Compensatory growth in white shrimp from the Pacific Litopenaeus vannamei (Boone, 1931), cultivated at different temperatures under feed restriction in a biofloc system
Compensatory growth, which is defined as a physiological process where the organism goes through a phase of rapid growth after a period of restricted development, is a strategy developed by some species due to frequent stressful situations in the environment, with an allocation of resources between maintenance, growth and reproduction. In the context of aquaculture, the use of food restriction and low temperature cultivation as a trigger for compensatory growth can be considered strategies for reducing feed supply, costs and for increasing the period of cultivation of tropical species in regions where cultivation is restricted to summer only. In addition, the BFT (Biofloc Technology System) system can also bring advantages to cultivation, where, in addition to reducing water use and greater biosafety, the microbial community can serve as a supplemental food source for organisms 24 hours a day, contributing also for the reduction of food offered to the system. In this context, the objective of this study was to evaluate the occurrence of compensatory growth in Litopenaeus vannamei shrimp, at three different temperatures (20, 24 and 28ºC) and under food restriction, cultivated in a biofloc system, and to analyze the dynamics of energy reserves in the hepatopancreas and the immunological condition of the organisms throughout the experimental period. The experiment was carried out for 64 days and divided into two stages: (1) Restriction and (2) Recovery, which lasted 36 and 28 days, respectively. Initially, juveniles of L. vannamei were stocked with an average weight of 1.78 g (±0.38 g), at a density of 300 shrimp/m2 (n=54). In the first phase, the experiment had a factorial experimental design (three temperatures and two diets), totaling six treatments, which were carried out in triplicate. Three temperatures were chosen as optimal (28ºC), intermediate (24ºC) and low (20ºC) and, for each temperature, two feeding regimes were applied: control, where the organisms received 100% of the previously calculated feeding rate; and restriction, in which the feeding rate offered was 40% in relation to the control treatments. At the end of the first phase (day 36), all experimental units (n=18) were exposed to favorable cultivation conditions (28ºC and 100% feed rate) for 28 days. To determine the concentration of energy reserves (total proteins, glycogen and triglycerides) in the hepatopancreas and to assess the health status of the animals, based on the differential hemocyte count (DHC), three animals were collected per experimental unit at the beginning of the experiment. (day 0), at the end of the restriction phase (day 36) and at the end of the recovery phase (day 64). At the end of the first phase, the animals subjected to feed restriction and maintained at 24 and 28ºC had significantly lower weight compared to their respective controls (P < 0.05), while the shrimp maintained at 20ºC (control and restriction) had weights similar to each other (P > 0.05). In addition, the treatments at different temperatures and without feed restriction, had significantly different weights, where the treatment kept at 20ºC had a lower growth rate, followed by the treatments at 24 and 28ºC, respectively. The concentration of total proteins in the hepatopancreas was not affected in any treatment (P > 0.05), while triglyceride levels were affected in the restricted treatments maintained at 24 and 28ºC (P < 0.05), but not at 20oC (P > 0.05). The glycogen concentration was lower and all treatments that suffered food restriction compared to controls (P < 0.05) and showed an effect of temperature, where lower temperatures were more affected by restriction. The CDH showed differences between treatments (P<0.05), but all values were within the expected range for healthy animals cultivated in bioflocs. After the end of the recovery period (day 64), all treatments that had been restricted in the first phase had similar weights to their respective controls (P > 0.05). Regarding temperature, animals kept at 20 and 24ºC during the first phase and fed ad libitum throughout the experiment did not reach the weight of shrimp maintained at 28ºC during the entire experimental period and without food restriction (P < 0.05). The concentration of total proteins did not change in any treatment (P > 0.05) and the triglyceride stocks were recovered in the treatments kept in the first phase under restriction at 20 and 24°C, but not at 28°C, which showed lower values in compared to its control (P < 0.05). The glycogen concentration was recovered in the three treatments previously affected by the restriction period (P > 0.05). The CDH showed difference between treatments (P < 0.05), but the values remained within the ideal range for shrimp cultivated in a biofloc system. Thus, it can be concluded that the substrates used to deal with food restriction were glycogen and triglycerides, to the detriment of the use of protein. Regarding the health status of the animals, low temperatures and food restriction did not affect the innate immune system of the shrimp. Therefore, the results of the present work showed that it is possible to subject L. vannamei to partial feed restriction, as a trigger for total compensatory growth, in order to improve the feed efficiency of the animals and reduce production costs. Furthermore, in regions where low temperature is a limiting factor for production, it is feasible to explore partial compensatory growth, with the objective of increasing the annual cultivation period.