Tuna fisheries have become the most important contributor to the coastal central provinces of Vietnam since it was introduced in early 1990s. However, developing effective management guidelines for yellowfin (Thunnus albacares) and bigeye (Thunnus obesus) tuna, the main target species for longline and handline fisheries, is difficult because there is no information on its growth characteristics. In particular, length-length, length–weight, and weight-weight relationships, the important components of fisheries production models, do not exist for those species. This study first provided those relationship equations and represented an improvement in the knowledge of these species in its distribution range with maximum sizes updated.
Unwanted bycatch of sea turtles in the tuna fisheries is a global challenge. To evaluate whether the incidental catch of sea turtles could be reduced through changes in fishing gear, this study compared catch rates and bycatch in the tuna pole-and-line with the addition of above-water lights (PL) fisheries using a Japan tuna hook (JT-hook) and a circle-shaped hook (C-hook). There were two phases to this study. First, five PL fishing vessels that used traditional JT-hooks were compared with five PL fishing vessels that used circle-shaped hooks throughout 1 full year of fishing. Results showed that C-hooks significantly reduced bycatch of sea turtle, while negligibly increasing the catch of yellowfin (Thunnus albacares) and bigeye (Thunnus obesus) tuna. Second, we conducted the onboard research to investigate the effect of JT-hook v. C-hook on the catch rates of commercial PL fishery. Results showed that there were higher catch rates of long snouted lancefish (Alepisaurus ferox) and wahoo (Acanthocybium solandri), but lower catches of thresher shark (Alopias spp.) on C-hooks, with no significant differences for other species considered. Our results suggest that the use of C-hooks in the PL fishery is beneficial to protected endangered sea turtle species.
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