Standing Stock, Agar Yield and Properties of Gracilaria salicornia Harvested along the Tanzanian Coast

Abstract

Seasonal biomass variation and agar yield of G. salicornia (C. Ag.) Dawson from Oyster Bay, Dar es Salaam, and Chwaka Bay, Zanzibar, were determined, and some properties of the agar examined. Mean biomass and canopy cover values ranged from 21-60 g/m2 and 7- 20 %, respectively. The highest mean biomass and cover values were obtained during the SE monsoon. Agar yield varied from 13.7 to 30.2 % (dry weight) and was highest during the dry NE monsoon period. Alkali treatment generally reduced agar yield by 25-56 %. Gel strength of the agar gels ranged between 118 and 251 g/cm2 and was significantly higher during the NE monsoon period (t =2.2; P =0.04 and t = 8.9; P« 0.05) for samples collected from Oyster Bay and Chwaka Bay respectively). Mean gel strengths of native agar samples (205 :t 45 g/cm2) was about 42 % of that of the standard agar used. The 3,6-anhydrogalactose content was highest during the rainy season (37.4-44.3 %) and tended to coincide with low gel strengths. Sulphate content varied between 0.5 and 2.8 % in both populations. It was concluded that the best period for harvesting G. salicornia for agar production in Tanzania is the dry (NE monsoon) period. INTRODUCTION Gracilaria is one of the algal genera from which agar is extracted commercially (Lewis et aI., 1990). and the main source of agar in the world (Oliveira et aI., 2000). The algae are harvested by hand from natural populations in Spain, Portugal, Morocco, the Azores, California, Mexico, New Zealand, South Africa, India, Chile and Japan (Annisen, 1995). Currently, the commercial mariculture of agar-producing seaweeds is led by the production of Gracilaria chilensis Bird, McLachlan et Oliveira in Chile (Buschmann & Correa, 2001). The seaweed has also been cultivated in China, Taiwan, Namibia, Venezuela, Mexico (Armisen, 1995), Indonesia and Vietnam (Oliveira et aI., 2000). Seven species of Graci/aria including G. cornea J. Agardh (previously known as G. crass a Harvey, Buriyo et aI., in press), G. corticata 1. Agardh, G. arcuata Zanardini. G. sa/icornia, G. Corresponding author: AKK. E-mail: E-mail:akivaisi@amu.udsm.ac.tz edulis (1. Ag.) Silva, G. fergusoni 1. Agar