Studies on the development of low-cost and stable live food production technologies for tropical aquaculture: a case study of Rotifera (Family: Brachionidae)

Abstract

Live food resources e.g. rotifers, copepods, cladocerans and Artemia are critical for successful fish larviculture. However, the hatchery production of these live foods depends on the availability of high density microalgae, which is unstable, laborious and costly to produce, thus impedes the development of larviculture, especially in most tropical countries. This study developed three low-cost and stable live food production protocols, with specific reference to brachionid rotifers, for mass culture initiatives and, applications in the larviculture sector. Firstly, a study was conducted to investigate the population ecology of a common freshwater rotifer species in Kenya, using individual life table and small batch cultures, to advance the knowledge of rotifer fauna in Kenya (Chapter II). The rotifer cysts were sampled from pond sediments in Kenya and transported to Nagasaki University for further study. The rotifer was morphologically identified as Brachionus angularis, before conducting the ecological studies at 20, 25 and 30°C using fresh Chlorella vulgaris diet at 2.5×105 , 2.5×106 and 2.5×107 cells ml-1. The rotifer was most fecund (2.11±0.07 offspring female -1 day -1) and reproductive (8.43±0.24 offspring female -1) at 25oC with 2.5×106 algal cells ml-1. The highest intrinsic rate of natural increase (0.74±0.02 d-1), specific population growth rate (0.49±0.01), longest life expectancy at hatching (12.41±0.28 d) and shortest generation time (2.87±0.03 d) also occurred at 25oC with 2.5×106 algal cells ml-1 . The duration of hatching to first spawning was shortest (2.86±0.21 h) at 30oC with 2.5×107 algal cells ml-1 and longest (8.83±0.39 h) at 20oC with 2.5×105 algal cells ml-1 . The highest population density (255.7±12.6 ind ml–1) occurred at 25oC with 2.5×106 cells ml-1 on day 8. The Kenyan strain of B. angularis has small lorica size (length: 85.6±3.1 µm; width: 75.4±3.6 µm) and, reproduces optimally at 25oC with 2.5×106 algal cells ml-1; thus suitable for feeding small-mouth freshwater fish larvae. To enhance mass production of this rotifer species, for aquaculture, a chicken manure extract (CME) technique was developed, and its effects on the population growth, mixis induction and body size of the rotifer was determined (Chapter III). Four concentrations of CME (i.e. 0.5, 1.0, 2.0 and 3.0 ml l-1) were added to different glass jars containing 20 ml of sterilized pond water, in which 30 clones of rotifers were cultured at 25oC, and daily fed with 2.5×106 cells ml-1 of C. vulgaris for 7 days without aeration or water exchange. The rotifer’s specific population growth rate and population density increased significantly with 2.0 ml l-1 of CME, without altering the lorica size. ii The mictic response decreased with increasing concentrations of CME. Therefore, 2.0 ml l-1 of CME is optimal for enhancing the mass culture of the rotifer B. angularis. To reduce overdependence on the freshly cultured microalgae for live food production, dried algae have often been used but cases of culture crash are common, thus requiring techniques to stabilize them. In chapter IV, dried Nannochloropsis oculata and Chlorella vulgaris were used to culture the euryhaline rotifer Brachionus rotundiformis (SS-type) with gamma-aminobutyric acid (GABA) supplementation. Firstly, the efficacy of GABA was tested during the lag phase of rotifer growth and every 2 days in small cultures (300 ml). Then, the rotifers were exposed to GABA for 24 and 48 h before up-scaling to 20 l cultures. GABA enhanced rotifer population density and egg/female ratio in both foods compared to the control. Pre-GABA incubation for 48 h caused higher rotifer population densities on days 5 and 6 (with both foods) and 8 and 10 (with C. vulgaris), than their respective controls. In chapter V, a study was conducted to eliminate the expensive microalgae from aquaculture production chain. Here, a protocol was developed for making a low-cost fishwaste diet (FWD) that was used to culture the rotifers B. rotundiformis (SS-type), after several culture failures with B. angularis. About 0.5 g l-1 of fishwaste was wrapped in plankton net (200 µm) and placed in the rotifer culture medium. Then, 0.2 g l-1 of starch (wheat flour) was added into the culture as carbon source. Three diets i.e. FWD1 (fishwastes only), FWD2 (FWD1 + starch) and control (C. vulgaris only) were each triplicated in 30 l polycarbonate tanks containing sea water (22 ppt), in which 20 rotifers ml-1 were stocked and cultured at 28±1oC without aeration for 18 days. The FWD, as food source, was used to determine the population growth, mixis rate and nutritive value of the euryhaline rotifer, B. rotundiformis (SS-type) (Chapter V.1). Half of the culture medium was replaced at every rotifer exponential growth phase and, fresh treatment added. The stability of the cultures was determined by the coefficient of variation (CV) of the mean specific growth rate (SGR). The total lipid content of the rotifers and the microbial flora were also analyzed. The FWD suppressed rotifer mixis rate but favoured parthenogenetic reproduction. FWD2 produced significantly higher rotifer density than FWD1 and control diet, where up to 1,188±69.7 rotifers ml -1 was obtained between 8-13 days with FWD2. There was no significant effect of FWD on the CV (0.08-0.11) of the cultures. About 0.35 and 0.39 mg g-1 of DHA and EPA, respectively was obtained in the FWD-fed rotifers and, both were under detectable limit in the control-rotifers. The DHA/EPA ratios were 2.7, 0.9 and 0.0 for bioflocs, FWD-fed rotifers and control-