Fish population size estimates from a small intermittently open estuary in South Africa, based on mark-recapture techniques

Population sizes of the migratory marine-spawning fishes in the small intermittently open East Kleinemonde Estuary (33-32'S,27 degrees 03'E) were estimated during two independent mark recapture studies between 1994 and 1996. Seines were used to sample fishes during the closed-mouth phase of the estuary. All individuals above a stipulated minimum size were marked by fin-clipping. Population estimates were obtained by use of two mark recapture models ( Schnabel and a maximum-likelihood estimator) and a derived method. The total population size was estimated at similar to 18 000 and similar to 133 000 individuals during the two study periods respectively. The large temporal ( interannual) variability was ascribed to both abiotic ( estuary mouth conditions) and biotic -conditions. The dominant species during both periods was Rhabdosargus holubi, which accounted for similar to 75% and similar to 80% of the total population of marine fishes in the estuary. The success of this species is attributed to an extended breeding season and the ability to recruit during mouth overwash events.