Reducing seabird bycatch in the South African joint venture tuna fishery using bird-scaring lines, branch line weighting and nighttime setting of hooks

Several seabird bycatch mitigation techniques were tested aboard distant water tuna fisheries in the South African EEZ.  Two types of bird scaring designs, light lines with short streams and  hybrid lines with a mix of streamer lengths were compared to unweighted branch lines. In addition, unweighted to weighted branch lines, as well as  night versus day setting with a combination of bird scaring lines were tested for their effects on seabird avoidance and targeted fish catch rates.  A total of twenty seven seabird species were observed, but white-chinned petrels (Procellaria aequinoctialis) were the most common seabird observed and had the highest seabird mortality rate in the study. Albatross (black-browed (Thalassarche melanophris), yellow-nosed (Thalassarche chlororhynchos) and shy (Thalassarche cauta)) also suffered high mortality rates while stealing bait from the petrels.  Other common surface foraging species included cape petrels (Daption capense). Hybrid lines did appear to reduce sea bird attacks, but the results are not conclusive because the birds could still attack the line at points not protected by the bird scaring lines.  Sets made at night caught far fewer seabirds (0.439 birds/1000 hooks vs. 2 birds/1000 hooks) and catch rates were 18 times higher on the unweighted compared to weighted branch lines (1.07 birds/1000 hooks and 0.06 birds/1000 hooks respectively). The weighted and unweighted branch lines did not affect targeted fish catch rates.