International Journal of Scientific and Research Publications (IJSRP) | 2021
Effect of Ocean Currents and Temperature Variability on Yellowfin Tuna Catch Rates of Sri Lankan Longline Fishery
Abstract
Duration of Sri Lankan fishing fleets has been reported as 2-4 weeks to catch the average target of approximately 2 tones per trip. However, low catch rates are caused by the uncertainty of catches due to lack of technology available to find fishing grounds. This study is focused on the impact of temperature and ocean currents at the depth where Yellowfin tuna (YFT) has been caught. Currents and temperature data at different depth levels were obtained from Copernicus Marine Environment Monitoring Service (CMEMS). Longline tuna catch datasets were obtained from the Department of Fishery, Sri Lanka. The catch datasets were gridded into 0.25-degree spatial resolution to overlay with the oceanographic parameters such as temperature in the same resolution. The fishery and oceanographic datasets were composited in to 5-day intervals for data overlay and matching. The matched dataset was analyzed using R-statistical packages. Empirical cumulative distribution function (ECDF) was applied to identify the nature of the relationships between catch per unit effort (CPUE) and the relationship with the two oceanographic parameters were proved by the generalized additive model (GAM). The results showed that YFT prefers warmer waters, above 24°C, particularly fished at the temperature between 22.0-27.0°C and current speed less than 0.4 ms, peak around 0.25 ms. The result of the study also confirms that tuna is more likely to be aggregated at the depth of 60 – 75 m. The YFT fishable aggregations can be found in the above temperature ranges which occur at different depths depending on the seasonal monsoon. Thus, the spatial distribution of YFT is varying depending on the prevailing oceanographic conditions. These results are based on the Sri Lankan longline which has maximum depth penetration is about 120 m.