The puzzle of the mushy halibut

By Sara Kirstine Hald

They quiver like jelly when pressed. They are smooth and filled with a liquid that can be squeezed out. They shine and are sometimes so transparent that you can see through them. 

These are fillets from what the locals call ‘jelly fish’; Greenland halibut that, for unknown reasons, are unusually soft and may be of such low quality that they are inedible. This mushy halibut syndrome is what Natacha Severin is researching. 

“The fillets from these fish are difficult to hold in your hand – it’s like trying to hold onto a lump of jelly. I’ve crawled around on all fours on the factory floor while trying to handle them,” she says. 

Inedible halibut 

“Jelly fish” is a kind of fish slang for a syndrome seen in the Greenland halibut, among other species,” says Natacha Severin, a veterinarian and PhD student at the Section for Parasitology and Aquatic Pathobiology at the University of Copenhagen. 

The term covers the fact that some fish develop an unusual texture in their flesh. They are essentially ordinary Greenland halibut, but for unknown reasons, they become mushy and watery, allowing liquid to be squeezed out of the fillets, and they fall apart when cooked. Since the fillets from these fish also lack taste and texture, they are less suitable for consumption – and, in some cases, entirely inedible.

“Jelly fish” is a kind of fish slang for a syndrome seen in the Greenland halibut, among other species”
-Natacha Severin

The phenomenon occurs in several different fish species and various locations worldwide. However, the cause of the condition in the Greenlandic halibut is unknown. This is where Natacha Severin comes into the picture. Through her PhD project, she aims to understand what characterizes mushy halibut.

Natacha Severin calls this picture “4 shades of jelly”. On the left is a normal Greenland halibut fillet of good quality. Towards the right, they become gradually more gel-like.

Photo: Natacha Severin

Under-investigated topic 

The mushy halibut syndrome has been described in various sources since the early 1900s, and according to Natacha Severin, it is a well-known problem in the fishing industry. However, she notes that the topic is “under-explored.” 

“I won’t say that we know nothing about it because we do know some things, but we don’t know the cause or the significance for the fish, its surroundings, or the fish stock,” she says. 

She explains that there are several theories about what causes the condition. For example, it may occur during spawning, where fish use their energy reserves to reproduce. The syndrome could also be related to pressure, temperatures, or pollution in the sea. Or it could simply be due to genetics. 

“We have no methods to sort mushy halibut at harvest because we don’t know what to look for,”
– Natacha Severin. 

Similar conditions can occur in other fish species due to parasites. Although such parasites have not yet been found in Greenlandic halibut, this theory has not been ruled out. The condition could also be related to the fish’s diet, such as a lack of food or specific nutrients. Experienced fishermen also point out that geography and the timing of the catch may influence the occurrence. 

Mushy halibut leads to waste 

In some cases, the jelly condition may be recognizable in freshly caught fish, but usually, it only becomes apparent during processing. This means that Greenland halibut with the mushy syndrome may have traveled long before being discovered. 

Let’s consider a scenario: a whole, frozen Greenland halibut has been exported to Asia, where it is thawed, and the recipient begins to clean and fillet it. Only then is it discovered that it is a mushy halibut and may be unusable. 

In such a case, resources have been used to catch the fish, transport it to the factory, package it, send it across the world, and deliver it to the recipient. Afterward, the recipient uses resources to store and clean the fish, only to potentially – worst case – having to discard it.  

Resources, including labor, materials, and transportation, are wasted. This is not sustainable – neither in terms of economy, fish quota utilization, or fish stock conservation.

The video shows a Greenland halibut that is so mushy that it can be recognized before the fish has been filleted.

Video: Natacha Severin

How big is the problem? 

Natacha Severin collaborates with Royal Greenland on her PhD project. They have identified mushy halibut syndrome as a challenge. The fishing industry knows the problem, but how extensive is it? 

“It’s difficult to say how big the problem is. There are no hard numbers, but mushy halibut are caught regularly – it’s not something that happens once in a blue moon. And the problem is certainly big enough for Royal Greenland to want it investigated,” says Natacha Severin. 

No one knows what to look for

For now, one can only guess what causes the mushy halibut syndrome in Greenland halibut. Since almost nothing is known about mushy halibut, they are challenging to avoid. 

“We have no methods to sort mushy halibut at harvest because we don’t know what to look for,” says Natacha Severin.

In her project, she will investigate what characterizes the fish affected by the mushy halibut syndrome. If, for example, the mushy condition is often seen in younger fish or spawning females, this knowledge could be used for sorting. 

Utilizing mushy halibut

Natacha Severin hopes that more knowledge in the field will make it possible to utilize the mushy halibut caught. This way, both the resources of nature and the fishing industry will be used more efficiently – as will the fish quotas. 

“Without being absolutely certain, I don’t think we can eliminate the challenge of mushy halibut. And since they exist, we have to deal with them. The question is how we can handle them and ensure that these lower-quality fish are detected early so that they can be utilized and gain some value rather than just ending up in the trash,” she says. 

Natacha Severin is about a year into her Ph.D. project and will finish the mushy halibut syndrome project in two years. Funding for the project has been provided by the Danish state’s funds for Arctic research, GrønlandBankens Erhvervsfond, and Royal Greenland. If you have experiences with mushy halibut (jelly fisk) that may be relevant to the project, you can contact Natacha Severin at nls@sund.ku.dk.