Cultivation of the red seaweed Gracilaria birdiae (Gracilariales, Rhodophyta) in tropical waters of northeast Brazil

The exploitation of macroalgae of the genus Gracilaria for agar extraction has increased significantly in recent decades. In Brazil, Gracilaria birdiae is one of the species exploited commercially by fishing communities and the excess harvesting of seaweeds of this genus has had negative consequences on the sustainability of this natural resource. Thus, the aim of this study was to gather basic information about the growth rates of G. birdiae required for the development of mariculture in this region. Throughout the study period salinity and temperature remained relatively constant, with a mean of 35 PSU and 28 °C, respectively. Biomass values did not vary significantly between the periods studied (p > 0.05). The mean biomass recorded during the entire experimental period was approximately 1900 g Fw m−1. The highest relative growth rate (RGR) was recorded in the dry season (7.45% d−1) but there was no significant difference between the periods (p > 0.05). Mean RGR for the two periods were 4.4% d−1 and 4.3% d−1 for the rainy and dry seasons, respectively. Positive correlations were found between biomass (r = 0.39; p < 0.001) and RGR (r = 0.41; p < 0.001) and NH4+. Agar yield ranged from 22% to 15%. The highest mean was obtained in the rainy season (∼20%). Gel strength varied between 850 and 650 g cm2. The mean obtained for the two periods was approximately 750 g cm2. These results demonstrate the great potential of G. birdiae for mariculture and its possible economic use by coastal communities.