This blog post has now moved to my Marine Biology website, Marine-bio-images.com. It can be read at:
In 2008, the UK Government Department for Environment, Food and Rural Affairs (DEFRA) closed an area of Lyme Bay, some 60 square miles in extent, roughly 10 per cent of the bay, to mobile benthic fishing gear. By mobile benthic fishing gear I mean gear that is towed across the seabed, i.e. bottom fishing trawl nets and scallop dredges. This closure was brought in to protect fragile seabed habitats and the associated marine life, in particular the subtidal rocky reefs and boulder and cobble reefs, known to occur in the central and eastern part of the bay. We’ve known for a long time, at least the late 1980s, that such heavy gear could be highly destructive to some reef species, especially fragile or soft tissued attached species such as sponges and soft corals. Of greatest concern was the increase in scallop dredging. Changes in the quota system, markets and fish prices had lead to the number of boats working with scallop dredges increasing dramatically in the late 1980s. The number of boats operating solely as scallop dredgers had also increased (vessels will often switch gear thoughout the year as fish species migrate and quotas change); thus the overall intensity of scalloping had rocketed. As far back as 1991, I conducted dive surveys for the Devon Wildlife Trust; we had heard reports from recreational divers of swathes of destruction on previously pristine reef areas. What we found was even more disturbing, not only were areas of reef being scraped clean of attached life, the very stucture of the reef was changing.
Amongst the more interesting reef areas in Lyme Bay are the boulder and cobble reefs and the mudstone ledge reefs. Boulder and cobble reefs are basically level boulder fields, most of the boulders are small, roughly football-sized and so the heavy scallop dredges can bounce and rattle across these boulders without getting damaged themselves, picking up the occasional scallop as they go. What also happens though is that the boulders are lifted out of the sediment, rolled along and banged together. As this happens the sponges and soft corals growing on the boulders are ripped off or ground to shreds. The steel teeth of the dredges rake into the sediment as the dredge travels, stirring up clouds of sediment which then subsequently settles on top of the boulders. The attached species that managed to survive intact are then smothered in a layer of sediment, blocking their delicate filter-feeding organs.
Mudstone reefs are composed of blue lias clay. This is the same hard, slate-like clay that can be seen in the fossil-rich cliffs that line much of the coast of Lyme Bay. As this clay can easily be shattered by a fossil hunters hammer, the effects of half a ton of toothed dredges being hauled across such ledges by a powerful fishing boat are quite devastating. The ledges simply crumble. Now as any good marine biologist knows, most of the larger attached fauna on reefs are filter feeding organisms – sponges, hydroids, soft corals, bryozoans etc., and as every hydrographer (and diver) knows, currents accelerate around the edges of ledges and promentories due to entrainment. So of course all the life clusters around the edges of ledges where the rich feeding currents are. Grind away the edges and you remove maybe 80% of the attached fauna and – most importantly – virtually all the large colonies. The large colonies are the structure-forming ones that provide new niches for other species, they are often the slow-growing species that can take years to re-establish (where possible), and they are also the ones with the largest reproductive potential: for example, a big seafan colony that’s maybe 15 or 20 years old is going to release many time more eggs or larvae than a little one about 5 years old thats only just reached reporductive maturity. So when the big colonies are removed this alone may drastically affect the ability of a species to maintain or re-establish its population in an area.
So to put it mildly, the situation was not good, and was rapidly deteriorating. It only took one boat working across a reef to remove so much life that it would take years for recovery to occur. Six months of boats intensively working an area could (and did) irreversably change the structure of some reefs. As scallop stocks declined on the so-called ‘clean ground’ (areas of sandy seabed) boast began to work closer to the edges of reefs, nibbling away at the edges. Fish finders and echo sounders improved in quality, so skippers could see exactly which way ledges rose up, enabling them to work close around the most rugged ledges and pinnacles. DECCA became standard for position fixing, then was replaced by GPS, allowing every more accurate positioning of where the roughest points to be avoided were, so boats could work into reefs where previously they dare not enter. The power of vessels also increased, so when dredges did become stuck fast on a rocky ledge or large boulder the solution was often to turn on the power and pull free, often with devastating consequences for the reef. I personally witnessed this many times over the years. Boats would dredge right up the the edge of a reef, gradually extending further and further in to it through the day as the edges were ground down or boulders rolled away. Occasionally a dredger would come fast. You could see it stop dead in the water and list over to one side where the dredges on that side were caught. Watching from a couple of hundred metres away you would hear the engine rev, see the vessel sink deeper on the caught side, then suddenly lurch free to carry on around again.
On one occasion (whilst conducting dive surveys on a reef composed of large limestone boulders) we watched twelve dredgers work around in tight circles clipping in to the reef again and again for over six hours. We took position fixes and returned at dusk with stills and video cameras. It was about forty minutes steaming time to reach this offshore reef and the sun was disappearing as we descended to the reef and swum on a bearing towards where we had seen the boats working. It was quite black without lights when we hit the bottom at around 28 metres, visibility was very poor as the water was still full of suspended sediment from the dredgers working earlier. Even so the transition from pristine to ‘worked’ reef was clear cut and the devastating effects of that one day’s dredging were unmistakable. The entire seabed was carpeted in a layer of fine sediment, detached soft corals drifted loosely across the reef, detached seafans lay flat partially buried in sediment, fragments of the plates of ross coral (a bryozoan) littered the reef. Large boulders lay overturned with still attached seafans protruding from underneath. Only isolated patched of undisturbed reef remained where the dredgers had been working. Video footage taken during this dive, showing pristine reef from the start of the dive and damaged reef encountered later in the dive, can be downloaded from the link below (48Mb, plays in Windows Media Player or Quicktime Player).
East Tennants Reef following scallop dredging 2002
Thus began a long road to the establishment of protection for the reefs. An 18 year long campaign driven by the Devon Wildlife Trust finally lead to stautory protection for the reefs in 2008. So what has this acheived? I’ll address this in Part 2 of this blog.
Update 10th July 2012, New blog: Lyme Bay, what makes it special?
All images and text (C) Colin Munro Photography.