Composting fishwastes as low-cost and stable diet for culturing Brachionus rotundiformis Tschugunoff (Rotifera): Influence on water quality and microbiota

Abstract

The demand for the rotifers Brachionus spp., which is considered indispensable for larviculture, has increased considerably in the tropical countries. Although freshly cultured microalga is the common rotifer diet, the culture protocols are laborious and costly, thus limit sufficient production of rotifers, and consequently disrupt fish seedling production programs in the microalgae-based hatcheries. This study aimed at developing an alternative low-cost and stable fishwaste diet (FWD), for the rotifers. The diets i.e. FWD1 (fishwastes only), FWD2 (FWD1+carbon source) and control (Chlorella vulgaris only) were used to determine the population density, specific growth rate and dietary value of the rotifer Brachionus rotundiformis (SS-type) in 30l tanks of sea water (22ppt), in which 20rotifersml−1 were stocked and semi-continuously cultured for 18days at 28±1°C without aeration. The coefficient of variation (CV) of the mean specific growth rate was calculated to determine the stability of the cultures. The culture medium and rotifer gut were aseptically screened for bacteria, which were counted and reported as colony-forming units (CFUml−1). The rotifers and microflora were harvested and analyzed for total lipids. FWD2 produced significantly higher rotifer density than FWD1 and control diet (p<0.05), but no significant difference was detected between FWD1 and control cultures (p>0.05). Up to 1188±70rotifersml−1 were obtained on day 13day with FWD2. There was no significant effect of FWD on the CV of the cultures, which were 0.11±0.05, 0.07±0.02 and 0.08±0.04 for FWD1, FWD2 and control cultures, respectively (p>0.05). There was significantly higher DO in the control- than in the FWD-tanks, and higher NH3-N in the FWD- than in the control-tanks (p<0.05). There was no significant effect of FWD on the pH in the diets (p>0.05). However, the water quality values were within the acceptable limits for aquaculture. The FWD-rotifers ingested significantly higher CFUml−1 of bacteria (1.02±0.12×107) than the control-rotifers (8.25±2.19×106CFUml−1). About 0.35 and 0.39mg/g of DHA and EPA, respectively was obtained in the FWD2-fed rotifers, and both were under detectable limit in the control-rotifers. The FWD may have contained essential nutrients and probiotics, which explains the higher rotifer population density in the FWD cultures than in the control cultures. This study offers an opportunity to reduce or eliminate the need for the expensive on-site microalgal production, and rotifer enrichment emulsions, toward a more cost-effective aquaculture, especially in the tropical countries.