Marine News 1 - Marine Mega-fauna

Denis is the second most northerly island in the Seychelles archipelago and lies only a few miles from the edge of the Mahe plateau where the seabed drops from some 50 metres rapidly down into the dark oceanic depths. This means that Denis is often surrounded by upwellings of cold nutrient-rich water from off the plateau. This in turn leads to algal blooms, followed by plankton through a procession of life up through the marine food chain. In the 1820s and 30s whaling barks from the United States plied their trade in the waters north of the Seychelles bank hunting sperm whales and they would use Denis Island on occasion for re-victualling.
Denis itself sits on a shallow shoal which extends for an area some 10-12 times the size of the island to West and North West this shoal and its shallow habitats also serve to attract marine life such as dolphins.
With these possibilities in mind Denis Island and GIF teamed up with the Marine Conservation Society, Seychelles (MCSS) and a Masters student from the University of Bremen, Yvonne Dartsch, to undertake a preliminary survey of the occurrence of whales and dolphins around Denis Island.
Yvonne’s study looked at species occurrence relative to water depth, weather patterns etc… and also combined different monitoring methods such as visual sightings and sonar recordings.
Yvonne is still writing up her thesis but I do have some preliminary results on her sightings records that she left with me on her departure. During some 400 hours of observations Yvonne and her assistants, including Shannon Switzer who we have to thank for the accompanying photos, sighted 74 groups of dolphins and whales, including:
· 55 groups of Bottlenose dolphins
· 13 groups of Spinner dolphins
· 1 group of Risso’s dolphins, and
· 2 groups of False Killer whales.

These species were found in water depths from 7- 1000 metres but most frequently on the plateau in depths between 20 and 40 metres only. Group sizes varied between 1 – 200 individuals, with Yvonne stating that finding herself amongst a pod of some 200 or so False Killer whales was a truly extraordinary experience.

Sightings of Bottlenose dolphins were particularly common giving the impression that a pod may be resident around the island. Also of interest were the frequent sightings of Manta rays which when coupled with turtles etc… give broad appeal to divers visiting Denis.