Résumé
In order to develop a simple and accurate index of the salinity resistance of tilapia, batches of 10 juveniles (5 to 20 g) of two different species
Oreochromis niloticus and
Sarotherodon melanotheron reared in freshwater were subjected to gradual increases in salinity until 100% mortality. Seven daily increments of salinity were tested with 4 replicates: 2, 4, 6, 8, 10, 12 and 14 g l
−1 day
−1, while control batches were kept in fresh water. The temperature was maintained at 27 °C. The concentration of oxygen, ammonia and the pH were not limiting factors. The mortality, monitored on a daily basis, appeared after 2–51 days and was spread out over 1–20 days, depending on the increment of salinity. The higher the daily rate in salinity increase, then the shorter the time lapse before total mortality occurred. The cumulative mortality as a function of salinity fit well with simple linear regressions. The criterion of the resistance to salinity was the index MLS (median lethal salinity) defined at each daily rate as the salinity at which 50% of fish died. For
S. melanotheron, the mean MLS was 123.7±3.5 g l
−1 whatever the daily rate in salinity. For
O. niloticus, the MLS was 46.3±3.4 g l
−1 for daily increases in salinity ranging from 2 to 8 g l
−1 day
−1 and decreased significantly (
P<0.05) above this level. The MLS-8 g l
−1 day
−1 ,which takes into account the full capacity of the fish to adapt to the increasing salinity, appeared to be a simple, optimized and efficient criterion for assessing the resistance to salinity for
O. niloticus and
S. melanotheron. This criterion can be a useful tool for ranking the different parental strains and hybrids of different genus and species of tilapia used in programmes of genetic selection for growth and salinity tolerance.