Our work during the current fishing season has been to focus on supporting the monitoring programme for the swordfish fishery, which the Chilean fisheries investigation institute (IFOP) manages. This monitoring includes sea-trips carried out by scientific observers on 100% of the longline fishing vessels.
However, there is also another longline fleet, which includes vessels classed as 'artisanal' as the total length does not exceed 18 m, which does not fall under the IFOP study. Our support has mainly consisted in the provision of necessary data capture for seabird interactions that the observers should be generating on a daily basis, defining the principle characteristics of mitigation measures like bird-scaring lines and assessing them using our experience aboard in the fleet.
To complement this support we have been working onboard to carry out mitigation trials and at-sea experiments to identify the causes of seabird mortality associated with the fishery and trial bird-scaring line designs that are used in other fisheries, such as the Alaskan design.
To study the causes of seabird mortality, we measured the sink rate of the branch lines (how quickly do the lines which hold the hook sink?) by using Time Depth Recorders (TDRs). We attach these devices to hook lines and they take precise measurements of the depth each second, giving us a very detailed picture of how fast the hooks sink as the vessel sets the line.
This was done by Jorge Ruiz aboard the FV Estefanía Carolina, an artisanal longline vessel of 18 m. The results have been very pleasing in many respects; firstly because there is an excellent will to use mitigation measures in the fleet and to let ATF observers onboard as there is a very reduced living space on these vessels. The results from the experimental trials showed that the sink rate is very decent, which may explain why the seabird mortality on these smaller vessels is much less than on the larger, industrial ships.
Even so, we found that the sink rate was very variable even during the same set, which is not what we expected. Also we identified some important details to be considered, such as the exact place that the hook is thrown into the water as the line is set. If hooks are thrown into the propeller wash (turbulent water created by the propeller behind the boat) the sinking line appears to be affected. Although we have to keep up the investigations to further define recommendable fishing gear designs, this information will help define information in the National Plan of Action for Seabirds in Chile which has been officially adopted for the fleet.
For the bird-scaring lines, we took Alaskan designs from Washington Sea Grant and trialled them onboard two Chilean vessels; the FV Arauco II (18 m) and the FV Portugal II (28 m). The results showed that although the bird-scaring lines could be used on these vessels, the design could be improved, mainly to increase the distance that they cover behind the vessel, which is where the seabirds are caught.
The design is basically too heavy because of the materials used so objects that provide tension need to be dragged on the end of the line, which tend to cause entanglements with the fishing gear. Our current work is therefore focussed on designing a towed device that reduces the probability of the bird-scaring line becoming entangled with the fishing gear, and to find lighter weight materials to use in the line design. This may avoid the need for a towed device.