Effects of stocking density and artificial substrate on the cultivation of the pink shrimp Farfantepenaeus paulensis in enclosures
In order to evaluate the effects of stocking density and the use of artificial substrates on the production of the pink shrimp Farfantepenaeus paulensis grown in pens, two experiments were carried out simultaneously in the Lagoa dos Patos estuary, RS, between the months of January and April 2004, totaling 86 days of cultivation. In experiment 1, 12 fences with 50 m2 of bottom area were used. The enclosures, made with PVC coated polyester netting (5.0 mm mesh and 2.1 m high), were supported by bamboo. In each experimental unit, 25 m2 of vertical substrate (polyethylene mesh with 1.0 mm mesh and green color) was added. Shrimp juveniles with an average weight (± SD) of 0.80 ± 0.28 g were stocked at densities of 10, 20, 40 and 60 shrimp/m2 with 3 repetitions per treatment. A commercial feed was supplied in trays twice a day.Every 14 days the chlorophyll a concentration and the dry weight of the biofilm adhered to the artificial substrate of the different treatments were evaluated. To monitor the growth of the animals, 30 shrimp from each experimental unit were weighed every 14 days. At the end of the cultivation, all the shrimp were removed from the pens to assess the survival, weight and biomass of each experimental unit. In experiment 2, 6 pens were used, similar to those in experiment 1, but with a storage density of 20 shrimp/m2. Two treatments were tested: with artificial substrate for fixing the biofilm (similar to experiment 1) and without substrate. Population of pens, food and monitoring of shrimp growth followed the same methodology used in experiment 1.The analysis of data from experiment 1 was performed with ANOVA followed by the Tukey test (α = 0.05), while in experiment 2 the results were analyzed using the “t test” (α = 0.05). In experiment 1, the concentration of chlorophyll a did not show a clear relationship with the different stocking densities. However, higher values of dry weight of the biofilm were observed in the lower stocking density, probably due to the lower predation of the shrimp on the biofilm in that treatment. Shrimp survival and growth were inversely proportional to stocking density. In experiment 2, the use of artificial substrates did not significantly affect shrimp survival, growth and feed conversion rates. However, in all of them I saw the treatments in experiment 1, between the 58th and 72nd days of cultivation,falls in the dry weight of the biofilm and in the rate of feed conversion were observed, which occurred simultaneously with an increase in the growth rate. This suggests that the shrimp consumed the biofilm in this period. Thus, the results of the present study suggest that although the use of artificial substrates has not resulted in significant improvements in the productivity of F. paulensis grown in pens at a density of 20 shrimp/m2, biofilm has an important role in feeding shrimp. In addition, the results indicate that the cultivation of F. paulensis in pens must be carried out with densities of a maximum of 20 shrimp/m2.