Experimental evaluation of the effectiveness of weighting regimes in reducing seabird by-catch in the longline toothfish fishery around South Georgia

A series of experiments was carried out in February 1999 at South Georgia ( Subarea 48.3) to examine the effects of different weighting regimes on the incidental mortality of birds caught on longlines fishing for toothfish (Di ssostichus spp.). Three regimes were examined, with weights of 4.25, 8.5 an d 12.75 kg attached at 40 m intervals on a Spanish-rigged longline. There w as a significant reduction in bird mortality when 8.5 kg was used compared to 4.25 kg, but no further significant reduction when 12.75 kg was used. Th erefore to minimise seabird by-catch, line-weighting regimes of at least 8. 5 kg at 40 m intervals should be used. Results suggested that seabird by-ca tch on lines using effective weighting regimes may be even further reduced where all lines being set in the vicinity use effective line-weighting regi mes. The importance of a good design in experiments of this type is emphasi sed, because even though conditions may be held as constant as possible wit hin experiments, the ability to separate treatment effects from other sourc es of variation (e.g, environmental factors) must be preserved.