Toleranceresponsetoammoniaandnitrite exposureinlarvaeoftwomarine®shspecies (giltheadseabream Sparus aurata L.andSenegal sole Solea senegalensis Kaup) G Parra & M Yu Âfera Instituto de Ciencias Marinas de Andalucõ Âa (CSIC), Apartado O®cial, 11510 Puerto Real, Ca Âdiz, Spain Correspondence: M Yu Âfera, Instituto de Ciencias Marinas de Andalucõ Âa (CSIC), Apartado O®cial, 11510 Puerto Real, Ca Âdiz, Spain Abstract Gilthead seabream Sparus aurata L. and Senegal sole Solea senegalensis Kaup are two species of ®sh that are of primary importance in Mediterranean aqua- culture. In the present study, the larvae of these species were exposed to different concentrations of total ammonia and nitrite for 24h to examine the toxicity of both nitrogenous compounds to larvae under rearing conditions. The non-ionized ammo- nia (NH 3 ) and nitrite (NO 2 ± ) 24-h LC 50 values were 0.24p.p.m. and 1997p.p.m., respectively, in 12- day-old S. aurata larvae. According to the criteria of 10% of LC 50 value, the rearing water for gilthead seabream larvae has to have levels below 0.024p.p.m. non-ionized ammonia and 200p.p.m. nitrite-N to be safe. In 7-day-old S. senegalensis larvae, the non-ionized ammonia (NH 3 ) 24-h LC 50 value was 1.32p.p.m. No deaths occurred when S. senegalensis larvae were exposed to nitrite concen- trations of up to 2000p.p.m. for 24h. Both species showed different tolerance levels to the two nitrogenous compounds and also showed differ- ences in resistance to toxicity tests. S. senegalensis larvae were more resistant. This is probably associated with the greater amount of mucus in their skin. Introduction Most ®sh species are very sensitive at larval stages to any change in environmental conditions (Westernhagen 1988). Changes in water character- istics can increase the rate of mortality during the ®rst developmental stage. Water quality is also of special interest in aquaculture. Waste in aquacul- ture systems is generated primarily by food and excretion processes (Cho, Hynes, Wood & Yoshida 1994). Ammonia is the major end-product of protein catabolism excreted by ®sh, and it is toxic to marine ®sh. Ammoni®cation of unconsumed food and organic detritus may also increase the ammo- nia content in rearing water. Ammonia is present in both non-ionized (NH 3 ) and ionized (NH 4 + )formsin sea water, the proportions of both forms being dependent on salinity, temperature and pH (Bower & Bidwell 1978). Therefore, variations in these environmental factors may change the effect of ammonia. The non-ionized form is the more toxic, but toxicity of the ionized form has also been reported (Meade 1985). Nitrite is not an important toxicant in an open culture system but, in water- recirculating systems, the failure of water treatment could be followed by an increase in nitrite concen- tration. The gilthead seabream Sparus aurata L. and the Senegal sole Solea senegalensis Kaup are two ®sh species of great importance for Mediterranean aquaculture. Larvae of these species are produced in the hatchery using live prey as food during the ®rst weeks (Pascual & Yu Âfera 1987; Yu Âfera, Polo & Pascual1993;Yu Âfera,Parra,Santiago&Carrascosa 1999). However, research on new rearing pro- cedures using inert diets for this stage requires a better knowledge of tolerance to changes in water quality. Formulated diets are more susceptible to R # 1999 Blackwell Science Ltd 857 Aquaculture Research, 1999, 30, 857±863