The best practice suite of mitigation measures for pelagic longline fisheries is the combination of a bird-scaring line, appropriate line weighting and night setting. This combination protects the area where hooks are still close to the surface (bird-scaring line), sinks baited hooks rapidly out of the reach of foraging birds (line weighting) and prevents seabirds from seeing baited hooks (night setting).
On my last trip here in southern Brazil the combination of measures was put to the test when the support pole for the bird-scaring line broke! We departed from the port of Rio Grande on board the pelagic longliner Ana Amaral I under the command of captain Marinho. The Ana Amaral I is a 29 m solid wooden vessel and usually targets swordfish and shark off the continental slope.
Despite spending 15 days at-sea, we only managed seven sets due to some awful weather conditions, which is quite normal during the winter season in the Southwest Atlantic. I needed to collect data on the sink rate of baited hooks under different line weighting configurations, so I had to make the most of the few fishing days we had. I was comparing the fishing gear when set with and without battery powered lights, which are used by some fisherman to attract swordfish. Over the seven sets I obtained excellent data on a total of 44 hooks to compile a decent sink profile, which I am currently analyzing.
Why look at the sink rate? It is important to understand the sink rate of baited hooks because the hooks must sink beyond the dive capability of most seabirds (at least 10 m depth) within the area the bird-scaring line protects. Sink rates can be improved by adding mass to the weights or moving the weights closer to hook. We are therefore testing to see if battery powered lights help sink baited hooks faster 8as they add mass to the line).
On this trip, the sink rate was important as the recent installation of a new bird-scaring line support pole proved to be too weak to cope with the tensions created by the towed device. Bird-scaring lines need a towed device to create enough resistance in the water to maintain appropriate aerial coverage of the setting area behind the vessel. The support pole must be strong enough to withstand that tension.
Below: The broken bird-scaring line support pole, with Black-browed albatross in the background
Because the bird-scaring line support pole broke during the first minutes of the first set, all the remaining seven sets were conducted without a bird-scaring line. Despite high abundances of albatrosses (up to 300 Black-browed albatrosses) no bird was caught. This was thanks to the moon. We were fishing under the new moon, which meant the nights were very dark. Under these conditions line weighting and night setting are a good combination of measures.
ATF studies over the past three years show clearly that the full moon is one of the main variables that contribute to high seabird bycatch in pelagic longline fisheries as it helps foraging seabirds to detect sinking hooks at night. Without the use of a bird-scaring line, this can mean heavy bird mortality even at night.
As we cannot control the moon phase, we need the additional measure of a bird-scaring line to protect sinking hooks during phases of the moon that illuminate the water during setting operations.
I will help fix and improve the tori pole of the FV Ana Amaral I with some stronger materials. We are also developing a guide to help disseminate tori pole design and construction in Brazil, something that this trip helped highlight as a necessary initiative.
Below: Black-browed albatross