animals were observed on Los Coronados during 

 August or on Ano Nuevo Island during August and 

 September. However, four juveniles with fresh 

 wounds, among 700 juveniles present, were ob- 

 served on Ano Nuevo Island during four censuses 

 in November (1. 9, and 30 November). One animal, 

 sighted on 1 November 1986, was marked; a 22-mo- 

 old juvenile born on Ano Nuevo Point on 11 Feb- 

 ruary 1984 and tagged 1 month later. 



As mentioned above, through examination of the 

 holdings of the Scripps Institution of Oceanography 

 Fish Collection, we uncovered additional Pacific 

 specimens of Isistius brasiliensis. The eight speci- 

 mens from seven lots included six males and two 

 females. The largest, a 470 mm (standard length) 

 female (SIO 69-345) with jaw width of 38 mm, was 

 collected by IKMT (Isaacs-Kidd midwater trawl) be- 

 tween the surface and 2,000 m from north of Easter 

 Island (lat. 25°58.5'S, long. 108°50.7'W). Another 

 eastern Pacific specimen (SIO 78-183) is from off 

 Isla de Guadalupe (29°26.5'N, 119°44'W) and was 

 collected by phytoplankton net. The other eastern 

 Pacific specimen (SIO 52-413) is from west of the 

 Galapagos (00°00", 100°00"W) and was captured at 

 the surface by dip net. 



Discussion 



The fresh crater wounds we observed on juvenile 

 northern elephant seals resemble those reported on 

 beaked whales, sperm whales, several species of por- 

 poises, and most of the baleen whales (Mackintosh 

 and Wheeler 1929; Van Utrecht 1959), as well as 

 those from a variety of pelagic fishes (Jones 1971) 

 and a nuclear submarine (Johnson 1978). Jones 

 (1971) and others have demonstrated conclusively 

 that those wounds are the result of bites inflicted 

 by the small squaloid shark, Isistius brasiliensis, or 

 possibly by its congener, /. plutodus. To date, the 

 only Pacific record of/, plutodus is from off Okinawa 

 (Compagno 1984), so we therefore presume that the 

 more wide-ranging and topotypical /. brasiliensis 

 is the culprit. Isistius brasiliensis is epipelagic to 

 bathypelagic and is known from all tropical oceans, 

 extending northward to off Japan and Baja Califor- 

 nia and southward to Lord Howe Island. It is typical- 

 ly caught by midwater trawl at depths between 85 

 and 3,500 m; however, it is occasionally found at the 

 surface at night. The shark is thought to be a diur- 

 nal vertical migrator, perhaps traveling a distance 

 as great as 2,000-3,000 m in each direction; in so 

 doing, it apparently encounters feeding Mirounga. 

 As noted by Compagno (1984, p. 94), Isistius is 

 highly specialized as a facultative ectoparasite in its 



dentition, suctorial lips, and modified pharynx that 

 allow it to attach to the side of large prey, drive its 

 sawlike lower jaw teeth into the skin and flesh of 

 its victim, cut a conical plug of flesh, and then pull 

 itself free with the plug cradled by its scooplike 

 lower jaw and held by the hooklike upper jaw teeth. 

 The scar patterns of juvenile Mirounga support the 

 scenario described above. A comparison of jaw width 

 of Isistius of known size with the scar patterns 

 observed on Mirounga suggests that the attacking 

 sharks were at least 50-60 cm long. 



Northern elephant seals would appear to be easy 

 prey for Isistius. They are slow swimmers, com- 

 pared with large pelagic fishes, and they spend 85% 

 of their time at sea underwater at depths of 400-650 

 m (LeBoeuf et al. 1985; Le Boeuf et al. 1986). 

 Juvenile seals that use Isla de Guadalupe during the 

 fall are evidently most prone to being parasitized. 

 Juvenile seals hauling out on other islands, especially 

 those to the north, are evidently not exposed to 

 Isistius to the same degree. Until recently, no fresh 

 crater type wounds were observed on seals at Ano 

 Nuevo despite 16 years of observations by B. J. Le 

 Boeuf. Some wounds observed may have been old, 

 healing crater type wounds, suggesting that the 

 animals bearing them may have been immigrants 

 from the south, the predominant direction of disper- 

 sal (Bonnell et al. 1979). Fresh crater wounds have 

 not been observed on the northern elephant seals 

 at the Farallones since their breeding began in 1972 

 (H. Huber, pers. commun.^). Le Boeuf never ob- 

 served fresh crater wounds on northern elephant 

 seals breeding on San Miguel and San Nicolas 

 Islands during 1968-78, despite annual visits to these 

 islands. 



Mexican northern elephant seals of juvenile age 

 fall prey to Isistius shortly before they haul-out in 

 late August. Le Boeuf and coworkers never ob- 

 served crater wounds on seals at these Mexican 

 rookeries during the winter breeding season (13 

 visits since 1968) or summer molt (4 visits). The 

 juveniles may be exposed to Isistius while feeding 

 or while returning to the island. The marked dif- 

 ference in distribution of shark wounds is consistent 

 with the observation that Mexican, southern Califor- 

 nian, and central Californian juveniles feed in dif- 

 ferent locations, and each "subpopulation" feeds 

 north of its birthplace (Condit and Le Boeuf 1984). 

 Juveniles, trapped in fishing gear, have been caught 

 around 200 m below the surface and captured 16-224 

 km offshore. 



iR. Huber, Point Reyes Bird Observatory, 4900 Shoreline 

 Highway, Stinson Beach, CA 94970. 



389 



