Survival and growth of bighead carp fry exposed to low salinities.
Bighead carp (Aristichthys nobilis) fry of various ages (11, 18 and 35 days post-hatch) were exposed to the low salinities encountered during the annual intrusion of seawater in Laguna Lake, Philippines. Practical indices of salinity tolerance assessed the effect of a 96 h direct exposure to low salinities (0-16 per mil). Mean (MST) and median survival times (MST50) of fry decreased as salinity of rearing medium increased. Younger fry were less able to tolerate exposure to these salinities than their older cohorts. Median lethal salinity after 96 h (MLS) revealed higher tolerance among 35-day old fry (7.6 per mil) than 11 (2.3 per mil) and 18-day old fry (6.0 per mil), demonstrating that survival in saline water depends on their age at initial exposure to low salinities. Mean body weight of 18-day old fry reared in 0 and 2 per mil for 3 and 4 weeks was higher than for those reared in 4 and 6 per mil for the same period. Growth over these periods was inversely related with the range of salinities tested. These results demonstrate that, despite their known stenohalinity, bighead carp fry possess some degree of osmoregulatory capability, allowing them to survive and grow in lakes subjected periodically to saltwater inflow.