Small sole

If you walk along the coast in The Netherlands you will see that a vast majority of restaurants have small sole on their menu. However, there are many problems that go accompanied with the fishery for small sole.

Small sole is caught using bottom trawls, a fishing method that has serious consequences for all life on and at the bottom of the ocean.  The nets used to catch small sole have very small mesh sizes which leads to a high amount of by-catch, consisting mainly of young and undersized fish.

VISwijzer advice

Due to all the consequences the fishing method has on all life on and at the bottom of the ocean, the high amount of by-catch of undersized fish, and the amount of juvenile sole that is caught it is better to avoid eating this species. A good alternative for small sole is Lemon sole. This is the same species, however lemon sole is an adult sole that has reproduced. Visit the extensive VISwijzer for details.

Why is 'sliptong' listed red on the VISwijzer?

The VISwijzer shows common sole as being MSC certified as well as red and yellow coded. Although the common sole stock in the North Sea is doing well and is within biologically safe limits, the main fishing methods beam trawlof fishing sole are associated with various issues.  

The popular ‘ sliptong’ is actualy a juvenile common sole. Because this young sole is far more popular to eat than its older, bigger version, fishermen are after this young sole. The problem is that this ‘ sliptong’ are very slim in shape and tend to hide in the sediment very quickly in case of danger. So in order to catch this popular target bottom trawling fishermen need small meshsizes and a method that startles them enough to get out of the sand and into the net. This leads to fishing practices that have a lot of bycatch of undersized fish of other species (which are less slim) and considerable bottom disturbance and corresponding fuel consumption.  

Overall we can state that the taste for young sole (sliptong) leads to very unsustainable fisheries practices and that it would be much better if we would aim for their bigger versions (zeetong). 

More details: 

Common sole is MSC certified (see the question MSC for more details). It is coded yellow for anchored gillnets and red for beam trawling in Skagerrak and Kattegat. The main reasons for this colour coding are presented below:  

Anchored gillnets: Yellow 

Common sole caught with anchored gillnets scores yellow on the VISwijzer, but only make up less than 5% of the catch. Although this fishing method is relatively selective and has little bycatch of undersized species, it does allow for incidental bycatch of porpoises

Beam trawling: Red 

Common sole caught with beam trawls scores red on the VISwijzer. The sole stock has recovered in past years and the fishing pressure is almost sustainable long-term. However, use of 80mm mesh sizes leads to significant bycatch of endangered, threatened, and undersized species. In addition, beam trawls have a negative effect on the seafloor and can change the composition of seabed specie communities. This fishery is managed at EU level and is working towards long term management plans.  

Electric pulse fishing: Yellow 

Common sole caught with electric pulse methods scores yellow on the VISwijzer. The sole stock has recovered in past years and the fishing pressure is almost sustainable long-term. Although pulse fishing is more selective than beam trawling, bycatch of species remains an issue and the survival of this bycatch is estimated to be low. Pulse fishing also causes less damage to the seabed than the conventional beam trawl. It is now prohibited to use electric pulse methods.  

Is common sole overfished?

No, although the common sole stock in the North Sea has been overfished in the past, it is currently not being overfished. This common sole stock is within biologically safe limits and is doing well.  

Is MSC certified common sole available?

Common sole from the North Sea is certified under the ´Joint demersal fisheries (JDF) in the north sea and adjacent waters´. This certification combines several sole fisheries that were previously individually certified by MSC. It is estimated that roughly 17% of total sole catches is MSC certified. The fishing gear that is included under this certification are set nets (SN), which can be gillnets and trammel nets, Danish (anchor) seines (SDN), longlines or handlines (LL), and the beam trawl (BT) with a required mesh size of more than 120 mm.  

Despite the motivational effects a MSC certification has for fishermen to improve their practices, the “ Joint demersal Fisheries” certification is one that is controversial, since some state that beamtrawl is not sustainable in essence.  

Dutch retailers have committed to only selling MSC certified fresh fish. This means that supermarkets will most likely sell MSC certified sole, while restaurants and fish mongers will be likely to sell common sole without MSC certification, which scores almost certainly red on the VISwijzer 

Which fishing methods are used to catch sole?

The Dutch cutter fleet consists of 290 vessels. Beam trawls are used 94% of the time. The other 6% of sole fishing methods include anchored gillnets, electric pulse fishing, otter trawling, and Danish seins. However, pulse fishing has been banned in the European Union since June 2021.  

Use of beam trawls is of particular concern. Numerous scientific articles have found beam trawling to have destructive effects to the seafloor and its benthic ecosystem. 


Figure 1: Percentage of fishing gear used to catch common sole in the North Sea; directed mobile gear including beam trawls and pulse gear (94%), static gear including gill and trammel nets (2.5%), bottom trawls (2%), and other gears (1.5%) (ICES, 2021).  
How do fishing methods, used to catch sole, affect the ecosystem?

As 94% of common sole in the Northeastern part of the Atlantic Ocean is caught by beam trawling, this section will focus on that specific fishing method. Since 2017, a lot of scientific articles have been published regarding the destructive effects of beam trawling on the ecosystem.  

Beam trawling is a fishing technique in which beams with ‘tickle’ chains attached are dragged along and raked through the seabed to startle the fish which then swim into the net. These chains disturb the upper 4-8cm of the seabed, which causes significant damage and can even lead to mortality of benthic organisms. This fishing method not only causes harm to the seabed but can also make the seabed ecosystem more uniform, leading to loss of biodiversity and habitat.  

The common sole beam trawl fishery in the North Sea is considered to be a fishery with one of the largest ecological impacts on the benthic ecosystem. In addition, studies have shown that the beam trawl fishery has significant CO2 emissions. Due to all of these negative effects, the European Commission is deliberating implementation of a bottomtrawling ban by 2040.  

Was pulse fishing sustainable?

Pulse fishing is a technique that uses electric pulses to startle fish, making them swim up off the seafloor and making flatfish easier to catch. Catching common sole through electric pulse fishing was commonly used by the Dutch fleet but has recently been banned due to political and economic reasons.  

One of the benefits of pulse fishing is that it causes less physical disruption to the seabed compared to conventional beamtrawl methods. In addition, the pulse fishing gear is also lighter in weight, which reduces fuel consumption. In 2020, ICES concluded that using electric pulse to catch common sole is more sustainable than using the traditional beam trawl. This video, created by WUR, answers the question whether pulse fishing is a good alternative to beam trawling. 

The ecological improvements to this pulse gear have resulted in a receiving a yellow score on the VISwijzer. While use of beam trawls still scores red, most pulse fishermen have now returned to using beam trawls. Due to lack of political support it is unlikely that the ban on pulse fishing will be lifted in the foreseeable future.  

What is the carbon footprint of sole?

As common sole is mainly caught using beam trawls, this section will specifically focus on this fishing method. Besides vessel operations, the added weight of the fishing gear and processing and refrigeration of fish onboard, increases CO2 emissions significantly. 

Due to the extreme weight of the fishing gear, the beam trawl fishery has the highest fuel consumption. Beam trawl vessels require larger and more powerful engines to operate which further increases the weight of the vessel. As beam trawls are most commonly used to catch common sole in the North Sea, catching these fish is contributing to climate change.  

Besides the release of CO2 emissions through fuel consumption, carbon that has been sequestered in the seabed is released through seabed destruction. This, now free, carbon is resuspended and can re-enter the atmosphere, further contributing to climate change. The extent to which this contributes is still unclear and under discussion. 

Nevertheless, a new invention, the sumwing, an aerofoil-shaped beam that replaces the traditional beam trawler beam, has been found to reduce fuel consumption by 13%. However, replacement of beam trawlers with pulse fishing, which has recently been banned, can reduce fuel consumption by 33%.  

What is the extent of the bycatch issue in the common sole fishery and can this be avoided?

Trawling with small, although legal, mesh sizes near the bottom where fish are occuring in a mixed order of species and sizes is prone to high bycatch rates. The common sole catch of the beam trawl fishery in the North Sea consisted of 5-30% undersized fish and 5-30% non-target species.  

It is difficult to avoid this since the target of this fishery, the sliptong is such a slender young fish. There are various studies being done to develop a method that decreases the amount of bycatch while remaining the wanted young sole sliptong. In this movie more about one of these initiatives: 

Illegal activities  

Some fisheries use nets with mesh sizes that are too small according to the standards set by the Common Fishery Policy (CFP) and the landing obligation. The size regulations are difficult to regulate due to lack of observer programmes, and thus illegal use of smaller mesh sizes does take place in the common sole fishery.  

Is common sole farmed?

Yes, common sole is being farmed in numerous extensive and intensive aquaculture farms in China, France, Italy, Portugal and Spain. The production is small in comparison to the amount that is wild caught. Common sole that is farmed has received a green score on the VISwijzer as the farming operation has very little impact on the environment.  

Where does common sole come from that is available in NL?

Common sole is one of the most important commercial species for the Dutch fishing sector, and is caught in the Atlantic or Arctic Ocean (FAO 27). Out of all countries that target common sole, the Netherlands has the highest catch rates, as it is a popular fish in the Netherlands. In 2020, 30% of the Dutch fish auctions consisted of sole.  

In comparison to Atlantic salmon and cod, common sole is not widely available in retail outlets in the Netherlands. There are some products sold in Dutch supermarkets, but most sole is sold by fish mongers and restaurants and is therefore not always MSC certified.  

What season is sole available?

Common sole from the North Sea is available year-round. However, the most optimal seasons are from June to January, as common sole spawn outside of this period.  

Are there vegan alternatives available for common sole?

Currently, there are no vegan products that specifically aim to replace common sole in the Netherlands. Common sole is often replaced by cod and there are vegan alternatives for this species such as: “Lekker Veggie Kibbeling Vegan” (Jumbo), “Salt & Vinegar Battered Fishless Filets” and “Lemon and Pepper Breaded Fishless Filets” (Quorn) “Tasty Codd”, or “De Lekkerbeck” (Vegan ZeaStar). In addition, there are blogs that discus how to recreate vegan sole dishes using products such as tofu.