| 英文摘要 |
一、The targeting effect and catch statistics of Taiwanese tuna longline fishing fleets including large and small longliners as well as observers data, collected in the south Pacific ocean were analyzed using cluster, similarity analyses, and multiple dimensional scaling methods, to understand the fishery dynamics, filter out different targeting effects and identify the effective fishing effort for this fleet. The results indicated that Taiwanese fleet targeted on albacore (ALB) before 2001, and shifted to bigeye tuna (BET) targeting thereafter. However, such a trend was changed again after 2008 when effort target on ALB increased again. Some vessels which deployed deep hooks and target on YFT seem also evident in recent years. The number of hooks per basket and the baits used by the YFT targeting fleet also were different from other fleets. Such a seasonal targeting on the YFT was due to abundant, instead of the size of the species. The nominal CPUE trend estimated from the fleet target on ALB showed stable or slightly increasing trend instead of decrease. The standardized CPUE also show a similar trend, and was consistence with recent status of the stock reported from the south Pacific region. The difference in size distribution pattern of fish in 2011-2013, with those in 2002-2010 periods also can be seen from observers’ data due to a possible change in oceanographic conditions among years. Our results also showed that distribution of catch rate was related to the sea surface temperature (SST), and may shift for 5 degree latitude between years. The analyses on the tonnage- and latitude-specific catch rates reveal that catch of albacore in recent years was in general, increase, except for waters of 10-20S, when catch rate decreased in 2010-2012. The size of albacore caught in lower latitude was slightly smaller than the average, but fish caught by small vessels in mid- to high latitudes was larger. A further analysis on the size distribution of albacore in a specific area between 150 and 180E showed an opposite trend between catch rate and mean size of fish caught, indicating that except for a possible increase in fishing pressure in 10-20S, changes in distribution of fish in relation to the differences in oceanographic condition among years may also be the cause. Such effects can also be seen from differences in catch rate and size of fish between EEZ and non-EEZ zones of the Taiwanese small tuna longline database, and should be studies in more details in future. |