Humpback whales can be recognized and identified by distinguishing marks. Identifying specific animals enables biologists to chart their migrations and movements, their preferences for specific feeding areas, and their reproductive histories and activities. Many of the whales that spend summers in the Glacier Bay-Icy Strait area in Southeast Alaska have been identified and assigned individual numbers, and some have been tracked for almost 40 years.
The number of whales that visits the Glacier Bay area varies from summer to summer, ranging from a low of 40 different whales to as many as 160. Between 1974 and 2008, 575 different individuals were identified. In that same 34-year period, 217 calves were born and documented, and many have returned multiple years. A female humpback whale is sexually mature at five years old, and can live 40 to 50 years. There are seven known grandmothers in the group, and there have been several cases where both grandmother and daughter returned to the Bay with calves in the same year.
A mature female humpback whale is either pregnant or lactating almost all the time - most have a calf every other year and nurse it for a year before having another calf. But five Glacier Bay whales have given birth every year for successive years. One of these whales, Number 581, calved three years in a row, twice. She is the most prolific of the Glacier Bay females and had 12 calves between 1984 and 2007.