10 September 2009 Fig. 1. Crustacean larval rearing system where Mithraculus forceps larvae were raised to juvenile. of 10 larvae/L promotes 90 percent survival to the juvenile stage, while using a stocking density of 40 larvae/L promotes 60 percent survival to the juvenile stage. Based on survival, the lower stocking density appears to be a better choice but the lower stocking density only produces 90 juveniles, while the higher stocking density produces 240 juveniles. The prediction of this and other aspects are fundamental for good management of an aquaculture facility. The knowledge of ornamental species culture has not been utilized in conjunction with models to predict and increase productivity. To reduce the collection of wild specimens, protocols and information on culture productivity should be addressed and be available to the aquaculture industry (Figueiredo and Narciso 2006). Through the use of models, one can predict and maximize production of captive raised animals. Survival to juvenile, larval duration and synchronism of metamorphosis are some of the various aspects that can be predicted using models. Finding out the optimal conditions to raise an animal at each stage of development is not always straight forward because it is almost impossible to test one factor, such as temperature, over its entire range. Instead, researchers have been selecting a few values within a range and choosing as optimum the value that yields higher survivorship and/or growth. Following the example of temperature, a researcher cultures the desired species at four temperatures, within the range of temperatures the species occurs in nature, for instance 21, 24, 27 and 30ºC, compares the results achieved at each temperature, detecting if survival and growth significantly differed between treatments, through an analysis of variance, and finally suggests that producers use the tested temperature that promoted the highest survival and growth, for instance 27º C. An inherent problem arises because the optimum temperature may not be one of the tested values; who can guaranty the optimum temperature was not 28ºC? Instead, researchers should select the best conditions by extrapolating results that could be achieved within the temperature range of 21-30º C through the use of a response curve and use it to estimate the optimum temperature (see sidebars). To exemplify how models can help improve productivity of aquaculture protocols we will use the larval and juvenile stages of the red clinging crab Mithraculus forceps. To construct the models, we will be using data obtained from the literature (Rhyne et al. 2005, Penha-Lopes et al. 2005, Penha-Lopes et al. 2006) where experiments were carried out using a larval rearing system developed and described by Calado et al. (2003). The system uses cylindrico-conical tanks and benefits from upwelling water flow that allows larvae and prey to remain in suspension and the use of screens that permit removal of prey items from the tank without manipulation of the larvae. Salinity of 35 g/L, pH of 8.0-8.2 and a photoperiod of 14L:10D were used in all experiments (Figure 1). Temperature, diet, stocking density and prey density tested during Mithraculus forceps larval and juvenile culture are presented on Table 1. Data obtained for survival and growth during larval and juvenile culture were used in the development of the models. Larval survival to juvenile was modeled with an asymptotic model. For stocking density, Survival to juvenile(%) = Φ 1x(1 – e-e(0.25)x(DPH -8)) (Figure 2), where Φ1=84.28% for 10 larvae.L-1; Φ 1=76.71% for 20 larvae.L-1; Φ 1=63.93% for 40 larvae.L-1; Φ 1=31.83% for 80 larvae.L-1. While for prey density, Survival to juvenile(%) = Φ 1x(1-e-e(0.0041)x(DPH-8)) (Figure 3), where Φ 1=8.54% for 1 nauplii.mL-1; Φ 1=42.61% for 4 nauplii/mL; Φ1=62.96% for 7 nauplii/mL; Φ1=66.09% for 12 nauplii/mL. Response curves were used to find the optimum stocking density and prey density during larval culture. Productivity was calculated by multiplying final survival to juvenile (%) by stocking density (SD) and tank volume (10 L) and the response curve is: productivity = -4.129 + 9.258 x SD – 0.076 x SD (Figure 2), Fig. 2. Effect of stocking density (SD, larvae./L) on larval rearing. Fig. 3. Effect of prey density (PD, nauplii or prey/ml) on larval rearing.
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