DIETARY pH PREFERENCE IN JUVENILE MARBLE GOBY Oxyeleotris marmorata AND THE POSSIBILITY OF WEANING THEM WITH ACIDIFIED DIETS

Marble goby Oxyeleotris marmorata is a popular freshwater fish species cultured mainly in the Southeast Asian region. However, fish mass mortality usually occurred in the hatchery during the early juvenile stage when the weaning process take place. In our previous study to identify species-specific feeding stimulant from organic acids for this fish, we discovered that O. marmorata showed high ingestion ratio of the acidic agar gel pellets. This finding suggested that dietary pH may play an important role in the food selection of O. marmorata. Therefore, the present study was aimed to identify the preferred dietary pH of O. marmorata juveniles (Exp. 1), and to determine the possibility of weaning them with acidified diets (Exp. 2).

In Exp.1, behavioral test was conducted on the O. marmorata juveniles (50 individuals or replicates; total length 6.6-7.0 cm) using agar gel pellets. Eight pH levels of agar gel pellets were prepared (pH 2.4, 3.0, 3.2, 3.4, 4.1, 4.9, 5.1, and the pure agar gel pellet without pH modification—pH 5.9). The ingestion ratio for each pH treatment of agar gel pellet was calculated, and the binary data (ingested or rejected) was analyzed using binomial test. Negative relationship was found between the ingestion ratio and pH level of the agar gel pellets. The highest ingestion ratio was found in the agar gel pellet with pH 2.4 (ingestion ratio, 94%), following by pH 3 (85%), pH 3.2 (3% agar gel powder; 65%), pH 3.2 (2% agar gel powder; 58%), pH 3.4 (24%), pH 4.1 (6%), pH 4.9 (6%), pH 5.1 (6%), and pH 5.9 (2%). The ingestion ratio of pH 2.4 agar gel pellet was significantly higher (P<0.05) than that in the other treatments except that of pH 3.0. These results confirmed the taste preference of O. marmorata for acidic foods, and the most preferred was the one with pH 2.4-3.0.

In Exp. 2, a 20-day feeding trial was conducted. Five fish meal-based diets were prepared: 4 of them were acidified diets (AD) pH 5.3, 4.3, 3.2, and 2.5 by hydrochloric acid (HCl), and the control diet was at pH 6. Each of these dietary treatments was fed to 10 individuals of wild-caught O. marmorata juveniles (BL 7.7 ˗ 9.3 cm) in separate aquarium (each fish represented a replicate), and the daily ingestion ratio (IR) was calculated (total number of the ingested pellet/ total number of the given pellet - 5). This data was analyzed using the Kruskal - Wallis test and the Conover-Inman post-hoc test. At the end of the feeding trial, the fish within each dietary treatment were categorized into the "Good feeding performance" or "Poor feeding performance" group according to the highest IR (Poor = 0.0, 0.2 or 0.4; Good = 0.6, 0.8 or 1.0) that they ever achieved, and the data was analyzed using Chi-square test. Along the feeding trial, O. marmorata juveniles showed clear increment in the IR of AD 2.5 (the highest IR among all dietary treatments at most of the time), AD 3.2 and AD 4.3. The control diet and AD 5.3 were almost totally rejected by the fish. Although there was no significant difference (P>0.05) among the IR of all diets at day 9, the IR of AD 2.5 (0.46) and AD 3.2 (0.36) was significantly higher (P<0.05) than those of AD 5.3 (0.1) and the control diet (0.02). In addition, 70% of fish fed AD 2.5 achieved the "good feeding performance", and this was the best result among all treatments (AD 3.2, 60%; AD 4.3 50%; AD 5.3 and control, 10%). These results suggested that acidified diets can be used to wean O. marmorata juveniles, and the most recommended pH level was 2.5.