Counting whales 3. Censuses

Census surveys aim to count every individual in the population. This method is best suited to species that aggregate at certain times of year, for example, when seals must haul out on land to moult or when birds are incubating eggs.

This isn’t usually an option for populations of whales and dolphins because they tend to range over wide areas and, even then, aren’t all at the surface and available to be counted at the same time. During their migrations, however, gray whales travel along the coastline making this approach feasible for monitoring their abundance. Particularly during the northward migration, gray whales travel very close to shore. From a high vantage point the whales can be tracked and counted as they pass.

The Gray Whale Census and Behaviour Project has been monitoring the gray whales migration since 1979. The cliffs of the Palos Verdes peninsula in southern California provide an excellent vantage point from which scientists and trained volunteers keep watch during daylight hours from the 1st of December to late May, recording the number of gray whales travelling in each direction. The numbers of calves are also recorded, along with behavioural observations and any sign of human impact, such as disturbance from boats or whales entangled in fishing gear.