Total weight losses produced during the industrial precooking of skipjack (Katsuwonus pelamis) and yellowfin (Thunnus albacares) tuna in the two most commonly used commercial precookers, atmospheric (APC) and vacuum precookers (VPCs) were studied. The purpose of precooking is to stabilize the fish flesh for cleaning. The tuna were precooked to a target backbone temperature of 60°C to balance the prevention of histamine formation and weight loss from heating. The weight losses were significantly different for fish of various sizes between precooking the tuna in an APC and the VPCs. We postulate that the difference is due to the venting phase preceding the heating and cooling phases. The overall difference in weight loss for skipjack was 1.1% and for yellowfin was 1.5% and fish of the same size that were precooked in the APC lost more weight than fish precooked in the VPCs.