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Preliminary observations on the spatial distribution of humpback whales off the north coast of Margarita Island, Venezuela—south-east Caribbean

Noemí Silva, Romina Acevedo and Lenin Oviedo*

Proyecto Golfo de la Ballena, Centro de Investigación y Conservación de la Biodiversidad Tropical, BIOTRÓPICA, Caracas 1001, Venezuela. *Corresponding author, e-mail: leninovi1@gmail.com

Megaptera novaeangliae sightings were gathered in the 2002 winter season, between February and April throughout several seismic survey cruises in a study area located north of Margarita Island. The data were processed by GIS (MapInfo 4.5) considering environmental features as underwater topography and bathymetry. The spatial and temporal distribution of M. novaeangliae in the study area suggested a significant concentration of whales within the Margarita – Los Testigos submarine platform region in a transition zone along the shelf edge between the north coast of Margarita Island and la Blanquilla Island. This apparent wintering habitat includes protected calving areas such as the Los Frailes Archipelago and Margarita Island itself. The seasonal presence of this species in the territorial waters of the north-east coast of Venezuela indicates the biological value of these animals, as well as the vulnerability of the whales that dwell in this area to oil and gas exploration and development activities.

INTRODUCTIONThe distribution and migration patterns of humpback whales have been documented through the

comparison of photographs, songs and genetic samples between different geographical areas where these whales occur. The abundance of the North Atlantic humpback whale stock is estimated at 10,600 whales (Smith et al., 1999). This stock has two major wintering areas: off Cabo Verde Islands and western Africa for the eastern North Atlantic stock, and the Caribbean basin for the western North Atlantic (WNA) stock. Aggregations of whales occur each winter in the West Indies at Silver Bank, Navidad Bank, Mouchoir Bank and other areas within the northern Caribbean. These areas represent sites where a large proportion of the WNA stock congregates each winter (Whitehead & Moore, 1982). Smaller groups occur around Samana Bay (Mattila et al., 1994), in the Virgin Bank, and within Mona Passage (Mattila & Clapham, 1989; Swartz et al., 2002). The Lesser Antilles and the south-east Caribbean is referred as a minor wintering area (Winn et al., 1975; Swartz et al., 2003). Although historical records show extensive whaling activity in this area during the 19th Century, the south-eastern region of the Caribbean basin is apparently utilized less extensively by humpback whales than the northern portions of the Caribbean, suggesting a shift to the Greater Antilles as their winter habitat (Reeves et al., 2001).

While humpack whales are included in the cetacean fauna of Venezuela, Linares (1998) did not mention Megaptera novaeangliae (Borowski, 1781) among the confirmed cetacean species based on scientific collections in museums. Naveira (1996) reported nine humpback sightings and the first stranding of the species in the north-east coast of the country. Acevedo (2001) and Romero et al. (2001) described cetacean occurrence and distribution patterns along Venezuela’s coast, and both noted humpback whales occurred in territorial waters. Acevedo (2001) considered the possible oc-currences of both the WNA population and the western South Atlantic one based on recent sight-ing records, and provided a detailed summary of all the environmental features that could support a wintering habitat for the species. Visual and acoustic surveys in Venezuelan waters were conducted by Swartz et al. (2003) from the north-east coast to the central coast up to Las Aves Archipelago. Singing male humpback whales (N=14) were detected along their entire survey track in Venezuelan waters, but they sighted only a single mother and calf plus a companion off the north-east coast of Margarita Island. The authors pointed out that the detection of singing whales and a mother and calf sighting suggested that the humpbacks continue to utilize the Caribbean coast of Venezuela and the Lesser Antilles as a nursery, mating and calving ground. They also noted the absence of humpback whales in the Gulf of Paria, where this species was frequently found in the 1800s, highlighting the historical importance of this locality.

MATERIALS AND METHODSStudy area

Surveys were conducted at the shelf edge on the north coast of Margarita Island and La Blanquilla Island (approximate area 6752 km2). This location is a transition zone between the neritic and pelag-ic habitats, and it is included into the topographical unit Margarita – Los Testigos submarine platform (Llano et al., 1999). This habitat supports pelagic cetacean particularly teuthivorous species.

N. Silva et al. Humpback whales off Margarita Island, Venezuela

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Cetacean surveyOpportunistic sightings of hump-

back whales were obtained during a cetacean survey and seismic study con-ducted during 9 February to 25 March 2002, around an area of 1800 km2. The surveys were conducted by cetacean observers aboard the RV ‘Gulf Service’ and RV ‘Western Patriot’. Daily ob-servation periods averaged six hours. Observers used 7x50 binoculars, and photographs were taken with a Canon Eos-3000N camera with a 35 to 80 mm zoom telephoto lens. Sighting records were plotted using a geographic infor-mation system (GIS), software package (mapinfo 4.5) which provided details of underwater topog-raphy and bathymetry features. Records of surface temperature were obtained from satellite images and compared with the locations of whale sightings.

RESULTSSix sightings of humpback whales were obtained along with two unidentified whale sightings (Ta-

ble 1; Figure 1). The first record on 9 February were two whales at 19.7 km south of the 200 m iso-bath, then on 1 March three more whales were seen at 2 km south of the shelf edge in front of the north-west side of Margarita Island. Another whale was seen breaching on 2 March at 9.2 km south of the same location. A similar sighting was recorded with the same behaviour on 7 March at 9 km south of the shelf edge. Three days later on 10 March another single sighting of a breaching whale

Latitude/Longitude DateNumber of whales

Distance (km) to 200 m

Isobath

Distance (NM) to 200 m

IsobathDepth

(m)

11°14'N 64°23'W 09 February 2002 2 19.7 14.44 38

11°13'N 64°30'W 01 March 2002 3 2.1 1.29 91

11°12'N 64°40'W 02 March 2002 1 9.2 4.75 73

11°19'N 64°19'W 07 March 2002 1 9 4.66 43

11°30'N 63°57'W 10 March 2002 1 7.4 3.71 64

11°20'02"N 63°48'50"W 15 March 2002 3 3 1.38 82

Table 1. Humpback whale sightings at the shelf edge on the north coast of Margarita Island.

Figure 1. Sighting distribution of Megaptera novaeangliae (filled circles) at the north coast of Margarita Island along the shelf edge.

Humpback whales off Margarita Island, Venezuela N. Silva et al.

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was registered, this time at 7 km south of the 200 m isobath, but in front of the north-east coast of Margarita Island. The last confirmed humpback whale sightings were three whales breaching on 15 March at just 3 km south of the shelf edge, also by the north-east coast of Margarita and Los Frailes Archipelago. Two additional sightings of breaching whales were recorded on 8 March north of the shelf edge towards La Blaquilla Island (11°31'07"N 63°50'07"W and 11°29'05"N 63°50'07"W). The unidentified whales were very likely humpbacks considering their behaviour. The mean distance of the whales to the shelf edge was 8.40 km (5.17 nautical miles), at a mean depth and temperature of 65 m and 25°C, respectively.

DISCUSSIONThese sightings were located in a transition zone between the neritic habitat and the pelagic zone,

along a track not covered by the RV ‘Gordon Gunter’ cruise in 2000, which only reported a single sighting for the area (Swartz et al., 2003). The presence of humpback whales at the north coast of Margarita Island concurs with humpback whale presence observed elsewhere in major winter-ing areas with similar topography and bathymetric features, which are characteristic of humpback calving and mating areas (Whitehead & Moore, 1982; Mattila & Clapham, 1989; Mattila et al., 1994). Concentrations of humpbacks are commonly found in winter along the shelf edges but within shal-low water submarine banks. The sightings obtained in this study near the submarine platforms of Margarita–Los Testigos are consistent with sightings reported from Silver and Navidad Banks by Whitehead & Moore (1982), where the average depth of the sighting locations ranged between 15 and 60 m, while in the study area the mean depth was 65 m. Frankel et al. (1995) considered that social factors (e.g. calving females) could influence the distribution of whales along with oceano-graphic features.

Previous studies of humpback whales in the northern Caribbean wintering ground note the importance of calm waters for calving whales (Whitehead & Moore, 1982; Mattila & Clapham, 1989; Mattila et al., 1994; Mignucci-Giannoni, 1998). Around Virgin Bank, mother and calves seemed closely associated with the bank’s archipelago due to the protecting effect against the prevailing winds (Mattila & Claphan, 1989). Similar results have been shown in Margarita Island and Los Frailes archipelago. The spatial and temporal distribution of humpback whale sightings in this investigation suggests that the portion of Margarita and Los Testigos islands’ submarine platform at the north coast of Margarita Island is also a wintering area. There is a need for additional research efforts to provide a better understanding of the seasonal abundance of humpback whales and their contin-ued use of this historical major wintering ground. It is significant that this area is currently under consideration for oil and gas industry exploration and exploitation activities, and that it represents a key area for other cetacean species reported in this locality, such as: Stenella frontalis, Stenella attenuata, Tursiops truncatus, Delphinus sp., Ziphius cavirostris, Globicephala macrorhynchus, Grampus griseus, Physeter macrocephalus and Balaenoptera edeni (Acevedo, 2001).

The authors would like to thank the crews of RV ‘Gulf Service’ and RV ‘Western Patriot’, for their valuable cooperation and kindness. Dr Steven Swartz and Cindy Renkas kindly reviewed the manuscript providing help-ful comments.

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Submitted 22 August 2005. Accepted 4 September 2006.