THE STATUS OF THE BOTTLENOSE DOLPHIN (TURSIOPS TRUNCATUS) POPULATION OF BOCAS DEL TORO, PANAMA: PRELIMINARY RESULTS BASED ON A THREE YEAR ONGOING STUDY

Abstract

The Bottlenose dolphin (Tursiops spp.) among the best known dolphin species worldwide, largely for being the ‘favorite’ species to have in aquaria (Defran and Pryor 1980). Bottlenose dolphins are easily identified by their robust body, curved dorsal fin, gray coloration, and short rostrum. Adults range from 2-3.8 m (mean weight of 242 kg) (Culik 2004). Two species are recognized, T. aduntus known as the Indo-Pacific Bottlenose dolphin, found in waters of Australia, the Indian Ocean, China, and South Africa; and T. truncatus known as the Bottlenose dolphin found in temperate and tropical waters around the world (Culik 2004, Natoli 2006). In some areas several well-defined coastal communities have been described (resident dolphins inhabit a mosaic of overlapping home ranges). Two ecotypes of T. truncatus are recognized: coastal and oceanic, that appear to be distinct in terms of morphology, prey choice, (Mead and Potter 1995) and possibly social structure. Within these communities resident and non-resident animals can be found. Resident animals can show patterns of site fidelity of up to 17 years (Rogers et al. 2004). Bottlenose dolphins live in social fission-fusion societies where individuals tend to associate in small groups and group composition varies throughout the day (Connor et al. 2000). The strongest social bonds are among adult males that tend to form complex hierarchical alliances which facilitate them to compete over estrus females (Connor et al. 1992, Connor et al. 2006). Females also form associations but they appear to be weaker and more variable. While some are solitary other can form associations of ‘moderate strength’ (Smolker et al. 1992). A wide variety of sounds are produce by these dolphins which have been categorized into (1) broad-band echolocation clicks used to monitor their environment, prey and predator detection, (2) broad-band burst-pulsed sounds and (3) tonal sounds (often referred to as ‘whistles’) which are used for communication (Cook et al. 2004). Caldwell and Caldwell (1965) discovered a special type of tonal sound in captive bottlenose dolphins, so called ‘signature whistles’. They noted that when captive bottlenose dolphins were separated during medical inspection, the animals emitted highly stereotyped whistles, unique for each individual. Based on these observations they proposed that signature whistles could serve for individual recognition and group cohesion. Since their work signature whistles have been documented in both wild and captive animals (Sayigh 2002, Janik and Slater 1997, Janik 2000). When all members of a group are together a variety of whistles are produced, but signature whistles are rarely used (Janik and Slater 1997). Recognition patterns are strongest between mothers and calves (Sayigh et al. 1998). When the calf voluntarily gets separated from its mother, it will emit signature whistles to facilitate a reunion. Newborn calves are capable of whistling within days of birth, but the signature whistle is not developed until the first to third month of life. Once the signature whistle is developed it remains stable for the rest of the dolphin’s life (Tyack 1999). Tyack and Sayigh (1997) proposed that the development of signature whistles had an important learning component. Several factors have been proposed to influence the acoustic structure of these signals: social structure (May-Collado et al. submitted, Podos et al. 2002), body size (May-Collado et al. in press, Matthews et al. 1999, Wang et al. 1995), the environment [ambient and anthropogenic noise] (Wang et al. 1995), and zoogeographical relationships (Steiner 1981). Because coastal bottlenose populations are often in contact with humans, their populations tend to be more susceptible to human activities such as whale-watching, pollution, boat traffic, fisheries, etc. (Culik 2004). In several countries around the world, including Costa Rica, Guatemala, Mexico, and Venezuela bottlenose dolphins have been directly caught (Culik 2004), often for bait for shark fishing. Incidental mortality in gillnets and purse-seine fisheries occurs but the magnitude is unknown. According to Reyes (1991) Bottlenose dolphin annual mortality in the eastern tropical Pacific appears to be a small fraction (<5%) of the total small cetacean mortality. Live catches also occur worldwide. Despite many international treaties and local protection through laws and regulations bottlenose dolphins are still caught from wild populations for aquaria exhibition. Many companies direct their efforts to Latin American countries where presumably there is little regulation (e.g. Guyana, Panama, Costa Rica), or limited enforcement of regulations.