Modeling Of Bycatch Occurrence Rate Of Seabirds For Japanese Longline Fishery Operated In Southern Hemisphere

We modelled the bycatch occurrence rate in consideration of factors of year and season and examined longitudinal changes in the rate across years. We used operational data obtained by scientific observers from 1997 through 2015. As a preliminary analysis, differences in species composition of seabirds bycaught between northern and southern regions of waters south of 20 degrees S were examined through hierarchical cluster analysis. Bycatch species composition was changed at the boundary of 40 degrees S, 35 degrees S and 40 degrees S, off Cape, in Indian Ocean and in the Tasman Sea, respectively. Presence/absence of seabird bycatch data by set was modelled with the generalized additive model (GAM). The data for the GAM analysis were split in two by a boundary dividing the data into northern and southern areas. Estimated bycatch occurrence rate varied at relatively low level in the model of the northern area, while that varied at relatively high level in the model of the southern area. Bycatch occurrence rates in an east-west direction differed not only among year periods but also among seasons in both waters north and south of 35 degrees S. It was suggested the importance of consideration of longitudinal variation of bycatch occurrence rate among year and season to estimate total bycatch number.