**1. Introduction**

240 Aquaculture

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In nature, zooplankton is the main nutritional source of poslarvae and young fish. The natural food offers essential nutrients to guarantee the survival and the growth of fish during their first development stages (Furuya et al. 1999). The description of feed value of living food, has been made by Watanabeetet al., (1998); Kraul, (2006). Living food has a vital job on seed production in fish farms. Without this living food, it is not possible to overcome an adequate survival rate, in species exclusively dependent (Kubitza, 1997; Lahnsteiner et al., 2009).

Micro crustaceans are highly important in aquaculture, mainly the freshwater genera *Moina* and *Daphnia spp,* these two are found in diverse natural environments (FAO, 1996)*.Daphnia* genera includes *D. magna, D. pulex, D. longispina* among others. In crops of freshwater spices, poslarvae are fed with 2 or 3 organisms during the beginning of their first hexogen feeding, during their first 10 to 30 days (Lubzens & Zmora, (2003), as cited in Stottrup & Mc Evoy, 2003; Botero, 2004; Prieto, 2006). It is evident the importance of *D. magna* as live food. Authors such as Emmens, (1984), have been reported that *Daphnia spp.* is the best foodstuff for tropical fishes, frequently used food source in the freshwater larviculture (i.e. for different carp species) and in the ornamental fish industry (i.e. guppies, sword tails, black mollies and plattys etc.) (Delbare & Dhert, as cited in Lavens & Sorgeloos, 1996), native and foreign species, for example White Cachama *(Piaractusbrachypomus),* Black Cachama*, (Colosoma macropomum),* Bocachico *(Prochilodus magdalenae),* Yamú *(Brycon amazonicus),*  Sabaleta *(Brycon henni),* Dorada *(Brycon moreii),* Striped Bagre *(Pseudoplatystoma fasciatum),*  Pacu *(Piaractus mesopotamicus),* among others (Botero; 2004; Prieto, 2008).

It is known of certain difficulties on live food production, due to the fluctuations on the natural conditions of aquatic environments and the elevated infrastructure requirements, equipment, maintenance spending and working labor. It is not possible to produce a constant amount of live food in a regular basis (Kanasawa, 2000). On the other hand the laboratory research to develop culture techniques *of Daphnia magna* has been widely studied, this is because it is easy to cultivate and has a low cost in high densities. Additionally their maintenance in a small space makes them an economically viable alternative culture (Terra, et al., 2010).

The secondary production in lakes is supported by zooplankton, zoobenthos and fish; this means that this group is diverse from the taxonomic and functional point of view. On scale work of authors such as Stotz and Pérez (1992) and Andrade et al., (2009), emphasizes the necessity to recognize the production of a secondary source to determine variables such as maximum extraction. In fact secondary extraction is considered as one of the most important parameters to evaluate the population utilization sustainability (Andrade et al., 2009).This section presents production secondary variables, following the method designed by González, (1988) that is specific for the cladocerans, showing the results obtained at the laboratory scale.

This chapter presents an experimental evaluation of two *Daphnia magna* populations, the first population integrated by neonates and the other by adults in early reproductive stage under stress conditions. This stress condition on the test was made by using 3 cm³ multicells, on each treatment, under controlled conditions of room temperature (21 – 25 C), water temperature (22 – 23 C) and pH (7.6). The diet used was *Saccharomyces cereviseae*, potatoes (*Solanum tuberosum*) and a fatty acid enriched environment n-6 (soy oatmeal). The diet and enrichment concentrations were 30 ppm and 15 ppm, factorial arrangement of 2³, in concentrations of 15 and 30 ppm mixture of nutrients: yeast and potato and the same concentration for enrichment. Four replicas/treatments were made (32). The Feeding was on a daily basis for 20 days to determine the population performance effect. The productive variables were evaluated: maximum density (Dmáx) daily average density (Dmd), doubling time (Td),specific growth rate (k), performance (r) , numeric growth (PN), birth rate (b), (Edmodson equation), individuals average number (N�), biomass productivity (Pw), mortality rates (d), biomass (B), production rate (I de P) and final weight. Reproductive variables were: egg number/female (HPP), neonates number/female (NPP), egg maturity time (tm), first reproduction age (EPR), litter number (NC), reproduction frequency (FR), net reproduction rate (Ro) and generation time (Tc).

There were significant differences (p<0.05) on T2 from the population of adults, with concentrations of 15 ppm *S. cereviceae*, potato 15 ppm, soy oatmeal 30 ppm, with the highest specific growth, 0.50 ± 0.05 per day, less doubling time with 1.39 ± 0.14 days and the highest mortality rate with 0.49 ±0.07 per day. In the rest of the treatments there were no significant differences (p>0.05). There was evidence that the highest nutrient combinations strengthened the population growth in both adults and adults in juvenile reproductive stage. They reached in T6 with concentrations of 30 ppm *S. cereviceae*, potato 15 ppm, soy oatmeal 30 ppm, and 8.25 ± 1.70 and 15.0 ± 9.76 *Daphnias*/mL for each population respectively. Likewise in T6, was observed a higher value on egg number/female, 3.83 ± 0.82 and 3.55 ± 0.98, for each population respectively. *D. magna* presents a favorable adaptation under stress conditions, turning it into an excellent alternative for the living food for poslarvae production, with minimum infrastructure.

In order to use the *S. cereviseae* probiotic as an alternative of real control strategy, a meticulous evaluation must happen, evaluating their competing and functionality on the living food utilized with the different poslarvae species and their environments. It is imperative prerequisite to develop pathogenicity studies not only with the living food (*D. magna*) but also the selected probiotic from any commercial consideration; this means is necessary to explore this matter deeply (Austin & Brunt, 2009, as cited in Montet & Ray, 2009).
