Amônia

Ammonia is the main product of nitrogen excre-tion in teleosts and is produced mainly in the livervia the catabolism of dietary protein (Foss, Si-ikavuopio, Sæther & Evensen 2004; Benli, K€oksal&€Ozkul 2008).Most fish species do not tolerate high levels ofammonia; thus, high concentrations of this nutri-ent can cause several problems for fish in rearingsystems, such as reduced growth rate and massmortality (El-Shafai, El-Gohary, Nasr, Steen & Gij-zen 2004; Benli et al. 2008; Le Francßois, Savoie &Siikavuopio 2008). In addition, a high load ofammonia from fish farms can contribute to watereutrophication (Boyd 2003). Therefore, afterextended time periods, it is possible that ammoniapollution can disturb aquatic ecosystem stability(Sinha, Giblen, AbdElgawad, De Rop, Asard, Blust& De Boeck 2013).Centropomus parallelus Poey, 1860 (fat snook) isa euryhaline fish that is native to the centralSouth Atlantic and lives in marine, brackish andfreshwater environments. Based on this character-istic, these fish can be farmed at different salinities(Alvarez-Lajonchere & Tsuzuki 2008). Thus, theobjectives of this study were to determine ammo-nia excretion in fat snook at different salinities andto assist in the development of models that canpredict the maximum load of this nutrient in theenvironment.

The fish (n = 90; weight, 5.84 Æ 0.162 g; totallength, 9.02 Æ 0.125 cm) were purchased from aprivate hatchery and transported to the labora-tory. The animals (n = 18 for each aquarium)were divided and acclimated in five 25-L aquariums (salinity, 35 g LÀ1) with constant aeration.Every 24 h, desalination or addition of artificialsalt (~20% of the initial salinity per day) was per-formed until the respective salinities of 0, 10, 20,30 and 40 g LÀ1were achieved. When water inthe aquariums reached the desirable salinity, theanimals were maintained in the respective salini-ties for 20 days. During salinity acclimatization,the water was completely changed twice a week,and feed was offered three times a day. The feedwas a commercial diet usually used for fat snookcontaining 45% crude protein (45 Propescado-Nutriave Foods).Before the start of the experiment, two fish weretransferred to 3-L aquariums at each salinity levelwith constant aeration (n = 8 aquariums for eachsalinity level, totalling 40 experimental aquari-ums). The fish were anesthetized with a solutionof 0.1 g LÀ1of benzocaine and weighed. The aver-age biomass of each aquarium was then deter-mined. The fish were fed at the rate of 2% of thebiomass (maximum feed intake observed) andacclimated under those conditions for 1 month.After the acclimatization period, fish were fastedfor 48 h. Subsequently, the fish were transferred toa clean aquarium at the same salinity level andmaintained for 2 h before beginning the experimentto avoid interference of ammonia excretion due thetransference stress. To evaluate excretion, watersamples were obtained from each aquarium at thebeginning and at the end of the experiment (24 h)to measure the concentration of ammonia. Duringthe excretion experiment, the animals were notfed.

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