The Influence of Environmental Factors and Mitigation Measures on By-Catch Rates of Seabirds by Japanese Longline Fishing Vessels in the Australian Region

Most seabirds caught and killed by longline fishing are captured during line setting. Data collected by Australian observers on Japanese longline vessels from April 1992 to March 1995 were used to investigate the influence of various environmental factors and mitigation measures on seabird catch rates. Generalised linear models were used to test the significance of the effect of each factor. The environmental factor that most influenced the seabird catch rate was whether line setting was carried out at night or during the day. From the data examined, the chance of catching seabirds during day sets was five times greater than for night sets. For night sets, the chance of catching seabirds during the full half-phase of the moon was five times greater than during the new half-phase. The area and season fished were also significant, while wind, cloud and sea conditions were not. Considerable variation in the seabird by-catch rate among vessels was found. This was probably due to differences in their implementation of mitigation measures, as well as the clumped distribution of seabirds by area and time. Although the by-catch rate was significantly different among years, the differences were small in comparison to other factors. An examination of the influence of mitigation measures for sets made during the day in summer in the Tasmanian area showed that the level of bait thawing and unidentified factors related to individual vessels were most significant in determining the seabird by-catch rate, followed by the use of a bait throwing device. For this data set, the amount of cloud cover had an influence, while moon phase, sea conditions and wind strength did not. The effect of using bird scaring tori poles and lines was not examined, as these were used during all sets examined in detail.