Dusky dolphins playfulness
Dusky dolphins (Lagenorhynchus obscurus) are possibly the most playful, and certainly one of the most – if not the most – acrobatic of all dolphin species. An encounter with a pod of duskies is an experience that stays with you for a long time. I’ve been fortunate enough to see dusky dolphins on a number of occasions, always around New Zealand, although they are widely distributed in cooler, coastal waters in the southern hemisphere. Kaikoura Peninsula, in the south-eastern corner of South Island, is famous for the large numbers of cetaceans that are found there, including pods of dusky dolphins, and it is here that I have had the best interactions with duskies. Many dolphin species will interact with boats, with groups changing direction to bow-ride for a few minutes. When it happens, no matter how many times you may have witnessed dolphins bow-riding, it’s still an uplifting experience when it happens. But duskies are something else! They don’t simply bow-ride; when they decide to play with a passing vessel, it’s like the the acrobats from Circus Soleil have teamed up with a bunch of olympic gymnasts to put on a show for you. It is impossible to watch and not be convinced that they are performing simply for the pleasure of showing off and letting you know just how good they are. While travelling at speed a dolphin will fly 3 metres high, performing a back-flip, to tail-slap hard on the water’s surface. I’ve watched three dolphins leap high in the air, describing a perfect arc, each dolphin precisely following the path of the one less than a body’s length in front.
There is also a tangible sense of competition between them. Groups of young male dolphins, five or six abreast, will suddenly sprint 50 metres, clearly racing one another.
Like many other species of cetacean, including sperm whales, dusky dolphins are attracted to Kaikoura because of the rich feeding in in deep water nearby. Kaikoura Canyon, a 60 kilometre long underwater trench, comes to with 1000 metres of the shore, but plunges steeply to up to 1.2 kilometres in places. Recent studies have shown that Kaikoura Canyon holds some of the greatest concentrations of biomass of deepwater species of anywhere in the World (De Leo, et al., 2010) with biomass concentrations up to 100 times greater than similar deepwater habitats. Within deeper waters, a phenomenon known as the DSL, or Deep Scattering layer occurs. This refers to the effect on the echo sounders used on ships to detect the depth of the seabed. A layer occurs mid-water that reflects the acoustic signal of these sounders. This reflection, or scattering, is caused by the gas-filled swim bladders of millions of fish, mostly types of lanternfish, the most abundant fish in the mesopelagic zone (the oceans between around 200-1000m depth, sometimes called the twilight zone). The layer also contains creatures such as squid and crustaceans, but they have no swim bladders, so do not contribute to the bounce of acoustic signals. This layer is where the dusky dolphins are feeding. This dense aggregation of fish, squid and mid-water crustaceans attracts predators, including the dolphins. This living layer is not static however, it undergoes a vertical migration of hundreds of metres every day, rising towards the surface at night, then descending back into the depths as the sun rises. Dusky dolphins feed in this layer at night. They are believed capable of diving to below 150 metres, but prefer to feed when the layer rises to within 130 metres of the surface (Benoit-Bird et al. 2004).
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Benoit-Bird, Kelly & Würsig, Bernd & Mfadden, Cynthia. 2004. Dusky dolphin (Lagenorhynchus obscurus) foraging in two different habitats: Active acoustic detection of dolphins and their prey. Marine Mammal Science. 20. 215 – 231. 10.1111/j.1748-7692.2004.tb01152.x.
De Leo Fabio C., Smith Craig R., Rowden Ashley A., Bowden David A. and Clark Malcolm R., 2010. Submarine canyons: hotspots of benthic biomass and productivity in the deep sea. Proc. R. Soc. B.2772783–2792