Determination of age and growth of Opsaridium microlepis (mpasa) and the influence of water quality parameters on its catches in the Linthipe river in Central Malawi

A study on the determination of age and growth of Opsaridium microlepis, locally known as the mpasa, and the influence of water quality parameters on its catches in the Linthipe river in Central Malawi was conducted between February 2006 and January 2007. Data collection was carried out at two sampling sites: closer to the river mouth at Mkama and upstream at Kamuzu bridge. The data collected included: fish catches and duration of net cast, water temperature, dissolved oxygen, salinity, pH, conductivity, turbidicity, alkalinity, total hardness, total suspended solids and total dissolved solids. Age of O. microlepis was estimated from the sectioned otoliths (lapilli) by counting annuli. Validation of otolith annuli formation was done using the marginal zone analysis. A total of 546 male and female fish ranging from 6-53 cm SL were sampled on monthly basis. Growth was best described by simple regression model with the following parameters: Fish total length (cm)=12.56+6.931* Fish age; r=0.799, r2=0.639, p=0.00. The maximum age for O.microlepis was estimated at five years. Marginal zone analysis indicated that otolith annuli (opaque zones), visible in the cross-section otoliths, formed yearly, primarily in August. The age and growth information showed that this species is short lived and first- growing, reaching sexual maturity between one to two years. The catch per unit effort was influenced by `dissolved oxygen, electrical conductivity, and total suspended solids (p<0.05). Dissolved and total suspended solids were positively correlated to the catch per unit effort, while electrical conductivity was negatively correlated, suggesting that river bank cultivation which involves cultivation in the marginal areas of the river is a major factor affecting the total suspended solids levels in the river. A management strategy for the Linthipe O. microlepis fishery has to place emphasis on the control of water quality in the river and its catchment such as total suspended solids and electrical conductivity, which could be due to improper land use practices in the river catchment area.