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With high-quality pictures of the stomach content of the fish, fish buyers bidding on catches at auctions can avoid buying a “pig in a poke”.

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Lidunn Mosaker Boge  

Read in Norwegian

Researchers at Nofima have developed a prototype of a photo box with standardized lighting which can provide fish buyers at auctions with important information.

What information is provided at fish auctions?

Herring and mackerel are caught in seine nets, and the fishing boats can often land hundreds of tonnes at a time. Fish buyers bid on the catches through an online auction system, often while the vessel is still out at sea. The boat can then go directly from the fishing grounds to the reception station that won the auction.

When the buyers bid on the catch, they are informed of the amount of stomach content the mackerel has. The fishermen take random samples and indicate the stomach content amount on a scale from one to four. If the fish has a low stomach content it is suitable for food products, where as fish with a high stomach content are often used as bait and therefore sold at a lower price. This information therefore helps buyers determine how much they are willing to bid.

It is only when the fishermen bring the catch ashore that the buyers can take a look at what they have bought. If they are unlucky, they may find that they have bought a “pig in a poke”.

“The buyers are only told how much the fish has eaten. The problem is that they are not given any information about the actual stomach contents”, says senior researcher Stein Harris Olsen.

For what the mackerel has been feeding on is quite important.

Video blog

The scientists opened up a mackerel to study its stomach content in a video blog (texted in English):

A bothersome diet

Mackerel eat fry from herring, sprat, sand lance and other fish species, but also krill, zooplankton and small shelled swimming snails, also called sea butterflies. The swimming snails are beautiful to watch, and are quite tasty to the mackerel, but they contribute to a deterioration in the quality of the mackerel.

When a swimming snail enters a mackerel’s stomach, the digestive system of the mackerel is put to the test. The snail’s shell is very hard, made up of aragonite (a calcium carbonate), which forces the mackerel’s digestive enzymes are put into full gear while there is a significant increase in stomach acid in order to break down the shell.

If the fish is caught and dies before the stomach content is digested, the digestive process continues nonetheless. The enzymes keep breaking down tissue and muscle fibres, which can in turn lead to deterioration of the abdominal and intestinal walls of the fish. If stomach acid and enzymes leak into the fish’s abdominal cavity, the abdominal wall will deteriorate in a matter of hours, and the bones in the abdominal wall will fall apart. In some cases, the enzymes and stomach acid will create holes in the abdomen, and the fish meat will become soft, damaged and eventually spoilt.

When swimming snails are found in the stomach, we often call it “kruttåte”, which means “gunpowder feed”, because the entire stomach contents develop a dark grey or even black colour. If you rub the stomach contents between your fingers it will feel grainy, and if you taste it you will experience a stinging sensation on your tongue. These are strong indications that the catch will be prone to belly bursting as it comes ashore.

A big fish catch where all the fish have been feeding on swimming snails benefits no one. The buyers cannot use the fish for their intended purpose, and the fishermen risk complaints and lower prices. Ideally, we would like to know in advance what the mackerel has been eating, or at least agree on what the stomach content was at the time the fish was sold.

We can achieve this by taking a sample of the fish in the net before the catch is taken on board. If the mackerel is full of undesirable stomach contents, the catch as a whole can be released without injuring the fish, and the fishermen can leave the fishing grounds and try their luck someplace else.

“In this way, we will not only gain better utilization of quotas and a higher quality of mackerel that is brought ashore, but we can also avoid needlessly wasting fish resources”, says Olsen.

Simple, objective methods

Collecting information about stomach contents is not only a question of avoiding fish feeding on swimming snails. Sometimes it’s advantageous if the fish to be well fed. Mackerel used as bait must have stomach contents which can help spread the fish smell as the enzymes get to work in the stomach. Herring which is to be used for soused herring products should ideally have fed on copepods. A stomach full of copepods triggers enzyme activity which ensures an ideal maturation process for this product.

Senior researcher Geir Sogn-Grundvåg has interviewed fish buyers, fishermen and sales teams in order to find out if the information they get about what the fish has eaten at the fish auctions can be trusted. In most cases, the answer to this is no.

“The industry has long called for an objective method for identifying stomach contents in order to avoid complaints from dissatisfied buyers. Our experiments show that a simple picture taken under good lighting conditions can provide the solution, says Sogn-Grundvåg.

The standardised light in the photo box ensures a detailed picture, and makes it easy to compare with other photos.
The standardized light in the photo box ensures a detailed picture, and makes it easy to compare with other photos. Any camera type, including mobile cameras, can be used. Photo: Stein Harris Olsen © Nofima

The photo box solution

The photo box the researchers have developed is very easy to use. The box has an open end where the fish, which has been sliced open so the stomach content is visible, is inserted. There is a hole at the top for placing a camera lens – either a mobile camera or another type of camera. The important thing is that the box ensures that all the photos are taken under the same lighting conditions, so the pictures of the fish can be compared against each other. The pictures can then be uploaded directly to the online auction with information about the catch – such as the size of the catch, average size of the fish, cooling temperatures, vessel type and position.

“The pictures we took using a mobile camera were so good that we don’t just see the stomach content, but also blood and bruises, and any signs of the muscle becoming soft or cracked”, says Olsen.

This means that the pictures can actually give buyers a very useful impression of the fish they are bidding on, while the boat is still in the fishing grounds. The pictures can also be used in support or defence of claims or complaints later.

Better tools are underway

Previously, some fishermen have been taking pictures of the fish while at sea, but lighting conditions and other conditions differ, making it difficult to compare the pictures. Our researchers have shown that by using this specially designed photo box, the pictures will be able to provide the required information at the auctions.

“We have received very positive feedback from people in the industry when we have told them about our prototype. It will be interesting to see if photos will become a part of the auctions in the future”, says Sogn-Grundvåg.

The Code of Practice for Fish and Fishery Products does not currently include any requirements for photographic documentation of the catches that are sold at auctions. Since the regulations don’t require it, it remains uncertain whether the fishermen will make use of these photo boxes. However, in September a circular from the Norwegian Fishermen’s Sales Organization for Pelagic Fish (Sildesalgslaget) was issued where they asked the fishermen to describe the stomach contents of the catch, with a notice that image material will be used in complaint and claims analysis. The organization wants to get useful tools for providing relevant information at auctions, especially with regards to the stomach contents of the fish.

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