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.

Sole or 'sliptong', what's the difference?

Good question. Menus often distinguish between sole and ‘sliptong’, even though both are the same species! The sole is an adult, while a ‘sliptong’ is a juvenile, or baby sole.

Adult sole is usually larger than 27 cm, while ‘sliptong’ ranges between 24 – 27 cm. So to catch a ‘sliptong’, you need a net through which it cannot slip!

Also distinction on the VISwijzer

On the VISwijzer, Good Fish also distinguishes between sole and sole (sliptong). This is because there are different sustainability ratings for both. Sole can score yellow or red, while ‘sliptong’ always scores red. This is because the ‘sliptong’ fishery has more negative impact on the ecosystem.

Why is 'sliptong' listed red on the VISwijzer?

Sliptong always scores red on the VISwijzer. This is due to a number of sustainability issues. Good Fish lists them here:

Sliptong is very slender and flexible in shape, and quickly hides in the bottom in case of danger. So to catch this popular target, demersal fisheries need nets with small mesh sizes and a method that scares the sole enough to get out of the sand and into the net. This leads to fishing practices with high bycatch of undersized sole and other fish species, and considerable seabed disturbance to catch the sliptong. Pulling such a heavy rig over the seabed in this way also requires a lot of fuel, contributing to high CO2 emissions per kilo of sliptong.

Adult sole on the VISwijzer

Although sliptong always scores red on the VISwijzer, the sustainability of adult sole can vary. Of all sole, 92% is caught using the unsustainable beam trawl. Some sole is MSC certified (see the question on MSC for more details), and others score yellow if caught with gill nets in the Skagerrak or Kattegat. However, you won’t be likely to come across these, as only 5% are caught this way.

Is common sole overfished?

No. The sole stock in the North Sea is no longer overfished, although it was from 2005 to 2020. In the past few years, fishing pressure has come down considerably and the sole stock is now recovering. The sole stock is also managed by minimum sizes, also known as Minimum Conservation Reference Sizes (MCRSs). Adult sole has a minimum size of 27 cm and ‘sliptong’ of 24 cm. Although it is not known whether this minimum size of 24 cm is ideal for the sole stock, the stock currently has full reproductive capacity. However, since the stock is only just at maximum sustainable yield (MSY), further research needs to be done on these minimum catch sizes to confirm its long term sustainability.

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?

The sole fishery has a very high bycatch of undersized fish and other marine life. Bycatch can be divided into three groups. The first of these is bycatch of starfish, mussels and crabs. In sole fisheries, the average bycatch of these animals is about 45% and goes straight overboard. This process is called discarding. Then you have fish that are not subject to the landing obligation and are not landed because they are not commercially interesting. This involves about 25% of the entire catch. Then there is another part (about 15%) of fish that are landed but are legally undersized. This is not allowed to be used for human consumption and is destroyed.

In short, you can say that per kilo of sole, at least 30 – 40 kilos of marine life is fished and wasted (Van Marlen et al., 2014¹).

How do you reduce this bycatch?

Several studies are being conducted to develop a method to reduce bycatch in (slip) sole fisheries. In this video, more about one of these initiatives.

¹Van Marlen, B., Wiegerinck, J. A. M., van Os-Koomen, E., & Van Barneveld, E. (2014). Catch comparison of flatfish pulse trawls and a tickler chain beam trawl. Fisheries research151, 57-69.

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.