Bottom trawls are mainly either "otter" trawls or "beam" trawls (beamers). Otter trawls keep the net open with heavy steel plates (otter boards) which act as hydrofoils. The foot rope carries
"rockhopper" discs of rubber which roll along the sea bed, lifting the front edge over obstructions and scaring the bottom fish up and into the net. Beam trawls do the same with a heavy steel beam with shoes on each end and a chain mat behind it. The beam holds the net open and the chain digs out any fish hiding on or in the sea bed. Beamers currently work the whole bottom of the North Sea three times annually and there is concern about the ecological impact of this.
coasts have reached over 600 and 100 respectively in some years and are mainly due to this form of trawling, although they can show no external damage, even under expert inspection.
Many pelagic trawlers pump their fish catch aboard and never get a clear view of what may have become caught in the net. Theirs is a moderately selective fishing method in terms of species but it can catch and destroy large numbers of undersized fish - a million mackerel in a single trawl was reported by the Cornwall Wildlife Trust's observer programme for the European Commission on these fisheries.
Bony fish produce huge numbers of mostly planktonic eggs but few survive for a year to count as "recruits" into the fish stock. These small post-juvenile fish benefit from the micro-landscape of the sea bed, with innumerable little heaps and holes from the activity of various animals, as well as populations of sea fans, tube worms and the like, to provide a degree of cover from predators. Little work has
been done so far to measure the effect on this age group of fish of flattening their habitat with a trawl. Newlyn has a large fleet.
There are many variants of trawling, such as fly-seining in which a rope laid on the bottom from each side of a trawl net in a wide arc is drawn together as the boat moves forward and concentrates the fish in front of the approaching net. This is a low-energy and relatively bottom-friendly method but not quite as profitable as other forms of trawling. As in other forms of fishing, the learning curve is a real obstacle to adoption of new methods and non-fishers may not realise how finely tuned many of the gear types are.
Purse-seiners encircle shoals of pelagic fish such as mackerel and herring with a very large strong net which falls around them like a curtain. The net is hauled so that the open bottom and edge of the curtain are brought up and the fish are enclosed. None of these work around Cornwall now, having been displaced by pelagic trawling, but this is the method that became notorious for its dolphin by-catch in tuna fisheries and it does still result in cetacean by-catches around the UK, even including reports of a pod of orcas (killer whales) on one occasion.
Potters set a variety of different designs of pot or trap to catch lobsters and crabs. They suffer from gear conflicts with trawling unless the ground is too rough for trawling. Storms can also result in pots being unrecoverable, and lost pots go on fishing for months or years. Crustaceans and fish can enter lost pots and die there. Pot designs with doors that are held by a string that can rot away have been shown to be perfectly practical but are not compulsory
despite good evidence of prolonged "ghost fishing" by lost pots. Fish traps are hardly used here but have some very desirable features.
Line fishing can be with simple lines with one weight and many hooks, or longlines which have weights and perhaps floats at intervals. Handlining for mackerel is rare among fisheries in that non-target fish can be thrown back alive, and is as ecologically perfect as a fishery can get. Set longlines are used around Cornwall for conger, and some netters fish for blue shark with hooks off the net buoys.
The regulation and monitoring of fisheries is another story . . .
Nick Tregenza
Illustrations: Sarah McCartney
