Bottom dragging.

Scotland’s coastal fisheries are focused on shellfish. Scallops and Nephrops (otherwise known as Dublin Bay prawns, langoustine or Norway lobster – we’ll call them prawns), caught mostly for the European export markets, make up the majority of fish and shellfish coming from this area. Although both species can be caught in a few different ways, most scallops are caught by dredgers, and most prawns are caught from bottom trawlers, normally otter or twin rig. These fisheries can have damaging impacts on the seabed and the wider marine ecosystems – more on that below.

There are always opportunities to make these fisheries more sustainable. By avoiding the sensitive areas of our seas, these fisheries would dramatically reduce the damage caused to the marine environment. Further minimisation of environmental impact can be achieved by promoting lower impact alternatives such as creeling and diving. Technical changes to fishing gears to avoid bycatch and habitat damage might also have a role to play.

Dredge the bed.

Scallop dredging is thought to have the most severe ecological effect of all UK marine fisheries because the damage and mortality it causes the seabed habitat and species that live there.

Scallop dredgers in the UK use heavy toothed dredges, that can weigh more than two tonnes and penetrate between 3 – 10 cm into the seabed. They are an inefficient way of capturing scallops so dredgers tend to repeat tows within the same area, exacerbating the initial impacts they have on the seabed. With powerful engines and heavy wires, they can be towed over most seabed types including areas of rocky reef, but mostly target areas of sands, gravels and cobbles where the scallops are found in greatest abundance. In some cases dredgers target areas which were once biogenic reefs where scallops live in small patches between the now damaged reef.

The amount and severity of damage and the rate at which the seabed and ecosystem can recover is considered to depend on habitat type and can range from 1 to more than 10 years.

Biogenic reefs (such as shell, sponge and coral beds) and other biogenic habitats (like kelp and seagrass beds) are normally considered the most sensitive. Unfortunately the sand, gravel and cobble seabeds where these occur (or formerly occurred) are also where scallops are found and dredging takes place.

Coral-like maerl is amongst the most vulnerable with 70% removed from an area by a single dredge tow, recovery takes many decades. Sponges, like the Elephant hide sponge also suffer and, in places where dredging is permitted in the Sound of Jura and Firth of Clyde, have nearly all been damaged. These (and other) slow growing sponges and soft corals can take up to 8 years to recover from a single dredging event. Mollusc species like Flame shells are capable of building large reefs and beds important for the wider ecosystem but again, a single pass can entirely remove a flame shell bed and recovery has been recorded at only 12% a year. The extent, density and overall biomass of seagrass beds is reduced by dredging, and this can take up between 6 – 20 years to recover, even in areas which are already considered wave, exposed and naturally disturbed. Unfortunately just one pass with a dredge can be just as damaging as persistent dredging.

Sandy habitats in areas with lots of wave and current action are thought to be of lower sensitivity due to their adaptation to natural disturbance. In the offshore sands and gravels of Cardigan Bay, studies have found that there are few differences between the species living on the seabed inside and outside a dredge closure. That said, decreases have been recorded in larger and immobile species in these habitats and, as acknowledged by the studies, the study closures have potentially not been closed long enough to see significant recovery. In similar Scottish “high energy coast, shallow and very exposed to wave action” habitats mollusc and crustaceans were also adapted to disturbance, yet large numbers of crabs and sandeels and other species were still killed by dredge tows.

Rocky reef areas are peripheral fishing grounds for scallop dredgers but in Scottish areas where dredging is permitted, they held 30% less fauna living on the rocky reefs than in areas where it is banned. That said, bedrock reefs can also sometimes provide protection. In Lyme Bay they appeared to shelter fragile pink sea fan, which persisted in areas despite dredging.

Whilst some of these habitats are protected from dredging pressure others, such as the reefs of Eagamol or even the Small Isles Marine Protected Area remain open. Many other areas will have also been home to these habitats up until the main expansion of the dredge fishery (about 40 years ago) and still have the potential to recover.

Trawl the bottom.

Where dredgers operate on the sands and the gravels, trawling for prawns occurs on the muds and muddy, bouldery habitats. These habitats hold less fragile biogenic reefs, but in Scotland’s inshore seas they are often a lot less dynamic (i.e. they naturally receive much less wave and tidal disruption), making them less able to adapt to change.

Prawn trawls are wide-mouthed nets, weighted to fall to the seabed, and often fitted with ‘rock-hoppers’ (heavy rubber disks that act kind of like wheels) to help them run along seabeds without snagging on rocks. To address some of the issues regarding unwanted catch (discussed below), nets are often fitted with escape and sorting panels to improve ‘selectivity’.

Whilst scallop dredging is considered to be the most damaging UK marine fishery, prawn trawling is considered second. Modelling estimates that Scottish prawn trawls cause a larger amount of disruption beneath the top 2cm of seabed (per unit effort) than any other European demersal fishery.

The impact of trawl on the species that live in and on the seabed has been well studied and argued about since at least the 14th century. Intensive trawling simplifies the ecosystem, reducing the diversity and abundance of marine life. In the North Sea trawling has changed the biomass and species richness and diversity. Species like seapens and molluscs suffer the worst, whereas species like polychaete worms actually benefit. The species and habitats can and do recover, but only when there is sufficient time between trawls.

Trawling is patchily distributed and focuses on some highly productive grounds. This means that there are some areas which are very heavily impacted, but it also means that there is potential to protect those areas on the periphery.

The Risk to Recovery.

As well as having an immediate impact, damaging or removing species living on the seabed, dredge and trawl fisheries have the potential to alter seabed habitats in ways that impacts the way the ecosystem works.

The diversity of the seabed habitat is important for many species’ reproduction; dead shells, rock crevices and maerl reef act as shelters for small mobile animals including small but important fish prey species such as mud lobsters, and provide ‘essential fish habitat’ for many fish species. Dredging generally leads to a flattened, less varied habitat and removes essential fish habitat function and performance.

In experiments significantly fewer juvenile cod survived on seabeds that resembled dredged ones, than on others where sponge habitats were present. The abundance of these species living in and on the seabed significantly reduces the mortality of year old Atlantic cod and these juveniles actively select seagrass beds as nursery grounds which can hold an entire order of magnitude more juvenile cod than bare sandy areas. Similar preference for biologically complex habitats has also been found in the Firth of Clyde for haddock, and in fact has just been further supported by very recent research showing the abundance of whiting and cod is correlated with the diversity of the seabed. Herring distribution in Shetland is also closely linked to the nature of the seabed, and their spawning is associated with maerl and gravels shallower than 100m.

This functional structure can be fundamentally changed by long-term trawling. Impacts are cumulative, and can lead to profound changes in benthic communities with potentially far-reaching implications for the integrity of marine food webs. Damage to fish prey caused by trawling has been found to decrease the condition (health and size) of the plaice themselves through increased competition. Similar effects are thought to be experienced by dab and whiting, but are somewhat masked by the fact that these two species are bycaught or caught in such numbers that there is no competition. In general, intense trawling can cause major changes to habitats, in turn affecting the fish populations and changing ecosystems.

Prawn trawling also has a significant impact on marine ecosystems through the amount of fish, shellfish and other marine species accidentally caught (known as bycatch) (Seafish, 2016). The scale of bycatch problems is different in different areas. In the Clyde studies have found that between 66 – 80% of the catch (by weight) was unwanted, in other areas only 12% of the original catch was kept. Much of the fish being discarded are potentially commercially valuable, edible species such as hake, sole, plaice, cod, whiting and haddock.

Trawler skippers, the industry and others have been trying – and continue to try – to catch fewer unwanted fish, with improved net designs and other innovations. However, approaches have not been entirely successful or broadly adopted to date and so the risk to the inshore ecosystem remains significant.

Coastal Margins

It’s not just about protection for the environment’s sake. Sustainability is about assessing the entire system and working out what’s in society’s best interests.

Around the world, companies are thinking about their operations in terms much broader than just the economic bottom-line, hardwiring sustainability and reporting on financial and non-financial performance.

The current intensity and distribution of bottom-towed fishing around our shores is unsustainable, not just ecologically, but economically and socially. We’ve seen similar things happen in the past; the boom of inshore herring lasted for well over a century but eventually crashed and was entirely banned in 1977. This seriously affected the villages that were built upon it or which had grown to depend on it.

Intensive bottom-trawling and dredging may have been commercially viable for the past 40 years, but many communities and some within the broader fishing industry are waking up to the tell-tale signs that this intensive method of bottom-towed fishing is not in the long-term good.

In the past few years we are fishing much harder but for fewer fish and lower profits. Scallop landings per unit effort grew to a peak in the late 2000s and early 2010s, but have been falling in the past few years. Income, operating costs and total profits have all declined, in some areas by as much as 92% for larger vessels.

In many cases, lower impact fishing methods provide better returns; creel-caught prawns yield both higher value and profits than those caught by trawls for example. However, as stated by a Report to Select Committee in 2004, “Trawling is now completely dominant [and] reduces the ability of other fishing methods to continue.” This opportunity cost is rarely considered in policy decisions.

It is also important to consider the impact damaging fishing might have on other sectors and communities. Fishing in Scotland (including both inshore and offshore) is estimated to generate about £215m (a further £461m if you also include all fish processing). Meanwhile spending on sea angling, wildlife watching, hotels and other aspects of the coastal economy are estimated at around £3.7 billion per year. Many communities have already raised concerns about their reliance on healthy seas and the impact of damaging fishing.

Stand still or make a change

The issues here are significant, and have been around for a long time. However, we have an opportunity to do better.

Some have recommended that fisheries and habitat conservation can both be achieved through an appropriate use of total and partial exclusion of bottom-towed fishing gears, avoiding displacement onto other sensitive areas. Others contend that switching from trawling to lower impact creels would allow Nephrops landings to continue without damaging wider fish stocks. Elsewhere in the UK, zoned fisheries management has allowed commercially viable fishing to continue whilst also protecting the benthic ecosystem and its function. Recent collaboration between seafood retailer, foodservice, leading academics and members of the fishing industry concluded that better understanding of the distribution of sensitive habitats is needed to improve the trawl fisheries.

Protection of the most biologically diverse and vulnerable parts of our inshore seas is needed, but we must acknowledge that current management does not consider fully the wider ecosystem services provided by our entire inshore area. Marine Protected Areas may cover roughly 20% of the inshore, but dredging and trawling can still legally take place in the vast majority (96%) of our inshore waters.

Government commissioned reviews have made similar recommendations. In 2013, a review of the entire Scottish scallop fishing industry concluded that the current system did not provide sufficient protection to sensitive areas of the seas (“priority marine features”), and recommended reserves and night time curfews on fishing in the inshore. Similar analysis, on a UK-wide basis, concluded that the costs of managing scallop dredging are outweighed by the benefits. Moreover a report investigating economic opportunities for the inshore found that they were significant opportunities to improve profitability in the inshore by moving away from damaging fisheries in sensitive areas.

Annual advice from the UK’s Seafood Authority states that “dredging must be done only with full understanding of the fisheries concerned, together with a practical application of mitigating measures”. In 2009 the UK Scallop Industry Good Practice Guide agreed commitments to reporting, avoiding and maintaining a protection zone around areas where sensitive species are encountered and observed. Unfortunately it appears this good practice is not being followed in Scotland.

Perhaps most importantly, collaboration between seafood retailer, foodservice, leading academics and members of the fishing industry concluded that better understanding of the distribution of sensitive habitats is the highest priority knowledge gap if we are to better understand the consequences of bottom trawling and dredging and to appropriately protect our seas.

We’re interested in protecting our marine environment and the things that live in it. We’re interested in buying and eating sustainable fish and shellfish. We can do much better, why don’t we?

Why should we care?

About 5 million seabirds come to breed in them every summer, 24 species of whale and dolphins have been sighted off the West Coast, and extremely rare seabed species like serpulid reefs exist in our sheltered sealochs (other than two lochs in Ireland, and a coastal lagoon in Italy, these exist nowhere else in the world.)

The seabed is the foundation of a healthy marine ecosystem and Scotland’s is a mosaic of different habitats. The east coast Firths like the Firth of Forth, once held massive oyster beds and are now shallow muds and gravels. Further offshore are sandbanks which provide the perfect habitat for flat fish and sandeels, a keystone species underpinning the food web for a wide range of species.

All along the west coast and in Shetland and Orkney to the North, the different rocks, the currents and waves and the impact of the last ice age have meant the seabed is more varied. Deep sealochs hold steep shelves of shells, sponges and coral-like species above the muddy depths. Beyond the mouths of these sheltered waters, sands and gravels surround the islands and can be important spawning grounds for fish like herring, whilst the strong tidal currents provide vital feeding grounds for basking sharks that visit in the summer. Jutting out from islands and in deeper waters, submerged rocky reefs are vital habitats for crabs and lobsters.

The seas are by no means in a natural state. The oyster beds mentioned above have been almost entirely removed from our seas by industrial harvesting. Similarly whaling, seabird hunting and fishing have all played a part in changing the range of life in the seas today. Some ecosystems have been fundamentally transformed because of these human impacts: herring no longer run up the west coast of Scotland in huge numbers as they once did. Others ecosystems have absorbed the change or adapted to it and rebounded.

Each habitat plays a part in the mosaic of Scotland’s seabed.