818 I. C. Avila et al.
PLATE 1 Some of the behaviours of sperm whales Physeter macrocephalus that we recorded in the Colombian Caribbean: (a) two adults swimming slowly (photo: Nohelia Farías-Curtidor); (b) an adult resting (photo: Javier Alarcón); (c–d) a juvenile breaching (photo: Nohelia Farías-Curtidor).
After data cleaning, 61 occurrences of spermwhale groups
were available forMaxentmodelling, butwith differing num- bers of records at different depth levels (Table 2). Allmodels generated had an AUC .0.77 and true skill statistic .0.47, indicating a good performance in
general.However, whenwe considered the optimal threshold, models of the surface and Level 1 failed to assign presence to 59.3%and 17.2% of real spermwhales occurrences used as training data, respectively, whereas the other models failed to assign,12%. The result- ing maps indicated that the probability of occurrence of sperm whales differed between the surface model and the models for Levels 1–5 (Supplementary Figs 4 & 5). The Level 3 model, at c. 1,000m depth, best represented the potential distribution of sperm whales in the Colombian Caribbean,with the highestAUC(0.84) and true skill statistic (0.55), and assigned presence to .88% of real sperm whale occurrences (Table 3). The map resulting from this model indicated there is high probability of occurrence of sperm whales in the south and north-east Colombian Caribbean over the shelf break to waters up to c. 3,000m deep, and near the Archipelago of San Andrés, Old Providence and Saint Catherine in the north-west (Fig. 3). The area of high probability of sperm whale occurrence is characterized by a distance to shore of 9.8–220.5 km (median 106.9km) and an oceanmixed layer thickness of 5.2–50.3m(median 29.7 m). The area of occurrence resulting from the model for Level 3 (Fig. 3) had similar features to the real occurrence data (Supplementary Table 5).
Discussion
This study provides the first assessment of the occurrence of sperm whales in the Colombian Caribbean, which appears to be an important habitat for this species. The mean en- counter rates of 0.8 individuals and 0.4 groups per 100 h are similar to rates reported in the Gulf of Mexico (1.3 indi- viduals and 0.6 groups per 100 h), where the sperm whale is
considered the most abundant large cetacean (Barkaszi et al., 2012). The sperm whale density we recorded in the Colombian Caribbean (1.42 individuals per 1,000 km2)is similar to that reported worldwide (1.4 individuals per 1,000 km2;Whitehead, 2002) and to that of other American trop- ical regions of the Pacific Ocean, but lower than reported for the Colombian Pacific (3.8 individuals per 1,000 km2; Gerrodette & Palacios, 1996; Supplementary Table 6). Previous studies in the Caribbean had reported lower en- counter rates for sperm whales (e.g. 0.35 individuals per 1,000 km2; Mullin & Fulling, 2004). However, most of these data were from coastal research platforms. Our find- ings highlight the value of marine mammal occurrence data obtained during seismic surveys, which cover offshore areas that researchers may not usually be able to survey. Whitehead et al. (2012) found that sperm whale social
units in the North Atlantic are based around 6–12 often matrilineally related individuals that move together, raise their calves communally, and probably share important knowledge among themselves. Our findings for the Colombian Caribbean are similar, with groups of up to 10 individuals, and similar to the Gulf of Mexico where groups have up to 16 individuals (Barkaszi et al., 2012). Given our documentation of juveniles and the stranding of a juvenile in 2009 in the Urabá Gulf on the Colombian Caribbean coast (Trujillo et al., 2013), it is possible that the waters of the Colombian Caribbean are a breeding area for Atlantic sperm whales. Further studies are required to examine this possibility. Genetic and photo-identification studies are also required, to assess whether sperm whales sighted in the Colombian Caribbean belong to either of the better-known populations of the eastern Caribbean (Gordon et al., 1998; Gero et al., 2007) or the Gulf of Mexico (Weller et al., 2000), or whether the Colombian Caribbean is an area of connectivity between the eastern and western Atlantic po- pulations. It has been proposed that the eastern Caribbean is a sink with favourable conditions for sperm whales (Whitehead & Gero, 2015), but current threats related to
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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