Method for estimating buoyancy of midwater float required to standardize hook depth in pelagic longline
Article first published online: 2 JUN 2008
© 2008 Japanese Society of Fisheries Science
Volume 74, Issue 3, pages 479–487, June 2008
How to Cite
SHIGA, M., SHIODE, D., HAYASHI, S., TOKAI, T. and HU, F. (2008), Method for estimating buoyancy of midwater float required to standardize hook depth in pelagic longline. Fisheries Science, 74: 479–487. doi: 10.1111/j.1444-2906.2008.01549.x
- Issue published online: 2 JUN 2008
- Article first published online: 2 JUN 2008
- Received 19 October 2007. Accepted 17 December 2007.
- hook depth;
- incidental catch;
- midwater floats;
- pelagic longline;
- sea turtle;
ABSTRACT: A midwater float is a small float attached to the mainline of pelagic longlines to standardize the hook depth. In this study theoretical equations are presented for estimating the buoyancy of the midwater float required to lift the joints of the midwater float line on the mainline to the target depth. Sea trials using full scale tuna longline gear with midwater floats were carried out in the Indian Ocean in December 2004 and 2005, in order to examine the validity of the theoretical equations. In the sea trials, two types of midwater float settings, single midwater float setting and double midwater float setting, were tested and compared with the conventional setting. As a result, the joints of the midwater float line on the mainline were successfully lifted to the target depth as expected, demonstrating the validity of the theoretical equation. The range of hook depths in the midwater float setting was less spread over depths than in the conventional setting, and therefore, use of long float lines (100 m) with the midwater float setting allows all hooks to avoid entering the sea turtle habitat of shallower than 100 m depth. Factors affecting shoaling of the longline with the midwater float are also discussed.