Small cetaceans may become trapped when purse seiners fish on free-swimming schools of tunas. The backdown procedure and Medina panel are used in combination with rafts, trained divers and skiffs to create an escape route and shepherd the cetaceans to safety.
Small cetaceans may become trapped when purse seiners fish on free-swimming schools of tunas. Early observation of trapped cetaceans is key to releasing them with minimal harm.
Hamer and Minton (2020) describe the behaviour of encircled animals thus:
Typically, encircled animals initially swim back and forth inside the cork line, at the surface and at the furthest point from the vessel. Despite the jumping behaviour that small cetaceans are often known for (especially the dolphins), they almost never use this ability to escape, most likely due to their instinct to group together when under threat. It is at this time that efforts should be prioritised to remove the animals from the net. If delays occur, then the risk of stress related ‘passive behaviour’ developing will increase, which is known to lead to death soon after.
The ‘backdown procedure’ has greatly contributed to the reduction of bycatch of small cetaceans in purse seine fisheries in the eastern tropical Pacific and is widely used in that region’s tuna fishery . The backdown occurs after most of the net is on board. At this point net retrieval is stopped, the net is tied to the vessel and the engine is put into reverse. This creates a water current that causes the remaining net to form a long channel in the water. The water current pulls the end of the channel underwater, with the corkline sinking a few meters, thereby providing an area for dolphins to escape (dolphins remain close to the surface while the tunas are lower in the net) .
Rafts and skiffs are used to maintain the shape of the seine net (folds in the net can entangle cetaceans), and, often in conjunction with highly trained divers swimming inside the net, to shepherd dolphins to help them escape. Cetaceans that cannot be released must be rescued by hand. A rescuer in a rigidly inflated raft can rescue dolphins/porpoises effectively at any time during a net set .
The Medina Panel, or dolphin safety panel, is a section of small-meshed webbing (net liner) at the apex of the net, which helps keep the dolphins from entanglement. It helps to increase resistance to the water flow and increase sinking of the corkline [1,3].
Copyright - Pacific Alliance for Sustainable Tuna
The use of skiffs should be carefully considered, as Hamer and Minton (2020) explain:
The placement of the skiff needs to be carefully considered, so its presence (i.e., the noise caused by the motor and the bubbles caused by propeller wash) does not deter the encircled dolphins from approaching the escape route. It may also be useful to use the skiff to shepherd the animals towards the opening, although it should be moved slowly and carefully to avoid further stress and strike injuries. On vessels with high lookout points (e.g., the wheelhouse roof, or a crow’s nest) one or more crew members should keep watch to ensure (i) the seine net remains open and net folds do not form, (ii) all animals observed encircled are accounted for during the release operation and (iii) skiff operations remain safe for the operator and for the encircled animals.
Extracts - links to pdfs
- Hall, M and Roman, M. 2013. Bycatches and non-tuna catches in the tropical tuna purse seine fisheries of the world. Technical Paper, FAO, Rome.
- Hall, M.A. 1998. An ecological view of the tuna-dolphin problem: impacts and trade-offs. Reviews in Fish Biology and Fisheries (1): 1-34.