effort which were 0-10, 11-149, 150-299, and 300+(Fig. 2a).

Additional dolphin sightings made by persons aboard 5 fishing vessels operating in the Monterey Bay area were collected and compiled by R. Ternullo, aboard the MN Pt. Sur Clipper on a year-round basis, and were screened for reliability based on observer experience. Data collected were date, time, position, temperature, estimate of Pacific white-sided dolphin group size, and 2 Figure 2. (a) Survey effort: number of km traversed in each quadrat (4x4km ) during dedicated Pacific white-sided dolphin surveys and opportunistic surveys. Quadrats are shaded relative to four effort categories. Numbers in lower left of quadrats indicate number of km surveyed in each quadrat. (b) Additional effort covered by charter vessels. Fishing destinations are shaded. Approximate effort represents shaded areas and transit to these areas from Monterey harbor (source: R. Ternullo). 13

presence of associated species. Effort was not quantified but was fairly localized and constant year-round. Sightings of cetaceans from these vessels occurred while transiting to fishing locations and while on site (Fig. 2b).

~

Distribution and Relative Abundance Analysis

Number of dolphins per km {relative individual abundance), number of groups per km (relative group abundance), mean depth, distance to shelf-break, and Contour Index value were calculated for each 4x4 km2 quadrat. Pacific white-sided dolphin sightings were categorized by depth: shelf {0·200 m), slope

{201-1000 m), and oceanic {>1000 m) waters; distance to shelf-break, from_.::; 5 km inside to > 15 km beyond the break; temperature, from 9.0°C to 19.0°C; and

Contour Index (CI), or degree of bottom relief. Contour index was calculated

(Evans 1975, Hui 1979, Selzer and Payne 1988) according to the formula:

Cl=1 OO{M-m) M where m=minimum depth, and M=maximum depth within a quadrat. Contour

Indices ranged from nearly no slope (0.01) to a steep slope (1.0). Index values were grouped into five equal categories from 0.01 to 99.99 (1 to 5). Observed Cl values, or frequency of occurrence of Cl categories in which dolphins occurred, were compared to expected values, or the frequency of occurrence of Cl categories, assuming dolphins were evenly distributed across all categories.

Chi-square analysis was used to compare the observed vs. expected frequency of occurrence categories among the Cl, depth, distance to shelf-break, and temperature classes. 14

To determine if dolphins were distributed differentially according to group size, mean group size for dolphin sightings was calculated for each quadrat with more than 20 km of effort, and shaded relative to five group size categories.

This was evaluated visually to assess trends. Group size means were compared by season and month with a Kruskai-Wallis test. Dolphin occurrence and relative abundance were compared by season and month with a Kruskai­

Wallis test to determine any seasonal patterns.

Behavior Analysis

Group size, behavior, degree of group cohesiveness, occurrence and type of aerial activity, and presence of multi-species groups were recorded during 15-min scan samples (Altman 1974). Behavior was observed between 0800 to 1400 hrs. Group size was the number of dolphins sighted within a 2 km area. This entire group often was composed of distinct subgroups of 5 to 25 dolphins. Dolphin behavior was categorized into five general states: feed, travel, social, mill, and rest (Shane 1990). Dolphins synchronously diving in a localized area, often with flukes emergent before diving, were considered feeding. Other factors that indicated feeding included detection of prey by the vessel's depth sounder and aggregations of scavenging seabirds near the dolphins. Surface feeding dolphins were easily classified as they pursued and caught fishes. A dolphin group moving in a single direction was considered traveling. Social activity was characterized by physical and/or sexual contact between two or more individuals, including rubs, nudges, chases, and object play (usually kelp). Milling behavior, or frequent direction changes, were those 15

activities not classified as feeding, traveling, socializing, or resting. Resting dolphins were tightly grouped and highly synchronous in their respiration . patterns {Norris and Dohl1980a, Wursig and Wursig 1980, Norris et al. 1994).

Mean sighting depth, distance to shore, and Contour Index were calculated for each behavior and compared with Kruskai-Wallis or Chi-square tests.

Differences in frequency of behaviors {mill, travel, socialize, or feed) among quadrats were totaled for all quadrats with more than 60 min. of behavioral observations. Frequency of each behavior in each of these quadrats

was compared with Chi-square analysis, and where significant, the primary behavior was noted. Quadrats were grouped according to primary behavior.

Mean depth, distance to the shelf-break (from center of the quadrat), and

Contour Index values were determined for each group of quadrats and analyzed with a Kruskai-Wallis test to determine environmental differences among these quadrats. Chi-square analysis was used to determine if frequency of behaviors differed by time of day.

Mean group size of dolphins engaged in each of the five behaviors was compared with a Kruskai-Wallis test. The observed number of occurrences of dolphins engaged in each behavior among four group size categories was compared to the percentage of total observations for all behaviors per group size category and multiplied by the total observations lor each behavior

{expected) with a Chi-square test.

Group characteristics of each behavior were compared; group cohesiveness, aerial behavior, and multi-species categories with a Chi-square test. Expected values were 50% of total frequency of observations for each 16

category. Group cohesiveness was scored at each 15 minute sample as (1) scattered • inter-individual distance greater than 100 m, or (2) tight - inter­ individual difference less than 100m. Aerial behavior occurred when all or part of the dolphin's body emerged above the water's surface, excluding normal respiration. Two types of aerial behaviors were distinguished (Norris and Dahl

1980a, Wursig and Wursig 1980): (1) single aerial leaps, creating little splash and (2) percussive repetitive leaps, characterized by rapid repetition of a particular leap creating distinct splashes. Aerial behavior was scored by type and occurrence within the previous 15-minute interval.

A multi-species group was considered as two or more species of marine mammals either intermixed as a single group, or In close proximity (within 0.5 km) of each other. The coefficient of association of mutli-species groups was calculated according to the formula:

Coefficient of association- Nab (Na+Nb+Nab) where Nab=number of occasions species a and species b were seen together,

Na=number of occasions species a was seen without species b, and Nb= number of occasions species b was seen without species a. Scores range from

0 (no association) to 1.0 (complete association; Martin and Bateson 1986). To determine if there was any difference in multi-species associations among

Pacific white-sided, Risso's, and northern right whale dolphins by season, coefficients of association were compared with a Chi-square test. Observed values were the number of occurrences of each multi-species group per season. Expected values were the percentage of Pacific white-sided dolphin 17

sightings (with no other species) per season multiplied by the total (all seasons)

number of occurrences of a particular multi-species group. This was based on

the assumption that if multi-species groups occurred equally throughout the

seasons, the number of occurrences of these groups should be proportional to

the number of Pacific white-sided dolphin sightings per season. Mean group

sizes for the three species, when alone and in association with each other, were

compared with a Mann Whitney U test. All combinations of multi-species groups

also were evaluated with Chi-square analysis to determine if associations were

significantly related to specific behaviors.

Radio-Tag and Track

Three dolphins {01, 02, 03) were captured and radio-tagged in

Monterey Bay. Tohe first dolphin was captured by B. WOrsig using a tail-grab

device {WOrsig 1982). The other two dolphins were captured on the same day

within the same school by J. Hall using a break-away hoop net (Evans 1974).

The radio-tags {148 MHz) were 8.0 by 1.5 em, with a 35.0 em whip

antenna, a battery life of one month, and an estimated reception range of up to

10 km from a boat, 30 km from shore, and about 100 km from a plane (B.

WOrsig, pers. comm.). Tags were attached to the dolphins' dorsal fin by boring two 0.5-cm holes through the fin. Tags were secured with two corrodible magnesium nuts, expected to break apart within two to four weeks. Dolphins were tracked from boats, shore, and planes.

Pulses from the radio-tags were only detected when the dolphin surfaced. Time and number of pulses, therefore, were recorded for each surface 18

period. When possible, the behavior of the dolphin group associated with the tagged dolphin was recorded. Two dive variables were analyzed and compared to behavior: 1) dive duration - the interval b-etween pulses; 2) respiration rate • the number of pulses or sequence of pulses per min. Two dive patterns were distinguished and compared to behavior: 1) regular dives • relatively consistent dive durations; 2) clumped -relative short dive durations interspersed by relatively long durations (30 sec+). Distances traveled and travel speeds were calculated for each hour and for each behavior.

Environmental Data Analysis and Correlations To determine how Pacific white-sided dolphins may be influenced by oceanographic conditions, nine environmental variables were assessed by month and season for each year during the study. Mean sea surface temperature was obtained from NOAA buoy #46042 (36°45'N, 122°25'W,

Fig.1 ). Sea surface temperature gradient was calculated by subtracting the greatest mean temperature from the lowest mean temperature from four locations within the study area. Locations of these temperature readings included Santa Cruz (36°57.5'N, 122°01.0'W), Hopkins Marine Station in

Pacific Grove (36°37.3'N, 121 °54.2'W), Granite Canyon (36°25.9'N,

121°55.0'W), and NOAA buoy #46042. Sea surface temperature gradient (or relative position of thermal fronts) was calculated with eight temperatures at 50 km intervals from a line extending west of Pt. Pinos {NOAA-Oceanographic

Monthly Summary). Each temperature value was subtracted from the adjacent

one to the west, creating seven temperature gradient values (numbered 1 to 7 19

extending east to west for reference). The thermal front was located where the

greatest temperature difference occurred. Relative intensity of the near-shore thermal gradient was the difference between the near-shore and 50 km offshore

value. Mean coastal upwelling indices were obtained from NOAA buoy #46042

(David Husby/NMFS). Mean salinities were obtained from Granite Canyon

(CDFG). Mean depth of mixed layer was obtained from F. Chavez (MBARI).

Mean temperature anomaly was obtained from the Pacific Grove site (Surface

Water Temperatures, Salinities, and Densities at Shore Stations, Marine Life

Research Group, Scripps Institution of Oceanography). The percentage of days

with intense upwelling was calculated by dividing the number of days with an

upwelling index value of 100 or greater by the total number of days within a

season.

Relative individual and group abundance, mean group size, and mean

distance to the shelf-break for each season were correlated to the nine

environmental variables with Spearman's rank correlation. 20

RESULTS

Distribution and Relative Abundance

Ninety-three percent (n=239) of 256 quadrats were surveyed, totaling

19,338 km of trackline. Pacific white-sided dolphins were observed in 108

(42.2%) surveyed quadrats. Dolphins were sighted on 133 days ( 201 separate sightings), or during 63% of dedicated and opportunistic surveys. In addition,

268 dolphin sightings collected by other observers resulted in 469 total sightings for the study period.

Pacific white-sided dolphins relative individual abundance (number dolphins/km) was greatest off Carmel Bay, across the outer Bay waters, and near the northern Monterey Canyon rim (Fig. 3).

Pacific white-sided dolphins relative group abundance (number of groups/km) was greatest near the canyon edge from Pt. Pinos to Pt. Lobos over water 50 m to 1,500 m deep (Fig. 4).

Dolphin sightings were not equally distributed among depth

(Chi2=34.314, df=2, P<0.001), distance to the shelf-break (Chi2=227.397, df=7,

P<0.001), temperature (Q.!J.[2=173.5, df=4, P<0.001), and Contour Index categories (Qbl2=10.228, df=4, P<0.05). Most sightings (43.7%) occurred over inner slope (201-1 000 m) waters, with fewer sightings over shelf (34.6%; 0-200 m) and outer slope waters (21.7%; 1001+ m). Mean (±SD) water depth for all sightings was 450.9±451.15 m and ranged from 50 to 2,012 m. Dolphins were most frequently sighted over the shelf-break and up to 2.0 km beyond it (Fig. 5).

Mean (±SD) distance to the shelf-break for all sighlings was 4.7±6.63 km, 21

#dolphinslkm oo D o.1-5.o 0]5.1-10.0 l!llill10.1-15.0 11111115.1+

------

Figure 3. Relative abundance of Pacific white-sided dolphins represented as number of dolphins per km per quadrat. Quadrats with more than 10 km effort are shaded relative to four abundance categories. Numbers in the lower left ~orner of each quadrat represent actual number of dolphins per km per quadrat. Numbers were rounded to the nearest 0.1. 22

#groupslkm oo fill 0.01-0.04 II o.os+

36"20'N

Figure 4. Relative group abundance of Pacific white-sided dolphins represented as number of dolphin groups per km per quadrat. Quadrats with more than 10 km effort are shaded relative to two occurrence categories. Numbers in the lower left corner of each quadrat represent actual number of dolphin groups per km per quadrat. Numbers were rounded to the nearest 0.01. 160 n=469 140 en Q) u 120 c: ...Q) :::1 100 u u 0 80 -...0 Q) 60 .c E :::1 40 z 20

0 + ...... 0 0 0 0 + ..... 0 l[) 0 l[) ..... u) C\1 C\1 u) ' ' 0 0 ...... ' 0 .f!, 0 0 ..... 0-- ..... 0- 0 .....- ..... C\1 - l[) C\1 l[) ' ' .....0 Distance to Shelf-break (km) Figure 5. Number of occurrences of the distance Pacific white-sided dolphin groups were sighted from the shelf-break (km). Negative values represent distances inshore of the shelf-break. 1\)w 24

ranging from 10.0 km inside to 41.0 km beyond the shelf-break.

Pacific white-sided dolphins were sighted in waters of 10.0°C to 18.9°C.

Fewer than expected sightings occurred in the 9.0°C to 12.9°C range, and more in the 15.0°C to 17.0°C range (Fig. 6).

Dolphins were observed more often in high relief areas than in low relief areas (Fig. 7). The Contour Index averaged (±SO) 3.9±1.21 for all sightings.

Dolphins were frequently sighted (50.6%) in small groups of 1 to 50 individuals, only 5.7% of groups contained a thousand or more dolphins (Fig.

8). Mean (±SO) group size was 203±395.4, with one to 4000 dolphins per group.

Based on dolphin mean group size by quadrat, groups greater than 300 individuals occurred only in the outer Bay waters, especially near Carmel Bay and the north rim of the' Monterey Canyon (Fig. 9). Quadrats with relatively large mean group sizes extended from Carmel Canyon across the Bay to the north rim, surrounded to the inside and outside by quadrats with lower mean group sizes. Within the Bay, relatively large groups ( 151 to 300) occurred only in quadrats over the main Canyon's southern edge and Soquel Canyon. Dolphin group sizes were lower in other quadrats inside the Bay, especially in shallow shelf waters.

Although there was no significant difference between group size and season, due to high variability among group sizes, there were some observable trends. Based on sightings plotted by group size and season, the upwelling season was characterized by smaller group sizes both inside and outside the

Bay, with only 3.4% of the groups containing 500 or more dolphins. Pacific 45 n=469 • Expected 40 (1.) 0 bJ Observed t:: 35 (1.) .... :::J 30 0 0 0 25 -0 ;::... 20 0 t:: ~ 15 C" ....CD 1.1.. 10

';:/!.,0 5

0 9.0-10.9 11.0-12.9 13.0-14.9 15.0-16.9 17.0+ Temperature (°C)

Figure 6. Observed and expected percent frequency of occurrence of Pacific white-sided dolphin sightings among five temperature categories. 80 n=469 • Expected 70 (I) [ill Observed cCJ (I) a.. 60 :sa.. CJ CJ 50 0 0 40 > cCJ (I) 30 :s tr ~ 20 1.1.. 0~ 10 0 1 2 3 4 5 Contour Index

Figure 7. Observed and expected percent frequency of occurrence of Pacific white-sided dolphin sightings among five contour index classes. 250 n=443

U) 200 Cl) 0 c Cl) ..... :I 150 0 0 0 0 -..... 100 Cl) .c E :I z 50

0--L-- ' 1-50 51-100 101-300 301-500 501-1000 1000+ Group Size

Figure 8. Number of occurrences of Pacific white-sided dolphin group sizes. 28

37"00711

group size 01-50 051-150 ~151-300 !11301-500 Ill 501+

Pt. Sur

Figure 9. Mean group size of Pacific white-sided dolphins perquadrat, for quadrats with greater than 20 km effort. Quadrats are shaded relative to five group size categories. Numbers in the lower left corner of each quadrat represent actual mean group size per quadrat. Numbers are rounded to the nearest 1.0. 29

white-sided dolphin groups during the oceanic season also predominantly contained less than 50 individuals, but were more evenly distributed among the classes, with groups over 500 dolphins comprising 9.1% of the sightings. Most sightings during the oceanic season were concentrated around the southern

Canyon rim and near Carmel Canyon, with relatively large dolphin groups observed in the outer Bay waters. The Davidson season was characterized by a relatively high proportion of large groups; 20.6% contained over 500 dolphins.

Most sightings were in the outer Bay waters except for a few sightings around

Soquel Canyon and the inner southern canyon rim, and were concentrated north of Pt. Pifios and around Carmel Canyon (Fig. 10, 11 ). The occurrence of large group sizes (500+) was greatest during the Davidson season ( 14. 7%), followed by the oceanic season (9%), and least during the upwelling season

(2.9%) (Chi2=6.646,' df=2, P<0.05).

Mean group sizes were greater during the Davidson season, mainly because the largest groups of the year were observed in February. Smallest mean group sizes occurred during the upwelling season, especially during

June and July (Fig. 12). Dolphins were nearest to the shelf-break during the

Davidson season (H=15.045, df=2, P<0.001 ).

Dolphin group abundance differed significantly among oceanographic seasons (H=7.419, df=2, P<0.025) and rnonths (H=22.419, df=11, P<0.025).

More dolphin groups per krn occurred during the oceanic season, especially during September and October (Fig. 13). Fewer sightings occurred during the upwelling season, especially during May. 30 z 0 en <1: w Q) N en "05 z c. 0 ::J en e c Ol "0 ~ c c C1l ~c 0 C/) "0 "> C1l 0 ·;::u C1l Q) u 0 z Ol 0en c ~ en l ::J 0 ~ c z 0 <1: C/) w re 0 C/) 0 >­ .0 C/) Ol c :;:: ;:::: Ol "05 c :cc. 0 "0 "0 Q) "0 "05 Q)' :c- ;;: u ;;:::: -~ a.. .,...0 ...w ::J u::Ol 31

80 Upwelling Season n=B9

1-50 51-100 101-300 301-500 501-1000 1000+ 80 Oceanic Season n=252

1-50 51-100 101-300 301-500 501-1000 1000+

Davidson Season n=102

1-50 51-100 101-300 301-500 501-1000 1000+ Group Size

Figure 11. Percent frequency of occurrence of Pacific white-sided dolphin group sizes for each oceanographic season. 32

1000 100

900 IJ Group size 800 E) EHort "tl Ql 700 N ... iii (')"' 0. 600 ::l ::l "' 0 500 -m ~ 50 =I: C!J 0 1: 400 ::+ Ql :::!:"' 300 200 100 0 Month I M A M J J A s 0 N D J F I I I Season ,,

Ql N 100 "tl iii ... (') 0. "' ::l ::l 0 200 50 "' ... -m C!J =I: 1: ...0 Ql 300 :::!:"' -

400 100

Figure 12. Mean Pacific white-sided dolphin group size (±SE) for each month and oceanographic season. Number of groups per month and season are indicated above error bars. The dashed boxes indicate approximate correspondence between months and seasons. Percent days of effort for all months and seasons during the study period are represented as white bars. 33

0.01

0.012 E ~... (!) 0.01 c. U) g. 0.008 ...0 Cl c 0.006 :E c. 0 0.004 0 "" 0.002

0 M M J J A N D J Month 1 A s 0 I Season •'

U) c. :::J ...0 Cl c .c c. 0 0 ""

Figure 13. Relative group abundance of Pacific white-sided dolphins, repre­ sented as the number of dolphin groups per km each month and oceanographic season. The dashed boxes indicate approximate correspondence between months and seasons. 34

Number of dolphins per km also differed among seasons (H=11.175, df=2, P<0.005) and months (H=21.377, df=11, P<0.05; Fig. 14). Dolphins were most abundant during the Davidson season, especially during November and

December. There was a significant difference in number of dolphins sighted per km among years during the oceanic (H=17.674, df=3, P<0.001) season, and no significant difference among years during the upwelling (P<0.1 0) and Davidson seasons (P<0.25).

Behavior Observations

Behavioral observations were collected during 201 sightings (134 days and 219 hours). Observations were from 30 to 360 min duration. Mill was the most frequentlY" observed behavior (33.3% of observations), followed by feed

(23.9%), travel (21.9%), social (17.9%), and rest (3.0 %).

Environmental Factors and Behavior

Mean depth (H=18.943, df=3, P<0.001 ), mean distance to the shelf-break

(H=11.817, df=3, P<0.01), and mean Contour Index values (H=13.271, df=3,

P<0.005) for dolphin sightings differed among behaviors (Fig. 15). Feeding dolphins occurred in shallower depths (mean (±SO), 529.7±477.53 m), closer to the shelf-break, (mean (±SO), 4.0±5.46 km), and in areas with greater bottom relief (mean (±SO), 4.2±1.12; Chi2=7.535, df=3, P<0.05) compared to other behaviors. Traveling dolphins occurred in the deepest waters, further from the 35

6

5

~4... Illc. '"c: 3 :cc. i5 c

Month I M A M J J A S 0 1 N D J F I I Season -+---

#dol=10,714 km=3043

Figure 14. Relative abundance of Pacific white-sided dolphins, represented as the number of dolphins per km for all months and seasons during the study period. The dashed boxes indicate approximate correspondence between months and seasons. The number of dolphins counted and number of km under effort for each oceanographic season are indicated below bars. 36

1000

900 'E ~BOOa c" .." 700 ::;" 600

500 Mill Travel Social Feed 'E :::. "" "'~ .c" ,.!. iii .:: rn .S! u" .s" c.. " ::;""' Mill Travel Social Feed

~ "C .5 ~ ::l 0 E u0 " ::;""'

Mill Travel Social Feed 289 196 149 198 Behavior

Figure 15. Mean depth (m), mean distance to shelf-break (km), and mean contour index for Pacific white-sided dolphins engaged in milling, travelling, socializing, and feeding. Standard error is indicated above bars. Numbers below behaviors indicate number of scan samples. 37

shelf-break, and in areas with less bottom relief. Pacific white-sided dolphins fed significantly more in waters over depths of 1 to 200 m than in deeper water

(Q.b12=10.051, df=2, P<0.01). whereas they traveled more often in waters

greater than 1000 m depth (Chi2=6.544, df=2, P<0.05). There was no difference

among categories lor socializing and milling dolphins.

Behavioral differences with a significance of P<0.20 occurred among 66

of 123 quadrats with observations (Fig. 16). Feeding was the predominant

behavior in nine quadrats (range p=0.0001 to 0.11 ), traveling in 22 quadrats

(range p=0.0004 to 0.16), socializing in 11 quadrats (range p=0.0002 to 0.16),

and milling in 24 quadrats (range p=0.0001 to 0.14). Dolphins frequently fed in

quadrats overlying the shelf-break off Pt. Pinos, over the head of Soquel

Canyon, and over the Canyon rim west of Moss Landing. Dolphins traveled in

outer Bay waters offshore of the shelf-break, except near Pt. Lobos, where they

traveled over Carmel Canyon. The predominant behavior within a quadrat

differed significantly according to depth (H=10.677, df=3, P<0.01 ), distance to the shell-break (H=7.974, df=3, P<0.025), and Contour Index values (H=10.677, df=3, P<0.01; Table 1).

For all seasons, Pacific white-sided dolphins fed more often in the

morning (Q.bl2=19.6, df=2, P<0.001) and socialized (Q.bl2=32.137, df=2,

P<0.001) more in the early afternoon. Dolphins were observed milling and

traveling with similar frequency in the morning and early afternoon (Fig. 17).

During the oceanic season, feeding often occurred in the morning, with

socializing observed more in the early afternoon. However, feeding occurred throughout the day during the upwelling and Davidson seasons. 38

37"00'N

Behavior D Mill osocial ~Feed II Travel

36"20'N

Pt Sur

Figure 16. Quadrats where Pacific white-sided dolphins exhibited predominant behaviors (mill, social, feed, travel) with a significance of P=.20 or less. Table 1. Mean depth (m), mean distance to the shelf-break (km), and mean contour index for quadrats where Pacific white-sided dolphins predominantly fed, milled, travelled, or socialized. Standard deviations, sample sizes (number of quadrats), and level of significance are shown.

FEED MILL TRAVEL SOCIAL SIGNIFICANCE Mean Depth (m) 373.5 914.9 1017.7 1068.1 H=10.677 so 391.4 570.1 529.7 497.7 .025

UJ (lJ Ill 0800-1 000

~ 1000-1200 40 Ill 1200-1400 Q) 35 (J ...5i 30 :I 8 25 0 0 20

>(J 5i 15 :I 0'" I!! 10 Ll.. 0~ 5

0 Mill Travel Social Feed 289 196 149 198

Figure 17. Percent frequency of occurrence of behaviors (mill, travel, social, feed) exhibited by Pacific white-sided dolphins during morning, mid-morning, and afternoon. Numbers below behaviors indicate number of scan samples. 41

Social Factors and Behavior - Group size differed among behaviors (H=37.625, df=3, P<0.001; Fig. 18).

Pacific white-sided dolphins traveling "(mean(±SD), 438.0±462.73) and resting

(mean (±SD), 366.7±294.58) were in larger group sizes than dolphins feeding

(mean (±SD), 204.0±268.72), socializing (mean (±SD), 166.5±224.34), and milling (mean (±SD), 115.6±202.64). The frequency of dolphin group size categories were different when feeding (Chi2=12.177, df=3, P<0.01); milling (Chi2=13.391, df=3, P<0.005); and traveling (C.h.i2=29. 754, df=3, P<0.001) but not socializing (Fig. 19).

The cohesiveness of dolphin groups differed according to behavior category (Chi2=132.585, df=1, P<0.0001; Fig. 20). Dolphins were in scattered subgroups while milling, socializing, and feeding; whereas dolphins were tightly grouped while traveling and resting.

The frequency of occurrence of aerial activity varied among behaviors

(Chi2=110.15, df=1, P<0.0001; Fig. 20). Dolphins were moraaerially active while traveling and socializing, and less so while milling and feeding, with no difference during resting. Percussive repetitive aerial behavior occurred most often while dolphins traveled; single leaps occurred most while dolphins socialized and fed.

While resting, traveling, and feeding, dolphins were associated with other species greater than 50% of sightings, and while milling and socializing, dolphins associated with other species less than 50% of sightings

(C.hl2=23.779, df=1, E.<0.001; Fig. 20). 196 500

400

.f!j 00 c. 300 ::I ....0 CJ s:: 200 ct1 (I) ~ 100

Mill Travel Social Feed Rest

Figure 18. Mean Pacific white-sided dolphin group size for each behavior. Standard error is indicated above bars. Numbers above bars indicate number of scan samples. • Mill Social 60 Em Travel

B 5o • Feed c Ill ...!!! :Iu 40 u 0 0 30 ~ c ~ 20 I:T !!! u.

0~ 10

0 <50 55-199 200-499 500+ Group Size

Figure 19. Percent frequency of occurrence of Pacific white-sided dolphin group size categories among behaviors (mill, social, travel, feed). Sample size for each behavior is indicated above bars. 44

D% Tight Ill %Scattered

Group Cohesiveness

Mill Travel Social Feed Rest EJ %Aerial ..u c: %No Aerial I!! II ~ :::1 u... Aerial Behavior 0 -0 u .."'c ...:::1 ! IL ;f.

Mill Travel Social Feed Rest 1 [ill %Mix Ill %No Mix

Mulli·Spe<;ies Groups

Mill Travel Social Feed Rest ""' 289 196 149 198 44 Behavior

Figure 20. Percent frequency of occurrence of group cohesiveness (tight vs. scattered), aerial behavior (no aerial vs. aerial), and multi-species associations (no associations vs. associations) among behaviors exhibited by Pacific white­ sided dolphins. Sample size is indicated below behavior. 45

Multi-Species Associations

Thirteen other species of cetaceans and four species of pinnipeds were sighted in addition to Pacific white-sided dolphins during the study period. In

67% (n=314) of the sightings, Pacific white-sided dolphins were the only species observed, 33% (n=155) of the time one or more other cetacean species (6 total) or pinniped species (1 total), also were observed with Pacific white­ sided dolphins. Northern right whale dolphins, Risso's dolphins, and California sea lions (Zalophus californianus) were most frequently sighted in association with Pacific white-sided dolphins. Although Dall's porpoise {Phocoenoides dalli) and harbor porpoise (Phocoena phocoena) inhabit Monterey Bay year­ round (Jefferson 1991, Dorfman 1990), only 4.8% of the time were Pacific white­ sided dolphins seen with Dall's porpoise and they were never seen with harbor porpoise.

Of all cetacean multi-species combinations, Risso's dolphins and northern right whale dolphins were associated most frequently (coeff. of assoc.=0.202), followed by Pacific white-sided dolphins and northern right whale dolphins (coeff. off assoc.=0.184; Table 2).

Pacific white-sided dolphins occurred significantly more than expected with northern right whale dolphins (Chi2=9.211, df=2, P<0.01) during the oceanic season, and less than expected during the upwelling season. Pacific white-sided dolphins were sighted in significantly larger groups when mixed with northern right whale dolphins (Z.=-5.121, P<0.001) than when sighted alone. Group size of Pacific white-sided dolphins when alone was not different than when mixed with Risso's dolphins (Fig. 21 ). Northern right whale dolphin Table 2. Coefficient of association values for all cetacean multi-species groups. Lo=Pacific white-sided dolphin, Lb=northern right whale dolphin, Gg=Risso's dolphin, Dd/Dc=common dolphin, Pd=Dall'.s porpoise, Pp=harbor porpoise, Oo=killer whale, Bb=Baird's beaked whale, Bm=blue whale, Mn=humpback whale, Ba=minke whale.

SPECIES Lo Lb Gg ", Dd/Dc Pd P_p Oo Bb Bm Mn Ba Lo Lb 0.184 Gg 0.108 0.202 Dd/Dc 0.01 0.01 0.006 Pd 0.018 0.01 0.013 0 Pp 0 0 0 0 0 Oo 0.002 0 0.005 0 0 0 Bb 0 0 0 0 0 0 0 Bm 0.024 0.005 0 0 0.011 0 0 0 Mn 0.057 0.021 0 0 0 0 0 0 0.013 Ba 0.004 0 0 0 0 0 0 0 0 0 Lb/Gg 0.062 0.01 0.01 0 0 0 0 0 0 450 B Pacific white-sided dolphin = Lo 400 B Northern right whale dolphin = Lb 350 , D Risso's dolphin = Gg :!! 300 Ci5 g. 250 ...0 " 200 t: Ill ~ 150 100 50

0 Lo Lo/Gg Lo/Lb Lb Lb/Lo Lb/Gg Gg Gg/Lo Gg/Lb 324 57 93 20 93 50 165 57 50 Dolphin Species

Figure 21. Mean group size of Pacific white-sided dolphins, northern right whale dolphins, and Risso's dolphins for single species groups (Lo, Lb, Gg) and multi-species groups (Lo/Gg etc.) when associated with one another. Standard error is indicated above bars. Sample sizes are indicated below bars. 48

group sizes tended to be greater when sighted alone than in association with

Pacific white-sided or Risso's dolphins. Northern right whale dolphins were seen more often in mixed groups (84"/o) than alone. Risso's dolphins were seen in significantly smaller group sizes when sighted alone than when associated with Pacific white-sided dolphins (b=-7.101, P<0.001) or northern right whale dolphins (Z=-7.164, P<0.001). This indicated that northern right whale dolphins tend to join large Pacific white-sided dolphin groups and that both Pacific white­ sided dolphins and northern right whale dolphins tend to join large groups of

Risso's dolphins.

When Pacific white-sided dolphins and northern right whale dolphins were associated, there was no significant difference among the five behaviors

(Fig. 22). Behaviors of Pacific white-sided dolphins differed when associated with Risso's ddlphins (Chi2=8.755, df=3, P<0.05); usually Pacific white-sided dolphins were milling and traveling. In association with blue whales, Pacific white-sided dolphins predominantly were socializing (Chi2=25.984, df=3,

P

Chi2=39.519, df=3, P<0.001, respectively). Although sample sizes were small, when Pacific white-sided dolphins were mixed with common dolphins or Dall's porpoise, they were either milling or feeding.

Killer whales (Orcinus orca) were never observed in direct association with Pacific white-sided dolphins (Table 2). However, on two occasions Pacific white-sided dolphins exhibited a flight response, swimming rapidly away, when sighted near killer whales. On one occasion, a group of 400 Pacific white-sided Ill Mill ~ Travel 90 Social 80 Ill Q) u l!lll! Feed lii 70 .... ~ 60 u 0 50 -0 ~40 c: g: 30 C" 3: 20 ;,g 0 10 0 Lo/Lb Lo/Gg Lo/Dc Lo/Pd Lo/Bm Lo/Mn Lo/Zc 78 32 4 5 7 12 53

Figure 22. Percent frequency of occurrence for behaviors that Pacific white-sided dolphins (Lo) were engaged in while associated with other species. Sample size is indicated above bars. Species codes: Lb=northern right whale dolphin, Gg=Risso's dolphin, Dc=common dolphin, Pd=Dall's porpoise, Bm=blue whale, Mn=humpback whale, Zc=California sea lion. Sample sizes are indicated below bars. l

50

dolphins was sighted traveling rapidly south (porpoising), with a large amount

of white water. A group of 10 killer whales was subsequently sighted about 1.5

km from the dolphins. On another occ·asion, a small mixed group of Pacific

white-sided dolphins and Risso's dolphins were observed, traveling rapidly

away from an adult male killer whale that was following them within 0.5 km. No

attack or aggressive behavior was observed.

Calves

Small calves with fetal folds were first observed in mid-June and were

seen throughout August to early September. Calves sighted in October

appeared less muted in color and larger than those seen in early summer. In all

instances when young calves were observed, they were closely associated with

an adult, presumed to be the mother, and often were in subgroups containing

other mother/calf pairs. Northern right whale dolphin calves and Risso's dolphin

calves were sighted from late October through February.

On 22 August 1987, epimeletic behavior in Pacific white-sided dolphins

was observed. A dead calf with fetal folds was found among a subgroup of 1o

other Pacific white-sided dolphins. There were at least three other dolphin

subgroups within a 0.5 km area. A large school of 1 ,500 white-sided dolphins

was present 18.5 km away (R. Ternullo, pers. comm.). It appeared that the adult

dolphins were taking turns supporting the calf. One dolphin would support the

calf on its beak for up to 1 min, release the calf, and as the calf began to sink,

another dolphin would swim below it and again support it on its beak. At least 51

five different dolphins in the subgroup exhibited this behavior. This continued for 1.5 hours, at which time the wind increased and observations ended.

Photo-Identification One hundred sixty-two Pacific white-sided dolphins with distinctive nicks or markings on their dorsal fins, and fifteen others with an anomalous coloration pattern were identified by use of photographs. Because the trailing edge of most dolphin dorsal fins were not nicked, dolphins were frequently sighted in groups of 100 or more, and there were limited observation periods, it became apparent that identifying most individuals would be difficult, resulting in few resightings. However, anomalous colored dolphins were much easier to sight and photograph even in groups numbering more than 1000 individuals. Although these dolphins were not always photo-identified, it was common to sight at least one "white" dolphin within large groups. Thirteen of these anomalous-colored dolphins were predominantly white with small areas of black-pigmentation on their sides, heads, and fins (Fig. 23). Dolphin #13 was orangish-tan dorsally where a normally colored dolphin would be gray. Another dolphin (#12) was normally colored except for an unusual white stripe extending up from its flank and widening over each eye, similar in coloration to the dolphin reported by Brownell (1 965).

These individual dolphins were identified from one to eight times within a particular season and between two seasons in a year, as well as among similar seasons in different years. Dolphin #4 was identified on six occasions over a two-month period during one oceanic season, and several others were sighted 52

Figure 23. Example of anomalous-colored Pacific white-sided dolphin. 53

several times during one oceanic season. Another dolphin {#B) was sighted

during three consecutive years, all during the month of October (oceanic

season). Dolphin #6, identified by D. Ekdahl {Shearwater Journeys) in October

1980 was resighted seven years later during this study, also in October. Fifty­

seven percent of initial sightings and resightings of anomalous individuals

occurred during September and October (oceanic season), 27% during

November and December (oceanic and Davidson seasons), only 7% during the

upwelling season, and the remainder during other months within the oceanic

and Davidson seasons {Table 3).

Ten anomalous-colored dolphins were sighted in the same group with up to three other anomalous colored dolphins, and were found within the same subgroup in a few instances. One such pair {#8, #11) was resighted together 14 months after they were initially photographed together. One "white" dolphin

{#11) was photographed with a normally colored newborn calf in early August, again in December of the same year, presumably with the same calf, and was last sighted with this calf the following year during December. Dolphin #4 was identified in Monterey Bay several times during the oceanic season, photographed again in Monterey Bay four years later, and near the Farallon

Islands (P. Pyle, pers. comm.) the following year.

Radio-Track

Pacific white-sided dolphins were tracked (not continuously) a maximum of 42.7 hr., and were tracked a maximum of 865 min. Mean (±SD) dive duration for all dolphins was 23.5 ±1.92 sec. Maximum dive duration recorded for D1 54

Table 3. Anomalous-colored Pacifie: white-sided dolphins, including dolphin number, date, group size, associated species and their group size. Lo=Pacific white-sided dolphin, Lb=northern right whale dolphin, Gg=Risso's dolphin, Ze=California sea lion.

9/l;Al7 250 Lb-10 9114187 1000 Ll>-100 wilh Dolphin #5 9124187 50 Wllh Dolphin #5 10113!87 300 Llr10 11/15/87 40 Lb-200 11/S/91 500 Ur30 with Oolpt!in #7 Islands

with DOlphin #11{same subgroup), #12, #13

#13 55

was 372 sec. Mean (±SO) respiration rate for all dolphins was 2.5±.32. Mean

(±SD) speed for all dolphins was 7.6±2.19 km/hr for all dolphins. The minimum distance traveled was 38.0 km in 5.8 hr. (Table 4).

For each dolphin, seventy percent of dives were less than 20 sec. Mean dive duration significantly differed among behaviors; slow travel, fast travel, mill/feed (H=44.326, df=2, P<0.001). Dolphins exhibited the regular dive pattern

90% of the time during slow or fast travel, and the clumped pattern 60% of the time during mill/feed behavior (Fig. 24).

Environmental Correlations Interannual variability among eight environmental variables was most constant during each Davidson season, whereas variables within the upwelling seasons fluctuated the most among years. Group and individual abundance of

Pacific white-sided dolphins were greatest during the 1987 oceanic season.

Compared to the other oceanic seasons, this period had the lowest upwelling index value, percentage of days of intense upwelling, and salinity. Also, the greatest mean sea surtace temperature, nearshore frontal gradient, and the greatest temperature anomaly for the study period were recorded during the

1987 oceanic period. Dolphin group and individual abundance were low during the 1991 upwelling season, corresponding to the lowest salinity, low near-shore frontal gradient, high upwelling index value, percent days of intense upwelling, and the shallowest mixed layer among upwelling seasons. The lowest mean temperature and temperature anomaly for the study period occurred during this season (Fig. 25). RADIO TRACK SUMMARY DOLPHIN 1 DOLPHIN 2 DOLPHIN 3 Sex F F M Length (em) 165 172 185 Tag Date 8/10/88 1/25/90 1/25/90 Track Time (min) 521.4 865.3 595.0 Contact Time (hr) 38.1 42.7 35.3 Number of Dives 1050 2324 1515 Mean Dive Duration (sec) 25.7 22.0 22.9 so 33.7 23.0 26.6 Max Dive Time (sec} 372 208 196 (6.2 min) (3.5 min) (3.3 min) Respiration Rate 2.1 2.7 ' 2.6 Mean Speed (km/hr} 5.0 8.9 8.9 Max Speed (km/hr) 14.8 20.2 20.2 Travel Speed (km/hr} 5.6 9.9 9.9 Mill/Feed Speed (km/hr) 4.7 2.6 2.6 Min Distance (km) 32.6 (19.2 hr} 38.0 (5.8 hr) 38.0

Table 4. Summary of three radio-tagged and tracked Pacific white-sided dolphins. Sex, length, track time, dive variables, and speed of movement are included. Ulen 57

Ul1 QJ a -~, o, DOLPHIN 2 ..... n=2324 ....0 QJ .c E ::J z

60 80 1 00 120 140 160 , 80 200 u Dive Duration QJ (/) EJ Dolphin 1 -c D Dolphin 2 0 -.:: IIlli Dolphin 3 ~ ::J Cl QJ -~ Cl c ctl QJ ::2: Fast Trav Slow Trav Mill/Feed

c DOLPHIN 3 ....c QJ lllll Clumped -ctl 1111 1111 Ill 0... Regular ~0 Ill I II II I II Min Fast Trav Slow Trav Mill/Feed BEHAVIOR

Figure 24. a) Frequency histogram of dive duration for 02; b) Mean dive (±SO) duration by behavior for 01, 02, 03; c) Percentage pattern type (clumped or regular) by behavior for 03. ~0.7 150 i o.e 14(1" ~,::: 0 0.5 J120 14 s .ll,oo F 13.5 )o• ~ !3 loo ~ 0.3 "so ~ 12.5 ]02 : 12 J40 ~ 5 11.5 A ~0.1 ~ 11L-~0~------0 • ~ ,_,.c"------,._------0 p u,.., upwan Oceanic DaMon

s "'45 0.9 40 ! " I : 1 ~ li 10

0.4 =------E 2 0-'.~·--"------Ocannlt: Oav!d.!lon Ocnanlc 0itllldaan Davidson 0.0170 500 • 450 400 0 ~350 cg_300 i!.so "200 ~150 100 50 " o K ·~------Oceanic UpwcU O:aanlc Davida011

Figure 25. Mean environmental conditions (A) sea surface temperature, (B) sea surface temperature gradient in bay, (C) nearshore frontal gradient, (D) upwell index, (E) salinity, (F) depth mixed layer, (G) sea surface temperature anomaly, (H) percentage days intense upwelling, and (I) number of dolphins per km, (J) number dolphin groups per km, (K) group size, (L) distance to shelf-break, for each oceanographic season (Upwell, Oceanic, Davidson) during each year of the study. 59

There was a significant correlation between dolphin group and individual abundance, group size, sighting distance to the shelf-break, and environmental variables for seasons. Pacific white-sidEld dolphin group abundance was

positively correlated to temperature (Is=0.622, n=13, P<0.001) and the near­

shore frontal gradient (rs=0.472, n=13, P<0.01 ). Relative abundance also was

positively correlated to temperature, and negatively correlated to the upwelling

index and percentage days of intense upwelling per season. Dolphin sighting distance to the shelf-break was positively correlated to temperature (Is=0.486,

n=13, P<0.01) and the near-shore frontal gradient (Is=0.305, n=13, P<0.05).

When the temperature anomaly was high, dolphins were more abundant (rs=0.471, n=13, P<0.01 ), and occurred closer to the shelf-break (Is=0.519,

n=13, P<0.01 ). 60

DISCUSSION

Distribution

Line transect methodology was not used because the main purpose of this study was not to estimate actual abundance but instead to spend time observing dolphins. Although surveys were "casual", observer effort was consistent throughout all trips providing data on distribution, and relative individual and group abundance. Biases may have been incurred in the relative abundance and distribution because pre-established random or consistent transects were not used.

Based on other surveys off California, Oregon, and Washington, Pacific white-sided dolphins are concentrated between 200 m and 2000 m, usually within 180 km of shore (Doh I et al. 1983, Brueggeman 1992, Green et al. 1993,

Barlow in press). The study area off Monterey encompassed shallow shelf waters out to just beyond 2000 m depth. Due to the near-shore proximity of the

Monterey Submarine Canyon, the prime habitat of these dolphins was regularly surveyed and accessed for detailed distributional patterns. However, more effort in the southern Bay may have resulted in underestimating the importance of the northwest portion of the study area. Dolphins were not randomly distributed throughout the Monterey Bay area, but instead frequented, and were relatively abundant over, inner slope waters in areas of high relief, around the northern edge of the Monterey Submarine Canyon and especially between Pt. Pinos and

Pl. Lobos. These areas are characterized by complex circulation patterns, and steep, heterogeneous bathymetry encompassing several mini-canyons 61

overlying the shelf-break and oriented perpendicular to the coastline. Dolphins infrequently occurred in small groups over Monterey Bay shelf waters. Dahl et aL (1983) and Leatherwood et aL (1984) al~o found that Pacific white-sided dolphins were uncommon and usually in groups of less than 10 over shell waters. As in the Monterey Bay area, especially in Carmel Bay and south of Pt.

Lobos, Pacific white-sided dolphins occurred closest to shore where the shelf­ break approached the coastline (Dahl et aL 1983, Bruggeman 1992,, Green et al. 1993, Barlow in press). Beca!Jse specific behaviors appeared more frequently in particular quadrats, it is likely that dolphins differentially use microhabitats within the Bay. High dolphin abundance extended from near-shore Carmel Canyon and offshore, crossing the Bay to the northern outer rim of Monterey Canyon. This relatively deep water area was frequented by traveling dolphins, which tend to occur in larger groups. This area could represent groups of dolphins bypassing the Bay while moving up or down the coast, possibly in search of prey within their preferred inner slope habitat. Hawaiian spinner dolphins (Stene/la longirostris) use extensive areas of the coast for feeding, and concentrate daily activities in certain coastal regions, occasionally moving away from these areas for several days (Norris et aL 1994).

Pacific white-sided dolphins frequently milled in quadrats adjacent and offshore of feeding areas which were located in shallower waters near the shell­ break, especially in high relief quadrats. Pilot whales (Giobicephala macrorhynchus; Shane 1984), Hawaiian spinner dolphins (Norris and Dahl

1980a), and humpback dolphins (Sousa chinensis; Saayman and Taylor 1979) 62

also exhibited particular behaviors in specific locations. Heimlich-Baran (1988) found that habitat use patterns of resident killer whales in the Pacific Northwest were centered around feeding areas. Feeding quadrats of killer whales were characterized by steep underwater slopes rising to within 10m of the surface.

The main prey of these whales was salmon, which fed on smaller bait fish. Killer whales traveled between feeding areas over deep water areas with low relief.

Pacific white-sided dolphins near Monterey Bay also traveled across deeper outer Bay waters with relatively low relief.

Risso's dolphins (Kruse 1989), and Dall's porpoise (Jefferson 1991) in the Monterey area, other dolphin species (Hui 1979, Evans 1982, Kenney and

Winn 1986, Cipriano 1992) and seabirds (Ainley and Jacobs 1981, Briggs el al.

1987, Heinemann el al. 1989) frequent high relief, heterogeneous, and shelfedge habitats: .Hui (1979) found that common dolphins were abundant over complex submarine topographies, similar to the situation with Pacific white-sided dolphins in Monterey Bay. He suggested that complex bottom topography often results in modified currents, increased mixing; therefore, greater food abundance. Evans (1971) suggested that common dolphins use passive listening, differences in currents, and thermal structure to locate specific bottom features, such as escarpments and seamounts, associated with abundant prey. Alternatively, orientation to these areas may involve learning and past experiences (Kenney and Winn 1986, Wi.lrsig 1986). High relief areas, and likely abundant food sources, occur throughout the Monterey Bay area, probably accounting for the frequent presence and abundance of Pacific white­ sided dolphins. 63

Although dolphins in Monterey Bay were in groups of 1-50 individuals, similar to the findings of Dahl et al. (1983), mean group size was greater in

Monterey Bay than in most other locations (Dahl et al. 1983, Leatherwood et al.

1984, Brueggeman 1992, Hill and Barlow 1992, Barlow 1993b, Buckland et al.

1993, Green et al. 1993). Wells et al. (1980) suggested that group size may vary

greatly among regions, and was related to season and habitat type.

Leatherwood et al. (1984} summarized all Pacific white-sided dolphin sightings from 1949 to 1979 in the eastern North Pacific by season and mean group size, and found that herds were significantly larger in southern (<30°N) and northern

(>55°N) areas than in their central (30-55 °N) range.

The difference between mean group size reported by Leatherwood et al. (1984), and that reported by Dahl et al. (1983) and in this study may represent less effort obtained for this area before 1979. The greater mean group size

reported here may be due to a higher proportion of larger dolphin groups near

Monterey. Large group sizes in bottlenose dolphins (Tursiops truncatus) are considered advantageous for feeding on abundant and patchy prey (WOrsig

1979, Shane et al. 1986, Scott and Chivers 1990). When dusky dolphins

(Lagenorhynchus obscurus) and common dolphins fed on anchovies, they occurred in large groups consisting of 300 or more dolphins, compared to smaller groups when feeding on other prey (Hui 1979, WOrsig and WOrsig

1980}. Similarly, where Pacific white-sided dolphins predominantly led on prey that congregate into large schools, such as anchovy, Pacific whiting, and Loligo, in southern and central California, dolphin group sizes were relatively large

(Stroud et al. 1981, Leatherwood et al. 1984, Walker et al. 1986, Ch. 2). In 64

contrast, off Washington and in the northern North Pacific where these dolphins fed mainly on cephalopods and mesopelagic fishes, prey that may not congregate into dense schools, group si.zes were relatively small (Stroud et al.

1981, WOrsig et al. 1989, Buckland et al. 1993, Walker and Jones 1993). Given

Monterey Bay's complex physiography and oceanography, and great abundance and diversity of seabirds and marine mammals, the Bay must contain an extremely abundant and aggregate food source.

Throughout the year, most large Pacific white-sided dolphin groups

(>150) were found in outer bay waters, whereas groups inside the bay were predominantly small (<150). Larger groups of Pacific white-sided dolphins, however, occurred inside the Bay over the edge of Soquel Canyon and near the southern rim of Monterey Canyon, areas of high relief and jagged bathymetric lines:· Large dolphin schools of several species frequent offshore waters with smaller groups found coastally in Japan (Kasuya 1971). Group size of bottlenose dolphins increased with water depth, distance to shore, and openness of habitat (WOrsig and WOrsig 1979, Wells et al.1980, Shane et al.

1986, Scott et al. 1990). The relatively limited deep water between shallow shelves in the north and south of Monterey Bay may result in smaller Pacific white-sided dolphin group sizes, presumably reflected in prey type or abundance here compared to outer bay waters.

Seasonality

Although group sizes of Pacific white-sided dolphins were not significantly different among oceanographic seasons due to high variability, 65

small groups frequently occurred during the upwelling season and relatively

larger groups occurred during the Davidson season, Leatherwood et al. (1984)

found seasonal differences in group size of Pacific white-sided dolphin among

five latitudinal belts along the eastern North Pacific. In some cases, differences

by season were extreme, particularly off Baja, California where the largest

mean group sizes occurred from July to September and the smallest from April

to June; and from Oregon to British Columbia where mean group size was

greatest from April to June and smallest from January to March. Seasonal

differences in group size are common in other dolphin species, and in most

cases represent seasonality of particular prey species (WOrsig 1978, Hui 1979.

WOrsig and WOrsig 1980, Ballance 1990, Cipriano 1992).

In the Monterey Bay area, Pacific white-sided dolphins differed

significantly in relative individual and group abundance among the three '' oceanographic seasons. Although Dohl et al. (1983) used the four solar

seasons in comparing dolphin abundance off central and northern California,

their results were similar to this study despite greater effort during the oceanic

season. During the upwelling season, Pacific white-sided dolphins were least

abundant and occurred less often, usually in small groups. Dahl et al. (1983)

counted the fewest number of Pacific while-sided dolphins during spring and

summer months off central and northern California. During spring, Pacific white­

sided dolphins appeared equally distributed off central California. relatively

close to shore (<15 nm), and in small groups, with half their sightings consisting

of groups less than nine dolphins. For three survey years, counts during May

were consistently low, even though Dahl et al. (1983) reported good sighting '~f'2 '

66

conditions. This corresponds to peak upwelling in the area, possibly suggesting

a reduced abundance or availability of food at this time. By summer, Dohl et al.

(1983) found that 90% of dolphin sighting~ occurred in central California, still in

relatively small groups. In Monterey Bay, Pacific white-sided dolphins occurred most frequently

during the oceanic season. Their frequent presence during this season,

especially from September through October, indicated that they were exploiting abundant food sources. Dahl et al. {1983) also found that Pacific white-sided

dolphins were most abundant during the fall, with counts about three times

greater than at other seasons. Although dolphins were distributed along the

central and northern California coastline, they were most concentrated off

central California, within 10-30 km of shore in relatively large groups.

More reproductive behavior and more young of the year were observed

during fall than at other times (Doh! et al. 1983). Norris and Prescott (1961)

reported small calves from May to September in southern California. During this

study, young Pacific white-sided dolphin calves were seen from mid June to

early September. These nursery groups, which could represent the

congregation of breeding and calving dolphin groups, may frequent Monterey

Bay at this time, for predictable prey during an increased need for food by

lactating females. Breeding appears seasonal, as indicated by the extreme

enlargement in male testis size from mid to late summer (Ridgway and Green

1967); with a gestation period of 10 to 12 months (Leatherwood and Reeves

1983). ~· !

67

During the oceanic season, Pacific white-sided dolphins fed

predominantly in morning hours and socialized more in the afternoon. No

similar pattern occurred during the other two seasons. Brager (1993) found that

bottlenose dolphins exhibited diurnal behavior patterns in summer only,

possibly because of differences in prey seasonality. In contrast, Norris et al.

(1994) suggested that spinner dolphins in Hawaii exhibited distinct diurnal

behaviors because of their year-round, abundant prey in the deep scattering

layer.

When present, Pacific white-sided dolphins were frequently found in large cohesive groups during the Davidson season. Dohl et al. (1983) also

found that group sizes were greater during winter than during spring and

summer. During the Davidson season, large but separated groups of Pacific

white-sided dolphins may reflect the presence of abundant but patchy prey.

Norris and Dohl (1980b) suggested that some species of dolphins travel in

large schools that are broader than long, allowing them to search a wide area

for prey. Locating a large, single prey school could provide food for hundreds of

dolphins. Also, because dolphins were observed only during daytime, it is

possible that they remain together during daytime and disperse later to feed at

night. Two Pacific white-sided dolphins radio-tagged from the same school in

the Monterey Bay area during this season, remained together during the day

and through the late night, but separated during the early morning hours before

dawn, and joined again later that day.

Pacific white-sided dolphins, although present year-round, probably

were not daily residents of the Monterey Bay area, as suggested by 68

Leatherwood and Reeves (1983). Photo-identification evidence gathered during this study indicated that certain dolphins frequent the Monterey area, particularly during the oceanic season, rat~er than new groups of dolphins continually passing through. This idea fits with frequent sightings of scattered and milling subgroups in the area during the oceanic season, and contrasts to the Davidson season, where larger groups of traveling dolphins were frequently sighted. Identified dolphins usually were seen only once during this season.

One Pacific white-sided dolphin opportunistically photographed near the Farallon Islands, 130 km north of Monterey Bay, was identified several times in the Monterey Bay area during this study. This indicated, as Evans (1982) found for common dolphins, that white-sided dolphins may frequent areas tor variable periods, then move to other productive feeding areas. A radio-tagged common dolphin released off southern California was resighted ott Baja, California, 500 km from the release site 10 days later (Evans 1982). Also, a radio-tagged dusky dolphin was released off north-central South Island, New Zealand, and tracked over 222 km to the North Island in a period of three days (B. WOrsig, pers. comm.) Two Pacific white-sided dolphins, radio-tagged near Monterey Bay during this study, traveled 38 km in 5.8 hours, indicating that they are capable, just as common dolphins and dusky dolphins, of traveling great distances.

Behavior The behaviors of Pacific white-sided dolphins were quite diverse and appear highly variable, as Wilrsig and Wilrsig (1980) suggested tor dusky 69

dolphins, a species closely related and similar in appearance to white-sided dolphins. Pacific white-sided dolphin behavioral states differed among group sizes, degree of school cohesiveness, frequency and types of aerial activity, and multi-species associations. Evans (1 971) suggested that estimates of delphinid group size could be affected by the tendency of some species to fluctuate between dispersed subgroups and cohesive large groups. Subgroups that appear independent and separated by several kilometers or more, can be actually part of a larger school. During aerial surveys of common dolphins, Dohl et al. (1986) found two subcategories of large schools; dispersed schools with many distinct subgroups, and tightly grouped, rapidly moving, compact schools. These temporary dispersals into smaller subgroups may represent the basic social unit with long term integrity (Norris and Dohl1980b, Wiirsig and Wiirsig 1980).

Similar to Wiirsig and Bastida's (1 986) finding of long-term affiliation between a pair of dusky dolphins, a pair of Pacific white-sided dolphins found within the same subgroup in Monterey Bay were sighted again together after a year.

Perrin (1 972) suggested that these large groups may be isolated from other large groups of the same species, forming primary breeding herds. Pacific white-sided dolphins tended to form scattered subgroups during periods when they occurred for several consecutive days in the Monterey Bay area, and frequently led in this dispersed pattern. Although prey type was often unknown, fishes occasionally were identified as dolphins led near the surface.

During the oceanic season, dolphins occasionally led in scattered subgroups on Pacific saury (Cololabis saira) 5+ km offshore. Individual dolphins chased, 70

abruptly blocked, and disoriented fish by a quick turn at the surface with

< considerable whitewater. Also during the oceanic period, large groups in excess of 500 individuals were observed feeding on anchovies. Also feeding there were pelicans, gulls, shearwaters, California sea lions, and humpback whales. As suggested by Wursig (1986), dolphin feeding strategy is probably flexible according to the amount and type of prey available, often involving the coordination of many individuals. He suggested that where prey is relatively constant, such as in the deep scattering layer preyed upon by Hawaiian spinner dolphins and New Zealand dusky dolphins. foraging strategy and group structure is fairly constant year-round. Where prey type is more variable, the dolphins' strategy must also change (Wursig et al. 1989). Killer whales fed on Dall's porpoise and pinnipeds in relatively small groups by rapid "surprise" attacks, compared to larger groups involved in the attack and pursuit of baleen whales during periods of several hours (Ternullo et al., unpubl. ms).

Pacific white-sided dolphins frequently exhibit aerial activity, as is common in many delphinid species (Pilleri and Knuckey 1969, Norris and Doh!

1980b, Wursig and Wursig 1980, Wursig 1986, Cipriano 1992). Heimlich-Baran

(1988) suggested that percussive behaviors may serve a social signaling function in dispersed traveling groups of killer whales. Leaps may produce short-range omni-directional sounds important for communicating among dolphins that are visually isolated. Leaping often occurs at night and in dispersed schools of Hawaiian spinner dolphins (Norris and Doh! 1980a).

Percussive repetitive leaps can be heard up to 0.5 km away (WOrsig and Wursig

1980). 71

While engaged in this type of aerial activity, Pacific while-sided dolphins were frequently found in large traveling groups, as opposed to widely dispersed groups engaged in other activities. If these aerial behaviors function to facilitate communication or structuring of the school, this activity would probably be most advantageous when dolphins are in these large tight groups, as individual vocalizations may be "drowned out", making more distinct signals necessary.

While engaged in this activity, an individual Pacific white-sided dolphin exhibits one of about 13 different types of leaps, often repeating the same type up to 20 times in quick repetition. However, single noiseless leaps occurred most when

Pacific white-sided dolphins were socializing, probably resulting from excitement and exuberance rather than communication. In this case, Pacific white-sided dolphins performed single leaps, often in unison with one or more dolphins, entered· nose first with little splashing, probably to surface and gain more momentum while diving back down again. This behavior also was exhibited by dusky dolphins (Wursig and WOrsig 1980).

Many multi-species associations involving Pacific white-sided dolphins probably are prey related. Food-based associations are common among widely diverse species types, such as dolphins, whales, pinnipeds, birds, and fishes

(Perrin et al. 1973, Au and Perryman 1985, Martin 1986, WO rsig 1986, Katona and Whitehead 1988, Norris and Schilt 1988, Scott and Chivers 1990). Norris and Dahl (1980b) suggested that mixed schools occur in areas with a predominant prey source. In southern California waters, anchovy and squid are dominant prey and mixed species groups are common. Evans (1982) suggested that the large opportunistic multi-species aggregations of seabirds, 72

dolphins, and tunas were related to high biological productivity. Spotted dolphins and yellowfin tuna (Thunnus albacares) in the eastern tropical Pacific have similar diets and are closely associated (Perrin et al. 1973). California sea lions, humpback whales, and birds associated with Pacific white-sided dolphins in Monterey Bay mainly during feeding episodes. Antonelis et al. {1984) observed sea lions feeding with Pacific white-sided dolphins in southern California when anchovies were abundant. It is possible that Pacific white-sided dolphins search for large groups of Risso's dolphins possibly for enhanced cephalopod foraging, just as bottlenose dolphins appear to join pilot whales (Shane 1984).

The reasons for species associations that are not primarily food based are less clear. Scott and Chivers (1990) suggested that in pelagic waters, mixed species herds ctluld provide a similar protective function as large single­ species herds, and provide more options in feeding strategies. The eastern tropical Pacific association among spinner dolphins, spotted dolphins, and yellowfin tuna is possibly due to the fact that spotted dolphins and tuna are diurnal feeders, preying on epipelagic species, whereas spinner dolphins feed nocturnally on mesopelagic species. Spinner dolphins join spotted schools in the morning and spend the day resting within spotted dolphin schools (Fitch and Brownell 1968. Perrin et al. 1973). Similarly, northern right whale dolphins off California fed predominantly on mesopelagic fishes and cephalopods (Leatherwood and Walker 1979) probably at night, whereas Pacific white-sided dolphins fed on epipelagic fishes and cephalopods (Stroud et al. 1981, Ch. 2), often during the day. Although in the northern North Pacific, these two species ''~J"''"~1

73

fed on very similar prey, it is not clear to what extent they associate (Walker and

Jones 1993).

Photo-identification

Anomalous-colored Pacific white-sided dolphins have been reported

elsewhere (Brown and Norris 1956, Brownell 1965, Hain and Leatherwood

1982, Leatherwood and Reeves 1983, Walker et al. 1986, Stacey and Baird

1991), although no one has previously photo-identified such individuals. Several anomalous-colored (white) dusky dolphins exist in New Zealand and

have been tracked for at least five years (B WOrsig, pers. comm.). Photo­

identification in Monterey Bay of "white" Pacific white-sided dolphins provided

valuable information on movements, associations, and residency, that

otherwise might' be difficult to obtain without actually tagging dolphins. At least

one of these dolphins had a calf that survived over a year, and several "white"

dolphins were sighted throughout three years. It is possible that these dolphins

could be used as "herd" markers to determine residency patterns, movements,

and seasonal shifts of large groups of dolphins. However, considerably more

effort is needed in the Monterey Bay area and other areas frequented by Pacific

white-sided dolphins.

Radio-Track

The maximum dive recorded for 01, 372 sec, was more than twice the

maximum dive duration previously published for this species. This was the first

dive recorded after the dolphin was released, and may represent an unusually 74

long dive time due to the stressful situation. The remainder of dives were less than 196 sec. Hall (1970) recorded a maximum dive duration of 134 sec for a female Pacific white-sided dolphin trained for open ocean release. This occurred when the dolphin dove to 214m and activated a sound device. After a

3-min dive, a trained bottlenose dolphin had a low percentage of lung oxygen, and was virtually anaerobic after a 6 min dive and spent 4 to 5 min recovering at the surface. This dolphin could dive continually if dives were less than 2 min duration. Kanwisher and Ridgway (1983), therefore, suggested bottlenose dolphins limit dive duration to avoid oxygen debt, with long dive durations the exception. Radio-tagged dusky dolphins, very similar in appearance and slightly smaller than Pacific white-sided dolphins. dove a maximum of 182 sec duration (Wursig et al. 1985, Cipriano 1992). Because Pacific white-sided dolphins exhibited relatively short dive durations (70% less than 20 sec) and dive durations greater than 90 sec were rare, indicated that dives greater than this may create an oxygen debt They are probably not deep divers, and presumably feed on prey at the surface or prey that vertically migrate at night, matching the habits of most of their major prey items (Ch. 2).

The two distinct dive patterns observed in radio-tagged Pacific white­ sided dolphins were similar to those of radio-tagged spotted dolphins in the eastern tropical Pacific (Leatherwood and ljunblad 1979, Scott and Wussow

1983}. Spotted (Stenel/a attenuata) and common dolphins observed feeding, spotted dolphins milling over a sharp dropoff in depth, and foraging bottlenose dolphins exhibited the clumped pattern of dive durations. Regular short dives were associated with traveling spotted and bottlenose dolphins, and mass >'if''' ! ' 75

movements of common dolphins (Leatherwood and Ljunblad 1979, Evans

1971, Scott and Wussow 1983, Lockyer and Morris 1987). Similarly, Pacific

white-sided dolphins exhibited the clumped pattern most often while they milled

or fed, and the regular pattern while they traveled.

Environmental Conditions Pacific white-sided dolphins varied in relative group and individual

abundance, and behavior among the three oceanographic seasons in the

Monterey Bay area. Each oceanographic season in Monterey Bay can

generally be characterized by specific environmental features (Table 5; Bolin

and Abbott 1963, Breaker and Broenkow 1994, Chavez et al. 1991 ). The mean

of eight environmental variables for each season during this study (Table 6)

appeared consistent with •normal" oceanographic conditions, although, dolphin

occurrence and abundance was not consistent among similar seasons in

different years. Interannual variability occurs among these seasons in both

onset and intensity of characteristic features (Ainley and Boekelheide 1990),

which may affect variability in dolphin ecology among years. An extreme

example of this occurred during an El Nino period corresponding to

anomalously high temperatures when some bottlenose dolphins moved from

southern California into Monterey Bay (Wells et al. 1990).

Some seasonal differences in Pacific white-sided dolphin occurrence,

relative abundance, group size, and distance to the shelf-break may be

explained by sea surface temperature. This environmental variable is

commonly used to explain biological effects because of its distinct physiological Table 5. General oceanographic characteristics (months, sea surface temperature, salinity, coastal fronts, mixed layer, upwell index, surface currents, winds, temperature gradient in bay) of the three seasons (upwelling, oceanic, Davidson) as related to Pacific white-sided dolphin relative individual and group abundance. The designations of high, med, and low are relative comparisons among the seasons.

OCEANOGRAPHIC

strong temp gradient deeper isotherms, deepest mixed layer, temp (surface-50m), increased stratification, uniform to considerable steep rise in isotherms, strong gradient, distinct ·depths

onshore CA Current Davidson Current

GROUP

low Table 6. Mean ±SD of eight environmental conditions (sea surface temperature, salinity, nearshore front intensity, relative front intensity, depth mixed layer (m), upwell index, percent days of intense upwelling, temperature gradient in bay) as related to Pacific white-sided dolphin relative individual and group abundance among the three oceano­ graphic seasons (upwelling, oceanic, Davidson) during the study period.

ENVIRONMENTAL

RELATIVE FRONT INTENSITY shore DEPTH MIXED

PERCENT DAYS INTENSE UPWELL .62±.10 .34±.15 .12±.03 TEMPERATURE

GROUP ABUNDANCE INDIVIDUAL 1.42 78

effects (Bakun and Parrish 1980). The occurrence and movements of several dolphin species were related to temperature (Gaskin 1968, Leatherwood et al.

1980, WOrsig and WOrsig 1980, Kasuya -and Jones 1984, Au and Perryman

1985, Selzer and Payne 1988, Breese and Tershy 1993). Dohl et al. (1986) thought the movements of common dolphins in and out of the Southern California Bight were temperature related, with peak numbers of dolphins f occurring 3 to 5 weeks after the intrusion of warm water. When the water cooled, dolphins shifted their distribution south into warmer waters. In Monterey Bay,

Pacific white-sided dolphins were more abundant, occurred more frequently, and were sighted closer to the shell-break as the temperature increased, with larger group sizes occurring most often in warmer temperatures. Dahl et al.

(1983) also found that Pacific white-sided dolphins were most abundant during warmer temperature periods, although not observed in water greater than

18.3°C.

In the Monterey Bay area, the temperature is warmest during the oceanic period, especially when the wind relaxes and warmer offshore water is advected onshore. When this occurs, frontal gradients can intensify in near­ shore waters. Fronts can be located further from shore during persistent upwelling events. Pacific white-sided dolphin occurrence was high when the intensity of near-shore thermal gradients increased. Along-shelf fronts form between warm, low salinity (>13°C, <33.5psu) well stratified offshore water, and cool, high salinity, vertically homogeneous upwelled water (Kinder et al. 1983,

Schwing et al. 1991 ). Mobile zooplankton are concentrated at fronts, providing T

79

increased food for fishes, cephalopods and seabirds (Brown 1980, Briggs et al.

1984, Haney 1985; ). Dolphins of the genus Stene//a"migrate near Sagami Bay, Japan, in

years when the Kuroshio current is near-shore, with dolphins frequenting the

frontal area where the colder Kuroshio Current meets warm water (Miyazaki et

al. 1974). Sightings of Stene/la spp. in the eastern tropical Pacific are

concentrated in warm water near the equatorial front, with movements

correlated to changing oceanographic conditions (Au and Perryman 1985).

Smith et al. (1986) found an abrupt change in birds and mammals near these

fronts off California, and suggested that the tilting and shallowing of the mixed

layer near fronts could increase food availability. In outer waters of Monterey

Bay, these fronts may approach the shelf-break, especially if winds have been

light for many days. Frontal convergences near shelf-breaks often intensify

aggregations of prey and predators (Fournier et al. 1979, Herman and Denman

1979, Ainley and Jacobs 1981 ), possibly explaining why dolphins were sighted

closer to the shelf-break in warmer temperatures when thermal gradients

increased.

Fronts also may be topographically induced near headlands, causing

eddies in the lee of the headland to congregate fishes, seabirds, and mammals

(Dykstra et al. 1984, Wolanski and Hamner 1988). It is possible that this

phenomenon could occur on a small scale near Pt. Pinos, Cypress Pt., or Pt.

Lobos, contributing to the high abundance of dolphins frequently sighted there.

Strong frontal gradients in Monterey Bay occur most often during the upwelling

and oceanic seasons, although the upwelling season generally has fewer 80

periods of wind relaxation events compared to the oceanic season. Although frontal gradients occur during the upwelling season, the lower abundance of dolphins at this time may be the result of increased and intensely turbulent periods. Prey may disperse and there may not be adequate time for an abundant prey base to stabilize (Reilly 1990). The fall of 1987 was characterized by calm winds for relatively long periods, with a strong near-shore frontal gradient and a high temperature anomaly possibly resulting in the concentration or stabilization of prey. Dolphins were highly abundant, frequently sighted, and occurred closest to the shelf-break during this season compared to all other seasons during the study period.

The presence, strength and depth of a thermocline also influence marine organisms. Some species of dolphins aggregate in areas of the eastern tropical

Pacific where the thermocline shoals under tropical surface water (Au and

Perryman 1985). The shallow thermocline in this area may act as a vertical aggregating mechanism for squids and fishes (Green 1967, Reilly 1990). The prey are concentrated near the surface instead of spread through the water column as in other tropical areas (Pryor and Shallenberger 1991 ). In the

California Current area, the thermocline slopes upward toward the coast (Hicky

1979, Mclain and Thomas 1983) and varies seasonally (Chavez et al. 1991 ).

The thermocline is used as a boundary for some species of zooplankton, affecting the distribution of their fish and cephalopod predators. Areas or seasons characterized by a deeper thermocline or none at all may result in lower prey concentrations due to dispersal throughout the water column (Briggs et al. 1987). T '

81

The thermocline is least stable during the upwelling period in Monterey

Bay, corresponding with lower occurrence and smaller group sizes of Pacific

white-sided dolphins. This contrasts with the oceanic season, in which surface

heating during periods of light wind can produce strong stratification and a

shallow mixed layer (Husby and Nelson 1982). Increased occurrence of Pacific

white-sided dolphins during this season may reflect a stable and concentrated

prey base. In certain years, more frequent upwelling events during the oceanic

season could destroy stratification, possibly disrupting prey and therefore

influencing dolphins. This may have occurred during the 1989 oceanic season,

corresponding to the greatest percentage of days with intense upwelling and

the lowest dolphin occurrence for any oceanic season. The greatest mean

group sizes also occurred during this season, indicating that dolphins were

probably just traveling through the area.

The thermocline during the Davidson Current period is persistent and

relatively deep, but strong winter storms can disrupt this, destroying stratification

and further deepening the mixed layer (Nelson 1977). The higher abundance of

dolphins during this period was due to relatively larger group sizes, but a lower

occurrence, indicating dolphins may exploit abundant food patches during

periods with a strong and relatively shallow thermocline, then move as local

conditions change.

Biological gradients at eddies also influence phytoplankton, zooplankton,

cephalopods, and fishes (Haney 1986). Eddies occur frequently in the

California Current (Fiedler 1986), and often approach the coast. A warm, low

salinity eddylike feature consistently appears west of Monterey Bay (Schwing et 82

al. 1991, Breaker and Broenkow 1994). The close approach of this eddy near­ shore appears unique to the area, as no similar features are found off central or northern California (Schwing et al. 1991). This phenomenon may contribute to higher dolphin abundance around Monterey compared to other coastal areas. Each oceanographic feature considered separately may not explain dolphin occurrence patterns, but the combination of these features with physiography may provide ideal conditions for cetaceans (Brown and Winn

1989, Bruggeman 1992). Surface eddies and convergences caused by complex topographies (Neumann 1960) may concentrate dolphin food (Au and

Perryman 1985). Within the study area, the shelf-break from Pt. Pinos to Pt. Lobos is relatively steep, with interspersed mini-canyons running perpendicular to the break, a region of high dolphin concentration. The importance of the shelf-break habitat in Monterey Bay has been demonstrated for organisms found concentrated there, including euphausiids, shortbelly rockfish (Sebastes jordam) and blue whales (Chess et al. 1988, Schoenherr 1991).

Strong upwelling events and increased nutrient availability are seasonal, creating a temporal lag in productivity. Phytoplankton production in Monterey

Bay peaks from March to August (Schrader 1981). The abundance and distribution of higher trophic level organisms are spatially separated from physical indicators of primary production (Croll1990). This is exemplified by the seasonality of northern anchovies (Engraulis mordax) and common dolphin distribution in the Southern California Bight (Hui 1979). The seasonal abundance of common murres, (Uria aa/ge) in Monterey Bay was associated with peak juvenile rockfish abundance (Croll 1990). Some of the main prey 83

species of Pacific white-sided dolphins also tend to be seasonally abundant, including northern anchovies (Engraulis mordax), Pacific whiting (Merluccius productus), and Loligo opalescens. -

Anchovies are one of the most abundant pelagic schooling fishes in the northeast Pacific and are a common prey item for fishes, birds, and mammals in

Monterey Bay (Morejohn et al. 1978). High concentrations occur over submarine canyons and escarpments in upwelling regions (Mais 1974, Baxter

1966). Anchovies differed in abundance, fish school size, and distribution among seasons (Baxter 1966, Mais 1974, Smith 1981, see Ch. 2). Patterns of these fishes were related to Pacific white-sided dolphin distribution, grouping patterns, behaviors, and seasonal abundance in southern and central

California (Dohl et al. 1983, Leatherwood et al. 1984, this study, see Ch.2).

Horizontal movement of anchovies may be blocked by sharp discontinuities at fronts (Biaxter and Hunter 1982), possibly contributing to the greater occurrence of dolphins in the study area when frontal gradients are great near-shore. This information strongly indicates that Pacific white-sided dolphins are influenced by anchovies.

Pacific whiting is another important prey item for Pacific white-sided dolphins. The high biomass of juvenile fishes concentrated near the shelf-break

(Alverson and Larkins 1969, Bailey et al. 1982) likely contributes to the high abundance of dolphins found off central California compared to other areas of their range (see Ch. 2). Productive fishing grounds for whiting are associated with prominent geographical sites such as banks and sharp curves in the r !

84

continental slope and canyons (Bailey et al. 1982), features prominent in

Monterey Bay. Cephalopods also comprise a major portion of the dolphins' diet, but

patterns of their abundance and distribution are poorly known, except for Loligo

opalescens. Lo!igo moves inshore to spawn in shallow waters (<100m) year­

round, with a peak in Monterey Bay from May-July; otherwise, schools occur widely in coastal and offshore waters (Frey 1971, Cailliet et al. 1979, Hardwick and Spratt 1979, Fiscus 1982). During the spawning season, squid are inshore

and therefore unavailable to dolphins, corresponding to a time of low dolphin abundance in Monterey Bay, and central California in general (Dohl et al.

1983).

Pacific white-sided dolphin abundance, distribution, and behavior are

reflected by the behavior and abundance of certain prey. Dolphin presence may be explained by a few predominant prey species. Also, the relatively large

groups of dolphins observed in the Bay compared to other locations within their

range could be influenced by the infrequent but year-round occurrence of killer whales. Killer whales in this area are the transient type, often occur over or near

the shelf-break, and frequently feed on several species of marine mammals in

the Bay (Black et al. 1993, unpub. ms, Ternullo et al. 1993, unpub. ms.).

Although killer whales have not attacked Pacific white-sided dolphins in

Monterey Bay, dolphins have exhibited distinct flight responses when in close proximity to these whales, and killer whales are known predators of white-sided

dolphins (Dahlheim and Towell 1994). The deep water but near-shore habitat in

Monterey Bay may provide some degree of protection from predators. T I

85

Monterey Bay appears to be important for Pacific white-sided dolphins,

but they are not abundant year-round. Therefore, the dolphins move elsewhere

or change their grouping patterns, probably reflecting seasonality and behavior

of prey (Norris and Prescott 1961, Shane 1984, Kenney and Winn 1986).

Although Pacific white-sided dolphins are less abundant during the

cooler spring upwelling season in the Monterey Bay area, it is not known if they

tend to move further offshore, north, or south. Because peak upwelling periods

progress seasonally northward from Baja, California to Washington, dolphins

may move into more favorable, less intense upwelled areas. Upwelling off

Oregon and Washington occurred during summer and fall, peaking during July

and August (Bakun 1973), and encounter rates for Pacific white-sided dolphins

were relatively low at this time (Green et al. 1993). Upwelling was most intense off Baja during April and May (Bakun 1973). and Pacific white-sided dolphin

abundance peaked from July-September (Leatherwood et al. 1984).

There are four possible explanations for low spring dolphin abundance

in the Monterey area. Dolphin distribution shifts north, as suggested by

Bruggeman (1992), but dolphins do not appear to be abundant enough off

Oregon and Washington (Bruggeman 1992) to account for a shift from

California (Barlow 1993, in press). Secondly, dolphins could shift offshore, as

suggested by Dahl et al. (1983). However, Dahl et al. (1983) conducted

extensive aerial surveys in central and northern California and found dolphins

mostly within 110 km of shore, even though survey effort extended to 278 km

offshore. Barlow (in press) surveyed California waters out to 556 km during T!

86

summer/fall and found white-sided dolphins within 185 km of shore. There was

no evidence of a major shift offshore during spring/summer.

Pacific white-sided dolphins co-uld shift into southern California.

Leatherwood et al. (1984) reported Pacific white-sided dolphins to be most

abundant in southern California from November to May. He suggested that

dolphins may shift south into Baja, California during summer and fall. Pacific

white-sided dolphins are abundant in central California from September to

February, overlapping their abundance in part off southern California. Because

peak densities off southern California are greater than reported for Baja

California (Leatherwood et al. 1984), it is possible that some of the southern California dolphins also may shift north into central California during summer

and fall. Adding to the complexity, the two forms (large-southern, small­

northern) of Pacific white-sided dolphins, appear to overlap in southern

California (Walker et al. 1986). It is possible that the southern form shifts south

into Baja, and the northern form shifts into central California. Both forms may

occur in abundance off southern California when anchovies are most

concentrated there.

Lastly, in addition to a partial shift south, dolphins may change their

grouping patterns and disperse along the coast. This idea is supported by the

observation of Dohl et al. (1983) that Pacific white-sided dolphins were

distributed along the entire central and northern California coastline during

spring. In other seasons, dolphin distribution was more localized.

Although the complex linkages between environmental processes and

biological consequences are difficult to observe (Bakun and Parrish 1980), an 87

investigation of environmental variables, prey abundance and distribution, and dolphin patterns of abundance and behavior can elucidate apparent trends.

The behavior, movements, and occurrenc"e of Pacific white-sided dolphins must be largely prey related, but also influenced by habitat type and predators.

Because Pacific white-sided dolphins are present year-round, and often in groups in excess of 500 individuals, with frequent feeding activity, certain locations in the Monterey Bay are clearly important for dolphins, providing a predictable and abundant food source. 88

CHAPTER 2

FOOD HABITS OF PACIFIC WHITE-SIDED DOLPHINS, OFF CENTRAL CALIFORNIA 89

INTRODUCTION

Pacific white-sided dolphins (Lagenorhynchus obliquidens) occur

throughout temperate waters of the North Pacific Ocean, 23°N to 61 aN

(Leatherwood et al. 1984) and are the most abundant cetacean off central and

northern California (Doh! et al. 1983). Pacific white-sided dolphins are

commonly found in groups of several hundred to more than a thousand

individuals, often in the company of northern right whale dolphins (Lissodelphis

borealis} and Risso's dolphins (Grampus griseus; Dohl et al. 1983,

Leatherwood and Reeves 1983). Pacific white-sided dolphins reach 2.3 m in

length and 150 kg in weight, have a relatively short beak, and an average of 30

teeth in each jaw (Walker et al. 1986).

Pacific white•sided dolphins are known to eat 24 taxa of fish and 10 taxa

of cephalopods, mainly schooling epipelagic species in California and

Washington (Jones 1981, Stroud et al. 1981, Walker et al. 1986). Walker and

.···Jones (1993) described 36 taxa of fishes and 12 taxa of cephalopods in dolphin

stomachs, dominated by mesopelagic species, especially myctophids, in the

offshore waters of the northern North Pacific. In the western Pacific, Wilke et al.

(1953) found that white-sided dolphins ate predominantly myctophids. Because

these dolphins live in a variety of habitats, encompassing a wide range of

oceanographic conditions, prey types probably vary according to region and

season.

Although several studies of food habits exist for this species, there is little

information on the estimated length, weight, and relative importance of T

90

particular prey species. Further knowledge of these variables and the

seasonality, abundance, and food habits of prey, should increase our

understanding of the role of Pacific white:sided dolphins in the food web.

Therefore, the objectives of this study were to: (1) identify prey types, (2)

estimate standard fish lengths and weights, (3) estimate cephalopod mantle

lengths and weights, and (4) compare prey types from central California to other

coastal locations. 91

METHODS

Stomach contents were examirled from 18 Pacific white-sided dolphins that stranded on beaches of Sonoma Co. (1 ), Marin Co. (1 ), San Francisco Co.

(2), San Mateo Co. (3), Santa Cruz Co. (3), and Monterey Co. (8). Sixteen dolphins were collected as dead strandings and two dolphins stranded alive but later died. Dolphins were all adults, from 172 to 225 em standard length, and were collected from March through October, 197 4-1992. Dolphin stomach contents and stranded dolphins were obtained from Moss Landing Marine

Laboratories, Long Marine Laboratory (U.C. Santa Cruz), Robert Jones (U.C. Berkeley), and California Academy of Sciences.

After measurements and dissections were performed on these dolphins, both ends of the stomachs were tied off, the stomachs removed, and frozen for later analysis. Stomachs were thawed and contents strained through a 0.01 mm sieve, and otoliths and beaks removed. Otoliths were washed, counted (right and left), measured with vernier calipers to the nearest 0.1 mm length, and stored dry in glass vials. Upper and lower cephalopod beaks were separated, counted, measured (lower rostral length for squids, lower hood length for octopus), and placed in 50% isopropyl alcohol for storage.

Otoliths were identified by the author using Fitch {1964, 1968, 1969) and the reference collection of J.T. Harvey at Moss Landing Marine Laboratories.

Lower beaks of cephalopods were identified by the author using Wolff (1984),

Clarke (1986a), and a reference collection of J.T. Harvey at Moss Landing

Marine Laboratories, with assistance and confirmation to voucher specimens 92

from E. Hochberg, Santa Barbara Museum of Natural History and W. Walker,

National Marine Mammal Laboratory, Seattle.

Prey items were identified to the lowest possible taxon. Number of each prey species/genus for each stomach was determined from the number of lower cephalopod beaks and the most numerous left or right otolith. The estimated length and weight of each prey item were calculated using regressions of fish standard length (SL) and weight to otolith length (Harvey et al. in press) and cephalopod dorsal mantle length (DML) and weight to lower beak rostral length

(LRL; Wolff 1984, Clarke 1986a, W. Walker, pers. comm.) for species in which values have been determined. For those species with no regressions, the genus/family or closest similar species' length/weight equations were used. The index of relative importance (IRI= (%number+% reconstituted mass) x% frequency of occurrence) for each prey item was calculated following Pinkas et ' al. (1971) with the replacement of mass for volume (Hyslop 1980, Finley and

Gibb 1982, Cockcroft and Ross 1990, Dorfman 1990). 93

RESULTS

Sixteen stomachs of Pacific wh1te-sided dolphins contained prey with one to 12 different prey species. A total of 1,048 otoliths and 503 lower cephalopod beaks was identified to species or genus. Seven fish species and one fish taxon (67.6% number, 87.5% occurrence, 80.2% mass), and 10 cephalopod species and three cephalopod taxa (32.4% number, 68.8 % occurrence, 19.8% mass) were identified. Pacific whiting, Merluccius productus, was the most frequently occurring fish species in all stomachs (69%) followed by northern anchovy, Engraulis mordax, and plainfin midshipman, Porichthys notatus. Loligo opalescens and Onychoteuthis borealijaponicus were the most frequently occurring cephalopod species. Based on IRI values, Pacific whiting, plainfin midshipman, northern anchovy, Sebastes sp., Gonatidae, L. opalescens, and 0. borealijaponicus were the most most important prey species, respectively (Table 7). Fish species were an average 15.9 ± 7.53 em

Sl and 111.8±118.56 g weight (Fig. 26}. Cephalopod species averaged

8.6±5.11 em DMl and 75. 7±127.65 g (Fig. 27}.

Stomachs contained two to 66 otoliths of Pacific whiting, one to 118 otoliths of northern anchovy, and three to 192 otoliths of plainfin midshipmen.

Although Sebastes sp. were low in occurrence, one stomach contained 178 otoliths of this species. The remaining fish species, Pacific sanddab

(Citharichthys sordidus), jack mackerel (Trachurus symmetricus), white croaker

(Genyonemus lineatus}, and Pacific sardines (Sardinops sagax) were low in Table 7. Prey of 16 Pacific white-sided dolphins collected off central California.

Number Occurrence Mass (g} IRI IRI O!oilth/Beak Est Length Est. Weight Preyftam N "k N % Total % rnnk Length{mm) (em) (g) Tolai 15 1000 03,709.3 tOM

Fish 1040 6Hi 1o1 1175 75, Hi£1.4 nn.2 maon(lSO}mnge mMn{iSO) mnga msen(±SO) rnnga Batracholdki.:IS Pvrichlhys flola!us J63 23.7 0 1,990 2 4.9±1.0 2.3-9.5 12At2.5 G.2·~H.I 35Jl±2U .t.2·1SO 9olhldaa Cifherichlhys sordidus lG 1.0 12.5 62(L0 0.7 21 15 3.8±1.2 13.7±3.7 10.3·20.2 4l.!H29.!i 19.1·121 Cnmngldae Tmchutm S}'Ttlfl'll!tricUS 5 02 2 125 1,167.1 1.9 I 4 5Jrl0_J 5.D-5Jl 29.2!:0.1 29.0-29.3 353.4.±3.6 347-357 Clupllitii\G Snrd:'fWPS ~ ... 0.1 OJ 4.t 0.0 <1 20 2.5 4.1 Etlgraultd!dan Engmulis f1JOfiW: 291 10.6 553 2,961.5 32 1,239 3 J.atoJt 9.0±1.3 5.2·13.7 10. H:JA 2.0·26.9 Mnriucclldaa ' Marfucdus pnxtuctus lUI 11 GaB 21,\?25.2 2.2.7 2.374 10.7±3.7 Sclasnldnn "" Benjltl\1omus linusrus 0.3 53 559.1 0.6 0 JB O.Rl:L3 Scorpaanldna StJbaslas sp" 160 1U.i '2 12.5 34,SJ1.5 37.2 010 4~·13.2 l5J>±J.9 9.3·2lMJ 193.0H59 31·1115

Cephalopods sro 32.4 1' oo.e 18.539.5 19.8

Ghlrateulhldaa Ghim;ou/1115 sp. 11 0.7 ., 11:10 O.t 5 17 2.4:«UI 1.4·3.5 7.0±1.9 ~-0· 8.7 Ctanch~dao G,;tli/aurhfs sp. 42 .2.7 5 313 597.2 0.6 9 2.0llil.B 1.0-3.5 liJ.ru2.fl: S.J.t5.S 14.2:UUl .2.1·311.1'1 Enoplo!O\llhldan 00 ,. '"' Abroliopsls fells ' 250 14111 0.2 10 1.lli:0.2 0.7·1.'1 3JUR6 2.6-5.4 2.5±1,0 LQ-.54 Gona«dna 159' 10.3 G ::175 5,514.9 5.9 600"" GevtllM$i.tA 63 4-1 ' 250 1,421.6 1.5 ... g 3.1:t0.J 2.3-4.1 ti.BftA .5.5·13.2 22.6±7.8 0.3·57.0 Goostusbef'/}'1 .Bl 5r2 4 25.0 781.1 o.o Hill 7 2.4.Hl.l 1.643 S.!liL5 2.:S.9.B 9.frt4.5 2.5·.25.0 Goootopsis borealis 15 1.0 J HV3 267.0 J.S 11 4.3±1.7 2.Ht3 l6.7i5.B 0.()-.JOJ:I 228.0±224 34·061 Hls!iotoulhlda!! "' Histlotrmth!.f ha/oropsis O.J 63 133 OJ} 19 LG 3.5 1~l3 ln!!glnlda-e Lo!iDO cplllll$CC!rl5 143 92 7 ..,. J,o;m.e 3.3 5 1.2±0.2 0,7·19 11J.7t1A 7.5·14J} 21.7±5.5 9,3·39.5 Octopolnulhlduo Octopolnvlhls deltJttOn 4 0.1 ., 159.2 0.2 2 18 1.0±0.5 1.2·2..2 4.!l.tl.4 J9.Bt.2t.O B.7·52.4 Oclopodldoa Sil7.t Q.9 Octopus rutmsc

45

~ 183 E 40 ,£. :535 "'1: 5 180 ..J 30 ~ J:"' "' 25 u:: 368 4 'D 16 -.."' 20 E 291 EE :;:; 15

~450 -E"' 400 ~"' 350 -:ii 300 250 u::'D a;"' 2oo E "iii 150 w .,1: 100 ::;:"' 50 Eb .,J;, 0 ~ . Ul .!!l c. Ul .0 E 0 :::J "' .c >. >. c: "S Ul . .c ~ c: £ u ::;: ., C3 ~ CJ 1- Fish Species

Figure 26. Mean estimated standard lengths (em) and estimated weights (g) for eight fish species. Standard deviations are represented by boxes and ranges by vertical lines. Sample sizes are indicated above vertical lines. 96

40 'E.., - 35 ,. 3!1 'J.1: 30 !l 25 5 !c: ::;:.. 20 '!!I 42 143 ~ 15 .E ;; 10 ~ UJ $ 4 c: 39 .. 5 ai$$~$ 1 $ 1 ::;;" lil 0

509

+ ~ <¥ ,!,! ~"' ~ ~"' c. "' E 1ii"' "' "' "' .!!!"' g. "' "' 1ii a :; '2 'l:i ~ .!!l D !:! e " "' 0 Q) .!!!""' !:! 0 n :; '§ c. :> D '0 .0" 0.. "' "' 2"' "' .0 "' .!2 1ii .!!! .t:; n ".!!! 2 t~ " -~ t: .. "0 .t:; " 05"' <: .a 2 1ii 1ii 0 0.. " 1ii 0 "' 0 !!l 0 :E (!) D (!) 0 'S c. ..0 :!! "' u < (!) 1ii .!2' .<: 0 0:: <: ~ 15 0"' .g D 0 "' -' c. ~ f.!l ~ 0 0 0 .!!! u :X: 0 ~ 0"' Cephalopod Species .t:; u c: 0""

Figure 27. Mean estimated mantle lengths (em) and estimated weights (g) for 13 cephalopod genus/species. Standard deviations are represented by boxes and ranges by vertical lines. Sample sizes are indicated above vertical lines. 97

occurrence and few in numbers. Estimated standard fish length was 3.3 to 41.8 em (Fig 28), and estimated weight was 2.6 to 1,114.8 g (Fig. 29).

Stomachs contained one to 92 lower beaks of L. opalescens. one to 17

lower beaks of 0. borealijaponicus, and one to 59 lower beaks of Gonatus sp.

A, Gonatus berryi, Galiteuthis sp., Abraliopsis felis, and Octopus rubescens. The

remaining cephalopod species, Gonatopsis borealis, Ocythoidae tuberculata,

Chiroteuthis sp., Octopoteuthis deletron, Histioteuthis heteropsis , and Rossia pacifica were low in occurrence and few in numbers. Cephalopod estimated mantle length was 2.5 to 30.6 em (Fig. 30), and estimated weight was 1.0 to

861.4 g (Fig. 31 ). The largest cephalopods were G. borealis (30.6 em DML and

861.4 g weight) and 0. borealijaponicus (30.0 em DML and 608.9 g). 60 60

60 Engrau/ls mordax 60 Merlucc/us productus ~11}3 40 """"' 40

30 30

20 20

> ..uc 10 10 ::J .,. 0 0 l!! ... 1 3 5 7 9 111315171921232527 29313335373941 -..r: 60 60 .,1:! Q. 60 60 Sebastes sp. Porlchthys notatus n=1S() n...:368 40 40

30 30

20 20

10 10

0 Qi------L------=~~~------1 3 57 911131517192123252729313335373941 1 3 57 911131517192123252729313335373941 Estimated Standard Length (em)

Figure 28. Frequency histograms of estimated standard lengths (em) for four fish species found in Pacific white-sided dolphin stomachs. 99

Engraulls mon:lax ~·

10 0~------,.

12

"' 25

5

ol---~=£~------

16

Figure 29. Frequency histograms of estimated weights (g) for four fish species found in Pacific white-sided dolphin stomachs. 100

50 60 45 Chlroteuthls sp. 50 4{) ~" Gomrtus berryi g'35 ~· • 40 t: 30 ~20 u 20 ~ ~5 10 10 5

0~--~----J------'O~J-______J______1 3 5 7 9 11 13 15 17 19 21 23 25 'Z1 29 31 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 4{) ~

30 Gonstopsis borealis ~" 25

20

15

10

5 5

o~~L______L ______, 0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 70 50 45 60 Loligo opalescens Abnlllopsi!J fells 4{) ~"' - ~ 30 25 20 15 10 10 5 0~~--J_------0 1 3 5 7 11 13 15 17 19 21 23 25 27 29 31 1 3 5 7 9 11 13 15 17 19 21 23 25 'Z1 29 31 70 • 20

16 Onychotcuthls boreslijsponlcus Gonstus sp. A ~ """' 16' r- 14 - 12 ' 10 - ., - '- ~ 4 2 ~ - '--- 0 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 1 3 5 7 9 11 13 15 17 19 21 23 25 'Z1 29 31 Estimated Mantle Length (em)

Figure 30. Frequency histograms of estimated mantle lengths (em) for eight cephalopod genus/species found in Pacific white-sided dolphin stomachs. 1

101

60 70

50 Chlroteuthls sp. 60 Gonatus berry/

u~ 50 ~· ,s 40 0" 40 0::30• c 30 •220 ...• 20 10 10

0 l----L------,~~~~~~Ol---~------~------50 •••••••••••------60 ··········------45 Gallteuthls sp. 50 40 Gonatopsls borenlls ~35 - ~· c 40 ~30 0::25• 30 ~20 u 20 ~ 15 10 10 5

0 ~----~~~~~~L------Ol---~-L-LJ-L-L-~L_--l_~L--L~

100 ··········------60 ··········------90 BO Lollgo opalescens u~ 70 c 40 ~"' 0• 60 0" :r. 50 30 c • 40 ...~ 30 20 10 10 0 ~L------,L------0~----L------r------­ 60 ············------50 ········------50 Onychoteuthls boresfl}aponlcus Gonatus...., sp. A 40 '""' 30

20

Estimated Weight (g)

Figure 31. Frequency histograms of estimated weights (g) for eight cephalopod genus/species found in Pacific white-sided dolphin stomachs. 102

DISCUSSION

In evaluating the food habits of·marine mammals based on stomach contents, inherent biases must be considered. First, cephalopod beaks are

resistant to digestion and may be retained in stomachs longer than otoliths, which are more easily eroded by stomach acids (Clarke and Kristensen 1980,

Finley and Gibb 1982, Harvey 1987). The importance of cephalopods, therefore, may be overestimated compared to fishes (Ross 1984, Bigg and

Perez i 985). Because otoliths are susceptible to dissolution and small otoliths

may be digested faster than larger ones, the estimated length of fish prey based

on otolith length may be considerably smaller than the actual fish length,

although this appears most significant in samples collected from scats (Jobling

and Breiby 1986, Harvey 1989a). This may be especially true for anchovy, with

small and numerous otoliths in stomachs, which may be easily dissolved or

quickly passed through dolphin stomachs.

Secondary prey, species in the stomachs of dolphin prey, may occur in

stomach samples. Walker and Jones (1993) found some intact squid in northern fur seal (Caflorhinus ursinus) stomachs that contained myctophid otoliths and

other cephalopod beaks. They suggested that isolated otoliths and beaks of

small prey may be considered secondary prey. Fiscus et al. (i 989) suggested

that single L. opalescens and A. felis beaks found in the stomach of sperm whales (Physeter macrocephalus) are probably secondary, because both are

schooling species that when present usually comprise a large proportion of

stomach contents of marine mammals. 103

Another problem that could result from analysis of stranded dolphin stomachs is the occurrence of prey species that are not normally part of their diet (Clarke 1986b). However, Leatheiwood et al. (1978) and Barros and Odell

(1990) found no difference in food habits between stranded and healthy

(fisheries caught) dolphins. Perhaps species found in low numbers in white­ sided dolphin stomachs could be anomalous prey of sick individuals.

Lastly, the sample size in this study may not be adequate for completely describing dolphin prey. Pacific white-sided dolphins rarely strand, and since

1965 there were only 41 stomach samples previously analyzed from dolphins found in central and northern California (Fiscus and Niggol 1965, Morejohn et al. 1978, Jones 1981, Stroud et al. 1981). Despite the low sample number, all four studies also reported northern anchovy, Pacific whiting, L. opalescens, and a few other oceanic cephalopod species as common prey for these dolphins.

Pacific white-sided dolphins feed opportunistically on abundant species of fishes and cephalopods, and based on prey habits, feed diurnally and nocturnally. The predominant prey of these dolphins are pelagic schooling species, particularly those less than 30 em in length. Stomachs have been examined from 101 Pacific white-sided dolphins in coastal waters of the eastern

North Pacific. Northern anchovy was the most frequently occurring species, followed by L. opa/escens, Gonatus sp., Pacific whiting, and O.borea/ijaponicus.

A division of the samples into three areas; south of Point Conception, Monterey

Bay, north of Point Conception, and Washington State, revealed some differences among regions (Table 8). Off southern California, samples were dominated by a diversity of fish species and few cephalopods. Table B. Percent frequency of occurrence of Pacific white-sided dolphin prey from southern California, Monterey Bay, northern California, and Washington State.

FISH SpECIES sc MB NQ ws All FISH SpECIES Icon! l sc Ml NQ ws AIL Numbor ol samplos 3J 26 57 11 101 Salmonldae Oncclfi}'T'Chus kola 9.1 1.0 Anop!oporn.aUdno Oncorl1ynchusldsurrh 18.2 2.0 Anopfopoma fimbria 30 1.0 Onyrorllynctlus spp. 72.7 7.9 Athorinldao Sclmm!daa Bothy1agldao '· Gon)OOomus /fnastus 9.1 3.0 1.8 48 LourogiO:SSus sfffblus 38 1.0 Sariphus polirus 6.1 , Ba\rnchoididao Scomberasocldoo Porlchthys notarus 15.2 "19.2 15.8 13.9 Co/oiEbls saim 11.5 17.5 9.9 Porldl!hys myrill.SI!Jr 12.1 4.8 ScombridBB Bothldao Pnoumntophol'lr.J japoll!CU!l 3.8 1.8 Citllarichthys sordldus 6.1 7.7 5.3 5.0 ScombDr japook:us 3.0 1.0 Calilngidao ScorpaanldBo TrnO'Iurus syrrmolrla.J$ 30.3 7.7 7.0 13.9 Sobrulos sp. 15.2 "7.7 7.0 8.9 Controloph!dao Sorrnnldeo /c:ictrltlys loddngtonl 1.8 1.0 Pnra.fabrax dalhrotus 3.0 1.0 C!upoldi!O Pamlabmx nobuffor 6.1 2.0 So!eldao Sarrtioops sngnx 9.1 1.8 4.0 S)mphurus slrlaum 3.0 1.0 Emb!otoddao""""""""" 3.0 " 1.0 Sphymon!da11 Engroulldaa Sphyrnanllnrgflnloo 3.0 1.0 Engrnlfisl'1'10fdal' 69.7 "69.2 54.4 53.5 Stromatoldon ExocooUdao Poprllus sJmllus B.! ~0 Cypsofurus c.a!ifomlcus 3.0 1.0 Tracfl!plolldao Gobndoo 30 10 lrm;!JII2Wo/S. a/Jb:Illi$. ao 10 Kyphocldao 3.0 1.0 li!bridno CEPHAlOpoD SpECIES sc MB m ws ALL Oxylui!LlS caBiomlcll 3.0 1.0 Merluo:::lklao Chlrotouthldao Marluccius productus 48.5 '38.5 43.9 40.6 Chlrcteuthls sp. 3.0 7.0 27.3 5.9 Myctophldao Crnnchlldna 15.4 14.0 27.3 10.9 Dlophus rhatn 3.0 1.0 Enoplotoulhldoo S~opiKJrtJS c.afifomlans/s 1.8 1.0 Abra/1opsls foils 3.0 '30.8 35.1 100.0 31.7 Trlpha/IJnJS moldcanus 6.1 2.0 G0110lldae 6.1 38.5 28.1 100.0 28.9 Ophldlldao Galai'!Jssp. 12.1 '5o.o 50.1 100.0 45.5 Gilamln)bl 12.1 4.0 Hlstlo!outh!dao Osmeridao Hislloleulhls hoteropsJs 3.B 1.8 1.0 0/oph/di!XTI scrlppsJ 3.0 1.0 loflg!n!doo 5pirlnch~n starks/ 1.8 1.0 Lol/go opa/oScm!s 54.5 '61.5 47.4 27.3 47.5 Plouronocllform Octopodldao Plouronoclldna Octopus sp. 21.2 '23.1 15.8 27.3 18.8 Hypsop!lotfa gulfala 3.0 1.0 OctopotoulhldaB Microstomus paci(ICUS 3.0 1.0 Dctopofoutflfs sp. 9.1 23.1 24.8 81.0 2"-ll Pfauronactos 11allllus 3.0 1.0 Onychotauthldno Pomncantr1dao Onyr:hoteurhls Chromis punctipinnls 6.1 2.0 boroaWjoponlcus 12.1 '34.6 38.6 100.0 36.6 Soplolldno _B~fa..m1t;;i • from Mora]ohn ol Dl. 1978, no data on lnd!vlduDI samples allailnblo

~ .,.0 105

Northern anchovy was most frequent, followed by Pacific whiting, L. opalescens, and jack mackerel. A mixture of fish and squid predominated off central and northern California. Gonatus sp. was most frequent followed by northern anchovy, Loligo opa/escens, Pacific whiting, and 0. borea/ijaponicus. Squid species predominated off Washington State; 0. borealijaponicus, A felis,

Gonatus sp., and Gonatidae occurred with equal frequency, followed by

Octopoteuthis sp. and Oncorhynchus sp. Specifically for Monterey Bay, northern anchovy and L. opalescens dominated the samples followed by

Gonatus sp., Gonatidae, and Pacific whiting (Table 8; Scheffer 1953, Brown and

Norris 1956, Houck 1961, Fiscus and Niggol 1965, Fitch and Brownell 1968,

Morejohn et al. 1978, Stroud et al. 1981, Jones 1981, Walker et al. 1986, Schwartz et aL 1992, this study). Walker and Jones (1993) found that 0. borealijaponicus was the most frequently occurring prey item in Pacific white-sided dolphins found in offshore waters of the northern North Pacific, followed by Myctophidae, A fells,

Bathylagidae, Argentinidae, Pacific saury (Colo/abis saira), Gonatus sp., and G. borealis. Myctophidae were predominant in this area, similar to Pacific white­ sided dolphin prey off Japan (Wilke et al. 1953), compared to the virtual non­ occurrence of this lam ily in dolphin stomachs from coastal waters of the eastern

North Pacific.

Most of the fishes occurring in Pacific white-sided dolphin stomachs are abundant within the California Current system, which extends from 23°N to

50°N (Bailey et al. 1982). In CaiCOFI collections, northern anchovy and Pacific whiting comprised the majority of larval fish (Ahlstrom 1969). Deep-sea pelagic 106

fishes also were abundant, such as myctophid lanternfishes, gonostomatid lightfishes, and deep-sea smelts, but are generally small (a few em), more abundant in offshore waters than in the California Current region, and usually do not form dense schools (Ahlstrom 1869). Therefore, these fish are probably not important in the diet of white-sided dolphins inhabiting coastal waters. Fish diversity is greatest in southern California and declines northward (Horn and Allen 1978), which may explain why Pacific white-sided dolphins consumed the greatest variety of fishes in southern California.

Pacific white-sided dolphin distribution, relative abundance, habitat, and behavior off California appear most influenced by their primary prey (based on frequency of occurrence), the northern anchovy. Anchovy is one of the most abundant fishes in the northeast Pacific (Baxter 1966). Anchovy are most abundant off sot:Jthern California during the spawning season from February to

May, corresponding to a time when white-sided dolphins are most abundant

(Leatherwood et al. 1984). Anchovy migrate north after spawning and become important to seabirds in central California during late summer (Ainley and

Boekelheide 1980). Anchovies were one of the most abundant fishes collected in summer midwater trawls in Monterey Bay (Cailliet et al. 1979). Northern anchovy tagged off southern California were recovered in Monterey Bay primarily from September to January (Haugen et al. 1969), a time when white­ sided dolphins were most abundant in the bay. In Monterey Bay, dolphins frequented waters overlying the shelf-break, particularly along steep canyon edges (Ch. 1). 107

High densities of anchovy similarly occur over submarine canyons

(Baxter 1966, Mais 1974). Dolphins were most abundant and found in the largest group sizes during fall and winler, whereas during spring, dolphins were least abundant and found in small groups. Similarly, the number of anchovy schools per km and anchovy school size were greatest in fall, and the fewest anchovy schools per km and the smallest schools occurred during spring

{Smith 1981). Dusky dolphins (Lagenorhynchus obscurus} off Argentina fed in

relatively large groups (>300) when southern anchovy were most abundant, and fed in smaller groups when anchovy were not present (WOrsig and WOrsig

1980). Pacific white-sided dolphins were observed feeding on anchovy during the daytime in Monterey Bay and off southern California (Norris and Prescott

1961, Ch. 1), probably due to the dispersal of anchovy at dusk and their reformation into dense schools just before dawn (Mais 1974). Anchovy reach a maximum size of 23 em length and 60 g weight (Eschmeyer et al. 1983), juveniles are 2.5 to 14 em in length and become mature at 7.8 to 14 em between 1 and 2 years of age (Frey 1971, Hart 1973). Pacific white-sided dolphins fed on the juvenile and young adult size classes (peak at 10.4 em). which probably reflected a higher abundance of juvenile fish available rather than a selection for size. Similarly, Harvey (1989b) found anchovies with an average size of 10.5 em in the stomachs of blue sharks collected in Monterey Bay.

Pacific whiling, which had the greatest IRI and was the second most frequent prey item for Pacific white-sided dolphins, have an estimated biomass off California second only to the northern anchovy (Alverson and Larkens 108

1969). In Monterey Bay, whiting were the second most abundant fish in trawls

(Cailliet et al. 1979). Adults undergo seasonal migrations from summer feeding grounds in the Pacific Northwest to spawn·off southern California and Baja,

California, during winter. Mature whiting off California averaged 47 em length and greatest length was 90 em. The greatest biomass of whiting consisted of juveniles (1 to 4 years) off central California, which concentrate near the shelf­ break (Bailey et al. 1982), similar to white-sided dolphin distribution. Pacific white-sided dolphins in central California fed mainly on juvenile whiting (mean size 23 em, 0-4 year old fish). Occurrence of whiting in dolphin stomachs was low off Washington where adult fish predominate (Best 1963), but was common in stomachs from California. This indicated that white-sided dolphins select juvenile whiting. High juvenile fish biomass in central California likely contributes to the high abundance of dolphins found off this area compared to ' other areas of their range.

Plainfin midshipman was next in importance after Pacific whiting in

Pacific white-sided dolphin stomachs from central California. The occurrence of this fish in dolphin stomachs indicates that Pacific white-sided dolphins also feed nocturnally. Midshipmen bury in the bottom during the day and feed at night, forming schools that may rise to 150 m above the bottom (Lavernberg and

Fitch 1966). Midshipmen mature at 14 em length and reach 38 em (Love 1991 ).

Pacific white-sided dolphins fed on the juvenile and young adult size classes.

Jack mackerel occurred in nearly one third of dolphin stomachs off southern California (Brown and Norris 1956, Fitch and Brownell 1968, Scheffer

1953, Stroud et al. 1981, Walker et al. 1986, Schwartz et al. 1992), and was 109

rare in northern California samples. This probably reflects the opportunistic feeding nature of white-sided dolphins, because maximum densities of juvenile and young adult fishes occur from Point Conception to central Baja, California

(Blunt 1969).

Although the various species of Sebastes are difficult to distinguish from otoliths, Pacific white-sided dolphins probably fed on shortbelly rockfish

(Sebastes jordani) the most predominate rockfish prey of many other marine vertebrates (Chess et al. 1988, Morejohn et al. 1978). Shortbelly rockfish occur from northern Baja, California to Vancouver Island, British Columbia, but are most abundant off central California (Miller and Lea 1972) particularly near submarine canyons (Chess et al. 1988). They attain lengths of 32 em and are found in depths from 90 to 280 m (Miller and Lea 1972), overlapping the depths where white-sided dolphins frequently occur. Cailliet et al. (1979) found large numbers of juvenile rockfish, 7.6 to 15.3 em length, in Monterey Bay during summer. Juvenile rockfish are abundant seasonally; therefore, their importance to white-sided dolphins may be underestimated because of the few dolphin stomachs collected during peak juvenile fish biomass off central California. This may be exemplified by one dolphin collected in Monterey Bay during June that contained 328 rockfish otoliths, representing a minimum of 178 fishes.

Pacific white-sided dolphins collected during this study consumed 13 cephalopod genus/species, of which six families occurred in 25% or more stomachs. Except for L opa/escens, cephalopod occurrence in stomachs was rare off southern California and greatest off Washington (Stroud et al. 1981,

Walker et al. 1986). Lo!igo are most prominent in samples from Monterey Bay. 110

Loligo is possibly one of the most abundant cephalopods off California (Young

1972), and are most common from southern California to Monterey Bay (Fields

1965). Lo/igo was the most frequently occurring species in summer midwater trawls in Monterey Bay (Cailliet et al. 1979). This squid moves inshore to spawn in shallow waters (<100m) year-round, with a peak in Monterey Bay from May to July; otherwise schools occur widely in coastal and offshore waters (Frey

1971, Hardwick and Spratt 1979, Fiscus 1982). During the spawning season, squid are inshore, and therefore unavailable to Pacific white-sided dolphins, corresponding to a time of low dolphin abundance in Monterey Bay. Loligo mature between 1 and 2 years of age as they grow from 0.2 to 14 em DML. At maturity, males are 7 to 13 em length and females 8 to 14 em length, and can reach 23 em during their two-year life span (Fields 1965). Commercial catches of spawning squid in· Monterey Bay are dominated by squid with mantle lengths of 14 to 15 em length (Fiscus et al. 1989), larger than the average size of 10.7 em found in Pacific white-sided dolphin stomachs. Similarly, ·Harvey (1989b) found L. opalescens averaging 9.6 em DML in blue shark stomachs from

Monterey Bay.

Little is known about the other frequently occurring cephalopods in the diet of Pacific white-sided dolphins. Although none are commercially important, many are commonly found in the stomachs of other marine mammals and seabirds (Baltz and Morejohn 1977, Fiscus 1982, Antonelis et al. 1987, Fiscus et al. 1989, Lowry et al. 1990). Most are epipelagic to mesopelagic species, undergo vertical migrations, and at least one, A felis, is a schooling species

(Roper and Young 1975, Jefferis 1983). The family Gonatidae is the most 111

abundant cephalopod group in the epipelagic and mesopelagic subarctic

Pacific waters (Fiscus 1991 ). There are at least three genera and 12 species found in the Pacific (Young 1972), although most are difficult to identify by their beak. Pacific white-sided dolphins are probably not deep divers and most likely feed at night on many of these vertically migrating cephalopods. Histioteuthis heteropsis and R. pacifica both occurred in only one white-sided dolphin stomach, and have not been previously reported as prey for these dolphins.

They could, however, represent secondary prey.

Many of the cephalopods found in Pacific white-sided dolphin stomachs also occurred in the stomachs of sperm whales from California. However, several species frequently found in dolphins, such as L opalescens, Gonatus spp., 0. borea/ijaponicus, and A. felis were infrequent in sperm whale stomachs. Similarly, species prominent in the diet of sperm whales, Moroteuthis robusta, G. borealis, Histioteuthidae, Cranchiidae, and Octopoteuthis were rare or absent in the diet of Pacific white-sided dolphins. Where there was overlap, sperm whales fed on cephalopods with larger lower rostral lengths than those found in dolphins. This indicated that dolphins fed predominantly on juvenile cephalopods of certain species, while sperm whales may select the larger adults that occur deeper in the water column (Fiscus et al. 1989). The distribution of the two species reflects their differences in diet. Sperm whales in

California occur in offshore waters, usually beyond 1000 m, whereas Pacific white-sided dolphins frequent shelf and slope waters, primarily from 200 to

1000 m (Doh I et al. 1983). 112

In contrast to the sperm whale, several other odontocetes frequent shelf and slope waters off California, sharing a distribution similar to Pacific white­ sided dolphins. Northern right whale dolphins and Risso's dolphins do not completely overlap the diet of white-sided dolphins. Northern right whale dolphins fed primarily on L opalescens, myctophids, bathylagids, and cephalopods from the families Gonatidae, Enoploteuthidae, Histioteuthidae, and Onychoteuthidae (Leatherwood and Walker 1979, Sullivan and Houck

1979, Clarke 1986b). These cephalopods also occurred in white-sided dolphins, whereas the deep-sea fishes were rare or absent in white-sided dolphin stomachs off California. Risso's dolphins fed only on cephalopods, many of which occurred in white-sided dolphin stomachs (Stroud 1968, Fiscus

1993, Black, unpubl. data), however, the predominant species and size classes may differ.

Dall's porpoise off central California primarily fed on Pacific whiting, northern anchovy, Pacific saury, L. opalescens and 0. borea./ijaponicus (Loeb

1972, Morejohn 1979, Stroud et al. 1981); overlapping the primary prey of white-sided dolphins. In contrast to northern right whale dolphins and Risso's dolphins, Dall's porpoise rarely associate with Pacific white-sided dolphins in

Monterey Bay and occur only in small groups (<20). Similar to white-sided dolphins, however, they frequent waters overlying canyon edges in the bay

(Jefferson 1991 ). Northern fur seals off central California also fed on similar fishes and cephalopods as white-sided dolphins (Stroud et al. 1981 ), and like

Dall's porpoise occurred singly or in small groups. 113

Before 1991, common dolphins primarily overlapped the range of Pacific white-sided dolphins only in southern California. Similar to white-sided dolphins, common dolphins fed predominantly on anchovies and L. opalescens

(Evans 1975). Numbers of Delphinus peaked in January, June, September, and

October (Evans 1975), and Pacific white-sided dolphins were most abundant off southern California from November to April. Therefore, Pacific white-sided dolphins appear to occupy a distinct niche; associating with abundant species

(northern right whale dolphins, Risso's dolphins) that do not completely overlap their diet, and they may alternate their abundance with common dolphins that feed on similar prey. Those species that consume similar prey and overlap

Pacific white-sided dolphin distribution, occur in small groups (Dall's porpoise, northern fur seals) compared to the relatively large groups of Pacific white-sided dolphins.

Pacific white-sided dolphins are opportunistic predators, feeding primarily on abundant prey within the various habitat types they occupy. This is reflected by variations in primary prey of dolphins found off California,

Washington, offshore northern Pacific waters, and the western Pacific. Similarly, other members of the genus Lagenorhynchus also fed opportunistically on abundant fishes and cephalopods. Atlantic white-sided dolphins,

Lagenorhynchus acutus, co-occurred with peak abundance of sand lance in the

Gulf of Maine and also fed on short-finned squid, smelt, herring, and hake

(Sergeant et al. 1980, Seizer and Payne 1988), all abundant schooling species.

Dusky dolphins in New Zealand fed on lanternfishes, hoki, and cephalopods associated with the deep scattering layer around the Kaikoura Submarine 114

Canyon (Cipriano 1992). In a contrasting habitat off Argentina, dusky dolphins fed on anchovies during summer in a relatively shallow, low relief area (WOrsig and WOrsig 1980).

In Monterey Bay, Pacific white-sided dolphins fit into a complex food web, centering around euphausiids, particularly Thysanoessa spinifera and

Euphausia pacifica (Morejohn et al. 1978, Cailliet et al. 1979). Distinct assemblages of organisms occurred in pelagic waters of Monterey Bay. Loligo and anchovy were prominent in these assemblages, which also included plainfin midshipman, Pacific whiting, juvenile rockfish, Pacific herring, and

Pacific sanddab. Two species of Gonatus comprised an assemblage, but were not abundant (Cailliet et al. 1979). All species except herring occurred in dolphin stomachs. Cailliet et al. (1979) suggested that the euphausiid "link" may be the reason for these recurrent groups. Although dolphins do not directly feed on krill, krill is consumed by the primary prey of dolphins off central California found within these assemblages. Little is known about the prey of cephalopods other than L. opalescens. although their predators are diverse. It seems important that the ecology of pelagic cephalopods be examined off central

California to fully understand the ecological patterns of Pacific white-sided dolphins. 115

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