The influence of environmental variables and mitigation measures on seabird catch rates in the Japanese tuna longline fishery within the Australian Fishing Zone, 1991-1995

Long term fisheries observer data were used to analyse the influence of a r ange of environmental variables and mitigation measures upon catch rates of seabirds in the Japanese pelagic longline fishery. In the Australian Fishi ng Zone (AFZ) seabirds were most likely to be caught on longlines that were set during summer, in southern areas of the zone, and during daylight hour s. However, interpretation of changes in catch rates resulting from the use of mitigation measures or from weather effects were problematic due to the interrelationships between the many measured factors. Interpretation and a ccurate assessment was further complicated by ongoing changes to fishing pr actices and equipment, and due to changes to the priority that fisheries ob servers placed on the collection of seabird data. The data relating to fact ors affecting seabird bycatch which is currently collected incidentally by fisheries observers are not sufficiently robust to allow confidence in stat istical assessments alone to examine the efficacy of mitigation measures. D edicated observations may allow for more confident determination of the rea sons why seabirds were or were not caught, and to what degree mitigation me asures are effective. The use of these observations in combination with the analyses suggested seabird bycatch rates may be lowered by the use of bird lines, bait throwing machines and thawed baits. However, appropriate use a nd deployment of these measures are critical if they are to be effective. F urther work is required to better understand the effect of these measures, and their effect upon the catch rate of target and non-target species. (C) 1999 Elsevier Science Ltd. All rights reserved.