Differential loggerhead by-catch and direct mortality due to surface longlines according to boat strata and gear type

Juan A Camiñas,Raimundo Real,José C Báez,Xulio Valeiras

doi:10.3989/scimar.2006.70n4661

Juan A Camiñas, Raimundo Real + Show 2 more

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https://doi.org/10.3989/scimar.2006.70n4661

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Abstract

Surface longline gears are used to fish different species, mainly albacor e Thunnus alalung a (Bonnaterre, 1788), bluefin tun a Thunnus thynn us (Linnaeus, 1758), and swordfish Xiphias gladius Linnaeus, 1758, and are considered highly dangerous for threatened marine turtles. Loggerheads Caretta caretta (Linnaeus, 1758) can be incidentally captured by surface longlines. A number of captured individuals die during the fishing operation, which we consider direct mortality due to fishing. We analysed the relative loggerhead by-catch and direct mortality associated with each type of boat and gear from April to December during the period 1999-2004 in the Spanish surface longline fleet that fishes in the western Mediterranean Sea, an important fishing area for this fleet. We used different indices to compute the catch per unit effort (CPUE) according to the number of hooks and to the number of fishing operations for each type of boat and gear. Both by-catch and direct mortality differed significantly according to the type of boat and gear. With respect to the number of hooks, boats longer than 12 m not using a roller and targeting bluefin tuna captured the highest number of loggerheads, whereas boats longer than 12 m with a roller that targeted swordfish caused the highest direct mortality. With respect to the number of fishing operations, boats longer than 12 m without a roller that targeted albacore captured the highest number of loggerheads; the highest direct mortality was caused by this type of boat and by boats longer than 12 m using a roller and targeting swordfish.

Highlights

Caretta caretta (Linnaeus, 1758) is included in the red list of threatened species of the IUCN (2001), and is considered to be a species of interest in the European Union and Spain (Official Journal of the European Union 1997 8.11.97/L305/50-; B.O.E. 1990 -5.4.90/9470/82-)
We analysed the relative loggerhead by-catch and direct mortality associated with each type of boat and gear from April to December during the period 1999-2004 in the Spanish surface longline fleet that fishes in the western Mediterranean Sea, an important fishing area for this fleet
We used different indices to compute the catch per unit effort (CPUE) according to the number of hooks and to the number of fishing operations for each type of boat and gear

Summary

Introduction

Caretta caretta (Linnaeus, 1758) is included in the red list of threatened species of the IUCN (2001) (http://www.redlist.org/), and is considered to be a species of interest in the European Union and Spain (Official Journal of the European Union 1997 8.11.97/L305/50-; B.O.E. 1990 -5.4.90/9470/82-). Since 2003 a new gear type called “american” and “roller” has been introduced into the fleet, which implies a reduction in the number of hooks and an increase in the main line length and the depth of fishing (Báez et al, 2005) This modified surface longline increases the distance between hooks (21 m in traditional long-line SWB and 90 m in SWBr, abbreviations as in Table 1) and the length of the gear (30-40 nautical miles in traditional long-line and 60-70 nautical miles total length in SWBr), reducing the average number of hooks to 1000 (versus >2500 mean number of hooks in traditional surface longline), and the time needed to board the gear to 6-7 hours, which entails a 2-3 hour reduction with respect to the traditional longline

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