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LESSON 6:
DOMINICA'S CETACEANS

Dominica's Cetaceans are a group of sea mammals which include whales, dolphins and porpoises. Currently, 78 species of cetaceans have been identified around the world. In the waters surrounding Dominica, 14 cetacean species have been sighted, ranging in size from the magestic sperm whale to the small, acrobatic spinner dolphin.

Why are so many cetacean species found in the waters near Dominica?

Dominica's near-shore waters provide a concentration of food for many of the cetaceans that visit the island, despite the fact that, in general, tropical surface waters are considered nutrient poor or oligotrophic. These low nutrient levels are brought about by the warming of the tropical ocean's surface layer (to 25-30° C) in the intense tropical sunlight. The warming makes the surface layer less dense, causing it to float on top of colder water below. This separation of the warm upper layer from the cold layer below, creates a layering or stratification of the water column, that in tropical waters, is generally permanent. The depth of the thermocline, or point of separation between the two layers, is around 50 m. Because the two layers are kept separated from each other by their density differences, nutrients and organic material which sink into deep water are not easily recycled back to surface. This creates a nutrient shortage for phytoplankton, which must remain in the nutrient-poor top layer to obtain sunlight for photosynthesis. With less available nutrients, the primary productivity of phytoplankton in tropical waters is less than 25% that of colder seas (where seasonal cooling of the top layer allows for mixing between layers, providing more nutrients for phytoplankton productivity and the rest of the food chain).

The variable, rugged, underwater terrain around steep-sided islands like Dominica, help churn the ocean waters that strike them, causing upwellings and the mixing of nutrient-rich deep waters with nutrient-poor surface waters. Productive coastal ecosystems, such as coral reefs and estuaries, also add nutrients to near-shore waters, all of which results in a higher densities of plankton, fish, squid (and in the case of Dominica — cetaceans) around steep-sided islands. The increased productivity of waters surrounding tropical oceanic islands, in comparison to the lower productivity of offshore tropical waters, is known as the island mass effect.

NATURAL HISTORIES OF THE 14 CETACEAN SPECIES SIGHTED IN DOMINICA:

1. Pygmy Sperm Whale

2. Dwarf Sperm Whale

3. Humpback Whale

4. False Killer Whale

5. Pygmy Killer Whale

6. Cuvier's Beaked Whale

7. Long-Snouted Spinner Dolphin

8. Frasier's Dolphin

9. Pantropical Spotted Dolphin

10. Rough-Toothed Dolphin

11. Bryde's Whale

12. Short-Finned Pilot Whale

13. Bottlenose Dolphin

14. Sperm Whale

Answer questions associated with observations of Saucon Valley students in the field.
All Dominica lessons and photography © 1999 Lance Leonhardt