Browsing by Author "Rothman, Mark D"

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    Open Access
    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
    (2019) Fleischman, Michael J; Bolton, John J; Rothman, Mark D
    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 weight.day-1 ; 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 μm.day-1 ; 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 g.mm-1), 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.
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    Open Access
    How reliable is morphological species delimitation in kelp? : a study of two closely related South African Ecklonia species
    (2014) Levy, Sarah Bernadette; Bolton, John J; Rothman, Mark D
    Ecklonia maxima and Ecklonia radiata are both kelp bed forming macroalgae along the South African coast, and the latter is also found in considerable abundance in Australia and New Zealand. Genetically they exist as two distinct species and can usually be differentiated morphologically, especially when occurring as geographically separate entities. However, they do appear to intergrade when growing together, where plants of apparent intermediate and indeterminate morphology have been observed. This study tested the reliability of morphology in separating these two Ecklonia species across their intraspecific range of morphological variation, from locations where the species co-occur, as well as where they appear in isolation in South Africa (both species) and Australia (E. radiata only). No individual characters reliably separated between species, yet overall size distinctions as well as the morphometric separation of hollow and solid-stiped sporophytes provide good evidence for morphological differentiation of E. radiata and E. maxima. While E. radiata clusters morphometrically, a distinction between Australian and South Africa specimens is observed. In localities where the two species are reported to co-exist morphological distinction is less clear, particularly in deep water at Buffels Bay. The blade morphology of these deep water sporophytes is distinct from both E. radiata and E. maxima across all locations, while overall size distinctions contribute most prominently to the morphological separation of E. radiata and E. maxima at De Hoop. Environmental data in combination with more detailed genetic analyses, especially those aimed at hybrid identification, are necessary to resolve the nature of these subtidal plants as well as to investigate the relationship between genetic differentiation and overlapping morphology in plants at De Hoop.
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    Investigations into the harvesting ecology of the South African kelp Ecklonia maxima (Alariaceae, Laminariales)
    (2006) Rothman, Mark D; Bolton, John J
    This present study examines several questions that were not addressed by previous studies of South African Ecklonia maxima beds. Firstly, this thesis examined the distribution of kelp biomass, at various sites around the Cape Peninsula and on the west coast, and at different depths within sites. An attempt was made to calculate a single figure that could be used in determining the biomass of kelp beds.
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    Modelling distribution and associated environmental preferences of South African kelps and their close relatives
    (2018) Lutchminarayan, Kirtanya; Bolton, John J; Rothman, Mark D
    Kelp are central components of marine ecosystems that are found on the west coast of South Africa and in temperate regions around the world. They are ecologically important in providing vertical habitats, and sustaining diverse life-forms. The kelp species that are investigated in this study are Ecklonia cava, Ecklonia maxima, Ecklonia radiata, Laminaria pallida and Laminaria ochroleuca. Ecklonia maxima, E. radiata and L. pallida are dominant South African kelps, therefore studying these species provides information about their distributions and environmental preferences. E.cava and L. ochloleuca are of interest as they are close relatives to South African kelp species, and add to the understanding of Ecklonia and Laminaria in a more global context. The aims of this study were to map the biogeographical distribution of each species, and model their potential niches based on the environmental conditions that facilitate their growth. Species Distribution Modelling was used to map the fundamental niche of each species. The Maxent method was used to model predictive niches by layering known coordinates of where each species is found, along with the Bio-ORACLE dataset of 23 environmental variables, consisting of satellite and in-situ data. The Carto cloud computing platform was used to map the current distribution of each species. The major environmental predictors of each species distribution were identified in the modelling process and contributed to the final predicted habitat suitability maps. The results of this study show that Maxent identified maximum, minimum, mean and range in sea surface temperature to be associated with all species' distribution. Measures of irradiance or light availability (specifically Photosynthetically Active Radiation, Diffuse Attenuation and cloud cover), were important predictors of four of the species' distributions. Chlorophyll, a proxy for nutrients, emerged as an important correlate of most distributions, whilst silicate, phosphate and calcite were important, particularly to the distribution of kelp species found outside of South Africa. There were regions identified 7 where each species could potentially grow, but are not found. The results show that while E.maxima and L.pallida are distributed along the west coast of southern Africa, L.pallida could potentially grow along the west coast of South America, whilst E.maxima appears to have a niche endemic to South Africa. Ecklonia radiata is distributed along the south coast of South Africa as well as southern Australia, New Zealand, whilst it could potentially grow in the north Atlantic, northeast Pacific and southeast Pacific. Ecklonia cava's distribution is endemic to the Japanese and South Korean coast and is predicted to be able to spread down the coast of China. Laminaria ochroleuca is distributed along the coasts of Spain, Portugal, France and Northern Africa with a widespread distribution on the British coastline, with a predicted potential to extend to western Ireland. Other suitable habitats for L.ochroleuca were southern Australia and New Zealand. Ecklonia maxima and L.pallida have overlapping distributions, whilst all other species have different distributions. The ecological preferences of each of these species differed in parameter, but were related to the same variables; temperature, light and nutrients. The South African species, E.maxima, E.radiata and L.pallida, differed in their preferences, including those occupying the same coastline. All species of kelp had variably unique distributions and varying environmental preferences compared to their congenerics. The possible causes for species not being able to grow in identified niches are likely to be temperature limits, light and nutrient limits, physical barriers and limitations to movement and establishment, as well as geographical and associated evolutionary adaptations that may limit dispersal. Besides the environmental variables analysed, biological, oceanographic and in-situ data is deemed important in gaining a complete and causal understanding of species distributions and their drivers. This study provides insight into South African kelp systems and their close relatives in a global context, and shows the successful use of niche modelling methods that can be more widely applied in marine research, conservation and management