The effects of a simulated harvest on Porphyra (Bangiales, Rhodophyta) in South Africa

Abstract

In South Africa, Porphyra has, until recently, been little exploited, having been harvested only for a small health-food market. However, the advent of land-based abalone farming has increased the pressure on wild Porphyra populations, as Porphyra is in demand for abalone fodder. This paper reports on the effects of a simulated harvest on Porphyra populations and those of sympatric fauna. Harvesting, starting in autumn, was found to reduce the biomass of Porphyra, an effect detectable up to six months later. Porphyra had a patchy distribution, with patches having a mode of approximately 300 thalli m-2. The main effect of harvesting was the removal of patches, as mean thallus size changed little in response to harvesting. Nine months after the start of the experiment, control populations had been reduced, through loss of patches, to the level of treatment populations. Although harvesting Porphyra reduced populations of some sympatric fauna (amphipods, isopods and littorinid snails), natural Porphyra population decreases had a comparable effect. Some recommendations are discussed for the management and controlled harvesting of Porphyra populations in South Africa.