博士 / 國立臺灣海洋大學 / 環境生物與漁業科學學系 / 100 / Yellowfin tuna (YFT; Thunnus albacares) is one of the main target species of the commercial tuna longline (LL) fishery and has a long history of being the subject of scientific research in the Indian Ocean. In this study, we collected Taiwanese LL fishery data and environment variables during the period of 1980–2005. The principal component analysis (PCA) and wavelet analysis were used to investigate the relationship between LL catch data of YFT and oceanic environmental factors. The results were summarized as below:
In the Indian Ocean, YFT is one of the most important target species in the Arabian Sea and Western-Center Indian Ocean. The major fishing season in the Arabian Sea is in the first and second quarters with a average nominal catch per unit effort (CPUE) about 14.92 fish/103 hooks and a average catch about 401 metric tons. In the Western-Center Indian Ocean, the catch and effort were the highest in all of the Indian Ocean and the average nominal CPUE was about 3.13 fish/103 hooks. Although there were highest effort in the Southern Indian Ocean from June to September, but the average nominal CPUE was lower than 2 (fish/103 hooks) and the average catch was lower than 50 metric tons.
Results of the PCA showed that monthly variations in values were significantly correlated with the sea surface temperature (SST), subsurface temperature at 105 m and chlorophyll-a concentration. In April and May, the SST was generally higher with deep mixed layer depth. After July, a drop in the temperature below the preferred temperature range for YFT is probably the reason why the CPUE subsequently decreased. In addition, the CPUE at a given time was significantly affected by chlorophyll-a concentrations 1–3 months prior to that time were probably due to a lag effect of trophic transformation. The regular LL (RLL) CPUE had a negative coefficient and deep LL (DLL) had a positive coefficient with the mixed layer depth anomaly. This implies that the shallow mixed layer depth produces a high CPUE for the RLL and the deep mixed layer depth causes a high CPUE for the DLL. In the long-term time series analysis, the main factor causing interannual variations in the CPUE of the RLL and DLL might change with time. RLL and DLL CPUE values showed positive correlations with SST and Dipole Mode Index from the beginning of the 1980s to the middle of the 1990s. The RLL and DLL CPUE were found to have a significant coherence of the two phases with a periodicity of 3 yr with and mixed layer depth.
Finally, we investigated the catches and distributions of yellowfin tuna in relation to climatic and marine environmental variations in the Indian Ocean. The gravity of yellowfin tuna fishing grounds showed similar variations with a climatic index, and an advanced time series analysis also showed a significant negative correlation between the climatic index and the CPUE with a periodicity of 2–3 yr. It suggested that decreases in areas of SST and net primary production optimal for YFT during positive Indian Ocean Dipole events would decrease the CPUE in the western Indian Ocean, while an increase in optimal areas would result in an increased CPUE in negative Indian Ocean Dipole events, especially in the Arabian Sea and surrounding seas of Madagascar.
| Identifer | oai:union.ndltd.org:TW/100NTOU5451002 |
| Date | January 2012 |
| Creators | Kuo-Wei Lan, 藍國瑋 |
| Contributors | Ming-An Lee, 李明安 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | zh-TW |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 144 |
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