小型メモリー式水深計を用いたマグロ延縄漁具の深度測定

Abstract

The setting depth of tuna longlines was measured with time-depth recorders (18mm in diameter, 127mm in length, 33.6g in air, 5.0g in water). Two different materials (the main lines and branch lines were either polyamid [nylon] monofilament or polyester multifilament) were used for the longlines (53 were operated in total). The number of hooks used in each hachi (i.e., a unit set of main line) was five, seven, or ten for both longline types. The time-depth recorders were attached to the under end of the branch lines (hook positions) and the under end of the float lines (main line positions). The depth fluctuation of the branch lines could be divided into six types. The float lines showed very little upward and downward movements. The setting depth of the branch lines was shallower than the estimated depth calculated by a catenary curve, especially with the polyamid monofilament gear. Sinking speed on the polyester multifilament gear was faster than that of the polyamid monofilament gear. The depth movments (as recorded via a hachi-attached time-depth recorder) of the longlines that fished for large fishes (e.g., Prionace glauca, Thunnus alalunga, Istiophorus platypterus, Tetrapturus angustirostris, and Coryphaena hippurus) showed significant upward and downward depth fluctuation, especially with the polyamid monofilament gear.