Acute exposure of Siberian sturgeon (Acipenser baeri, Brandt) yearlings to nitrite: median-lethal concentration (LC50) determination, haematological changes and nitrite accumulation in selected tissues
Introduction
Nitrite can reach high concentrations in recirculating aquaculture systems in which high densities of fish are kept and bacteria transform ammonia, the main nitrogenous waste product of fishes, to nitrite (Palachek and Tomasso, 1984). Nitrite concentrations can also be high in streams which receiving effluents from wastewater treatment plants because of imbalances among species of nitrifying bacteria (Scarano et al., 1984). In both cases, nitrite levels can become toxic and even low concentrations of this compound are toxic for many fish species (Russo and Thurston, 1977, Tomasso, 1994).
Nitrite is actively taken up across the gills in competition with chloride (Bath, 1980; Eddy and Williams, 1987). Nitrite accumulation in blood plasma causes haemoglobin (Hb) oxidation to methaemoglobin (MetHb) and ensuing hypoxia (Tomasso, 1986), as MetHb does not bind and transport molecular oxygen (Scarano et al., 1984). However, nitrite-induced methaemoglobinemia does not seem to be the primary cause of death, since several studies have revealed that some species can survive for long periods with low concentrations of functional Hb (Bath and Eddy, 1980, Colt et al., 1981, Palachek and Tomasso, 1984, Doblander and Lackner, 1997).
Recent studies have shown that the primary cause of nitrite toxicity may vary among fish species (Doblander and Lackner, 1997). This is not only due to a sole mechanism but to a combination of several toxic mechanisms, such as a reduction in the functional Hb concentration in blood and the subsequent functional anaemia (Tomasso, 1986), hepatocyte degeneration as a consequence of liver hypoxia (Arillo et al., 1984a), damage of lysosomal and microsomal membranes in the liver (Mensi et al., 1982, Arillo et al., 1984b, Doblander and Lackner, 1996), the formation of DNA-damaging nitroso derivates (De Flora and Arillo, 1983) and changes in plasma homeostasis, such as extracellular hyperkalemia (Doblander and Lackner, 1997, Knudsen and Jensen, 1997).
The present study aimed to determine median-lethal concentrations (LC50) of environmental nitrite and assess the effects of acute exposure on several haematological and tissular parameters in Siberian sturgeon (Acipenser baeri, Brandt), one of the most farmed acipenserid species in Western Europe (Williot et al., 1993).
Section snippets
Fish acclimation
Siberian sturgeon juveniles were obtained from the CREA (Centre de Recherche Aquacole, Cemagref, France) and transferred to the Laboratory of Aquaculture (Facultat de Biologia, Universitat de Barcelona, Spain) where they were acclimatised for 30 days in rectangular 500 l concrete tanks (filled with 440 l) with dechlorinated tap water. During acclimation, fish were fed, according to Williot et al. (1988), a dry commercial diet containing 45% protein, 12% lipid, 10% ash and 3.5% carbohydrate
Median-lethal concentration (LC50)
At 72 h, the median-lethal concentration of NO2−–N for Siberian sturgeon juveniles was calculated using sigmoidal regression [x0=139.4 (31.3 S.D.); a=99.5 (21.7 S.D.); b=43.7 (27.7 S.D.); r=0.99; P<0.01] was 130 mg/l (Fig. 1). Almost all the fish survived the 72-h exposure to 0 and 25 mg/l NO2−–N, while 66 and 10% of fish survived the 72-h exposure to 180 and 275 mg/l NO2−–N, respectively.
Haematological parameters
The percentage of MetHb increased significantly (ANOVA; P<0.05) in all fish exposed to different NO2−–N
Discussion
At 72 h, the median-lethal concentration of NO2−–N for Siberian sturgeon fingerlings was 130 mg/l, a value similar to that reported for Micropterus salmoides (Palachek and Tomasso, 1984) but much higher than values reported for other freshwater species, such as A. brevirostrum (Fontenot and Isely, 1998), Oncorhynchus mykiss (Margiocco et al., 1983, Lewis and Morris, 1986), Ictalurus punctatus, Tilapia aurea (Palachek and Tomasso, 1984), Cyprinus carpio (Lewis and Morris, 1986, Jensen, 1990) and
Acknowledgements
The authors thank M. Pelard, D. Mercier and T. Rouault (Cemagref) for rearing fish. This research was sponsored by an ABM/acs/ACI-98 grant from the Direcció General de Recerca (Generalitat de Catalunya).
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