Salinity tolerance of Nile tilapia fry ( Oreochromis niloticus), spawned and hatched at various salinities

Abstract

Fertilized eggs of the Nile tilapia ( Oreochromis niloticus L.) spawned in freshwater, were removed from mouthbrooding females, 1 day post-spawning and artificially incubated at elevated salinities. At 6 days post-hatching, mean survivals of 85.5, 84.4, 82.5, 56.3, 37.9, 20.0 and 0% were recorded for broods incubated at salinities of 0, 5, 10, 15, 20, 25 and 32 ppt, respectively. Fertilized eggs exhibited a 96-h median lethal salinity (MLS-96) of 18.9 ppt, a value identical to that of 7- to 120-day-old fry and fingerlings. Fertilized eggs exhibited a higher median survival time (ST 50 = 978 min) than 7- to 395-day-old fry and fingerlings (ST 50 = 28.8–179.0 min). The salinity tolerances of fry spawned at various salinities and fry spawned in freshwater but hatched at various salinities, were determined using the median survival time (ST 50), mean survival time (MST) and 96 h-median lethal salinity (MLS-96) indices. For comparative purposes, fry spawned and hatched in freshwater were acclimatized to various salinities and their salinity tolerance determined. Fry salinity tolerance progressively increased with increasing salinity of spawning, hatching, or acclimatization. However, at equivalent salinity, early exposure (spawning) produced progeny of comparatively higher salinity tolerance than those spawned in freshwater and hatched at elevated salinity. Similarly, at equivalent salinity, progeny spawned in freshwater but hatched at elevated salinity exhibited higher salinity tolerance than those spawned and hatched in freshwater, then acclimatized to an elevated salinity. The utility of these methods of early salinity exposure toward the saltwater culture of tilapias is discussed.