By Isabel C. Avila, 20th April 2022
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Sperm whales Physeter macrocephalus are large marine mammals, characterized by a pseudo-torpedo shaped body and a large forehead. They are the largest toothed predator, can dive to depths of over 3,000 m and feed primarily on cephalopods (squids and octopuses). Sperm whales are distributed worldwide, usually in waters over 1,000 m deep, but distribution patterns vary between sexes. Whereas females and young males live together in groups in temperate and tropical habitats, adult males lead solitary lives in polar habitats, migrating periodically to mate.
Sperm whales were hunted from the late 1700s until the 1980s. The hunting of a mythical albino sperm whale was made famous in Herman Melville’s novel Moby Dick, supposedly based on a real sperm whale called Mocha Dick, from Chilean waters. These whales were targeted mainly for oil and ambergris, but also for meat. Thousands of them were commercially hunted, until the International Whaling Commission moratorium came into force in the 1980s, granting them protection. Although whaling is no longer a major threat, sperm whales are facing new anthropogenic threats from incidental catch, interactions with fishing gear, collisions with boats, and pollution. The global population trend of the species is unknown, but researchers have documented a decline in some subpopulations, including those in the eastern Caribbean and the Mediterranean. Currently sperm whales remain protected by the moratorium, and are categorized as Vulnerable on the IUCN Red List.
Sperm whales have been studied in the Caribbean waters of The Bahamas and Dominica since the 1990s. Research has suggested that the eastern Caribbean may be an ecological trap for the species. However, little is known about the sperm whales in this area. The Colombian Caribbean comprises c. 30% of the Caribbean Sea and is characterized by a mean depth of 2,700 m and a maximum of 4,500 m. Twenty-nine species of marine mammals have been reported in this region, including sperm whales. These reports correspond to c. 23% of the global marine mammal diversity. This region has an important commercial and tourism potential but the basic ecological information necessary to support management decisions is scarce.
Given the importance of the eastern Caribbean region as sperm whale habitat, it is vital to monitor Caribbean Colombian waters. Our study was the first to do this. We estimated sperm whale density, recorded their behaviour, and identified areas that may be important for the species. We identified the potential distribution of sperm whales in this using sighting and acoustic data obtained during our surveys, published information, and opportunistic encounters during 1988–2020. We conducted observations during surveys on seismic vessels over an area 68,905 km2 and 703 days of observation effort during 2011–2016. We recorded 98 individuals in a total of 50 groups, a density of 1.42 individuals per 1,000 km2, which is similar to that reported worldwide (1.4 individuals per 1,000 km2). Sperm whales exhibited slow and fast swimming, exposure of pectoral and caudal fins, resting, spyhopping (putting the head out of water and looking around) and breaching behaviours.
To determine the potential distribution of the species, we built models with uncorrelated environmental variables at five depths from the surface to c. 2,000 m. The model for 1,000 m depth best described the species’ distribution, whereas the surface model was relatively poor, suggesting that the analysis of sea surface conditions alone is insufficient to describe the distribution of sperm whales. This is not unexpected, as the sperm whale dives deeply to feed. Of the environmental variables that influenced the sperm whale’s distribution, the most important were distance to shore, and ocean mixed layer thickness (a layer with a homogeneous density, temperature and salinity), which plays an important role in phytoplankton and food chain dynamics.
We identified that areas of high probability of occurrence of sperm whales are in the south and north-east Colombian Caribbean over the shelf break to waters of up to 3,000 m deep, and near the Archipelago of San Andres, Old Providence and Saint Catherine. These areas may provide an important tropical habitat for sperm whales in which they can socialize, rest, breed and feed. Our study underlines the importance of monitoring marine mammals offshore and describes the potential distribution of sperm whales in the Colombian Caribbean, supporting conservation actions there for this Vulnerable species.
The article The Colombian Caribbean Sea: a tropical habitat for the Vulnerable sperm whale Physeter macrocephalus? is available open access in Oryx—The International Journal of Conservation.