Na. Strydom et Ak. Whitfield, The effects of a single freshwater release into the Kromme Estuary. 4: Larval fish response, WATER SA, 26(3), 2000, pp. 319-328
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.