Survival and growth of bighead carp fry exposed to low salinities

Bighead carp (Aristichthys nobilis Oshima) 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 ind ices of salinity tolerance assessed the effect of a 96 h direct exposure to low salinities (0-16 parts per thousand). Mean (MST) and median survival t imes (MST50) of fry decreased as salinity of rearing medium increased. Youn ger fry were less able to tolerate exposure to these salinities than their older cohorts. Median lethal salinity after 96 h (MLS) revealed higher tole rance among 35-day old fry (7.6 parts per thousand) than 11 (2.3 parts per thousand) and 18-day old fry (6.0 parts per thousand), demonstrating that s urvival in saline water depends on their age at initial exposure to low sal inities. Mean body weight of 18-day old fry reared in 0 and 2 parts per tho usand for 3 and 4 weeks was higher than for those reared in 4 and 6 parts p er thousand for the same period. Growth over these periods was inversely re lated 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 subje cted periodically to saltwater inflow.