Spatial and Temporal Variability in the Larval Fish Assemblage of a Warm Temperate South African Estuary, with Notes on the Effects of Artificial Channelling

Occurrence of larval fishes sampled by drifting light traps in the lower reaches of a South African estuary

The tidal occurrence of larval fishes was investigated in the permanently open Kowie Estuary on the warm-temperate coast of South Africa. Larval fishes were sampled in the mouth region using two drifting light traps deployed on the ebb and flood tides every second night for two consecutive 14-day periods, coinciding with the dark moon phase. A total of 553 larval fishes were caught, representing nine families and 26 species, of which Blenniidae and Clupeidae dominated. The prevalence of different estuarine association fish guilds was also tide-specific. Marine and estuarine species, such as Omobranchus woodi, were more dominant during flood tides, while marine straggler species, such as Sardinops sagax, which are not dependent on estuaries, were dominant on the ebb tide. Marine estuarine-dependents were only present during flood tides, potentially indicating ingress and entrainment within the estuary. The results confirm that light trap catches yield a different composition of species compared to towed ichthyoplankton net studies. Additionally, drifting light traps allow for better targeting of species with a phototactic response and reduction of incidental catch. Consequently, a mixture of gear is encouraged for more comprehensive surveys of larval fish occurrence.

Estuarine ecocline function and essential habitats for fish larvae in tropical South Western Atlantic estuaries

Estuarine gradients rule the dispersal of larval fishes leading to community replenishment and the recruitment of juveniles to adult populations. Here, the variations in density and diversity of fish larvae communities were assessed to understand whether the seasonal variability of environmental forcings in two tropical estuaries express the estuarine function for larvae. Spatial differences ruled larval dispersal. Larval recruitment to the Caeté Estuary occurs in the upper estuary in the late-dry season. Species richness is higher when temperature, salinity and precipitation increase, while changes in diversity is more pronounced in the lower estuary due to salinity variability. Larval recruitment to the Goiana Estuary occurs in the lower estuary, with peaks during wet warmer conditions. Species richness and diversity are also higher seawards. Thus, the seasonal fluctuation of the salinity ecocline had a greater power to predict larvae distribution and diversity by retaining larvae in essential habitats with suitable environmental conditions.

Fish assemblages along a morphodynamic continuum on three tropical beaches

A comparative study involving three mesotidal beaches from reflective to estuarine states was conducted between October 2010 and September 2011 in the estuary of Mamanguape River, northeastern Brazil. The aim was to test for differences in fish assemblages along a morphodynamic continuum, particularly the effects of wave action. In total, 3,732 fish from 71 species were examined; Rhinosardinia bahiensisand Atherinella brasiliensisdominated the estuarine beach, whereas Trachinotus carolinus dominated the reflective beach. Fish assemblages were structured by spatial differences among the sites, and overall abundance, richness and biomass demonstrated a clear increasing trend from the reflective to the estuarine beaches, indicating a primary influence of wave exposure in the measured parameters. The Mamanguape River estuary plays a crucial ecological role as a nursery for several species that depend on its resources and have adapted to the high variability of its physical and chemical factors and the diversity of its habitats.