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 vs. hybrid lines with a mix of streamer lengths were compared with unweighted branch lines. In addition, unweighted to weighted branch lines were tested and night vs. 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 were the most common seabird observed and had the highest seabird mortality rate in the study. Albatross (black-browed, yellow-nosed and shy-type) also suffered high mortality rates while stealing bait from the petrels.  Other common surface foraging species included cape petrels. Hybrid lines did appear to reduce sea bird attacks for both species, 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.