222 
PACIFIC SCIENCE, VoL XIII, July 1959 
Laysan Island 
Laysan is a raised atoll in which the lagoon 
has no connection with the sea. The dimen- 
sions of the island are 1 by 2 miles, of the 
lagoon, 14 by 1 mile. It is surrounded by sub- 
merged coral reefs. We made aerial counts on 
January 7 (1100-1130) and April 15 (1405- 
1430). David Woodside camped on Laysan 
from June 25 to July 3. 
French Frigate Shoals 
This crescent-shaped atoll is 20 miles long 
by 6 miles wide. It contains about 12 perma- 
nent islets and several sandspits which occa- 
sionally shift position. The largest islet is 
occupied by a U. S. Coast Guard unit. Seal 
counts here were made by Paul Breese and 
by POFI scientists in 1956 and 1957. 
From near extinction in the 19th century, 
the monk seal has increased slowly in num- 
bers. In 1923 , Alexander Wetmore visited all 
the islands and atolls in the Leeward Chain 
and estimated the total population at about 
400 (Allen, 1942). Galtsoff counted 68 seals 
at Pearl and Hermes Reef in 1930 (Galtsoff, 
1933 ). In 1951 , the total population was 
placed at about 477, based on estimates made 
by Robert L. Sheehan on Kure, and Vernon 
E. Brock on all the other atolls (Bailey, 1952). 
In 1954 , Johnson A. Neff and Philip A. 
DuMont made an aerial count of 334 seals on 
all the atolls except French Frigate Shoals. 
Biologists of POFI have made counts of seals 
during various cruises to all the Leeward Is- 
lands, except Kure, from 1950 to 1957. The 
total of their highest counts for each location 
is 589 seals. The 1957 total of 1,013 (Table 4) 
does not mean that there has been a recent 
and rapid population increase but that more 
intensive observations were made. 
POPULATION FACTORS 
Sex Ratio 
The lack of obvious secondary sex char- 
acters makes it difficult to ascertain the sex of 
monk seals in the field. If a seal is not lying 
with the ventral surface exposed, it can usu- 
ally be made to roll over on its side by poking 
it sharply with a stick. Thus, under field con- 
ditions, it is practicable to identify sex of only 
a small number of seals. Males are recogniz- 
able by the penile opening (Fig. 2). With 
practice, this cannot be confused with the 
less conspicuous umbilical scar on females. 
The females are recognizable by two pairs 
of teats. 
JUVENILE SEX RATIO: Five pups were born 
in Midway Atoll in spring, 1957. Four were 
males; one prematurely- born pup found dead 
on the beach was a female. At Kure Atoll, we 
sexed 17 pups, of which 10 were males and 7 
were females. At Laysan Island, on June 26 
and 28, 1957, David Woodside tagged and 
sexed 23 young seals. Most of these were 
pups but a few were possibly yearlings. There 
were 8 males and 15 females. The foregoing 
counts total 22 males and 23 females. 
SUBADULT SEX RATIO: Young seals of the 
yearling class and older examined at Midway 
and Kure atolls totaled 35, of which 21 were 
males and 14 females. 
ADULT SEX ratio: At Midway, seals were 
sexed whenever possible as groups (see Table 
5) and individually. Total observations in- 
cluded 38 males and 31 females (single ob- 
servations are not shown in Table 5). Unfor- 
tunately none of the above was marked and 
it is impossible to say how many were seen 
more than once. The largest single day’s ob- 
servation included 9 males and 3 females. In 
TABLE 5 
Sex Ratio of Adult Hawaiian Monk Seals 
ON Midway Atoll^ 
DATE, 1957 
MALES 
FEMALES 
January 14 
5 
5 
February 2 
5 
2 
March 5 
9 
3 
March 14 
4 
2 
May 2 
6 
3 
Total observations^ 
29 
15 
On each of 5 occasions when 6 to 12 seals were examined. 
