An In-Depth Look at Bottlenose Dolphins

Description

Bottlenose dolphins are recognized by their medium-sized, robust body, moderately curved dorsal fin, and dark coloration, with a sharp demarcation between the melon and the short rostrum. Adult lengths range from 2 - 3.8 m, weights from 220-500 kg (mean of 242 kg), varying geographically. Body size also seems to vary inversely with water temperature in many parts of the world. The animals are coloured light grey to black dorsally, with a light belly (Bloch and Mikkelsen, 2000; Wells and Scott, 2002).

Despite the wide distribution, abundance, and popularity of bottlenose dolphins, their taxonomy remained muddled for a long time (Rice, 1998).

Geographical variation in bottlenose dolphins is only vaguely comprehended, and in most parts of the world subspecific designations are best avoided. The name T t. truncatus (type locality: Great Britain) may be applied to the offshore populations on both sides of the North Atlantic, and some authors have used it for similar animals that live in the temperate waters of the western North Pacific, South Africa, Walters Shoal, southern Australia, and New Zealand. Often, there are size differences between neighbouring populations: The dolphins that live in the Black Sea (named T t. ponticus Barabash-Nikiforov, 1940) are smaller than those in the North Atlantic, while those in the Mediterranean are intermediate in size. In some parts of the world, sharply differentiated inshore and offshore populations live in close proximity. Results of mtDNA analyses do not indicate genetic isolation among offshore populations from different ocean basins, but do show that there are differing coastal or inshore populations which are genetically isolated from offshore populations (Rice, 1998 and refs. therein).

Recent genetic work by Le Duc et al. (1999), osteological comparisons by Wang et al. (2000) and morphological analyses by Hale et al. (2000) support the view that that bottlenose dolphins of the tropical Indian Ocean, T. aduncus, are reproductively isolated from the widespread T. truncatus.

Distribution

In the Atlantic T. truncatus occurs north to the Gulf of Mexico, George's Bank off Massachusetts, the Azores, the British Isles, The Faroe Islands, the Baltic Sea including the Gulf of Finland, the Mediterranean and Black seas. In the Pacific it ranges north to the Bo Hai (Gulf of Chihli), East China Sea, central Honshu, Kure Atoll, Hawaii, Isla Guadalupe, and Monterey Bay in California. In the Southern Hemisphere T. truncatus occurs south to Golfo San Matias in Argentina, 18°S in northern Namibia, Port Elizabeth in Cape Province, Walters Shoal (33°20'S, 43°30'E) in the south-western Indian Ocean, the southern coast of Australia including Tasmania, South Island in New Zealand,

and Concepción, Chile (Rice, 1998). Recent evidence (Möller and Beheregaray, 2001), however, suggests that coastal Tursiops off south-eastern Australia belong to the aduncus type.

Bottlenose dolphins are found primarily in coastal and inshore regions of tropical and temperate water s of the world, and population density seems to be higher nearshore. There are also pelagic population s, such as those in the eastern tropical Pacific and around the Faroe Islands. Except for their occurrence around the United Kingdom and northern Europe, they generally do not range poleward of 45° in either hemisphere (Jefferson et al. 1993). The bottlenose dolphins occurring around the Faroe Islands (62°N 7°W) seems to be the most northernly of the North Atlantic offshore populations (Bloch and Mikkelsen, 2000).

In the North Atlantic, Tursiops truncatus is vagrant to Newfoundland and Norway, and in the North Pacific it ranges as far north as Puget Sound in Washington State (Rice, 1998). The species is rare in the Baltic Sea, and there is some question as to their occurrence in the Barents Sea (Wells and Scott, 1999 and refs. therein)

Sykes et al. (2003) investigated the variables that best predict the seasonal distribution of sightings of Bottlenose dolphins along the Dorset coast (England). The factors investigated included salinity, sea surface temperature, chlorophyll a (an indicator of primary productivity) and fish distribution (inferred from landing catch data). Local data sampling validated the use of historical data sets for all the variables. They found that chlorophyll a and fish distribution were the main factors influencing Bottlenose dolphin distribution. Of the 29 fish species investigated, Brill (P< 0. 005), Cuttlefish (P<0.0001), Plaice (P<0.0001), Pollack (P<0.005), Red and Grey Mullet (P< 0.005),Sole (P<0.001), Sprat (P<0.0001) and Spurdog (P<0.0001) were found to be significant predictors and could explain 87.63% of the frequency of dolphin sightings. Stepwise Multiple Regression also identified historical chlorophyll a (P<0.05) as a significant predictor of sightings, explaining 13.5% of the frequency of dolphin sightings. These findings indicate that feeding is an important factor affecting Bottlenose dolphin distribution along the Dorset coast.

Population Size

Only a few abundance estimates are available for Tursiops from parts of the species' range. In the northern Gulf of Mexcio, the population estimate ranges from 35,000 - 45 000 Tursiops inshore of the 100-fathom contour, an area that extends to more than 250 km from shore. Off the northeast coast of North America, the overall population is approximately 10,000-13,000, of which the inshore form comprises around 4%. Large-scale research vessel surveys by the US National Marine Fisheries Service (NMFS) produced an estimate of 243,500 Tursiops in the eastern tropical Pacific.

Japanese surveys found 316,935 dolphins in the Northwest Pacific.

Reports for various areas, such as the Mediterranean, identify T. truncatus as the most common and abundant dolphin, but estimates of population size are not given. A Russian survey of the Black Sea estimated a population size of 7,000 Tursiops, although the details of the surveys were not presented. Approximately 900 bottlenose dolphins inhabit the 400 km stretch of coastal waters off Natal, south-east of southern Africa (Wells and Scott 1999 and refs. therein; Reyes, 1991 and refs. therein). In the eastern Sulu Sea, Dolar (1999) estimated the population size at 2,200.

From the North Atlantic Sightings Surveys in 1987 and 1987 (NASS-87 and NASS-89) a very cautious estimate of the bottlenose dolphins around the Faroe Islands comes to about 1,000 animals (Sigurjónsson et al., 1989; Sigurjónsson and Gunnlaugsson, 1990, Bloch and Mikkelsen, 2000).

Biology and Behavior

Habitat: As a result of increased pelagic survey efforts over the last 20 years, researchers have come to recognise Tursiops as a truly cosmopolitan species. Although they tend to be primarily coastal, they can also be found in pelagic waters (Wells and Scott, 1999). Bottlenose dolphins exploit a wide variety of habitats. The inshore form frequents river mouths, bays, lagoons and other shallow coastal regions (between 0.5-20 m). Occasionally they may travel far up into rivers. The offshore form is apparently less restricted in range and movement, and can be found in many productive areas, particularly in the tropics. Some offshore populations are residents around oceanic islands. A coastal habitat seems to be preferred in the Black Sea, with limited movements into offshore waters (Reyes, 1991 and refs. therein). Limits to the species' range appear to be temperature related, either directly, or indirectly through distribution of prey. Off the coasts of North America, they tend to inhabit waters with surface temperatures ranging from about 10 °C to 32°C (Wells and Scott, 1999 and refs. therein).

Food: The differences between inshore and offshore Tursiops are also reflected in their feeding habits. The inshore form feeds primarily on a variety of fish and invertebrates from both the littoral and sub-littoral zones, whereas mesopelagic fish and oceanic squids are commonly reported as the diet of animals of the offshore form (Reyes, 1991 and refs. therein).

Diet also varies with local prey availability. Along the central US Atlantic coast 31 genera of fish and two species of invertebrates were reported from stomach samples. The four most common prey items were fish: Cynoscion regalis, Micropogonias undulatus, Leiostomus xanthurus, and Bairdiella chrysoura. Stomach contents of dolphins caught off South Africa were composed of at least 50 genera of fish and

at least three genera of cephalopods. The most important prey included fish: Trachurus delagoae, Pomadasys olivaceum, Pagellus bellotti, and Scomber japonicus, and the cephalopods Sepia officinalis and Loligo sp. This extensive variety of prey inhabits an equally diverse selection of habitats, and includes benthic-reef and sandy-bottom prey and their associated predators, pelagic schooling fish and cephalopods, and deeper-water fish (Wells and Scott, 1999 and refs. therein).

Off Peru, both coastal and offshore dolphins consumed Pacific sardines, anchoveta, and hake, but demersal species such as sciaenids and toadfish were found only in coastal dolphins. By contrast, the offshore animals were the only ones with mesopelagic fishes and squids in their stomachs (Wells and Scott, 1999 and refs. therein).

The stomachs of bottlenose dolphins stranded on the Mediterranean coast of Spain contained mainly cephalopods and fish , hake (Merlucciusa merluccius) being the most important single prey species. Based on stomach contents, eeding habits were considered to be mostly demersal (Blanco et al. 2001).

Although individual feeding is perhaps most prevalent, co-operative herding of schools of prey fish has been reported from a number of regions. In Mauritania and Brazil, dolphins regularly drive schools of mullet towards fishermen wading with nets in shallow water, and in other regions they have been observed feeding behind shrimp trawls and in the vicinity of small purse seines, collecting discarded fish from these operations after the nets are retrieved, and stealing fish from a variety of fishing gear (Wells and Scott, 1999 and refs. therein).

Schooling: Group size Is commonly less than 2010, but large herds of several hundred to a thousand are regularly seen offshore (Bloch, 1998; Wells and Scott 2002). Bottlenose dolphins are commonly associated with other cetaceans, such as pilot whales, white-sided, spotted, rough-toothed and Risso's dolphins, and humpback whales, and hybrids with other species are known from both captivity and in the wild (Jefferson et al. 1993; Bloch, 1998; Wells and Scott, 1999).

Reproduction: Spring and summer or spring and autumn calving peaks are known for most populations (Jefferson et al. 1993; Wells and Scott 2002).

Migration

According to Wells and Scott (1999, and refs. Therein; 2002), little is known about the ranging patterns of pelagic bottlenose dolphins, but coastal dolphins exhibit a full spectrum of movements, including 1) seasonal migrations, 2) year-round home ranges, 3) periodic residency, and 4) a combination of occasional long range movements and repeated local residency. Long term residency may take the form of a relatively permanent home range, or repeated occurrence in a given area over many years. For example, the residents of several dolphin communities along Florida's west coast have maintained relatively stable home ranges during more than 28 years of observations. In other areas, residency is long term but more variable: Dolphins seen frequently during 1974 - 1976 in Golfo San Jose, Argentina, showed a subsequent decline in frequency of occurrence, but were still occasionally identified in the area 8 -12 years later.

Along the central west coast of Florida, communities of resident dolphins appear to inhabit a mosaic of overlapping home ranges. The home range of the Sarasota dolphins encompasses an area of about 125 km². Most of the activities of the residents are concentrated within the home ranges, but occasional movement between ranges occurs also. The same applies to bottlenose dolphins off San Luis Pass, Texas (Maze and Würsig, 1999). Within the home range, habitat use varies with season, with shallow estuarine waters frequented during the summer and coastal waters and passes between barrier islands used during the winter (Wells and Scott, 1999 and refs. therein). However, behaviour may also vary among animals within the same area: Simoes-Lopez and Fabian (1999) found that in Laguna, southern Brazil 88.5% of the individuals were resident and the rest were non-resident.

Dolphins living at the high latitude or cold water extremes of the species' range may migrate seasonally, as is the case along the Atlantic coast of the United States. It has been suggested that

some dolphins may use seasonal home ranges joined by a travelling range: a 4-month cycle of occurrence of dolphins was observed in Golfo San Jose, Argentina (Wells and Scott, 1999 and refs. therein). Wood (1998) investigated a group of bottlenose dolphins in the coastal waters of Cornwall, UK in 1991. The dolphins demonstrated a seasonal residency pattern, spending the winter in southern Cornwall and moving further north-eastward during spring and summer. Residency was flexible with a number of individual dolphins using the region intermittently. The dolphins occupied a linear coastal range of 650 km. Within this range they repeatedly made long- distance journeys. The longest journey recorded covered 1076 km and took 20 days.

Threats

Direct catch: Directed fisheries taking bottlenose dolphins have previously occurred around the Black Sea as well as in Mexico, Guatemala, Costa Rica, the West Indies, Venezuela, Sri Lanka, and off southern Africa, India and Peru. Drive fisheries for bottlenose and other dolphins were also reported from the People's Republic of China (Taiwan), but the numbers are not known. The species is taken in a drive fishery in the Faroe Islands which dates back to 1803, annual takes numbering from 1-308, often in mixed schools with long finned pilot whales (Globicephala melas) (Reyes, 1991 and refs. therein; Bloch, 1998).

In Peru, coastal fisheries still take Tursiops and other cetaceans for human consumption, using gill nets, purse seines, and harpoons and a similar fishery occurs in Sri Lanka (Wells and Scott 1999 and refs. Therein; Wells and Scott 2002). Although direct killing has noticeably decreased since dolphin hunting was banned by law in 1996, around a thousand dolphins and other small whales are still falling victim annually to fishermen to supply bait meat for the shark fishery (2003, see mundo azul in "links"). The most significant take probably occurs off Japan, where bottlenose dolphins are killed for human consumption, bait and because of perceived competition with fisheries (Wells and Scott 2002). Reported catches were: 230 in 1986; 1,813 in 1987 and 828 for 1988. (Reyes, 1991 and refs. therein).

Live captures: More than 530 Tursiops have been taken from US waters since the passage of the Marine Mammal Protection Act of 1972 (MMPA), particularly from the south-eastern USA. Present federal regulations limit the annual allowable take to less than 2% of the minimum estimated population in designated management areas, but no bottlenose dolphins have been collected in US waters since 1989. Some small scale live-capture fisheries continue in other countries (Wells and Scott, 1999 and refs. therein).

Incidental catch: Fisheries around the world account for incidental takes of bottlenose dolphins, but the present level of take remains unknown. Gillnet and purse - seine fisheries off Peru take an unknown number, but rough estimates are in the hundreds.

In the western Mediterranean incidental catches have been reported in trawl fisheries and in the driftnet swordfish fishery. Some tens are also taken in several other fisheries throughout the range.

Read et al. (2003) report that in North Caroliona interactions between dolphins and gill nets are common and that many of these interactions are food-based. Surprisingly, however, dolphins engaging in depredation do not appear to become entangled; instead it seems more likely entanglement occurs as a result of dolphins failing to change course around the net.

The estimated annual incidental mortality in the eastern tropical Pacific fishery for tuna ranges from 0 to almost 200, although it makes up only a very small fraction (less than 5%) of the total small cetacean mortality in the fishery (Reyes, 1991 and refs. therein).

Incidental catches in Chinese fisheries reach several hundred per year (Yang et al. 1999), and a large incidental take of Tursiops has apparently occurred in the Taiwanese gill net fishery off Australia, with an annual mortality perhaps exceeding 2000 animals.

The use of shark nets to protect bathing beaches in South Africa and Australia has caused mortality as well. Dolphins were found with full stomachs, indicating recent feeding in the vicinity of the nets and there was a correlation of mortality rates with the direction of the prevailing current. Attempts to prevent the animals from entangling by incorporating active and passive devices in the net were not successful. The relatively high incidental catches of coastal dolphins off South Africa has prompted concerns that the take is not sustainable (Wells and Scott, 1999 and refs. therein).

According to Northridge (2003) a high proportion of the common dolphins that strand on the south coast of England in winter months bear evidence of fishery interactions. It is not known which fisheries are involved, but the number of stranded by-caught dolphins has raised concerns for their conservation status. Observers have monitored 149 days at sea since 2000, and have recorded 61 common dolphins taken in trawl nets. All of these animals were recorded in trawl tows targeted at bass. Preliminary mitigation trials using pingers were not effective, with no reduction in dolphin catch rate when pingers were deployed around the mouth of the trawl. Current work is focussed on using exclusion grids to allow dolphins to escape from the sleeve of the trawl.

Killing: Tursiops have been intentionally killed by fishermen in Japan and Hawaii and presumably such practices are found elsewhere in their range (Reyes, 1991). The Japanese drive fishery off Iki Island and the Kii Peninsula takes several hundred Tursiops annually to reduce the perceived competition with the commercial fishery for yellowtail, Seriola sp (Wells and Scott, 1999 and refs. therein).

Pollution: Their worldwide distribution and great adaptability to diverse habitats make this species a good indicator of the quality of inshore marine ecosystems. There are reports of DDT, PCBs and heavy metals in bottlenose dolphins from the western Mediterranean, with higher levels of DDT and its metabolites (Reyes, 1991 and refs. therein).

Concentrations of polychlorinated biphenyls and dieldrin were measured in the blubber of South African specimens. First-born calves received 80% of their mother's body burden of contaminant residues, perhaps leading to increased neonatal mortality. Accumulation of contaminants in tissues of males reached levels that theoretically could impair testosterone production and thus reduce r eproductive ability (Wells and Scott, 1999, and refs. therein). Preliminary results of research by Lahvis et al. (1995) indicate that even relatively low levels of PCBs and DDT such as those found in the blood of Sarasota dolphins can result in a decline in immune system function.

Tourism: Excessive and unregulated visiting of wild dolphins habituated to humans has raised concern in several areas, in particular in Europe (Reyes, 1991 and refs. therein). Nowacek et al. (2001) conducted focal animal behavioral observations during opportunistic and experimental boat approaches involving 33 well-known identifiable individual bottlenose dolphins off Sarasota, Florida. Dolphins had longer interbreath intervals (IBI) during boat approaches compared to control periods (no boats within 100 m) and the duration was inversely correlated with distance to the nearest boat in opportunistic observations. Dolphins decreased interanimal distance, changed heading, and increased swimming speed significantly more often in response to an approaching vessel than during control periods. These findings provide additional support for the need to consider disturbance in management plans for cetacean conservation.

However, in Zanzibar waters, in the Western Indian Ocean, dolphin tours are organised from Kizimkazi, since local fishermen realised that their touristic value far exceeded that of using them as bait for sharks. As many as 2,000 tourists visit the dolphin site at Kizimkazi per month and dolphin-tourism is currently becoming a popular economic activity. Successful management of the dolphin-tourist trade will ensure continued visitors to the villages where dolphins are present and thus add income to these villages while contributing to management and conservation (Ali and Jiddawi, 1999).

Overfishing: Reduction of fish stocks by pollution or overfishing may affect dolphin populations such as those in the Black Sea, which has been severely depleted by intense hunting which continued until 1983 (Reyes, 1991 and refs. therein).