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Please note. I should probably add the ‘don’t try this at home’ caveat. This is NOT intended as ‘how to dive on a trawl’ article. I have been careful to leave out most detail on methodology for that very reason. Diving on moving heavy gear does incur certain risks and should not be approached lightly. I should probably also add that this was quite a few years ago and we were a little fast and loose with safety. I’m quite a bit older and possibly a little wiser now; this is not how we dive on trawls today, where safety is paramount, and I would not encourage anyone else to do so. Enjoy the story.

Every now and then I go out on a trawler and dive on the trawl net. Sometimes its been for scientific research purposes, sometimes its been filming (most recently for Ocean Odyssey, a two-hour special by Impossible Pictures for BBC/Discovery, and providing clips for Countryfile, BBC). I’ve been doing this for nearly twenty years, and I reckon I’ve almost got the technique sussed now. We should have been out filming on nets again this summer, but various things got in the way: I was busy, the skipper was busy, some of the team were busy, the weather blew up, and then winter arrived. So it’s currently on hold for a little while. However it made me think about the first time I tried it.

Way back in the early 1990s, I was looking for a way to document the effects of trawl nets and other mobile fishing gear on fragile seabed habitats in a way that would really grab peoples’ attention. When the idea came it was so obvious. True I had never done it before, but it couldn’t be that difficult could it? This was an excellent example of the Dopeler shift, where stupid ideas seem smarter when they come at you rapidly (apologies to Christian Doppeler).

I had the big advantage that a good friend happened to be a trawler skipper and together we hatched a plan. I persuaded an old diving buddy of mine, Joe, that this would be a hoot and that of course I knew exactly what I was doing. We picked a bright summers day with only a slight breeze ruffling the water. We loaded our SCUBA gear aboard, slipped our mooring and headed out to sea.

A few miles offshore John, the skipper, shot the trawl. Now unless you happen to be a trawlerman you’re unlikely to have detailed knowledge of how a trawl net works, so this is probably as good a place as any to explain a little about the mechanics of it.

We were going to dive on a bottom-set otter trawl. Bottom-set meaning it dragged along the bottom catching near bottom dwelling fish (as opposed to a mid-water trawl that catches mid-water…ok, you get the picture). The mouth of the net spreads very wide, tapering off into the net ‘wings’ that act rather like outspread arms, herding fish towards the mouth. The wings then attach to chain bridles, which in turn attach to wire warps that run all the way back to the boat. There is a slight flaw with a simple design like this however; once the boat starts to tow the net the wings would be pulled together and the mouth of the net close up as the towing warps took up strain. To counteract this inward pull on the trawl wings, trawl ‘doors’ (or ‘otter boards’ as they are also known) are used to provide an opposing force. Trawl doors are large flat (or very slightly curved) plates that act as hydrofoils. They are attached to the net bridles and align vertically, sliding along the seabed on steel ‘shoes’ welded to their lower edge. On very small trawlers the ‘doors’ may be constructed out of wood and look rather like square-ish house doors with metal edges. On our trawler they were steel plates, around four feet high and five feet long by about an inch thick. As the net is towed forward, water pressure on the faces of the trawl doors forces then outwards, thus spreading the net wings and keeping the trawl mouth open. Remember the ‘doors’, they’re important.

The trawl was set. Joe and I had donned our gear. John killed the engine and we sat motionless on a perfect sea. With a nod to John we rolled over the boat’s gunnel and swam to the warps at the stern. Our plan was simple. We would pull ourselves down along one of the trawl warps until we reached the net. John would give us five minutes to get there. He would then fire up the engine and tow at an agreed speed for fifteen minutes, then knock the boat out of gear and come to a halt. I would film the trawl in action. Once the trawl stopped we would then surface and wait for John to circle back and pick us up. What could possibly go wrong?

A trawl net is towed at quite a low angle; although we were in relatively shallow water we had around a hundred metres of warp to haul ourselves along. Although the upper layers were very clear a thick plankton bloom filled the mid-water layers and I found myself able to see only a couple of metres in front of me before the warp disappeared into a green gloom. I hauled myself one-handed, carefully keeping my video housing clear of the grease covered warp with my free hand. It felt like a very long five minutes before I spotted the seabed. The warp changed to two lines of chain bridle lying on the seabed and I knew we were almost there. The chain jerked slightly in hand; I felt the links go tight. The chain started to move. I caught hold and saw Joe do likewise. In seconds the World changed. The two metres visibility we had moments previously disappeared, to be replaced by utter blackness. The warps and chains scuffing along the seabed churned up a cloud of sediment that excluded all light. I could feel the chain links in my hand but could see neither chain nor hand, although both were only inches from my nose. Joe was somewhere out there in the blackness. The chain whipped up and down, momentarily going slack then snapping tight again. Somewhere near me I could hear the other chain scraping and snapping similarly. A loud metallic banging started up very close by. I realised it was the trawl door. When stationary the door would lie flat on the seabed. Only once the trawl was up at towing speed would the door rise up and slide vertically. As the trawl picked up speed the door was starting to flap up and down on the seabed. I idled away the next few moments musing over the likelihood of my fingers being sheared off by snapping chains, or how long it would be before I slammed into a boulder, lost my grip and slid backwards to have the trawl door plough a furrow through the back of my skull as it bounced over me. Just stay cool I told myself, this sediment will clear in a minute and you’ll be able to see what to do then. One minute passed, two; actually I have no idea how much time passed as I couldn’t see my watch. It felt like an hour but was probably about 90 seconds. The blackness wasn’t clearing and I was sure the banging was getting louder and the motion of the chains more violent. I bailed. Drifting up out of the sediment cloud I looked up, to see Joe floating way above me. Even at a distance I could see Joe’s weary expression. The look in his eyes said ‘Christ! Why on earth did I allow myself to be talked in to something so stupid’ more clearly than words.

By the time we were back on board we had started to see the funny side. It had been an elementary error but easy to rectify. So we came up with plan B. Joe’s ears were truly blocked now after bouncing about on the seabed and a swift ascent, so he could not dive again. However John the boat skipper had been itching to get in the water, thus this was a perfect excuse (I should add, lest I am thought completely mad, that John is also a fully qualified and very experienced diver). That only left the minor detail of who would drive the boat. Joe was given a quick lesson in how to start and stop a trawler, John climbed into his suit and we were ready to go again.

Our second dive went like clockwork. We got the trawl speed and our positioning just right so were able to move freely about the net. Indeed I became so relaxed my thoughts occasionally strayed to what was happening on the surface as we gaily careered across the briny in the hands of our novice skipper. I pushed them to one side telling myself to concentrate on the job in hand. Our dive ended uneventfully and I gained some excellent footage. As John and I began out long surface swim back to the trawler I thought to myself ‘I must try that again sometime’.

Images and HD video from our latest trawl dives will be available early in the New Year. Watch this space

Close up of rock-hopper trawl travelling along the seabed. Colin Munro / Impossible Pictures