Fish as Primary Consumers

Four investigations were made in the laboratory on the nutrition of Sarpa salpa, an herbivorous fish from the rocky shoreline of Cape Peninsula, South Africa: Assimilation efficiency while the fish fed on a green alga Ulva lactuca; cellulase activity of aerobic gut microflora; epiphytic growth on the alga; and maintenance ration. The fish assimilated 61% of the dry weight, 59% of the ash‐free dry weight, 81% of the protein, and 65% of the energy content of the alga. No statistically significant differences in assimilation were apparent in tests between large (length = 156 mm) and small (83 mm) fish or among fish tested singly or in groups of 2, 4, or 5 individuals. No cellulase activity was detected in the gut microflora. A slight epiphytic growth of bacteria was revealed by scanning electron microscopy, but it was not sufficient to cause a difference in protein content between freshly collected, untreated fronds of Ulva and fronds treated with streptomycin and washed. Sarpa salpa is able to digest, with its own enzymes, significant quantities of the nutritive components of the green alga, Ulva lactuca, particularly protein. A feeding experiment designed to estimate the maintenance ration suggested that the fish could maintain their body weight by consuming the alga at a rate of about 5.6–5.9% of the body weight/day. Received May 21, 1983 Accepted March 21, 1984