2011 11 22- Poster presentation on the timing of humpback whale migrations- Soc for Marine Mammology |
![]() |
![]() |
Written by Andrew Stevenson | ||||
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
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. MethodsIndividual 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and calf presence were obtained through sighting record for that individual, at Bermuda or elsewhere (NAHWC, WB & PCCS, unpublished data).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. Best, P.B., Sekiguchi, K., Findlay, K.P. (1995) A suspended migration of humpback whales Megaptera novaeangliae on the west coast of South Africa. Marine Ecology Progress Series 118:1-12 Craig, A.S., Herman, L.M., Gabriele, C.M., Pack, A.A. (2003) Migratory timing of humpback whales (Megaptera novaeangliae) in the central North Pacific varies with age, sex, and reproductive status. Behaviour 140, 981-1001 Dawbin, W.H. (1956) The migrations of humpback whales which pass the New Zealand coast. Transactions of the Royal Society of New Zealand 84(1): 147-196 Dawbin, W.H. (1997) Temporal segregation of humpback whales during migration in southern hemisphere waters. Memoirs of the Queensland Museum 42(1):105-138 Katona (1979) Katona, S. K., B. Baxter, O. Brazier, S. Kraus, J. Perkins, and H. Whitehead. 1979. Identification of humpback whales by fluke photographs. Pages 33-44 in H. E. Winn and B. L. Olla, editors. The Behavior of Marine Animals. Volume 3. Cetacea. Plenum Press, New York. Stevenson, A.D and Stevick, P.T. 2009. Habitat use of humpback whales at Bermuda. Presented to the 18th Biennial Conference on the Biology of Marine Mammals, Quebec City. Stockin, K.A. & Burgess, E.A. (2005) Opportunistic feeding of an adult humpback (Megaptera novaeangliae) migrating along the southeastern Queensland, Australia Stone, G.S., Katona, S.K., Tucker, E.B. (1987) History, migration and present status of humpback whales Megaptera novaeangliae at Bermuda. Biological Conservation 42: 133-145.
|