The Hawaiian monk seal on Laysan Island, 1987 and 1989

The endangered Hawaiian monk seal, Monachus schauinslandi, was studied on Laysan Island in the Northwestern Hawaiian Islands during 8 April-20 July 1987 and 30 March-16 July 1989. Data were collected on haul-out, behavior, and reproductive patterns; population structure; and factors affecting survival (primarily male aggression toward adult female and immature seals). In 1987, mean ( 2 SD, n = 26) beach counts were 147.1 seals (2 19.6) including pups and 130.1 (+ 17.7) excluding pups, and in 1989 (n = 26) were 100.2 (+ 12.4) including pups and 82.8 (+. 12.5) excluding pups. Five adult males were removed from the Laysan Island population and brought into captivity in May 1987. The total 1989 spring-summer population was 283 seals, and the sex ratio of adult males to adult females was 1.64:l. Thirty-four and 32 pups were born in 1987 and 1989, respectively. Fifty-two percent of the adult-sized females gave birth in 1989. The mean nursing periods were 42.1 days (2 5.7, n = 6) and 3 7 . 6 (-+ 6.6, n = 18) for pups monitored in 1987 and 1989, respectively. The mean axillary girth and straight dorsal length measurements for 23 weaned pups in each year were 112.3 cm (2 8.2) and 136.3 cm (+ 7.9) in 1987 and 102.0 cm ( 2 10.6) and 127.7 cm (+ 7.4) in 1989, respectively. Interatoll movement was documented for ten seals in 1987 and six in 1989. These animals moved between Laysan Island and either Lisianski Island, French Frigate Shoals, or Pearl and Hermes Reef. Sixteen potentially life-threatening conditions were observed in 1987, resulting in the death of 7 animals, while 47 incidents were documented in 1989, resulting in the death or probable deaths of 14 animals. Injuries caused from mounting by males accounted for 57% (4 of 7) of the deaths in 1987 and 71% (10 of 14) of the deaths and probable deaths of seals in 1989. Five seals in 1987 and 4 seals in 1989 were seen entangled, and 353 and 648 pieces of debris capable of entangling seals were inventoried and destroyed in 1987 and 1989, respectively.