Article,
A new birth-interval approach to estimating demographic parameters of humpback whales
Ecology, 78 (2): 535--546 (1997)
Abstract
A demographic model is developed based on interbirth intervals and is applied
to estimate the population growth rate of humpback whales (Megaptera novaeangliae)
in the Gulf of Maine. Fecundity rates in this model are based on the probabilities of giving
birth at time t after a previous birth and on the probabilities of giving birth first at age x.
Maximum likelihood methods are used to estimate these probabilities using sighting data
collected for individually identified whales. Female survival rates are estimated from these
same sighting data using a modified Jolly–Seber method. The youngest age at first parturition
is 5 yr, the estimated mean birth interval is 2.38 yr (SE 5 0.10 yr), the estimated noncalf
survival rate is 0.960 (SE 5 0.008), and the estimated calf survival rate is 0.875 (SE 5
0.047). The population growth rate (l) is estimated to be 1.065; its standard error is estimated
as 0.012 using a Monte Carlo approach, which simulated sampling from a hypothetical
population of whales. The simulation is also used to investigate the bias in estimating birth
intervals by previous methods. The approach developed here is applicable to studies of
other populations for which individual interbirth intervals can be measured.
Key words: birth intervals; birth rate; demography; fecundity; growth rate; humpback whale;
Megaptera novaeangliae; modelling; population; survival.
