Scallop dredging: why is it considered so damaging to reefs?

I first wrote this blog back in 2012.  If moved off-site for several years, but in 2020 I’ve reinstated it, with a few very minor changes.  Lyme bay now has statutory protection from scallop dredging, and all towed bottom fishing gear. However the majority of the UK’s coastal seas, and indeed coastal seas around the World, do not.  Scallop dredging is still conducted around the World: Iceland, Ireland, Faeroes, North America , to name just a few. So I believe this is still a relevant piece in relation to the massive amounts of damage caused to reef, boulder and indeed all seabed habitats. 
Colin Munro April, 2020.

I’ve written this in relation the the Lyme Bay Closed Area (see my earlier blogs about this: Part 1 and Part 2) and the concerns about the impacts of scallop dredging on the nearshore reefs that lead to this statutory closure. There are numerous studies identifying damage on seabed habitats from scallop dredging, and much indeed about the impacts in Lyme Bay in particular. But why exactly is scallop dredging so damaging on reef communities compared to other forms of bottom fishing? There is probably not spelled out as clearly as possible; those familiar with the gear perhaps assuming it is obvious. So I’ve decided to post this article for those who may have heard about scallop dredging being very destructive on sensitive habitats but are maybe rather hazy as to why this is. Much of the following I have actually lifted from a report I wrote for the Devon Wildlife Trust about twenty years ago (An investigation into the effects of scallop dredging in Lyme Bay, 1992) and although there is much that I have had to update, it is depressing how much remains unchanged.

A scallop, Pecten maximus, lying buried in gravel, with only its tentacles visible. Lyme Bay.

A scallop dredge in basically a weighted rectangular bag dragged across the seabed. the upper part of the bag is made from a synthetic fibre stretched mesh; the underside has to withstand constant abrasion as it travels across the seabed. To withstand this abrasion it is composed of heavy interlinked steel rings, each about 8cm diameter. This is the chain belly of the dredge. The mouth of the bag is a letterbox shaped steel frame, which then connects to a towing eye. The individual dredges are then attached to a dredge beam, a heavy steel tube with bobbin wheels at either end. This in turn is connected to a steel wire bridle, and then a towing warp which will normally be deployed from a beam. beams are set either side of the boat (just as in a beam trawler); with this set a typical scallop dredger will tow up to sixteen dredges, eight either side (four to six either side is the norm). Now all this chain, wire and metalwork makes the gear very heavy, so clearly if this is dragged across fragile habitats it’s going to do a lot of damage. One feature not yet mentioned is the tooth bar. Scallops, unlike the otter and beam trawls, are trying to catch relatively static animals (scallops do move, but not that readily) that mostly live half buried in sediment. Scallops will try and dig themselves into the seabed until their flat upper shell is flush and covered with a fine layer of sand or gravel, only the line of fine tentacles around the rim of the shell betrays its presence. Even a heavy chain bellied dredge will simply pass straight over the scallop as it clamps its two valves shut and sits tight. So the scallop has to be, quite literally, dug out of the seabed. To do this scallop dredges are fitted with a tooth bar. This is bolted to the framework at the bottom of the dredge bag mouth, the teeth, about 9-10cm in length, are downward pointing rake through the seabed as the dredges are pulled along. Now of course this tends to lift partially buried stones and small boulders into the dredge as well as scallops. Indeed when they work particularly stony areas the dredges have to be hauled regularly to empty out the stones that have filled up the dredges. If instead they were fishing for bottom dwelling fish (like traditional trawls) then of course the boats simply couldn’t work such areas as the fish caught would end up so badly damaged they couldn’t be sold for anything other than fish paste, but scallops are tough blighters and are encased in a pretty heavy duty shell (check out those ash trays or soap dishes) so can withstand being smacked and clattered about in a bag fill of stones for thirty minutes or so until hauled. Even so, the main reason (apart from being far more environmentally friendly) that diver caught scallops command a much higher price is that they look better; no-one wants to pay top dollar in a plush restaurant for scallops served up in a shell all broken around the edges or looking like it’s been opened using a lump hammer. In fact scallop dredging is known to pretty inefficient. many scallops are missed as the dredges bounce along, many are also fatally damaged as dredge teeth coming down hard smash through the shell leaving them broken on the seabed for scavengers to feed on (see the pictures of broken shells below).
A modification to this design occurred in the late 1960s. Instead of having rigidly fixed teeth, the tooth bar was spring-loaded (the Newhaven dredge). A problem with the fixed tooth design was that if it hit something very hard, like a raised outcrop on rocky ‘hard ground’ it tended to come fast or damage the teeth. By contrast when the spring loaded teeth are hit hard against an immovable object the teeth will pivot backwards against the springs, allowing the dredge to lift over the obstruction. This is sometimes erroneously interpreted as being less damaging to reef habitats since the teeth spring back. When I say hit hard I mean HARD, we’re taking about maybe a ton of ironwork being slammed against a rock at maybe 3-4 knots by a vessel of maybe 200 or more horsepower). For any sponge, soft coral, sea squirt or seafan that happens to be attached to the outcrop in question it’s rather akin to saying it’s okay because the sledge hammer that just pulverised you has a spring-loaded handle. What it did allow though was for scallop dredgers to work areas of low rocky ledges, boulder reefs and low rock outcrops that previously had been off limits due to the risk of snagging or damaging gear. The law of unintended consequences: suddenly reef habitats and their associated life that had been more or less undisturbed for thousands (perhaps millions) of years had massive steel structures being dragged across them. By the very fact that that had been untouched by mechanical disturbance, species with delicate growth forms flourished: large delicate diaphanous feeding apparatus and colonies forming marshmallow soft cushions, eggshell brittle plates and slender fragile branching structures carpet such tide swept but undisturbed reefs; until the passing of a scallop dredger sweeps them away.

A scallop dredger, with five dredges aside, hauling its dredges.

Illustration showing the key parts of a spring-loaded scallop dredge and how it works on the seabed, including how it affects marine life on boulder reefs. Colin Munro Photography

Illustration showing the key parts of a spring-loaded scallop dredge and how it works on the seabed, including how it affects marine life on boulder reefs.

The comparison to loggers clearing virgin rainforest is compelling, and also quite valid. From the available evidence many of the species occurring in such habitats are long lived, the soft coral Alcyonium digitatum (aka dead men’s fingers) is known to live for at least 28 years and with colonies changing little in that time, so its actual lifespan is probably much longer, the pink seafan (Eunicella verrucosa) also appears to have a lifespan of several decades, some axinellid sponges (e.g. Axinella dissimilis) occurring in these habitats also appear to change little over many years study. We are very much in the infancy of our understanding of the ecology of species living in the temperate rocky reef habitats but it appears that they are mostly species well adapted to competing for space and resources in a very stable environment. They are not (or at least unlikely to be) adapted to rapid recolonisation following major physical disturbance. So the likelihood is (borne out by the evidence on the ground) that when these habitats are disturbed these long lived species do not come back any time soon, instead they are replaced by short lived rapid colonisers (the weeds of the reef, if you like). And if disturbance continues, they do not come back at all.

Relatively undisturbed boulder reef, Lyme Bay, rich in branching sponges and large Phallusia tunicates (sea squirts).

A final question that needs to be addressed is ‘why do scallop dredgers work reef areas in the first place?’ Scallops are pretty widely distributed, but their preferred habitat is not rock but sand or gravel where they can hide by partially burying themselves. Scallop fishermen are not unpleasant people (contrary to the views of some conservationists) with a burning desire to destroy the environment. Nor is working over reef areas risk free; there is always the chance that dredges are going to come fast and the vessel being unable to free them. This may result in the boat losing the gear (more than once I have come across old dredges lying abandoned, firmly wedged under a rocky ledge). Worse still, the gear catching on one side will cause the boat to slew sharply and list violently to that side; once seas start pouring over the gunwales the vessel can sink in minutes. This can and does happen (for example, the 12 metre scalloper Guyona that went down in exactly this manner off the Channel Islands in 2008, with all crew plunged in to the water before they had a chance to get out a Mayday or even grab their life jackets, fortunately all were picked up safely). A third consideration is that when dragged across rocky ground the dredges are often bouncing over the rocks and small boulders. Any scallops present will generally be located in the sand or gravel accumulations in hollows between outcrops; dredges will literally bounce over many of them.

An area of ‘worked’ boulder reef in Lyme Bay. Almost all larger and slow growing species have been removed. Broken scallop shells and a live scallop buried in a sediment hollow can be seen.

Empty and broken scallop shells swept in to a pile, possibly due to the raking action of dredges, on a heavily worked area. Lyme Bay.

Empty and broken scallop shells swept in to a pile, possibly due to the raking action of dredges, on a heavily worked area. Lyme Bay.

So apart from being highly damaging to the seabed fauna, scalloping on rocky ‘hard ground is a risky and inefficient business. And yet it happens. The reasons are simply that there are insufficient scallops on the surrounding sandy or gravelly ‘clean ground’ to support the number of vessels. As scallops are a non-quota species, there are no set limits on the numbers that can be landed, so vessels will switch to non-quota species, such as scallops, when quotas for other fished species are exhausted. When scallopers clean out most of the scallops on sandy seabeds, the temptation is then to start working around the edges of the rocky ground, working ever closer. As they work the boulders are rolled away, the ledges are ground down, and so the following year they can work that much further into the reef areas. And so it goes.
This was essentially the point we were at in 2008, when the statutory closed area was established in Lyme Bay, protecting the nearshore reefs. How that has changed things in the four years since will be the subject of another blog.

Related blog: Lyme Bay, what makes it special?
All images and text (C) Colin Munro/Marine Bio-images.



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