Growth rates and agar properties of three gracilarioids in suspended open-water cultivation in St. Helena Bay, South Africa

Relative growth rates (RGRs), yields and agar characteristics of threegracilarioid isolates (Gracilariopsis sp. from St. Helena Bay, and Gracilaria gracilis isolates from Langebaan Lagoon and Saldanha Bay) weremeasured to assess the suitability of a site in St. Helena Bay for suspendedcultivation. The gracilarioids were grown on polypropylene ropes and`netlon' lines, and the RGRs were 4.0–11.0% d<sup>-1</sup> and 5.0–7.0%d<sup>-1</sup>, respectively. The RGR of the Langebaan isolate of G. gracilis grown on ropes was significantly higher than the RGR of otherisolates. The mean net yield for the Langebaan isolate grown on `netlon'lines was 2.6 ± 0.9 kg wet wt m^-2 30 day^-1. Thecultured gracilarioids were extracted for native and alkali treated agars. Themean native agar yield over the entire period was 39.0% dry wt. Alkalipretreatment reduced the yield by 55%, but significantly increased gelstrength. High gel strengths (>750 g cm^-2) were measured inagars from Gracilariopsis sp. and Saldanha Gracilaria gracilis inmid-summer and winter. The dynamic gelling and melting temperatures ofnative and alkali treated agars varied among the gracilarioids. The meangelling and melting temperatures of agars were about 39.0 ^°C and86.0 ^°C, respectively. The 3,6-AG content ranged from 29% to38% for native agars and 34–45% for alkali treated agars. While theseresults indicate that this site is suitable for gracilarioid cultivation, occasionallow-oxygen events in St. Helena Bay lead to production of hydrogensulphide in the sea water (`black tides'). Such events killed most inshorebiota (including seaweeds) in 1994 and 1998. This frequency (on average1–2 per decade) and duration (maximum 2 weeks) would have to beconsidered in planning commercial seaweed farming in St. Helena Bay.