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As nitrate is the final product in nitrification process, it may reach high concentrations, especially in recirculating aquaculture systems (Hamlin, 2006). In this case, sublethal or lethal effects might occur in fish (Poerch et al., 2007), affecting growth and reproduction (Hamlin et al., 2008), endocrine functions and secondary responses of stress (Hamlin, 2006; Pottinger, 2017) and also histopathologies in gills, esophagus and brain (Shimura et al., 2004; Rodrigues et al., 2011). However, little attention has been given in literature (Rodrigues et al., 2011) and mechanisms of nitrate toxicity are still poorly understood in marine fish (Hamlin, 2006).
Among centropomid fish, the common snook Centropomus undecimalis (Bloch 1972) is one of the most promising species for aquaculture (Souza-Filho and Cerqueira, 2003) and has great potential considering the Brazilian coast (Cavalli et al., 2011). Found in tropical and subtropical estuaries and coastal environments of the Atlantic Ocean (Brennan et al., 2006), it has been studied in America, as the United States (Hauville et al., 2016; Yanes-Roca et al., 2009), Mexico (Ibarra-Castro, Jimenez-Martinez), Colombia (Cruz-Botto et al., 2018), Venezuela (Figueredo-Rodrigues and Fuentes, 2018) and Brazil (Cerqueira et al., 2017; Pedrotti et al., 2018; Michelotti et al., 2018, Passini et al., 2018). Because no information is available about nitrate safe levels for this species, the present study aimed to evaluate nitrate acute toxicity in cultured common snook juveniles.
All animal handlings were in according accordance with to the Ethic Committee on the Animal Use of the UFSC (PP00861 n° 82/CEUA/PROPESQ/2013). Fish (20.35±6.10 g and 13.90±1.75 cm) (Passini et al., 2016) were acclimated for ten days fed ad libitum with commercial feed (45 % PB), in 100 % renewed water per day with aeration. Treatments consisted of a control, without nitrate addition, plus 20 increasing concentrations of nitrate (100, 250, 400, 550, 700, 850, 1000, 1150, 1300, 1450, 1600, 1750, 1900, 2000, 2200, 2300, 2400, 2500, 2600 and 2735 mg L-1) obtained by addition of sodium nitrate (Dynamics, Sâo Paulo, Brazil). Trial was carried out in triplicate for 96 h, using 60 circular fiber tanks filled with 60 L of marine water, containing five fish in each, observed twice a day (08 am and 06 pm). System was semi-static, with 100 % water renewed daily and sodium nitrate addition to maintain the respective concentrations. Fish were maintained at 20.99±0.55 °C, dissolved oxygen 6.79±0.21 mg L-1, pH 8.23±0.10, alkalinity 141.80±7.68 mg L-1 CaCO3 salinity 33.47±3.75 g L-1, total ammonia and nitrite less than 1 mg L-1.
During experimental period (96 h), no mortalities were observed in fish with or without nitrate addition. Nonetheless, considering toxicant safety levels are equivalents to 10 % of LC50 96 h (Sprague, 1971), fish may have suffered sublethal effects. Despite mechanisms of nitrate toxicity in fish are not completely understood (Hamlin, 2006), increasing methaemoglobin levels appears to be associated, causing mortality due to suffocation (Camargo et al., 2005).
It is known nitrate toxicity varies mainly among species (Rodrigues, 2011), fish size (Hamlin, 2006) and water salinity (Tisai and Chen, 2002). Considering the survival rate presented in this study, the common snook presents great resistence to nitrate when compared to other species (Knepp and Arkin, 1973; Rubin and Elmaraghy, 1977; Kincheloe et al., 1979; Frakes and Hoff, 1982; Pierce et al., 1993; Tilak et al. 2007). Regarding other marine fish, nitrate median lethal concentration (LC50) 96 h were 1006 mg L-1 for Florida pompano juveniles (Pierce et al., 1993), 1522 mg L-1 for mullet fingerlings (Poersch et al., 2007) and 1829 mg L-1 for cobia juveniles (Rodrigues et al., 2011). Thus, as nitrate toxicity significantly depends on specific variations, it is essential to study its effects on different species (Poerch et al., 2007).
In this context, acute toxicity tests are important tools for determining nitrate safety levels in aquaculture, especially considering recirculation systems (Rodrigues et al., 2011). Based on the present findings, the acute exposure of nitrate up to 2735 mg L-1 does not present lethal risk for common snook juveniles.
