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 Doppler shift, where stupid ideas seem smarter when they come at you rapidly (apologies to Christian Doppler).

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.

Filming John ‘shooting’ the trawl net before we dive. Most of the net is already in the water, the two wings remaining. The trawl doors can be seen hanging from the stern gantry, still to be attached to the trawl bridles. (This picture is from a subsequent trawl dive, many years later).

We were going to dive on a bottom-set otter trawl. Bottom-set meaning it dragged along the seabed 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.

John ‘shooting’ the trawl. The net is now completely underwater and the trawl doors can be seen just disappearing. The towing warps have been lifted in to the pulley blocks hanging from the transom. (Again this picture is from a trawl many years later).

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.

A bottom trawl, otter trawl. © Colin Munro

Diagram of the bottm trawl used (not to scale)

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 eight 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. Twenty three minutes total seemed about the right balance between allowing sufficient time to get some useable footage, and not too long in case we got ourselves in to trouble. 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?

Preparing my video camera in the trawler wheelhouse. There’s not a lot of room (from a subsequent trawl dive, quite a few years later).

A trawl net is towed at quite a low angle; although we were in relatively shallow water, around 15 metres, we had around a hundred metres of steelwire 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 time before I finally spotted the seabed. The warp changed to two lines of chain bridle lying on the seabed and I knew we were almost there. The trawl doors must be close, and then it was only a few more metres on to the wing on the trawl net.  Suddenly the chain jerked slightly in hand, and I felt the links snap tight. The chain started to move, jangling and clattering over the sand and stones. I caught firm hold of the chain and saw Joe do likewise. In seconds everything 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.  Holding on with just one hand, my other was gripping tightly on to my video housing, I flapped up and down in the blackness, like a beach towel having the sand shaken out of it. 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 the trawl was stationary the door would fall 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 lift and slam back 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. Maybe I would simply be scooped up by the net, falling inexorably back, to be slowly macerated and suffocated in the cod end.  Just stay cool I told myself, this sediment will clear in a minute, you’ll be able to see, and then you can judge what to do. One minute passed, maybe 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 more than a couple of minutes. The blackness was still absolute, and showed no sign of clearing.  I was convinced 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.

John, of couse, was completely unaware of our little misadventure, so the trawler was gaily disappearing in to the distance when we surfaced.  But it was a calm, sunny day, and I’d known John a long time and knew how good a skipper he was, so I wasn’t really worried. About half a mile away I saw him stop, then after a few minutes slowly turn the boat and start heading back towards us.  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.  Unfortunately, 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 a recent biology graduate about to start teacher training.  Unsurprisingly, he had never driven a trawler, or any boat for that matter, before.  No matter, Joe was given a quick lesson in how to start and stop a trawler,  and how to point it in a straight line.  Back then, I wasn’t fully aware of just how badly this could have gone, so I wasn’t nearly as worried as i should have been. John dragged his diving gear out of the hold, climbed into his drysuit, 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.  It was fascinating and exillarating to be able to haul myself across the top of the net and down the other side, to lie alongside the heavy chain and rubber footrope, watching fish being herded in by the outer wings of the net swim hard in front of the gaping net mouth. 

Close up of ground rope of the trawl, showing large rubber rock-hopper discs that allow the trawl to bounce over small boulders. Still taken from video footage.

Just how lucky we had been was brought home to me in future dives.  Not long after this trial I dived John’s trawl again (with John at the helm this time).  A slight miscalculation on speed and tides left me pinned to the top of the trawl for entire dive, unable to let go or move in any direction.  The water rushing past my spare regulator was causing it to freeflow like crazy, but I couldn’t change grip to stop it, and I was gasping for breathe with the effort of holding position.  In 18 minutes I almost emptied two ten litre air cylinders before managing to get to my buddy and signal to bail. 

The cod end of the trawl, full of fish as it is hauled at the end of a trawl. (This image was taken many years later, on a subsequent trawl dive).

But that was the future.  This dive progressed perfectly, ended uneventfully, and I gained some excellent footage.  All too soon we felt the trawl slow and then stop.  The adrenaline was still buzzing in my veins as John and I broke surface and began the long surface swim back to the trawler. 

The stupid grin you wear when it all works out.

Note: most of the photographs I have used here for illustration are from subsequent trawl dives, some 15 or more years later. I’ve used these simply because I don’t have any useable stills from back then.