This is a poster presentation for the upcoming conference of the Society for Marine Mammology we have done with Leah Crowe and Dr Peter Stevick at Allied Whale, College of the Atlantic.


Below is the text of the presentation:

Individual migration timing of humpback whales (Megaptera novaeangliae)

Leah M. Crowe1( This e-mail address is being protected from spambots, you need JavaScript enabled to view it ), Andrew W. Stevenson2, Peter T. Stevick1
1College of the Atlantic, 105 Eden Street, Bar Harbor, ME 04609, U.S.A.
2Whales Bermuda, 16 Skyline Road, Smiths, FL 08, Bermuda

The factors that trigger the initiation of migration in humpback whales are not well

understood. A tendency for individual whales to be present on breeding ground at

particular times has been suggested, but it is unclear if there is any individual tendency

to migrate at particular times. Factors including the age of an individual, its gender, and

its reproductive status have been shown to influence migration timing (Dawbin, 1997).

However, as much of this work was done based on whaling catches, multiple

observations of the same whale were not possible, and no information on individual

patterns has been presented.

In many areas, the majority of humpback whale migration is pelagic, so there is limited

opportunity to observe it, and thus little information on migration routes or timing. We

report on the timing of individual whales sighted during multiple years in Bermuda.

Bermuda offers a unique opportunity to observe the North Atlantic humpback

population on their northbound migration.

Individual animals were identified by the markings on the ventral fluke (Katona et al.,

1979). Identification photographs from the North Atlantic Humpback Whale Catalog

(NAHWC), a central curation facility for fluke photographs collected throughout the

North Atlantic Ocean, were used for this study. The NAHWC has records from

Bermuda dating to 1970. The number of animals identified has increased dramatically

since 2007 as the result of dedicated photographic effort by Whales Bermuda (WB).

Individuals with sightings from multiple years around Bermuda were used in these

analyses. In those cases where an individual was seen more than once within a year at

Bermuda, the first sighting date of that year was used for analysis, as this date will be

closest to the arrival time of the animal. Details associated with age, gender, and calf

presence were obtained through sighting record for that individual, at Bermuda or

elsewhere (NAHWC, WB & PCCS, unpublished data).

Results and Discussion

Thirty-seven individuals were identified in Bermuda in more than one year. Seven

individuals were sighted in more than 2 separate years, with a maximum of 5 sightings

at Bermuda. Four individuals had a sighting record spanning at least 22 years, with one

individual identified over a 34-year span.

The temporal difference between sightings in different years ranged from 0 to 36 days

with a mean difference in sighting dates of 10.3 days. There was no significant

relationship found between the first and subsequent sighting dates on an individual

level (F1,35 = 1.68, p = 0.204, see figure below). Similarly, there was no consistent trend

in the dates for animals seen in three or more years. Thus, this study provides no

evidence suggesting that individual humpback whales show any inclination to migrate

at a particular time of the season.

There was also no relationship between the number of years between sightings and the

difference in day of year sighted (F1,35 = 1.12, p = 0.281, see figure below). Animals

with a shorter resighting history display greater variability in day sighted than those

with longer spans. However there does not appear to be any plausible biological

explanation for this reduced variability with time, and it is likely an artifact of sample

size. The lack of correlation between date of sighting and span between sightings

suggests that there is no tendency to shift migratory date over time or with maturation.

The sex of eleven individuals was known. Nine were male and two female. One female

(NAHWC# 6030) was sighted at Bermuda in1983 and again, 12 days later, this time

with a calf, in 1985. This is consistent with the widely-documented pattern of females

with calves leaving the breeding grounds later than females without calves.

Conclusion

This work represents the largest data set yet examined on the timing patterns of

individual whales during migration, yet these data show no evidence for individual

timing preference in migration.

There is substantial data from whales killed along migration routes (Dawbin, 1997) and

from observations of individual whales on the breeding grounds (Craig et al.2003) that

different reproductive classes migrate at different times. Thus reproductive fitness, and

the changing age and reproductive class of individuals may overwhelm any individual

tendency.

Opportunistic feeding on migration has been recorded off East Australia (Stockin et al.,

2005), South Africa (Best et al., 1995), and New Zealand (Dawbin 1956). Bermuda is

not on a direct route to most feeding grounds, the northbound migration coincides with

a period of high productivity, many animals are observed to be present over several

days, and some feeding has been observed, all suggesting the exploitation of a minor

food source (Stone et al., 1987; Stevenson and Stevick, 2009). The presence of whales

at Bermuda, therefore, may reflect the timing and availability of prey. Also, if animals

remain in the area to feed, rather than passing directly, this will mask the arrival date,

introducing variability into these data.

While the lack of evidence for any individual preference for migration timing, shown

here, does not preclude the presence of such a pattern, it suggests that, at best, this

effect is small in comparison to other factors that may act on migration timing and

presence at Bermuda.

Acknowledgments

This dataset is a result of many years of dedication and contribution to the NAHWC. E. Tucker, G.

Stone and S. Katona were instrumental in early identification studies at Bermuda. The dedicated work

of Whales Bermuda made this analysis possible. Many, many thanks go to P. Onens for all his hard

work organizing this poster and presenting the material. Thanks also go to J. Allen, J. Robbins, and S.

Todd for their comments and review.

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