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 three grac ilarioid isolates ( Gracilariopsis sp. from St. Helena Bay, and Gracilaria gracilis isolates from Langebaan Lagoon and Saldanha Bay) were measured to assess the suitability of a site in St. Helena Bay for suspended cultivatio n. The gracilarioids were grown on polypropylene ropes and 'netlon' lines, and the RGRs were 4.0-11.0% d(-1) and 5.0-7.0% d(-1), respectively. The RGR of the Langebaan isolate of G. gracilis grown on ropes was significantly h igher than the RGR of other isolates. 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). The cultured gracilarioids were extracted for native and alkali treated ag ars. The mean native agar yield over the entire period was 39.0% dry wt. Al kali pretreatment reduced the yield by 55%, but significantly increased gel strength. High gel strengths (>750 g cm(-2)) were measured in agars from G racilariopsis sp. and Saldanha Gracilaria gracilis in mid-summer and winter . The dynamic gelling and melting temperatures of native and alkali treated agars varied among the gracilarioids. The mean gelling and melting tempera tures of agars were about 39.0 degreesC and 86.0 degreesC, respectively. Th e 3,6-AG content ranged from 29% to 38% for native agars and 34-45% for alk ali treated agars. While these results indicate that this site is suitable for gracilarioid cultivation, occasional low-oxygen events in St. Helena Ba y lead to production of hydrogen sulphide in the sea water ('black tides'). Such events killed most inshore biota (including seaweeds) in 1994 and 199 8. This frequency (on average 1-2 per decade) and duration (maximum 2 weeks ) would have to be considered in planning commercial seaweed farming in St. Helena Bay.