Copy Right: T. Eguchi

This summary covers the following topics:

1. Taxonomy and systematics
2. Distribution and movements
3. Abundance
4. Morphology, hematology, and growth
5. Diet
6. Human interactions
7. Literature cited

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1. Taxonomy and Systematics

The bottlenose dolphin (Tursiops truncatus) belongs to the Order Cetacea, Family Dephinidae, and Genus Tursiops. The genus Tursiops is polymorphic and at least 20 species have been described (Mead and Potter 1990). Because systematic works have been conducted on relatively small sample sizes and restricted geographic areas, however, there is not a general consensus on the validity of these species (Mead and Potter 1990, Ross and Cockcroft 1990). Throughout the range of the species, researchers have found two morphological types among adult dolphins, which often are referred to as ecotypes (e.g., Perrin 1984, Van Waerebeek et al. 1990).
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The bottlenose dolphin (Tursiops truncatus) is a cosmopolitan species and found in almost all oceans except for the very high latitude. In the Pacific, the species ranges along the both sides of the ocean from northern Japan and central California southward to Australia and Chile (Leatherwood and Reeves 1983, Wells et al 1990). In the Atlantic, bottlenose dolphins range from Nova Scotia and Norway southward to Patagonia and the tip of South Africa (Leatherwood and Reeves 1983). Bottlenose dolphins also are common in the Mediterranean Sea and the Indian Ocean (Leatherwood and Reeves 1983).

Bottlenose dolphins often exhibit offshore-inshore separation. Inshore, or coastal bottlenose dolphins usually are found shoreward of the 10-fathom contour and often enter harbors, inlets, bays, lagoons, estuaries, and rivers (Leatherwood and Reeves 1983). Several studies have indicated that these coastal dolphins have limited home ranges (e.g., Connor and Smolker 1985, Scott et al. 1990, Hammond and Thompson 1991). The offshore form, however, is less restricted in its range and movements (Leatherwood and Reeves 1983, Leatherwood et al. 1988, Scott and Chivers 1990).

Kenney (1990) reported a distinct pattern in the distribution of bottlenose dolphins off northeastern United States. Inshore sightings were located in the coastal waters (< 20 m) from Cape Lookout to Delaware Bay, whereas the offshore sightings were distributed between the 200 and 2000 m bottom contours, from Cape Hatteras to the eastern end of Georges Bank (Kenney 1990). The mean depth of the inshore sightings was 10.3 m, whereas that of the offshore sightings was 845.6 m (Kenney 1990). The inshore-offshore distribution pattern, however, was less distinct south of Cape Hatteras (Scott et al. 1988) and no distinctive offshore groups of bottlenose dolphins have been described south of Cape Hatteras.

Some coastal bottlenose dolphins, north and south of Cape Hatteras, seem to exhibit north-south seasonal migration. Researchers have hypothesized that coastal dolphins migrate latitudinally in response to the change in the ocean temperature caused by the ocean currents (Mead and Potter 1990). Because water temperature offshore is not influenced by the incursion of the Labrador Current during winter, offshore dolphins may not migrate seasonally (Mead and Potter 1990). During summer, coastal bottlenose dolphins are distributed as far north as Long Island, New York. The main concentration of coastal bottlenose dolphins during summer, however, is along the coast of North Carolina to New Jersey (Scott et al. 1988). During winter, bottlenose dolphins are distributed south of Cape Hatteras to the northern and central Florida coast, with main concentrations at the southern end of this range (Scott et al. 1988). No dolphins were sighted during winter in the coastal water north of Cape Hatteras during the aerial surveys conducted between 1978 and 1983 (Kenney 1990). Some bottlenose dolphins, however, have been reported north of Cape Hatteras during winter (Blaylock and Hoggard 1994, Wiley et al. 1994). Seasonal movements, extent of offshore distributions, and inshore-offshore movements of the offshore form have not been determined because sampling has been limited to areas within 200 km of shore (Wang et al. 1994).

Bottlenose dolphins also are found throughout the year in some estuaries, inlets, and rivers south of Cape Hatteras (North Carolina to Florida; Scott 1988, Hohn 1997, Sayigh et al. 1997, A. Hohn and L. Hansen, pers. comm.). Results of photo-identification indicated that there was a difference in the extent of residency among dolphins: residents through out the year, seasonal residents (returning to the same area during one season every year but not present during other seasons), migrants or transients, and possible groups with great home ranges (Hohn 1997). Resident dolphins are found only in the southern part of their range (North Carolina to Florida; Sayigh et al. 1997, Hohn 1997). Seasonal residents and transients (migrants) also occur in the same area as where residents occur (Odell and Asper 1990, Hohn 1997).

Movements and home ranges of bottlenose dolphins have been studied using radio telemetry, tagging, satellite telemetry, and photo identification of natural marks (e.g., Würsig and Würsig 1977, Tanaka 1987, Wells et al. 1987, Scott et al. 1990, Mate et al. 1995). Extensive works on home ranges of bottlenose dolphins come from the Sarasota Bay area, Florida, in which some marked dolphins have maintained fidelity to the area for over 17 years (Scott et al. 1990). In the Sarasota Bay area, home ranges of males are greater than females. Adult females create several relatively discrete groups, with each band occupying different and relatively limited core areas that broadly overlap, and adult males travel from one female band to another (Scott et al. 1990). Odell and Asper (1990) captured and freeze-branded 133 bottlenose dolphins in Indian River, Florida, between March 1977 and October 1981. Marked dolphins were searched for actively and opportunistically after branding from September 1979 through March 1982. Eight branded dolphins were never identified positively following their release. Of 81 freeze-branded dolphins with sighting records, 60 were sighted exclusively within the Indian River (mean linear range = 32.8 km, SD = 18.0 km) and the remainder were seen in both Indian and Banana rivers (mean linear range = 55.6 km, SD = 20.5 km).

Continuous movements of bottlenose dolphins have been estimated by using satellite telemetry. Mate et al. (1995) deployed a satellite transmitter on a bottlenose dolphin in Tampa Bay, Florida, and found that the dolphin traveled at least 581 km during 25 days, and the longest distance traveled in a day was 50.2 km. Similar results have been reported for resident dolphins in Sarasota Bay, Florida (< 30 km/day; Irvine et al. 1981), and a satellite-tagged bottlenose dolphin off Japan (604 km/18 days; Tanaka 1987).

The extent of movements by transient dolphins, however, has not been determined. More information on movements will be available as all the previously taken photographs of dorsal fins are catalogued and analyzed in a systematic manner.
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To estimate the abundance of bottlenose dolphins along the east coast of the United States, aerial surveys have been conducted. In the areas north of Cape Hatteras, Kenney (1990) used data from the surveys conducted by the Cetacean and Turtle Assessment Program (CETAP) between 1978 and 1983. Data were collected from the shoreline to 5 nautical miles seaward of the 1,000 fathom isobath, using random line-transect procedures (Kenney 1990). He defined the inshore as all dolphins found in waters less than 20 fathoms, whereas all dolphins found in waters deeper than 20 fathoms were considered the offshore form. No bottlenose dolphins were observed in inshore waters during winter. The greatest number of dolphins were found in the coastal water during summer, with estimates between 400 to 700. In the offshore water, the number of dolphins were least during winter (1500 to 2,300) and greatest during summer (9,700 to 12,800; Kenney 1990).

From 1992 through 1994, several aerial surveys were conducted along the east coast of the United States between Cape Hatteras and southeast Florida (Blaylock and Haggard 1994, Blaylock 1995). The abundance of bottlenose dolphins during the winter of 1992 was estimated to be 12,435 (95% CI = 9,684 - 15,967) between Cape Hatteras and southeast Florida using the line-transect method (Blaylock and Haggard 1994). During the summer of 1994, two types of aerial surveys were conducted. One type was a direct count of bottlenose dolphins within approximately 1 km of shore, and the other was the simple-random line transect method (Blaylock 1995). Direct count surveys indicated the minimum of 2,482 dolphins along the shore. The estimated abundance of dolphins from the line-transect suvery was 25,841 (95% CI = 13,010 - 51,329; Blaylock 1995). During these surveys, however, offshore-inshore types were not distinguished.

From June 1987 to March 1988, an epizootic of morbilliviruses caused mass die-off of bottlenose dolphins along the Atlantic coast of the United States from New Jersey to Florida. Between June 1987 and April 1988, 742 dolphins were found dead from New Jersey to Florida, which was 10 times greater than the previous 3-year historical level of carcass detection (mean = 73 dolphins; Wang et al. 1994). Although exact number of death is not known, Scott et al. (1988) estimated that greater than 50% of the coastal migratory stock between New Jersey and Florida died during this epizootic. Consequently, on 6 April 1996, the National Marine Fisheries Service listed the migratory stock of Atlantic coastal bottlenose dolphins as depleted under the Marine Mammal Protection Act (Wang et al. 1994). The estimated abundance during the winter of 1991, however, was not significantly different from the comparable estimate during the winter of 1983 (Blaylock and Hoggard 1994, Waring et al. 1997).
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Male bottlenose dolphins are larger than females. Tolley et al. (1995) reported the sexual dimorphism in bottlenose dolphins in waters near Sarasota Bay, Florida. Males were larger than females in 20 out of 29 measurements. Only one out of seven skull measurements (rostral girth), however, indicated a sexual dimorphism. It was greater in males than females. Males also were proportionally more robust and possessed larger appendages than females. Sexual dimorphism of offshore dolphins in the Atlantic has not been determined.

Although bottlenose dolphins are the most studied cetacean species, few studies have been conducted to estimate the growth and reproduction of the species. The majority of the information comes from dolphins in Gulf of Mexico and around Florida, especially near Sarasota, where virtually all the individuals in the community have been examined at least once since 1970 (Scott et al. 1990). Read et al. (1993) used the longitudinal data set of near Sarasota Bay to estimate growth of bottlenose dolphins (47 males and 49 females) in the community. Ages of captured dolphins were either known from field observations or estimated by the method described by Hohn (1980, 1990). Male dolphins showed greater asymptotic values in length (266.4 ± 2.86 SE vs. 249.2 ± 1.30 SE (cm)), girth (154.0 ± 5.67 SE vs. 141.7 ± 1.36 SE (cm)), and mass (259.0 ± 14.6 SE vs. 194.4 ± 4.72 SE (kg)) than females (Read et al. 1993). During the first six years of life, however, most females were larger in length and mass than males of the same age (Read et al. 1993). Males grow continuously for several years, especially in mass and girth, after females reach their asymptotic body size (Read et al. 1993).

Fernandez and Hohn (1998) aged 195 stranded dolphins (78 males, 81 females, and 36 unknown sex) along the Texas coastline. The asymptotic lengths for these dolphins using the Gompertz model were 263.5 cm for males and 244.7 cm for females, indicating no significant differences from the estimate for dolphins near Sarasota, Florida (Read et al. 1993, Fernandez and Hohn 1998). Data from the Cape Hatteras area, however, indicated no sexual dimorphism and the asymptotic length of 240 cm (44 females and 38 males; Mead and Potter 1990). In Florida, female bottlenose dolphins reach sexual maturity at 5 to 12 years of age and a length of 220 to 235 cm, whereas males at 10 to 13 years and 245 to 260 cm (Odell 1975). Mean length at birth for a sample of both sexes was estimated to be 109.4 cm (SD = 8.5, n = 42; Fernandez and Hohn 1998), which was comparable to another estimate from the Cape Hatteras area (mean = 114.3, SD = 7.8, n = 26; Mead and Potter 1990).

The number of stranded neonates have been used as an indicator of calving in the region. In the Cape Hatteras area, 20 out of 32 stranded neonates were found during March and April, indicating a prolonged calving season with a peak during the spring (Mead 1975, Mead and Potter 1990). In waters around Florida and Texas coast lines, however, calving appeared to occur almost throughout the year and varied among locations (Urian 1996, Fernandez and Hohn 1998).

Morphological and hematological differences exist between coastal and offshore forms (Leatherwood and Reeves 1982, Duffield et al. 1983, Hersh and Duffield 1990). Duffield et al. (1983) compared hematological variables of 70 Atlantic and 35 Pacific bottlenose dolphins, which were maintained at Sea World facilities in San Diego and Florida. Although no offshore type dolphins from the Atlantic were included in the sample, distinctive differences were found in hemoglobin concentration, packed cell volume, and red blood cell counts between coastal and offshore types from the Pacific (Duffield et al. 1983). Hersh and Duffield (1990) compared hemoglobin profiles and morphometrics among fresh dead stranded dolphins along the east coast of Florida and found distinctive differences. Four dolphins that were found in the area close to the deep water exhibited electrophoretically different hemoglobins from dolphins found in areas with no immediate deep water access. They also found significant differences in skull measurements between the two groups. In general, offshore dolphins had longer body lengths with a proportionately shorter snout and proportionately smaller flippers and wider skulls and rostrums than coastal dolphins (Hersh and Duffield 1990).
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Because bottlenose dolphins are found nearshore and often interact with fishing activities, their diet has been described by several researchers. Bottlenose dolphins are often described as opportunistic foragers and they feed on a wide variety of fish and invertebrates. Stomach samples (n = 76) from stranded dolphins in the southeastern United States indicated that main prey items of bottlenose dolphins included silver perch (Bairdiellachrysoura), Atlantic croaker (Micropogonias undulatus), mullet (Mugil spp.), brief squid (Lolliguncula brevis), and spot (Leiostomus xanthurus), all occurring in 20% or more of the stomachs (Barros and Odell 1990). Although most prey were benthic species (e.g., drums, croakers, seatrouts, toadfishes, and midshipman), pelagic (e.g., jacks, bluefish, and cutlass fish) and surface (e.g., mullets and clupeids) fish also were included in their diet. Main prey items of bottlenose dolphins differed among locations, reflecting the abundance of local fish species (Barros and Odell 1990). To compare prey items found in stranded dolphins and that of free ranging dolphins, Mead and Potter (1990) compared the stomach samples of stranded dolphins and dolphins that were incidentally killed in gill nets or haul seines. No differences in prey items were found between these two groups (Mead and Potter 1990).

Although few studies have been conducted to describe food habits of the offshore type bottlenose dolphins, Barros and Odell (1990) reported that ommastephid squids (Illex sp. and Ornithoteuthis antillarum) were the primary prey items of one offshore dolphin. Another stomach sample of an offshore type dolphin contained 150 beaks from the shortfin squid (Illex illecebrosus), three octopodid squids, and an unidentified fish otolith (Mercer 1973).

Anecdotal reports indicated bottlenose dolphins steal several fish species from fishing lines of fishermen, including king mackerel (Scomberomorus cavall), Spanish mackerel (S. maculatus), kingfish (S.regalis), tarpon (Megalops (Tarpon) atlanticus), sailfish (Istiophorus americanus), hammerhead shark (Sphyrnazygaena), speckled trout (Cynoscion nebulosus), roballo (Centropomus undecimalis), spotted eagle-ray (Aetobatus (Stoasodon) narinari), mullet (Mugil sp.), sea catfish (Aris (Galeichthys) felis), sheepshead (Archosargus probatocephalus), and flounder (Paralichthys sp.; Gunter 1942, Odell 1975). Gunter (1942) also examined stomach samples collected from 34 dolphins killed in a small-scale harpoon fishery in Aransas County, Texas, and found striped mullet (Mugil cephalus), gizzard shad (Dorosoma cepedianum), spot (Leiostomus xanthurus), croaker (Micropogon undulatus), sand trout (Cynoscion arenarius), puffer (Sphoeroides marmoratus), sheephead (Archosargus probatocephalus), needle gar (Strongylura marina), black drum (Pogonias cromis), spotted trout (Cynoscionnebulosus), and flounder (Paralichthys lethostigmus).
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Blaylock, R. A. 1995. A pilot study to estimate abundance of the U.S. Atlantic coastal migratory bottlenose dolphin. NOAA Technical Memorandum NMFS-SEFSC-362, 9 pp.

Blaylock, R. A. and W. Hoggard 1994. Preliminary estimates of bottlenose dolphin abundance in southern U.S. Atlantic and Gulf of Mexico continental shelf waters. NOAA Technical memorandum. NMFS-SEFSC-356, 10 pp.

Barros, N. B. and D. K. Odell. 1990. Food habits of bottlenose dolphins in the southeastern United States. In. Eds. S. Leatherwood and R. R. Reeves. The bottlenose dolphin. Academic Press, San Diego, California.

Connor, R. C. and R. A. Smolker. 1985. Habituated dolphins (Tursiops sp.) in Western Australia. Journal of Mammalogy 66: 398-400.

Duffield, D. A., S. H. Ridgway, and L. H. Cornell. 1983. Hematology distinguishes coastal and offshore forms of dolphins (Tursiops). Canadian Journal of Zoology. 61: 930-933.

Fernandez, S. and A. A. Hohn. 1998. Age, growth, and calving season of bottlenose dolphins, Tursiops truncatus, off coastal Texas. Fishery Bulletin 96: 357-365.

Gorzelany, J. F. 1998. Unusual deaths of two free-ranging Atlantic bottlenose dolphins (Tursiops truncatus) related to ingestion of recreational fishing gear. Marine Mammal Science. 14: 614-617.

Gunter, G. 1942. Contributions to the natural history of the bottlenose dolphin, Tursiopstruncatus (Montagu), on the Texas coast, with particular reference to food habits. Journal of Mammalogy. 23: 267-276.

Hammond, P. S. and P. M. Thompson 1991. Minimum estimate of the number of bottlenose dolphins Tursiops truncatus in the Moray Firth, NE Scotland. Biological Conservation. 56: 79-87.

Hersh, S. L. and D. A. Duffield. 1990. Distinction between Northwest Atlantic offshore and coastal bottlenose dolphins based on hemoglobin profile and morphometry. In. Eds. S. Leatherwood and R. R. Reeves. The bottlenose dolphin. Academic Press, Inc. San Diego, California.

Hohn, A. 1980. Age determination and age related factors in the teeth of western North Atlantic bottlenose dolphins. Scientific Reports for Whales Research Institute, Tokyo. 32: 39-66.

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Hohn, A. 1997. Design for a multiple-method approach to determine stock structure of bottlenose dolphins in the mid-Atlantic. Report of a workshop, 11-12 February 1997. NOAA Technical Memorandum NMFS-SEFSC-401. 22 pp.

Kenney, R. D. 1990. Bottlenose dolphins off the northeastern United States. In. Eds. S. Leatherwood and R. R. Reeves. The bottlenose dolphin. Academic Press, Inc. San Diego, California.

Leatherwood, S. and R. R. Reeves. 1983. The Sierra club handbook of whales and dolphins. Sierra Club Books, San Francisco, California.

Leatherwood, S., R. R. Reeves, W. F. Perrin, and W. E. Evans. 1988. Whales, dolphins, and porpoises of the eastern north Pacific and adjacent Arctic waters. Dover Publications, Inc. New York, New York.

Mate, B. R., K. A. Rossbach, S. L. Nieukirk, R. S. Wells, A. B. Irvine, M. D. Scott, A. R. Read. 1995. Satellite-monitored movements and dive behavior of a bottlenose dolphin (Tursiops truncatus) in Tampa Bay, Florida. Marine Mammal Science. 11: 452-463.

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Odell, D. K. 1975. Status of aspects of the life history of the bottlenose dolphin, Tursiopstruncatus, in Florida. Journal of Fishery Research Board of Canada. 32: 1055-1058.

Odell, D. K., and E. D. Asper. 1990. Distribution and movements of freeze-branded bottlenose dolphins in the Indian and Banana rivers, Florida. In. Eds. S. Leatherwood and R. R. Reeves. The bottlenose dolphin. Academic Press, San Diego, California.

Perrin, W. F. 1984. Patterns of geographical variation in small cetaceans. Acta Zoologica, Fennica. 172: 137-140.

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Ross, G. J. B. and V. G. Cockcroft. 1990. Comments on Australian bottlenose dolphins and the taxonomic status of Tursiops aduncus (Echrenberg, 1832). In. Eds. S. Leatherwood and R. R. Reeves. The bottlenose dolphin. Academic Press, San Diego, California.

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Wells, R. S., L. J. Hansen, A. Baldridge, T. P. Dohl, D. L. Kelly, and R. H. Defran. 1990. Northward extension of the range of bottlenose dolphins along the California coast. In. Eds. S. Leatherwood and R. R. Reeves. The bottlenose dolphin. Academic Press, San Diego, California.

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