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 fish es were recorded in the Kromme Estuary during the study period. Dominant sp ecies included Atherina breviceps, Caffrogobius gilchristi, Diplodus sargus capensis, Gilchristella aestuaria, Glossogobius callidus, Rhabdosargus hol ubi and Rhabdosargus globiceps. The larval fish catch in the estuary indica ted a marine dominance with a relatively high species diversity. The introd uction of a regulated freshwater pulse of 2 x 10(6) m(3) into the estuary f rom the Mpofu Dam was monitored. Pre- and post-release collections of fish larvae were made on a weekly basis for a two-month period between October a nd December 1998. The regulated freshwater release into the estuary resulte d in no significant changes to the fish family composition, species diversi ty or estuarine association of the larval fish assemblage. No significant i ncrease in total larval fish abundance or recruitment response by estuarine -dependent species was recorded. A limited breeding response by estuarine-r esident fish species such as Caffrogobius gilchristi, Gilchristella aestuar ia and Glossogobius callidus was documented. The increases in larval abunda nce of estuarine-resident species were mainly attributed to spawning events in the Geelhoutboom Tributary. This tributary received freshwater inflow f rom rainfall, which coincided with the dam release. It appears that the tri butary serves to supplement the Kromme Estuary ichthyoplankton with large n umbers of larvae belonging to estuarine-resident species. Physical conditio ns in the estuary returned to marine dominance within two weeks of the fres hwater release. It is concluded that the riverine pulse and salinity gradie nt 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 larg er amount of freshwater would be required to produce a positive response by the larvae of estuarine-associated marine species.