Spatio-temporal variation in length-frequency and salinity tolerance of dominant fishes utilizing the Orange River-Estuary Continuum
The length-frequency distribution of the dominant species of euryhaline marine Chelon richardsonii, estuarine Gilchristella aestuaria and freshwater species Labeobarbus aeneus, Mesobola brevianalis and Pseudocrenilabrus philander were investigated seasonally along the Orange River Estuary Continuum (OREC) during high-flow and low-flow periods, conducted in eight different years, within a 15-year period (2004-2018). Fish were sampled using a seine net at 18 sites spanning from the mouth of the estuary up to 35 km upstream. The length-frequency distributions of all species did not differ between the high-flow and low-flow season. Spatial differentiation in size frequency distribution of all freshwater taxon was recorded along the river-estuarine continuum but not for C. richardsonii and G. aestuaria. Total catch-per-unit-effort (catch per haul, CPUE) was low for both M. brevianalis and P. philander and their abundance declined downstream into the estuarine region as expected for these two freshwater species. A negative correlation between salinity and fish length of all five species was found, demonstrating that juvenile fishes, in particular, freshwater species are less salt-tolerant than the adults. It is concluded that the lower OREC provides an important fish nursery habitat suitable for growth and development, particularly for marine C. richardsonii and estuarine G. aestuaria.