Mullet grazing on surf diatom accumulations

Abstract

Grazing by southern mullet, Liza richardsoni (Smith), on surf diatoms occurring in bloom concentrations off an ocean‐exposed East Cape beach, South Africa, was investigated. Field observations and stomach content analysis demonstrated that surf diatoms taken from the air‐water interface were a principal source of food. A qualitative examination of stomach contents revealed a feeding transition from planktonic carnivore in juveniles to a diet consisting entirely of surf diatoms in larger fishes. This change in diet commonly occurred at a standard length of 50–135 mm. Fish larger than 135 mm fed entirely on surf diatoms which were ingested together with large quantities of beach sediment. Grazing on surf diatoms only took place during daylight hours. Energy, ash, protein, fat and carbohydrate content determinations indicate a high food quality of surf diatoms. It is concluded that surf diatom accumulations form a richly concentrated and reliable food source of high nutritional quality for these fish. Possible widespread grazing on surf diatoms by mullets is considered.