Ectoparasites are key agents responsible for causing diseases in farmed fish. The Nile tilapia, in particular, has been shown to be susceptible to infestation by trichodinids and monogeneans. These agents are the main parasites that impact super-intensive tilapia culture, although other ectoparasites may also be involved, such as Epistylis, Apiosoma, Chilodonella, Ichthyophthirius, Ichthyobodo, Cryptobia and Piscinoodinium.
Ectoparasites are traditionally responsible for causing lesions on the skin and gill, which facilitate the access of opportunistic bacterial infectious agents such as Streptococcus, Aeromonas, Pseudomonas, Flavobacterium, Acinetobacter and Francisella. These bacterial agents act as the main causes of mortality in the different stages of tilapia rearing. For greater efficiency in the control of these bacterial diseases, the control of ectoparasite infestations (Pádua and Cruz, 2014) is fundamental.
The incidence of parasitic infections in farmed fish is strongly related to the environmental conditions and water temperature. Poor water quality is often detected during the masculinisation of tilapia in hapas, since the very fine mesh size of hapas favours the rapid clogging from a proliferation of periphyton. Subsequently, this drastically reduces water movement and water renewal. The use of powdered feed during this rearing phase also promotes the proliferation of periphyton besides a loss of nutrients into the water column. Under these conditions, high infection rates of ectoparasites in fry and fingerlings occur, leading to large losses and mortality of animals.
In this context, the aim of this present study was to evaluate the effect of a natural nutritional additive (Aquate FishTM, Alltech, USA) on the incidence of parasites and its effect on the productive performance of Nile tilapias during masculinisation (sex reversal).
Study design and methodology
The test was performed in the hatchery of New Fish, Alterosa municipality, State of Minas Gerais, Brazil. In this study, 42,000 larvae of Nile tilapia were used after absorption of the yolk sac. Fish were stocked in 6 hapas with 1 mm mesh installed in a single earthen tank that was in continuous use throughout the 2013/2014 harvest. As such it contained remnants of fish escapees from hapas from previous lots. The test comprised two groups, G1: test group containing 6 g of Aquate Fish/kg of dry feed (Supra®) + masculinising hormone; and G2: control group which received the same diet + masculinising hormone, but omitting the nutritional additive. These animals were maintained in this system for 12 days until the first hapa exchange, in which they were distributed into 12 hapas (1 mm mesh) for a further 18 days.
Diagnosis of parasites
Parasitological analyses were performed in situ on days 0, 10, 20 and 30, by a random capture of fish and assessment of fresh shaved skin, fins and gills using optical microscopy. In each evaluation, 60 animals were analysed per treatment, while 42 animals were analysed in the baseline collection. A total of 402 fish were subjected to diagnostic tests. The diagnosis of the parasite prevalence rate was calculated as proposed by Bush et al. (1997), and then unranked in the Parasite Intensity Score or PIS (working methodology established by Aquivet Saúde Aquática).
Every 10 days 100 larvae/hapa were captured and biometry performed. The weighing included the evaluation of a pool of 10 larvae/fry, using a scale of 0.001g accuracy. Total length was performed using a digital pachymeter. For productive performance the following parameters were evaluated:
- Weight Gain, where WG = (final body weight - initial body weight)
- Specific Growth Rate, where % SGR=100 [(ln final body weight - ln initial body weight) /experimental days]
- Survival Rate, where %SR = (final number of fish / initial number of fish) *100
Data were subjected to analysis of variance (ANOVA) and when significant, the means were compared by the Student t-test (0.05).
Diagnosis of ectoparasites
Table 1 presents data related diagnoses of ectoparasites in Nile tilapia during the masculinisation phase. The use of nutritional additive showed a positive effect on the control of some ectoparasites, such as reduction of (p < 0.05) the prevalence rate of Trichodina compacta and Apiosoma sp., both ciliated parasites, and reduction (p < 0.05) of the PIS (Parasite Intensity Score) for the group of trichodinids.
Figure 1: Layout of experimental hapas and study design
Table 1: Effect of nutritional additive on the incidence of ectoparasites in Nile tilapia during masculinisation.
Table 2: Production performance of Nile tilapia fed nutritional additive during masculinisation in hapas.
In the assessment on the incidence of ectoparasites among different days of collection, it was observed that treatment diets provided specific responses in the control of Trichodinids, Monogeneans, Piscinoodinium pillulare, Apiosoma sp. and Ichthyobodo sp. (Figure 2).
Figure 2. Effect of nutritional additive on the incidence of ectoparasites on Nile tilapia during masculinisation. The presence of an asterisk indicates statistical difference by Student’s t test (p < 0.05).
The addition of the nutritional additive during the masculinisation of Nile tilapia did not significantly influence weight gain or specific growth rate among the tested groups (Table 2). However, a significant increase (p < 0.05) in the survival rate of fish fed with the additive (Figure 3) was observed.
Figure 3. Effect on the survival rate of Nile tilapia during masculinisation in hapas. The presence of an asterisk indicates statistical difference by Student’s t test (p < 0.05).
Optimisation of production
The use of the nutritional additive as a food additive resulted in a decreased PIS of trichodinids, as well as a decrease in the prevalence rate of the group of monogenean worms, P. pillulare and Apiosoma with 20 days of feeding. With 10 days after the initiation of a diet containing the additive, lower prevalence rates of P. pillulare and Ichthyobodo sp. were observed. During the initial phase of rearing Nile tilapia, these ectoparasites are responsible for lesions on the skin and gills, which in turn favours the occurrence of secondary infection by opportunistic bacteria (Valladão et al., 2014). Therefore, the decrease in ectoparasitic infestation has optimised productive performance, obtaining higher survival rates, as observed in this study in which fish fed diet with diet G1 showed an increase of 12.3% in the survival rate.
Decrease in parasitism
The incidence of ectoparasites on farmed fish is strongly related to the rearing conditions, climatic aspects and environmental quality (Hossain et al., 2008; Jerônimo, 2010; 2011). In the present study, there was a decrease in the PIS for trichodinids, as well as the prevalence of P. pillulare and Ichthyobodo for treated and control groups on the 30th day of analysis which coincided with a period of an abrupt drop in water temperature. This decrease on ectoparasites incidence is related to ecological aspects and seasonality during the cold season. However, in the period with water temperature ranging between 25-20°C when there was increased pressure by parasites, differences in the incidence of ectoparasites were observed among the groups with and without the nutritional additive.
In this study, no significant difference (p > 0.05) was observed in the productive performance of fish fed with G1 diets compared with the control group. On the other hand, the group fed with the additive showed a higher survival rate (p < 0.05), which has resulted in increased stocking density obtained in hapas with 486 fish/m2, while the control group showed a mean of 378 fish/m2. We showed that despite a difference in the final stocking density (it was 28.6% higher in fish fed with diet G1), both groups showed equivalent productive performance. Currently, it is known that the stocking density represents a key parameter on the productive aspects of tilapia breeding, since the higher stocking density generally yields lower productive performance (Garcia et al., 2013).
The addition of 6g Aquate Fish/kg of diet resulted in lower parasitism of tilapias during the masculinisation process in hapas, optimising the survival rate.
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This article first appeared in Aquaculture Asia Pacific magazine, September/October 2014, Volume 10 Number 5.