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 recrui tment success. We examine the seasonal pattern in access between the surf z one and an estuary, and the implications for recruitment success and proces s in fish that spawn at sea but spend their juvenile phase in estuaries. Ab out 70% of South African estuaries are closed by barrier sand bars that ope n intermittently; an alternative but brief access opportunity is marine ove rwash (overtopping), when the surf zone extends over the bar to contact est uarine waters. Larval fish have limited ability to wait for an access oppor tunity (overtopping or opening event), so timing of settlement is important with respect to the seasonal distribution of waiting times for access oppo rtunities. Periodic regression on daily observations (1993-1999) at the Eas t Kleinemonde Estuary profiled the seasonal variation in expected waiting t ime. The data set is dominated by overwash events. Waiting time (for all ev ents) is significantly related to both the first and second harmonics of se ason, and tends to be longest in December-January, and shortest in April-Ma y. If the analysis is restricted to openings alone waiting times are shorte st near November, and longest from February to September. The seasonal vari ation shown has implications for recruitment processes, population sizes, a nd productivity in estuaries. (C) 2001 Academic Press.