A method of surveying incidental mortality of seabirds during longlining operations and the effectiveness of mitigation measures is described. An alternative design of streamer line for use with the 'Spanish' method of longlining (separate hauling and fishing lines) is described. The results of observations of incidental mortality of seabirds made during the fishing operations of the Korean longliner lhn Sung 66 are presented. Preliminary findings suggest that the phase of daylight during which the longline is set is more important than the use of a streamer line in the mitigation of incidental mortality. The use of a streamer line during setting of longlines during the day reduced the observed rate of bird deaths from snagging and drowning by 79%. More data are required to develop the investigation further.