Trawling for More: Bottom Trawling Impact on Seabed

A while back AccuPixel director Simon Brown had a letter published in the weekend supplement of the Financial Times. The magazine had published a piece about a fishing practice known as damage bottom trawling. The piece addressed the impact bottom trawling was having on the seabed in Orkney. This triggered Simon’s letter. Simon has first-hand experience of the impact of Bottom Trawling. The sonar surveys conducted by Grahame Knott bear witness to what goes on out of sight on the seabed.

A few folks have been in touch asking for more details…so here goes…with a look at the background before looking at a few examples of aircraft crash sites.

Bottom Trawling

Bottom trawling involves a fishing boat dragging a net across the seabed. The lower half of the net is weighted to hold it down and is often fitted with a set of steel rakes to cut into the seabed. The net tapers back and is fully enclosed, holding whatever is caught.

The method is both indiscriminate – catching everything in the path – and is sufficient to leave near-permanent scars in the seabed.

These scars are deep enough to be visible on side scan sonar…and the marks are clear track lines in the seabed.

Large steel wrecks will present a significant obstacle and risk of lost fishing gear. Most are known and marked on the charts and fishing boats will avoid them.

But low lying ship wrecks, typically the older wooden ships now little more than a pile of ballast, cargo or armaments, will be unknown and unmarked. Their charted position just does not exist and trawlers will be unaware of their presence.

The Vulnerable

The seabed sites most vulnerable to bottom Trawling impact are aircraft crash sites. Most will date from the Second World War but the Cold War recorded a few losses and even as recently as 2019 the tragedy of Emiliano Sala’s last flight saw another aircraft crash in the Channel.

Compared to a cargo ship or submarine an aircraft is exceptionally fragile. Only heavier items such as main landing gear, engines and weapons will typically remain intact after impact. But, occasionally aircraft ditch and are intact. However, a trawl running into an aircraft crash site will cause no end of damage. The wreck of Wildcat JV751 bears witness to this.

JV751 was located by Kevin Heath of Sula Diving. You can read more about the story of this aircraft here.

The irony is that a fishing boat discovered JV751 in Scapa Flow when it landed a small section of airframe snagged in its net. The part number identified it as a Grumman Wildcat. In the case of the Wildcat we have substantial remains; engine, oil cooler, fuel tank, landing gear, wings and machine guns. But, the rear section of the airframe and tail is missing.

The trawl has gone through the aft section of the aircraft. Where it lies now is anyone’s guess.

It’s often a fishing snag that reminds us of the potential of what is out there. Here’s another example, this time of a propeller landed at Weymouth harbour:

The Impact of Bottom Trawling on the Seabed

The impact of trawling is not to be underestimated. The recovery of the aircraft carrying Sala was thwarted by the aircraft remains being dragged away from its recorded crash position. This occurred within a matter of weeks from discovery to disturbance.

The thing is…fishermen have no desire or intention to snag cultural history. Snagged, lost or damaged fishing gear costs time and money and it’s not what commercial fishing is about. And it can be more serious than a loss of catch or gear. At least one fishing vessel sank with all hands when its nets snagged what we believe to be the remains of an aircraft.

Knowing where the history lies, and sharing the position, would be the perfect way to help fishermen avoid damage and loss of earnings. But, as Grahame and Simon both know, no one is willing to cover the costs of a sonar and investigative survey to figure out what is on the seabed and record it.

Digital Preservation: Digitizing Bottom Trawling’s Impact on the Seabed

If there was ever a tool for recording submerged sites then photogrammetry is *the* tool and applications like 3DF Zephyr deliver amazing results. For underwater sites, the power to record to very high levels of detail – 1mm per pixel – delivers a fantastic visual record and very powerful tool for explanation and interpretation for non-divers.

Moreover, a good example is the P47-D crash site in Weymouth Bay. So, it has taken 20 years but we have identified the names of two possible pilots – you can read the full story here.

This site has fishing gear snags. A lobster pot has caught on a section of the airframe, and a long length of rope wraps around one of the main landing gear legs. Both are visible in the orthomosaic:

Additionally, we do not know what we have already lost. Submerged history is out of sight and only the dedicated maritime archaeologists, divers and fishermen will have an idea.

In summary, We are losing it faster than we can find or record it. Grahame and Simon continue to do what they can. Mostly for the personal satisfaction of adventure, to satisfy the incurable curiosity and discovery.