TURBIDITY GRADIENTS IN TWO INDO-PACIFIC ESTUARIES AND THEIR INFLUENCE ON FISH DISTRIBUTION

Summary

Research undertaken in two widely separated estuaries in the Indo-Pacific, showed that both have distinct turbidity gradients. In the turbid St Lucia System, on the South East Coast of Africa (range 2 to 568 Nephelometric Turbidity Units (NTU)), different turbidity patterns were observed during summer and winter. The Embley Estuary, on the eastern side of the Gulf of Carpentaria, Queensland, Australia, although also turbid, had a much more restricted range (0.6 to 55.0 NTU), and showed three distinct seasonal patterns, referred to as the Wet, Early Dry and Late Dry.

Studies undertaken at St Lucia illustrated that turbidity had a direct influence on fish distribution in the system. The Embley Estuary was found to have species groups which were associated with specific turbidity levels. As only a limited range of turbidity occurred, these groups were somewhat restricted in composition. Comparisons made of the turbidities occupied by 13 fish taxa common to the different systems showed that nine had similar patterns irrespective of locality. It is concluded that, dependent on the levels, turbidity gradients have profound effects on the distribution of certain fish species utilizing estuarine systems. These results may well provide an indication that the influence of turbidity on the distribution of a number of fish species throughout the Indo-Pacific could be similar.

Keywords:

• turbidity gradients
• fish distribution
• estuaries
• Indo-Pacific