Empirical yield predictive models for the fisheries of irrigation reservoirs in Sri Lanka

As fisheries production in reservoirs of most countries is a secondary use, challenges for improved management of fisheries should be addressed by building partnership between fisheries and other interested groups such as agriculture concerned with water management. Attempts were therefore made to develop empirical fish yield predictive models in ten irrigation reservoirs of Sri Lanka incorporating morphological, edaphic and hydrological parameters together with fishing intensity, with a view to investigating their influence on fish yields.

Reservoir fish yield was found to be significantly correlated with two formulations of morpho-edaphic index (i.e., conductivity in μS cm^-1/mean depth in m [MEI_c] and alkalinity in m. equiv. l^-1)/mean depth in m [MEI_a]), and a relative reservoir level fluctuation index (RRLF), defined as the mean amplitude of the annual reservoir level fluctuations divided by the mean depth of the reservoir. Both MEI_c and MEI_a also had significant positive ln-ln relationships with RRWL, indicating that RRWL can be used as an independent variable in reservoir fish yield prediction. Reservoir fish yield was also related to fishing intensity (FI in boat-days ha^-1, yr^-1) conforming to a ln-linear regression model (p<0.05). When MEI_a, MEI_c and RRWL were used as predctor variables together with FI, reservoir fish yield (FY) was multiply correlated as follows:

Ln FY = 3.245 + 0.327 Ln MEI_a + 0.023 FI (R² = 0.355; p< 0.01)

Ln FY = 3.403 + 0.249 Ln MEI_c + 0.019 FI (R² = 0.369; p< 0.01)

Ln FY = 1.330 + 0.650 Ln RRWL + 0.016 FI (R² = 0.593; p< 0.001)

The empirical yield predictive model based on RRWL and FI as independent variables was more robust than those based on MEI_a and MEI_c, and the former has significant management implications because RRWL can be manipulated by irrigation authorities whereas control of FI is under the jurisdiction of fisheries authorities. Hence, through an effective dialogue between irrigation and fisheries authorities, there is a considerable potential to optimize fish yields in irrigation reservoirs of Sri Lanka.