Occurrence of Larval and Early Juvenile Fishes in the Surf Zone Adjacent to two Intermittently Open Estuaries, South Africa

I monitored larval and early juvenile fishes in the surf zone adjacent to two warm temperate, intermittently open estuaries in South Africa. Mugilidae (59–67%) dominated fish assemblages in the surf zones adjacent to the Van Stadens and Kabeljous estuaries. Other dominant families included the Atherinidae, Sparidae and Soleidae. The most abundant species caught during the study were the catadromous mugilid Myxus capensis, in a postflexion or early juvenile stage of development, and the sparid Rhabdosargus holubi, in a postflexion stage of development. Species displaying a degree of dependence on estuaries during the early phase of their life cycle dominated (96–98%) surf zone catches. Most fish encountered were at the postflexion stage of larval development. Larval and early juvenile fishes were more abundant during the summer and autumn months with density peaking several-fold during natural estuary opening events, particularly in the surf zone adjacent to the Kabeljous Estuary. Decreases in salinity showed a significant relationship with increases in fish density during a March opening event in the surf adjacent to the Kabeljous Estuary where catches were concentrated along a current-driven estuarine plume. Catches adjacent to the Van Stadens Estuary also increased considerably during two smaller opening events in March and November although salinity played no statistically significant role in these increased catches. Results from the surf adjacent to the Van Stadens Estuary suggested that either very small amounts of estuarine water entering the surf zone can elicit an accumulation response by estuary-dependent larval and early juvenile fishes or that an additional characteristic of estuary/river water, other than salinity per se, is aiding the cueing process.