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Abstract
In this study, we collected environmental variables to investigate their effects on the catch per unit effort (CPUE) of yellowfin tuna in the Arabian Sea during the period 1980–2005. We used an advanced time series analysis, including a state-space approach to remove seasonality, and wavelet analysis to investigate transient relationships. For large-scale environmental effects, we used the dipole mode index (DMI) to represent the Indian Ocean dipole; for local environmental factors, we investigated sea surface temperature (SST), thermocline depth and chlorophyll-a (chl-a) concentration. The main factors causing interannual variations in the CPUE might change with time. CPUE showed positive correlations with SST and DMI from the beginning of the 1980s to the middle of the 1990s. It also showed a significant coherence with chl-a, especially a long-term positive correlation for the regular longline fishery in 1998–2005 with a periodicity of 2 years. Both regular and deep longline CPUEs were found to have significant coherence with thermocline depth having a periodicity of 3 years. The relations were of opposite signs such that the shallow thermocline depth produced a high CPUE for the regular longline fishery and deep thermocline depth caused a high CPUE for the deep longline fishery.
Acknowledgements
This study was financially supported by the Council of Agriculture (95AS-14.1.2-FA-F2 and 97AS-15.1.2-FA-F3) and National Science Council (NSC95-2611-M-019-016) and the Centre of Marine Bioenvironment and Biotechnology of the National Taiwan Ocean University. We are grateful to the OFDC of Taiwan for providing us with the data from the Taiwanese longline fishery and the National Research Institute of Far Seas Fisheries, Fisheries Research Agency of Japan for the model-simulated vertical sea temperature data. Prof. C.T. Shih of the National Taiwan Ocean University kindly helped in revising the grammar of this article.