Deborah Robertson-Andersson

Can kelp extract (KELPAK®) be useful in seaweed mariculture?

The addition of low concentrations of commercial kelp extract (Ecklonia maxima: Kelpak®) in addition to fertiliser has proven to be beneficial in agriculture. It triggers rooting in field crops, increases yields and has other useful effects, such as parasite reduction. Its efficacy has been attributed to the fact that Kelpak® is produced by a cold process, and is a high auxin/low cytokinin product. The aim of this study was to investigate if seaweeds (which do not have a root system) grown in culture systems, would benefit from the addition of Kelpak® or a combination of Kelpak® and fertilizer. A preliminary laboratory experiment was carried out by growing excised 15 mm tips of the red alga Gracilaria gracilis in culture dishes containing Provasoli Enriched Seawater medium to which various concentrations of Kelpak® were added. Gracilaria tips in some of the Kelpak® treatments (1:2500; 1:1000; 1:500) grew significantly better than the control. Further experiments were carried out on a pilot commercial scale at Jacobsbaai Sea Products Ltd. on the South African west coast. Ulva lactuca was grown in effluent from fish (turbot) culture, with additions of 1:5000, 1:2500 and 1:500 concentrations of Kelpak® once a week. The intermediate Kelpak® concentration (1:2500) produced the highest growth of Ulva in the turbot water, while the highest Kelpak® concentration (1:500) inhibited Ulva growth. In another Ulva experiment, various combinations of aquaculture effluent water, commercial fertiliser and Kelpak® at 1:2500 were used. Best growth of Ulva was obtained in turbot water containing both fertiliser and Kelpak®. The results suggest that Kelpak® could be useful in commercial seaweed mariculture operations.

Rapid assessment of tissue nitrogen in cultivated Gracilaria gracilis (Rhodophyta) and Ulva lactuca (Chlorophyta)

Tissue nitrogen content and thallus colour were quantified using a rapid assessment method based on the Pantone® matt uncoated formula guide for raft-cultivated Gracilaria gracilis Steentoft, Irvine ef Farnham at Saldanha Bay and tank-cultivated Ulva lactuca Linnaeus at Jacobsbaai in 2001–2002. For Gracilaria a relationship between thallus colour and tissue nitrogen, as well as a transition between green-yellows and yellow-browns that occurs between 0.8 and 1.3 mg N per g tissue (Pantone® colours 460U-455U), was found, with the green-yellow colours indicating nitrogen-starved material and the yellow-browns indicating nitrogen-replete material. For Ulva a transition between green-yellow and green occurred at a tissue nitrogen content of between 1.5 and 1.7 mg N per g tissue (Pantone® colours 585U-583U). This relationship can be used by seaweed farmers for cultivation management as a quick guide to determine nutritional status of the seaweeds, and as an indication of protein content when the seaweeds are used as feeds.

Effects of wild and farm-grown macroalgae on the growth of juvenile South African abalone Haliotis midae Linnaeus

The effect of various macroalgal diets on the growth of grow-out (>20 mm shell length) South African abalone Haliotis midae was investigated on a commercial abalone farm. The experiment consisted of four treatments: fresh kelp blades (Ecklonia maxima (Osbeck) Papenfuss) (c. 10% protein); farmed, protein-enriched Ulva lactuca Linnaeus (c. 26% protein) grown in aquaculture effluent; wild U. lactuca (c. 20% protein); and a combination diet of kelp blades + farmed U. lactuca. Abalone grew best on the combination diet (0.423 ± 0.02% weight d−1 SGR [specific growth rate]; 59.593 ± 0.02 −m d−1 DISL [daily increment in shell length]; 1.093 final CF [condition factor]) followed by the kelp only diet (0.367 ± 0.02% weight d−1 SGR; 53.148 ± 0.02 −m d−1 DISL; 1.047 final CF), then the farmed, protein-enriched U. lactuca only diet (0.290 ± 0.02% weight d−1 SGR; 42.988 ± 0.03 um d”1 DISL; 1.013 final CF) that in turn outperformed the wild U. lactuca only diet (-0.079 ± 0.01% weight d−1 SGR; 3.745 ± 0.02 −m d”−1 DISL; 0.812 final CF). The results suggest that protein alone could not have accounted for the differences produced by the varieties of U. lactuca and that the gross energy content is probably important.

Biofiltering and Uptake of Dissolved Nutrients by Ulva armoricana (Chlorophyta) in a Land-based Aquaculture System

An on-land flow-through cultivation system was designed for the macroalgal species Ulva armoricana (Chlorophyta) to reduce the environmental impact of aquaculture effluent in coastal ecosystems as part of an integrated aquaculture system. The macroalgae was cultured in various enriched media at a stocking density of 500 kg wet weight/pond. Overall, U. armoricana was able to remove a greater percentage of inorganic nitrogen in the double fertilizer ratio. The total dissolved phosphate was higher in standard seawater. U. armoricana showed preference for bioaccumulation, with ranges as follows: zinc (9.908 – 32.942 mg.kg); copper (1.893 – 5.927 mg.kg); cadmium (0.254 – 1.500 mg.kg); and lead (none detected). Apart from the presence of cadmium (Cd), the algal biomass produced at the end of the experiment was of a relatively good quality with limited heavy metal contamination so that U. armoricana could be successfully used as a plant stimulant but not as part of a feed formulation for livestock and for the food industry. This study showed that U. armoricana can effectively be used as a biological filter for dissolved nutrient uptake from aquaculture effluents. The prospect of better management practices, based on the utilization of Ulva mariculture designs, bodes well for the aquaculture industry.