Cetacean occurrence near an offshore oil platform in southern Brazil

Information about cetaceans in offshore Brazilian waters is scarce, and oil-rigs could provide an important opportunity to obtain new data. The present work was conducted on the P-XIV oil-rig (Petrobrás) (26o46’02.2”S; 46o47’02.15”W), located on the border of the continental slope, in an area of 200m depth. In the period between July 2000 and August 2002, 75 sightings of cetaceans were recorded during 38 days of effort. Among the species identified, Tursiops truncatus was the most common, corresponding to 53.3% of the records. Among Short Communication Biotemas, 22 (3): 247-251, setembro de 2009

the misticets, only Balaenoptera acutorostrata was identified with accuracy, with 4 records (5.3%).These were the only species that approached and stayed close to the oil-rig.Many records were made at night, when the gas burner illuminated the area around the oil-rig.We recorded an aggressive interaction involving T. truncatus and B. acutorostrata.Key words: Tursiops truncatus, Balaenoptera acutorostrata, oil platform, aggressive interaction Oil platforms are known to be structures that attract a large amount of fish in ocean waters (Hostim-Silva et al., 2002).Thus, cetaceans can occupy areas around these structures in order to exploit the local availability of food resources.Information about the cetaceans that live in offshore Brazilian areas is scarce and limited to few species, mainly because of the operational limitations of this kind of research (Cremer and Simões-Lopes, 1997;Fertl et al., 2003;Moreno et al., 2005;Zerbini et al., 2006;Dalla-Rosa and Secchi, 2007).Observations of cetaceans from oil platforms provide a great opportunity to develop studies concerning the occurrence, diversity and seasonality of species in this area and even their behavior.
The study site was an area of around 3 km from the P-XIV oil platform (Petrobrás), with 200m in depth, on the border of the continental slope (26 o 46'02.2"S;46 o 47'02.15"W),southern Brazil (Figure 1).Day surveys were conducted from the platform during all daylight hours (6:00 am to 6:00 pm) from the bow or the stern, with the presence of one or two researchers.Night surveys were conducted only near the gas burner that was turned towards W or E, depending on the wind direction, from 6:00 pm to 6:00 am.The burner's pipes were often cooled with salt water that dropped onto the surface of the sea from 25m above, like a warm shower.The burner provided an intense pool of light around it, attracting large amounts of organisms, especially fish, and allowing cetacean sightings at night.To avoid the fatigue of the researchers, observation efforts of less than 1:30h were made, with intervals of half an hour.Under adverse weather conditions (sea state > 4 on the Beaufort scale) the effort was interrupted.Upon sighting a cetacean, the species was identified and the number of individuals was recorded, in addition to information related to its behavior.
During the study (July 2000-August 2002), 38 days of observation were conducted, totalling approximately 318 hours of effort.The samplings were conducted during 10 months, but were not systematized because of logistic limitations.Each month, samplings were made on 3 to 6 days.In May, June, September and November, no samplings were conducted.A total of 75 records of cetaceans were made, including 40 (53.3%) of the bottlenose dolphin, Tursiops truncatus, and 4 (5.3%) of the dwarf minke whale, Balaenoptera acutorostrata (Table 1).These were the only species that approached the oil platform, remaining near it.Many species were not identified because of their distance from the platform, and constituted mere transitory sightings.
In the Gulf of Mexico, T. truncatus seems to be the most common species occurring near oil platforms (Jefferson and Schiro, 2008).In general, there is little published data on the marine mammals near these structures, nor is there much data on underwater noise levels produced by the platforms and their effects on marine mammals (Richardson et al., 1995).Twenty-two sightings of T. truncatus, corresponding to 55% of total sightings, occurred at night.In this period, group size ranged from 1 to 7 dolphins.During the daytime sessions, 18 sightings (45%) were made, with group sizes ranging from 3 to 40 dolphins (Table 2).At night, the dolphins stayed for longer periods near the platform, under the light of the gas burner, remaining more than four hours (during the daytime, they remained for only around one hour).The large amount of fish attracted by the burner light probably attracted the dolphins that spent most of the time foraging on the surface.On a few occasions, dolphins were seen catching flying fish (Exocoetidae) slightly above the surface.Tursiops truncatus is a common species on the central and southern Brazilian coast, including some estuaries and rivers (Pinedo et al., 1992).In offshore waters, the occurrence of the species has only been registered opportunistically(e.g.Zerbini et al., 1998).During our daytime observations, it was possible to observe that all T. truncatus individuals showed an intense despigmentation in the caudal peduncle area (Figure 2).Possibly, this could be related to a population pattern of offshore bottlenose dolphins in the South Atlantic.The existence of offshore and coastal types of this species has been identified by certain researchers in other areas based on a number of biological parameters (Ross, 1977;Walker, 1981;Waerebeek et al., 1990).In Brazil, the existence of an offshore population was first proposed by Simões-Lopes (1996), considering color patterns.Barreto (2000) proposed species variations considering only the analysis of coastal populations.For the dwarf minke whale, 4 sightings were made, two of which were of lone animals (August and January), occurring during the night period.During the day, a group with two adult animals and a mother-calf pair, both in October, were recorded.Minke whales have a tendency to approach ships (Leatherwood et al., 1976), and this could explain the species' behavior toward the platform.The period that the individuals remained near the platform varied between 30min and 3h.The species is distributed along the eastern coast of South America (5º -56º S), and has a seasonal occurrence on the coast of Brazil with peaks in winter and spring (Zerbini et al., 1997).Sightings made in October and January are probably related to southward movement, from breeding areas to feeding areas, and the record made in August could be related to the opposite movement (Kasamatsu et al., 1995).Northern Brazil (7º) is a breeding area for the species in the South Atlantic (Zerbini et al., 1997).Little is known about the migration routes of B. acutorostrata.Minke whales are more common in coastal and inshore areas, but they can be seen offshore (Nowak, 1999;Jefferson et al., 2008).Pinedo et al. (2002) observed B. acutorostrata on two occasions on the Rio Grande do Sul state coast during a cruise (survey) in the spring (November-December).Records of B. acutorostrata carcasses in Santa Catarina state were made along the coast (Cherem et al., 2004).
In August, in the night period, an interaction involving T. truncatus and B. acutorostrata was observed.At 8:36 pm a group with five T. truncatus adults came near the gas burner, remaining in the area and performing an intense feeding activity.They were observed with a fish in their mouth on 10 occasions.At 9:00 pm, a dwarf minke whale came into the area near the gas burner, after the group of dolphins had left.Feeding and resting behaviors were observed and five breathing intervals ranging from 1min and 11sec to 2min and 32sec were also recorded.A few moments later, the group of dolphins came closer again, exhibiting aggressive behavior towards the whale, such as tail slaps and head strikes, as described by Shane (1990), Simões-Lopes (1998) and Wedekin et al. ( 2004) for the species.The dolphins constantly chased the whale and threatened it with physical violence, apparently changing its route to oblige it to leave the area.The interaction lasted about 3 hours and was videotaped.At 00:45 am, the whale left the area in a NW direction after many attacks by the group of dolphins.It is possible that the dolphins were defending a known place with easily exploitable resources, showing some territoriality.It might be that a part of the T. truncatus population had become specialized to exploit food resources associated with the oil platform.The species is known to develop specialized feeding strategies, as described by Shane (1990) and Simões-Lopes et al. (1998), for example.Inter-specific interactions involving aggressive behavior on the part of bottlenose dolphins have been recorded in connection with other cetaceans such as Stenella frontalis (Herzing and Johnson, 1997), Phocoena phocoena (Ross and Wilson, 1996;Patterson et al., 1998) and Sotalia guianensis (Wedekin et al., 2004).In these situations, it was not exactly clear why attacks took place, but they could be related to practices of infanticide or to food competition.This is the first record concerning interactions between T. truncatus and B. acutorostrata.These species are not considered as direct competitors, given the fact that their strategies for obtaining food are different.However, the oil platforms could be used opportunistically by both species as an area of prey concentration in the offshore habitat.

TABLE 1 :
Cetacean sightings made between July 2000 and August 2002 from P-XIV oil platform (Petrobrás), southern Brazil.

TABLE 2 :
Group size and number of sightings of Tursiops truncatus made between July 2000 and August 2002 from P-XIV oil platform (Petrobrás), southern Brazil.