The main purpose of the present work is to determine physico-chemical differences (dissolved oxygen, pH, salinity, nitrite, ammonia, temperature) between sea water and the tank water used for keeping live bait in a tuna fishing vessel. The study was conducted onboard the tuna vessel “Kowalski V”, around the Macuco island and Bombinhas coast (Santa Catarina, Brazil) in January 2000. The main live bait were clupeid fishes (Sardinella brasiliensis and Anchoa spp.). The results revealed the presence of ammonia in tank water, which, associated with high values of pH, can reduce the fish immunity and increase mortality. In general, the dissolved oxygen in the tank indicated a negative anomaly of 2 mg/l. Temperature, salinity and pH showed nonsignificant differences.