While it is often stated that the dusky dolphin was first described by Charles Darwin, from a specimen harpooned in Saint Joseph's Bay of Argentina (Delphinus fitzroyi, C. Darwin as cited by Waterhouse 1838), it was earlier-described as Delphinus superciliosus off Tasmania (Lesson and Garnot 1826) and briefly thereafter as Delphinus obscurus off the tip of Africa (Gray 1828). Thus, it was described for science from its three major areas of occurrence, Australia/New Zealand, southern Africa, and South America, within the space of about ten years. These three accounts are fascinating but not truly remarkable, due to the extensive voyages of scientific discovery in that vibrant era. While several genera and species names were used for the next 50 or so years, Frederick True (True 1889) established the name as the present Lagenorhynchus obscurus. Largely due to modern molecular evidence, there is debate about the status of several species of the presently-recognized genus Lagenorhynchus, and we can expect to see changes as evidence is gathered and systematic affinities are refined (LeDuc et al. 1999; see Harlin-Cognato 2010 for an excellent taxonomic overview). Morphological (e.g., differences in skull and body length, tooth size, and teeth counts) and molecular differences between populations suggest differentiation of the species at the subspecies level.
Size, shape and distinctive characteristics
Dusky dolphins have a stocky body shape with a head that slopes evenly from the blowhole to the short rostrum. The dorsal fin is falcate in shape and bi-colored, consisting of a dark leading edge and light gray trailing edge. The white throat and belly are in contrast to the bluish-black to dark gray dorsal side, including dark lips and a light to dark gray patch over the eye. Along each side there is a light gray dorsal flank patch or "blaze" which extends caudally from the tailstock but generally does not project further than the leading edge of the dorsal fin. There is little sexual dimorphism other than a tendency for adult males to have wider rostrums, dorsal fins that are more curved with broader bases, and to be slightly larger in length and weight.
While dusky dolphin size varies between populations, average length is approximately 180 cm. Dusky dolphins in Peru have an average adult length of 185 cm; maximum length for males and females are 210 cm and 205 cm, respectively. Dusky dolphins in New Zealand and southern Africa are typically 8-10 cm shorter than dusky dolphins in Peru. Maximum lengths for males and females in New Zealand and southern Africa range from 180 to 191 cm. Data from Peru indicate that dusky dolphins measure 91.2 cm and 9.6 kg at birth (Van Waerebeek & Read 1994). They generally weigh less than 100 kg. Males and females in New Zealand weigh 69-78 kg and 70-85 kg, respectively (Cipriano & Webber 2010).
Dusky dolphins have approximately 30-32 teeth on each half of the upper and lower jaw. In New Zealand, they tend to have teeth which are smaller and more numerous than dusky dolphins in southern Africa. Dusky dolphins may occur in very large groups (>1,000) and are known for their acrobatic leaping behavior, with frequent somersaults that distinguish them from all other dolphins in the field (except the non-sympatric Pacific white-sided dolphin, L. obliquidens).
The dusky dolphin is a semi-pelagic species, inhabiting coastal zones and shallow shelves and slopes of the continental shelf. It is associated with landmasses of southern hemisphere continents or islands, and rarely occurs in waters deeper than 2,000 m (Cipriano and Webber 2010). It is well-known off especially the south island of New Zealand, Chile and Peru, Argentina and the Malvinas (Falkland) Islands, and the southwest coast of southern Africa. However, it also occurs off Tasmania (and occasionally the southern-most extent of Australia proper), Tristan de Cunha and Gough Islands of the South Atlantic, and Prince Edward, Marion, Amsterdam, and St. Paul Islands of the Indian Ocean. It may be associated with other southern/temperate water islands, but residency has not been clearly established. The northernmost extent of the species is off the coast of Peru at 8° S. In New Zealand, dusky dolphins inhabit waters ranging from 11 to 16 °C (Gaskin 1968).
Ecology and Behaviour
Dusky dolphins occur in groups of <12 to several thousand. In shallow water environments where schooling prey are less predictable, dusky dolphins forage in close coordination (Vaughn et al. 2007, 2010). In deep water environments where prey is associated with the deep scattering layer (DSL), dusky dolphins aggregate primarily for predator protection. Behavior and ecology has been studied intensively in three areas: Golfo San José, Argentina; and Kaikoura and Admiralty Bay, New Zealand.
Golfo San José occurs in the continental shelf region of central Patagonia, measuring c.a. 750 km2. Dusky dolphins occur <5 km from shore at depths of <200 m (Würsig et al. 1989). Fission-fusion dynamics are driven by diurnal foraging strategies for southern anchovy (Engraulis anchoita). Groups of 8-12 individuals rest near shore at night. In the morning, groups spread out in search of prey, and once detected, individuals coordinate foraging behaviors by encircling the prey ball and noisy leaping to create loud slapping sounds. Large prey balls require increased group coordination and noisy leaps may function to call other dolphins to join, forming groups of ≤300 individuals. After a feeding bout, they exhibit a high level of social activity which includes acrobatic leaping, high intensity vocalizations, and sexual activities (Würsig & Würsig 1980, Dans et al. 2010).
Admiralty Bay, at the northern tip of New Zealand's South Island, measures c.a. 120 km2 and is inhabited by dusky dolphins from late fall to early spring when they feed on pilchard (Sardinops neopilchardus). Most individuals are males (Shelton et al. 2010). They coordinate foraging activities to herd prey balls; however, group size is smaller (max=50 individuals) and does not oscillate as dramatically between small resting and traveling groups and large foraging and socio-sexual groups as in Golfo San José. Potential reasons include the relatively small bay size and low predation risk (Pearson 2009).
Most dusky dolphins in Admiralty Bay travel c.a. 275 km south to Kaikoura during the summer (Markowitz 2004, Markowitz et al. 2004, Shelton et al. 2010), where they move between shallow nearshore and deep offshore waters. At night, they move in large groups to feed on the DSL in the Kaikoura Canyon. Within the large group, individuals form subgroups of 1 to 5 which may coordinate foraging behaviors (Benoit-Bird et al. 2004). During the day, they move near shore to rest and socialize, remaining in large groups or forming smaller groups. Mating groups are composed of several males mating with several females, likely exhibiting a polygynandrous (multi-mate) system (Markowitz et al. 2010b). Mother-calf pairs form nursery groups and rest in shallow nearshore waters, likely to evade predation and male harassment (Weir et al. 2010).
Based on tooth ring analysis, maximum age for dusky dolphins in New Zealand was recorded at 35-36 years old (Cipriano 1992). In Peru, males and females reach sexual maturity at 3.8-4.7 years and 4.3-5 years, respectively. In Argentina, females do not reach sexually maturity until 6.3 years. The low age at sexual maturity in Peru may represent a density-dependent response to decreased abundance due to overexploitation or unpredictable oceanographic events associated with the El Niño Southern Oscillation (Van Waerebeek and Würsig 2009). Age at first reproduction in New Zealand was estimated at 7-8 years of age (Cipriano 1992).
Dusky dolphins in New Zealand give birth in late spring and early winter (November-January) but newborns are occasionally seen during other months (Cipriano 1992, Weir et al 2010). Peak calving season is August-October in Peru (Van Waerebeek and Read 1994) and during the austral summer in Argentina (Würsig and Würsig 1980). In Peru, the female reproductive cycle consists of a 12.9 month gestation period and 12.0 month lactation period followed by a 3.9 month resting period (Van Waerebeek and Read 1994). Dusky dolphins in New Zealand have a gestation period of 11 months and lactation period of 18 months (Cipriano 1992). Fetuses grow at a rate of 0.261 cm/day.
Adult male testes weight increases dramatically during the breeding season as compared to the non-breeding season. Testes weight off New Zealand during the peak breeding season ranged from 995-2143 g each, which contrasts with a range of 252-479 g during the non-breeding season (Cipriano 1992). Maximum weight of a single testis collected off Peru during the peak mating season was 5120 g, which is approximately 8% of total body weight (Van Waerebeek and Read 1994). This is the largest testis to body mass ratio reported for any mammal and indicates a promiscuous or multi-mate mating system with sperm competition (see Ecology and Behavior).
Southern anchovy (Engraulis anchoita) are major prey off Patagonia (Würsig & Würsig 1980). In Peru, anchoveta (E. ringens) is a primary prey species, in addition to horse mackerel (Trachurussymmetricus), hake (Merluccius grayi), sardine (Sardinops sagax), and squid (Loligo gahi, Dosidicas gigas; McKinnon 1994). Diet in New Zealand is dominated by pilchard (S. neopilchardus) in Admiralty Bay and deep-scattering layer prey such as squid (Nototodarus/Todaroides spp.) and lanternfish (Symbolophorus spp., Diaphus spp., Myctophum spp., Hintonia spp.) off Kaikoura (Würsig et al. 2007).
There are currently few abundance estimates available for dusky dolphins. In South America, the dusky dolphin is reported to be the most common small cetacean off Patagonia and one of the top three most abundant species off the Peruvian coast. Current and historic mortality in directed catch and bycatch (see Conservation Issues) may have population-wide impacts in Peru but more data are needed. Abundance in a 15,000 nmile2 area off Argentina was approximately 7,000 dolphins (Van Waerebeek and Würsig 2009). From "mark-recapture" analysis of recognizable individuals, an estimated 12,000 dusky dolphins occur off Kaikoura, New Zealand (Markowitz 2004), but never all at one time. An aerial survey off South Africa produced an abundance estimate of approximately 1,100 dolphins; however, this is a rough estimate which may include double-counts of some animals (Best and Ross 1984).
The dusky dolphin is listed as "data deficient" by the IUCN and the population trend is unknown. Status in 1994 was "insufficiently known" and in 1996 was "data deficient" (Hammond et al. 2009). Comprehensive photo-identification, genetic and survey studies are needed throughout the entire range of the dusky dolphin in order to define populations and obtain abundance estimates.
During 1982-95, 852-1377 dusky dolphins were caught in the red shrimp mid-water trawling fishery off Patagonia, Argentina; a maximum of 550 individuals caught in 1984. These estimates were equal to or exceeded replacement thresholds and likely caused a population decline. Increased regulations in 1994 reduced bycatch mortality, but continued mortality at the current estimate of 70 dolphins/year will lead to a 20% population decline in 50 years. Reproductive females are disproportionately caught in nets, exacerbating the rate of population decline. Dusky dolphins are incidentally caught in midwater nets of the Patagonian southern anchovy fishery, but no mortality estimates are available. In Buenos Aires Province, Argentina, purse seine nets are intentionally set around dusky dolphins herding prey; an estimated 100 dusky dolphins died in purse seine nets each year during the 1990s. In New Zealand, an estimated 200 dolphins were caught in gill net fisheries in 1984 (Cipriano & Webber 2010, Markowitz et al. 2010a).
In Peru, dusky dolphins were intensely hunted for human consumption after the 1972 anchovy fishery collapse. During 1991-93, an estimated 7,000 dusky dolphins were hunted annually. After the hunting ban in 1996, directed takes decreased but ≤1,000 dolphins are still hunted each year (Read et al. 1988, Van Waerebeek & Würsig 2009, Markowitz et al. 2010a).
Historically, a small number of dusky dolphins were captured for aquaria display. During 1961-81, 37 individuals were captured off South Africa (Best & Ross 1984, Cipriano & Webber 2010).
Green-lipped mussel (Perna canaliculus) farming in New Zealand is a major and growing industry and may limit habitat and interfere with foraging behaviors. Dusky dolphins do not use areas within mussel farms (Markowitz et al. 2004, Pearson 2008). Mussel farms may alter food webs as artificial increases in mussel abundance may lead to unsustainable removal of plankton from the prey base.
Tourist operations may cause behavioral change, interruption of feeding and resting behaviors, and habitat displacement. Off Kaikoura, New Zealand, swim-with-dolphin programs result in dusky dolphin exposure to tourist operations up to 70% of daylight hours, but tour operator adherence to a voluntary mid-day rest period has mitigated some deleterious effects of tourism. A moratorium on dusky dolphin tourism since 1999 is under review (Würsig et al. 2007; Markowitz et al. 2010a). In Argentina, there is greater potential for negative tourism impacts on dusky dolphins due to a lack of regulation and overlap between tours and dusky dolphin diurnal foraging behavior.
Dusky dolphins bioaccumulate toxins, potentially resulting in decreased reproductive capacity and immune function. Elevated DDT and PCB levels are reported from South Pacific and South Africa (Best and Meÿer 2010, Markowitz et al. 2010).
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