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 hock 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.
Similar content being viewed by others
References
Brothers N. Albatross mortality and associated bait loss in the Japanese longline fishery in the Southern Ocean. Biol. Conserv. 1991; 55: 255–268.
Williams P, Anninos PJ, Plotkin PT, Salvini KL (eds). Pelagic Longline Fishery-Sea Turtle Interactions: Proceedings of an Industry, Academic and Government Experts and Stakeholder Work Shop Held in Silver Springs. Maryland, 24–25 May 1994. NOAA Technical Memo NMFS-OPR-7. US Dept. Commer. Washington, DC. 1996.
Food and Agriculture Organization of the United Nations (FAO). Expert Consultation on Interactions Between Sea Turtles and Fisheries Within an Ecosystem Context. FAO Fisheries Report No. 738. FAO, Rome. 2004.
Gilman E, Zollett E, Beverly S, Nakano H, Davis K, Shiode D, Dalzell P, Kinan I. Reducing sea turtle bycatch in pelagic longline fisheries. Fish Fish. 2006; 7: 2–23.
Kiyota M. Incidental take of seabirds in longline fisheries: nature of issue and measures for mitigation. J. Yamashina Inst. Ornithol. 2002; 34: 145–161.
Yoshihara T. Distribution of fishes caught by the longline II. Vertical distribution. Nippon Suisan Gakkaishi 1951; 16: 370–374.
Yoshihara T. Distribution of catch of tuna longline. IV. On the relation between K and ϕ0 with a table and diagram. Nippon Suisan Gakkaishi 1954; 19: 1012–1014.
Bigelow K, Musyl MK, Poisson F, Kleiber P. Pelagic longline gear depth and shoaling. Fish. Res. 2007; 77: 173–183.
Polovina JJ, Howel E, Parker DM, Balazs GH. Dive-depth distribution of loggerhead (Caretta caretta) and olive ridley (Lepidochelys olivacea) sea turtles in the central North Pacific: might deep longline sets catch fewer turtles? Fish. Bull. 2003; 101: 189–193.
Dagorn L, Bach P, Josse E. Movement patterns of large bigeye tuna (Thunnusobesus) in the open ocean, determined using ultrasonic telemetry. Mar. Biol. 2000; 136: 361–371.
Schaefer KM, Fuller DM. Movements, behavior, and habitat selection of bigeye tuna (Thunnus obesus) in the eastern equatorial Pacific, ascertained through archival tags. Fish. Bull. 2002; 100: 765–788.
Bach P, Dagorn L, Bertrand A, Josse E, Misselis C. Acoustic telemetry versus monitored longline fishing for studying the vertical distribution of pelagic fish: bigeye tuna (Thunnus obesus) in French Polynesia. Fish. Res. 2003; 60: 281–292.
Shiode D, Hu F, Shiga M, Yokota K, Tokai T. Midwater float system for standardizing hook depths on tuna longlines to reduce sea turtle by-catch. Fish. Sci. 2005; 71: 1182–1184.
Miyamoto Y, Uchida K, Orii R, Wen Z, Shiode D, Kakihara T. Three-dimensional underwater shape measurement of tuna longline using ultrasonic positioning system and ORBCOMM buoy. Fish. Sci. 2006; 72: 63–68.
Mizuno K, Okazaki M, Nakano H, Okamura H. Estimation of underwater shape of tuna longline by using micro-BTs. Bull. Nat. Res. Inst. Far. Seas. Fish. 1997; 34: 1–24.
Mizuno K, Okazaki M, Miyabe M. Fluctuation of longline shortening rate and its effect underwater longline shape. Bull. Nat. Res. Inst. Far. Seas. Fish. 1998; 35: 155–164
Hanamoto E. Effect of oceanographic environment on bigeye tuna distribution. Bull. Jpn. Soc. Fish. Oceanogr. 1987; 51: 203–216.
Nakano H, Okazaki M, Okamoto H. Analysis of catch depth by species for tuna longline fishery based on catch by branch lines. Bull. Nat. Res. Inst. Far. Seas. Fish. 1997; 34: 43–62.
Author information
Authors and Affiliations
Corresponding author
Rights and permissions
About this article
Cite this article
Shiga, M., Shiode, D., Hayashi, S. et al. Method for estimating buoyancy of midwater float required to standardize hook depth in pelagic longline. Fish Sci 74, 479–487 (2008). https://doi.org/10.1111/j.1444-2906.2008.01549.x
Received:
Accepted:
Issue Date:
DOI: https://doi.org/10.1111/j.1444-2906.2008.01549.x