1 

ESTIMATING RESIDENCE TIMES OF 

HUMPBACK WHALES IN HAWAII

Prepared by 

Alison S. Craig, Ph.D., Research Associate 

Louis M. Herman, Ph.D., President 

Adam A. Pack, Ph.D., Vice President 

THE DOLPHIN INSTITUTE 

420 Ward Ave., Suite 212 

Honolulu, HI 96814 

For the 

Hawaiian Islands Humpback Whale National Marine Sanctuary 

Office of National Marine Sanctuaries 

National Oceanic and Atmospheric Administration 

U.S. Department of Commerce 

and the 

Department of Land and Natural Resources 

State of Hawaii 

June 2001 

This report is used for documentation and timely communication of preliminary results. It has not 

undergone external scientific review and should not be cited without permission of the authors. 
2 

ABSTRACT 

A review of previous literature on humpback whales in the Hawaiian Islands revealed 

that relatively few whales were identified on more than one day within the same season, but 

there was little information about the residence times of different classes of whale. We 

addressed this information gap by examining the sighting histories of individual whales 

photographed by Kewalo Basin Marine Mammal Laboratory off west Maui and the Kohala 

Coast of the Big Island from 1976 through 1996. Our analyses of resighting rate (the proportion 

of individual whales identified on more than one day within the same season) showed that (a) 

overall resighting rates were low (11.94%), (b) resighting rates did not differ significantly among 

males, mothers, females without calf, and juveniles for the Big Island, Maui, or all data 

combined, and (c) resighting rates were significantly greater off the Big Island than off Maui. 

Our analyses of the resighting intervals (intervals between first and last identifications within the 

same season) of all whales identified on more than day showed that (a) most whales (65.98%) 

had an interval of 2 weeks or less, (b) no juvenile had an interval of more than 4 weeks, and no 

mother or female without calf had an interval of more than 5 weeks, but some males (N = 10) 

and adults of unknown sex (N = 2) had intervals of 8 weeks or more, the longest interval being 

almost 11 weeks (76 days), (c) differences in resighting intervals among classes of whale were 

not significant overall or for the Big Island, but off Maui resighting intervals for females without 

calf were significantly shorter than resighting intervals for males and adults of unknown sex, and 

(d) inter-island resighting intervals tended to be longer than intervals recorded off a single island. 

Our results suggest that in Hawaii, humpback whales of all classes rarely remain resident in the 

same locale for long periods. Whales may be more transient off Maui than the Big Island, but it 

is also likely that differences in the extent of preferred habitat caused sampling differences 

between these two areas. Resighting rates and resighting intervals of females without calf 

relative to other whales off Maui and the Big Island suggest that these females may spend more 

time off the Big Island than off Maui, although further research is required. Finally, inter-island 

resighting intervals appear to be more representative of residency in the Hawaiian Islands as a 

whole than are intervals based on resightings of whales in the same location. Future studies 

combining photo-identification and large-scale satellite telemetry may provide the most precise 

data on humpback whale residency in the Hawaiian Islands, although obtaining sample sizes 

sufficient to compare different classes of whale would be financially and logistically challenging. 
3 

INTRODUCTION 

In the North Pacific, three main wintering areas are utilized by humpback whales. In the 

eastern North Pacific, humpback whales winter in Mexican waters, specifically in the waters of 

the southern coast of Baja, California, northern Gulf of California, Revillagigedo Archipelago, 

and the mainland coast of Mexico including the area around the Islas Tres Marias and the Isla 

Isabel (Urban & Uguayo 1987). In the western North Pacific, humpback whales winter in 

Japanese waters, specifically the waters of the Ryukyuan and Bonin Islands (Nishiwaki 1966). 

However, the main wintering area for the majority of North Pacific humpback whales is in 

Hawaiian waters, with the most dense concentrations occurring on Penguin Bank and in the 

“four-island” region between Maui, Lanai, Molokai, and Kahoolawe (Baker & Herman 1981; 

Herman & Antinoja 1977; Mobley, Bauer, & Herman 1999). 

 Although the peak period of humpback whale aggregation in Hawaiian waters is typically 

between January and April, less is known about the length of residency of individual whales. By 

identifying individual whales from the patterns of pigmentation and scarring on their tail flukes 

(see Katona et al. 1979), it is possible to calculate minimum residence times by computing the 

interval between first and last sightings of individual whales within the same winter season. In 

this report, we review existing literature on residence times of individually-identified humpback 

whales in the Hawaiian Islands. We then expand on existing knowledge of residency by 

examining data collected by researchers from Kewalo Basin Marine Mammal Laboratory 

(KBMML) over 21 winter field seasons in Hawaii. We determine the proportions of whales 

identified on more than one day within the same season for west Maui and the Kohala Coast of 

the Big Island, classifying whales into the following categories: male, mother, female without 

calf, juvenile, and adult of unknown sex. We then examine in detail the intervals between first 

and last identification within the same season for all these classes of whale for Maui and the Big 

Island. Finally, because some whales were photographed off both the Big Island and Maui 

within the same season, we present details of inter-island resighting intervals. Altogether, this 

represents the most comprehensive analysis to date of humpback whale residency in the 

Hawaiian Islands. 

Review of existing literature

 All available literature pertaining to humpbacks in Hawaii was reviewed for examples of 

repeated sightings of individual whales within a given year. Several previous studies by 

researchers from KBMML have included information about residency (e.g., Baker & Herman 

1984; Mobley & Herman 1985; Gabriele 1992). However, because each of these studies was 

based on a subset of the data analyzed in the next section of this report, they are not examined 

individually here. 

Darling et al. (1983) identified individual humpback whales off west Maui from 1977-

1979. Only 28 of 264 (10.6%) individuals were identified on more than one day within the same 

season during this time. Of these 28 whales, 26 were identified on dates less than 6 weeks apart. 

The longest interval between first and last sighting within the same season was 74 days, and the 

second longest interval was 55 days. The mean of the intervals presented by Darling et al. 
4 

(1983) was 20.7 days, but these authors did not distinguish between different classes of whale 

when presenting within-season resighting intervals. 

 Glockner and Venus (1983) identified individual humpback whales off west Maui from 

1977-1979. When discussing resightings of individual whales, they focused on sightings in 

different years rather than repeat sightings within the same year. However, from the data that 

they presented in Appendix Table 2 it is possible to calculate resighting intervals for whales 

identified on more than one day within the same season. From 1977 – 1979, eight mothers were 

identified more than once within the same season. The longest resighting interval for a mother 

was 27 days, and the mean interval for all eight mothers was 12.1 days. One male was identified 

with an interval of 22 days in 1977 and an interval of 18 days in 1978. Two whales of unknown 

sex had intervals of 1 and 21 days. 

 Based on data collected off west Maui from 1977-1981, Glockner-Ferrari and Ferrari 

(1984) reported that 25% of individually-identified mothers were observed on two or more 

occasions within the same year. Intervals between successive sightings ranged from 1 to 48 

days, with a mean of 13.7 days. Glockner-Ferrari and Ferrari (1985) reported that from 1982-

1983 only 9 of 42 (21.4%) mothers were observed more than once within the same season. The 

longest interval between first and last sighting of a mother during this time was 56 days, and the 

next longest was 28 days. The mean of all intervals reported for mothers was 16.4 days. 

Glockner-Ferrari and Ferrari also reported an interval of 45 days for an escort (presumably male) 

and intervals ranging from 3 to 22 days for three whales of unknown gender. 

 Cerchio (1998) collected data around Kauai’s north, south, and west coasts from 1989-

1993. As with the studies conducted off west Maui, he found that few whales were identified 

more than once within the same season. The percentage of whales identified on more than one 

day within the same season ranged from 5% in 1989 to 14% in 1992 and 1993. For the 82 

whales identified on two different days within the same season, the mean resighting interval was 

15 days. For the 19 whales identified on three different days within the same season, the mean 

interval between first and last sighting was 23 days. For the three whales identified on four 

different days within the same season, the mean interval between first and last sighting was 37 

days. However, the standard deviations in each case were large, indicating that the resighting 

intervals varied considerably among whales. No information was presented with respect to the 

classes of whale identified more than once within the same year. 

 The studies discussed thus far were each focused on a single island location, and any 

information about residency was restricted to that location. However, some studies have 

compared data collected off different islands within the same year in order to investigate interisland movements. These provide some information about residency within the Hawaiian 

Islands as a whole. The most comprehensive analysis of inter-island movements to date was 

conducted in a collaborative study by researchers from Moss Landing Marine Laboratories and 

KBMML (Cerchio et al. 1998). In this study, photographs of humpback whales identified from 

1989-1991 off the islands of Kauai and the Big Island were compared. Of 1072 individual 

whales, only 15 (1.4%) were identified in both locations within the same year. The shortest 

inter-island resighting interval was 8 days, and the longest was 71 days. Overall, the mean interisland resighting interval was 33.5 days. Of the 15 whales identified in both locations within the 

same year, 8 were believed to be male based on their sighting histories, and the remainder were 
5 

of unknown sex. These results suggest that males may be more mobile than females within the 

Hawaiian Islands, although the sample size is relatively small. When the eight known males 

were considered separately, the mean inter-island resighting interval was 49.8 days. Overall, the 

inter-island resighting intervals reported in this study are longer than most resighting intervals 

reported by studies conducted in one location only. This suggests that more representative data 

on residency within Hawaii as a whole may be produced by comparing data collected in more 

than location within the Hawaiian Islands, rather than relying solely on data collected in a single 

location. 

 Other data on within-season inter-island movements in Hawaii are meager. Baker and 

Herman (1981) reported inter-island resighting intervals for five whales that traveled from the 

Big Island to Maui (mean interval = 26.4 days), for one whale that traveled from Hawaii to 

Penguin Bank (interval = 25 days), and one whale that traveled from Maui to Oahu (interval = 38 

days). Darling and Morowitz (1986) reported that five whales traveled from Maui to the Big 

Island within the same season, but the resighting interval was documented for only one whale. 

This whale was first identified on Maui, and was then identified 20 days later on the Big Island. 

Two days later, the whale was identified once again in Maui waters. Mate et al. (1998) reported 

that one humpback whale satellite-tagged off Kauai traveled to Oahu in 4 days, while another 

individual tagged off Kauai spent the next 10 days travelling through the coastal waters of Oahu, 

Molokai, Lanai, and Maui in addition to visiting Penguin Bank. Finally, Craig (1995) presented 

details of several inter-island resighting intervals gleaned from the KBMML database. Rather 

than summarizing those data here, we present them in full in the next section of this report. 

 In each of the studies reviewed here information about residency was sparse, largely 

because relatively few whales were identified on more than one day within the same season. 

This was true for both west Maui and Kauai. Because most studies collected data in the same 

waters each day, the dearth of resightings within the same year strongly suggests that individual 

whales are quite mobile and rarely remain resident in the same area for extended periods of time. 

This suggestion is supported by satellite telemetry of tagged humpback whales: at the most 

recent Biennial Conference on the Biology of Marine Mammals, Mate (1999) reported that most 

whales tagged in the Hawaiian Islands did not remain in one area for very long. Although these 

studies presented some basic data on resighting intervals, in general there was little information 

regarding possible differences in residency among different classes of humpback whale. This 

information gap will be addressed in the next section of this report, in which we present data 

collected over 21 field seasons by researchers from KBMML. 

 
6 

ESTIMATING RESIDENCE TIMES OF HUMPBACK WHALES IN THE HAWAIIAN 

ISLANDS, 1976 – 1996 

Methods

Humpback whale tail flukes were photographed by KBMML staff between January and 

April of each year from 1976 through 1996. Photographs were obtained mainly off the Kohala 

Coast of the Big Island and in the waters of west Maui (see Table 1), but a few photographs 

(0.6%) were taken around Niihau, Oahu, and Penguin Bank. Each photograph was allocated a 

unique observation number and assigned to one of five color categories. Each photograph was 

then compared to all others in the same color category, and all photographs that were found to 

match were allocated a common “resight” number. This resight number was used to link all 

photographs of a known individual, so that individual sighting histories could be compiled. 

 In some cases, the sex of individual whales was determined by observing the genital 

region directly underwater. More commonly, sex was inferred from social role. An adult 

observed while escorting a mother and calf was assumed to be male, as was any whale identified 

as a singer. An adult observed in close proximity to a calf was assumed to be its mother, and 

therefore female. Biopsy research and underwater observations of the genital region have 

validated these assumptions (Glockner 1983; Glockner-Ferrari & Ferrari 1985; Clapham et al. 

1992; Medrano et al. 1994). When sex was determined, it was applied retrospectively to all 

previous identifications of an individual. 

 For the purposes of this study, all sightings of individual whales were assigned to one of 

five categories: male, mother, female without calf, juvenile, and adult of unknown sex. 

Juveniles were perceived by experienced observers to be unambiguously small relative to adults, 

but too large to be calves. In many cases, these would have been yearlings that had recently 

separated from their mothers. 

 Residency was examined for these classes of whale by using two measures: resighting 

rates and resighting intervals. First, the proportion of individual whales identified on more than 

one day within the same season (resighting rate) was calculated for Maui, the Big Island, and for 

all data combined. This provides some basic information pertinent to residency: if whales tend 

to reside in the same area for extended periods of time, they are likely to be photo-identified on 

more than one day. Conversely, if whales tend to be quite mobile, they are less likely to be 

photo-identified in the same area on more than one day. Next, the actual resighting intervals of 

whales identified on more than one day were examined for each class of whale off Maui, the Big 

Island, and for all data combined. Resighting intervals were calculated by subtracting the date of 

last identification from the date of first identification within a given season. Finally, because 

data were collected off Maui and the Big Island within the same season in some years (see Table 

1), data on inter-island resighting intervals are presented. 
7 

Table 1. Dates and locations of data collection by KBMML in the Hawaiian Islands. 

Year Location Study Period 

1976 Maui 13 March - 16 March 

1977 Maui 12 February - 20 April 

1978 Maui 1 February - 8 April 

1979 Maui 3 February - 31 March 

1980 Maui 27 January - 2 April 

 Big Island 15 January - 5 April 

1981 Maui 11 January - 12 April 

 Big Island 21 January - 24 February 

1982 Maui 3 January - 28 March 

1983 Maui 1 February - 29 March 

1984 Maui 26 January - 28 March 

1985 Maui 6 March - 6 April 

 Big Island 18 January - 12 March 

1986 Maui 1 March - 22 April 

 Big Island 19 January - 27 February 

1987 Maui 23 January - 12 April 

1988 Maui 17 March - 16 April 

 Big Island 13 January - 10 March 

1989 Big Island 7 January - 3 April 

1990 Big Island 11 January - 13 April 

1991 Big Island 9 January - 15 April 

1992 Big Island 11 January - 3 April 

1993 Big Island 15 January - 7 April 

1994 Big Island 26 January - 19 March 

1995 Big Island 23 January - 19 March 

1996 Maui 9 January – 17 April 
8 

Results

I. Proportion of whales identified on more than one day during the same season. 

As with the previous studies reviewed earlier, few whales were identified on more than 

one day within the same season (Table 2). Overall, only 11.94% of whales were identified on 

more than one day within the same season. Off Maui, resighting rates were highest for males, 

followed by mothers, females without calf, adults of unknown sex, and juveniles. However, off 

the Big Island the highest resighting rate was for females without calf, followed by mothers, 

males, juveniles, and adults of unknown sex. The differences in resighting rates for males, 

mothers, females without calf, and juveniles were not significant for the Big Island (χ2 = 1.40, df 

= 3, p = 0.71, N = 684), Maui (χ2 = 5.42, df = 3, p = 0.14, N = 947), or overall (χ2 = 1.71, df = 

3, p = 0.63, N = 1652). 

Table 2. Proportion of whales identified on more than one day (resighted) within the same season in the 

Hawaiian Islands: males, mothers, females without calf, juveniles, and adults of unknown sex. 

 

Overall * 

 

Big Island 

 

Maui 

 

Total 

no. 

whales 

Percentage 

resighted 

 

Total 

no. 

whales 

Percentage 

resighted 

 

Total 

no. 

whales 

Percentage 

resighted 

 

Males 1099 19.75 441 24.94 643 15.09 

Mothers 404 17.82 157 25.48 243 12.76 

Females without calf 94 17.02 42 28.57 50 6.00 

Juveniles 55 14.55 44 18.18 11 0.00 

Adults, unknown sex 2853 7.89 1699 9.30 1124 5.25 

Total for all whales 

 

4505 11.94 

 

2383 13.76 

 

2071 

 

9.17 

* Overall data include a small number of whales identified in other locations within the Hawaiian Islands. 

The proportion of whales identified on more than one day within the same season off the 

Big Island (13.76%) was significantly greater than the corresponding proportion for Maui 

(9.17%) (χ2 = 22.27, df = 1, p < 0.0001, N = 4454). This may suggest that whales in west Maui 

waters are relatively transient, whereas off the Kohala Coast whales are more likely to linger in 

the same area. However, there are other potential explanations for this difference. Because the 
9 

density of whales is much greater in Maui waters than off the Big Island (Herman et al. 1980; 

Mobley, Bauer & Herman 1999; Craig 2001), it is possible that a consistently smaller proportion 

of the population was photographed off Maui than off the Big Island. This would tend to reduce 

within-season resighting rates off Maui relative to the Big Island. In order to investigate the 

potential influence of population size on the resighting rates reported here, we compared 

resighting rates for data collected off Maui in 1987 and 1996. These years were selected because 

they were the years of greatest research effort off Maui, and they are spaced far enough apart that 

the population size should have increased significantly over this time period. Based on the 7% 

annual rate of increase suggested by the aerial surveys of Mobley, Grotefendt, et al. (1999), the 

population size in 1996 would be about 1.8 times the population size in 1987: i.e., the 

population would almost have doubled over this time. Table 3 summarizes the proportions of 

whales identified on more than one day for these two seasons off Maui. The overall resighting 

rates are very similar: in 1987 8.84% of whales were identified on more than one day and in 

1996 the figure was 9.05% (χ2 = 0.00, df = 1, p = 1.00, N = 911). Thus, an increase in 

population size appears to have had little effect on the within-season resighting rates of 

humpback whales identified off west Maui. This suggests that the difference between Maui and 

the Big Island with respect to the proportion of whales identified on more than one day is not due 

simply to regional differences in population size. 

Table 3. Proportion of whales identified on more than one day (resighted) within the same 

season off Maui: 1987 and 1996 compared. 

 

Maui, 1987 

 

Maui, 1996 

 

Total 

no. 

whales 

Percentage 

resighted 

 

Total 

no. 

whales 

Percentage 

resighted 

 

Males 63 17.46 204 13.73 

Mothers 31 9.68 91 13.19 

Females without calf 10 10.00 6 0.00 

Juveniles 3 0.00 4 0.00 

Adults, unknown sex 108 3.70 391 5.88 

Total for all whales 

 

215 8.84 

 

696 9.05 

 Another potential explanation for the finding that whales are resighted within the same 

season more commonly off the Big Island than Maui is that the extent of preferred habitat is 
10 

much smaller off the northwest coast of the Big Island than it is off west Maui (Craig & Herman 

2000). Humpback whales are typically found in waters less than 183m (100 fathoms) deep 

(Herman & Antinoja 1977). Almost the entire area between west Maui and the islands of 

Kahoolawe and Lanai is less than 183 m deep, but off the northwest coast of the Big Island the 

water deepens to more than 183 m within 5 km of the coastline. Consequently, it is likely that 

our research vessel consistently sampled a larger proportion of preferred habitat off the Big 

Island than was possible for Maui. This would tend to increase the probability of identifying 

whales on more than one day within the same season. However, the possibility that individual 

whales tend to be more transient in Maui waters than in Big Island waters cannot be discounted. 

In addition to the regional difference in overall resighting rates, it appears that the 

resighting rate of females without calf relative to other whales differed between Maui and the 

Big Island. Females without calf were the class of whale most likely to be resighted in the same 

year off the Big Island, whereas they were much less likely than males and mothers to be 

resighted off Maui (Table 2). It is possible that the behavior of females without calf differs in 

these two locations. Specifically, it may be that females without calf are less transient off the 

Big Island than in Maui waters. We have shown previously that individual females identified off 

Maui and the Big Island in different years were likely to be seen off the Big Island in the years 

when they had no calf, and off Maui in the years during which they were mothers (Craig & 

Herman 2000). This would be consistent with the suggestion that females without calf may 

spend more time in Big Island waters than Maui waters. 

II. Resighting intervals of whales identified on more than one day during the same season. 

Figure 1 illustrates the intervals between first and last identification for all whales 

photographed on more than one day during the same season in the Hawaiian Islands. Intervals 

are expressed in weeks. For example, an interval of 1 represents 1-7 days, 2 represents 8-14 

days, and so forth. It is striking that for every class of whale, the majority of individuals had 

resighting intervals of 2 weeks or less. Overall, 44.98% of whales had an interval of 1 week or 

less, 65.98% had an interval of 2 weeks or less, 81.04% of whales had an interval of 3 weeks or 

less, and 89.40% of whales had an interval of 4 weeks or less. No juvenile had an interval of 

more than 4 weeks, and no mother or female without calf had an interval of more than 5 weeks. 

However, some males (N = 10) and adults of unknown sex (N = 2) were seen over periods of 8 

weeks or more, including one male whose dates of first and last identification were almost 11 

weeks (76 days) apart. These data combine all identifications from all locations available in the 

KBMML database. As such, they include some inter-island resighting intervals. These are 

presented in detail below. 
11 

Figure 1. Intervals between first and last identification of whales photographed on more than 

one day (all data). 

 Figure 2 illustrates the intervals, again in weeks, between first and last identification for 

all whales identified on more than one day within the same season in west Maui waters, and 

Figure 3 illustrates the corresponding information for the waters off the Kohala Coast of the Big 

Island. It should be noted that no juvenile was photographed on more than one day within the 

same season off Maui. Figures 2 and 3 reveal broadly similar trends, with one notable 

difference: no female without calf had a resighting interval of more than 1 week off Maui, 

whereas off the Big Island over 40% of females without calf had a resighting interval in excess 

of 1 week. This is consistent with the suggestion, discussed previously, that females without calf 

may be less transient off the Big Island than off Maui. The sample size of females without calf 

in Figure 2 is small (n = 3). However, this is because these females were rarely identified more 

than once during the same season in Maui waters. A closer examination of the data reveals that 

of the 50 females without calf that were identified in Maui waters, only three (6.0%) were 

identified on more than one day. Of these three, none had an interval greater than 3 days. By 

contrast, of the 42 females identified in Big Island waters, 12 (28.6%) were identified on more 

than 1 day. Of these 12, eight (66.7%) had resighting intervals greater than 3 days and six 

(50.0%) had resighting intervals of 7 days or more. The difference in resighting rates of females 

without calf off Maui as compared to the Big Island was significant (χ2 = 6.95, df = 1, p = 0.008, 

N = 72), although it will be recalled that overall resighting rates were higher off the Big Island 

than off Maui. A comparison of resighting intervals is of limited utility because there are only 

three data points for Maui, due to the rarity with which females without calf were identified on 

more than one day in this location. However, a two-tailed Wilcoxon rank sum test showed that 

even with this minimal sample size, the difference approached statistical significance (S = 12.00, 

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

1 2 3 4 5 6 7 8 or more

Resighting interval (wks)

%

Female with no calf (n=16)

Juvenile (n=8)

M ale (n=217)

M other (n=72)

Adult, unknown sex (n=225)
12 

df = 1, p = 0.095). More data are required to permit a definitive conclusion, but these data do 

suggest that females without calf may spend more time in waters off the Big Island than off 

Maui. 

Figure 2. Intervals between first and last identification of whales photographed on more than 

one day (Maui). 

Figure 3. Intervals between first and last identification of whales photographed on more than 

one day (Big Island). 

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

100

1 2 3 4 5 6 7 8 or more

Resighting interval (wks)

%

Fem ale without calf (n = 3)

M ale (n=97)

M other (n=31)

Adult, unknown sex (n=59)

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

1 2 3 4 5 6 7 8 or more

Resighting interval (wks)

%

Fem ale without calf (n=12)

Juvenile (n=8)

M ale (n=110)

M other (n=40)

Adult, unknown sex (n=158)
13 

 Table 4 summarizes the resighting intervals (in days) for each class of whale identified on 

more than one day within the same season. Data are presented separately for Maui and the Big 

Island. Rather than calculating mean intervals and standard deviations, we calculated median 

intervals and interquartile ranges. With skewed distributions, as are evident in Figures 1-3, 

medians are more representative of the data than are means, and interquartile ranges are the 

appropriate measure of variability. 

Table 4. Median intervals (in days) between first and last identification for all males, mothers, 

females without calf, juveniles, and adults of unknown sex identified on more than one day 

during the same season off the Big Island, Maui, and overall. 

Location Median 

(days) 

Interquartile 

range 

Minimum Maximum N 

 

Males Big Island 8.0 13.0 1 76 110 

Mothers Big Island 8.5 13.5 1 35 40 

Females without calf Big Island 6.5 10.0 1 23 12 

Juveniles Big Island 6.5 10.5 1 28 8 

Adults, unknown sex Big Island 8.0 13.0 1 57 158 

All whales Big Island 8.0 13.0 1 76 328 

 

Males Maui 13.0 20.0 1 64 97 

Mothers Maui 5.0 12.0 1 33 31 

Females without calf Maui 2.0 2.0 1 3 3 

Juveniles Maui - - - - - 

Adults, unknown sex Maui 10.0 15.0 1 45 59 

All whales Maui 10.0 17.0 1 64 190 

 

Males Overall 11.0 17.0 1 76 217 

Mothers Overall 7.0 12.5 1 35 72 

Females without calf Overall 5.0 10.5 1 31 16 

Juveniles Overall 6.5 10.6 1 28 8 

Adults, unknown sex Overall 8.0 14.0 1 65 225 

All whales Overall 9.0 15.0 1 76 538 

Note: Overall data include a small number of whales identified at other locations within the Hawaiian Islands.

 
14 

For all classes of whale the intervals were very variable, as indicated by the relatively 

high interquartile ranges. For the Big Island, mothers had the longest median interval (8.5 days), 

followed by males and adults of unknown sex (8 days), and then females without calf and 

juveniles (6.5 days). In the case of Maui, males had the longest median interval (13 days), 

followed by adults of unknown sex (10 days), mothers (5 days), and females without calf (2 

days). No juvenile was identified on more than 1 day off Maui. When all data were combined, 

males had the longest median interval (11.0 days), followed by adults of unknown sex (8 days), 

mothers (7 days), juveniles (6.5 days), and females without calf (5 days). 

The differences in resighting intervals among classes of whale were not significant 

overall (Kruskal-Wallis, F[4,533] = 1.92, p = 0.106) or for the Big Island (Kruskal-Wallis, F[4,323] = 

0.34, p = 0.851), but were significant in the case of Maui (Kruskal-Wallis, F[3,186] = 2.96, p = 

0.034). A Dunn test (Zar 1984) revealed that off Maui, resighting intervals for females without 

calf were significantly shorter than resighting intervals for males and adults of unknown sex. 

This is consistent with the suggestion that females without calf are more transient off Maui than 

off the Big Island, although again it should be noted that only three females without calf were 

identified on more than one day off Maui. 

III. Inter-island resighting intervals. 

 Despite the fact that data were collected off both Maui and the Big Island during the same 

season in 5 years (Table 1), almost all whales identified on more than one day were observed in 

only one location within a given season. However, there were several instances of movements 

between different islands within the same season (see Table 5). Fourteen whales were identified 

initially off the Big Island and subsequently off Maui, and one was identified initially off Maui 

and subsequently off Niihau. Although the data may appear to suggest a northwesterly direction 

of travel, the apparent direction of movement of whales in many cases is likely to be a result of 

the timing of data collection in each location (see Table 1). Of the fifteen whales, eight were 

known to be male, one was known to be female, and the remaining six were of unknown sex. 

Overall, the mean inter-island resighting interval was 36.2 days. When males alone were 

considered, the mean interval was 35.5 days, and for whales of unknown sex the mean interval 

was 38.0 days. The only female in this data set had an inter-island resighting interval of 31 days, 

and was unaccompanied by a calf. 
15 

Table 5. Resighting intervals of whales identified off more than one Hawaiian Island within the 

same season. 

Whale Sex First identification 

 

Second identification Interval (days) 

 

Location Date 

 

Location Date 

 

4 Unknown Big Island 23 Jan 1980 Maui 26 Feb 1980 34 

6 Male Big Island 1 Mar 1988 Maui 27 Mar 1988 26 

8 Unknown Big Island 6 Feb 1980 Maui 1 Mar 1980 24 

23 Male Big Island 12 Feb 1980 Maui 1 Mar 1980 18 

48 Male Big Island 10 Mar 1988 Maui 12 Apr 1988 33 

68 Female Maui 7 Mar 1981 Niihau 7 Apr 1981 31 

96 Unknown Big Island 31 Jan 1980 Maui 26 Feb 1980 26 

152 Unknown Big Island 30 Jan 1981 Maui 10 Mar 1981 39 

433 Male Big Island 13 Feb 1981 Maui 3 Apr 1981 49 

498 Male Big Island 5 Mar 1988 Maui 21 Mar 1988 16 

979 Unknown Big Island 3 Mar 1988 Maui 13 Apr 1988 41 

1019 Male Big Island 21 Feb 1988 Maui 12 Apr 1988 51 

1026 Unknown Big Island 19 Jan 1988 Maui 24 Mar 1988 64 

1031 Male Big Island 18 Feb 1988 Maui 15 Apr 1988 57 

1043 Male Big Island 19 Feb 1988 Maui 24 Mar 1988 34 

Note: These data were previously presented in Baker and Herman (1981) and Craig (1995).
16 

Discussion

Resighting rates 

 Overall, the results showed that only 11.94% of whales were identified on more than one 

day within a single season. Thus, the vast majority of whales were identified on only one day 

during the winter reproductive season. This is consistent with the studies reviewed earlier, and 

by considering the results of all studies together we can conclude that this is true for all locations 

within Hawaii in which whales have been studied intensively: west Maui, the Kohala Coast of 

the Big Island, and the north, south, and west coasts of Kauai. Similar findings have been 

reported from the winter grounds of the North Atlantic. For example, Mattila et al. (1994) 

reported that only 15.8% of humpbacks were identified on more than one day within the same 

season at Samana Bay, Dominican Republic, and Mattila et al. (1989) reported that only 9.1% of 

humpbacks were identified on more than one day on Silver Bank. If humpback whales on the 

winter grounds tended to reside in the same area for protracted periods, resightings of whales 

within the same season would be much more common than they are for Hawaii and other winter 

grounds. Thus, all available data on within-season resighting rates suggest that humpback 

whales on the winter grounds do not tend to reside in the same area for long periods, and may be 

quite transient at the local level. It should be noted that this apparent transience is very different 

from the residency behavior of humpback whales in the summer feeding grounds, where whales 

are often resighted in the same location many times throughout the feeding season (Clapham 

2000). For example, Baker et al. (1985) reported that 54% of whales identified in southeastern 

Alaska in 1982 were sighted on more than one day, and Clapham et al. (1993) reported that 

77.3% of whales identified in the Gulf of Maine between 1979 and 1988 were resighted on more 

than one day during the same year. 

When we compared resighting rates for Maui and the Big Island, we found that 

significantly more whales were identified on more than one day off the Big Island than off Maui. 

This may indicate that humpback whales are more transient in west Maui waters than off the 

Kohala Coast of the Big Island. However, there are other potential explanations, as discussed 

previously. It is possible that the difference in resighting rates off Maui and the Big Island is 

related to regional differences in population density. However, although the population size of 

humpback whales in Hawaii appears to be increasing annually at a rate of 7% (Mobley, 

Grotefendt, et al. 1999), a comparison of overall resighting rates off Maui in 1987 and 1996 

showed very little difference. Consequently, it is unlikely that the difference in resighting rates 

between Maui and the Big Island is due simply to differences in population density. However, it 

is possible that the regional difference in resighting rates was due to differences in the extent of 

preferred habitat. Because the extent of preferred habitat is much greater in the west Maui 

region than off the northwest coast of the Big Island (e.g., see Craig & Herman 2000), it is 

probable that our research boat consistently sampled a greater proportion of preferred habitat off 

the Big Island than off Maui. This would tend to increase the probability of identifying 

individual whales on more than one occasion. However, the possibility remains that humpback 

whales are indeed more transient off west Maui than off the Big Island. 
17 

We presented a comprehensive analysis of resighting rates for the following classes of 

whale: males, mothers, females without calf, juveniles, and adults of unknown sex. Off Maui, 

resighting rates were highest for males, followed by mothers, females without calf, adults of 

unknown sex, and juveniles. Off the Big Island, the highest resighting rate was for females 

without calf, followed by mothers, males, juveniles, and adults of unknown sex. When all data 

were combined, the highest resighting rates were for males, followed by mothers, females 

without calf, juveniles, and adults of unknown sex. However, statistical analysis revealed that 

there were no significant differences in resighting rates between these classes of whale for Maui, 

the Big Island, or all data combined. 

The resighting rate of females without calf relative to the resighting rates of other classes 

of whale was quite different off the Big Island as compared to Maui. Off the Big Island, females 

without calf were the class of whale most likely to be identified on more than one day, whereas 

off Maui females without calf were much less likely to be resighted than were males and 

mothers. As was noted earlier, the suggestion that these females may spend more time off the 

Kohala Coast of the Big Island than off west Maui is consistent with our previous research 

showing that individual females are likely to be identified off Maui in the years during which 

they have a calf and off the Big Island in the years when they have no calf (Craig & Herman 

2000). 

Resighting intervals 

 Our analysis of resighting intervals for whales identified on more than one day within the 

same season showed that for the majority of whales (65.98%), 2 weeks or less elapsed between 

first and last identification within the same season. Moreover, 81.04% of whales had intervals of 

3 weeks or less, and 89.40% of whales had intervals of 4 weeks or less. The longest interval 

between first and last identification for any whale was exhibited by one male whose dates of first 

and last identification were almost 11 weeks (76 days) apart. This interval is 2 days longer than 

the longest interval previously reported for humpback whales in the Hawaiian Islands (Darling et 

al. 1983). No mother or female without calf had an interval in excess of 5 weeks, and no 

juvenile had an interval greater than 4 weeks. However, 10 males and two adults of unknown 

sex had intervals of 8 weeks or more. 

 When resighting intervals were considered separately by island, the trends were generally 

similar for Maui and the Big Island. However, there was a difference in the resighting intervals 

of females without calf in the two locations. Off Maui, no female without calf had a resighting 

interval of more than 3 days. By contrast, off the Big Island 50.0% of all females without calf 

that were identified on more than one day had intervals of 7 days or more. This again suggests 

that females without calf may spend more time in waters off the Big Island than off Maui, 

although more data are required before a definitive conclusion can be reached. 

 Resighting intervals for all classes of whale were very variable. Off the Big Island, 

mothers had the longest median interval, followed by males and adults of unknown gender, and 

finally by females without calf and juveniles. However, statistical analysis indicated that the 

differences between these classes of whale were not significant. It should be noted that the 

variability of intervals within each class of whale would reduce the chance of finding significant 
18 

differences between classes. Off Maui, males had the longest median interval, followed by 

adults of unknown sex, mothers, and females without calf. In this case, statistical analysis 

indicated that the resighting intervals of females without calf were significantly shorter than the 

resighting intervals of males and adults of unknown sex. Again, this is consistent with the 

suggestion that females without calf are relatively transient in Maui waters. 

 Overall, biological considerations predict that females without calf should spend little 

time in Hawaiian waters as compared to males and mothers. The absence of an exploitable food 

resource in the winter grounds and the rarity of feeding during most of the migration mean that 

wintering humpback whales must subsist on stored energy reserves for extended periods of time 

(Nishiwaki 1959; Chittleborough 1965; Dawbin 1966). Male humpback whales are likely to 

remain on the winter grounds for as long as their energy reserves allow, in order to maximize 

their mating opportunities. However, after a female without a calf has become pregnant there is 

no need to remain on the winter grounds. Given that she will have to face the extreme energetic 

demands of lactation while she herself is not feeding the following winter, her best strategy is 

probably to return to the feeding grounds and begin accumulating energy reserves as soon as 

possible. In the years when a female has a calf, however, she presumably remains on the winter 

grounds until such time as her calf is sufficiently mature and robust to survive the migration back 

to the high-latitude feeding grounds. Juveniles are unlikely to remain in the winter grounds for 

extended periods of time because they are physiologically incapable of reproducing, and because 

their smaller body size will restrict their ability to accumulate stored energy reserves (Calder 

1984). 

Inter-island resighting intervals 

There were fifteen cases in which individual whales were identified off more than one 

island within the same season. Of these fifteen whales, eight were known males, six were of 

unknown sex, and only one was a known female. Overall, the mean inter-island resighting 

interval for these whales was 36.2 days. The mean inter-island resighting interval for males only 

was 35.5 days, and the mean interval for whales of unknown sex was 38 days. The interval for 

the only female (without a calf) in this data set was 31 days. The most notable aspect of these 

intervals is that they tend to be longer than the resighting intervals reported for whales identified 

off only one island during the same season. For example, off the Big Island only 4 of 110 (3.6%) 

males had a resighting interval of 35 days or more, and only 3 of 158 (1.9%) adults of unknown 

sex had a resighting interval of 38 days or more. In the case of Maui, only 11 of 97 (11.3%) 

males had a resighting interval of 35 days or more, and only 3 of 59 (5.1%) whales of unknown 

sex had a resighting interval of 38 days or more. This underlines the fact that resighting intervals 

provide only minimum estimates of residency, and suggests that inter-island resighting intervals 

are more representative of residency in the Hawaiian Islands as a whole than are intervals based 

on resightings of whales in the same location. 
19 

Directions for future research 

 While providing the most comprehensive analysis to date of humpback whale residency 

on the winter grounds, our study has raised some interesting issues that should prompt further 

research. The finding that inter-island resighting intervals tend to be longer than resighting 

intervals recorded off a single island suggests a need for more studies comparing data collected 

simultaneously in different locations throughout Hawaii. The possibility that whales, especially 

females without calves, may be more transient in west Maui waters than in the waters off the 

Kohala Coast of the Big Island merits further investigation. Future studies combining photoidentification with large-scale satellite telemetry would provide the most precise data on 

residency. However, obtaining a sample size sufficient to allow statistical comparison of 

different classes of whale would be financially and logistically challenging. 

ACKNOWLEDGEMENTS 

 Funding for the preparation of this report was provided in part by the National Oceanic 

and Atmospheric Administration (NOAA) of the U.S. Department of Commerce through 

contract number 50ABNC-8-00063 between the Hawaiian Islands Humpback Whale National 

Marine Sanctuary and the State of Hawaii, Department of Land and Natural Resources. Funding 

for data collection over the years was provided by grants from Earthwatch and its Research 

Corps, The Dolphin Institute, Sea Grant of the University of Hawaii, the National Science 

Foundation, and the National Marine Fisheries Service. Data were collected under National 

Marine Fisheries Service permit numbers 223, 576, 810, and 941. We are grateful to the 

numerous staff, students, and volunteers of the Kewalo Basin Marine Mammal Laboratory who 

have participated in our humpback whale research from the 1970s to the present day. The large 

number of people involved makes it impossible to thank everyone by name, but among those 

who made substantial contributions to the data set analyzed here are Scott Baker, Gordon Bauer, 

Brooks Bays, Adam Frankel, Tom Freeman, Paul Forestell, Chris Gabriele, David Helweg, Mike 

Hoffhines, Hilary Maybaum, Joe Mobley, Anjanette Perry, Neil Polehn, Sue Lynn Reif, Scott 

Spitz, and Suzanne Yin. We would also like to thank the numerous volunteers from Earthwatch 

and the Dolphin Institute who have assisted with our research, and Trimble Navigation Company 

for donation of equipment. 
20 

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