The science of sustainable seafood, explained

Will Scallop Dredging in Cardigan Bay be an Environmental Disaster?

Recently in the UK, George Monbiot, a well-known environmental journalist, criticised the Welsh Government for undertaking a public consultation about the possibility of allowing restricted amounts of scallop dredging (dragging nets along the bottom) within a legally protected marine conservation zone. In particular, Monbiot used a quote from Callum Roberts at the University of York to imply that the scientific study which led to this consultation was flawed.

Comment by Michel J. Kaiser, Jan Geert Hiddink & Gwladys Lambert

Conservation zones in Europe

Across Europe, there exist a wide range of different types of conservation zones. In this particular case, Cardigan Bay Special Area of Conservation (CBSAC) was designated to protect a number of features of which dolphins and cobble reefs are the most relevant in the case of Monbiot’s article. When Special Areas of Conservation (SACs) are proposed, it is understood that the features within them may be able to exist alongside existing levels of activities such as fishing.

The CBSAC and wider marine environment of Cardigan Bay is undoubtedly one of the best locations to see dolphins in the UK. The cobble reefs within the CBSAC are primarily confined within 3 nautical miles off the coast and extensive survey work shows that there are few examples of cobble reefs beyond this boundary. The area within the 3 nautical mile boundary has been closed to scallop dredging for over 7 years as it is an area that appears to be used extensively by the dolphins within the area. The remainder of CBSAC beyond 3 nautical miles from the coast is relatively shallow (15-30 m deep) and the seabed is composed of a mixture of mixed sediment (gravels and sands) overlaid with highly mobile sand waves. The area is subjected to seasonal storms that result in considerable sediment resuspension. As a result of high levels of wave erosion, the seabed is dominated by opportunistic species such as small bivalves and worms and ephemeral surface dwellers like crabs and starfishes.

In Monbiot’s article, he makes reference to the scientific reports that provide some of the evidence base to which the Welsh Government consultation refers and uses Callum Roberts’ assessment to discount the validity of the science:

This is a dreadful piece of science. Imagine that you stop cutting the lawn for five years. Would you have a highly biodiverse oak forest at the end? No, it would be a scrappy patch of weeds. Protect a heavily dredged piece of seabed for five years and you will have the underwater equivalent of weeds … We lost the oak forests long ago – ie the seabed encrusted with fabulously diverse communities of invertebrates and coralline seaweeds that built up over centuries … All that is left today is muddy bottom with scattered rocks and the odd horse mussel.

Roberts seems to overlook the fact that there is a wide diversity of terrestrial systems that are shaped by their local environmental conditions (e.g. tundra, alpine meadows, conifer forests, oak forests, swamps, marshes, other wetlands, deserts). Each of these habitats has differing levels of diversity and the resident organisms are adapted to their local environment. The implication of Roberts’ statement is that all ‘lawns’ would revert to oak forests if left unmanaged or unaffected by humans for long enough. Think of a manicured lawn in sunny Las Vegas, left alone for a hundred years this would revert to scrub desert. Clearly we know that Robert’s statement is rather simplistic, as does any first year student of ecology who has studied succession in natural systems.

The current scientific understanding of the effects of bottom fishing

The scientific evidence-base supports the view that bottom fishing with trawls and dredges alters the seabed, but that the effects vary from strong to minor depending upon the type of habitat and environment in which they occur. These effects also differ considerably among different fishing gears, of which scallop dredging seems to have the most negative initial effects on living organisms. Some habitats are so sensitive to fishing with towed bottom fishing gear that they may need to be protected from these activities. These habitats are typically dominated by reef or habitat forming organisms. A nice illustration of the latter is Hall-Spencer & Moore’s paper of scallop dredging that had occurred in a maerl (calcareous algal) habitat. However, at the other end of the spectrum, some areas of the seabed are subjected to tidal and wave scour or other forms of natural disturbance (e.g. sediment laden riverine discharges). In these environments the resident organisms are adapted to cope with the high intensity and frequency of natural disturbance (e.g. Stokesbury & Harris 2006; McConnaughey et al. 2014) and additional disturbance by trawling may only have minimal effects beyond the removal of the target species.

An important study by Diesing et al. (2014) modelled the interplay between fishing and natural sources of disturbance at a continental shelf scale and demonstrated precisely in which environments fishing had an effect that was either greater or lesser than the local natural disturbance regime. Another large scale study by Hiddink et al. used empirical observations across gradients of fishing in different habitats to demonstrate how the direct effect of fishing on biomass and production of animals living on the seabed depends upon the environmental regime that prevails at each site. Extending this study further Hiddink et al. were able to model habitat sensitivity at a sea basin scale which demonstrated how we could identify areas of the seabed where bottom trawling and dredging are least compatible (i.e. where the most sensitive benthic communities occur). Each of these studies show that shallow dynamic seabed habitats are less affected by towed fishing gear and recover more quickly.

What are the effects of scallop dredging in Cardigan Bay SAC?

Our findings in relation to CBSAC have shown convincingly that the seabed and its inhabitants can sustain quite high levels of fishing activity before negative effects occur and have provided a clear basis to set upper thresholds for fishing activity that are in line with the conservation objectives of the CBSAC. The findings provide the basis to inform an ecosystem-based approach to management in the area which is possibly a first for a scallop fishery anywhere in the world.

At the time that CBSAC was designated for its conservation features, those features existed alongside existing fishing activities (including scallop dredging) that had occurred and where occurring within the proposed SAC area. Clearly the features designated at that time (dolphins, cobble reefs and sand banks, lampreys) existed together with the prevalent fishing activities. Since designation of the SAC there has been no suggestion that the status of the bottlenose dolphin population has deteriorated. Nevertheless in 2008-2009 a large fleet of scallop fishing vessels arrived in the area to exploit a particularly large settlement of scallops. The sudden increase in contemporary fishing activity triggered protests from environmental groups in Cardigan Bay who made a complaint to the European Commission. As a result of this complaint the Welsh Government closed CBSAC (and all other SACs in Wales) to scallop dredging to allow time for evidence to be gathered to determine if scallop dredging could resume.

In 2009 Bangor University undertook an extensive survey of part of the CBSAC and the subsequent report was submitted to the Welsh Government who had the report externally reviewed and validated. The survey showed that there were no cobble reef features within the area studied and that the area was dominated by mobile sand waves and opportunistic fauna that were resilient to scallop dredging. This was used by the Welsh Government to make the case to the European Commission that they were managing the area responsibly and in a manner that did not impact negatively on the features within CBSAC. As a result a small area of the CBSAC was reopened to scallop dredging and has remained open (seasonally) since that time.

Bangor University continued to monitor changes both within the open and permanently closed areas of the CBSAC and determined that scallop dredging was having no detectable effects in the open area. This was largely due to the highly dynamic nature of the seabed. These results led to a peer-reviewed published paper and mirrors other findings from similar environments elsewhere in the world.

The current scientific study highlighted by Monbiot was carefully conceived with the input of Natural Resources Wales (NRW) (the statutory conservation advisory body to the Welsh Government), Welsh Government and fishing representatives. 18 months of additional survey work was undertaken to ensure that NRW were in agreement that the study could proceed without impacting the features of the CBSAC. Bangor University designed the study in consultation with NRW. The idea was to see if it was possible to develop an ecosystem-based approach to managing fishing activities in a way that did not compromise the conservation objectives of the area. In this way, perhaps moderate levels of fishing could occur while ensuring the achievement of conservation objectives.

The experiment was designed to create a gradient of scallop dredging intensity across 12 different areas of the seabed within the CBSAC. An additional four areas were used as ‘controls’ where no scallop dredging occurred. The experimental design was extremely robust because we identified at the outset any habitat differences that might have compromised the integrity of the study (this was one of the outcomes of the preliminary 18 month study). Secondly we sampled all areas (including the controls) before any experimental scallop dredging occurred and then again several weeks after the dredging had occurred. The site was resampled 5 months later and resurveyed using acoustic techniques 11 months later. In addition, and somewhat uniquely, we employed five commercial fishing vessels to create the fishing disturbance up to an intensity of 6.5 times (this means an area of seabed was fished completely on average 6.5 times). The findings show that the immediate effects of scallop fishing became apparent when an area was fished more than twice. More importantly recovery of the invertebrate communities that live in the seabed (prey species on which fish feed) was very rapid and occurred after five months. All geological traces of scallop dredging were eradicated through natural sediment movements in all but two areas where we could still perceive faint traces of scallop dredge marks after 11 months. Both of these areas where fished intensively in the inshore or offshore area of the seabed.

We cannot know for certain whether, historically speaking, the area was much more diverse and productive with biogenic reef structures, but equally we do not have any evidence to suggest that this was the case. We do have strong evidence that protection of the area does not result in large changes (unlike for example the Start Point area on the coast of Devon UK where the effect of scallop dredging was studied using similar methods and over similar time scales) and that the effect of scallop dredging in Cardigan Bay is very limited. The evidence therefore shows that it is unlikely that the area was much more diverse in the past compared with the present day, and it also shows that even if this was the case, recovery to this state is not happening. Regardless, the evidence shows that protection of the area is unlikely to result in recovery from a ‘lawn‘ to an ‘oak forest.’

Are there any other effects on dolphins?

The results of the study above demonstrate the point at which scallop dredging has short term effects on benthic invertebrates, but also shows that recovery is mostly very rapid indeed. Thus provided that scallop dredging is kept within these limits the likely consequences for fish that feed upon these invertebrates is negligible. Furthermore, if the scallop dredging was limited within a small area of the seabed (perhaps as part of a rotational management regime) the effects would be minimised still further (as occurs presently within the current area that is open to scallop dredging).

Of course there could be other effects of the fishing on bottlenose dolphins, e.g. from the presence of fishing activities, competition for food (through removal of fish), noise generated by fishing boats, interactions with fishing gear (e.g. dredges). However a study by Evans & Hintner could find no evidence to support negative interactions between scallop dredging activities and marine mammal features in Welsh waters and they conclude that the current management measures would appear to be effective in ensuring no negative effects on the status of the bottlenose dolphin population. The final statement in the Evans & Hintner report fits very neatly with the potential recovery cycle identified in our current study and the current seasonal (six months closed – six months open) fishing regime.

The current technical and spatial constraints implemented by the 2010 Scallop Order seem to be limiting fishing effort to a level and to areas where this activity is unlikely to have a significant impact on marine mammals. The important thing is that scallop dredging should not occur close inshore, and should be restricted to relatively small areas which are not repeatedly dredged before the benthic communities have had an opportunity to recover fully.

Michel J. Kaiser is a Professor of Marine Conservation Ecology at Bangor University. Find him on twitter here.
Jan Geert Hiddink is a Reader of Marine Ecosystem Ecology at Bangor University. Find him on twitter here.
Gwladys Lambert is an Affiliate Post-Doctoral Fellow at NOAA in Seattle


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