Effects of the interaction between temperature and dissolved oxygen in juveniles of Pacu Piaractus mesopotamicus (Holmberg 1887)
The pacu, P. mesopotamicus, is a species that, due to its characteristics, has great potential for intensive culture, in which dissolved oxygen (DO) is one of the main limitations. In addition, the culture of this species in regions of subtropical climate, where there is wide thermal variation, the interaction of factors such as DO and temperature can cause biochemical and physiological changes in fish. Thus, the aim of this study was to verify the effects of interaction between different temperatures and DO levels on hematological parameters and oxidative stress in pacu juveniles. Two experiments were carried out (EXP): EXP 1 - interaction between different temperatures(~18,~23and~28°C)and DO levels(~2-hypoxia,~4.5 - moderatehypoxia and ~7 mg DO L-1 normoxia) for a period of 12 h. Immediately after this period, blood, gills, liver, muscle and brain samples were collected from 9 fish per treatment. In EXP 2 the fish were exposed to interaction between the same temperatures mentioned above, hypoxia and normoxia (~ 2 and ~ 7 mg OD L-1) for 12h. At the end of this period, they were submitted to different speeds of reoxygenation: abrupt (0.5 h) and gradual (5 h).After 1, 3 and 12 h of the end of reoxygenation blood samples were collected (n=9), as well as samples of gills and liver at 1 and 12 h. The results demonstrate that the effects of the interaction between different temperatures and DO levels in pacu juveniles are organ specific. The analysis of blood parameters during exposure to different temperatures and levels of hypoxia indicated more evident changes in blood glucose and pH. The abrupt reoxygenation at 23 and 28 oC provoked changes in blood glucose and at the highest temperature in blood lactate in up to 3 hours. However, at 23 oC, the blood lactate increase occurred after 3 h and remained elevated until 12 hours after the end of reoxygenation. An induction of pro oxidant condition was observed in the liver, muscle and brain driven by the interaction of different temperatures and hypoxia levels, evidenced by increase in oxidative damage levels (lipoperoxidation - LPO and proteinthiol content - PSH) associated with an inhibition of antioxidant defense system (total antioxidant capacity - ACAP and glutathione-S-transferase - GST). Regarding reoxygenation speeds, pacu juveniles kept at 18 °C were able to adapt better with abrupt reoxygenation, evidenced by the lower oxidative damage levels (LPO) in the gills and liver. After 12 h from the end of reoxygenation, only fish kept at 23 °C and subjected to abrupt reoxygenation and those kept at 28 °C and subjected to gradual reoxygenation showed all blood parameters within baseline levels. Thus, the interaction between different temperatures and DO levels for a period of 12 h causes metabolic and acid base disturbances, in addition to inducing oxidative stress. However, this species proved to be tolerant to hypoxia and reoxygenation. Fish kept between 23 and 28 °C and subjected to gradual reoxygenation are less susceptible to oxidative damage, present100% survival. A period of 12 h is sufficient to reestablish the biochemical and physiological parameters depending on the temperature and speed of reoxygenation that were submitted the pacu.