Hawaiian Monk Seal Research Articles

Effect of variable oceanic productivity on the survival of an upper trophic predator, the Hawaiian monk seal Monachus schauinslandi

Effect of variable oceanic productivity on the survival of an upper trophic predator, the Hawaiian monk seal Monachus schauinslandi

REPRODUCTIVE PATTERNS OF THE HAWAIIAN MONK SEAL

AbstractWe evaluated reproductive patterns of the Hawaiian monk seal (Monachus schauinslandi) using a combination of fitted age‐specific reproductive curves and analysis of reproductive patterns of individual females. We review the difficulties inherent in the acquisition and modeling of reproductive data with emphasis on the significance of reproductive senescence to populations with dissimilar age/sex compositions. Validation of the fitted reproductive parameters was accomplished by Monte Carlo sampling of parameter distributions to compare the expected number of pups with the observed production. Although the fitted reproductive functions appear to provide an acceptable fit to the raw reproductive data, we found that the fitted curves did a poor job of predicting the actual pup production in individual years because of high variability among years. To further verify, and elaborate on, the patterns in the pooled (multi‐seal, and multi‐year) rates, we examined attributes of the reproductive performance of individual seals. The attributes included age of primiparity, reproductive rates computed over several age ranges, and the relationship between reproductive performance and seal longevity. Analysis of individual seal patterns reinforced the conclusion that reproductive senescence is operative in monk seal populations.

INFECTIOUS DISEASE MONITORING OF THE ENDANGERED HAWAIIAN MONK SEAL

As part of conservation efforts between 1997 and 2001, more than 25% (332 animals) of the endangered Hawaiian monk seal (Monachus schauinslandi) population was sampled in the northwestern Hawaiian Islands. Serum samples were tested for antibodies to viruses, bacteria, and parasites known to cause morbidity and mortality in other marine mammal species. Antibodies were found to phocine herpesvirus-1 by using an enzyme-linked immunosorbent assay, but seropositive results were not confirmed by virus neutralization test. Antibodies to Leptospira bratislava, L. hardjo, L. icterohaemorrhagiae, and L. pomona were detected in seals from several sites with the microagglutination test. Antibodies to Brucella spp. were detected using 10 conventional serologic tests, but because of inconsistencies in test results and laboratories used, and the lack of validation by culture, the Brucella serology should be interpreted with caution. Antibodies to B. canis were not detected by card test. Chlamydophila abortus antibodies were detected by complement fixation (CF) test, and prevalence increased significantly as a function of age; the low sensitivity and specificity associated with the CF make interpretation of results difficult. Antibodies to Toxoplasma gondii and Dirofilaria immitis were rarely found. There was no serologic evidence of exposure to four morbilliviruses, influenza A virus, canine adenovirus, caliciviruses, or other selected viruses. Continuous surveillance provides a means to detect the introduction or emergence of these or other infectious diseases, but it is dependent on the development or improvement of diagnostic tools. Continued and improved surveillance are both needed as part of future conservation efforts of Hawaiian monk seals.

Ranking coral ecosystem "health and value" for the islands of the Hawaiian Archipelago

An evaluation of the "health" and "value" of the Northwestern Hawaiian Islands (NWHI) in relation to the main eight Hawaiian Islands (MHI) was undertaken as part of the process for evaluating the NWHI for possible designation of this area as a National Marine Sanctuary. Biological information for the NWHI region is very limited due to its extreme Isolation, but sufficient data on five important biological Indicators were developed for both the NWHI and the MHI. These Include: reef fish biomass, reef fish endemics, total living coral cover, population of the endangered Hawaiian Monk Seal Monachus schauinsandi, and the number of female Green Sea Turtles Chelonia mydas nesting annually on each island. These diverse data sets were used in a simple Integrated scoring and ranking scheme for all the islands of the archipelago. The resulting composite scoring is essentially an index of biological integrity. The final result graphically demonstrates the value of the NWHI in a manner easily understood by the public, government decision makers and managers. Further, the contrast of the NWHI to the MHI illustrates the diminished condition of reefs close to human population within the Hawaiian Archipelago. This approach proved to be very useful in the Integration of diverse data sets.

Survey for Selected Pathogens and Evaluation of Disease Risk Factors for Endangered Hawaiian Monk Seals in the Main Hawaiian Islands

A recently reestablished and increasing population of Hawaiian monk seals in the main Hawaiian Islands (MHI) is encouraging for this endangered species. However, seals in the MHI may be exposed to a broad range of human, pet, livestock, and feral animal pathogens. Our objective was to determine the movement and foraging habitats of Hawaiian monk seals in the MHI relative to the potential exposure of seals to infectious diseases in near-shore marine habitats. We captured 18 monk seals in the MHI between January 27, 2004 and November 29, 2005, tested them for various infectious diseases, and then monitored the foraging movements of 11 of them using satellite-linked radio transmitters for the next 32–167 days. All seals tested negative for canine adenovirus, calicivirus, four morbilliviruses, phocine herpes virus, Leptospira sp., and feline and canine heartworm antigen/antibody. Six of the seals tested positive on complement fixation for Chlamydophila abortus (formerly Chlamydia psittaci). Four seals demonstrated positive titers to Sarcocystis neurona, two to Neospora caninum, and two to Toxoplasma gondii. Fecal cultures showed approximately half (n = 6) positive for E. coli 0157, no Salmonella sp., and only one with Campylobacter sp. Satellite monitored seals spent considerable time foraging, traveling, and resting in neritic waters close to human population centers, agricultural activity, and livestock ranges, and sources of land-based water runoff and sewage dispersal. Consequently, Hawaiian monk seals in the MHI may be at risk of exposure to several infectious disease agents associated with terrestrial animals that can contaminate marine habitats from runoff along drainages and that are known to cause disease in marine mammals. Further, some seals overlapped substantially in their use of coastal habitats and several moved among islands while foraging and were seen on beaches near each other. This suggests that diseased seals could infect healthy conspecifics throughout the MHI.

Moult in the Mediterranean monk seal from Cap Blanc, western Sahara

At birth, the Mediterranean monk seal (Monachus monachus) is covered with dark lanugo This is shed earlier and faster in females, so they complete the moult when they are significantly younger (64 days) than males (82 days). Moulted pups present a greyish dorsal and a white ventral pelage. Only the hair is shed in the first moult. In subsequent moults, the hairs are shed along with large sheets of cornified epidermis. A series of sequential stages were identified, which included a long pre-moult phase (brown pelage) and a post-moult phase (dull appearance). The shedding process lasted 15 days. It is different to that of the Hawaiian monk seal and elephant seals, as Mediterranean monk seals partly moult in the water. The intermoult period was close to one year except in females nursing a pup. Such females have longer intermoult periods and can even moult whilst still lactating. In males, the process of developing the mature pelage pattern of bulls is gradual. It involves at least two annual moults and can be completed by the age of 4 years.

Moult in the Mediterranean monk seal from Cap Blanc, western Sahara

At birth, the Mediterranean monk seal (Monachus monachus) is covered with dark lanugo This is shed earlier and faster in females, so they complete the moult when they are significantly younger (64 days) than males (82 days). Moulted pups present a greyish dorsal and a white ventral pelage. Only the hair is shed in the first moult. In subsequent moults, the hairs are shed along with large sheets of cornified epidermis. A series of sequential stages were identified, which included a long pre-moult phase (brown pelage) and a post-moult phase (dull appearance). The shedding process lasted 15 days. It is different to that of the Hawaiian monk seal and elephant seals, as Mediterranean monk seals partly moult in the water. The intermoult period was close to one year except in females nursing a pup. Such females have longer intermoult periods and can even moult whilst still lactating. In males, the process of developing the mature pelage pattern of bulls is gradual. It involves at least two annual moults and can be completed by the age of 4 years.

Phocine distemper virus in the North and European Seas – Data and models, nature and nurture

Two outbreaks of phocine distemper have severely affected harbour seal ( Phoca vitulina) populations in European and UK waters. The first occurred in 1988 when the causative virus was identified as a new member of the genus morbillivirus. The second outbreak in 2002 was first detected on the same Danish Island of Anholt and involved similar populations and geographical locations. However, despite the obvious similarities between the epidemics, differences in viral transmission and case mortality were found. Harbour seals are highly susceptible to infection while sympatric grey seals ( Halichoerus grypus) are resistant but could be important asymptomatic carriers of the disease. Arctic phocid seals remain the most likely source of the virus and grey seals could be the link between these primary hosts and the harbour seal populations further south. Future epidemiological models should therefore consider including multiple host species. The future conservation and management of harbour seal populations vulnerable to PDV relies on the ability to accurately predict the long-term impact on population abundance and distribution. Although knowledge about the behaviour and pathogenesis of the virus has increased substantially and data on host movements and contact rates are accumulating, studies into the determinants of the host range have lagged behind. The development of more realistic epidemiological models should be combined with studies into the factors controlling species and individual susceptibility. Assessing the risk of infection to endangered but currently unexposed potential host species (such as the Hawaiian monk seal, Monachus schauinslandi) is essential for guiding potential conservation management options, such as vaccination

Gastrointestinal Helminths in the Hawaiian Monk Seal (<I>Monachus schauinslandi</I>): Associations with Body Size, Hematology, and Serum Chemistry

Gastrointestinal helminth parasites are found commonly in the Hawaiian monk seal (Monachus schauinslandi), the most endangered seal in the United States and one of the world’s most endangered pinnipeds. We studied potential associations between gastrointestinal helminth infections and body size, and hematologic and serum chemistry variables in a sample of 282 monk seals captured between 1998 and 2002 as part of a population health assessment. Based on the presence of eggs in feces, the highest prevalence of infection (78%) was for cestodes belonging to a complex of several Diphyllobothrium spp. Infections with the nematode Contracaecum turgidum were found in 29% of samples tested. Eggs of the acanthocephalan Corynosoma rauschi were found in 4% of seals examined; and the feces of four weanling seals on the French Frigate Shoals contained eggs of the recently described trematode, Heterophyopsis hawaiiensis. We used a general linear model and analysis of variance techniques with adjustment for subpopulation and age to determine whether infections with Diphyllobothrium spp. or C. turgidum were associated with changes in hematologic or serum chemistry variables and found little evidence of an effect when we compared infected with negative seals or seals in the highest quartile of egg counts with negative seals. We also conducted analyses of associations between infection and morphometric values in adjusted, age-stratified data. Infection with Diphyllobothrium spp. was associated with a decrease in axillary girth and an increase in dorsal standard length/axillary girth ratio in seals less than 2 y of age, with the effects most pronounced in seals less than 1 y of age. After adjustment for Diphyllobothrium spp., C. turgidum infection was not associated with morphometric parameters. Co-infection with Diphyllobothrium spp. and C. turgidum was not associated with differences in body size greater than those found with diphyllobothriid tapeworm infection alone. These findings suggest that intervention strategies to reduce the prevalence of tapeworm infections in immature Hawaiian monk seals should be considered as a conservation measure for this highly endangered marine mammal.

USE OF DISCOVERY CURVES TO ASSESS ABUNDANCE OF HAWAIIAN MONK SEALS

AbstractWe investigated the pattern of first sighting of individual seals over the course of a field season, or the “discovery curve,” as a means for estimating abundance of the endangered Hawaiian monk seal, Monachus schauinslandi. We empirically derived a criterion to determine whether or not total enumeration had been accomplished at a given site and year. When greater than 100‐h field effort was expended without a new individual being identified, we concluded that total enumeration was likely achieved. To evaluate the potential for estimating abundance through extrapolation of nonlinear asymptotic functions fitted to discovery curves, we conducted simulations under a range of capture probability scenarios, including some based on observed individual variability in monk seal sighting frequencies. We demonstrated that if capture heterogeneity existed among individuals, the fitted asymptotes tended to yield biased estimates of abundance. Moreover, the levels of bias and uncertainty tended to increase inversely with the proportion of the population identified. While extrapolation shows little promise for generating unbiased abundance estimates, discovery curves have practical appeal for determining whether total enumeration has been achieved, and for optimizing field effort allocation. This is especially true for relatively small, closed populations of marked individuals.

Distribution and frequencies of shark‐inflicted injuries to the endangered Hawaiian monk seal (Monachus schauinslandi)

AbstractI analyzed the frequencies of wounds inflicted by sharks to the endangered Hawaiian monk seal Monachus schauinslandi at three colonies in the Northwestern Hawaiian Islands between 1990 and 2000. I applied specific criteria typical of shark bites to all injuries to avoid bias. Pooling data from all years and the three colonies revealed several patterns. Sharks injured more pups (nursing and weaned) and juveniles (1–2 years old) than adults and subadults. More female pups and male juveniles than any other size classes were injured by sharks. Almost all (97%) of the injured pups were from French Frigate Shoals. More juveniles than expected were wounded at Laysan Island and Lisianski Island. Most shark wounds were between the diaphragm and the pelvic girdle, but pups were bitten most often just behind the pelvis compared with adults who were bitten more often near the head and neck.

Paucity of class I MHC gene heterogeneity between individuals in the endangered Hawaiian monk seal population

The Hawaiian monk seal population has experienced precipitous declines in the last 50 years. In this study, we provide evidence that individuals from remaining endangered population exhibit alarming uniformity in class I major histocompatibility (MHC) genes. The peripheral blood leukocyte-derived mRNA of six captive animals rescued from a stranding incident on the French frigate shoals in the Hawaiian archipelago was used to characterize genes in the monk seal class I MHC gene family, from which techniques for genotyping the broader population were designed using degenerate primers designed for the three major established human MHC class I loci (HLA-A, HLA-B, and HLA-C), and by sequencing multiple clones, six unique full-length classical MHC class I gene transcripts were identified among the six animals, three of which were only found in single individuals. Since The low degree of sequence variation between these transcripts and the similarity of genotype between individuals provided preliminary evidence for low class I MHC variability in the population. The sequence information from the class I transcripts from these six animals was used to design several primer sets for examining the extent of MHC variability in the remaining population using a combination of polymerase chain reaction and denaturing gradient gel electrophoresis (DGGE). Several DGGE assays, each one amplifying subtly different class I MHC gene combinations, were designed to compare exons encoding the highly polymorphic domains of the putative peptide-binding region of MHC class I. In combination, these assays failed to show interindividual variability at any of the class I MHC gene loci examined in either the six captive seals or in 80 free-ranging animals ( approximately 6.7% of the estimated population) representing all six major subpopulations of Hawaiian monk seal.

DOMOIC ACID CAUSES REPRODUCTIVE FAILURE IN CALIFORNIA SEA LIONS (ZALOPHUS CALIFORNIANUS)

Exposure of marine mammals to toxins can influence their survival and reproduction. A number of marine mammal mortality events have been attributed to naturally occurring algal toxin exposure, including deaths of manatees (Trichechus manatus latirostris) in Florida due to brevetoxin exposure (O’Shea et al. 1991, Bossart et al. 1998), Hawaiian monk seals (Monachus schauinslandi) due to ciguatoxin (Gilmartin et al. 1980), humpback whales (Megaptera novaeangeliae) due to saxitoxin (Geraci et al. 1989), and California sea lions exposed to domoic acid (DA) (Scholin et al. 2000). Although anthropogenic toxins have been shown to reduce reproductive success in marine mammals (Reijnders 1986) and mortality of marine mammals due to exposure to a variety of marine biotoxins is becoming increasingly recognized (Van Dolah 2000), effects of biotoxins on reproduction have not been reported to date. DA is a potent excitatory neurotoxin produced by marine diatoms in the genus Pseudonitzschia that can affect humans and marine mammals (Perl et al. 1990, Van Dolah et al. 2003). Large-scale mortality of California sea lions has occurred after ingestion of prey that fed on toxin-producing algae (Scholin et al. 2000). DA-producing algal blooms have increased on the west coast of the United States in

Potential effects of sea level rise on the terrestrial habitats of endangered and endemic megafauna in the Northwestern Hawaiian Islands

ESR Endangered Species Research Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials ESR 2:21-30 (2006) - doi:10.3354/esr002021 Potential effects of sea level rise on the terrestrial habitats of endangered and endemic megafauna in the Northwestern Hawaiian Islands Jason D. Baker1,2,*, Charles L. Littnan1, David W. Johnston3 1Pacific Islands Fisheries Science Center, National Marine Fisheries Service, NOAA, 2570 Dole Street, Honolulu, Hawaii 96822, USA 2University of Aberdeen, School of Biological Sciences, Lighthouse Field Station, George Street, Cromarty, Ross-shire IV11 8YJ, UK 3Joint Institute for Marine and Atmospheric Research, 1000 Pope Road, Honolulu, Hawaii 96822, USA *Email: jason.baker@noaa.gov ABSTRACT: Climate models predict that global average sea level may rise considerably this century, potentially affecting species that rely on coastal habitat. The Northwestern Hawaiian Islands (NWHI) have high conservation value due to their concentration of endemic, endangered and threatened species, and large numbers of nesting seabirds. Most of these islands are low-lying and therefore potentially vulnerable to increases in global average sea level. We explored the potential for habitat loss in the NWHI by creating topographic models of several islands and evaluating the potential effects of sea level rise by 2100 under a range of basic passive flooding scenarios. Projected terrestrial habitat loss varied greatly among the islands examined: 3 to 65% under a median scenario (48 cm rise), and 5 to 75% under the maximum scenario (88 cm rise). Spring tides would probably periodically inundate all land below 89 cm (median scenario) and 129 cm (maximum scenario) in elevation. Sea level is expected to continue increasing after 2100, which would have greater impact on atolls such as French Frigate Shoals and Pearl and Hermes Reef, where virtually all land is less than 2 m above sea level. Higher elevation islands such as Lisianski, Laysan, Necker, and Nihoa may provide longer-term refuges for species. The effects of habitat loss on NWHI biota are difficult to predict, but may be greatest for endangered Hawaiian monk seals, threatened Hawaiian green sea turtles, and the endangered Laysan finch at Pearl and Hermes Reef. This study marks the first effort to detail the topography and evaluate sea level rise effects on NWHI species. KEY WORDS: Sea level rise · Habitat loss · Northwestern Hawaiian Islands · Hawaiian monk seal · Green sea turtle · Laysan finch Full text in pdf format PreviousNextExport citation Mail this link - Contents Mailing Lists - RSS Facebook - Tweet - linkedIn Cited by Published in ESR Vol. 2. Online publication date: May 24, 2006 Print ISSN: 1863-5407; Online ISSN: 1613-4796 Copyright © 2006 Inter-Research.

Photogrammetric Estimates of Size and Mass in Hawaiian Monk Seals (<I>Monachus schauinslandi</I>)

A non-invasive photographic technique was devel- oped to estimate the body mass of Hawaiian monk seals (Monachus schauinslandi). Newly weaned monk seal pups (n = 31) were photographed and measured at Kure Atoll in the northwestern Hawaiian Islands. Length, side area, anterior/pos- terior area, and perimeter were measured from pho- tographs to establish predictive relationships with body mass using regression analyses. Photographs were subjectively ranked in terms of quality, based on the degree to which the seal's body position deviated from the ideal position used to obtain standardized photographs. Results indicated that deviations in body positioning (e.g., a seal rolled on its side) did not significantly alter photogram- metric (surface area or perimeter) values compared to those obtained in a standardized position. Although the most reliable models (based on information criterion analysis and 95% CIs) were based on directly measured morphological vari- ables, models using only photogrammetric vari- ables also yielded practical and reliable models with 95% CI, ranging from ± 4.95 to 9.12 kg and R 2 values from 0.93 to 0.77. This finding indicates that the use of photogrammetry alone to assess body condition is suitable to estimate body mass in 10- to 120-kg weaned pups.

Toxoplasmosis in a Hawaiian Monk Seal (Monachus schauinslandi )

Toxoplasma gondii infection in marine mammals is intriguing and indicative of contamination of the ocean environment with oocysts. T. gondii was identified in a Hawaiian monk seal (Monachus schauinslandi) that had visceral and cerebral lesions. Tachyzoites were found in the lymph nodes, spleen, diaphragm, heart, adrenal glands, and brain. A few tissue cysts were found in sections of the cerebrum. The diagnosis was confirmed serologically, by immunohistochemical staining with T. gondii-specific polyclonal rabbit serum, and by the detection of T. gondii DNA. The genotype was determined to be type III by restriction fragment length polymorphisms of the SAG2 gene. This is the first report of T. gondii infection in a Hawaiian monk seal.

A COMPARISON OF FOUR SEROLOGIC ASSAYS IN SCREENING FOR BRUCELLA EXPOSURE IN HAWAIIAN MONK SEALS

A survey for Brucella spp. antibodies was undertaken on 164 serum samples from 144 Hawaiian monk seals (Monachus schauinslandi) from the northwestern Hawaiian Islands collected between 1995 and 2002. The buffered antigen plate agglutination test (BPAT), the indirect enzyme immunoassay (I-ELISA), the competitive enzyme immunoassay (C-ELISA), and the fluorescence polarization assay (FPA) were compared with regard to their ability in detecting antibodies to Brucella spp. in the serum samples. Overall, antibodies were detected in 28 (17.1%) animals, using the BPAT test, 25 (15.2%) by the C-ELISA, and 19 (11.6%) in the I-ELISA and the FPA test, using thresholds established for cattle. No evidence of gross pathology consistent with clinical brucellosis was noted in any of the seropositive animals tested. Although further work would be necessary to validate these tests for use with monk seals it appears that both the C-ELISA and the FPA tests would be appropriate as diagnostic screening tests for detection of antibodies to Brucella spp. in this species.

FORAGING OF JUVENILE MONK SEALS AT FRENCH FRIGATE SHOALS, HAWAII

Emaciation and poor survivorship of juvenile Hawaiian monk seals at French Frigate Shoals atoll prompted a study of their foraging, using video camera technology (crittercam). Nine juveniles between the ages of 1 and 3 yr (six males, three females) were fitted with crittercam to identify their foraging habitat and feeding behavior. All feeding was directed at small (≤ 10 cm), cryptic, benthic prey. Older seals (ages 2 and 3), varied in their foraging intensity with most of their attention directed at shallow atoll depths (10–30 m). In contrast, the three yearlings focused all their feeding in the sand fields (50–100 m) on the atoll's outer slope. Bottom trawls were used to assess the prey abundance of the sand habitat and found 70% of the numerical catch was flounder (Bothidae). Extrapolating the yearlings' prey capture rate (0.13/min, derived from the crittercam video) over their total bottom time yielded an estimated 1–1.3 kg/day of flounder. The mean size of flounder (5 ± 1.7 cm) caught in the bottom trawls was close to the size at which larval flounder settle from the plankton (3 cm), suggesting that localized changes in oceanography could directly impact the seals' prey supply. Extensive use of sand communities by young seals may be the strongest link yet identified between juvenile survivorship and oceanographic dynamics.

MORPHOMETRY, HEMATOLOGY, AND SERUM CHEMISTRY IN THE HAWAIIAN MONK SEAL (MONACHUS SCHAUINSLANDI)

AbstractWe studied morphometric, hematology, and serum chemistry variables in 140 Hawaiian monk seals (Monachus schauinslandi) to establish normal baseline values for these variables among free‐living seals. We compared seals at French Frigate Shoals (FFS), Midway Atoll (MID), and Pearl and Hermes Reef (PHR) because these subpopulations differ in their rates of population recovery. Dorsal standard length and axillary girth differed significantly between immature (1–4 yr old) and adult (≥5 yr old) seals among sex and island subgroups. Immature seals at FFS were shorter than those at MID and PHR; adult seals at FFS had smaller dorsal standard lengths and axillary girths compared to the other subpopulations. The differences in size were more pronounced among adult females. Significant differences were also found for hematology and serum chemistry variables among seals at FFS, MID, and PHR. Monk seals at FFS had an absolute lymphopenia and eosinopenia compared to those at MID and PHR, compatible with a stress response. Seals at FFS also had lower blood urea nitrogen than seals at PHR, and a lower plasma potassium than seals at MID or PHR. Monk seals had an absolute and relative eosinophilia compared to previously published values. Analysis of subpopulation differences is useful for population health assessment and for long‐term monitoring of an endangered species.

EVALUATION OF CLOSED CAPTURE–RECAPTURE METHODS TO ESTIMATE ABUNDANCE OF HAWAIIAN MONK SEALS

Numerous capture–recapture methods have been developed to estimate abundance, yet the performance of these models is only rarely judged by comparison with true abundance. This study presents a rare opportunity to assess capture–recapture estimates in a free‐ranging population with known minimum abundance. Hawaiian monk seal abundance historically has been characterized using a trend index or has been estimated using simple enumeration. Here, I evaluate the applicability of various closed‐population capture–recapture models to estimating Hawaiian monk seal abundance and its associated error. I analyzed 12 data sets (two years from each of six subpopulations) representing a wide variety of sampling and logistical scenarios, using models that explored the effects of animal size class (juvenile, subadult, or adult), tag status, and sighting location on initial capture and recapture probabilities. I also explored various models to account for capture heterogeneity among individuals. Size and sex effects always significantly improved model fits, and tag status and location effects were also frequently influential. In most cases, abundance estimated from capture–recapture models was substantially lower than known minimum abundance, suggesting the influence of individual capture heterogeneity. Attributes of individuals known to be alive, but not captured during systematic surveys, did not reveal patterns that explained sources of capture heterogeneity. In some cases, mixture models produced estimates that were less biased but were associated with very large confidence intervals. Among the model types examined, those available in Program CAPTURE performed best; although they are still prone to negative bias, these models nevertheless may prove useful in characterizing population trends in Hawaiian monk seals. This study demonstrates that selection of appropriate closed capture–recapture models can be substantially improved by independent validation.