Macrocystis (Laminariales, Phaeophyceae) in South Africa: potential for cultivation through holdfast fragmentation and use as feed for the aquacultured abalone, Haliotis midae

Abstract

Macrocystis pyrifera (Linnaeus) C. Agardh, known in some regions as ‘giant kelp’, occurs along temperate coastal rocky reefs throughout the southern hemisphere, as well as along the north-east Pacific coastline. In Chile, it serves as a major source of abalone feed within the abalone industry, with considerable literature on cultivation of the species. Here we attempt to cultivate South African M. pyrifera through vegetative propagation and investigate its use as a feed for the abalone, Haliotis midae. Kelp holdfasts were attached to a rope-raft system in Saldanha Bay, South Africa, and monitored over 3 months. In addition, a 6-month H. midae feed inclusion trial was carried out, which compared the use of M. pyrifera with the predominant commercial kelp feed species, Ecklonia maxima. Young M. pyrifera grew well on the culture system, with mean upright length nearly doubling over the culture period (55.6–102.2 cm) and displayed mean upright relative growth rates between 0.99 and 1.22% day-1. Additionally, holdfasts developed new uprights and holdfasts increased in size, growing around culture lines. Simulated partial harvesting of samples stopped upright elongation, but samples survived, whereas complete removal of uprights caused mortality. Haliotis midae fed with M. pyrifera + formulated feed (SAF 3000®) displayed comparable growth rates to the E. maxima + SAF 3000® diet. No significant differences in mean specific growth rate (M. pyrifera 0.19, E. maxima 0.18% body weight day-1), daily increment in shell length (M. pyrifera 31.19, E. maxima 24.64 μm day-1), condition factor (M. pyrifera 1.37, E. maxima 1.40 g mm-1) and mortality rate were identified between the two diets. Additionally, H. midae consumed significantly less M. pyrifera than E. maxima, suggesting potential differences in feed conversion ratio/kelp nutritional composition among the kelp species. These findings provide baseline information for the potential cultivation and use of M. pyrifera as a new source of fresh seaweed feed in the South African abalone industry.