Perch larvae (Perca fluviatilis L.) survive better in dilute sea water

Larvae of perch (Perca fluviatilis L.) reared in tanks for 27 days survived better in lake water to which 2% of artificial sea water had been added (46.6% average survival rate) than in lake water only (13.5% survival). One week after hatching, over 90% of the larvae had their guts filled in all tanks. During the following week the populations divided into “floaters” (fish with an inflated swimbladder) and “sinkers” (fish apparently lacking a functional swim-bladder). Although they continued feeding, sinkers remained smaller than floaters and most of them died.