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Peter Nilsson New Brighton Beach Daniela Maldini Aptos Santa Cruz Rio del Mar La Selva Beach Manresa Beach University of Alaska Fairbanks School of Fisheries and Ocean Sciences Fairbanks, AK 99775 Alaska SeaLife Center P.O. Box 1329 Seward, AK 99664 Myhouse Sunset Beach
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New Brighton Beach
Rio del Mar
La Selva Beach
University of Alaska Fairbanks
School of Fisheries and Ocean
Fairbanks, AK 99775
Alaska SeaLife Center
P.O. Box 1329
Seward, AK 99664
Pajaro River Mouth
Monterey Dunes Colony
Social Ecology of Bottlenose Dolphins In Monterey Bay, California
Monterey Bay (Figure 1) is located along the central California coast, about 180 km south of San Francisco. It is California's second largest bay, and one of the few major bays along the entire Pacific coast of the United States. Another distinguishing characteristic is the presence of the deepest and largest submarine canyon along the west coast of North America. The bay is approximately 37 km long, north to south, and 16 km wide, east to west. The biological importance of Monterey Bay lies in its nutrient rich waters, which support extensive fish, invertebrate, seabird, and marine mammal populations.
The area selected for the current study stretched from Marina State Beach, on the south side of the bay, to New Brighton Beach, on the north. The area covered approximately 34.8 km along the coast to 1 km offshore.
Between 1990 and 1995, Pacific coastal bottlenose dolphins (Tursiops truncatus gilli) were studied using photo-identification during 228 boat-based surveys of the coastal strip (<1 km offshore) between Marina and New Brighton 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-El Nino years, and periods of high school fluidity.
Encounter rate was significantly higher in El Niño (81%) than non-El Niño years (61%). School size averaged 16 individuals (C.I.3, =0.05) and was significantly larger in El Niño years. Schools where calves were present were twice as large (mean=15; S.D.=8) than schools without calves (mean=8; S.D.=6). Newborns represented 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 once a year for the duration of the study, possibly indicating a high rate of mortality for calves in this area.
Individuals often traveled as subgroups of more consistent composition than the school itself, possibly indicating that a stronger social bond exists within these units which may function as “bands” (sensu Wells 1991) of same sex individuals traveling within a larger school of mixed composition.
Figure 1 – Monterey Bay , Study Area
How The Presence of Calves Shapes School Structure
In wild ranging odontocetes, school structure is highly dependent on the social status of individual animals, as well as their age, sex and reproductive condition. Mother-calf schools appear to play a crucial role in the complex social structure of a dolphin community.
In Monterey Bay calves were seen year round with a slight peak in summer and fall. Number of calves per school ranged between 0 and 6. The average number of adults present in a school when calves were present was significantly higher than in schools with 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 specific arrangement when traveling. All mother-calf pairs swam together, generally in one or two defined subgroups, which were located at the front or at the rear of a school. These sub-groups tended to travel closer to shore than the rest of the school, generally hugging the shoreline at a depth of 2-3 meters.
These findings suggest that the social structure of a school is altered by the presence of calves in a way that increases their safety and possibly provides additional support to the mothers.
Residents, Transients or Simply Shifting Communities?
It has been common practice in odontocete studies to refer to animals that remain in a study area for most of their time as “residents”, and to animals that come through an area with less regularity as “transients”. These terms should be interpreted with caution and in the framework of the study duration and the size of the study area. These definitions are often satisfactory only to describe the static picture provided by the short term (a few years). From this 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 of their lifetime.
Within this framework, it is easier to understand the data collected in Monterey Bay. The rate of discovery of new individual animals using photo-identification (Figure 2) never leveled off throughout the duration of the study (1990-1995).
At least half (52.4%) of the individuals present in 1991 were also present in 1995 (Figure 3). The community changed gradually from the first year by influx/efflux of individuals over time. The one above is a conservative figure, because photo-identification effort varied between years and it is, therefore, possible that individuals recorded as missing during a year were actually present but never photographed.
Figure 6 – Average school size (counting only adults) of bottlenose dolphin schools sighted in Monterey Bay between 1990 and 1995, comparing schools were calves were present to schools were no calves were found. Schools where calves were present were almost twice as large (X=14.87±8.2) than schools without calves (X=8.08±6.27).
Figure 2 - The rate of discovery of new dolphins using photo-identification suggests a continued influx of new dolphins into Monterey Bay over time
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.
From these findings it appears that Monterey Bay hosts a shifting community of bottlenose dolphins, where individuals mix and separate with a periodicity that is determined by the total extent of each individual’s preferred range and by the amount of time each individual spends in each portion of such range.
Figure 7 – Average school size in Monterey Bay increased with increasing number of calves present.
Births Occur Year Round while Birth Rate is Highly Variable
Figure 3 - The photo-identifiable dolphin community changed over time with dolphins present in 1991 gradually leaving the area (dark blue bars), and with new animals entering it.
Calves tended to be found in Monterey Bay year round with a summer peak and a smaller peak in the fall (Figure 8). The crude birth rate (number of calves per number of animals identified) varied widely among years (Figure 9) and the reasons for these variations are not detectable from the data available.
Environmental Factors Appear to Shape the Size of the Range
El Niňo Southern Oscillation events occur periodically (every 5-7 years) along the West Coast of the American Continent. In recent years this periodicity has increased, with three consecutive events occurring between 1992 and 1995. Coastal bottlenose dolphins were recorded in Monterey Bay for the first time in the 20th century, during the 1988 El Niňo event, although skulls recovered in central California and dated to be over 100 years old, suggest these animals ranged this far north in the past. Since 1988, dolphins have been continuously present in Central California suggesting a permanent range extension beyond Point Conception.
The rate with which new dolphins were discovered in the population in Monterey Bay during this study (Figure 4) was significantly higher during the three El Niňo years versus the non-El Niňo years. School size and encounter rate were also significantly higher in El Niňo versus non-El Niňo years (Figure 5).
Figure 9 – Crude birth rate per year for bottlenose in Monterey Bay.
Figure 8 – Rate of occurrence of bottlenose dolphin calves per survey in Monterey Bay.
Is there a High Mortality Rate in Monterey Bay's Calves?
Figure 4 - The rate of discovery for each year of the study indicates a marked difference among years in the influx of new dolphins into the Monterey Bay community with non-El Niňo years (1991 and 1995) having low to no influx versus El Niňo years (1992-1994) having high influx.
Twelve females were identified using the presence of calves with them over the years as an indicator of sex. The proportion of mothers in the identified population was similar for each year, being highest in 1992 and 1994, and lowest in 1993. Nine females had multiple births and seven of these dolphins had calves in successive years (Table 1) with no evidence of the presence of the previous year’s calf with them when the next newborn was identified.
It is apparent from these findings that environmental factors affect the extent of the dolphins’ range (perhaps due to shifts in distribution of the preferred prey).
One of the calves, Epoxy’s (1994), stranded in July 1994 and died in captivity a few days after (cause of death unknown). It is possible that the anomalous occurrence (normally the calving interval is between three and six years) of successive year births is a consequence of high levels of mortality in Monterey Bay’s calves. If this is the case, the possible reasons for such a phenomenon should be investigated.
Table 1 - Calving intervals of nine known females
Figure 5 – School size and encounter rate were significantly higher during the three El Niňo years (1992-1994) combined versus the non-El Niňo years (1990, 1991, 1995).
Social Units Within Shifting Schools
Monterey Bay is located at the upper limit of the dolphins’ geographic range and it is therefore an area of extreme shift. El Niňo appears to be a key factor in the shrinking and expansion of the population range. Because El Niňo causes a shift of prey items northward as the waters along the coast warm up, it is likely that the dolphins adjust their range following their prey. Water temperature and other 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 detectable.
We defined social units as highly stable subgroups that traveled in
highly cohesive fashion within a school. It appeared that these
subgroups within the school had a higher level of association over time than the fluid fission-fusion schools. We are currently investigating these sub-group level associations which appear, at the preliminary stage, to be defined by sex and or/ reproductive status.
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 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.
A NEW PROJECT AT THE ASLC
Two genetically distinct types of killer whale are found in Alaskan waters. These two types occupy two separate ecological niches and do not interbreed. The most commonly encountered killer whales are known as residents and feed exclusively on fish, with a marked preference for salmonids.
Residents travel in large pods which are organized in several distinct matrilines. These whales have been studied extensively and their affiliation and genealogy is well known.
Residents are very vocal, acrobatic, and surface active, and tend to frequent the same areas over time.
The killer whales of the transient type generally eat other marine mammals (such as seals, sea lions, dolphins, porpoises, whales and sea otters).
They are ambush predators or attack as a wolf pack, often traveling in small groups whose affiliation is unclear. Because of their lifestyle, transients are harder to study and little is known about their social organization, abundance and distribution patterns.
Transients dive for long periods of time (15-20 minutes), are generally silent and are less surface active. Often they go undetected.
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.
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.