Population size and mortality of juveniles of the marine teleost Rhabdosargus holubi (Pisces: Sparidae) in a closed estuary

The populations of juvenile Rhabdosargus holubi (Steindachner) present in a South-east African estuary which was closed off from the sea in 1971 and 1972 were estimated using mark-recapture techniques. The estuary opened to the sea in August, 1971, and did not close again until January, 1972, by which time a different population of R. hulubi had established itself. The Petersen method and a multiple markrecapture method gave comparable population estimates. Catch per unit effort data supported the trends shown by the population estimates. At the beginning of 1971, the population was about 55,000 but, at the end of 7 months, had declined to 11,000, the monthly mortality varying between 9 and 49%. Mortality was insignificant in 1972, when a different population of approximately 12,000 individuals was present. Evidence is presented to show that the decline in numbers in 1971 was possibly due to bird predation, and was densitydependent.