Social Ecology of Bottlenose Dolphins in Monterey Bay_ California

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Social Ecology of Bottlenose Dolphins in Monterey Bay_ California Powered By Docstoc
					                                                                                                                                                                                                                                                                                                                                                                                                            New Brighton Beach
                                                                                                                                                                                                                                                                                                                                                                                               Santa Cruz       Aptos
                                                                                                                                                                                                      STUDY AREA                                                                                                                                                                                                  Rio del Mar
                                                                                                                                                                                                                                                                                                                                                                                                                     La Selva Beach
                                                                                                                                                                                                      Monterey Bay (Figure 1) is located along the central                                                                                                                                                            Manresa Beach
                                                                                                                                                                                                      California coast, about 180 km south of San Francisco. It                                                                                                                                                          Sunset Beach
                                                                                                                                                                                                      is California's second largest bay, and one of the few                                                                                                                                                               Pajaro River Mouth
                                                                                                                                                                                                      major bays along the entire Pacific coast of the United
                                                                                                                                                                                                                                                                                                                                                                                                                             Zmudowski Beach
                                                                                                                                                                                                      States. Another distinguishing characteristic is the                                                                                                                                                                    Jetty
                                                                                                                                                                                                      presence of the deepest and largest submarine canyon                                                                                                                                                                      Moss

                                                                                                                                                                                                      along the west coast of North America. The bay is
                                                                                                                                                                                                                                                                                                                                                                                       MONTEREY                                 Landing
                                                                                                                                                                                                                                                                                                                                                                                                                               Monterey Dunes Colony
                                                                                                                                                                                                      approximately 37 km long, north to south, and 16 km                                                                                                                                BAY                                   Barge Wreck

                                                                                                                                                                                                      wide, east to west. The biological importance of Monterey
                                                                                                                                                                                                      Bay lies in its nutrient rich waters, which support                                                                                                                                                                    Sand Plant

                                                                                                                                                                                                      extensive fish, invertebrate, seabird, and marine mammal                                                                                                                                                               Marina

                                                                                                                                                                                                      The area selected for the current study stretched from                                                                                                                                                    Monterey
                                                                                                                                                                                                      Marina State Beach, on the south side of the bay, to New
                                                                                                                                                                                                      Brighton Beach, on the north. The area covered
     Between 1990 and 1995, Pacific coastal bottlenose dolphins (Tursiops truncatus gilli) were studied using photo-                                                                                  approximately 34.8 km along the coast to 1 km offshore.
 identification during 228 boat-based surveys of the coastal strip (<1 km offshore) between Marina and New Brighton                                                                                                                                                                                                                                                                            Figure 1 – Monterey Bay , Study Area
 Beach in Monterey Bay (18 km of coastline). The study period encompassed three non-El Niño years (1990, 1991
 and 1995) and three El Niño years (1992, 1993, 1994). Based on dorsal fin markings, 97 unique individuals were
 identified. Eighteen animals (19%) showed a high level of site fidelity (defined as presence in at least 5 of the 6
 years), although their overall range was larger than the study area. Thirty-eight animals (39%) were transient,
 leaving for periods of time, and 41 (42%) were occasional encounters. The rate of discovery indicated a pulsed
 recruitment of new individuals into the study area, with periods of stable school composition, especially during non-                                                                                                                                                                                                           In wild ranging odontocetes, school structure is highly dependent
 El Nino years, and periods of high school fluidity.                                                                                                                                                                                                                                                                             on the social status of individual animals, as well as their age, sex
     Encounter rate was significantly higher in El Niño (81%) than non-El Niño years (61%). School size averaged                                                                                                                                                                                                                 and reproductive condition. Mother-calf schools appear to play a
 16 individuals (C.I.3, =0.05) and was significantly larger in El Niño years. Schools where calves were present                                                                                                                                                                                                                crucial role in the complex social structure of a dolphin
 were twice as large (mean=15; S.D.=8) than schools without calves (mean=8; S.D.=6). Newborns represented                                                                                                                                                                                                                      community.
 12% of the sightings and were seen year round with a peak in summer and fall. Crude birth rate ranged between
 0.09 and 0.17 (mean=0.13; S.D.=0.03). Five females calved in consecutive years and a resident female calved                                                                                                                                                                                                                    In Monterey Bay calves were seen year round with a slight peak
 once a year for the duration of the study, possibly indicating a high rate of mortality for calves in this area.                                                                                                                                                                                                                in summer and fall. Number of calves per school ranged between
     Individuals often traveled as subgroups of more consistent composition than the school itself, possibly                                                                                                                                                                                                                     0 and 6. The average number of adults present in a school when
 indicating that a stronger social bond exists within these units which may function as “bands” (sensu Wells 1991) of                                                                                                                                                                                                            calves were present was significantly higher than in schools with
 same sex individuals traveling within a larger school of mixed composition.                                                                                                                                                                                                                                                     no calves (Figure 6). In addition, adult school size increased
                                                                                                                                                                                                                                                                                                                                 proportionately to the number of calves present (Figure 7). It has
                                                                                                                                                                                                                                                                                                                                 been speculated in several studies that an increased number of
                                                                                                                                                                                                                                                                                                                                 adults provides added protection from predation to a school
                                                                                                                                                                                                                                                                                                                                 where calves are present.
                                                                                                                                                                                                                                                                                                                                 In Monterey Bay, schools containing mother-calf pairs had a
                                                                                                                                                                                                                    25                                                                                                           specific arrangement when traveling. All mother-calf pairs swam
 It has been common practice in odontocete studies to refer to animals that remain in a study area for most of their                                                                                                                                                                                                             together, generally in one or two defined subgroups, which were
 time as “residents”, and to animals that come through an area with less regularity as “transients”. These terms                                                                                                    20                                                                                                           located at the front or at the rear of a school. These sub-groups
                                                                                                                                                                                           Average School Size

 should be interpreted with caution and in the framework of the study duration and the size of the study area. These                                                                                                                                                                                                             tended to travel closer to shore than the rest of the school,
 definitions are often satisfactory only to describe the static picture provided by the short term (a few years). From this                                                                                         15
                                                                                                                                                                                                                                                                                                                                 generally hugging the shoreline at a depth of 2-3 meters.
 and other studies in California we have learned that the same population of coastal bottlenose dolphins ranges from
 Encinada, Mexico, all the way to San Francisco, and that individual animals may use this entire range in the course                                                                                                                                                                                                             These findings suggest that the social structure of a
                                                                                                                                                                                                                    10                                                                                                           school is altered by the presence of calves in a way
 of their lifetime.
 Within this framework, it is easier to understand the data
                                                                                                                              120                                                                                                                                                                                                that increases their safety and possibly provides
 collected in Monterey Bay. The rate of discovery of new                                                                      100                                                                                                                                                                                                additional support to the mothers.
                                                                                       Rate of Discovery of
                                                                                         New Individuals

 individual animals using photo-identification (Figure 2)
                                                                                                                                  80                                                                                                 0
 never leveled off throughout the duration of the study                                                                                                                                                                                                                                                                                                 30
                                                                                                                                                                                                                                                         Schools with Calves               Schools without Calves
 (1990-1995).                                                                                                                     60

                                                                                                                                                                                                                                                                                                                                  Average School Size
 At least half (52.4%) of the individuals present in 1991                                                                                                                                                                                                                                                                                               25
                                                                                                                                  40                                                                    Figure 6 – Average school size (counting only adults) of
 were also present in 1995 (Figure 3). The community                                                                                                                                                    bottlenose dolphin schools sighted in Monterey Bay
 changed gradually from the first year by influx/efflux of                                                                                                                                                                                                                                                                                              20
                                                                                                                                  20                                                                    between 1990 and 1995, comparing schools were
 individuals over time. The one above is a conservative                                                                                                                                                 calves were present to schools were no calves were
                                                                                                                                      0                                                                                                                                                                                                                 15
 figure, because photo-identification effort varied between                                                                                                                                             found. Schools where calves were present were almost
 years and it is, therefore, possible that individuals                                                                                                  Unit of Time                                    twice as large (X=14.87±8.2) than schools without
                                                                                                                                                                                                        calves (X=8.08±6.27).
 recorded as missing during a year were actually present                                                                                                  (Month)
 but never photographed.                                                               Figure 2                                           - The rate of discovery of new dolphins                                                                                                                                                                        5
                                                                                       using photo-identification suggests a continued influx
                                 Animals Present in 1991                               of new dolphins into Monterey Bay over time                                                                                                                                                                                                                       0
                                 Number of Animals Photo-identified
                       70                                                              Eighteen animals (19%) showed a high level of site                                                                                                                                                                                                                              0           1       2                3          4              5          6
                       60                                                              fidelity (defined as presence in at least 5 of the 6                                                                                                                                                                                                                                        Number of Calves per School
  Number of Dolphins

                                                                                       years), although their overall range was larger than
                       50                                                              the study area. Thirty-eight animals (39%) were
                                                                                                                                                                                                                                                                                                                                 Figure 7 – Average school size in Monterey Bay increased with
                       40                                                              transient, leaving for periods of time, and 41 (42%)                                                                                                                                                                                      increasing number of calves present.
                                                                                       were occasional encounters.
                       20                                                              From these findings it appears that
                       10                                                              Monterey Bay hosts a shifting community of
                                                                                       bottlenose dolphins, where individuals mix
                        1990 1991 1992 1993 1994 1995 1996                             and separate with a periodicity that is
                                             Year                                      determined by the total extent of each
 Figure 3 - The photo-identifiable dolphin community                                   individual’s preferred range and by the
 changed over time with dolphins present in 1991 gradually                             amount of time each individual spends in                                                                             Calves tended to be found in Monterey Bay year round with a summer peak and a smaller peak in the fall (Figure 8). The
 leaving the area (dark blue bars), and with new animals                               each portion of such range.                                                                                          crude birth rate (number of calves per number of animals identified) varied widely among years (Figure 9) and the reasons
 entering it.                                                                                                                                                                                               for these variations are not detectable from the data available.
                                                                                                                                                                                                                 Rate of Calf Occurrence per Survey

                                                                                                                                                                                                                                                      0.80                                                                                                                  1.00
                                                                                                                                                                                                                                                                                                                                                         Crude Birth Rate

                                                                                                                                                                                                                                                      0.70                                                                                                                  0.80
 El Niňo Southern Oscillation events occur periodically (every
                                                                                         Rate of Discovery of New Dolphins

 5-7 years) along the West Coast of the American Continent.                                                                                                                                                                                           0.60
                                                                                                                             30                                                                                                                                                                                                                                             0.60
 In recent years this periodicity has increased, with three                                                                               non-El Nino                                                                                                 0.50
 consecutive events occurring between 1992 and 1995.                                                                         25           El Nino                                                                                                     0.40                                                                                                                  0.40
 Coastal bottlenose dolphins were recorded in Monterey Bay                                                                                                                  1993
                                                                                                                                                                                                                                                      0.30                                                                                                                  0.20
 for the first time in the 20th century, during the 1988 El Niňo                                                             20
                                                                                                                                                                        1992                                                                          0.20
 event, although skulls recovered in central California and                                                                                                                                                                                                                                                                                                                 0.00
 dated to be over 100 years old, suggest these animals                                                                       15                                                                                                                       0.10
 ranged this far north in the past. Since 1988, dolphins have                                                                                                                                                                                         0.00
                                                                                                                                                                                                                                                                                                                                                                                    1990   1991             1992           1993           1994       1995




 been continuously present in Central California suggesting a                                                                                                                                                                                                                                                                                                                                                     Year
 permanent range extension beyond Point Conception.                                                                                                1991
                                                                                                                              5                                        1995
                                                                                                                                                                                                                                                                                            Month                                                                      Figure 9 – Crude birth rate per year for bottlenose in
 The rate with which new dolphins were discovered in the                                                                      0                                                                                                                                                                                                                                        Monterey Bay.
 population in Monterey Bay during this study (Figure 4) was                                                                      0         5    10      15    20      25      30     35              Figure 8 – Rate of occurrence of bottlenose dolphin calves per
 significantly higher during the three El Niňo years versus the                                                                                                                                       survey in Monterey Bay.
 non-El Niňo years. School size and encounter rate were also                                                                                       Number of Surveys
 significantly higher in El Niňo versus non-El Niňo years
                                                                                       Figure 4 - The rate of discovery for each year of the
 (Figure 5).
                                                                                       study indicates a marked difference among years in the
                                                                                       influx of new dolphins into the Monterey Bay community
                       30.00                                   1.00
                                                                                       with non-El Niňo years (1991 and 1995) having low to no
                                Encounter Rate                                         influx versus El Niňo years (1992-1994) having high
                                                                      Encounter Rate

                                School Size                    0.80                    influx.
                                                                                                                                                                                                                 Twelve females were identified using the presence of calves with them over the years as an indicator of sex. The proportion of
  School Size

                       20.00                                                                                                                                                                                     mothers in the identified population was similar for each year, being highest in 1992 and 1994, and lowest in 1993. Nine
                                                               0.60                                                                                                                                              females had multiple births and seven of these dolphins had calves in successive years (Table 1) with no evidence of the
                       15.00                                                             It is apparent from these findings that                                                                                 presence of the previous year’s calf with them when the next newborn was identified.
                                                               0.40                      environmental factors affect the extent of the
                       10.00                                                                                                                                                                                                                                                                                                                One of the calves, Epoxy’s (1994), stranded in July 1994 and died
                                                                                         dolphins’ range (perhaps due to shifts in                                                                               Table 1 - Calving intervals of nine known females
                        5.00                                   0.20                      distribution of the preferred prey).                                                                                                                                                                                                               in captivity a few days after (cause of death unknown). It is
                                                                                                                                                                                                                    Dolphin  1990 1991 1992 1993 1994 1995                                                                                  possible that the anomalous occurrence (normally the calving
                        0.00                                   0.00                                                                                                                                              Miss         **   **   **   **   **   **                                                                                   interval is between three and six years) of successive year births is
                               non-El Nino          El Nino                                                                                                                                                      Topper       **   **                                                                                                       a consequence of high levels of mortality in Monterey Bay’s
                                                                                                                                                                                                                 Keyhole                **   **                                                                                             calves. If this is the case, the possible reasons for such a
Figure 5 – School size and encounter rate were significantly
                                                                                                                                                                                                                 Bitchin'                    **   **                                                                                        phenomenon should be investigated.
higher during the three El Niňo years (1992-1994) combined
versus the non-El Niňo years (1990, 1991, 1995).                                                                                                                                                                 Epoxy                            **   **
                                                                                                                                                                                                                 Simba                            **   **
Monterey Bay is located at the upper limit of the dolphins’                                                                                                                                                      Square Root                      **   **
geographic range and it is therefore an area of extreme shift.
                                                                                                                                                                                                                 Hook                   **        **
El Niňo appears to be a key factor in the shrinking and                                                                                                                                                                                                                      We defined social units as highly stable subgroups that traveled in
expansion of the population range. Because El Niňo causes                                                                                                                                                        Ripple                 **             **
                                                                                                                                                                                                                                                                             highly cohesive fashion within a school. It appeared that these
a shift of prey items northward as the waters along the coast
                                                                                                                                                                                                                 subgroups within the school had a higher level of association over time than the fluid fission-fusion schools. We are currently
warm up, it is likely that the dolphins adjust their range
                                                                                                                                                                                                                 investigating these sub-group level associations which appear, at the preliminary stage, to be defined by sex and or/
following their prey. Water temperature and other
                                                                                                                                                                                                                 reproductive status.
environmental factors and their effects on prey distribution
are likely to be important also during non-El Niňo conditions
but the changes, in this case, are more subtle and not easily
                                                                                                                                                                                                                 Our deepest appreciation goes to Kristie Brockway, Kathy Hough, Sue Lynn Konopka-Reif, Dida Kutz, Gene Kent and Dave Silver for their help in the field during data collection. This
detectable.                                                                                                                                                                                                      work would not have been possible without the tremendous help of Leann Eitreim who painstakingly analyzed hundreds of photographs, and without the contribution of D’Arcy
                                                                                                                                                                                                                 Glanzmann. This study was made possible by a grant from Earthwatch Expeditions and The Center for Field Studies.
Two genetically distinct types of killer whale                                                                       The killer whales of the transient type
are found in Alaskan waters. These two types                                                                         generally eat other marine mammals (such
occupy two separate ecological niches and                                                                            as seals, sea lions, dolphins, porpoises,
do not interbreed. The most commonly                                                                                 whales and sea otters).
encountered killer whales are known as
residents and feed exclusively on fish, with a                                                                       They are ambush predators or attack as a
marked preference for salmonids.                                                                                     wolf pack, often traveling in small groups
                                                                                                                     whose affiliation is unclear. Because of their
Residents travel in large pods which are                                                                             lifestyle, transients are harder to study and
organized in several distinct matrilines.                                                                            little is known about their social organization,
These whales have been studied extensively                                                                           abundance and distribution patterns.
and their affiliation and genealogy is well
known.                                                                                                               Transients dive for long periods of time (15-
                                                                                                                     20 minutes), are generally silent and are less
Residents are very vocal, acrobatic, and                                                                             surface active. Often they go undetected.
surface active, and tend to frequent the same
areas over time.
                                                 Killer whale societies are centered around the oldest female
                                                 (or matriarch) in a family unit. All offsprings, both males and
                                                 females remain with their mother throughout their lifetime. A
                                                 family unit composed of a mother and her offsprings (or
                                                 matriline), generally swims with other close female relatives
                                                 and their family, thereby forming a pod. In resident whales,
                                                 this association is so consistent that individuals not
                                                 photographed with their pod for more than a couple of weeks
                                                 have surely deceased.
                                                  AB POD                                  AB 10 ♀

                                                                                                     AB4 ♂
                                                  AB10 Sub-Pod                    AB5 ♂   AB11 ♂

                                                 The AB Pod is composed of three matrilines: AB10, AB17 and
                                                 AB 25. Older post-reproductive females such as AB 10 seem to
                                                 direct the activities of the pod. Often, when an old female dies,
                                                 her sons are not long to follow. The reason for this life-long
                                                 bond between an adult male and its mother is not understood.

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