Introduction
Compensatory growth is defined as a phase of accelerated growth when favorable conditions are restored after a period of growth depression caused by a stressing factor. It is a strategy developed in some species due to frequent stress situations in the environment, assuming trade-offs in resource allocation among growth, reproduction and self-maintenance. In aquaculture, the use of feed restriction and low temperatures as a trigger for compensatory growth can be considered strategies to reduce feed supply, costs and to increase the period of production of tropical species in regions that growing seasons is limited by low water temperature. In addition, Biofloc Technology System (BFT), can also bring several production advantages compared to the traditional systems in ponds, as increase the stock density, and improvement in water quality and biosafety. Furthermore, microorganisms community besides removing nitrogen compounds, also acts as a food supplement for shrimps, providing a constant feed supply 24 h a day.
Thus, the present study aimed to evaluate the occurrence of compensatory growth in Litopenaeus vannamei, in three different temperatures (20, 24 and 28º C) under feed restriction reared in a biofloc system, and its effect energy reserves and immunological condition of shrimps.
Materials and Methods
The experiment lasted 64 days and was divided in two phases: (1) Restriction and (2) Recovery. L. vannamei were stocked with 1.78 g (±0.38) in a stocking density of 300 shrimps/m³. In the first phase (36 days), the experiment was performed using a 3 x 2 (three temperatures and two feed regimes) experimental design, totaling six treatments (in triplicate). Three temperatures were chosen as optimum (28 ºC), intermediate (24 ºC) and low (20 ºC) and two feeding regimes were established for each temperature: the control that received 100% of the calculated feed rate and feed restriction, where the feed rate was 40% of the control. In the second phase (28 days), all the experimental units (n=18) were exposed to favorable conditions (100% of feeding rate and 28 ºC). Three shrimps per tank (nine per treatment) was collected at days 0, 36 and 64 to determine energy reserves content (total protein, glycogen and triglycerides) in hepatopancreas and assess differential hemocyte count (DHC) in hemolymph.
Results
In the end of the experiment, previously restricted shrimps held at 24 and 28 ºC displayed complete body weight catch-up through compensatory growth following the restriction period with depressed growth (P > 0.05). Shrimps maintained at 20 and 24 ºC with no feed restriction did not reach 28ºC treatment body mass when favorable temperature (28 ºC) was established (P < 0.05). Protein levels in hepatopancreas were not affected in any treatment over the experiment (P > 0.05) and glycogen was used as a metabolic fuel in all restricted groups during phase 1 (P < 0.05), but fully recovered when total feed supply was offered (P > 0.05). Triglycerides were also used in restricted shrimps held at 24 and 28 ºC in phase 1 (P < 0.05), and after recovery period, only treatment previously maintained at 24º C presented total recovery (P > 0.05). DHC presented differences among treatments (P<0.05) but was maintained in the safe range for healthy shrimps reared in BFT system over the experimental period. Also, survival was not affected by feed restriction or low and intermediary temperatures (P > 0.05).
Conclusion
Therefore, it is possible to submit L. vannamei to partial feed deprivation with a later recovery period as a trigger for total compensatory growth, in order to improve feed efficiency and decrease of feed supply. Also, in regions that low temperatures are a limiting to shrimp growing seasons, it is viable to explore partial compensatory growth to increase annual production.
Acknowledgements
The authors are grateful for the financial support provided by the European Union (ASTRAL Project – H2020 – Grant Agreement 863034), National Council for Scientific and Technological Development (CNPq), Foundation for Research Support of the State of Rio Grande do Sul (FAPERGS). Special thanks to GUABI Animal Health and Nutrition SA for donating the commercial diets.
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