Comparative studies on Macrocystis pyrifera and Ecklonia maxima (Laminariales) in South Africa; investigating the use of M. pyrifera as abalone feed in South African aquaculture and assessing the vulnerability of these two species to Marine Heat Waves

Master Thesis


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Macrocystis pyrifera is a kelp species found along many rocky temperate coastlines throughout the world. This species has been the focus of much research in recent decades, owing to its important ecological role, as well as its many commercial uses. Along the South African coastline M. pyrifera exists in eight known populations, over a limited ca. 200 km distribution. South African M. pyrifera differs from other global populations, as it is shorter in length and develops as small populations, often inshore of populations of the widely distributed and commercially valuable Ecklonia maxima. Very little research has been carried out on South African M. pyrifera. This thesis comprises two comparative studies on South African M. pyrifera, carried out with the aim of increasing our knowledge about the species in South Africa. South African abalone aquaculture has been increasing for many years and farmers are continually searching for new sustainable sources of abalone feed. As more abalone farms are built and existing farms expand, farms that use kelp as a feed, will likely require increased fresh kelp biomass in the future. The first study investigated M. pyrifera as a potential alternative source of supplementary fresh kelp feed for the South African abalone aquaculture industry. A six month comparative abalone feed inclusion trial and growth study was carried out, where the commercially farmed abalone species, Haliotis midae, was provided a diet of either E. maxima (the primary fresh kelp diet in the industry) + SAF 3000® formulated feed or M. pyrifera + SAF 3000® formulated feed. Haliotis midae fed with M. pyrifera + SAF 3000® displayed comparable growth rates to abalone fed a diet of E. maxima + SAF 3000®. No significant differences in the resultant mean Specific Growth Rate (M. pyrifera- 0.19 ± 0.01, E. maxima- 0.18 ± 0.01 % body ; F= 0.180, df1= 1, df2= 192, p= 0.672), Daily Increment in Shell Length (M. pyrifera- 31.19 ± 2.59, E. maxima- 24.64 ± 2.31 μ ; F= 0.853, df1= 1, df2= 192, p= 0.357) and mortality rate (t(10)= 0.263, p= 0.789) were identified between the two diets provided. Mean abalone Condition Factor also showed no significant differences among diets (M. pyrifera- 1.37 ± 0.01, E. maxima- 1.40 ± 0.01, with the exception of month 2 (F= 4.014, df1= 1, df2= 224, p< 0.05). Additionally, H. midae consumed significantly less M. pyrifera (31.76 ± 2.83 %) than E. maxima (72.70 ± 1.26 %) (t(18)= 13.218, p< 0.05), suggesting potential differences in the Feed Conversion Ratio and/or kelp nutritional composition amongst these two kelp species. Macrocystis pyrifera could thus serve as a possible alternative supplementary kelp feed for the South African abalone industry. However, a sustainable source of M. pyrifera would be required. Moreover, further studies that examine the Feed Conversion Ratio, nutritional content and other important parameters relative to the use of M. pyrifera in aquaculture, should be established to reinforce these findings. At present, anthropogenic climate change arguably poses the largest threat to the world’s oceans. Rising ocean temperatures will increase climatic variability, leading to increases in the frequency, duration and intensity of extreme climatic events such as storms and Marine Heat Waves (MHW’s). The second study investigated the impact of MHW’s on both M. pyrifera and E. maxima recruits. Recruits of both species were exposed to an experimentally simulated MHW for a 72 hour period. Recruits were subjected to four temperature treatments: 15, 17.5, 20 and 22.5 °C. The impact of the simulated MHW on the samples was measured via photosynthetic oxygen production. Evidence of thallus deterioration in the samples was also recorded. The findings demonstrated that the thermal tolerance ranges and optima varied among the two species, with M. pyrifera displaying a greater sensitivity to the simulated MHW. Both species indicated minimal thallus deterioration at 15 °C and 17.5 °C. However, at 20 °C, 80 % of the M. pyrifera recruits and 40 % of the E. maxima recruits displayed signs of thallus deterioration. This increased to 100 % of M. pyrifera recruits and 80 % E. maxima recruits in the 22.5 °C treatment, exhibiting thallus deterioration. Mean oxygen production by M. pyrifera was greatest at 15 °C (1.42 ± 0.12 mg O2.g-1 DW.h1), with oxygen production significantly declining above 17.5 °C (F= 18.410, df1= 3, df2= 16, p< 0.05). Therefore, temperatures just above 17.5 °C likely lie at the upper end of the thermal tolerance range of South African M. pyrifera, with a thermal optima potentially existing at a lower temperature than what was tested. Mean oxygen production by E. maxima was greatest at 17.5 °C (1.92 ± 0.19 mg O2.g-1 DW.h-1) and was significantly reduced at 22.5 °C (F= 4.987, df1= 3, df2= 16, p< 0.05). Ecklonia maxima thus possesses a larger thermal tolerance range in comparison with M. pyrifera, with temperatures between 20 and 22.5 °C representing the upper limit of thermal tolerance for this species. The findings of this study highlight the threat of MHW’s to South African M. pyrifera and E. maxima. Furthermore, the findings assist in explaining the limited distribution of M. pyrifera along the South African coastline. However, further research is required to fully understand the implications of MHW and other warming events on the persistence of these two species in a climate change future.