Ommatophoca rossu Gray 1844

8.

Ross Seal

Ommatophoca rossu

French: Phoque de Ross / German: Ross-Robbe / Spanish: Foca de Ross

Other common names: Bigeyed Seal, Ross's Seal, Singing Seal

Taxonomy. Ommatophoca rossi Gray, 1844,

“Antarctic ocean.” Restricted by G. E. H.

Barrett-Hamilton in 1902 to “pack ice, north of Ross Sea 68°S, 176°E.”

This species is monotypic.

Distribution. Circumpolar in the Southern Ocean.

Descriptive notes. Total length 170-210 cm (males) and 190-250 cm (females); weight 129-216 kg (males) and 159-201 kg (females). Newborns are 105-120 cm in length and weigh 17-27 kg. Ross Seals have relatively small but robust bodies with short, broad heads. Eyes are noticeably large and forward pointing, reflecting adaptations to their deep diving and foraging habits. Teeth are all small, and post-canines are simple without shearing or grinding structure. Canine teeth are very sharply conical, apparent adaptations for catching squid that seem to be their primary prey. Short pelage is dark brown dorsally and cream or tan ventrally, with several dark stripes radiating down throat from mouth and some spotting along boundary between counter-shaded, dorsal-ventral pattern. Ross Seals molt in late December—January and perhaps into mid-February.

Habitat. Confined to the Southern Ocean and sparsely distributed around the Antarctic Continent. In late autumn and early summer, breeding Ross Seals are distributed in heavily consolidated pack ice in the Ross, Amundsen, and Bellingshausen seas of West Antarctica and scattered around East Antarctica and perhaps in the western Weddell Sea. From late summer through early autumn, Ross Seals appear to be pelagic, spending most of their time north of sea-ice habitats and foraging at relatively deep depths of 200-500 m or more. Most sightings of Ross Seals have been ofsolitary individuals, but small groups and aggregations have been seen a few times. Some of these aggregations and groups were recorded in areas of sparse ice and evidently reflected absence of suitable sites to haul-out. Vagrant Ross Seals have been observed at several subantarctic islands, New Zealand, and Australia.

Food and Feeding. Ross Seals primarily eat deep-water squid and fish thatlive at depths of 100-500 m and occasionally krill when they are near sea surface. Based on stomach content analysis, their diet is ¢.47% squid, ¢.34% fish, and ¢.19% invertebrates.

Breeding. Very little is known about breeding biology of Ross Seals. Age at sexual maturity is thought to be 3-4 years for females and 2-7 years for males. The few observations on reproductive biology of Ross Seals suggest that offspring are born in mid-October-November. Mating may occurjust after that in December and early January. Ross Seals make two kinds of low-frequency (100-1000 Hz) vocalizations out of the water: an explosive exhalation noise, with mouth slightly open, and a chugging, siren noise from the throat and pharynx, with rising and lowering frequencies and apparently with mouth and nostrils closed. Underwater pulse and siren calls of Ross Seals are similar to in-air calls, but their frequencies are higher (1-4 KHz) and are consistently pulse modulated at c.650/s with two independently varying tones—similar to the Walrus (Odobenus rosmarus). These sounds might be used during aggressive intraspecific interactions and perhaps breeding displays. Ross Seals can live to 20 years, but c.12 years is probably typical.

Activity patterns. Ross Seals forage at depths of ¢.100-200 m and occasionally as deep as 500 m or more during continuous diving for 4-8 minutes/dive. When Ross Seals are in areas of ice in spring and summer, they haul-out during the daytime and forage mostly at night.

Movements, Home range and Social organization. Distribution of breeding Ross Seals in heavy pack ice has prevented study of their social organization. During the nonbreeding season, they appear to be solitary and widely dispersed in northern reaches of the pack ice and even farther north in open waters of the Southern Ocean. During a 1999-2000 survey in East Antarctic in December—February, 112 of 115 definite or probable sightings of Ross Seals were solitary individuals; the remaining three sightings were of pairs. Immature seals may spend most of their lives at sea, returning briefly to pack-ice habitats to molt in summer.

Status and Conservation. Classified as Least Concern on The IUCN Red List. Ross Seals are listed as “Specially Protected Species” in Appendix A of Annex II to the Environmental Protocol of the Antarctic Treaty, principally because of insufficient information to make a judgment about their population abundance and status. Commercial harvest of Ross Seals is also prohibited by the Convention for the Conservation of Antarctic Seals. Published global population estimates range from 20,000-50,000 individuals to 220,000 individuals. The most recent global estimate, based on analysis of ship and aerial sighting surveys carried out around the Antarctic continent between 1968 and 1983, provided an overall estimate of ¢.130,000 Ross Seals in the pelagic pack ice of the Southern Ocean. In a 1999-2000 survey of the East Antarctic in December—February, the regional population of Ross Seals was estimated at 41,300-55,900 individuals; however, 97-5 confidence intervals suggested the population could range from a low of 20,500 individuals to a high of 226,600 individuals, clearly reflecting the difficulty in surveying such a sparsely distributed and largely solitary species. Aside from possible changes to pack ice from climate change, conservation threats are believed to be limited because distribution of the Ross Seal occurs in areas of very low human activity and does not overlap with any commercial fisheries.

Bibliography. Ackley et al. (2003), Barret-Hamilton (1902), Bengtson & Stewart (1997), Bengtson et al. (2011), Bryden & Felts (1974), Curtis et al. (2007), Decker et al. (2002), King (1969), Lehman et al. (2004), Ling (1972), Piérard & Bisallion (1978), Skinner & Westlin-van Aarde (1989), Southwell (2003, 2005, 2008), Southwell et al. (2008), Thomas & Rogers (2009), Thomas et al. (1980), Watkins & Ray (1985).