Grass carp are a herbivorous species, so using màng tang leaves in their feed promises high efficacy. Therefore, this experimental evaluation is very necessary to propose effective and environmentally friendly disease prevention and treatment measures.
Introduction
Grass carp (Ctenopharyngodon idella) is one of the traditional aquaculture species, especially in the northern provinces. The fish meat is delicious, and in many areas, grass carp are stocked at very high densities (Dongmeza et al., 2009, Steinbronn, 2009). However, grass carp frequently suffer from bacterial diseases, among which *Aeromonas hydrophyla* is considered the main causative agent (Mayrhofer et al., 2010). Fish can die sporadically until complete mortality, causing losses for farmers.
To date, the use of therapeutic drugs for fish seems ineffective; while the use of antibiotics for prevention and treatment of diseases in grass carp can pose significant risks to the ecological environment, lead to drug resistance, and directly affect human health. Therefore, the use of herbal products is considered effective and environmentally friendly (Tuan et al., 2017).
Màng tang is a plant with high pharmacological activity, capable of effectively inhibiting pathogenic bacteria in aquaculture such as Aeromonas sp., Streptococcus sp., Edwardsiella sp (Nguyen et al., 2016). This study showed that common carp supplemented with màng tang leaf powder had significantly higher immunity compared to the control. Meanwhile, grass carp are herbivorous, so using màng tang leaves in their feed promises high efficacy. Therefore, this experimental evaluation is very necessary to propose effective and environmentally friendly disease prevention and treatment measures.
Màng tang plant
Experimental Methods
Preparation of experimental feed
Màng tang and banana leaves were collected from nature in Ha Giang province, washed, and dried before being ground into powder for feed mixing. Commercial feed (28% protein and 8% lipid) was used as the basal diet for the experiment. The basal diet was ground, supplemented with 2% cassava flour and varying proportions of banana leaf powder or màng tang leaf powder. The experiment was conducted with 3 replicates and 5 feed formulations as follows: CT1 (control) was commercial feed (30% protein and 8% lipid) supplemented with 2% cassava flour for binding, CT2: control feed + 8% banana leaf powder, CT3: control feed + 8% màng tang leaf powder, CT4: control feed + 16% banana leaf powder, and CT5: control feed + 16% màng tang leaf powder.
Growth experiment setup
Grass carp with an average size of ~10 g/fish were acclimated for 10 days before the experiment began. A total of 300 fish were evenly distributed in a recirculating system of 15 tanks (80 liters/tank). Fish were fed to apparent satiation 3 times/day. Environmental parameters were always maintained at optimal levels for the fish. Fish were weighed before the experiment and re-weighed weekly to monitor growth. Growth performance indicators such as specific growth rate (SGR), average daily gain (ADG), and feed conversion ratio (FCR) were determined according to the method (Tuan, 2010). After the growth phase, fish were subsequently challenged by injection with *Aeromonas hydrophyla* bacteria.
Challenge experiment with A.hydrophyla bacteria in fish
*A. hydrophila* bacteria were isolated from diseased fish at the Faculty of Aquaculture - Vietnam National University of Agriculture. The 50% lethal concentration was used to challenge fish after 45 days of feeding with different diets. Fish mortality was monitored thereafter, and pathological samples were isolated to reconfirm the cause of death. Data were calculated as mean ± standard deviation of the mean. Statistical differences were determined by ANOVA analysis with Tukey's test (P<0.05) using Minitab-16 software.
Results
Throughout the growth experiment, fish showed no abnormal signs and no mortality. Fish in CT3 showed the highest average daily growth, reaching 0.52 ± 0.17 g/day (Table 1). However, this result was not statistically different from the results obtained from CT4 and CT5 but was significantly higher than CT1 and CT2.
SGR growth results showed that all formulations supplemented with plant leaves were statistically higher than the commercial feed (CT1). Among the formulations supplemented with plant leaves, the highest SGR value was achieved in CT5 (0.31 ± 0.08 % day-1) and the lowest in CT3 (0.26 ± 0.06 % day-1). However, this difference among the feed formulations was not statistically significant. This trend was also similar to the FCR results after the experiment. FCR values ranged from 2.29 ± 0.11 (CT1) to 2.39 ± 0.28 (CT5), where formulations CT4 and CT5 in the experiment (supplemented with 16% plant leaves) showed significantly higher FCR results than formulations CT1 and CT2 (P<0.05).
The SGR results in this experiment were lower than those reported in the previous study by Nguyen et al. (2016) on common carp. However, it is possible that the higher proportion of plant leaf powder supplemented in the feed in this experiment led to lower growth results. Although grass carp are herbivorous, the dried plant leaves mixed into the feed can significantly affect the fish's digestibility. In practice, when grass carp are fed fresh grass and leaves, they show quite good growth performance. However, using dried grass results in much lower growth, even negative growth, as observed in Dongmeza's (2009) experiment, despite the nutritional composition of the leaves remaining unchanged when freeze-dried.
The FCR results of this study, although higher than previous studies (Tuan, 2010), were lower than the results published for common carp by Nguyen et al., 2016 and for grass carp by Dongmeza (2009). This could be due to the higher proportion of plant leaves in the feed used in this experiment compared to common carp feed, leading to high cellulose content and affecting digestibility. However, compared to the results for the same species by Dongmeza (2009), these results are significantly better.
Monitoring results of fish mortality after challenge showed that fish deaths primarily occurred within 3 days post-injection; this phenomenon gradually decreased, and after day 9, no further fish mortality was observed in any of the formulations. Pathological isolation results after challenge also showed a strong proliferation of the pathogenic bacterium *A. hydrophyla* in dead fish. The mortality rate in the control formulation was the highest, reaching 75%, similar to CT2. However, both of these formulations showed statistically higher mortality rates (74.9 ± 2.3 and 71.5 ± 3.1% for CT1 and CT2, respectively) compared to the other formulations supplemented with màng tang leaves (P<0.05). The lowest mortality rate was 46.6 ± 3.1%, recorded in CT5 (Table 1).
These results are quite similar to those previously published by Nguyen et al. (2016) on common carp. However, the study by Nguyen et al. (2016) showed that common carp did not grow very well when supplemented with 16% màng tang leaf powder. Nevertheless, that experiment also demonstrated that supplementing with 8-16% màng tang leaf powder could improve the antibacterial resistance of common carp.
Conclusion
The results show that màng tang has high antibacterial activity against *A. hydrophyla* bacteria. Supplementing màng tang leaf powder into fish feed does not affect fish growth; in fact, it increases fish growth rate and improves immunity against *A. hydrophyla* bacteria.
Source: Vietnam Aquaculture
