Marine Bio-images diver Dr Lin Baldock counts marine species with random quadrats at station 1 as part of the diving study investigating the changes occurring within the seabed communities now that these cobble reefs have been closed off to towed bottom fishing gear such as trawl nets and scallop dredgers. Photograph Colin Munro, 2009.
Final Report covering diver monitoring 2008-2010 now available here.
In 2008, DEFRA closed an area of Lyme Bay, southwest England, some 60 square miles in extent to all mobile benthic fishing gear, i.e. bottom trawling and scallop dredging. This closure was brought in to protect fragile seabed habitats, in particular subtidal rocky reefs and areas of boulder and cobble reefs and their associated flora and fauna, from damage caused by such gear. This was a hugely important step; the first such area in England closed specifically for nature conservation purposes, and the culmination of 18 years of data collection, eductaion and campaigning by organisations such as the Devon Wildlife Trust. Numerous studies conducted by ourselves and others had demonstrated that such habitats were particularly vulnerable to physical damage by mobile fishing gear. Marine Bio-images was part of the consortium (lead by Plymouth University) conducting a monitoring programme to study the recovery of the newly protected area of seabed. our particular study focussed on the monitoring of cobble reef areas. We chose to do this by SCUBA diving. This decision was based on our long experience in survey and monitoring, and are knowledge of the area. Most of the species of interest are quaite small small and difficult to spot and the three-dimensional nature of the habitat and the relatively turbid waters of the bay there was no way of collecting the required data remotely with the necessary accuracy. The study collected data on species at 10 fixed stations, 4 stations within pre-existing voluntary closues (also inside the new statutory closure), 3 stations within the new statutory closure (but outside the pre-existing voluntary closures) and 3 outside the new stautory closure. At each station 8 0.25m sq. quadrat counts were conducted and larger species were counted within an 8m belt transect. This study began in September 2008 and ended in August 2010; three annual data sets being collected. the final report has now been completed and we waiting to hear from DEFRA when the report will be published.
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Map depicting the Closed Area (yellow); pre-existing Voluntary exclusion areas (light green) and our monitoring station locations (red polygons). The pre-existing voluntary exclusion areas were agreed between local fishermen and the Devon Wildlife Trust between 2001 and 2006. They were partially successful but not all vessels appeared to abide by the agreement and damage to the reef habitats continued, hence the statutory cosed area was created.
Concerns about the effects of towed bottom fishing gear on the rocky and cobble reefs within Lyme Bay, and their associated fauna, have been expressed since the late 1980s. In response to these concerns and several studies indicating damage (e.g. Munro, 1992; 1993; Devon Wildlife Trust, 1998) a voluntary agreement was negotiated by the Devon Wildlife Trust whereby bottom fishing towed gear would not operate within three vulnerable reef areas, known as Beer Home Ground, Lane’s Ground and Saw-tooth Ledges. This agreement came in to effect in 1995. The agreement was considered a partial success, with many fishermen abiding by it. However this abiding by the agreement was not universal, and damage continued to be recorded.
As a consequence, in July 2008 a larger area of 60 square miles within Lyme Bay was closed to all towed bottom gear fishing by Statutory Instrument. This area enclosed to three existing voluntary areas.
In particular, regular scallop dredging activity was believed to be causing significant degradation of habitat and loss of epifaunal species within rocky reef and mixed ground (areas comprising mixtures of boulders, cobbles, pebbles, shells and shell and stone gravel).
An area of boulder reef within Lyme Bay badly damaged by scallop dredging. Scallop dredges, when used over rocky reefs leaves the area largely devoid of life with large amounts of broken rock. Even a single pass by such gear can cause large amounts of damage and recovery may take many years. Photograph Colin Munro.
The reefs of Lyme Bay
The ‘hard ground’ (as most local fishermen call it) within Lyme Bay comprises a mixture of low limestone ledges, mudstone ledges, boulder reefs and boulder, cobble and pebble patchworks. The deeper reefs (between 20 and 30 metres depth) support diverse communities of sponges, hydroids, soft coral, gorgonions, bryozoans and ascidians (sea squirts). The fauna communities present can be very different between different reefs, depending on the location, size, depth and relief. Cobble and small boulder reefs tend to support high densities of sponges, hydroids, anemones, tube worms and solitary and colonial ascidians.
- A nearby area of relatively pristine cobble reef, I took this image some years ago, before scallop dredgers had made major inroads in to Lane’s Ground Reef. Larger, longer-lived species such as the axinellid sponge Axinella dissimilis (yellow sponge, centre) and the large sea squirts Phallusia mammillata (white sea squirt, centre foreground) flourish on the undisturbed reef. In years to come Axinella dissimilis, a long lived and slow growing species, would become rare on the reef.
- This photograph (taken in 2009) shows an area of relatively undamaged area of Lane’s Ground Reef, one of the few patches still untouched by trawls and dredges. A wide range of branching and encrusting sponges can be seen covering the boulders.
Lane’s Ground Reef
The best known (and most studied ) of these boulder reefs in Lyme Bay is Lane’s Ground Reef. This is a narrow strip of ‘hard ground’ that runs parallel to the shore, approximately 3 nautical miles south of Lyme Regis. Lane’s Ground is an area of boulder reef, comprising small boulders, cobbles, pebbles, gravel and sand. Due to it’s low profile it has suffered extensive damage due to mobile fishing gear. The relatively flat reef presents little obstacle to scallop dredgers, there are no large rock outcrops on which to snag gear. Benthic trawls and scallop dredges will turn and roll small boulders and cobbles, destroying the fragile species growing on them. They will also mobilise large amounts of fine sediment, which then settles on the rock and attached species. as many of these are filter-feeding organisms they are effectively smothered by this layer of sediment. Such areas are especially vulnerable to damage by mobile fishing gear. Being relatively low lying they present little physical impediment to dredges or rock-hopper trawls (these are trawl nets fitted with large rubber discs along the footrope at the mouth of the trawl net, allowing the net to ride over small boulders without snagging). The smaller boudlers and cobbles present can also be overturned and rolled by the gear passing across them and consequently soft bodied or fragile attached animals are destroyed. Although Lane’s Ground was one of the initial voluntary exclusion areas agreed in 2001, damage continued. Much of Lane;s Ground reef has been very badly degraded between 1990 (when I first started diving there) and 2008. However the substrate, boulders and cobbles, still remains and pockets of relatively pristine reef can still be found. There is therefore good reason to be optomistic that the reef will recover over time now that the use of trawls and dredges across it is banned.
The next part of this blog will provide more detail on our study methodology and its findings.
Lyme Bay Monitoring Study: Lyme Bay Closed Area – Measuring Recovery of Benthic Species in cobble reef habitats. Marine Bio-images
Final Report covering diver monitoring 2008-2010 now available here.
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