Influence of rainfall and reservoir water level on inland fish production: A study in Magalla reservoir in Sri Lanka

Abstract

The study was conducted in the freshwater reservoir called Magalla located in Nikaweratiya in Sri Lanka. The study aimed to assess the changes in catch per unit effort (CPUE) (kg/h) of fish according to rainfall (mm) and whether the reservoir water level (m) affects the fish abundance and diversity. The data were collected from May to December 2023. A structured questionnaire survey was used to obtain the fish yield, abundance, and diversity data. Daily reservoir water level and rainfall data were collected from the irrigation office, Nikaweratiya. Fish diversity was calculated by using the Shannon-Wiener Diversity Index . Spearman's rank correlation analysis was conducted between monthly CPUE and monthly average rainfall, and between fish abundance and monthly average reservoir water level. Pearson correlation analysis was carried out between fish diversity and monthly average reservoir water level. The accepted level of significance was P > 0.05. During the study period, 18 fish species belonging to 9 families namely; 2 from Cichlidae, 8 from Cyprinidae, 2 from Channidae, 1 from Anabantidae, Gobiidae, Heteropneustidae, Mastacembelidae, Siluridae, and Osphronemidae were observed. Monthly CPUE was strongly negatively correlated (-0.929) with monthly average rainfall (P < 0.05). The highest monthly CPUE of fish (103.45 kg/h) was in August when recorded minimum rainfall (4.00 mm), and the lowest (21.55 kg/h) in November when recorded the highest rainfall (532.00 mm). The fish abundance (sites 1, 2, 3, 4: -0.976) and diversity (site 1: -0.821, site 2: 0.844, site 3: -0.803, site 4: -0.844) in all four sites were negatively correlated with the monthly average reservoir water level. The highest fish abundance (site 1: 3124, site 2: 6222, site 3: 1678, site 4: 15488) and diversity (site 2: 356, site 2: 2.359, site 3: 2.325, site 4: 2.364) were observed in August when recorded minimum monthly reservoir water level (2.0762 m). The lowest fish abundance (site 1: 609, site 2: 1415, site 3: 315, site 4: 4768) and diversity (site 1: 1.695, site 2: 1.910, site 3: 1.681, site 4: 1.987) were observed in November when recorded the highest monthly reservoir water level (5.5575 m). With the heavy rainfall and rising water levels, fishing frequency was decreased. Therefore, the present study reveals that the CPUE of fish changed according to rainfall patterns. Furthermore, the reservoir water level directly affected the fish abundance and diversity. Being aware of weather conditions and the seasonality of fish catch is useful for better fish harvest on a sustainable basis.