Which factors strongly influence the sinking speed of a demersal longline?

Sinking speed is one of the main factors affecting the performance of longlines. Demersal and pelagic longlines need to sink quickly, to prevent bycatch of marine animals and bait loss to seabirds. The incidental catch or bycatch of seabirds by longlining is a problem from the environmental conservation point of view. Thus, it is necessary to develop techniques to reduce the incidental catch or bycatch by longlining.

This study focuses on the sinking speed of longlines. To determine the significant factors that may influence the sinking speed, the study considers the various parameters for longlining such as bait properties, rope materials, rope thickness, anchor weights, shooting speeds, shooting methods, and the ratio between the depth and mainline length.

The results of the numerical analysis were verified by comparing them with observations from field experiments. The results suggest that the sinking speed can be increased and the bycatch during the sinking process can be reduced when baits have elliptical shapes with a small projected area relative to the sinking orientation, as well as when high-density and thicker materials are used. Heavier anchors also help to increase the sinking speed of the anchor part, excluding other parts such as the middle part of a mainline. If the water is shallow compared with the line's length, the anchor's weight has a smaller influence on the speed of sinking in the middle of the line. In this case, the factors that influence the sinking speed of a longline 10, 30, and 50 m from the stern are rope material, rope thickness, and shooting speed.

These results may be used to design techniques for bycatch prevention. Based on these results, accounting for these important factors when designing a longline is expected to increase the fishing efficiency. Copyright © 2013 John Wiley & Sons, Ltd.