Movements and feeding habits of juvenile yellowfin tuna (Thunnus albacares) associated with subsurface fish aggregating devices (FADs) off southwestern Taiwan

• Main Authors: Weng, Jinn-Shing, 翁進興
• Other Authors: Lee, Ming-An； Liu, Kwang-Ming
• Format: Others
• Language: zh-TW
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/73258892715680505654

博士 === 國立臺灣海洋大學 === 環境生物與漁業科學學系 === 104 === The yellowfin tuna, Thunnus albacares, is one of the major species caught around subsurface fish aggregation devices (FADs) in the waters southwestern of Taiwan. An increase in yellowfin tuna catch by danish seine fisheries around the subsurface FADs in southwestern Taiwan waters has been a concern of local government and environmental groups. However, the attraction mechanism of aggregating tunas at the subsurface FADs and the interaction between yellowfin tuna and others organisms are still poorly known. The objectivs of this study are (1) to examine the fine-scale vertical and horizontal movements of juvenile yellowfin tunas around subsurface FAD, (2) to examine the diet and feeding habits of yellowfin tuna. In total, 53 tunas (35–81 cm fork length) were tagged with ultrasonic telemetry tags and released at a subsurface FAD in the waters off Shiao-Liu-Chiu Island, southwestern Taiwan from October 2008 to December 2009. These tunas stayed at the subsurface FAD for up to 31 days, with daytime vertical movement depths averaging 60–80 m at a maximum depth of 250 m. At night, the tuna gathered at a shallow depth of 40 m. The mean depth of vertical movement in the daytime is significantly different from that of the nighttime (p < 0.05, t- test,). The maximum detectable distance of horizontal movement was 1600 m, with 80% of the long horizontal movements occurring in the daytime. It is likely that the purpose of these vertical and horizontal movements was for feeding. Moreover, the tagged tunas did not depart from the subsurface FAD simultaneously, suggesting distinct behaviors in their movements. Stomach contents analysis based on 1477 specimens with fork lengths (FLs) ranging from 24 to 108 cm and stable isotope analysis (202 specimens) collected around subsurface FADs in the waters southwest of Taiwan. Stomach contents analysis indicated that juvenile yellowfin tuna with FL < 50 cm mainly feed on larval Sthenoteuthis oualaniensis, larval shrimps, and zooplanktonic organisms such as Amphipods. Yellowfin tuna with FL of ~50 cm switch their diet to teleost fishes such as Lestrolepis japonica, Benthosema pterotum, Exocoetidae, and Scombridae. Stable isotope analysis indicated that the δ15N values ranged between 6.2and 12.6‰, and the estimated trophic position varied from 3.2 ± 0.2for tuna with FL < 30 cm, while it reached 4.6 ± 0.5 for those with FL > 50 cm and 4.8 ± 0.1 for those with FL > 90 cm. Based on the distinct diet shift of the juvenile yellowfin tuna, demonstrated by both stomach contents and stable isotope analysis, this study concluded that tuna shift their diet at approximately 50 cm FL.