1. The development and calibration of a land-based technique to measure inter-animal spacing in free-ranging coastal cetaceans is described here. The technique was developed to study the behaviour of killer whales Orcinus orca in Norway.
2. A theodolite was used to measure the surfacing location of one reference individual while simultaneous video recordings of the whole group were made. Digitized video frames were then used to estimate the locations of all individuals in the video frame relative to the reference animal.
3. The technique was calibrated using a line of towed buoys with known separations. Estimated inter-buoy distances were compared with actual values to calculate errors. There was no observable bias in measurements, with a mean error of -0.014 m (n = 304, SD = 0.880). At ranges up to 2 km from the observation site, 95% of measurements were accurate to within 1.7 m.
4. The accuracy of the measurement system was characterized with a set of Monte Carlo simulations. Simulations were run at offshore ranges from 100 m to 2000 m, with random perturbations applied to all variables. Errors in inter-animal distances for n = 16 whales were estimated using 10 000 simulation runs for every range value. The results from the simulations agreed with experimental findings. The results showed no bias in inter-animal distance measurements, with an overall mean error of 0.0864 m.
5. The results indicate that this technique is suitable for studies on a variety of coastal cetacean populations. It provides a new tool for quantitative studies on spatial behaviour of cetaceans, and will help underpin management efforts to monitor effects of anthropogenic disturbance. With modification, the technique might also be applicable to other coastal vertebrates where inter-organism distances are required.
- cetacean behaviour
- inter-animal spacing
- video camera