Report on research cruise of Regional Tuna Tagging in Indian Ocean-Executive phase for Oman Sea waters

Tuna catches in Indian Ocean have been increasing since 1980 by entrance of purse seiners into the area, making their catches about 10 times over 25 past years. Remarkable catches of the fish, especially the three main species yellowfin, Thunnus albacares, bigeye tuna, Thunnus obesus, and skipjack, Katsuwonus pelamis, have raised concern about overfishing of the stock. In spite of the importance of the valuable fish, the present stock assessment methods are not able to estimate population dynamics parameters precisely in order to understand the real status of tuna stock, and for this, presently precautious strategies are used for fishing management. In 1991, scientific committee of IOTC agreed that conducting a planned tagging project in large scale is the most effectiveness tool for obtaining the required data to improve the stock assessment of the main tuna species. The whole plan of the tuna tagging in Indian Ocean have been developed in two phases: pilot study in small scale, and the main tagging phase in large scale. After achieving successful results from the pilot study in 2001, executive programme for tagging operations in large scale known as Regional Tuna Tagging project-Indian Ocean (RTTP-IO)were developed for over two and a half years for the three main species above. The project was started in May 2005 and ended in 3 rd September 2007. The study area included Seychelles, Mozambique Channel, Kenya, Tanzania waters and Arabian Sea (Yemen and Oman). The objectives of the RTTO-IO were: To tag at least 80,000 fish from the main species, to achieve remarkable recovery (at least 15%), to study migration routs between different areas and stock structure, and to estimate growth rates. The present report was resulted from participation of research cruise for tuna tagging in Oman Sea waters in the line with RTTP-IO. In this cruise, 2835 of yellowfin were captured by two pole and line vessels using live bait, and then released after tagging. Different tagging techniques used in the project were: Conventional tagging (Single and double tagging), Otolith tagging, electronic tagging. The required descriptions for different sections including live bait fishing, capturing of tuna by pole and line fishing and different tagging techniques are given. Since the beginning of the RTTP-IO until the end of executive period around 170,000 tagged tuna from the main target species were released in Indian Ocean, which up to now 9.8% of the fish were recovered. The number of 8 tagged yellowfin was also recovered by Chabahar local fishermen in Oman Sea which the data in details are presented. Moreover, more information on the recoveries obtained from the RTTP-IO up to July 2007 is given.

Iranian Fisheries Science Research Institute

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