A COMPARISON OF METHODS FOR COUNTING SEABIRDS AT SEA IN THE SOUTHERN-OCEAN

Two methods for counting seabirds from ships were compared during a re search cruise in the marginal ice zone of the northwestern Weddell Sea in the Southern Ocean (European Polarstern Study 1988/1989-EPOS). The basic difference between the two methods concerns the way in which bi rds in Right are recorded. Methods using continuous counts of all bird s crossing the transect area in Flight result in bias in density estim ates because they measure flux rather than density. In the Southern Oc ean the continuous method has been used as the standard in the BIOMASS program. To avoid bias due to bird movement, the Scottish Seabirds at Sea Team (SAST) recommended the snapshot method which uses instantane ous counts of birds in flight at specific time intervals to cover cons ecutive subsectors of the census area. Comparative investigations duri ng EPOS showed that continuous counts resulted in bird density estimat es about twice as high as densities derived from snapshot counts. The level of bias caused by continuous counts fluctuated widely between an d within species. Effects of different variables such as species-speci fic behavior, ship's speed, and windforce are difficult to separate. T he use of instantaneous counts of birds in flight is strongly recommen ded because they yield better estimates of both relative and absolute densities of seabirds at sea.