The deep divers
by Becci Jewell
The northern transect lines of SCANS-III zig-zag between the edge of the continental shelf and the deep waters beyond and many of our sightings have been of the deep diving cetacean species that inhabit these waters. Deep diving species are often difficult to detect because of the long periods of time they spend foraging at depth. Beaked whales in particular can be elusive, with their long dives and cryptic surfacing behaviour. Despite this, beaked whales and sperm whales were seen multiple times during the survey, as were numerous groups of long-finned pilot whales.
Beaked whales are incredibly difficult to detect both visually and acoustically and very little is known of many of the 22 species of beaked whale. One of the better known species, Cuvier’s beaked whales, have been recorded diving to a depth of 2,992m during a dive that lasted for 137.5 minutes – the longest and deepest mammalian dive ever recorded. Sowerby’s beaked whales, a species found exclusively in the North Atlantic, were seen in the northern block of the survey and Cuvier’s beaked whales were seen in both the northern and southern blocks.
Sperm whales, the second deepest diving species of cetacean, often associate with the bathymetric features such as the continental shelf, sea mounts and submarine canyons where upwelling water brings nutrients from the depths, driving primary productivity. This productivity attracts the squid and fish that sperm whales feed on, at times diving for over an hour to forage at depths of more than 1,000m. Using a hydrophone array towed behind the M/V Skoven in the northern block, we detected the broadband echolocation clicks of sperm whales foraging in the depths as well as seeing them logging at the surface between dives. Our busiest sperm whale encounter was of at least 10 individuals at the surface in little over an hour.
Long-finned pilot whales were one of the most frequently sighted species in the northern block and were also seen in French waters in the southern survey block. Despite undertaking dives to depths of over 900m, this species is much easier to detect than the more stealthy beaked whales. With their broad black dorsal fins and conspicuous surfacing behaviour we often spotted groups of pilot whales well ahead of the vessel and could track them as we passed.
The data recorded during sightings of these deep diving species will allow new estimates of abundance to be calculated and will shed light on the current distribution and status of their populations in the northeast Atlantic.