Environmental preferences of Alopias superciliosus and Alopias vulpinus in waters near Marshall Islands

This paper reports on a survey carried out in waters near Marshall Islands and on the analysis of catch rates of bigeye thresher shark, Alopias superciliosus, and thresher shark, Alopias vulpinus. The optimum swimming depth, water temperature, salinity and dissolved oxygen range of bigeye thresher sharks were identified as 240-360 m, 10-16 degrees C, 34.5-34.7 and 3.0-4.0 ml/l, respectively, while for thresher sharks they were 160-240 m, 18-20 degrees C, 34.5-34.8 and 1.0-1.5 ml/l, respectively, during daytime. The bigeye thresher shark and thresher shark were widely distributed in areas where the dissolved oxygen was higher than 0.5 ml/l. Some mitigation measures were recommended to reduce the bycatch of bigeye thresher shark and thresher shark, including avoiding fishing in peak areas and periods of shark abundance, using fish as bait and using nylon monofilament leaders. Furthermore, the hook depth data could be input into the 'habitat model' to standardise the shark catch per unit effort (CPUE).