Mikura Island is a dormant volcano likely thousands of years old. While the near shore area is shallow (4 to 20 m), depths of over 100 m can be found within 150 m of shore. The coast is lined with boulders and pebbles. The Kuroshio current annually encompasses the island in its northerly flow and brings a variety of fish. Due to the deep waters nearby, the effect resembles an upwelling near the island. Thus, fish species are abundant. Dolphins can be seen in the surf zone near Mikura Island as well as in the deeper waters.
Mikura Island is a dormant volcano likely thousands of years old. While the near shore area is shallow, depths over 100 m can be found within 300 meters of shore. The coast is lined with boulders and pebbles. The Kuroshio current annually encompasses the island in its northerly flow and brings a variety of fish. Due to the deep waters around the island, the effect resembles an upwelling near the island. Thus, fish species are abundant.
Observations on bottlenose dolphins around Mikura Island have been ongoing since the early 1980’s (Moyer, 1997; Kogi et al., 2004). Systematic data collection in the form of video and photographic recordings has been conducted since 1994 (T. Iwatani, I.C.E.R.C. Japan, pers. comm., 1996; Kogi et al. 2004). Work by I.C.E.R.C. and Mikura Bottlenose Dolphin Research Association volunteers estimates the population size frequenting the waters within 300 m of Mikura shores at about 165 individuals. A 1:1 sex ratio of identified dolphins has been reported (Kogi et al., 2004), although this ratio does not hold within different age classes. There are many more young males than females and more adult females as compared with adult males. Early analyses suggest that adult females with calves are the most frequent group types observed around Mikura Island (R. Soeda, I.C.E.R.C. Japan, pers. comm., 1997; Moyer, 1997; Kogi et al., 2004).
Tourists have become increasingly interested in interacting with these dolphins over the past four years. Before 1993, fewer than 5000 people visited Miyake and Mikura Islands for the purpose of swimming with dolphins. In 1997, the number of dolphin-watching visitors rose to well over 10,000 people per year – mostly from May through October. The volcanic eruption on Miyake Island in 2000 dropped the number of visitors to about 1,000 people per year, and only to Mikura Island. From 2000 – 2005, the Mikura Tourism Board established more outlined dolphin watching and swimming with guidelines.
Calf/Infant – Dolphins less than 2/3 the length of an adult with which they consistently swim beside or in echelon. Calves were less than one year old and ranged in size from about 0.75 to 1.5 meters in length. Very young calves/infants (< 1 mo. old) had visible fetal folds, slightly folded dorsal fin and fluke tips, and “cork-bobbed” at the surface when breathing.
Juvenile – Dolphins ranging in size between 1.5 and 2.0 meters in length, swimming independently from adults, and often interacting with other similar aged and sized individuals were considered to be juvenile.
Subadult – Dolphins approximately 2.0 to 2.5 meters in length, and without the girth observed in adults. These individuals appear not to have reached sexual maturity. The distinction between older juveniles and subadults and young adults and subadults is difficult and classification is aided by observations of associates and behavioral activity. Once a female sub-adult has a calf, she is assumed to be an adult.
Adult – Dolphins approximately 2.5 to 3.0 meters in length with some degree of ventral spotting present.
DCP’s Studies on the Dolphins around Mikura Island, Japan
Since 2010, DCP has not conducted lengthy behavioral observations or direct data collection of the behavior and acoustics of dolphins around Mikura Island. Instead, we collaborate with Mai Sakai and Tadamichi Morisaka, Kyoto University, for research questions.
Work on the Mikura bottlenose dolphins (Tursiops aduncus) offers a unique opportunity to examine dolphin behavior in a setting far different from the Bahamas or Roatan. The clear water and habituation to humans by dolphins around Mikura Island offer predictable opportunities for observing the social behavior and structure of dolphins from underwater, similar to the setting in the Bahamas. Dudzinski’s research at Mikura Island, focused on dolphin signal exchange and use of contact behaviors between individuals. Related comparative questions focus on the similarities and differences in use and production of contact and vocal behavior as well as signal exchange among individuals between the Mikura Island bottlenose dolphins and the Bahamas’ Atlantic spotted dolphins, as well as all three captive dolphin study groups.
Do individuals from both groups exhibit similar group types? Do they exhibit similar behavioral activities in their given habitats? Do individuals within both study populations use vocalizations differently? Do they interact with conspecifics similarly given similar group types or behavioral activities? Could the difference in habitat for the two locations – far from shore, white sand in the Bahamas versus near-shore, boulder-covered sea floor around Mikura – play a significant factor in any observed differences in behavioral activity or signal exchange?
Study Animals: Bottlenose Dolphins (bandoiruka)
Observations on bottlenose dolphins around Mikura Island have been ongoing since the early 1980s. Systematic collection of data in the form of video and photographic recordings have been conducted since 1994 (T. Iwatani, I.C.E.R.C. Japan, pers. comm., 1996; Kogi et al., 2004). Work by ICERC and the Mikura Bottlenose Dolphin Kenkyukai volunteers estimate the population size frequenting the waters within 300 m of Mikura shores at about 165 individuals. A 1:1 sex ratio of identified dolphins has been reported, although this ratio does not hold within different age classes. There are many more young males than females and more adult females as compared with adult males. Early analyses suggest that adult females with calves are the most frequent group types observed around Mikura Island.
Justin Gregg joined DCP in 2005 to continue studying the dolphins around Mikura Island for his doctorate. His work related to echoic eavesdropping and joint attention behaviors among dolphins. DCP continues to collaborate with Japanese researchers (Sakai and Morisaka) who also study the dolphins around Mikura Island. Together, we continue to expand our knowledge and database on these amazing creatures.
Through DCP, Dudzinski spent 29 days on Mikura Island and had 15 boat trips to gather data observing dolphins interact and communicate. We spent 30 hours and 20 minutes looking for dolphins (i.e., effort) and recorded 3 hrs, 2 min. of video. With our preliminary analyses, we have identified: 8 Adult females, 3 Adult males, 6 sub-adult females, 10 sub-adult males, 4 juvenile females and 2 juvenile male dolphins. We saw 4 mother/calf pairs for which the calf was a year old. Merging our preliminary records with those of the Mikura Iruka Kyoukai for this season, we saw 88 different individual bottlenose dolphins: this represents a 61.54% re-sighting rate just for May and June this year, with respect to the study population size documented in 2000.
We were lucky to gather several hours of data with our new echolocation click detector (ECD) boards. Because these boards allow us to capture all information associated with dolphin echolocation clicks (~120kHz) that are not audible to our ears, we need to rely on computers to download the data. Preliminary checks of the audio tapes confirmed that we did indeed record at least something from the dolphins when they approached the camera, scanned with their heads and made sounds. With computers we have confirmed the frequency (kHz) that the dolphins were using when investigating the camera, Dudzinski, and the other swimmers. Analyses of the tapes are ongoing, and we should be able to post details by the end of 2001.
For those of you interested in some background information about Dudzinski’s research on the dolphins around Mikura Island, you can download her report covering her research from 1997 to 1999.
Work on the Mikura bottlenose dolphins (Tursiops truncatus) offers a unique opportunity to examine dolphin behavior in a setting far different from the Bahamas. Mikura Island is a dormant volcanic island with a boulder coastline. Depths range from four to 20 meters within 150 m of shore, dropping sharply to more than 100 m depth at about 200 m from shore. Dolphins can be seen in the surf zone near Mikura Island as well as in the deeper waters. In the Bahamas, Atlantic spotted dolphins are regularly observed between 40 and 60 km from shore in shallow (< 8 m) water with a white sandy bottom. The clear water and habituation to humans by dolphins in both locations offer predictable opportunities for observing dolphin social behavior and communication underwater.
Details of intra-specific interactions and signal exchange as related to dolphin age, gender and associates were examined for spotted dolphins (see Bahamas page for details & references). But it was not possible to determine if the observed exchanges were simply related to dolphin characteristics, or if external factors played a significant role in specific associations. Information gathered on the Mikura bottlenose dolphins will facilitate a comparison of vocal and behavioral activities with those of spotted dolphins as related to varying social associations and habitat differences (open, flat, sandy bottom vs. coastal, rocky sea floor).