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Unraveling the secrets of marine mammals

The first systematic survey information along the entire northern mid-Atlantic Ridge (MAR) on cetacean populations was conducted during the multidisciplinary survey with the Norwegian research vessel G.O.Sars in June 2004. The fruits from detailed planning, collection of sighting data in all kinds of weather and careful analyses and writing have now resulted in a scientific publication in the Journal of Cetacean Research and Management. This has truly been an international effort including authors from four different countries.

 Bow riding whales
Using naked eye or hand-held binoculars, we searched in a 140° arc centred along the ships’ heading. Eleven cetacean species and 10 other taxonomic groups were identified along 2321 km of transect effort. All observations were done in a rather remote area of the Northeast Atlantic Ocean, with limited scientific data previously available on marine mammals.

The sei whale (Balenoptera borealis) and sperm whale (Physeter macrocephalus) were the most commonly sighted species, with 53 and 48 sightings, respectively. Spatial distribution varied north to south, and the highest aggregations of baleen whales were sighted at the Charlie Gibbs Fracture Zone (CGFZ).  Fin whales (B. physalus) were sighted 12 times including 20 animals. There were 26, 13 and 12 sightings, respectively of common dolphin (Delphinus delphis), pilot whale (Globicephala sp.) and striped dolphin (Stenella coeruleoable).

Two sightings of single blue whales were made just south of the Faraday Fracture Zone (FFZ) and one sighting of one blue whale about 100 km further south. The Faraday region was characterized by krill patches and fin whales were seen actively feeding on these patches.

marine mammal observers

The survey identified a foraging hotspot for sei whales around the Charlie Gibbs Fracture Zone. A total number of 85 individuals were sighted, and most groups contained 2-5 animals, although aggregations of up to 10 animals were observed. Sei whales were most common over the slopes of seamounts and rises in waters with depth between 1500 m and 3000 m. Calanoids were abundant in the CGFZ, implying that this deep water canyon is an important feeding ground for sei whales in summer.

The spatial distribution of 83 sperm whales also suggests that the MAR is an important oceanic habitat for this largest toothed whale on our blue planet. Group sizes varied from 1-15 animals. They were common in waters shallower than 2000 m. Sightings of sperm whales and catches of the squid (Gonatus spp.) co-occurred in the northern part of the MAR. Gonatus spp. is an important prey organism for sperm whales in northern Atlantic waters.

Publication title: Distribution and density estimates of cetaceans along the mid-Atlantic Ridge during summer 2004.

Authors: Gordon T. Waring (USA), Leif Nøttestad (Norway), Erik Olsen (Norway), Henrik Skov (Denmark), Gisli Vikingsson (Iceland)

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