The effects of a single freshwater release into the Kromme Estuary. 4: Larval fish response

In total 17 families comprising more than 29 species of larval teleost fishes were recorded in the Kromme Estuary during the study period. Dominant species included Atherina breviceps, Caffrogobius gilchristi, Diplodus sargus capensis, Gilchristella aestuaria, Glossogobius callidus, Rhabdosargus holubi and Rhabdosargus globiceps. The larval fish catch in the estuary indicated a marine dominance with a relatively high species diversity. The introduction of a regulated freshwater pulse of 2 x 10 m into the estuary from the Mpofu Dam was monitored. Preand post-release collections of fish larvae were made on a weekly basis for a two-month period between October and December 1998. The regulated freshwater release into the estuary resulted in no significant changes to the fish family composition, species diversity or estuarine association of the larval fish assemblage. No significant increase in total larval fish abundance or recruitment response by estuarine-dependent species was recorded. A limited breeding response by estuarine-resident fish species such as Caffrogobius gilchristi, Gilchristella aestuaria and Glossogobius callidus was documented. The increases in larval abundance of estuarine-resident species were mainly attributed to spawning events in the Geelhoutboom Tributary. This tributary received freshwater inflow from rainfall, which coincided with the dam release. It appears that the tributary serves to supplement the Kromme Estuary ichthyoplankton with large numbers of larvae belonging to estuarine-resident species. Physical conditions in the estuary returned to marine dominance within two weeks of the freshwater release. It is concluded that the riverine pulse and salinity gradient induced by the release of freshwater was too short-lived and too weak to result in a cueing effect on larval fish in the marine environment. A larger amount of freshwater would be required to produce a positive response by the larvae of estuarine-associated marine species.