Caribbean sperm whales 815
of the species in Colombia despite its occurrence in both Pacific and Caribbean waters (Trujillo et al., 2013). The Colombian Caribbean region is important for cruise
tourism(Aguilera et al., 2006) and fishing activities (Suárez& Rehder, 2009), but there is little information on the ecology of this region to support management decisions. Studies of the occurrence ofmarine mammals in the Colombian Caribbean Sea have been limited to coastal areas, with few studies off- shore (e.g. Pardo et al., 2009; Farías-Curtidor et al., 2017). Information about sperm whales in Colombian waters is scarce, and the few studies on sperm whales in this region have been in The Bahamas (Ward et al., 2012) and close to Dominica and surrounding islands (e.g. Gordon et al., 1998; Gero et al., 2007, 2014;Gero&Whitehead, 2016),where small aggregations of up to 14 adult females and subadults of unknown sex have been reported (Ward et al., 2012;Gero et al., 2014). It has been suggested that the eastern Caribbean is an ecological trap for the species (Whitehead & Gero, 2015). Given the importance of the eastern Caribbean region for sperm whales and the documented population decline there, which potentially suggests migration to surrounding areas with better conditions (Whitehead & Gero, 2015; Gero &Whitehead, 2016), such as The Bahamas, it is important to determine the environmental conditions that affect sperm whale distribution in other areas in the Caribbean, such as Colombian waters. Here we report our research on the behaviour, encounter
rate, group structure and density of sperm whales in the Colombian Caribbean through the compilation and analysis of data from offshore surveys. To describe the potential dis- tribution of the species in this region, we investigate how environmental conditions at various depths influences its distribution. We identify areas of the Colombian Caribbean where sperm whales are present, and demonstrate the im- portance of offshore monitoring of marine mammals to generate data for management plans in this region.
Study area
The study area is the Colombian Caribbean, which com- prises a total area of 132,288 km2 (Fig. 1). This region is char- acterized by a mean depth of 2,700 m, with a maximum of 4,500 m, and a wide continental shelf (70–150 km) that ex- tends to 200m depth (Tabares et al., 1996). Temperature at the surface is 27–29 °C, and varies from 18 °C at 200m to 6 °C at 800 m; salinity is 35–37 ppt (Andrade et al., 2015).
Methods
Surveys During February–November in 2011 and 2013–2016 we recorded occurrences of sperm whales and collected
environmental data in the Colombian Exclusive Economic Zone in the Caribbean Sea from aboard seismic vessels in an area being explored for oil and gas (Supplementary Fig. 1). Visual surveys were during 06.00–18.30 by two biol- ogists trained in observation of marine fauna. Observations, using 10 × 50 binoculars, were made from the highest plat- form of five survey vessels (Veritas Viking, Osprey Explorer, Polar Duke, Oceanic Sirius and Oceanic Vega), with amean observation height of 18mand at amean speed of 4.2 knots. When possible, observed sperm whales were photographed. Sperm whales were located via their lateral blowing out and by their dorsal fins, tails or body (Farías-Curtidor et al., 2020). For each sighting, date, time, location (with a GPS), number of individuals, presence of any juveniles, follow-up time, depth of bottom, sea conditions (Beaufort), cloudiness and visibility were recorded. Behaviour was recorded using ad libitum sampling. Vessels halted any seismic activity when a marine mammal was close to the vessel (,500 m), to avoid or mitigate any potential negative impact of this ac- tivity on them (JNCC, 2017), and thereforewe did not evalu- ate the behaviour of whales in relation to seismic activity. For seismic surveys during 2013–2016, acoustic data were
collected by a passive acoustic monitoring operator during 18.31–05.59). Sperm whales were detected acoustically from their wideband clicks, which can be distinguished from other marine sounds (Mellinger et al., 2003).
Sperm whale occurrence and density data analyses
Encounter rate and group size statistics were estimated for sighting surveys, the former as the number of individuals and groups sighted per 100 h of observation effort and the latter as the number of individuals and groups observed per 1,000 km2.
Modelling sperm whale distribution
We used the maximum entropy algorithm, in Maxent (Phillips et al., 2017), to model the potential distribution of sperm whales in the Colombian Caribbean Exclusive Economic Zone using our observations combined with pub- lished data. Maxent estimates the geographical range of a species by finding the distribution that has the maximum entropy constrained by the environmental conditions re- corded at occurrence locations. Models were performed using the maxnet function in Maxent with a complementary log–log transformation, which appears to be most appropri- ate for estimating probability of presence (Phillips & Dudík, 2008; Phillips et al., 2017). As our data were not collected along survey track lines,
real absences were not available, and therefore to represent pseudo-absences we randomly selected locations lacking presence data (Phillips & Dudík, 2008; Merow et al., 2013).
Oryx, 2022, 56(6), 814–824 © The Author(s), 2022. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605321001113
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