Effects Of Environmental Variability On Fish Diversity And Composition Along The Southern Namibian Coastline During Summer

ABSTRACT

Species diversity, richness and composition of fish in bottom-trawling were investigated

off the Namibian coast between Oranjemund and Henties Bay. Sampling followed a

systematic transects design, along latitude gradients (28-22oS) at different seafloor

depths (100-500m). In total 18 transects were sampled containing 105 stations. At each

trawled station the whole catch was sorted into species type and the total body mass (kg)

of each fish species was recorded. A total of 91 fish species were sampled in the study

area. Environmental factors, in particular bottom water temperature, dissolved oxygen

and salinity were recorded automatically by the CTD instrument. Results indicated

significant differences in means of fish species diversity (F = 14.01, df = 4, p < 0.001)

and richness (df = 4, p < 0.001) at different seafloor depths. Deeper areas were more

diverse than shallower area. With regard to latitudinal transect gradients significant

differences were observed in means of fish species richness (df = 6, p < 0.05) but not

diversity (F = 1.28, df = 6, p > 0.05). The hierarchical cluster analysis on species

presence/absence data separated the fish species composition into six groups, formed at

38% similarity level. Indirect gradient analyses indicated a complex interaction of

gradients which have influenced the pattern of species composition. The direct gradient

analysis indicates significant influence of water temperature (F = 4.03, p < 0.05) and

salinity (F = 1.55, p < 0.05) on species composition at 100m and 200m seafloor depths.

Differences in species diversity and richness of fish at seafloor depths might be a result

of absence of disturbances by bottom-trawling at shallower depths. Insignificant

differences in species diversity along latitudinal gradients indicate inconsequential

effects of global latitudinal trends off the Namibian coast. It was concluded that

environmental variabilities off the Namibian coast influence fish species composition.