OCTOBER 1984 LiSRAPv 

Smthwest Fisheries jcience Center 

National Marine Fisheries Service 

P 0. BCX 231 

La Joila 178 o'*:: 2R 

OBSERVATIONS OF THE HAWAIIAN MONK SEAL 

ON LAYSAN ISLAND FROM 1977 THROUGH 1980 

Brian W. Johnson 

Patricia A. Johnson 

NOA A-TM-NMFS-S W FC-49 

/ 

U.S. DEPARTMENT OF COMMERCE 

uational Oceanic and Atmospheric Administration 

National Marine Fisheries Service 

Southwest Fisheries Center 
NOAA Technical Memorandum NMFS 

The National Oceanic and Atmospheric Adniinistration (NOAA), organized in 1970, hasevolved intoan 

agency which establishes national policies and manages and conserves our oceanic, coastal, and atniospheric resources An organizational element within NOAA. the Office of Fisheries is responsible fbr 

fisheries policy and the direction of the National Marine Fisheries Service (NMFS) 

In addition to its formal publications, the NMFS uses the NOAATechnical Menioranduni series to issue 

informal scienttfic and technical publications when complete formal review and editorial processing are 

not appropriate or feasible Documents withm this series, however, reflect sound professional work and 

may be referenced in the formal scientific and technical literature 
NOAA Technical Memorandum NMFS 

r 

This TM series is used lor documentallon and timely communication 01 preliminary results. imterlm reports. or 

special purpose information. and have not received complete lormal review. edbtorlal control. or detailed editing 

OCTOBER 1984 

OBSERVATIONS OF THE HAWAIIAN MONK SEAL 

ON LAYSAN ISLAND FROM 1977 THROUGH 1980 

Brian W. Johnson 

Patricia A. Johnson 

Aquatic Mammals Behavioral Research 

6843 SE Oakridge Dr. 

Gladstone, Oregon 97027 

Prepared for National Marine Fisheries Service 

Honolulu, Hawaii Contract 80-ABC-00 124 August 1984 

NOAA-TM-NMFS-SWFC-49 

U.S. DEPARTMENT OF COMMERCE 

Malcolm Baldrige. Secretary 

National Oceanic and Atmospheric Administration 

John V. Byrne. Administrator 

National Marine Fisheries Service 

William G. Gordon, Assistant Administrator for Fisheries 
PREFACE 

This report was prepared under Contract No. 80-ABC-00124 to the 

National Marine Fisheries Service by Brian W. Johnson and Patricia A. 

Johnson. The primary purpose of the contract was to summarize Hawaiian 

monk seal observational data collected on Laysan Island by the contractors 

from 1977 through 1980. The statements and findings in this report are 

those of the contractors and do not necessarily reflect the view of the 

National Marine Fisheries Service. 
ABSTRACT 

Behavioral observations of the Hawaiian monk seal on Laysan Island 

were made between 1977 and 1980. Primary objectives included determination 

of population size, reproductive patterns and survival rates. Individual 

seals were recognized by photographs of natural markings (particularly scar 

patterns) or by temporary bleach marks. . 

The 1977 mean count, 179 seals, was representative of the average 

count from the previous 10 years. During 1978 an unusual "die-off" of 

seals occurred, resulting in a 35X reduction in the mean count, to 112 by 

1980. Using nondisturbance population estimation techniques developed in 

1979, the mean count of 100 nonpups ashore in 1980 was shown to be well 

below the estimated population size of 269 seals. 

From the Laysan data it appears sexual maturity (as measured by first 

Known-age 

bring the 4 

birth) may not occur until after five years of age for females. 

males did not show courtship behavior until 8 or 9 years old. 

years of the study an estimated 64% of the adult females produced pups. 

The mean weaning age for pups was 36.2 days. Females outnumbered males at 

birth but first year survival w a s - s i g n i f i c a n t l y g r e a t e r for males. 

males outnumbered adult females by 3 to 1. Yearly survival rates of seals 

from birth to about age 4 was estimated at 75%, increasing to 85% for 

subadults and adults. 

Adult 

ii 
TABLE OF CONTENTS 

INTRODUCTION .............................. 1 

c 

METHODS ............. Seal Identification ..... Age Determination ...... Sex Determination ...... Molt ............ Censusing Techniques .... Population Estimation .... Natality and Reproduction . . 

.................... 3 

.................... 3 

.................... 3 

.................... 4 .................... 5 .................... 5 

.................... 7 

.................... 7 

RESULTS AND DISCUSSION ..... Island Counts ........ Population Estimation .... Molt Summation ..... Pups ........ Yearlings ...... Juveniles ...... Subadults ...... Adult females .... Petersen Estimate .... Marked animals ... Census procedure . . Results ....... Petersen Molt Estimate . Methodology ..... Results ....... Summary of Estimates . . Distribution ........ Molt 

Seasonal Patterns .... Island Usage ...... Reproduction ........ Sexual Maturity: Females 

Sexual Maturity: Males . Reproductive Rate . . Tagged-females ... Estimated-females . . Identified-females . Mature-females ... Discussion ..... Interval Between Births . Pupping ......... Sex ratio at birth . Timing of births . Rookery areas .... Age at weaning ... Factors affecting pup 

Percentage Ashore .... 

.................... 8 

.................... 8 .................... 12 

.................... 12 .................... 13 .................... 13 .................... 13 .................... 13 

.................... 13 

.................... 14 .................... 14 

.................... 14 

.................... 15 .................... 15 .................... 16 

.................... 17 .................... 17 

..................... 18 

.................... 21 

.................... 24 

.................... 24 .................... 30 

.................... 30 

.................... 30 

.................... 31 

.................... 32 

.................... 32 

.................... 32 .................... 32 .................... 33 

.................... 34 .................... 36 

.................... 37 .................... 37 .................... 37 .................... 38 

.................... 38 

survival ............... 39 

iii 
Breeding ............................ 39 

Survival .............................. 40 

Pups .............................. 40 

Survival through weaning .................. 40 

Survival through 4 months .................. 40 

Survival through 16 months ................. 43 

Yearlings ............................ 44 

Other Age Classes ........................ 44 

ACKNOWLEDGMENTS ............................. 45 

LITERATURE CITED ............................ 46 

A . 

B . 

C . 

c1 . 

c2 . 

c3 . c4 . D . 

E . 

F1 . F2 . F3 . F4 . 

APPEND1 CE S 

Itinerary . Arrival and departure dates. Laysan Island. 

1977-80 .............................. 47 

Biases affecting sex ratio data .................. 48 

Laysan Island count data ..................... 51 

Counts from 1977 on Laysan Island (age/sex breakdown not 

available for all seals seen on 10 March) ............. 52 

Counts from 1978 on Laysan Island (age/sex breakdown not 

available for all seals seen on 27 February) ........... 54 

Counts from 1979 on Laysan Island ................. 56 

Counts from 1980 on Laysan Island ................. 58 

Tag numbers read on Laysan Island ................. 60 

Reproductive patterns of mature females .............. 61 

Data used to calculate Figure 31 ................. 62 

Data used to calculate Figure 32 .................. 63 

Data used in Table F-3 ...................... 64 

Survival data ........................... 62 

LIST OF TABLES 

1 . 

2 . 

3 . 

4 . 

Date and time of Petersen Estimate censuses on Laysan Island. 

1980 ................................ 15 

Population estimate for each age and sex class using the 

Population estimate for each age/sex class using the 

Petersedmolt estimate at Laysan Island. 1980 ........... 17 

Comparison of results from estimation techniques used in 

1980 with the best population estimate from 1979 on 

Laysan Island ........................... 18 

Percentage of each age/sex class using the various areas of 

Laysan Island in 1980 ....................... 24 

Comparison between the percentage of each age/sex class 

molting in the various areas of Laysan with the 

distribution throughout the year. 1980 ............... 30 

on Laysan Island ........................... 31 

Petersen estimate at Laysan Island. 1980 .............. 16 

* 

5 . 

6 . 

7 . Number of births to known-age females between 1977 and 1980 

iv 
8. 

9. 

10. 

11 . 

12. 

13. 

14. 

15. 

16 . 

17 . 

18. 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

11. 

12. 

13. 

Subjective age classification of tagged, known-age males 

between 1977 and 1980 on Laysan Island . . . . . . . . . . . . . . . 31 

Yearly reproductive rates for females seen and classes as 

adult in at least 3 years on Laysan Island . . . . . . . . . . . . 33 

Reproductive status of females in years following the first 

observed birth. . . . . . . . . . . . . . . . . . . . . . . . . . . 33 

Observed and expected number of females showing each of four 

reproductive patterns, including only known-maturity females 

seen in at least 3 years on Laysan Island . . . . . . . . . . . . . 34 

Summary of reproductive success estimates for the Hawaiian monk 

seal population on Laysan Island, 1977 through 1980 . . . . . . . 35 

Yearly crude birth rate (births/mesn count) for 1977 through 

1980 on Laysan Island . . . . . . . . . . . . . . . . . . . . . . . 35 

Sex ratio at birth for all pups born from 1977 through 

1980 at Laysan Island . . . . . . . . . . . . . . . . . . . . . . . 37 

Number of pups born during each month on Laysan Island 

from 1977 through 1980 . . . . . . . . . . . . . . . . . . . . . . 38 

Preweaning disappearance and mortality on Island between 

1977 and 1980, excluding pups born dead . . . . . . . . . . . . . . 41 

Weaned pups which died or disappeared during their pup 

year on Laysan Island . . , . . . . . . . . . . . . . . . . . . . . 41 

Number and percentage of seals identified in 1979 that were 

resighted in 1980 on Laysan Island . . . . . . . . . . . . . . . . 44 

LIST OF FIGURES 

Hawaiian Archipelago . . . . . . . . . . . . . . . . . . . . . . . . 2 

The mean mid-March through mid-August seal count, 1977-80 . . . . . 9 

Mean island count from mid-March through mid-August, based 

on extrapolated data from Appendix C . . . . . . . . . . . . . . . . 10 

Mean count (excluding pups) from mid-March through 

mid-August . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 

Semimonthly counts (mean and standard deviation) for 1979 

and 1980 combined . . . . . . . . . . . . . . . . . . . . . . . . . 11 

Adult male counts (mean and standard deviation) for 1979 

and 1980 combined . . . . . . . . . . . . . . . . . . . . . . 11 

Adult female counts (mean and standard deviation) for 1979 

and 1980 combined . . . . . . . . . . . . . . . . . . . . . . . . 11 

Immature male counts (mean and standard deviation) for 1979 

and 1980 combined . . . . . . . . . . . . . . . . . . 12 

Immature female counts (mean and standard deviation) for 

1979 and 1980 combined . . . . . . . . . . . . . . . . . . . 12 

Percent of total population ashore on counts, 1980 

(mean and standard deviation) . . . . . . . . . . . . . . . 19 

Percent of adult males ashore on counts, 1980 (mean and 

standard deviation) . . . . . . . . . . . . . . . . . . . . 19 

Percent of adult females ashore on counts, 1980 (mean and 

standard deviation) . . . . . . . . . . . . . . . . . . . . 19 

Percent of immature males ashore on counts, 1980 (mean and 

standard deviation) . . . . . . . . . . . .*. . . . . . . . . 20 

V 
14. 

15. 

16. 

17 . 

18. 

19. 

20 . 

21. 

22. 

23. 

24 . 

25. 

26. 

27 . 

28. 

29. 

30. 

31. 

32. 

Percent of immature females ashore on counts,’ 1980 (mean 

and standard deviation) . . . . . . . . . . . . . . . . . . 20 

Mean percent ashore in each of four areas on Laysan Island, 

1977................................22 

Mean percent ashore in each of four areas on Laysan Island, 

1980................................22 

Map of Laysan Island taken from aerial photograph (PWS 1980) 

showing circumference divided into 14 areas . . . . . . . . . . . . 23 

The number of seals which molted (excluding pups) during each 

semimonthly period . . . . . . . . . . . . . . . . . . . . . . 26 

The number of seals which molted (excluding adult males and 

pups) during each semimonthly period . . . . . . . . . . . . . . 26 

The number of aduit females which molted during each V 

semimonthly period . . . . . . . . . . . . . . . . . . . . e . . . . 26 

The number of nonparturient adult females which molted 

during each semimonthly period . . . . . . . . . . . . . . . . . 27 

The number of parturient adult females which molted during 

each semimonthly period . . . . . . . . . . . . . . . . . . . . . . 27 

The number of subadult females which molted during each 

semimonthly period . . . . . . . . . . . . . . . . . . . . . 27 

The number of subadult males which molted during each 

semimonthly period . . . . . . . . . . . . . . . . . . . . . . . . . 28 

The number of juvenile females which molted during each 

semimonthly period . . . . . . . . . . . . . . . . . . . . . . . . . 28 

The number of juvenile males which molted during each 

semimonthly period . . . . . . . . . . . . . . . . . . . . . . . . 28 

The number of yearling males and females which molted 

during each semimonthly period . . . . . . . . . . . . . . . . . . . 29 

Mean number of molting adult males seen on censuses, 1978 . . . . . 29 

Semimonthly distribution of births on Laysan Island, 

1977-80 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -37 

Pup survival on Laysan Island, 1977 through 1980 . . . . . . . . . 42 

Pup survival for Laysan (1977-801, Kure (1964-651, and 

Midway (1958-59) . . . . . . . . . . . . . . . . . . . . . . . . . . 42 

Pup survival on Laysan Island, 1978 and 1979 pup 

cohorts . . . . . . . . . . . . . . . . . . . . . . . . . . . . -43 

%- 

vi 
INTRODUCTION 

c 

The Hawaiian monk seal, Monachus schauinslandi, was designated as a 

depleted species under the Marine Mammal Protection Act of 1972 and listed 

as endangered under the Endangered Species Act in 1976. As the basic biology and life history of the monk seal were incompletely known, the authors, 

aided by personnel from the National Marine Fisheries Service (NMFS) and 

U.S. Fish and Wildlife Service (FWS), designed a study to learn about population parameters and threats to survival of the Hawaiian monk seal. This 

report summarizes the results of 4 years investigation on Laysan Island. 

Information gathered on the French Frigate Shoals population of monk seals 

is included in a separate report (Johnson and Johnson 1984). 

Funding for the project was provided primarily by the Marine Mammal 

Commission, with additional support and equipment provided by National 

Marine Fisheries Service, U.S. Fish and Wildlife Service, the U.S. Coast 

Guard, the Easy Rider Corporation, and the State of Hawaii. 

Laysan Island was selected as the study site, with field research 

beginning in 1977. Laysan is a coral sand island located approximately 

1350 km from Oahu (Fig. 1). 

pelago, it is included in the Hawaiian Islands National Wildlife Refuge. 

Access to Laysan is strictly limited. The island is home to two endemic 

bird species listed as endangered as well as the monk seal, and to the 

threatened Hawaiian green sea turtle, Chelonia mvdas. Laysan is 1.7 by 2.8 

km, with a circumference of about 12 km. Low vegetation and a high saline 

lagoon cover much of the interior. (For a more complete description of the 

island and its history see Ely and Clapp 1973.) 

As part of the Northwest Hawaiian Island ArchiThe historical use of Laysan by the monk seal is incompletely known. 

No information is available on the size of the seal population prior to near 

decimation by sealers and other visitors using the island in the 1800's. By 

the early 1900's seals were rarely seen on Laysan. As summarized in Fiscus 

et al. (19781, the population gradually recovered, with counts occasionally 

exceeding 300 animals in the late 1950's and early 1960's. 

seals counted declined during the mid-l960's, but remained relatively stable 

during the 10 years prior to the start of this study, averaging just under 

200 seals. 

The number of 

Past studies have shown the Hawaiian monk seal to be adversely affected 

by human disturbance (Kenyon 1972). 

was carefully designed to record the behavior of a colony of monk seals relatively undisturbed by man. 

species not be altered by the presence of seal or bird biologists living on 

Laysan, or by visitors to the island. 

With this in mind, the present study 

Care was taken that the natural behaviors of the 

During each year from 1977 to 1980 5 to 7 months were spent on Laysan. 

Observations were limited to the months from late February through September 

except in 1978, when brief visits were also made in October and December. 

(Arrival and departure dates for all years are listed in Appendix A.) 

authors were the sole human inhabitants of Laysan during 1977 and 1978. In 

1979 and 1980 the size of the Laysan field party increased to five with the 

addition of three FWS biologists. 

The 
2 

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Camp was established on the northwest side of the island, east of the 

lone ironwood tree and well back from the beach. 

beach directly below camp, all human access to beaches was restricted and 

carefully monitored. No handling of seals occurred except during a 3-day 

visit by a team of scientists in May of 1978, when 11 seals were restrained 

for collection of blood samples and physical examination. 

With the exception of the 

During the 1978 field season an unusually large number of seals died or 

disappeared on Laysan (Johnson and Johnson 1981b). Later investigation 

determined the probable cause was a fish toxin, ciguatera (Gilmartin 1983). 

Dramatic population declines of unknown etiology had been recorded for seal 

popalations on other atolls in recent years (Gilmartin 1983). 

effects of a high mortslity on what had appeared to be a relatively healthy 

population. Observing the affected animals, the changes in population 

structure, and the overall effect of the mortality became a high priority. 

The Laysan 

die-off" provided the first opportunity to study the circumstances and It 

This report summarizes 4 years of research, but emphasizes data collected in 1980 as funding was available to computerize data from that year 

which allowed more complete analysis. 

been reported elsewhere (Johnson-and Johnson 1978, 1981a, 1981b). 

Results from the previous years have 

METHODS 

Seal Identification 

An extensive portfolio of photographs, sketches, and descriptions of 

natural scars and marks was compiled and constantly updated, allowing reliable identification of individual seals. Photographs were taken with both 

Polaroid and 35-mm cameras. Field identification was generally made on the 

basis of sketches, descriptions, or direct comparison of a photograph and 

the animal. Most recognizable animals were photographed repeatedly to monitor changes in scar patterns or visibility and to assess rate of acquisition of new marks. 

Many adult seals had conspicuous scars or other markings which were 

unique. However, young seals rarely had acquired identifiable scars. To 

facilitate individual identification of these animals it was necessary to 

apply artificial identification marks to the hair of some immature seals and 

all pups born during the study, using a commercial hair lightener preparation (Lady Clairol Ultra Blue). The marks were applied to sleeping seals 

and did not result in observable adverse physical or behavioral changes. 

Bleach marks lasted until the next molt of the animal. 

Age Determination 

During the first year of the study it was difficult to reliably assign 

seals to any age class other than adult, immature, and pup. The adult 

female size class was defined by the size of the smallest animal to bear a 

pup. The definition of an adult male was based on the size of the smallest 

animals showing courtship behavior. Nonadult seals were classified as 

subadult, juvenile large, juvenile small (assumed,to be yearlings), and 

pups. 

to serve as a reference, and distinctions based on size, color, and other 

In subsequent years tagged seals and known-age animals were available 
4 

features were used to age seals. The age classes were defined as follows 

for all observations made from 1978 through 1980. 

Adult: animal of breeding size, usually over 2.0 m in nose to tail 

length. Minimum breeding size for females was defined as the size of the 

smallest female to give birth to a pup. Minimum breeding size for males was 

defined as the size of the smallest male to engage in courtship behavior, 

including the "rolling bellow" (see Kenyon and Rice 1959, p. 235). 

Subadult: animal near breeding size, approximately 1.6 to 1.9 m in 

length. Data fron the few known-age animals in this age class indicate that 

most 4- and 5-year-3ld females and most 4- to 7-year-old males would be 

included as subadults. I 

a 

Juvenile: small seal, not a pup or identified 1-year-old animal, 

approximately 1-4 to 1.6 m in length. This age class includes all 2-yearold and probably most 3-year-old seals. 

Yearling: approximately 1.3 to 1.4 m in length. Includes only those 

animals known to have been born on Laysan during the previous year. 

year-old animals born on other atolls or with marks that were unreadable 

during censuses would have been classified as juveniles. 

OnePup: young of the year, classified as "nursing" or "weaned," approximately 1.3 m in length at weaning. 

Length measurements are listed as a basic guide, They are not a reliable measure of age, as there is a wide overlap in lengths of animals known 

to have been born in different years. For some purposes subadult, juvenile, 

and yearling animals were combined into a.single age class and called immatures. 

Because of the inherent subjectivity of the age classifications used, 

it is likely that the specific boundaries between age classes vary slightly 

from census to census. These differences are probably insignificant due to 

the continual check provided by the large number of identifiable animals in 

the population, and because the same observers conducted the counts throughout all 4 years. Further, counts conducted with other monk seal researchers 

(including DeLong , Fiscus, A. Johnson, Kenyon, Knudt son, and Rauzon) showed 

close agreement in age classification, indicating the basic age categories 

are relatively distinct, even when observers differ and identified animals 

are not used. b 

Sex Determination 

While subtle sexual differences in face, body proportions, and coat 

color may be present in monk seals, we considered a good view of the posterior ventrum the only reliable method of determining sex, with three exceptions. 

1. The sex of seals recognizable by natural or applied marks was 

routinely recorded without reconfirmation. 
5 

2. The sex of adults attending nursing (i.e., unweaned) pups was 

assumed to be female. 

3. The sex of vocalizing adults giving the characteristic adult male 

"rolling bellow" during courtship displays was recorded as male, 

even if no ventral examination was made. 

By recording the sex of an animal only if it met one of the above 

criteria, errors in sex determination are very unlikely, but it is also 

inevitable that many seals will be recorded as "sex unknown." 

icant proportion of the seals ashore are unsexed, the observed sex ratio 

will be biased unless the probabil5ty of sexing males and females is the 

same. In particular, counts of adults mede during the pupping seasan will 

be strongly biased toward females (if very many seals are unsexed) because 

- all adults attending pups will be recorded as female. 

discussion of other potential biases, and offers a method of correcting for 

the female-with-pup bias. The female-with-pup correction should be applied 

to counts from all studies when interpreting or comparing sex ratio data 

collected during the pupping season. 

If a signifAppendix B presents a 

. Molt 

During 1977, data collected on censuses included noting any obviously 

molting seals. From 1978 through 1980, all seals seen on censuses were 

recorded as either premolt, molting, or postmolt. Premolt seals generally 

had a brownish-yellow tinge to the pelage, and often had algal growth on the 

hind flippers or muzzle. Postmolt seals were similar in color to recently 

weaned pups, with a gray or silver coat color. 

the hair began to gradually develop a brownish-yellow tinge. 

At about 3 months postmolt, 

In 1977 and early 1978, a seal was classified as molting once the first 

patch of hair was shed. In July of 1978 the molt was redefined to include 

animals as molting only if a patch of hair at least 2 cm square was missing 

from the chest of the animal. This change was necessary because some animals showed signs of molting on the hind flippers or around scars as much as 

a week before they began to molt elsewhere. Other animals retained a small 

patch of old pelage on their backs for a few days after the rest of the old 

hair had been shed. In the latter case, we continued to record a seal as 

molting until the last patch of old dorsal hair was gone. (Occasionally 

patches of epidermis remained attached after the last hair was shed: 

animals were called postmolt . These 

From 1978 through 1980, molting seals were classified by the percentage 

of body hair that had been shed. 

from (202 to >80%. (For additional information on the molt in Hawaiian monk 

seals see Johnson and Johnson 1981a.l 

There were five main categories, ranging 

Censusing Techniques 

Knowledge of the specific methodology used during seal censuses is 

extremely important when making comparisons between studies. Differences in 

such factors as start time, counting seals in the water, or overall time to 

complete a count can seriously bias comparisons. Unfortunately, these factors are sometimes difficult to identify, and are frequently ignored. 
6 

The methodology used on Laysan during the 4 years of our study was patterned after that used in previous surveys of the Hawaiian monk seal, and 

included counting seals in the nearshore water. Both our counts and previous ground surveys attempted to count the maximum number of seals using 

the area. Our methods differed slightly from previous counts in that greater precautions were taken to avoid counting individual seals more than 

once. Comparison of counts made by the authors with concurrent counts made 

by researchers visiting Laysan during the 4 years of the study period 

showed close agreement, despite the slight difference in methodology. 

Counts made by National Marine Fisheries Service personnel since 1982 have 

excluded all animals in the near-shore waters (Gilmartin, personal communication) and are therefore not directly comparable to our Laysan counts or to 

other counts made prior to 1982. 

The methodology used on Laysan was consistent between 1977 and 1980. 

To minimize disturbance, most beaches were rarely visited between censuses. 

Since a major objective of the study was observation of an undisturbed population, we made every effort to remain undetected by the seals. Observations 

were made at the greatest distance possible, using binoculars. 

A once every 4-day censusing schedule was maintained throughout all 4 

years of the study. These "main" censuses were usually started between 1200 

and 1600 and allowed direct comparison of haul-out parameters between years. 

Additional censuses, with variable start times were conducted after 1977. 

All censuses generally started on the west shore below camp. The two observers walked in opposite directions; one observer (BWJ) counted seals to the 

south while the other (PAJ) counted seals around the northern half of the 

island (observers occasionally switched censusing directions). 

normally ended approximately 3 hours after they started, when the two observers met on the east shore. 

Censuses 

All seals onshore during the census were added to the total unless a 

recognizable individual seal was encountered which had been counted earlier 

on the census. Seals floating in the water or swimming directly toward or 

away from the island were included in the count, unless previously identified. Seals swimming along the shoreline presented a problem, as these 

animals often swam back and forth along a large section of beach, introducing the possibility of double counting. 

included all seals swimming from an area not yet censused (unless a previously identified animal) and excluded from the total all seals swimming from 

an area already counted (unless the animal was an identified individual 

which had definitely not been seen previously on the count; e.g., when all 

seals of that age/sex class seen previously had been identified). 

To minimize this possibility, we 

Data collected on each seal during censuses throughout the 4 years 

included: age class, sex (when possible), identity (if known), and location 

on the island. 

stage of molt, including pre- or postmolt and molting (as percent molted) 

was recorded. 

Molting seals were noted in 1977; in all other years the 

Because of the nature of the study there were slight differences 

between data collected in 1977 and data from later years. 

year of the study few seals were individually recognizable; therefore, fewer 

seals were sexed on counts in 1977 than in subsequent years. The presence 

During the first 
7 

of large numbers of identified individuals by the end of the study made 

sexing a large proportion of the seals easier, and increased the consistency 

of the age class assignment. Finally, our definition of the molt stages 

changed during 1978. Thereafter, a seal was not recorded as molting until 

hair loss began on the chest. 

molting for longer periods in 1977 and early 1978 than in subsequent years 

(Johnson and Johnson 1981a) . 

This change meant that seals were recorded as 

Population Estimation 

The techniques used to estimate population size were designed to avoid 

any handling or disturbance to the ,seals. This was important for two reasons. First, earlier studies indicated human disturbance might be a major 

factor in the decline of the species (Kenyon 1972). Secondly, data obtained 

using nondisturbance methods can serve as an important control to test the 

reliability of future studies involving tagging of animals. Any studies 

which require handling of animals must assume that capture and tagging do 

not affect the haul-out behavior of the animal. The ratio of mean count to 

population estimate during our study can be compared with data collected in 

future years to test this assumption if counts are made in a comparable 

manner. 

The methodology was designed so that data could be collected using binoc 

ulars at a considerable distance from the animals. The techniques which 

required individual recognition of animals (Petersen Estimate, Petersen/Molt 

and Molt Summation) were based on detailed identification files which contained photographs, sketches, and descriptions of scars and marks. 

No techniques were used to estimate the number of pups born, as regular 

censusing throughout the field season provided an accurate count of the 

total pup production. Also, no attempt was made to estimate the number of 

yearlings in the population, since all pups born on Laysan the previous year 

were bleach marked, thus identifiable. 

Natality and Reproduction 

As the risk of disturbance to all beaches each day was considered 

unjustifiable, many areas of the island were not visited on noncensus days. 

Accurate determination of birth and wean dates for pups was considered 

important, so daily visits were frequently made to a particular beach when a 

birth or weaning was expected. Despite these additional observations, the 

actual day of birth was not determined for many pups, and it was necessary 

to use a best-guess method to minimize the possible error in assigning a 

birth or weaning date. The day of birth was recorded as the day halfway 

between when the pup was first seen and the last time the area had been 

checked. The only exception occurred when tracks and/or a fresh placenta 

indicated the birth was very recent (same day) or definitely had not been 

that recent. The same approach was used to determine the day of weaning, 

with the day of weaning considered to be the day halfway between the date 

the pup was last seen with an attending female and the first day it was seen 

alone. Since censuses were conducted at least once every 4 days, this 

introduced a maximum error of plus or minus 4 days into determination of age 

at weaning (or lactation interval for adult females). 

as the departure of the attending female while the pup remained. 

Weaning was defined 
8 

One pup was weaned prior to our arrival in 1980, but it was possible to 

determine the probable mother as the physical appearance and alteration in 

haul-out and molt patterns for only one female fit the pattern expected for 

the mother of the early pup. The reproductive data for this female has been 

included in the calculations, classifying her as parturient in 1980. In the 

only other case of a weaning prior to our arrival (in 1977), no such assumption was made. Only females known to produce pups were included as parturient. To be listed as nonparturient the female had to be seen throughout the 

period she might be expected to pup, or be obviously not pregnant at that time 

(otherwise her reproductive status for that year was excluded from analysis). 

Although it is unlikely that any female seen regularly on Laysan Island during 

a field season gave birth on another atoll in that year, the possibility 

cannot be excluded. No seals seen during our study on Laysan Island matched 

the photographs, sketches, or descriptions of mothers seen in 1980.at Kure 

Atoll (NMFS files examined in November 1980) or French Frigate Shoals (FWS 

files examined in July 19811. Any female that became pregnant but aborted 

prior to the start of the field season would have been classified as nonparturient . 

u 

* 

A subjective assessment of the size of most mothers was made several 

times during the field season, including before, during, and after lactation. A five point scale was used, with 1 being very small, and 5 very 

large. The mean of all recorded sizes was calculated for each female each 

year. The assignment of a size was based on overall length and girth. 

RESULTS AND DISCUSSION 

Island Counts 

In 1977 the mean of all censuses conducted between mid-March and midAugust (the period allowing comparisons between all years of the study) was 

179 seals. The highest count (211 seals) was as high as any count since 

1965, indicating little change in the total number of seals using Laysan 

between 1965 and 1977. 

The mean count during the mid-March to mid-August period in 1978 

declined dramatically to 123 animals. 

to have died or disappeared during what has been termed the 1978 "die-off." 

The mean count during 1979 was 113 seals, showing a further decline. The 

mean count of 112 seals in 1980 was almost identical to the 1979 mean, 

suggesting the Laysan population may have stabilized, but at approximately 

35% below the 1977 level. 

study are included in Appendix C.) 

Over 50 identified animals were known 

(The results of all censuses conducted during the 

The age composition also changed with the die-off of 1978. Figure 2 

shows the decrease in total seals, immatures, and adults, taken from all 

censuses conducted in the mid-March through mid-August period of each year. 

The greatest decline occurred in the immature seal counts. Field observation showed the yearling cohort was especially affected, with only 7 of the 

42 pups born in 1977 known to have survived through 1978. 
9 

208- 

E 14Gi 

N 128 

C 

0 80-- 

GB-- 

" 

100 'a___ &j-TOTQL 

,A D U L T S 

-1MMATURES 

77 

I 

78YEAR79 

I I I 

80 ~ 

Figure 2.--The mean mid-March through mid-August 

seal count , 197 7-80. 

Changes in the sex structure were more difficult to document as 44% of 

the seals (excluding pups) were not sexed on counts in 1977, compared with 

only 8% unsexed in 1980. The increase in number of animals sexed over the 

study was due to the greater number of individually recognizable seals as 

well as to the increased experience of the observers. 

To ascertain the degree of change in the sex structure of the population, it is necessary to examine potential sources of bias which could 

result in either males or females being easier to sex on counts. If no 

biases exist, then extrapolating the sex ratio of animals sexed on counts to 

the unsexed animals in each age class would be a valid way to compare data 

from 1977 with other years. Appendix B lists several potential biases, and 

presents data showing the effect of these biases was negligible, except for 

the correctable female-with-pup bias. 

Figure 3 compares the mean count for the adult and immature age/sex 

classes from mid-March through August 1977 with the same period in 1980. 

The data include a correction for the female-with-pup bias (as described in 

Appendix B) and assume that the ma1e:female sex ratio for unsexed animals is 

the same as for sexed animals. These data show similar declines in male and 

female numbers in both age classes, with a greater decline in the number of 

immature seals than adults (44% and 26%, respectively). 

Seasonal changes in haul-out patterns for each age/sex class can best 

be shown using data from 1979 and 1980 (1977 data are excluded because of 

the large number of unsexed animals and 1978 data because major changes were 

occurring in the composition of the population throughout that year). 

Figure 4 compares semimonthly mean counts for 1979 and 1980. Data from both 

years show a similar pattern, with an initial high in April followed by a 

low in May and a second high in June followed by a decline in July. A 

difference can be seen between years in the counts during the April and June 

high periods. In early April the 1979 counts were about 10% below the 

counts, but in early July the situation was reversed with the 1979 counts 

higher than the 1980 counts. 
10 

1977=OPEN BARS 1988=SHADED BARS 

M 

E 

A 

N 

# 

e ci 

0 

R 

E 

G0 

20 

0 1 

Ami ADF IPy AGEySEZ: CLksS 

I MF 

Figure 3.--Mean island count from mid-March through mid-August, 

based on extrapolated data from Appendix C. 

e' : I I I I I 

4 5 G 7 8 

MOtJTH 

Figure 4.--Mean count (excluding pups) from mid-March through mid-August. 

Figure 5 combines the data from both years to illustrate the overall 

pattern. Although the standard deviations for most months overlap, the 

count data show the lowest counts occurred in late May (at the start of the 

molting season) and the highest counts occurred in September (during the 

peak of the adult male molt). 

the least variance in counts and thus may be the best time to conduct yearto-year comparative counts on Laysan. 

Late May, early June, and early August showed 

Figures 6 through 9 present seasonal haul-out patterns for the different agelsex classes, based on the combined counts of 1979 and 1980. Counts 

have been corrected for the female-with-pup bias, and unsexed animals have 

been included based on the sex ratio of seals that were sexed in each age 

class. Counts of adult males were lowest during June and July, and highest 

in September (Fig. 6). Adult female counts remained relatively stable 

throughout the study until a decline in August and September (Fig. 7). 

Counts of immature males peaked in August (Fig. 81, and immature female 

counts peaked in June (Fig. 9). 
11 

140 

60 

40 

28 

1980 ONLY- 

/ 

1979+88 

01 : 1 I I I I I 

4 5 G 7 8 9 

MONTH 

Figure 5.--Semimonthly counts (mean and standard deviation) 

for 1979 and 1980 combined. 

1980 ONLY- 

>>( 

0’ : I I I I I I 

4 5 G 7 8 9 

MONTH 

Figure 6.--Adult male counts (mean and standard deviation) 

for 1979 and 1980 combined. 

1388 ONLY 

5- - -b’ 

0’ : I I I I I I 

4 5 G 7 8 9 

MONTH 

Figure 7.--Adult female counts (mean and standard deviation) 

for 1979 and 1980 combined. 
12 

I I I I 

4 5 G 7 8 9 

a- 

"t 

MONTH 

Figure 8 .--Immature male counts (mean and standard deviation) 

for 1979 and 1980 combined. 

I 51 I . 19SO ONLY 

01 : I I I 1 I I 

4 5 G 7 8 9 

MONTH 

Figure 9.--Immature female counts (mean and standard deviation) 

1979 and 1980 combined. 

Population Estimation 

In 1979 several methods for estimating the size of the monk seal population on Laysan Island were devised and tested, all using nondisturbance 

techniques exclusively. 

reported elsewhere (Johnson and Johnson 1981a). One of the techniques (the 

8-day Molt Estimate) was tested at French Frigate Shoals during 1980 (Johnson 

and Johnson 1984). These sources should be consulted for detailed discussion 

of assumptions, methods, inherent biases, and results. The following summary 

reviews each technique as it applies to data collected on Laysan during 1980. 

The methodology used and the 1979 results have been 

Molt Summa t ion 

Since all Hawaiian monk seals undergo a conspicuous annual molt, during 

which they remain on or near shore, counting each molting seal would result 

in a total population count for that atoll, discrete from all other atoll 
13 

n 

populations (Johnson and Johnson 1981a). In 1979 and 1980, frequent censusing during the molting season allowed us to identify and count each individual seal that molted during each study period. Most molting seals had 

been previously identified on the basis of natural or applied marks. Because 

of the extended molting season, relatively few seals molted at any one time, 

thus animals without identifiable marks could still be differentiated on the 

basis of age/sex category, stage of molt, and location on the island. Occasionally molting seals were marked with bleach to facilitate identification. 

In 1979 the study period ended well before the end of the molting season, 

thus a complete count of molting seals was not possible. In 1980, the field 

season extended throughout most of the molting season for all age/sex classes 

except adult males, allowing a total population count of the other age/sex 

classes. Summing the animals in all age classes, the Molt Summation data 

suggest a total of 158 seals, excluding adult males, used Laysan during the 

summer and fall of 1980. The number of molting seals in each age/sex class 

was as follows. 

Pups.--Of the 33 pups born on Laysan during 1980, 28 survived through 

weaning (and their postnatal molt) including 9 males and 19 females. 

Yearlings.--Twenty-four yearlings (seals born on Laysan in 1979) molted 

on Laysan during the 1980 field season (14 males and 10 females). 

yearling female was seen regularly throughout the field season but had not 

molted by our departure, resulting in a total yearling count of 25. Two 

other yearlings (one male and one female) were seen on Laysan early in the 

field season but not after the start of the molting season, and are therefore excluded from the yearling total. 

One 

Juveniles.--A total of 8 juvenile males and 11 juvenile females molted 

during the study period. This includes all the identified juveniles seen in 

1980 except one male which died on 23 April and another male which was not 

resighted after 26 July. 

Subadults.--A total of 30 subadult males and 18 subadult females molted 

during the field season. This included all identified subadults seen in 

1980 except one male, last seen on 22 April, and one female, last seen on 3 

April . 

Adult females.--Thirty-five adult females molted during the study 

period. Three additional females which pupped late in the pupping season had 

not molted by mid-September, resulting in a total adult female count for the 

summer and fall in 1980 of 38. An additional 11 adult females identified 

during 1980 were not seen molting, thus were excluded from the molt total. 

(The unusually large number of adult females disappearing during 1980 is a 

matter of concern, needing further study and careful monitoring.) 

The Molt Summation count is an ideal method for determining population 

size as it provides an actual count of the total number of animals in each 

age/sex class and defines a population discrete from all other populations 

(eliminating the problem of interatoll movement). 

technique is the requirement for frequent censusing over many months. (Although the 1980 field season lasted over six months, it was not long enough 

to encompass the entire adult male or yearling molting season.) Therefore, 

future studies may have to rely on other population estimation techniques. 

The major drawback to the 
14 

The sighting data from Laysan indicate seals that molt on the island are 

probably year-round residents. However, other populations could be less 

stable, with the molting population not the same as the population using the 

area at other times of the year. To test this, population estimating techniques using data gathered in spring or winter months can be compared to the 

Molt Summation results. 

Petersen Estimate 

The Petersen Estimate (also known as the Lincoln Index) is a simple 

method for estimating N, the number of animals in a closed population. As 

described by Seber (19731, a sample of marked animals (nl) is released into a - 

population. 

marked and unmarked aniaals, a second sample of animals (1-121 is taken, a 

subset (m2) of which were previously marked. Assuming that the proportion of 

marked animals in the second sample is a reasonable estimate of the proportion of marked to unmarked animals in N, we can equate the two and obtain an 

estimate of N. Thus: 

After sufficient time has elapsed to nllow for random mixing of ? 

m2 nl h nl n2 

n2 N m2 

-=- -or N = - . 

Specifics of the methodology used in 1979 and a discussion of assumptions can be found in Johnson and Johnson (1981a). 

difference between 1979 and 1980 was an attempt in 1980 to increase the 

reliability of the estimate by increasing the number of marked animals in 

each age/sex class, and by increasing the number of recapture censuses. 

following reviews the methodology as it applies to the 1980 data. 

The major methodological 

The 

Marked animals.--Prior to the start of the Petersen Estimate censuses, a 

list was compiled of all seals with identifiable scars or marks that were 

seen on Laysan during the previous 2 months (15 March to 15 May). 

were listed by age/sex class and ranked according to the conspicuousness of 

their markings. 

actual number of animals in both age classes was known.) Excluded from the 

list were two animals which died before 15 May and three seals which were 

intermediate in size, sometimes being classed as subadult, sometimes as 

adult . 

The seals 

(Yearlings and pups were excluded from the lists as the 

b The seals chosen as the marked sample included the 40 adult males and 25 

adult females with the most conspicuous markings, and all 23 immature males 

and 19 immature females on the list. 

Census procedure.--The procedure was designed to ensure that any marked 

animals onshore would always be recognized. The perimeter of the island was 

divided into three roughly equal sections. 

times between 17 May and 5 June. 

in Table 1. 

Each section was censused nine 

The date and time of each census are listed 

Each census involved two observers walking onesof the three sections of 

the island perimeter together. 

him/herself so as not to be observed by any seals, and waited until the seal 

An observer stopped at each seal, positioning 
15 

L 

Table 1.--Date and time of Petersen Estimate censuses 

on Laysan Island, 1980. 

Time* North Southeast Southwest 

0730-1115 5-1 7 5-20 5-22 

5-26 5-28 5-30 

6-01 6-03 6-0 5 

1130-1515 5-22 5-1 7 3-20 

5-30 5-26 5-28 

6-0 5 6-01. 6-03 

1 53 0-1 91 5 5-20 5-22 5-17 

5-28 5-3 0 5-26 

6-03 6-0 5 6-01 

*Hawaii Standard Time. 

had moved sufficiently to allow a good view of all sides. 

the animal were compared with photographs and sketches of identified seals 

carried by the observer. 

count. All animals which entered the water during the observation period, 

including identified seals, were excluded, unless the observer had clearly 

seen all sides of the animal. To increase.the reliability of age classification, one of the observers (PAJ) was responsible for recording age/sex data 

on all seals. 

Scars or marks on 

Animals in the water were not included in the 

Results.--The formulas used to estimate population size and variance 

(Seber 1973, p. 61) are: 

h 

N = nl(n2+l)/(m2+1) 

The results of the Petersen Estimate (N), standard deviation (SI)), and 

upper (NU) and lower (NL) 95% confidence limits (from Pearson and Hartley 

1966, p. 228) for each age/sex class are shown in Table 2. An estimate of 

the total population (excluding pups) and confidence limits were obtained by 

summing the separate age/sex class values. 

Petersen/Molt Estimate 

The presence of animals marked prior to the molting season provided 

another opportunity for estimating the 1980 Laysan monk seal population. 

Since the actual number of seals which molted before our departure from 
16 

Table 2.--Petersen Estimate for each age and 

sex class at Laysan Island, 1980. 

Adult Adult Immature Immature Yearling Yearling 

male female male female male* female* Total 

nl 40 25 23 19 

n2 246 154 136 125 

m2 74 86 83 85 

h 

N 132 45 38 28 14 12 269 

SD 12.6 3.1 2.5 1.6 

NL 110 39 33 25 14 12 233 

Nu 160 53 44 32 14 12 315 

*The size of the yearling population was known, thus N = NU = NL. 

Laysan was 

molted was 

on Laysan) 

known, and the proportion of a "marked" sample of seals which had 

also known, then the total resident population (all seals molting . can be estimated by: 

m2 n2 , nln2 

-3- or N=- 

nl N m2 

where: 

nl = total number of marked animals 

n2 = total number of seals known to have molted 

m2 = marked animals known to have molted 

^N = estimated resident population. 

This is the same formula for calculation of the Petersen Estimate, discussed 

in the previous section. 

Methdbalogy.--For calculation of the Petersen/Molt Estimate, the nl 

sample of marked animals was comprised of all uniquely scarred or marked 

seals seen by both observers during May. (Because adult males molted later 

than the other age/sex classes, and because some adult males stayed away from 

the island for over a month at a time, the marked sample of adult males was 

taken from all adult males seen during May and June.) Of the total number of 

animals which had started the molt by 16 September (n2), all were examined 

for scars or marks, and the number of molted marked animals determined (m2). 

Sampling was considered without replacement, as each molting seal (whether 

marked or not) could only be counted once in the n2 sample. The formula for 

an unbiased estimate of N (from Seber 1973, p. 60) is: 
17 

4 

with a variance of: 

Results.--The results of thc'Petersen/Molt Estimate, with standard 

deviation (SD), upper (NU), and lower (NL) confidence limits, are listed in 

Table 3. 

Table 3 .--Petersen/Molt estimate for each age/sex 

class at Laysan Island, 1980. 

ADM ADF IMM IMF YM* YF* TOTAL 

nl 51 31 30 20 

n2 78 37 36 27 

m2 30 27 30 20 

N 132 42 36 27 14 12 263 

SD 11.6 1.5 0 0 

NL 100 37 36 27 14 12 2 26 

NU 182 55 45 34 14 12 3 42 

*The size of the yearling population was known, thus N = NU = NL. 

Summary of Estimates 

Table 4 compares the results of the 1980 Petersen and Petersen/Molt 

estimates with the 1980 Molt Summation count, and also presents the best 

population estimate from 1979 (from Johnson and Johnson 1981a). 

The best estimate (the one with the narrowest confidence limits) for 

the 1980 Laysan monk seal population is the Petersen Estimate. 

bility of the estimate is strengthened by its close agreement with the 

Petersen/Molt estimate results and the close agreement of both estimates 

with the Molt Summation count. Further, the Molt Summation count for immatures was within two animals (for each sex) of both estimates, indicating 

the resident population (defined as the seals which molted on Laysan) is 

the same as the number of seals using Laysan during earlier months. 

The relia- 
18 

Table 4.--Comparison of results from estimation techniques used in 1980 

with the best population estimate from 1979 on Laysan Island. 

each age/sex class include the estimate with the lower and upper confidence 

limits, when available. The yearling totals are not estimates, as the 

actual size of that age class was known. 

Numbers for 

ADM ADP IPM IUF YM YF TOTAL IHN+YH IUF+YF 

Petersen Estimate 132 110-160 45 39-53 38 33-44 28 25-32 (14) (12) 269 233-315 52 47-58 40 37-44 

Petersenlblt 132 100-1&2 42 37-55 36 36-45 27 27-34 (14) (12) 263 226-342 50 50-59 39 39-46 

Molt Summation --- 38 38 29 (14) (11) 52 40 b 

1979 Estimate 148 103-241 44 38-58 46 34-72 35 25-58 (8) (9) 290 217-446 54 42-80 44 34-67 

When compared with the 1979 estimates, the adult numbers fall well 

within the confidence limits of both years, indicating no significant change 

in the adult population. But within 1980, the lower confidence limit for the 

Petersen Estimate of adult females (early spring) was higher than the Molt 

Summation count (summer), indicating a serious decline occurred in the number 

of adult females using Laysan in 1980. 

Although the immature male and female confidence limits for both years 

overlap, the 1979 immature estimates for both sexes are greater than the 

upper confidence limits for 1980. But when the yearling count is added to 

the immature estimate for both years, there is no significant difference 

between years. 

Like the earlier 1979 data, the 1980 data indicate adult males 

outnumber adult females by approximately 3:1, while the immature sex ratio 

is almost equal. (Although immature males appear to outnumber immature 

females, sighting records of tagged animals indicate that most 7-year-old 

females are called adults, while most 7- and 8-year-old males are still 

classed as immatures. Further, even if an equal sex ratio were expected, a 

Chi-square analysis indicates that no significant difference exists between 

the observed frequencies (X2 = 1.2, df = 1, n.s.1.) 

Percentage Ashore 

The Petersen Estimate indicates the total 1980 Laysan seal population, 

excluding pups, was 269 seals. The mean count (excluding pups) for all 

censuses was 100 seals. Thus, on the average, just slightly more than onethird of the Laysan population was counted on censuses. 

during 1980 was 137, or approximately one-half the actual population. 

The maximum count 

. 

Figure 10 presents the semimonthly percent of the total population 

which was ashore during afternoon censuses. Figures 11 through 14 present 

the semimonthly percent ashore for each age/sex class. The total population estimates were taken from the 1980 Petersen Estimate except for adult 

females which were estimated at 42, the mean of the Petersen Estimate and 
19 

A 40-- 3 30-- 

0 28-- E 18-- 

0: I I I I I I 

4 5 G 7 8 9 

MONTH 

TOTQL POPULFtTION <MINUS PUPS) 

100 

p 90 

80 

70 

60 

50 

A 40 a 30 

0 28 

0 

E 10 

MONTH 

Figure 10.--Percent of total population ashore on counts, 1980 

(mean and standard deviation). 

ADULT MALES 

108- 

P 90-- E 

k 80.- 

4 5 G 7 8 

MONTH 
20 

IMMATURE MALES 

1 

P 

E 

R 

C 

E 

I4 

T 

A 

S 

H 

0 

R 

E 

80 

50 

40 

30 

I I I I I I 

4 5 G 7 8 9 

MONTH 

Figure 13.--Percent of immature males ashore on counts, 1980 

(mean and standard deviation). 

TMMATURE FEMALES 

74- I 

1 

P 

E 

R 

C 

6 GO 

T 50 

A 40 3 30 

0 28 R 

E 

0 0

4 5 G 7 8 9 

MONTH 

Figure 14.--Percent of immature females ashore on counts, 1980 

(mean and standard deviation). 

the later Xolt Summation (this was done to compensate for the large number 

of adult females disappearing during 1980). The mean semimonthly counts 

(with standard deviations) were taken from censuses started between 1200 

and 1600 (Hawaii standard time). 

unsexed on these censuses. For the purposes of this summary, the sex of 

unsexed animals was estimated, by first correcting for the female-with-pup 

bias, and then by assuming that the proportion of males to females in the 

sexed animals was the same in the unsexed animals (with adults and 

immatures calculated separately, see Appendix C). 

An average of 8% of the seals were 

For the period covered by the 1980 field season, adult males spent 

proportionately the least time ashore (30%) 

most time ashore (51%). Adult females spent about 42% of their time ashore, 

while about 40% of the immature females were ashore on counts. If data from 

the entire year were available this pattern might change, as seasonal variation in percent ashore was seen in each age/sex class. 

while immature males spent the 
21 

Distribution 

Seals did not use all haul-out areas around Laysan Island equally. The 

choice of haul-out area varied with age, sex, and time of year. 

The beaches around Laysan differed in several ways, including: amount of 

vegetation; exposure to trade winds; nearshore substrate (sand or reef) and 

water depth; onshore substrate (sand or rock), and other less obvious characteristics. 

During the 1977 field season the circumference of the island was subdivided into four "ecelogical" units, roughly equal in size. 

were based primarily on availability of vegetation and the type of nearshore 

substrate. The areas and descriptive characteristics were: 

The divisions 

NORTH - 

WEST - 

EAST - 

SOUTH - 

wide beaches, exposed to trade winds, with little or no 

vegetation at the beach crest. 

narrow beaches, on the leeward side of the island, with 

numerous shallow reef areas in the nearshore waters, and 

vegetation at the beach crest. 

wide beaches, with a shallow rock ledge extending the length 

of the nearshore area and no vegetation near the beach 

crest . sand beaches interspersed with rock ledges, rough surf, and 

some cover at the beach crest. 

Figure 15 shows the proportion of seals which used each area from March 

through August 1977, based on census counts. Figure 16 presents comparable 

data from 1980. The use of the south shore was similar in both years. Use 

of the east shore increased about 10% in 1980, while both the west and north 

areas showed a 5% decrease in use. The significance of this shift, if any, 

is unknown. 

Detailed data on the haul-out patterns for each age/sex class are 

available only for 1980. With a high proportion of seals sexed on censuses 

and the island circumference divided into 14 areas, it was possible to identify haul-out patterns unique to different segments of the population. 

Figure 17 presents a map of Laysan (from an FWS aerial photograph taken in 

May of 1980) showing the area breakdowns. Table 5 presents the percentage of 

seals in each age/sex class utilizing each of the 14 areas. These data 

indicate that all age/sex classes, except pups, preferred the northeast 

beaches (areas 5 and 6) but the second most utilized haul-out area varied for 

each age/sex class. 

Over 30% of the adult males hauled out along the southwest corner of the 

island (areas 11, 12, and 13). This same part of the island was used by less 

than 4% of the adult females and about 10% of the immature seals. A substantial number of adult males also used area 14, which was a preferred haul-out 

area for mothers attending pups. However, males used this area mostly during 

August and September (during the adult male molt), well after the pupping 

seas on. 
22 

P 

E 

R 

c 

E 

N 

T 

A 

S 

H 

O 

R 

E 

P 

E 

R 

N 

T 

A 

S 

H 

0 

R 

E 

E 

10-- e 

€i fl 

0 I I I I - 

3 4 5 i 

MONTH 

Figure 15.--Mean percent ashore in each of 

four areas on Laysan Island, 1977. 

68 

50[ 40 [ NORTH 

L-. 3_61 SOUTH 1°t I I I I I I 

3 4 5 G 7 8 

MONTH 

0' 

Figure 16.--Hean percent ashore in each of 

four areas on Laysan Island, 1980. 

In addition to areas 5 and 6, adult females were found primarily in 

areas 2, 3, 4, and 14. If all sightings of females attending pups are 

excluded, the percentage of females using areas 2 and 14 (the major pupping 

areas) decreased while the percentage of adult females using areas 5 and 6 

increased. It appears that the selection of rookery areas is not based on 

the same factors affecting choice of haul-out area at other times of the 

year. 

Immature males used areas 5 and 6 primarily, with the next highest numbers found in areas 4, 9, and 10. The immature males were the only age/sex 

class to haul out in substantial numbers on exposed rocks, 10 to 40 m away 

from the beach along the shallow rock ledge area of the east shore (areas 9 

and 10). Seven percent of the subadult male sightings, 4% of the juvenile 

male sightings, and 10% of the yearling male sightings were of animals on 

offshore rocks. Overall, 7X of immature male sightings were of animals 
23 

Figure 17.--Map of Laysan Island taken from aerial photograph 

(FWS 1980) showing circumference divided into 14 areas. 
24 

Table 5.--Percentage of each age/sex class and the total number 

of seals using the various areas on Laysan Island; based on mean 

monthly census counts in 1980. ---C--------L-------___________L________------------------ 

1 5 4 3 4 5 5 6 

2 4 13 8 3 5 23 12 

3 5 11 10 3 4 13 10 

4 4 10 12 * 11 8 7 15 

5 11 13 18 18 27 3 S 

6 15 21 29 17 24 6 8 

7 2 6 4 6 6 11 3 

8 1 2 2 3 2 2 2 

9 2 2 2 10 3 1 4 

10 6 3 3 12 6 5 6 

11 14 1 1 4 4 0 1 

12 10 1 2 2 3 0 1 

13 10 2 1. 5 2 2 15 

14 11 12 7 3 3 24 9 

5 

6 

6 

9 

15 

16 

5 

2 

5 

7 

7 

4 

7 

8 

hauled out on offshore rocks or sleeping in the water beside the rocks. 

Age/sex structure data based on counts which include animals using the offshore rocks should therefore not be compared with data which exclude these 

animals. 

Over 50% of the itmnature females hauled out in areas 5 and 6. Area 4 

was the only other area used by more than 6% of the immature females. 

1X of immature female sightings were of animals resting on or near exposed 

off shore rocks. 

(Only 

Areas 2 and 14 were the areas most often selected by females attending 

pups, but a substantial number of mothers with nursing pups were also found 

in areas 3 and 7. After weaning, pups continued to use the same areas (with 

the exception of area 7) and also began to congregate in areas 4 and 13. 

These latter areas had protected pools of water near shore which attracted 

many newly weaned pups. 

agelsex class not found in substantial numbers in areas 5 and 6. 

Pups, both suckling and weaned, were the only 

Other than the seasonal changes just mentioned, the haul-out patterns 

for the different age/sex classes were generally consistent throughout the 

field season. 

Molt 

Seasonal Patterns 

The molting season began in April when the first adult and subadult 

females started to molt and continued at least into December when a few 
25 

molting adult males and yearlings were seen (no visits were made to Laysan 

in January or early February). It is possible that seals molted in all 

months of the year as the molt was delayed in some animals with injuries or 

apparent illnesses, and adult females having pups late in the year would 

molt 2 to 3 months after weaning their pups (the normal delay for parturient 

females). 

The molt for individual seals (the period of visible hair loss) aver- ' 

aged 9 days. The molting season for the various segments of the population 

differed, with little overlap between sane age/sex groups. 

every seal molting on Laysan are available for the April through early 

August period of 1979, and the April through early September period of 1980. 

Molt dates for 

The following 10 figures present data on the number of molting seals 

seen through mid-September. Each data point represents the number of seals 

which reached the midpoint of their molt (50% of the old hair shed) during 

a particular semimonthly period. (Using a midmolt date reduced the chance 

of missing seals at either the beginning or final stages of the molt.) 

Because of the relatively small number of seals which molt during each 

semimonthly period (particularly when the age/sex classes are separated), 

considerable variation is 1ikely.from year to year. To increase sample 

size, molt data from 1979 and 1980 have been combined. 

also presented separately to show the pattern through mid-September as no 

data are available after mid-August of 1979.) 

(Data from 1980 are 

Figure 18 presents the semimonthly sightings of molting seals on Laysan. Few seals of any age/sex class molted in April or May. 

molting seals increased rapidly during June, showed a slight decline in 

July, and then increased again in August. 

to adult males. Figure 19 shows that excluding adult males changes the 

upward trend. 

The number of 

The August rise was due primarily 

The timing of molt for adult females was somewhat dependent on their 

reproductive status. The overall pattern for adult females is shown in 

Figure 20. Nonparturient females began to molt in April, with the majority 

molting in late June (Fig. 21). 

their pups were weaned, with the majority molting in late July and early 

August (Fig. 22). 

Parturient females did not molt until after 

Subadult females showed a molting season similar to that of the nonparturient females, with the greatest frequency of molters seen in June (Fig. 

23). Subadult males molted later than subadult females, with the onset of 

molt in June and the peak occurring in July and early August (Fig. 24). 

Juvenile seals and yearlings showed less sex difference in molting 

season than other age classes. Juveniles of both sexes began molting in 

early June, with the greatest number of molting juveniles seen in late June 

(Fig. 25 and 26). The sample size of molting yearlings was too small to 

separate males from females. Adding the molt data for both sexes shows a 

later molt than the older animals already described, with the highest number 

of molting yearlings not seen until August (Fig. 27). The only age class 

still molting in large numbers when the yearlings*molted were the adult 

males. 
26 

0 

M 

B 

E 

R 

M 

O 

L 

T 

I 

N 

G 

TOTAL POPULATION (MINUS PUPS) 

GOT 

Figure 18.--The number of seals which molted (based on 

midmolt dates) during each semimonthly period. 

N 

U 

M 

B 

E 

R 

M 

O 

L 

N 

G 

‘J; 

TOTAL (MINUS ADULT MALES & PUPS) 

60 

50 

48 

38 

20 

18 

8 

1979+88- 

4 5 G 7 8 9 

MONTH 

Figure 19.--The number of seals which molted (based on 

midmolt dates) during each semimonthly period. 

ADULT FEMALES 

M 

I! I 

N 

G 

8 

G ‘I 4 

2 0 

p”-p 1979+88- 

4 5 G 7 8 9 

MONTH 

Figure 20.--The number of seals which molted (based on 

midmolt dates) during each semimonthly period. 
27 

N 

U 

M 

R 

M 

IO 

L 

T 

I 

E! 

12 

12 

10 

NONPARTURIENT ADULT FEMALES 

8 1979+80- 

G 

4 

2 

0 

4 5 G 7 8 9 MONTH 

Figure 21.--The number of seals which molted (based on 

midmolt dates) during each semimonthly period. 

PARTURIENT ADULT FEMALES 

IY 

N 

G 

16 

14 

12 

10 

8 

G 

4 

2 

0 

1979+80- 

4 5 G 7 8 9 MONTH 

Figure 22.--The number of seals which molted (based on 

midmolt dates) during each semimonthly period. 

SUBADULT FEMALES 

- 1979+88 

G 7 8 

MONTH 5 

I 

9 

Figure 23.--The number of seals which molted (based on 

midmolt dates) during each semimonthly period. 
28 

SUBCSDULT MALES 

E 

R 

M 

F I 

N 

G 

18 

16 

14 

12 1979+80- 

10 

8 

G 

4 

2 

0 4 5 G 7 8 9 

MONTH 

Figure 24.--The number of seals which molted (based on 

midmolt dates) during each semimonthly period. 

i JUUEN I LE FEMALES 

' N 115 U 

M 14 

I 18 

E 12 

T4 I 

Ncz 

R 10 

8 1; 1979+88- L G 

0 

4 5 G 7 8 9 MONTH 

Figure 25.--The number of seals which molted (based on midmolt 

dates) during each semimonthly period. 

i 

Figure 26 

JUVENILE MALES 

18 

14 

14 

12 

10 

8 1979+88- 

G 

4 

2 

0 

4 5 6: 7 8 9 MONTH 

.--The number of seals which molted (based on midmolt 

dates) during each semimonthly period. 

I 
29 

t-4 

U 

M 

B 

E 

R 

'M 

0 

L 

T 

I 

N 

G 

YEARLINGS <MALE S, FEMALE) 

18 

115 

14 

12 

10 

8 1979+80- 

G 

4 

2 

0 

Figure 27.--The number of seals which molted (based on midmolt 

dates) during each semimonthly period. 

The 1979 and 1980 field seasons did not continue through the adult male 

molting season, so data from 1978 are presented in Figure 28. 

data presented for the other age/sex classes, these numbers are based on the 

mean number of molting seals seen on the censuses (a complete listing of all 

molting seals was not collected in 1978). Adult males did not begin to molt 

until August, with the number of molting seals reaching a peak in late 

September. 

October 13.) 

adult and adult females, there was little overlap in molting season for 

adult and subadult males. 

potentially be useful in distinguishing between reproductive and nonreproduct ive males. 

Unlike the 

(The early October "mean" is based on a single count made on 

Unlike the similarity in molting patterns seen between subThis difference (reflected in the coat color) can 

M 

E 

A 

N 

n 

M 

a 

ICi 

I 

ADULT MALES 41978) 

Figure 28.--Mean number of molting adult males seen on censuses, 1978. 

(October data based on one count.) 
30 

Island Usage 

Table 6 presents data on the percentage of each age/sex class which 

molted in the various areas around Laysan (based on 1980 counts). 

Table 6.--Comparison between the percentage of each age/sex class molting in 

the various areas of Laysan with the distribution throughout the year, 1980. 

Areas 

Adult-Males 

Mo 1 t Tc tal 

Adult-Females Immatxre-Males Immature-Females 

Molt Total Molt Total Molt Total 

1+2 

3+4 

5+6 

7+8 

9+10 

11+12 

13+14 

8 9 

0 9 

14 26 

2 3 

7 8 

26 24 

42 21 

8 16 4 7 11 10 

20 21 10 13 10 12 

43 34 42 35 43 52 

7 8 15 9 12 7 

9 5 7 22 14 9 

0 2 7 6 3 6 

14 14 15 8 8 5 

Over 40% of all sightings of molting adult females, immature males, and 

immature females occurred in areas 5 and 6. Areas 5 and 6 were also the areas 

most frequently used by these age/sex classes for haul out in 1980. 

with the exception of a substantial reduction in the usage of areas 9 and 10 

by molting immature males, the distribution of molting adult females and immatures of both sexes was similar to their distribution throughout the year. 

Overall, 

This was not true for adult males. The greatest number of adult males 

molted in areas 13 and 14 (42x1, twice the percentage of adult males normally 

hauled out in those areas. The increased use of areas 13 and 14 corresponded 

to a reduction in use of areas 3 through 6, the areas used by the greatest 

number of molting seals from the other age/sex classes. 

Reproduction 

Sexual Maturity: Females 

No pups born during the study had reached reproductive size by 1980. 

Only two females from the 1977 pup cohort were still alive and recognizable 

in 1980 (as 3-year-old seals); one was consistently classified as a 

juvenile and the other was generally classed as a subadult. 

Tag numbers of 10 females marked prior to 1972 were read in 1977. 

Data from one of these females have been excluded as the animal was not 

resighted after the first day of the study. (Tag numbers are listed in 

Appendix I>.> 

old in 1977. 

6 years old produced pups, while one of the two 7-year-old seals did have a 

pup. All tagged females 7 or older were consistently classified as 

adult size when observed, whereas the females seen when 5 or 6 years old 

The ages of the remaining animals ranged from 5 to 11 years 

As Table 7 shows, neither of the two females seen when 5 or 
31 

Table 7.--Number of births to known-age females between 1977 

and 1980 on Laysan Island. 

Age of Female 

5 6 7 8 9 10 11 12 13 14 

With Pups 00111 3 3 3 01 

Without Pups 21113 3 2 0 10 

Total Sample 21224 6 5 3 1 1 

- 

were recorded as subadult size. Additionally, of the six small juvenile 

females (probably 1- or 2-year-old seals) identified in 1977 and seen 

through 1980, none had produced pups by 1980. 

From these limited data it appears age at first reproduction for 

female Hawaiian monk seals (as measured by the birth of the first pup) may 

not occur until age seven or later. 

females which died on Laysan during the study, including a known-age 4-yearold female were given to NMFS for analysis. 

sexual maturity will become available when analysis of reproductive tracts 

and tooth sections from these animals is completed.) 

(Materials from subadult and adult 

Additional information on 

Sexual Maturity: Males 

It was more difficult to measure age at first reproduction for male 

A subjective age was recorded for the six tagged 

monk seals as sexual maturity of males did not result in an observable 

event such as a birth. 

males seen during the study, based on size and the presence or absence of 

reproductive behaviors normally shown by adult males (tag numbers are 

included in Appendix D). As Table 8 shows, males generally did not exhibit 

typical "courtship" behavior until 8 or 9 years old, although it is possible that physiological maturity occurred earlier. Of the three males 

still sighted from the 1977 pup cohort in 1980, one was generally listed as 

a subadult, while the other two were usually classified as juveniles. 

Table 8.--Subjective age classification of tagged, knownage males between 1977 and 1980 on Laysan Island. 

Age of Males 

5 6 7 8 9 

Subadult 2 3 3 3 0 

Adult 0 0 1 1 3 
32 

A potentially useful means of identifying sexual maturity in males is 

by the timing of their molt. 

overlap in molting seasons, as all but 5% of the subadult males had completed their molt by the end of August, when the first adult males were 

beginning to molt. 

Subadult and adult males showed minimal 

Reproductive Rate 

Ovulation and pregnancy rates could not be determined from observations of monk seals on Laysan, therefore reproductive success for females 

was measured by the birth of a pup. 

care should be taken to compare only similar data. 

In comparisons with other species, 

There are several ways to estimate the reproductive rate of the Laysan 

population. Four different approaches will be presented. 

Tagged-females.--The reproductive status was determined for nine 

tagged females, ranging in age from 5 to 11 years old in 1977. Of the four 

females still being seen in 1980, the youngest was 10 years old, the oldest 

14. Sighting records presented in Table 7 show the change in frequency of 

pupping with age. Of the 14 sightings of females aged 7 through 10, 43% 

had pups (5- and 6-year-old animals were excluded as they were classified 

as subadult in size). Of the 10 sightings of females 11 through 14, 70% 

pupped. While this difference indicates an increased fertility with age, 

the sample size is too small to tell if the trend is significant. The 

overall reproductive success was 13 pups to 24 adults, or 54%. As the 

Tagged-female sample is small, and includes only animals up to age 14, it 

is probably not representative of the total population. 

Estimated-females.--Population estimates for adult females during the 

pupping season provide another means of calculating reproductive rate. 

Population estimates are only available for 1979 and 1980 (the estimates 

with the narrowest confidence limits were used for each year). During 

1979, 32 pups were born to an estimated adult female population of 44 

(Table 4). 

tion of 45 animals (Petersen Estimate, Table 4). Using these numbers, the 

estimated reproductive rate for 1979 was 73%, with a range of 55 to 84% 

(using the lower and upper confidence limits for the adult female estimate). The estimate was also 73% for 1980, with a range of 60 to 89%. The 

disadvantages of this approach include: the wide range of the estimate (due 

to the confidence limits of the adult female estimate); a certain degree of 

subjectivity inherent in deciding which females to class as adult; and the 

availability of the estimate for only 2 years. 

In 1980, 33 pups were born to an estimated adult female populaIdentified-females.--Table 9 presents yearly reproductive rates for 

well-identified females seen, and classed as adult size, in at least 3 

years of the study (data taken from Appendix E). 

rate for the 4 years was 64%. The data for the individual years indicate 

an increase in reproductive success occurred during the study. When data 

from 1977 are combined with 1978 (for a 57% reproductive rate), and data 

from 1979 with 1980 (for a 72% rate), Chi-square analysis shows a 

significantly lower pupping rate for the first 2 years of the study than 

for the last 2 years (x2 = 4.01, pC0.05). 

the advantage of a large sample size and covers the entire 4-year study 

The overall reproductive 

The Identified-female sample has 
33 

period. One disadvantage of this sample is the subjectivity of size classification. A second disadvantage is the increased age over time, as 

animals had to have known reproductive histories in at least 3 years. This 

requirement did not allow recruitment after 1978 or mortality before 1980. 

As a check on the degree of this latter problem, a pupping rate was calculated, based on the number of pups born to well-identified females seen in 

1980. This sample includes all females of reproductive size in that year, 

not just animals of adult size in previous years. The resulting reproductive rate of 75% (30 pups born to 40 adult females) is only slightly lower 

than the 79% presented in Table 9 for 1980, indicating the increased age of 

the Identified-female sample was not solely responsible for the increased 

reproductive rate. 

Table 9.--Yearly reproductive rates for females seen and 

classed as adult in at least 3 years on Laysan Island. 

Total 

Year Births Females Rate 

1977 

1978 

1979 

1980 

23 

24 

29 

27 

39 

44 

44 

34 

0.59 

0.55 

0.66 

0.79 

To tal s 103 161 0.64 

Mature-females.--Some variability due to the subjectivity of age classification can be eliminated by including only those females known to be 

mature by a previous birth. The Mature-female sample included all females 

pupping in 1977, 1978, or 1979 that were seen in at least one additional 

year. 

calculations. 

in years 2 through 4. Animals first observed pupping in 1977 have up to 3 

years data, whereas females first observed pupping in 1979 are included in 

The data for the first observed birth (year 1) are excluded from the 

Table 10 presents the sample size and the percentage pupping 

Table 10.--Reproductive status of females in years following the first 

observed birth. 

of adult females (N), and percentage pupping (XI. 

Data for each year include numb%r of pups born (No.), number 

First Year 2 Year 3 Year 4 To tal s 

Birth No. N % No. N % No. N % No. N % 

1977 15 27 56 18 23 78 15 19 79 48 69 70 

1978 6 12 50 8 9 89 14 21 67 

1979 2 3 67 ---- 2 3 67 

---- 

---- 

Totals 23 42 55 26 32 81 15 19 79 64 93 69 
34 

only 1 year. The reproductive rate increased dramatically from year 2 to 

years 3 and 4, with an overall reproductive success for years 2 through 4 

of 64 births to 93 female sightings, or 69%. (Reproductive patterns for 

individual seals are listed in Appendix E.) 

Previous observation of Hawaiian monk seals had suggested that a large 

proportion of monk seal females missed pregnancy in the year following a 

birth (Rice 1960). 

lower, as all females in the sample pupped in year 1. Testing whether the 

probability of pupping in year 2 is independent of reproductive status in 

year 1 is possible by comparing the observed frequency of pupping (for 

females seen in 3 consecutive years) against the expected frequencies of 

a binomial distribution. As can be seen in Tabie 11, there is no significant difference; thus reproduction in year 2 can be considered independent 

of previous reproductive status. 

If true, then the year 2 reproductive rate would be 

Defining maturity by the actual birth of a pup guarantees that all 

animals in the sample are capable of reproduction; however, it also raises 

the age of the animals used to calculate the reproductive rate, and eliminates infertile animals, resulting in a higher reproductive rate than for 

the population as a whole. 

Table 11.--Observed and Expected number of females showing each of four 

reproductive patterns, including only females known to be mature which were 

seen in at least 3 years on Laysan Island (P = pup, N = no-pup). 

Ob 8 e yv e d Expected 

PPP 

PPN 

PNP 

PNN 

13 

2 

13 

5 

12.7 

7.8 

7.8 

4.5 

x2 = 4.7 p = n.6. 

Discussion.--Table 12 summarizes the results from the four reproductive rate estimates. Of the four, the Tagged-female estimate is the least 

reliable due to small sample size and nonrepresentative age structure. Of 

the other three estimates, the one most likely to be representative of the 

pupping rate between 1977 and 1980 is the Identified-female sample. The 

overall reproductive rate for the sample, 64%, was based on a large sample 

size covering all 4 years of the study. 

Support for the accuracy of the Identified-female estimate is provided 

by comparisons with the other estimates. Comparison with the Mature-female 

estimate (69%) shows the Identified-female estimate to be slightly lower. 

This is reasonable as nulliparous females (young adults and infertile animals) are excluded from the Mature-female sample but are included in the 

Identified-female sample. Comparing the Identified-female estimate for 1979 
35 

Table 12.--Reproductive success estimates for the Hawaiian monk seal 

population on Laysan Island, 1977 through 1980. 

Sample Range N Overall Rate 

Tagged-females 7-10 years = 43% 14 

11-14 years = 70% 10 5 4% 

Estimated-females 1979 = 55 - 84% 44 

1980 60 - 89% 45 3 3% 

~~ 

Ma tur e-f emales Year 2 = 55% 42 

Years 3+4 = 80% 32 6 9% 

Identified-females 1977 = 59% 39 

1978 = 55% 44 

- 1979 = 66% 44 

1980 = 79% 34 6 4% 

and 1980 combined (72%) with the estimate from the Estimated-female sample 

for the same years (73%), shows the two estimates are very close. 

Data from the Identified-female sample show a significant increase in 

reproductive success between the first 2 years of the study and the last 

2 years. If increased age were responsible for the increased reproductive 

rate, the Identified-female sample should overestimate the reproductive 

success in 1979 and 1980. The close agreement between the Identified-female 

and the Estimated-female rates for these years, however, indicate the effect 

of increased age within the sample was negligible. 

Further evidence for an increase in pupping rate during the study is 

by the mean mid-March through mid-August census counts, excluding 

provided by the crude birth rate (calculated by dividing the yearly pup product ion 

0.35 in 

pups) 0 These data show a change from 0.27 in both 1977 and 1978 to 0.34 and 

1979 and 1980 respectively (Table 13). 

Table 13.--Yearly crude birth rate (births/mean count) for 

1977 through 1980 on Laysan Island. 

- ~~~~~ ~ ~ 

Year No. of Births Mean count Birth rate 

1977 42 1 56 0.27 

1978 29 106 0.27 

1979 32 94 ' 0.34 

1980 33 95 0.35 
36 

Factors which could have resulted in an increased fertility rate 

include the fo 1 lowing. 

1. Illnesses associated with the 1978 die-off could have increased 

the number of missed pregnancies or abortions during 1977 and 1978 

(although only one aborted fetus was found in 1977 and none in 1978). 

2. The die-off of 1978 could have resulted in changes in the age 

distribution of adult females. If the animals affected by the 

1978 die-off were primarily the youngest and the oldest animals 

(the females that would be expected to have the lowest fertility 

rates), this could have contributed to the increase in reproductive ra-te seen in 1979 and 1980. 

3. The changes in reproductive success could be density dependent, 

with the higher reproductive rates in 1979 and 1980 a response to 

the 35% decrease in seal numbers from 1977 levels. 

Additional research is needed to identify which factors affect reproductive rates in the monk seal. Taken as a whole, the data from Laysan 

indicate pupping rates for the species are variable and may have the potential to respond quickly to changes in the environment. 

Interval Between Births 

Based on observations at Midway from 1957 to 1959, Rice (1960) reported 

that a high proportion of monk seal females missed pregnancy following a 

birth. As shown by the Mature-female sample previously described, this was 

not the case on Laysan. Data from Laysan show a variety of reproductive 

patterns. One female pupped in all 4 years and was photographed with a pup 

in 1976 (by DeLong, NMFS, Seattle) for a total of five consecutive births. 

Another female was known not to have pupped in any of the 4 years of the 

study, although appearing adult size in a photograph from 1976 (DeLong, 

NMFS, Seattle), as well as fully adult size throughout our study. 

On Laysan, females pupping in consecutive years generally pupped later 

in the second year. 

20.62, N = 60). All identified females pupping in consecutive years were 

included in this sample. The mean interval between the end of lactation for 

the female (whether due to pup's death, disappearance, or normal weaning) 

and birth the following year was 347.3 days (SD = 20.64, N =6 0). 

females had full term pups in 4 consecutive years (one additional female 

pupped in 3 years, but aborted in the fourth). For these six females, the 

birth dates in 1980 ranged from 38 to 99 days later than in 1977. 

change in birth date between 1977 and 1980 was 62 days, with each successive 

birth averaging 15.5 days later. Wirtz (1968) reported a mean interval 

between births of 382 days for Kure in 1964 and 1965. 

The mean interval between births was 380.7 days (SD = 

Six 

The mean 

Another way to look at the interval between births examines data from 

females seen during all years of the study. Twenty-nine females were 

classed as adult in 1977 and seen in all 4 years (data in Appendix E). 

The largest proportion, 45%, pupped twice during the 4 year period; 24% 

pupped in all years; 21% pupped in 3 of the 4 years; 7% pupped only once; 

and one female did not pup in any year. 
37 

Pupping 

Sex ratio at birth.--Between 1977 and 1980, 136 pups were born on Laysan 

Island. Seventy-five were females and 58 were males. The sex of three pups 

was not determined as they disappeared before weaning. There was no significant difference between the observed and the expected sex ratio at birth. 

Table 14 lists the sex ratio at birth for each year of the study and shows 

considerable variability between years. 

tended to be male, while pups born later were more likely to be female 

(t = 1.994, p<0.05). 

Pups born early in the field season 

Table 14.--Sex ratio at birth for all pups born from 

1977 through 1980 on Laysan Island. 

Year Male Female Unknown Total 

1977 17 23 2 42 

1978 12 17 0 29 

1979 19 ~ 13 0 32 

1980 10 22 1 33 

Total 58 75 3 136 

Timing of births.--Births were known to occur in all months from January 

through August. If any pups were born during the September to December 

period, they had disappeared by the time the next field season began. No 

attended or recently weaned pups were seen during winter visits made in 1978 

(BWJ) or 1980 (Knudtson, personal communication). 

Figure 29 shows the semimonthly distribution of births. More pups were 

born in April than in any other month. The yearly patterns are shown in 

Table 15. The mean pupping date was 17 April, ranging from 13 April in 1977 

N 

U 

M 

B 

E 

~R 

IP 11 

T 

H 

s 

30T 

25 

20 

15 

10 

5 

G3 - 

123456789 

MONTH 

Figure 29.--Semimonthly distribution of births 

on Laysan Island, 1977-80. 
38 

to 20 April in 1979. 

counts of mother-pup pairs see Johnson and Johnson 1984.) 

(For a discussion of the pupping season as defined by 

Table 15.--Number of pups born during each month on 

Laysan Island from 1977 through 1980. 

~~ 

Year JFMAMJ J A Total 

1977 1 5 12 12 7 2 12 42 

1978 0 2 7.10 7 3 00 29 

1979 03 7 993 10 32 

1980 1 1 713 4 6 10 33 

Total 2 11 33 44 27 14 3 2 136 

Rookery areas.--Pups were born on most beaches of Laysan, but pupping 

activity was not evenly distributed around the island. 

(76%) were born on the northwest quarter (areas 1 through 3 and area 14). 

Most of the northwest corner of the island was characterized by relatively 

narrow beaches with vegetation above the beach crest. 

tended to congregate near shallow water reefs in these areas. 

percent of the births occurred on the beaches along the east shoreline 

(areas 7 through 10) where little cover was available above the beach crest. 

A rock ledge extended 20 to 30 meters from the shoreline along the east side 

of the island, with the water depth rarely exceeding one-half meter. Eight 

percent of the pups were born on the northeast quarter (areas 4 through 61, 

and 3% on the south and southwest beaches (areas 11 through 13). 

The majority of pups 

Mothers with pups 

Twenty-two 

Age at weaning. The calculation of weaning age of pups was the same as 

that used for lactation interval of females, except in cases where a pup was 

exchanged or adopted. 

least one female other than their natural mother; two in 1977, nine in 1978, 

and the last in 1979, when a deserted pup was adopted by a female that had 

lost her pup. While at least 11 pups were involved in pup exchanges in 1977 

and 1978, no known pup exchanges took place in either 1979 or 1980. 

Twelve pups were known to have been nursed by at 

The mean weaning age for pups born during the study was 36.2 days 

(range 27 to 50 days). 

for male pups (36.6 days) and for female pups (35.8 days) did not differ 

significantly (t = 1.25, df = 118, n.s.1. 

Statistical analysis showed that the mean wean age 

The overall size and condition of pups at weaning generally depended on 

the number of days the pup suckled. Pups weaned at more than 36 days were 

generally fatter and larger than pups weaned at less than the mean weaning 

age. Some exceptions did occur, and the range in size of pups showed 

considerable overlap. 

The number of days a female remained with a pup depended on her size 

and on the behavior of the pup (some pups weaned themselves by moving away 
39 

from the female). Females ate little or nothing during lactation, so the 

larger the female, the longer she could afford to stay with a pup. 

sionally mothers chased fish and spent long periods submerged in reef areas, 

but no actual feeding behavior was seen.) 

number of days smaller-than-average and larger-than-average females remained 

with their pups. Excluding pups which died or disappeared before weaning, 

the larger females remained with pups for a significantly longer time than 

the smaller females (t = 3.59, df = 117, p<O.OOl). 

(OccaA t-test was run comparing the 

Factors affecting pup survival.--Of the 103 pups born in 1977, 1978, 

and 1979: 13% died or disappeared prior to weaning; another 6.5% died or 

disappeared before the fall of their birth year (when the field season 

ended); and an additional 18% disappeared before the next field season began 

(a few of these pups had insufficient markings to insure recognition as 

yearlings). The remaining 63% of the pups were resighted at least once 

during their yearling year. Chi-square analysis (2x2) showed no significant 

differences in the overall tendency of pups to survive to the end of the 

field season of their yearling year when compared to area of birth (prime 

versus other), size of mother (small versus large), season of birth (early 

versus late, mid versus early and late), or wean age (greater than versus 

less than mean). 

and females. The 103 pups born during 1977 through 1979 included 48 males 

and 53 females (with 2 pups of undetermined sex). Of the 50 pups known to 

be alive at the end of the field season of their yearling year, 30 were 

males and 20 females. 

survival during their yearling year than females (X2 = 6.18, df = 1, p<O.O5). 

However, a significant difference was found between males 

These data suggest males have a significantly greater 

Breeding 

Breeding can occur throughout much of the year for the Hawaiian monk 

seal as births are not limited to one season or time period. Males showed 

typical courtship behaviors throughout the field season, although a marked 

reduction in the tendency to haul-out with, or defend adult females took 

place from mid-August through October, corresponding to the adult male 

molting season. Based on haul-out patterns of adult females, estrus appears 

to occur between the time the pup is weaned and when the female comes ashore 

to molt. Females remain highly aggressive toward all seals, including adult 

males, throughout lactation. When ashore for the molt, approximately 3 

months after weaning, adult females are rarely accompanied by adult males. 

Actual breeding encounters were seen on only three occasions. Two of 

these observations were described in detail in Johnson and Johnson (1981b). 

The third involved a female and consort observed on 18 and 19 April 1980. 

Seen first in late afternoon, the female appeared unusually tolerant of the 

attentions of the consort, allowing the male to grasp her back and attempt 

to mount her while on dry sand. The pair moved into shallow water just 

before sunset and the female remained passive while the male repeatedly 

mounted her, but we could not confirm whether intromission occurred. After 

about 20 minutes in the nearshore water the male followed the female into 

the vegetation. The pair was watched until midnight, remaining in the 

vegetation. Near sunrise the following morning the pair could not be 

located. As dawn approached their shapes could be made out in shallow 

water. By the time it was light enough to see, the male followed the female 

back up to the vegetation. It appeared she had again been passive during 
40 

the time in shallow water. When the pair returned to the shoreline 2 hours 

later, the consort was driven off by another adult male. The female immediately showed the characteristic aggressive behavior toward the new male, 

resisting all attempts by the male for close approach. She remained onshore 

for most of the day, moving out to sea in the late afternoon, unaccompanied. 

The female had last pupped on 16 March 1979 and did not have a pup in 1980. 

Survival 

Well-identified seals disappeared from Laysan throughout the 4 years of 

the study. As bodies of dead animals were rarely found, it was not known if 

the disappearances were due to death cf the animal or to emigration. 

Records of tagged Hawaiian monk seals (johnson et al. 1983) indicate that 5% 

of resightings occur on atolls different from the tagging site. Therefore 

we assume that some of the identified seals that disappeared during the 

study could have emigrated to another atoll, but that most disappearances 

resulted from mortality . will be used to describe unexplained disappearances. With this uncertainty 

unavoidable, we concentrated on rates of known survivorship, which undoubtedly underestimate the actual surqivorship for the population. 

The abbreviation M/D (mortality/disappearance) 

Pups 

One emphasis of the study was to follow known-age cohorts as long as 

possible. Marking pups soon after weaning and yearlings soon after their 

molt made this possible. For most pups, the bleach mark allowed reliable 

identification throughout the first year of life. When the mark was lost 

during the molt, about 15 months after weaning, animals were re-marked. The 

identification of the entire pup cohort each year made collection of 

detailed data on rates of survivorship possible. 

Survival through weaning.--A total of 136 pups were born during the 

study. 

fore unmarked and unidentifiable). 

year. One of these was premature; the cause of death was not determined 

for the other three. 

One pup was not weaned by the time we left the field in 1977 (thereFour pups were stillborn, one in each 

Of the remaining 131 pups, 120 (89%) survived at least 27 days (the 

minimum age at weaning for the study) and were resighted after the departure 

of their mother. Survival to weaning ranged from a low of 82% in 1980 to a 

high of 97% in 1978. The age and circumstance of death for the 11 pups 

dying or disappearing before weaning, excluding those born dead, are listed 

in Table 16. 

Survival through 4 months.--After weaning, pups were generally seen at 

least once each census period (4 days) for several months. Only rarely were 

pups not seen for more than a week during their pup year. 

tendency to remain near shore, any pup that was not sighted during the last 

2 weeks of the study (and not resighted during the following year) was 

listed as having disappeared. 

one was found dead and seven disappeared before the,end of their pup field 

season. Information on these pups is listed in Table 17. 

Because of this 

Of the 120 pups surviving through weaning, 
41 

Table 16.--Pre-weaning disappearance and mortality on Laysan Island 

between 1977 and 1980, excluding pups born dead. 

Year Sex Age (days) Notes 

1977 ? 23 Appeared healthy 3 days before death 

? 22 Disappeared at night, mother remained 

M 8 Mother may not have produced milk 

1978 None 

1979 M 33 Disappeared, mother remained 

M 3 Mother lost pup to another mother hours 

before death 

F 8 Mother may not have produced milk 

1980 M 6 No observable cause of death 

F 6 No observable cause of death, stomach 

empty 

F 7 Wedged in bird burrow when found 

F 16 Appeared weak day before death 

? 1 Alive at birth. Pup gone while female 

remained ashore the next morning 

Survival data for the first 4 months postbirth are presented in Figure 

30. All pups born during the study were included in as many data points as 

their sighting data permitted. For example, data from a pup born 40 days 

before the end of a field season could only be included in the Birth through 

40-day period. (Yearly sample sizes are included in Appendix F.) 

Table 17.--Weaned pups which died or disappeared during 

their pup year on Laysan Island. 

Year Sex Wean Age Condition when last seen 

1977 F 47 days 

F 40 days 

M 37 days 

Heal thy 

Small dorsal infection 

Dead, massive dorsal infection 

1978 M 35 days Small for wean-age 

F 28 days Small seal 

F 38 days He a1 thy 

1979 M 47 days Healthy 

M 33 days Heal thy 

1980 None 
42 

As Figure 30 shows, 92% of the pups born on Laysan survived the first 

The M/D rate was low during the next 70 days, with 88% 

After 80 days the M/D 

10 days postbirth. 

of the pups known to be alive by 80 days postbirth. 

rate increased. 

resighted. 

By 140 days postbirth, 80% of the pups were still being 

S 

U 

R 

U 

I 

U 

A 

L I, 

-a 1,1.1.1.1.1.~.~ 

0 40 10 120 160 

AGE (DAYS) 

Figure 30.--Pup survival on Laysan Island, 1977 through 1980. 

The survivorship patterns seen on Laysan differ markedly from data 

obtained during previous studies on Midway (Rice 1960) and on Kure (Wirtz 

1968). 

for Midway and Kure (see Appendix F for specific data). 

mortality was similar to Laysan, with approximately 90% of the pups still 

alive at 10 days postbirth. 

high on Kure, with only 5% of the pups still being sighted at 50 days. 

Rice suggested that shark attack could have been responsible for the loss 

of pups, while Wirtz (1968) mentioned that attacks by adult male monk seals 

may have played a part in early pup mortality. 

disappeared before the mother left the area, a pattern not seen with normal 

weaning on Laysan. 

Figure 31 compares pup survivorship on Laysan with that reported 

On Kure, early pup 

However the M/D rate through weaning was very 

The pups at Kure generally 

P 

E 

R 

C 

E 

N 

1 

100 

90 

80 

70 

GO 

50 

40 

30 

20 

LAYSAN 

MIDWAY 

Figure 31.--Pup survival (# = resighted/births) for Laysan (1977-801, 

Kure (1964-651, and Midway (1958-59). 
43 

. 

Data from Midway during the late 1950's also showed a higher M/D rate 

than Laysan, but the pattern was not the same as that seen at Kure. On 

Midway, approximately 30% of the pups were either born dead or died within 

the first 10 days. 

much lower than at Kure, with 35% of the original cohort still sighted 

after 70 days. 

stillbirths could have been due to underwater explosions which were used to 

clear a channel at Midway during that time period. 

which were born alive and healthy at Midway were not being subjected to the 

same factors causing the high postbirth mortality seen at Kure. 

For pups that survived past 10 days the M/D rate was 

Kenyon and Rice (1959) suggested that the higher rate of 

Apparently the pups 

Survival through 16 months.--The best data for calculating micimum 

survivorship to 1 year of age are the resighting records for pups born in 

1978 and 1979. Data from 1977 are excluded as a few pups were not marked 

well enough to guarantee reliable resighting the following year, and because 

approximately 80% of the 1977 pups recognized as yearlings in 1978 died or 

disappeared during the unusual 1978 "die-off ." 

Figure 32 presents survivorship data to 16 months for the 61 pups born 

in 1978 and 1979. All pups disappearing during their pup year were given a 

disappearance date of 1 day after-their last sighting. 

seen throughout their pup year, but not resighted as yearlings. 

animals the M/D age was recorded as the mean between the last day of the 

pup field season and the start of the following field season. (Data for 

each animal are included in Appendix F.) 

Many pups were 

For these 

P 

E 

R 

IC 

I 

U 

A 

'L 

Of the 61 

594- 

40- - 

38- - 

28-- 

10-- I I I I I I 1 I I 03 104 209 313 417 

AGE <DAYS> 

Figure 32.--Pup survival on Laysan Island, 

1978 and 1979 pup cohorts. 

pups born in 1978 and 1979, 46 (75%) were seen during the 

following year. However, only 43 (70%) were resighted 1 year after their 

actual birthday. This should be considered a minimum survival estimate, as 

the fate of the 18 pups which disappeared after weaning is unknown. 

of the pups born in 1978 and 1979 were poorly marked as pups. 

possible that one or two could actually have survived on Laysan, but were 

unrecognizable as yearlings. 

A few 

It is 

Three previously unidentified juvenile seals 
44 

were seen on Laysan which did not have visible bleach marks, two in 1979 

and one in 1980. Although unlikely, if all three are assumed to be pups 

born on Laysan the previous year which had "lost" their marks, this would 

increase the 12-month survivorship to 75% (46 of 61). 

Year1 ing s 

The survival rate for yearlings was similar to that of pups. Eleven 

of the fourteen yearlings seen regularly throughout 1979 were also seen 

throughout 1980 as 2-year-old seals (79%). 

seven yearlings still seen regularly by the end of the 1978 field season 

(after the die-off), five were seen as 2-year-old seals in 1979. 

the data from the 1977 and 1978 pup cohorts gives a survival rate for 

yearling seals of 76% (16 of 21). 

For the 1977 pup cohort, of the 

Combining 

Other Age Classes 

Of the five 2-year-old seals seen in 1979, all were seen throughout 

1980. 

known-age and others) seen in 1979 were also seen in 1980. 

as juveniles averaged about 3 years old, with a range of 2 to 4 years for 

known-age animals. 

However, only 23 of 29 (73%) identifiable juvenile seals (both 

(Seals classed 

The survival rate was higher for older seals. Table 18 presents data 

on 1 year of survivorship for all reliably identifiable seals older than 

yearlings that were seen in 1979. 

In summary, yearly survival rates were about 75% for animals up to 4 

years of age (birth through juvenile size). The survival rate for seals 

over 4 years of age (subadults and adults) increased to about 87%. As 

mentioned in the Reproductive section, there was a significantly greater 

survival of male pups through their yearling year than of females. Chisquare analysis of the data presented in Table 18 showed no significant 

difference in male versus female survival for juvenile, subadult, or adult 

animals; but the data from the small sample of juveniles indicate a tendency toward lower survival for young female monk seals. The reason for 

this apparent sex difference in the M/D rate is unknown, and needs additional study. 

Table 18.--Number and percentage of seals identified in 1979 

that were resighted in 1980 on Laysan Island. 

1979 1980 x 

Juvenile males 

female s 

Subadult males 

females 

Adult males 

female s 

15 14 93 

14 9 64 

15 13 87 

15 13 87 

61 53 87 

53 ' 45 85 
45 

ACKNOWLEDGMENTS 

We would like to express our gratitude to the Marine Mammal Commission, especially R. Hoffman, for support and encouragement throughout this 

research project. 

study, including R. Shallenberger, W. Gilmartin, R. Shomura, A. Johnson, 

the officers and crews of the NOAA ship Townsend Cromwell and the Coast 

Guard buoytender Buttonwood. Finally, we especially wish to thank Skip 

Naftel and the crew of the research vessel Easy Rider for all their help 

and assistance. 

Many others have been helpful during all phases of the 

, 
46 

LITERATURE CITED 

Ely, C. A., and R. B. Clapp. 1973. The natural history of Laysan Island, 

Northwestern Hawaiian Islands. Atoll Res. Bull. 171:l-361. 

Fiscus, C. H., A. M. Johnson, and K. W. Kenyon. 1978. Hawaiian monk seal 

(Monachus schauinslandi) survey of the Northwest Hawaiian Islands, 

July 1978. National Marine Fisheries Service, Processed Report. 

Gilmai-tin, W. G. 1983. Recovery pian for the Hawziian monk seal (Monachus 

schauinslandi) . National Marine Fisheries Service Publication. 

Johnson, A. M., R. L. DeLong, C. H. Fiscus, and K. W. Kenyon. 1982. 

Population status of the Hawaiian monk seal (Monachus schauinslandi) , 197 8. J. Mammal. 3 : 41 5-421. 

Johnson, B. W., and P. A. Johnson. 1978. The Hawaiian monk seal on Laysan 

Island: 1977. U.S. Department of Commerce, National Technical 

Information Service, No. PB-285-428, 38 p. 

Johnson, B. W., and P. A. Johnson. 1981a. Estimating the monk seal 

population on Laysan Island. U.S. Department of Commerce, National 

Technical Information Service, No. PB-82-106113, 29 p. 

Johnson, B. W., and P. A. Johnson. 1981b. The Hawaiian monk seal on 

Laysan Island: 1978. U.S. Department of Commerce, National Technical 

Information Service, No. PB82-109661j 17 p. 

Johnson, P. A., and B. W. Johnson. 1984. Hawaiian monk seal observations 

on French Frigate Shoals, 1980. U.S. Dep Commer., NOAA Tech. Memo. 

. NMFS, NOAA-TM-NMFS-SWFC-50, 47 p. 

Kenyon, K. W. 1972. Man versus the monk seal. J. Mammal. 53:687-696. 

Kenyon, K. W., and D. W. Rice. 1959. Life history of the Hawaiian monk 

seal. Pac. Sci. 13:215-252. 

Pearson, E. S., and H. 0. Hartley. 1966. Biometrika tables for 

statisticians. Vol. 1. Cambridge Univ. Press. 

Rice, D. W. 1960. Population dynamics of the Hawaiian monk seal. J. 

Mammal. 41 :376-385. 

Seber, G. A. F. 1973. The estimation of animal abundance. Hafner Press, 

New York. 506 p. 

Wirtz, W. O., 11. 1968. Reproduction, growth and development, and 

juvenile mortality in the Hawaiian monk seal. J. Mammal. 49:229- 

238 . 
47 

Appendix Table A.--Itinerary. Arrival and departure 

dates, Laysan Island, 1977-80. 

~ ~~~ 

Ye 81: Arrive Laysan Depart Laysan 

1977 27 February 3 September 

1978 27 February 18 June 

~ 28 June 1 October 

12 October 13 October 

12 December 18 December 

1979 12 March . 17 August 

1980 14 March 18 September 
48 

Appendix B.--Biases affecting sex ratio data. 

It is rarely possible to determine the sex of all seals ashore on a 

census. If the probability of sexing males and females is equal, then the 

sex ratio of unsexed animals should be the same as the ratio of sexed animals. Unfortunately, several potential biases exist which can alter the 

relative probability of sexing males and females. 

large percentage of the animals are unsexed, these biases can significantly 

alter the calculated sex ratio. 

On censuses in which a 

Female-With-Pup Bias 

The ciearest example cf a bias which affects sex ratio data occurs 

during the pupping season. 

tinely recorded as female, without the prerequisite ventral examination 

required for sexing other animals. 

looking at a hypothetical adult population made up of 50 males and 50 

females, 25 of which are accompanied by black pups. Two counts are made. On 

the first count all animals are sexed, resulting in a 1:l sex ratio. On the 

second survey, conditions are such that the probability of sexing each individual seal is only 20%, except that all adults accompanied by pups are sexed 

as female. The results of these counts show that the second count, including 

the unsexed animals, will be strongly biased toward females. 

All adults accompanied by black pups axe rouThe effect of this bias can be seen by 

Females 

Male :Female 

Males Alone with Pup Total Unknowns Sex Ratio 

Count 1 50 (25) (25) 50 0 1:l 

Count 2 10 ( 5) (25) 30 60 1 :3 

In theory, this bias can be corrected by estimating how many of the 

mothers would have been sexed as female by the normal method of sexing 

animals, based on the ratio of sexed adults to total adults. 

A 

A/B = X/M or X = (MA/B) 

where: 

A = total adults sexed (excluding mothers) 

B = total adults (excluding mothers) 

M = females with pups (mothers) 

X = estimate of mothers which would have been classed as female 

without presence of pup. 

In the above population, where 20% of the nonmothers were sexed, X E 

Added tosthe 5 females sexed using 

(25)(15)/75 = 5, indicating that of the 25 mothers, only 5 would have been 

sexed using the normal sexing techniques. 

the normal methods, a total of 10 females results. When compared to the 10 

seals sexed as males, the corrected sex ratio becomes 1:l. 

. 
49 

In practice, this correction factor will not eliminate all problems, as 

other biases in both ageing and sexing animals may exist. 

reduce the problem during the pupping season, especially at atolls like French 

Frigate Shoals where a large percentage of animals are generally not sexed. 

However, it should 

Other Biases 

Several other biases may exist which could affect sex ratio data; but 

the relative magnitude of these biases is difficult to predict and probably 

varies from study to study. Some of these are listed below. 

Identified Animals.--In tha Laysan study, and others where a 

significant number of seals are individually recognizable a the sex of 

"marked" animals seen on censuses is not generally reconfirmed. 

research priorities resulted in proportionally more adult females being identified than males. 

this would bias the sex ratio in favor of females. 

those involving tagging, could also be affected by this bias. 

Laysan indicate that adult males spend less time ashore than other age/sex 

classes, and would tend to be underrepresented in random marking efforts.) 

If a good population estimate is available, it may be possible to correct for 

the identified animal bias, based on the ratio of identified animals to total 

population for each age/sex class. 

cated by such factors as differences between observers (in the number of 

animals they can identify) and the variability in the conspicuousness of 

markings (the identifying marks on some animals are inconspicuous and may 

occur only on the ventral area). 

a correction for identified animals in the Laysan study, but they should 

present less of a problem in tagging studies. 

On Laysan, 

On censuses in which 2 large number of seals were unsexed, 

(Other studies, such as 

Data from 

Unfortunately, this analysis is compliThese factors make it difficult to calculate 

Male Vocalizations.--There is a characteristic adult male vocalization 

(the "rolling bellow") given in threat and courtship displays which, when it 

occurred during a census, was often used to sex the animal as male without 

ventral examination. If a sufficient number of males were sexed in this way 

it would result in a count biased in favor of males. However, the general 

response of seals in the presence of a displaying adult male was to shift to a 

position exposing the ventral surface to the approaching male, thereby often 

allowing the observer to sex the other animal as well. 

Visibility Bias.--It is possible that males were, in general, easier to 

sex than females. 

penile opening and the penile ridge (a line extending from the penile opening 

to the anus), while females were sexed primarily by the absence of these features. On censuses where many animals were unsexed this would result in a 

bias favoring males. But this bias, if it exists, is likely to vary with 

viewing conditions and between observers. 

This was because males were sexed by the presence of the 

Incomplete Count Bias.--Data from Laysan, French Frigate Shoals, and 

Differences in sex ratio are likely when comparing 

elsewhere indicate various haul-out areas within an atoll are used disproportionately by one sex. 

complete atoll counts with partial atoll counts. 

the bias depends on which areas are excluded duringhpartial counts. 

would be particularly significant at atolls such as French Frigate Shoals, 

The specific direction of 

This bias 
where weather conditions and time constraints often make it impossible to 

visit all haul-out areas available to the seals. 

This bias does not affect data collected during the Laysan study because 

the entire island was covered on all censuses. But it could affect comparisons with future studies on Laysan if those studies exclude animals in the 

water or hauled out on offshore rocks, both groups being disproportionately 

male. 

Testing for Sex Ratio Bias c 

Excluding the Female-with-pup bias, the total effect of the above biases 

(if any) on sex ratio data is difficult to predict and may vary from study to 

study. Studies involving a series of censuses can be used to test for any 

overall bias by comparing counts in which a high proportion of the animals are 

sexed (minimizing bias) with counts in which large numbers of seals are 

unsexed (when the effect of any bias would by greatest). 

During 1977, an average of 43% of the seals were not sexed on censuses, 

increasing the probability of a sexing bias having a significant effect on the 

calculation of sex ratio. The adult counts made during the 1977 pupping 

season (March through June) clearly show the problem. 

Moment Correlation indicates there is a significant correlation between the 

percentage of sexed adults classed as female and the percent of adults which 

were unsexed (r = 0.666, t = 4.462, df = 25, p<O.OOl). This suggests a bias 

in favor of females. But when the same statistic is run comparing the percentage of adults sexed as female when a correction has been made for the 

Female-with-pup bias, the results are nonsignificant (r = -0.154, t = -0.782, 

df = 25, p>O.lO). Thus, when the Female-with-pup bias is taken into account 

and a correction factor applied to the counts, the resulting adult sex ratio 

data appear unbiased. 

A Pearson ProductCount Verses Population Sex Ratio 

Although sex ratio data obtained from ground counts can be useful when 

comparing counts in different months or years within an atoll, or between 

atolls, it should not be considered a direct measure of the population sex 

ratio. Data from Laysan indicate that adult males tend to spend less time 

ashore than other age/sex classes; thus would be underrepresented in sex 

ratios based on ground counts. Further, data indicate the sex ratio of seals 

ashore changes from month to month, a result of different haul-out patterns 

for the various age/sex classes (e.g., different molting seasons). As the 

data in Appendix C show, the sex ratio obtained on ground counts does vary 

considerably from month to month. 
51 

Appendix C.--Laysan Island count data. 

Appendix Tables C-1 through C-4 present the count data for all censuses 

conducted during the study, from 1977 through 1980. Two of the counts (10 

March 1977 and 27 February 1978) included animals which were not classified 

by age; therefore, no age/sex data are included for those dates. 

ing is a brief description of the headings found on each table. 

The followDATE: Date of count 

TIME: Time count began 

ADM: 

AI)F : 

AD? : 

SAM: 

SAF: 

SA? : 

JM: 

JF : 

J? : 

YM: 

YF: 

Y?: 

WP: 

MP: 

TOT : 

STOT : 

%UNK : 

AM/C : 

AF/C : 

IM/C : 

IF/C: 

%AD: 

XFEM: 

Number of 

Number of 

Number of 

Number of 

Number of 

Number of 

Number of 

Number of 

Number of 

Number of 

Number of 

Number of 

Number of 

Number of 

adult males counted 

adult females counted 

unsexed adults 

subadult males 

subadult females 

unsexed subadults 

juvenile males 

juvenile females 

unsexed juveniles 

yearling males (or yearling size male seals in 1977) 

yearling felpales (or yearling size female seals in 1977) 

unsexed yearling size seals, used only in 1977 

weaned pups 

pups still attended by a female (i.e., nursing) 

Total of all seals counted 

Total excluding WP and MF' counts 

Percentage of STOT counts that were not sexed 

Corrected adult male count corrected for female-with-pup bias 

(Appendix B) and with a sex assigned to the unsexed adults 

based on the resulting sex ratio 

Corrected adult female count (same as AM/C) 

Corrected immature male count (includes subadult, juvenile, and 

yearling males) with sexes assigned to unsexed animals on the 

basis of sex ratio of sexed immatures 

Corrected immature female count (same as IM/C) 

Percentage of STOT that were classified as adults 

Percentage of STOT that were sexed as female (based on the AF/C 

and IF/C totals) 

A summary table is provided at the end of the census data for each year. 

The table includes a total mean for all counts conducted during the year, and 

separate means for each month. 

The "corrected" (e.g., estimated) counts for the four age/sex classes can 

be misleading for censuses where large numbers of seals were not sexed (such as 

the count on 17 April 1977). 

are based on the mean of several counts (e.g., semimonthly or monthly means). 

These data are best used when the corrected counts 

Although only whole numbers are presented in the tables, all summary 

calculations for the last seven columns (%Urn to XFEM) are based on numbers 

carried out to several decimal places. 

may differ slightly from the sum of the columns. 

As a result, the summary calculations 

. 
52 

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60 

Appendix Table D.--Tag numbers read on Laysan Island. Sighting 

records of tagged seals seen on Laysan Island from 1977 through 1980. 

Originally Tagged Years Seen ~- - ~~~~ ~~- ~~~ 

Tag Sex Date Age At ol 1 1977 1978 1979 1980 

A5 

A43 

A5 9 

A389 

707 

708 

7 64 

818 

93 1 

944 

945 

1018 

1072 

1073 

1074 

1092 

F 

F 

F 

F 

F 

M 

M 

F 

F 

F 

M 

M 

M 

F 

M 

F 

3 / 121 67 

3/19/67 

3/19/67 

9/05/€8 

6 1021 69 

6/02/69 

71 170 

3 / 26 169 

8/17/70 

8/17/70 

8/17/70 

9/07/71 

9/ 10172 

9/10/72 

91 101 72 

51 172 

Year 

P 

P 

P 

P 

P 

P 

Year 

P 

P 

P 

P 

P 

P 

P 

P 

FFS 

Laysan 

Lays an 

Lays an 

'Lay s an 

Lays an 

FFS 

Lays an 

Lays an 

Laysan 

Laysan 

Lays an 

Lays an 

Lays an 

Lays an 

FFS 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X X 

X 

x X 

X X 

X 

? 

X X 

X X 

X X 

X 

a 
61 

Appendix Table E.--Reproductive patterns of mature females. 

ductive status of females classified as adult and seen in 3 or more years on 

Laysan Island (P = parturient, N 5 nonparturient, S = subadult size, - = not 

seen). 

Yearly reproFemale 1977 1978 1979 1980 

c 

01 

02 

03 

04 

05 

06 

07 

08 

09 

10 

11 

12 

13 

14 

15 

16 

17 

18 

19 

20 

21 

22 

23 

24 

25 

26 

27 

28 

29 

30 

31 

32 

33 

34 

35 

36 

37 

38 

39 

40 

41 

42 

43 

44 

P 

P 

P 

P 

P 

P 

F 

P 

P 

P 

P 

P 

P 

P 

P 

P 

P 

P 

P 

P 

P 

P 

S 

N 

N 

N 

N 

N 

N 

N 

N 

N 

P 

N 

S 

N 

N 

N 

N 

N 

N 

- 

- 

- 

P 

P 

P 

P 

P 

P 

P 

N 

N 

N 

N 

P 

*P 

N 

N 

N 

N 

N 

N 

N 

N 

P 

P 

P 

P 

P 

P 

P 

P 

P 

P 

P 

P 

P 

N 

P 

N 

N 

N 

N 

N 

P 

N 

N 

P 

P 

P 

P 

P 

P 

P 

P 

P 

P 

P 

N 

P 

N 

N 

N 

P 

P 

P 

P 

N 

.P 

P 

P 

P 

N 

N 

N 

N 

N 

P 

P 

P 

N 

P 

P 

P 

N 

N 

P 

P 

P 

N 

N* 

P Tag number 389. 

P 

P Also pupped in 1976. 

P 

P 

P 

P 

N 

N 

N 

N 

P 

? 

P Tag number 707. 

P 

P 

P 

P 

P 

- Also pupped in 1976. 

P 

P 

P 

P 

P Tag number A5. 

P 

P 

N 

P 

? 

? 

P 

P 

N 

P 

P 

- Tag number A59. 

Seen early in 1980 only. 

- 

Seen early in 1980 only. 

Seen early in 1980 only. 

- 

- 

- 

- 

P Tag number 944. 

N 
62 

Appendix Table F1.--Survival data. 

including age (in days), the number of pups which could have been seen at 

that age (N), and the number of pups known to be alive at that age (OBS). 

Data used to calculate Figure 30 

Age N OBS N OB S N OBS N OBS % Alive 

Birth 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100 

110 

120 

130 

140 

150 

160 

42 

42 

41 

41 

40 

40 

39 

39 

39 

37 

36 

34 

31 

29 

25 

21 

16 

41 

40 

39 

37 

36 

36 

35 

34 

34 

32 

29 

27 

24 

23 

19 

17 

13 

29 

29 

29 

29 

29 

29 

29 

29 

29 

29 

28 

28 

28 

24 

22 

20 

19 

28 32 

28 32 

28 32 

28 32 

28 31 

28 31 

28 30 

28 29 

28 28 

26 27 

25 23 

25 18 

24 15 

21 11 

19 9 

17 7 

16 5 

31 33 32 

29 33 28 

29 33 27 

29 33 27 

27 33 27 

27 33 27 

26 33 27 

25 33 27 

24 32 26 

24 30 24 

20 30 24 

16 26 20 

14 24 18 

10 23 17 

8 22 16 

6 21 16 

4 18 13 

97 

92 1) 

91 

90 

89 

89 

39 

88 

88 

86 

84 

83 

.82 

82 

80 

81 

79 

Appendix Table F2.--Data used to calculate Figure 31. 

from Appendix Table F1; Midway data from Rice (1960); and Kure data from 

Wirtz (1968). 

Laysan data are 

~ ~ 

Lays an Midway (N = 17) Kure (N = 56) 

Age 

(days) N OBS % Alive OBS % Alive OB S % Alive 

10 136 126 92 12 71 50 89 

20 135 123 91 11 65 46 82 

30 135 121 90 9 53 36 64 

40 133 118 89 8 47 15 27 

50 133 118 89 7 41 3 5 

60 131 116 89 6 35 2 4 

70 130 114 88 6 35 2 4 
63 

Appendix Table F3.--Data used to calculate Figure 32. 

which would have been at least "age" days old at the end of their yearling 

field season. OBS = the number of those pups which had not died or disappeared by that age. 

N = number of pups 

1978 1979 To tal 

Age 

(days) N OB S N OB S X Alive 

Birth 

52 

104 

157 

209 

26 1 

313 

365 

41 7 

47 0 

29 28 

29 28 

29 26 * 

29 25 

29 24 

29 21 

29 19 

29 17 

29 17 

20 10 

32 

32 

32 

32 

32 

32 

32 

32 

32 

29 

31 

28 

27 

27 

27 

27 

27 

26 

26 

22 

97 

92 

87 

85 

84 

79 

75 

71 

71 

65 
64 

Appendix Table F4.--Data used in Appendix Table F3. 

pups born on Laysan in 1978 and 1979. Pups are numbered in order of birth 

for each year. Column headings are: 

when last seen (if the seal was seen within 2 weeks of its "max. pos. age" 

it was listed as surviving to that age); 

between birth of a pup and the end of the field season the following year 

(the maximum number of days an animal could have been seen); Status: S = 

survived through the yearling field season, D = disappeared (migrated or 

died) prior to the end of the yearling field season, or M = known mortality; 

Assigned M/D as - estimated disappearance age (seals which disappeared 

during a field season were given an M/D age 1 day after their last sighting, 

, and seals which disappeared between field seasons were given an M/D age 

halfway between their last sighting and the start of the next field season). 

First year survival for 

&e last seen = age (in days) of a pup 

&. E. = the number of days 

1978 Age last Max. Assigned 

PUPS seen pos. age Status MID age Notes 

1 

2 

3 

4 

5 

6 

7 

a 

9 

10 

11 

12 

13 

14 

15 

16 

17 

18 

19 

20 

21 

22 

23 

24 

25 

26 

27 

28 

29 

559 

5 51 

5 21 

454 

211 

270 

508 

26 5 

117 

162 

500 

89 

4 93 

450 

489 

442 

482 

83 

473 

46 9 

3 54 

213 

459 

338 

0 

442 

442 

176 

482 

559 S 

5 51 S 

521 S 

519 D 

519 D 

515 D 

508 S 

5 08 D 

5 04 D 

501 D 

500 S 

498 D 

4 93 S 

491 D 

489 S 

483 D 

482 S 

482 S 

482 D 

473 S 

469 S 

46 2 D 

459 D 

45 9 S 

448 D 

443 M 

442 S 

442 S 

41 8 D 

M/D during 1979 season. 

M/D between October 1978 

and the 1979 season. 

M/D between December 1978 

and the 1979 season. 

M/D between December 1978 

and the 1979 season. 

M/D during 1978 season. 

M/D during 1978 season. 

M/D during 1978 season. 

M/D during 1979 season. 

M/D during 1979 season. 

M/D during 1979 season. 

M/D during 1979 season. 

M/D between December 1978 

and the 1979 season. 

M/D during 1979 season. 

Born dead. 

M/D between December 1978 

, and the 1979 season. 
65 

Appendix Table F4.--Continued. 

1979 Age last Max. As signed 

PUPS seen pos. age Status M/D age Notes 

1 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

16 

17 

18 

19 

20 

21 

22 

23 

24 

25 

26 

27 

28 

29 

30 

31 

32 

576 

57 6 

574 

56 2 

33 

554 

5 53 

548 

5 47 

442 

53 5 

527 

525 

5 24 

5 23 

518 

490 

101 

3 38 

3 

8 

506 

503 

497 

480 

490 

489 

0 

47 1 

46 5 

457 

43 4 

576 

576 

574 

562 

559 

554 

5 53 

548 

5 47 

53 8 

535 

5 27 

525 

5 24 

5 23 

518 

51 5 

513 

508 

507 

506 

506 

503 

497 

496 

4 90 

489 

485 

47 1 

46 5 

457 

43 4 

S 

S 

S 

S 

D 

S 

s 

S 

S 

D 

S 

S 

S 

S 

S 

S 

D 

D 

M 

M 

M 

S 

S 

S 

D 

S 

S 

M 

S 

S 

S 

S 

M/D during 1979 season. 

MID during 1980 season. 

M/D during 1980 season. 

M/D during 1979 season. 

Died during 1980 season. 

Died during 1979 season. 

Died during 1979 season. 

M/D during 1980 season. 

Born dead. 

. 
RECENT TECHNICAL MEMORANDUMS 

r 

Copies of this and other NOAA Technical Memorandums are available from the National 

Technical lnformatlon Service, 5285 Port Royal Road, Springfield, VA 22167. Paper copies vary in 

price. Microfishe copies cost $3.50. Recent issues of NOAA Technical memorandums from the 

NMFS Southwest Fisheries Center are listed below: 

NOAA TM-NMFS SWFC 39 

40 

41 

42 

43 

44 

45 

46 

47 

48 

Annotated references to techniques capable of assessing the 

roles of cephalopods in the eastern tropical Pacific Ocean, 

with emphasis on pelagic squids. 

J. B. HEDGEPETH 

(December 1983) 

Summary of environmental and fishing lnformatlon on Guam 

and the commonwealth of the Northern Mariana Islands: 

Historical background, description of the Islands, and revlew 

of the climate, oceanography, and submarine topography. 

L. G. ELDREDGE 

(December 1983) 

Diving patterns of the Hawaiian Monk Seal, Llslanskl Island, 

1982. 

F. V. SCHLEXER 

(February 1984) 

“The Hawaiian Monk Seal on Laysan island: 1982” 

D. J. ALCORN 

(April 1984) 

Atlas of airborne sea surface temperature observations in 

nearshore California waters, 1980-1983. 

P.N. SUND 

(July 1984) 

Potential impact of deep seabed mining on the larvae of 

tunas and biiiflshes. 

W.M. MATSUMOTO 

(July 1984) 

Sampling commercial rockfish landings In California. 

A.R. SEN 

(July 1984) 

Histopathological manual for monitoring health of striped 

bass In relation to pollutant burdens. 

J.A. WHIPPLE, M. JUNG, R.B. MacFARLANE and R. FlSCHER 

(August 1984) 

Hawailan monk seal population research, Llslanskl island, 

1982. 

H.S. STONE 

(August 1984) 

“interpreting spotted dolphin age distributions” 

J. BARLOW and A.A. HOHN 

(August 1984)