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Introduction

The dusky dolphin may look like a species familiar to those in Pacific Northwest, appearing remarkably similar to the local Pacific white-sided dolphin. In fact, some researchers have (and some still argue) that they are the same species, but there are key differences, both morphologically and in genetically.

Found in coastal environments of the Southern Hemisphere (typically in the higher latitudes, most famously in Peru and New Zealand), the dusky dolphin is smaller than most – up to 6 feet long and 200 pounds. It dark to black in coloration, with a white side and belly leading to hot-wheel style white zigzag pattern at the peduncle. The dorsal fin is two toned – the leading edge is black like most of the body, leading to a more white or lighter coloration in the trailing edge. As this is basically the same coloration scheme of the Pacific white sided dolphin (and similar to the hourglass and Peale’s dolphins, all in the same genus), researches sought out DNA structures as a method of proof of distinguishing the species definitively. Using cytochrome b sequence analysis, the two are sister species (have a recent common ancestor) separated by at least 2-3 million years, inhabiting separate ranges as well.

The dusky dolphin’s coloring looks remarkably similar to several other species in the same genus. Photo: Martha Rivera

Diet & Behavior

These beautiful coastal dolphins are probably best known for their amazing aerial behavior. Seen in groups of 2 to over 1000 (most commonly 10 – 20 or more), they regularly perform high jumps and breaches, tumbling and twisting in the air, so it makes sense when some call them one of the most active dolphin species and ‘insane acrobats’! They are also inquisitive – towards humans and other marine mammals. They often approach boats, enjoying a good bow ride. They have also been known to associate with many different species including common dolphins, long-finned pilot whales, bottlenose dolphins, Hector’s dolphins, killer whales, New Zealand fur seals (hunting together), sperm whales, Southern right whales and humpback whales. They may even bow ride on the larger whale species! They play with other species too: they grab kelp gulls and brown-hooded gulls and pull them beneath the surface of the water before releasing them. But sometimes their curiosity gets them in trouble. They have been documented interacting with the common octopus – likely started off as exploratory behavior that quickly turned to distress as the octopus attached itself to the dolphin! Makes you think of that old adage, ‘curiosity killed the cat’, or in this case, really annoyed the dolphin!

Two dusky dolphins interact with a free-swimming common New Zealand octopus off Kaikoura, New Zealand. Photo courtesy of snorkeler on a Dolphin Encounter expedition on 21 January 2012; accompanying figure in Orbach & Kirchener 2014.

There are four geographically separated subspecies, with the two most studied being in Argentina and Kaikoura, New Zealand. Most of the information on social structure and behavior we have comes from the New Zealand population. Dusky dolphins in Kaikoura, like other delphinids, are highly social and live in a structured fission-fusion society. In They have long-term associates that they hunt with (associated over 1,000 days together), and non-random associates (associated over 200 days). Mothers and calves are found in dedicated nursery groups, usually separate from the larger groups. There is evidence that gregariousness differs between individuals, so it is important to look at the individual level to understand their social structure, and there is still more to learn. In New Zealand the dolphins migrate between the inshore and offshore, both diurnally (day/night) and seasonally where the dolphins move offshore in winter. But this may not be the same for each population, as they differ in habitat and foraging techniques. They are able to adapt their foraging methods to the habitat and prey availability patterns.

Dusky dolphins show off their acrobatic nature. Photo: www.dolphins-world.com

Dusky dolphins in general feed on schooling fish such as anchovies, mackerel, sardines along with deep water species like hake, lantern fish and several squid species. In Argentina they feed mostly during the day, cooperatively feeding on schooling fish on the continental shelf; but in New Zealand they rest during the day and move offshore at night to feed on prey associated with the deep scattering layer – however when food is plentiful they will also cooperatively hunt in the shallower water during the day. During cooperative hunting the impressive acrobatic displays are believed to help synchronize cooperative movements and herd prey – the group can’t miss those leaps!

Dusky dolphins can live 30 years or more, and become sexually mature around 7-8 (maybe as young as 5-6). Like other dolphin species they mate year round for social reasons (not reproductive), but breed seasonally. They mate in spring/summer – which in the southern hemisphere is September – December. In New Zealand they have a particular time they like to mate, usually in the late afternoon, prior to nocturnal feeding. This happens in small groups usually without calves, and males will aggressively chase females, with 6 males chasing 1 female. Thus the quickest and most agile will likely be the winner! After about a 13-month gestation, calves are born in November to mid-January and they nurse for 1-1.5 years on average and moms will calve every 2-3 years.

Mating between species as also been documented as likely hybrids between dusky dolphins and both southern right whale dolphins and common dolphins have been observed.

Status

Dusky dolphins are listed by the IUCN as a “Species of Least Concern: Data Deficient”. This basically means that, although we have good information on specific populations, we don’t have enough data on total numbers to know whether they should be considered endangered or not! Unfortunately, however, it seems as though numbers might be in decline in many parts of their range due to human encroachment.

Threats

  • Predation: Predation by killer whales (orcas) and sharks is one of the main causes of death for the small dusky dolphin.
  • Bycatch: Despite changes in fishing regulations and use of gillnetting, bycatch of dusky dolphins is still a big problem. According to one report, from 1991-1993 an estimated 7,000 dusky dolphins were captured annually in Peru. In New Zealand unknown numbers are bycaught each year, however one port recorded 100-200 animals annually. Patagonian shrimp fisheries have been found to be particularly problematic and result in regular bycatch of these cetaceans.
  • Boat Strikes: This one might sound odd, as intuitively you’d think these agile creatures could easily move out of the way of any boats. However with a rapidly growing number of vessels on the water, and the dolphins’ propensity to bow ride and show curiosity towards boats, the rate of boat strikes is increasing.
  • Active Hunting: This occurs predominantly in South America, where many dusky dolphins are still hunted for food. Although commercial hunting has mostly ceased, these cetaceans are still at risk from hunters.

Fun Facts

  • The dusky dolphin’s Latin name is quite a mouthful: Lagenorhynchus obscurus. Lagenorhynchus literally means “bottle-beaked”, while obscurus refers to the species’ “dusky” or “dim” coloring.
  • Dusky dolphins are one of the most agile cetaceans in the world, and certainly one of the most acrobatic. They regularly perform leaps, backslaps, tailslaps, breaches, spins, head-over-tail leaps, etc. However it seems that calves are not born with an innate knowledge of how to do these moves! Calves have to learn these more complex maneuvers one by one in a specific sequence (just like human babies have to learn to crawl first, then walk, then run). Pretty cool, huh?!

cindy.elliser@pacmam.org

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