Diet of the mullets Planiliza macrolepis and Moolgarda cunnesius in the Mfolozi-Msunduzi Estuary, KwaZulu-Natal, South Africa

Abstract

The mullet species, Planiliza macrolepis and Moolgarda cunnesius, were collected seasonally from Mfolozi-Msunduzi Estuary on the east coast of South Africa in order to determine their diet. The gut contents of 178 fish with standard length (SL) ranging between 100 and 220 mm revealed strong evidence that in the Mfolozi-Msunduzi these fish rely heavily on benthic floc as a primary food source. Plant debris and small benthic organisms, including Assiminea, copepods, nematodes and ostracods also formed part of their diet. Some of these food items (e.g. Assiminea sp. nematodes) were only consumed by P. macrolepis. Differences in the occurrence of food items consumed by these two closely related species may be explained by their feeding ground preferences. Results suggest that P.  macrolepis forages in substrata mostly dominated by fine-to-medium sand grain sizes (<50 to 450 µm), whereas M. cunnesius prefers substrata dominated by fine sand to muddy grain sizes (50 to 250 µm). Based on these findings, it is concluded that differences in substrate preference by these two mullet species is an important strategy by which they circumvent any possible interspecific competition for food resources within the system.

Keywords: benthic floc, food, organic content, sand grain size