Optimizing the Width of Strip Transects for Seabird Surveys from Vessels of Opportunity

Marine bird at-sea distributions are surveyed using two standardized population surveying techniques: line transects and strip transects (Buckland et al. 1993). Line transects use the perpendicular distances to individual sightings to model a detection function, which quantifies the probability of observing an object given its distance from the trackline. This approach allows for speciesspecific differences in detectability (e.g. small-sized vs. largebodied taxa) under a variety of behavioral (e.g. flying vs. sitting on the water) and environmental (e.g. visibility, Beaufort sea state) conditions. The resulting perpendicular sighting distributions are then used to estimate the surface area effectively searched during surveys. Strip transects, on the other hand, assume that observers detect every target within the survey strip, and estimate seabird relative abundance by dividing the number of individuals sighted by the area of ocean surface surveyed. Ultimately, the width of the survey strip represents a compromise between the desire to cover as much surface area as possible and the ability to detect every bird within the area surveyed (Tasker et al. 1984, van Franeker 1994, Becker et al. 1997).