The effects of kelp harvesting on its regrowth and the understorey benthic community at Danger Point, South Africa, and a new method of harvesting kelp fronds

Abstract

At Danger Point, South Africa, mariculture of the abalone Haliotis midae has led to increased demand for freshly harvested kelp Ecklonia maxima. This study estimates the biomass of E. maxima available for harvesting, investigates the effects of harvesting on the recovery of kelp and on the benthic understorey biota, and tests an alternative method of harvesting kelp by cutting off only fronds rather than whole plants. Estimates from diver sampling between May 1992 and November 1994 and infra-red maps indicate an E. maxima biomass at Danger Point of 22 973 tons fresh mass in 336 ha of kelp beds, insufficient to sustain projected abalone feed requirements in the area if harvesting is done in the traditional manner of cutting the stipes. However, sufficient frond material may be harvested sustainably if only fronds are cut, at distances 20-30 cm from the bases of fronds. Cutting at 10 cm from the base significantly reduced regrowth rates, and there was little regrowth after cutting at 2 cm. Yield per unit effort was higher for a four-month interval between harvesting them for one-, two- and three-month intervals. Two years after harvesting whole plants (in two 100 × 20 m lanes), biomass and density of kelp had recovered (similar in control and harvested lanes), and there were no detectable differences in the understorey communities, as measured by detrended correspondence analysis. Although harvesting of whole kelp has a minimal effect on the understorey biota and kelp itself recovers within two years, selective cutting of fronds allows plants to remains alive and produce a higher yield. This method is likely to minimize damage to the kelp bed system.