Effect of bait species and color on sea turtle bycatch and fish catch in a pelagic longline fishery
The effects of bait species (mackerel and squid) and color (blue-dyed and non-dyed) on the loggerhead turtle Caretta caretta bycatch in a pelagic longline fishery in the western North Pacific were assessed in shallow-set longline fishing experiments. The loggerhead turtle catches were analyzed using a generalized linear model (GLM) with a Poisson distribution. The potential factors (bait species, bait color, other species catch, and sea surface temperature) affecting loggerhead turtle catch were incorporated as explanatory variables. The model analyses indicated that bait species affected loggerhead turtle catch, while bait color did not. The model predicted that catch rates of loggerhead turtles were 75% less on mackerel bait to squid bait. This study demonstrated that fish bait choice was very effective in reducing loggerhead turtle bycatch in pelagic longline fisheries, but that the use of blue-dyed bait was not. Similar model analyses were also performed on target and by-product fish species, such as swordfish Xiphias gladius, striped marlin Tetrapturus audax, bigeye tuna Thunnus obesus, blue shark Prionace glauca, and shortfin mako shark Isurus oxyrinchus, and other non-target species. The remarkable differences between bait species and color that were found for loggerhead turtles were not found for these species catches.