The influence of bait quality on the sink rate of bait used in the Japanese longline tuna fishing industry: an experimental approach

Experiments were conducted to determine the effects of bait size (large versus small) and bait condition (frozen versus thawed) on the sink rate of five species of bait used in the Japanese longline tuna fishery in Tasmanian waters. The effect of adding lead sinkers to baited hooks as also examined. While all factors (bait species, size, condition) were statistically significant, the most powerful effect was due to bait condition: thawed bait sank whereas frozen bait floated. Fish with inflated swim bladders floated, however, even when thawed. The addition of a 20 g lead sinker to a conventional 20 g tuna hook reduced the sink time of bait bearing a hook alone by 50% to 2 sec. m-'. The results highlight the importance of careful bait selection (to avoid fish species with swim bladders which cause the bait to float), the proper thawing of bait and the addition of small amounts of weight to baited hooks before deployment from longline tuna vessels.