Figure 1. Two green laser dots (10 cm apart) projected onto the dorsal fin of an adult female resident killer whale in Prince William Sound (Whale AK2, estimated birth in 1958). Click to enlarge.
Figure 2. Photograph showing morphometric measurements for an adult male resident killer whale from Prince William Sound (Whale AJ21, born in 1976). Click to enlarge.
Figure 3. Photograph showing differences in the size of whales from three generations within a transient killer whale matriline in the eastern Aleutian Islands. Click to enlarge
Data on the size of killer whales (Orcinus orca) are important for resolving taxonomic
differences, estimating energetic requirements
to infer predation rates, and assessing nutritional status and health of protected populations. However, until recently, it has proved difficult to obtain measurements from free-ranging cetaceans. Researchers with the Cetacean Assessment and Ecology Program (CAEP) have developed a simple approach for obtaining morphometric measurements based on photographing two green laser dots that have been projected
onto the body of a whale using two small laser-pointers. These laser-pointers are mounted in a parallel orientation on the camera to maintain a fixed and known separation distance, and the dots therefore provide a scale of known dimension on the image of the whale.
For the past 3 years, CAEP researchers and collaborators have adopted this approach
to estimate the size structure of killer
whale populations in Alaska. Specifically, CAEP collaboration on surveys conducted by the North Gulf Oceanic Society (NGOS) in Prince William Sound has provided laser-metric photographs from fish-eating “resident” killer whales; and both CAEP and NGOS surveys have been used to obtain laser-metric photographs from mammal-eating “transients” in the eastern Aleutian Islands. The latest round of data collection was recently completed during a survey of transient killer whales in May 2008, onboard an NGOS research charter in the eastern Aleutian Islands.
Laser-metric photographs have now been obtained from 161 individual whales (69 transients and 92 residents) in these two study areas. These include a primary set of photographs capturing the projected lasers on the dorsal fin of target animals (Fig. 1). These images can be used to measure
features of the dorsal fin, such as the absolute width and height, using the scale of known separation between the lasers. Once the dorsal fin size is known, further photographs
displaying the dorsal fin can be used to estimate additional morphometrics such as the distance between the blowhole and the anterior insertion of the dorsal fin (Fig. 2). This blowhole-dorsal fin measurement
provides a consistent index of length and is a standard measurement taken from stranded individuals.
Because the identity of killer whales can also be determined from the same photographs,
measurements can be cross-referenced
with established photo-identification catalogs and associated life-history data to assess the size-at-age structure of these populations. We have obtained laser-metric
images of whales from different age classes within both resident and transient populations (Fig. 3), including juveniles (Transients (T) = 14, Residents (R) = 43
individuals), adult females (T = 33, R = 25 individuals), subadult males (T = 6, R = 8
individuals), and adult males (T = 16, R = 16
individuals). It is likely that growth is a key indicator of nutritional status for these long-lived top predators, and it can be assessed
through monitoring size changes of young animals and differences in the adult size of successive cohorts. Future analyses of these data will allow us to assess size trends within and between populations of killer whales in Alaska.
