7. Dall’s9 Porpoise. Phocoenoides dalli.

French: Marsouin de Dall / German: Weilflanken-Schweinswal / Spanish: Marsopa de Dall

Other common names: Dall Porpoise, True Porpoise; Dalli-type Dall's Porpoise (dalli); Truei-type Dall’s Porpoise, True's Porpoise (true)

Taxonomy. Phocaena dalli True, 1885, USA, Alaska, “in the strait west of Adakh [sic] Island, one of the Aleutian group.”

Two subspecies are recognized.

Subspecies and Distribution.

P. d. dalli True, 1885 — cool temperate regions of the N Pacific Ocean from Japan to Alaska and S to N Baja California, Mexico. P d. truee Andrews, 1911 — NW Pacific ÚCCZUI waters ¢.35-534° N, including the Okhotsk Sea.

Descriptive notes. Total length 165-240 cm; weight 60-200 kg. Dall’s Porpoise is the largest member of the family, reaching lengths of up to 240 cm for males and 220 cm for females. It is very stocky, with a very small head and small appendages. There is a very slight, poorly defined beak. Dorsal fin is wide-based and triangular, and in males,it is extremely canted forward. Tailstock is deepened, exceedingly in adult males. Flukes may have a convex trailing edge, especially in adult males. Color pattern is contrasting black and white, with a large, ventrally continuous, white flank patch (extending farther forward in the subspecies truei) and white frosting on upper margins of dorsal fin and trailing edge of flukes. In newborns, color pattern is muted, consisting of various shades of gray, and frosting on fin and flukes is absent. Color pattern becomes bolder as individuals age, and frosting (or trim) begins to appear in the first year of age. Tooth counts generally are 23-28 in each half of each jaw.

Habitat. Deep, cold waters. Dall’s Porpoises are generally found far offshore, but they do occur in nearshore waters where there is water deeper than 100-200 m nearby. They are commonly seen in the inshore waters of north-western USA (Washington, Alaska) and west Canada (British Columbia), where they inhabit deep channels and passes.

Food and Feeding. Dall’s Porpoises are opportunistic feeders, taking a wide variety of prey items. These include mostly mid-water and surface-living fish and cephalopods, such as herring (Clupea), anchovies (Engraulis), sardines (Sardinops, Clupeidae), hake (Merluccius), sauries (Cololabis, Scomberesocidae), lanternfish (Myctophidae), and gonatid squid (Gonatidae). They occasionally consume invertebrates such as krill, decapods, and various species of shrimps, but these latter items are not considered to be their primary prey.

Breeding. Dall’s Porpoise is a seasonal breeder, with all populations known to have a very strong birthing peak in summer. Age and length at sexual maturity appear to vary among different populations, but they generally occur at 4-7 years of age and lengths of 170-190 cm for females and 3-5-8 years and 180-200 cm for males. Gestation lasts c.11 months, and offspring are generally nursed for less than one year. Mating system of Dall’s Porpoise is strongly polygynous, with males apparently using their sexually dimorphic morphologic characters to compete directly for females, and then guarding the female from other potential suitors. In inshore waters of Washington, where they overlap in distribution with Harbor Porpoises (Phocoena phocoena), the two species regularly appear to interbreed and produce hybrids, and this also is known to occur elsewhere in their distributions. In these cases, a Dall’s Porpoise is generally the maternal parent. Longevity is rarely much greater than 20 years.

Activity patterns. Dall’s Porpoise is an active, energetic species. It may be the fastest swimmer of the cetaceans, at least for short bursts. They are most frequently seen swimming very quickly and throwing up a large V-shaped rooster-tail of spray as they surface to breathe at high speed, bringing only very little of their body above the surface. When riding bow waves of vessels, which they do frequently, they rooster-tail on virtually every surfacing, but at other times, they slow down and surface in a slow roll, with no splash. Breaching, spy hopping, and other types of aerial displays are not common, and they very rarely porpoise completely clear of the water.

Movements, Home range and Social organization. Dall’s Porpoises are most commonly found as singletons and pairs, or in small groups of up to a dozen. They occasionally gather in large groups of several dozens or even hundreds (although these opportunistic aggregations do not have any particular structure). Groups of up to several thousand have been reported, but there is some doubt as to correct species identification in these cases. Seasonal movements of population segments of Dall’s Porpoises are known, with increased densities near shore and in more southern areas in winter, and corresponding northern and offshore shifts in summer. Little is known about home ranges or movements of individual Dall’s Porpoises because specimens have rarely been identified, either through photo-identification studies or tagging work. Based on what little is known, however, it would appear that they can undergorather extensive movements of at least several hundred kilometers in pelagic portions of their distribution.

Status and Conservation. CITES Appendix II. Classified as Least Concern on The IUCN Red Last, reflecting the large numbers that occur throughout the North Pacific Ocean. The subspecies dalli and true: have not been evaluated separately on The IUCN Red List. Dall’s Porpoise has been hunted heavily in Japanese waters, where they are the main target species of a harpoon fishery based in Hokkaido. Meatis used for human consumption and pet food. It has also been incidentally killed in very large numbers in several pelagic driftnet fisheries that have operated in waters ofJapan, Russia, and the USA (Alaska). Other potential threats include occasional kills in trawl nets, environmental contamination, anthropogenic noise disturbance, and habitat deterioration, but fishery kills are major factors affecting populations. Several populations in the western Pacific have been depleted by these kills, and their future is uncertain because Japan has often set kill quotas that are unsustainable, ignoring advice from their own nationals and the international community. The total number of Dall’s Porpoises in the North Pacific may be over 1-2 million individuals. Specific populations are estimated at 104,000 in waters offJapan, 554,000 in the Okhotsk Sea, 83,000 off Alaska, and 100,000 off the US West Coast.

Bibliography. Amano & Hayano (2007), Amano & Miyazaki (1996), Curry (1992), Escorza-Trevino et al. (2004), Ferrero & Walker (1999), Forney & Barlow (1998), Hanson & Baird (1998), Houck & Jefferson (1999), Jefferson (1990, 1991, 2009c), Kajiwara et al. (2002), Kasuya (1978), Kubota et al. (2005), Miller (1990), Miyashita (1991), Ohizumi et al. (2003), Rosel et al. (1995), Szczepaniak et al. (1992), Temte (1991), Turnock & Buckland (1995), Turnock et al. (1995), Walker (1996, 2001), Willis & Dill (2007), Willis et al. (2004), Yang Jian et al. (2007).