Effects of the Level and Frequency of Fertilization with hen Droppings on Zooplanktonic Density and Growth Performance of Common Carp Post-Larvae (CyprinusCarpio)

The study took place from March to May 2018 at the IRAD pisciculture Station at Foumban, more precisely at the Koupa-matapit fish farm (5º21 'to 5°58' LN: 10º17 'to 11º02' to 10°48.82‘ LE) and an altitude of 1147m in the Western Region of Cameroon. The climate is of the Sudano-Guinean-type and includes a rainy season (March-October) and a Dry season (November-February). The average values ​​of the temperature and the rainfall recorded annually respectively are 22°C and 1800 mm ¹².

The test was carried out in 12 identical concrete tanks, 1.2 m long, 0.75 m wide and 1 m high each. These tanks were covered with plastic material in order to keep the temperature of the ambient environment constant. Two weeks before the start of the test, the tanks were washed and disinfected with bleach and then left to dry for 3 days. In addition, each tank received three days after draining a volume of 400 liters of drilling water and 21 grams of quicklime.

The droppings of laying hens (analyzed composition: 19.24% of crude cellulose; 0.87% of nitrogen and 0.61% of phosphorus) were used.

A concentrated sample of zooplankton was collected between 6 and 7 a.m in a Tilapia pond at the Station using a 40 µm mesh trawl pulled from one side of the pond to the other. This zooplankton sample was homogeneously distributed in 13 test tubes of 25 ml each. 1200 post-larvae of Cyprinus carpio were caught in the nursery tanks of the Koupa-Matapit Station just after absorption of the yolk sac. These post-larvae came from a semi-artificial reproduction according to the model described by ¹².

One week after liming, each of the tanks was fertilized randomly and in three repetitions at one of the following doses of droppings: D450-F1 (fertilization at 450 g/m³ applied every 2 two weeks), D450-F2 (fertilization at 450 g/m³ at weekly frequency), D600-F1 (fertilization at 600 g/m³ at biweekly frequency) and D600-F2 (fertilization 450 g/m³ weekly). Three days after this fertilization the tanks were sown with phytoplankton (food of the future zooplankton). For this purpose, 100 liters of water from a tilapia storage pond were withdrawn, filtered to 50µm of meshes in order to let only the phytoplankton pass, then added in each tank. Three days later, a sample of zooplankton was collected between 6 and 7 a.m in a Tilapia pond from the same Station. As described above, this zooplankton sample was concentrated and distributed homogeneously in 13 test tubes of 25 ml each. The 12 tanks received each the contents of one tube. The content of the 13th tube was fixed with 5% formalin and then immediately sent to the IRAD aquaculture laboratory at Foumban for a qualitative and quantitative inventory of the zooplankton under the MOTIC type optical microscope at the 10X objective ⁹. It emerged that the zooplankton was inseminated in the tanks at a density of 7 ind/l (that is 1; 2 and 4 ind/l of rotifers, cladocerans and copepods, respectively). 12 days after fertilization, each 1 m² concrete tank received 100 Cyprinus carpio post larvae aging 5 days.

Zooplankton production was assessed every three days during the 30 days of the trial following the protocol described by several authors ^{10, 11}. For this, every three days, between 6 and 8 a.m., 20 liters of water were withdrawn from each tank using a one-liter container (therefore 15 sampling points on the edges and 05 points in the middle of the tank). This water was filtered through a 50µm zooplankton screen to retain the zooplankton which was then stored in a 25 ml test tube then fixed with 5% formalin and sent to the IRAD aquaculture laboratory at Foumban for observation according to the protocol described by ¹¹. To follow the evolution of the zootechnical performances of common carp larvae, for 30 days, control fisheries were carried out every two weeks, within at least 10% of the individuals in each tank were sampled and the lengths (total and standard) were measured using a millimeter ichtyometer. The weight was measured using a 0.05g precision electric balance.

The physico-chemical characteristics of the water were measured at the start of the test and then weekly between 6 and 8 a.m such as the pH, the dissolved oxygen, the temperature, the conductivity, the transparency and the depth of the water have been relieved in situ. On the same dates, a water sample was collected for the determination of nutrient salts, namely: nitrates, nitrites, ammoniacal nitrogen and phosphates. All these physico-chemical characteristics are within the acceptable range for zooplanktonic production the best growth of Cyprinuscarpio.

At the end of trial, the live weight, the total and standard lengths as well as the number of zooplankton made it feasible to evaluate the following growth characteristics:

Zooplankton density (D): With: n = number of individuals counted; v₁ = volume of the subsample taken; v₂ = volume of the concentrated sample; v₃ = total volume of filtered water); Weight gain (WG) = final weight - initial weight; Average daily gain (ADG) = (final weight - initial weight) / (time (day)); Specific growth rate (SGR) = ((ln final weight - ln initial weight)*100) / (day time); Condition factor K = 100*(Weight / (Total length)³) and Survival rate = (Number of initial fish - final number of fish)*100 / (Number of initial fish);

The data were submitted to the one-way analysis of variance (ANOVA 1). When the effect of the dose associated with frequency was significant, the Duncan test was used to separate the means clustering at 5% threshold. The regressions tests were done using Excel 2007. All analyzes were Performed using the SPSS software version 21.0.

The evolution over time of the average density of zooplankton according to the dose of chicken droppings and the frequency of fertilization in carp nursery tanks is illustrated in Figure 1. It shows that independently of the dose of droppings and the frequency of fertilization the evolution of the density of zooplankton follows the same profile, trend and pace. Thus, we observe rapid growth from the insemination of zooplankton between day 6 and day 12, followed by an abrupt drop of the density of zooplankton three days after fish stocking (15th day), then occurs a slow and regular growth phase.

Figure 1.Evolution of the average density (ind/l) of zooplankton according to the dose and the frequency of fertilization with hen droppings in nursery

The total density of the different groups of zooplankton is summarized in Table 1. It appears that with the exception of the copepods whose highest significant values ​​were obtained with the dose 600 g/m³ applied every 2 weeks, all the other groups presented the highest density values ​​with the dose 600 g/m³ applied each week

The influence of the level and frequency of fertilization with hen droppings on the survival rate of Cyprinus carpio post-larvae is illustrated in Figure 2. It follows that the survival rate was not significantly influenced (p <0.05) by the level and frequency of fertilization.

Figure 2.Survival rate according to the level and frequency of fertilization with hen droppings

The influence of the dose of hen droppings and the frequency of fertilization on the growth and survival of the Cyprinus carpio post-larvae is presented in Table 2. It appears that the tanks having received 600 g/m³ each 2 weeks presented the highest significant values ​​of bodyweight, weight gain, average daily gain and condition factor K.

It appears from Table 3 presenting the correlations between density of zooplankton groups and growth characteristics that the zooplankton density of the medium did not significantly influence the zootechnical characteristics of common carp post-larvae, except for copepods and cladocerans which were significantly negatively and very strongly correlated (P <0.01) with the specific growth rate of post-larvae from tanks receiving 450 g/m³ and 600 g/m³ every two months and which were significantly positively and strongly correlated (P <0.05) with the survival rate of the post-larvae from the fertilized tanks at 600 g/m³.

During this test, the evolution of the zooplankton density showed rapid growth from the insemination of the zooplankton until the introduction of the fish post-larvae, which corresponds to the usual scheme described by ⁹ and ¹¹. It then dropped rapidly from day 15 before stabilizing from day 21. The drop in zooplankton density on the 15th day would no doubt be due to the predation action exerted by the newly introduced post-larvae (at 12th day) on them.

During this test, the total length (LT) values ​​ranged from 29.10 and 36.20 mm. These values ​​are comparable to the respective values ​​of 30 and 25 mm reported by ¹⁴ in Clariidae and ¹⁴ by in post-larvae of Cirrhiusmrigala (20 days old). They nevertheless remain well below the values ​​(44-50 mm) reported by ¹⁵ in post-larvae of Labeobarbusbatesii aged 45 days. The differences in this result would be explained not only by the species used but also by the more intermediate duration of the nursery which was 30 days in this test.

The values ​​obtained during this work included between 420-620 mg; 180-431 mg and 12-29 mg/day respectively for bodyweight, weight gain and average daily gain were very low compared to those obtained after 45 days of nursery by ¹⁵ in the Labeobarbusbatesii post-larvae (675-1275 mg; 525-1126 mg and 12-26 mg/day respectively) feed on different types of food. The same observations were made by ¹⁶ in ornamental fish Puntusvittatus (370- 840 mg and 5 - 11.2 mg/day respectively for weight gain and average daily gain) after 75 days of nursery. These values ​​are also much lower than those reported by ¹⁷ in Cyprinus carpiofingerlings (7680- 13410 mg; 170-300 mg/day respectively for weight gain and average daily gain) after 45 days of breeding and by ¹⁹ in Osteochilusvittatusjuveniles of (13740-20690 mg and 2560-3110 mg/day) for the same parameters. The same is true for the work of ¹⁹ in the Aspikutum hybrid with respective values ​​of weight gain and average daily gain of 5190-8620 mg and 280- 460 mg/day. These differences could be explained by the species used, the duration of production and the stage of breeding.

The specific growth rate between 3.6 and 10.85% obtained during this work is high compared to that (1.4-2.04%) reported by ²⁰ with the post-larvae of the keurelian fish Tor tamba, as well as that (3.33-4.72%) obtained by ¹⁵ in post-larvae of the African carp Labeobarbusbatesii. The same trend was observed by ²¹ in post-larvae of Pethiareval (1.3-1.49%). These differences are essentially linked to the genetic determinism of the species.The values ​​of the condition factor K comprised during this test between 0.92 and 1.24 were comparable to the values ​​(0.89-1.096) obtained by ²² in the wild milieu with Labeobarbusbatesii. This value remains relatively high compared to that (0.73-0.77) obtained with the Aspikutum hybrid by ¹⁹. This factor also remains very low compared to the values ​​of 0.87-3.14 obtained by ²³ with Garraruffa. The low values ​​of K observed could be explained by the high density used in this test, which was 100 individuals/m². In short, regardless of the level and frequency of fertilization, the fish showed good nutritional and health status since almost all K values ​​remained greater than 1 (K≥1).

The values ​​of this parameter during the test varied significantly between 29.6 and 37%. This result is lower than that observed with Cirrhiusmrigala by ¹, which reports survival rates varying from 30-50% during the first 20 days of nursery and 60-70% during the 2-3 months of pre -grossing in this species. However, it remains very low compared to those (79-92% and 78-93%) obtained with the fry ofPoecilia reticulata and Pethiareval, respectively by ²⁴ and ²¹. This variation could be explained by the difference in development stage considered and therefore due to the fragility and difficulty of nutrition of the post-larva used in breeding.