Seasonality in Frequency of Marine Access to an Intermittently Open Estuary: Implications for Recruitment Strategies

Timing of life-history stages and environmental conditions is key to recruitment success. We examine the seasonal pattern in access between the surf zone and an estuary, and the implications for recruitment success and process in fish that spawn at sea but spend their juvenile phase in estuaries. About 70% of South African estuaries are closed by barrier sand bars that open intermittently; an alternative but brief access opportunity is marine overwash (overtopping), when the surf zone extends over the bar to contact estuarine waters. Larval fish have limited ability to wait for an access opportunity (overtopping or opening event), so timing of settlement is important with respect to the seasonal distribution of waiting times for access opportunities. Periodic regression on daily observations (1993–1999) at the East Kleinemonde Estuary profiled the seasonal variation in expected waiting time. The data set is dominated by overwash events. Waiting time (for all events) is significantly related to both the first and second harmonics of season, and tends to be longest in December–January, and shortest in April–May. If the analysis is restricted to openings alone waiting times are shortest near November, and longest from February to September. The seasonal variation shown has implications for recruitment processes, population sizes, and productivity in estuaries.