Pelagic longline fishing trials to shape a mitigation device of the depredation by toothed whales

Two types of depredation mitigation devices (DMD) were designed and tested aboard pelagic longline fishing vessels targeting tunas and swordfish off the Seychelles.  The DMD's were designed to reduce the damage and removal of fish from the fishing gear by predators such as toothed whales and sharks. The first DMD was termed "spider" and was made up of a 100 mm diameter plastic disc with sixteen holes in its outer range and a 37 mm diameter central hole.  Eight 1200 mm long hanging legs were created by placing four polyester strands into the outer holes and the entire system is triggered by a biting fish.  The second DMD, "sock", was designed to cover the fish to hide it from predators through either a net made up of fiberglass mosquito netting or of propylene fiber net.  Shark depredation was significantly higher throughout the surveys than toothed whale depredation, but toothed whales caused more damaged to fish through depredation events. The sock DMD did not affect the catch rates of targeted species or depredation events by either sharks or toothed whales.  These results for the spider DMD were not presented. Trigger rates for the two DMD's were up to 87.3% for the spider and 69.2% for the sock.  The protection rate was as high as 80% for spider but only 15.5% for the sock. Entanglement rates of the DMD's with the fishing gear were low for the spider (3.6%) and higher for the sock (17.8%).  In addition, the use of these DMD's signficantly slowed down the fishing process, and therefore additional work into the design and application of DMD's needs to be explored.