Stock density and mortality assessment of Sarotherodon mossambicus (Cichlidae) in Plover Cove Reservoir,Hong Kong

Synopsis Two methods were used to estimate stock density and two to estimate mortality of Sarotherodon mossambicus (Peters) in Plover Cove Reservoir. The first method for density estimation (the capturemark-recapture method) involved tagging and pelvic fin clipping. The former technique was eventually abandoned in favour of the latter. Using the capture-mark-recapture method the population was estimated to be approximately 50,000. The number of fish recaptured was low (six recaptures out of 1590 fish marked) and undoubtedly led to underestimation of the density. The second method, using differences between catch per unit of effort values, gave a much more acceptable estimate of density—702,900.Mortality data were obtained by routine collections of dead fish and interpretation of catch curve results. The former method revealed that whereas natural mortalities (that is, fishing mortality excluded) of all other stock and indigenous fish reach a maximum in May and June, Sarotherodon mortalities were at a maximum in February but with a smaller post-spawning summer peak between August and October. It is suggested that temperature is the major factor involved in these natural mortalities, though post-spawning mortality is thought to be responsible for the summer peak. Mortality data obtained by the second method, when compared with results of dead fish collections and gill net surveys, suggested that illegal fishing operations caused the greatest mortality and represented 97.75% of the overall mortality of Sarotherodon.