Dusky dolphin

Distribution

Dusky dolphins are limited to the Southern Hemisphere, and the four distinct sub-species are non-overlapping, and in some cases, separated by entire ocean basins¹.  They appear to prefer coastal waters associated with the continental shelf or slope, but can also be found over deep water where it occurs close to shore².

Dusky dolphins are native to the following countries and territories: Argentina; Australia; Chile; Falkland Islands (Malvinas); French Southern Territories (Amsterdam-St. Paul Is.); Namibia; New Zealand; Peru; Saint Helena, Ascension and Tristan da Cunha (Tristan da Cunha); South Africa (Marion-Prince Edward Is., Northern Cape Province, Western Cape)

Biology and Ecology

Feeding

Dusky dolphins appear to be adaptable in their feeding strategies and preferred prey².  In Argentina they forage mostly during the day, using cooperative strategies to herd and catch schooling fish in waters over the continental shelf².  In New Zealand, on the other hand, dolphins often rest during the day and move offshore at night to feed in deeper waters on prey associated with the deep scattering layer.  But these same dolphins can also feed during the day when prey is abundant closer to shore³.  Preferred prey can include lantern fish from the deep scattering layer, anchovies, pilchards, mackerel and various species of squid². Dusky dolphin feeding, which may vary by time of day and by season, is often marked by spectacular acrobatic behavior, including chases and aerial displays. These displays are thought to help coordinate the movements and prey herding in the dolphin groups that can number hundreds or even (in New Zealand) thousands of individuals^3-5.

Social structure, Reproduction and growth

Dusky dolphin groups vary in size from only 3-5 individuals to the  more common 10-20, but can also reach up to 2000 animals off of Kaikoura, New Zealand in the months of April-May³.  Life history characteristics of this species have been well studied in Peru, where a directed hunt for dusky dolphins has yielded a high number of specimens for detailed examination.  Females are able to start reproducing between the age of 4.3 and 6.3 years (varying by population), and pregnancy lasts 12.9 months.  Calves nurse for approximately 12 months, and a female normally rests for 3-4 months before conceiving again⁶.

Threats and conservation

Natural Predators

Killer whales prey on dusky dolphins, which have been known to enter into shallow water to avoid detection and predation when killer whales are in the area².  A wide range of diseases and parasites have also been documented in dusky dolphins, particularly in Peru where many specimens have been available for detailed study^7-10.

Human induced threats

Like most small cetacean species, particularly those that inhabit coastal waters where trawling and gillnet fisheries are common, dusky dolphins are subject to bycatch in fishing gear.  In the past bycatch has been high in New Zealand and Argentina¹¹, but the most significant bycatch has occurred in Peruvian fisheries^12-14.  Historically Peru also hosted a targeted catch of thousands of individuals per year^12,13.  Although targeted catches have been banned since 1994, bycatch continues, and it is believed that a targeted hunt for human consumption and use as bait in shark fisheries persists as well¹⁴.  In New Zealand, as well as Argentina, there are concerns that pervasive and persistent presence of whale-watch tour vessels is impacting populations ^15-19 (see more below).

Conservation status

Despite the fact that catches off the coast of Peru have been well documented and are almost certainly not sustainable^12-14, at the time of the last IUCN Red List assessment, this species was considered Data Deficient due to a lack of accurate abundance data from throughout its range²⁰.  Dusky dolphins are listed on Appendix II of the Convention on Migratory Species (CMS).

Dusky dolphins and dolphin watching

Please see the IWC Whalewatching Handbook