PS - Purse Seine

A purse seine is a large net used to surround a shoal of pelagic fish. Once shot, the bottom of the net is drawn together by hauling in a long wire called the ‘purse line’ to form a huge cup shape of netting just below the surface of the water with the targeting fish inside. The net is gradually hauled onboard the vessel and the catch taken onboard the vessel.

One of the main criticisms of purse seine is the use of fish aggregation devices (FADs) this is usually confined to the ocean going tuna purse seiners. FADs congregate the target species but can also encourage many other species around them, and attract large predators such as sharks into the vicinity, all of which may be caught in the purse seine (by-catch) when it is shot around the FAD.

Apart from the by-catch issue when FADs are used, the purse seine can be considered as environmentally friendly.

It is very species selective in that for the smaller pelagics, purse seine can usually only be used on a seasonal basis on particular areas when the particular target species is known to shoal up in that area. The skipper can also tell by shape and behaviour of the shoal whether it is the target species or not. they can also aim to catch the size of fish they are looking for by picking a certain section of the shoal. Larger fish are often on the outside of the shoal. Very occasionally there may be some larger fish or cetaceans encircled in the net but usually these can be released by ‘dipping’ the float line and allowing them to swim out without too much loss of target species.

If the catch is the wrong species or the wrong size the whole catch can be released unharmed before the net is hauled completely.

A purse seine does not come into contact with the seabed therefore minimal seabed impact. Very occasionally in shallow water the bottom of the net may lay on the seabed but as the gear is not dragged across the seabed there should very little effect.

As a purse seine is not towed through the water by the vessel, it has relatively low fuel consumption, limiting green house gas emissions.

A purse seine is shot in a circle around a shoal of fish, to form a deep curtain of netting hanging vertically in the water. The net is fitted with rings (purse rings) along its lower edge, through which a strong cable is passed. As this cable is hauled in it closes up the bottom of the space encircled by the purse seine, preventing the fish from escaping downwards. The net will now form a bowl-like shape in the water containing the fish.

In both ring net and purse seine, the skipper will firstly locate and track a suitable shoal of the target species. Nowadays, this is done using a package of sophisticated electronics including sonars, echo sounders and GPS systems. However, this is still a very skilful part of the fishing operation, success depending very much on the skipper’s ability and experience. Once the shoal is located, the vessel will drop one end of the net, with a dhan on it, and shoot the net in a circle around the shoal of fish forming a curtain of netting around them.

As the vessel completes the circle, they will pick up the dhan and first end of the net. In some fisheries, a second smaller boat called a skiff is used to take the end of the net, instead of just using a dhan, and tow it round to meet the main vessel and complete the circle. Once both ends of the net are back on board the purse seiner, they will then start to close up the bottom of the net by hauling in the purse line to prevent the fish from escaping downwards. Once this is hauled in, the fish should be contained in a huge bowl shape of netting alongside the boat.

The net is slowly hauled on board, gradually decreasing the size of the ‘bowl’ containing the fish, until all the fish are alongside the boat in the strengthened section of the net called the ‘bunt’. They will then start to take the fish on board the vessel. Originally, this was done using a brail that is similar to a large version of an angler’s landing net to scoop the fish out and dump on deck. This method is still used for larger pelagic fish, but for small pelagics such as herring and mackerel, nowadays, it is normal practice for the larger, more modern, boats to use fish pumps to pump the fish on board.

As the catch comes on board, it will pass through a water separator, the surplus water flows directly back overboard and the fish will be channelled into large tanks of refrigerated sea water (RSW tanks) for storage. Smaller boats may use a simpler system of chilled sea water (CSW tanks) that contains a mixture of sea water and ice to cool it down. Larger pelagic fish, such as tuna, tend to be stored below decks in a refrigerated or freezer hold.

As the purse seine is hauled, it is passed from the side of the vessel to a pound, aft, where it is stowed ready to shoot again. The larger UK registered purse seiners (60m-70m in length) will use very large nets that can be large enough to fill the back of an articulated lorry. These will be in the region of 700 metres long and 200 metres deep, that would encircle an area approximately 250 metres across. The cost of the gear will be about £250,000. The nets are made very heavy at the bottom by adding several tons of zinc weights (in many countries they are no longer allowed to use lead) to get the net to sink quickly as it’s shot away. Many floats, often several thousand, will be fixed to the top of the net to keep it on the surface, and ensure that the netting hangs vertically in the water. Spaced out along the weighted footrope are many short lengths of ropes with steel rings attached, to run the purse wire through.

The purse wire will be a heavy steel cable that is hauled on to a winch to close the bottom of the net. The skipper will be very careful not to allow the net onto the seabed in case of damage, as repairs to such a large net can be very time-consuming and expensive. The physical size, weight and cost of the net will be scaled up or down to suit the size of vessel using it. The design of the net will be slightly different in different areas, and to suit the target species and how the vessel handles it.

In the UK, the larger purse seine vessels are based in North East Scotland and Shetland where they mainly target herring and mackerel. Over the past 10 or so years many of these vessels have been using pelagic trawling to catch these species, but there is a trend back to purse seine as demands on the industry change. Both purse seine and ring net are used in many fisheries throughout the world to target pelagic fish. One of the main fisheries is the ocean-going purse seiners targeting various species of tuna. In this fishery, there can be a mix of tuna species congregated together, creating the possibility of a by-catch of other tuna species in the purse seine. In some fisheries, the vessels make use of fish aggregation devices (FADs) that encourage the tuna to shoal up around them, making an ideal target for the purse seiner.