International Journal of Aquaculture, 2017, Vol.7, No.17, 112-121
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4 Discussion
The mean temperature in buckets surrounded 34.51 ± 1.78°C during the full period of experiment with a pH value
in the range of 6.87 ± 0.94 and 7.33 ± 0.94. The limit of tolerance of fresh water zooplankton (rotifer for instance:
Brachionus calyciflorus) is in the range of 15 and 31°C. The optimal value of pH is situated between 6 to 8
(Ludwig, 1993). Temperatures recorded during experiment were close to those found by Park et al. (2001) and
Kabir et al. (2010) that fluctuate between 28 and 32°C during fish larvae feeding with great densities of rotifers.
The conductivity was function of the area richness in minerals (NH
4 +
, PO
4 -
, NO
3 -
, NO
2 -
). It was higher than those
obtained by Akodogbo et al. (2015) with pig dung and close to what was reported by Bokossa et al. (2014) with
pig manure. This difference may be explained by the greatest richness of rabbit manure (compared to pig dung) in
mineral salts necessary to primary production that is the base of organisms’ development. These results are
comparable to those obtained by Agadjihouèdé et al. (2011) with poultry manure. Indeed, the use of rabbit manure
had significantly increased nutrients concentration in the water and accordingly enabled a good primary
production for zooplankton development. These results are comparable to those obtained by Dalme et al. (2011)
during zooplankton production process from the use of animal wastes. The effects of rabbit manure are revealed
by the concentration of chlorophyll-a in treatments T
1
, T
2
, T
3
, T
4
and T
5
. Concentration of chlorophyll-a obtained
in these different treatments were higher than those reported by Canovas et al. (1991) that is 2 mg/L as minimal
value for a good phytoplankton production. Thus, treatments T
1
, T
2
, T
3
, T
4
and T
5
are favorable to zooplankton
development. The most important zooplankton concentration was obtained in treatment RM
1500
and could be
explained by its strong load in organic matter due to the strongest concentration of orthophosphate and ammonium.
Indeed, phosphorus in the shape of orthophosphate (assimilable) is the prime element required for vegetal (algae)
development (Reynold, 1980; Schlumberger et al., 2002). In addition, the low transparency value obtained in
treatments T
2
, T
3
, T
4
and T
5
indicate the strong load in algae in treatments these areas particularly in T
3
and T
5
.
This great load could be the source of low density recorded in these treatments. However, oxygen levels recorded
in different doses were slightly higher than those recorded by Agadjihouèdé et al. (2011), in the range 5.5±0.9 and
6.10±1.21. Such difference could be due to different in time of parameters measurement. Highest chlorophyll-a
rates were obtained in doses of T
5
though, the density of zooplankton was low. Reversely, the treatment T
2
provides excellent nutritive conditions to zooplankton.
4.1 Production and specific growth rates of zooplankton
Preferable densities of zooplankton were given by doses in T
2
and T
4
(600 g/m
3
or 0.6 g/L and 1200 g/m
3
or 1.2
g/L). Several studies were based on the relationship between algae biomasses and zooplankton production
(Sendacz et al., 2006; Ekelumu et al., 2011). This relationship could be characterized by optimal concentrations
lower than which zooplankton growth is exponential and upper than which this growth is inhibited (Ovie et al.,
2002; Liady et al., 2015). This matter could explain low productions of zooplankton observed in T
5
and T
3
compared to those observed in T
2
and T
4
. The inhibition of zooplankton growth by high biomasses of algae might
be due to their respiration (especially at night) that provokes zooplankton asphyxia. Indeed, high densities of algae
tend to impact negatively the availability of oxygen for zooplankton that is competed by bacteria that also need it
to deteriorate (mineralize) rabbit manure in order to make nutritive elements available to algae. In addition, the
development of an important algae biomass is not sufficient for good fish farming; the nature of algae species and
their height is to be considered (Lazzaro et al., 1995; Barbe et al., 2010). In the same way, treatments T
2
and T
4
provided the best specific growth rates with a best daily production among the different zooplankton species in
culture areas with an abundance of rotifers. Indeed, this abundance of rotifers is a trump for larval rearing by
considering their small height in relation to the diameter of larvae muzzle. Specific growth rate recorded were
upper than those obtained by Agadjihouèdé et al. (2014) that are 0.74, 0.85 and 0.21 respectively for
Brachiomus
calyciflorus
,
Moina micrura
and
Thermocyclosps sp.
This difference could be explained by the nature of fertilizer
that is function of the quality of the animal diet. It results from this analysis that treatment T
2
(600 g/m
3
) would be
the favorable dose for an optimal plurispecific production of fresh water zooplankton.
