Lyme Bay, Lane’s Ground Reef: sponge species recovery and opportunities lost

As part of a small study looking in to gear impacts on seabed species, we recently conducted a few dives attempting to record HD video of bottom trawls and crab pots working on the seabed. Unfortunately we picked a period of one of the thickest plankton blooms this year (either very late or very early for thick plankton, but this has been rather a strange summer, weather-wise). Hanging on to a moving trawl net holding a bulking camera in 0-2 metres visibility certainly keeps you alert! However, it’s not really conducive to great images, so we’ll be trying again once the waters clear.

One very useful aspect of this however was a checkout dive on Lane’s Ground Reef. As described in previous blogs (Lyme Bay, what makes it special, Lyme Bay Closed Area Pts 1 and 2)., Lane’s Ground Reef previously supported rich and diverse sponge assemblages, which largely disappeared as scallop dredging intensified within Lyme Bay. Co-incidentally we dived on an area of lane’s Ground Reef that I had surveyed 17 years ago, before scallop dredgers and other mobile fishing gear had significantly degraded the reef, and again in 2007, immediately prior to the implementation of statutory protection from bottom fishing mobile fishing gear within the Lyme Bay Closed Area (within which Lane’s Ground Reef lies). In 2007 the condition of the reef appeared very poor. Although not a detailed survey (as was the 1995 study) the visual appearance was of far fewer sponge species and much lower densities of sponges and ascidians (sea squirts), with many other attached species appearing to have dramatically declined. (See blog: Scallop dredging: why is it so damaging to reefs for more info on effects)

Lane's Ground Reef, a circalittoral boulder reef rich in sponges and ascidians, within Lyme Bay Closed Area, Lyme Bay, southwest England. Colin Munro Photography

Lane’s Ground Reef, an undisturbed patch of reef rich in sponges and ascidians.

Our three year monitoring, funded by Natural England as part of the study to look at whether the reef habitats recovered following cessation of scallop dredging, centred around Lane’s Ground Reef (Report here as PDF). One reason being it was one of the hardest hit of all vulnerable reefs within Lyme Bay but was also one where the basic reef structure (small boulders on mixed sand and gravel) remained intact, thus the potential for recovery was there. Another reason was that Lane’s Ground Reef, of all the reefs in Lyme Bay, was the one reef highlighted as previously supporting particularly rich sponges assemblages and that these rich sponge assemblages were, probably more than any other feature, what made the reefs of such high conservation importance, with many unusual or rare species and others not yet fully identified. We knew that sponges, being soft-bodied filter feeding organisms, were particularly vulnerable to physical impact (i.e. the passing of a scallop dredge would completely destroy them). The available evidence from other monitoring studies (e.g. Lundy Island Marine Nature Reserve and Skomer Marine Nature Reserve) also indicated that many of these sponge species reproduced and grew very slowly indeed (some colonies being decades old and with little or no recruitment over many years). Thus recording and measuring recovery in the sponge assemblages within Lane’s Ground Reef would seem one of the top priorities for assessing recovery of the reefs species assemblages in Lyme Bay Closed Area after scallop dredging and bottom trawling has stopped. It would also provide invaluable information of rates of recruitment and growth of these species during recovery following prolonged disturbance. At the end of our three years of monitoring (summer 2010) we believed we were just starting to detect such a recovery in sponges on Lane’s Ground. The change was small, and not (at least then) statistically significant, but given the expected slow recovery of sponges this was hardly surprising. That we might, after three year just be starting to see a recovery was therefore extremely encouraging. Unfortunately Natural England decided not to fund further years work. In my view this was a serious error of judgement; in essence they have paid for all the set up and groundwork, then said ‘Ok, let’s stop there and not bother getting the meaningful data’. The closure of 60 square nautical miles of Lyme Bay to mobile bottom fishing gear for conservation purposes is unprecedented in U.K. waters and provided a unique opportunity to study the changes that occurred following the removal of these impacts. The uniqueness of this opportunity also lay in the fact that so there was so much existing diver-collected survey and monitoring data for Lane’s Ground; including accurately positioned data going back to the early 1990s. Thus, probably more than just about anywhere else one could think of in Southwest British waters, we knew what they area had been like prior to intensive scallop dredge and trawling; not just anecdotal diver observations but detailed survey reports and quantitative species counts by experienced marine biologists.

Three years monitoring would, at best, only lay the foundations for detecting recovery by providing a baseline against which recovery of the most impacted, slower growing species could be measured. Real change is far more likely to be observed over a 5-10 year period. We hope to start limited monitoring again in 2013, on a self-funded basis, because we believe that understanding the changes that occur on these boulder reefs is crucial to our understanding of how the reef species assemblages are recovering. As the prime reason that for establishing the Closed Area was to protect these reef assemblages and allow their recovery, and was also the driver behind a 16 year campaign (notably by the Devon wildlife Trust) to achieve this, then it seems to me a little absurd not to measure whether this is actually achieving the desired effect. Currently there is no study running that is capable of detecting these subtle changes in species such as sponges, ascidian and other small turf-forming species that create the richness and diversity of species for which these reefs were previously known.

I bring this up now because, during our brief dives on Lane’s Ground last week, our observations did suggest that quite significant recovery, especially within the sponge assemblages, was indeed now occurring. Unfortunately these dives were not on any of our 2008-2010 monitoring stations as this was not practical, so direct comparison is not possible.

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