Time heals fillet wounds

Janne Karin Brodin

Within a year after an outbreak of pancreatic disease (PD) in a salmon fish farm, fillet quality is back to the same standard as in non-infected fish. Suppositions of lasting fillet quality loss due to this illness can by and large be repudiated.

PhD-student Jørgen Lerfall looking for fillets that show signs of PD infection Photo: Vegard Stensvold

The fish farming industry in is sceptical to buying fish material from fish farms that have had PD infection, since it is supposed that fish health and fillet quality are linked.

The results from the experiments conducted by Jørgen Lerfall – PhD student at the University of Lift Sciences in Norway and Sør-Trøndelag University College (HiST) show that even though quality is reduced for a few months after an outbreak, fillet quality is restored within 7-12 months. This goes for colour, firmness and the occurrence of brown spots and gaping (ruptures in the salmon fillets) in the fish meat.

It has been suspected that PD leads to increased fillet gaping, but our results show that this is incorrect. Salmon with a case history of illness actually displayed less fillet gaping than fish that had not been ill. What is more, the fillet was firmer than usual, says Lerfall.

PD is a highly infectious and notifiable disease, and has mostly been found in West Norwegian salmon fish farms. Whereas inoculation is possible for bacterial diseases, there is as per today no efficient treatment for this disease.

Although PD has been seen in Norway for some years, it is during the past 5 years that is has become more active. It is therefore relatively new as a problematic illness for the industry.

The results from Lerfall’s PhD studies give the fish farming industry a pointer to the development of salmon fillet quality after PD infection. This knowledge is important in order to avoid reduced quality salmon fillets being released into a market of demanding customers, causing complaints and economic losses for the industry.

The results show that quality deviation (especially poor colour) is most pronounced soon after an outbreak of PD, and that quality is largely normalised within half a year. After having been infected by PD the salmon is very sensitive to stress, and it is important to treat the fish extra gently to avoid quality loss, especially in connection with slaughter. After an outbreak of PD, easily digestible feeds can help the salmon recover more quickly. Preferably, this feeding regime should be continued for more than half a year before slaughter.

Extended quality control before slaughter
Fish farms have standard procedures for quality control prior to slaughter.

If PD is suspected, it is important to examine the entire fillet – not just one selected cutlet. The reason for this is among other things that brown spots are found in other places than just the cutlet one normally takes samples from. It is recommended that at least 30 fillets from each netpen should be examined, says Lerfall.

A fish that looks muscular and rounded is normally in good condition and of good quality. If the fish has a lean exterior, the probability that the interior will be of poorer quality is greater.

There is good reason to keep watch. In some cases, the fish does not regain its colour properly after the illness – probably due to lasting muscle damage. This should be investigated further, and possibly the question of adding extra pigment to the feed could be considered, says Lerfall.

Quality issues are not always visible straight after filleting, but can surface after smoking and vacuum packing.

The observed quality loss is all visual. A sensory test was performed where infected as well as healthy fish was prepared. As it turned out, there was no difference in taste between the two.

Salmon fillets with PD infection may vary in shape and colour. PD infection was found in both fillets Photo: PhotoFish AS

Important financial consequences
PD is the disease that is most costly to the fish farming industry. It can strike hard with high mortality, or last for a long time in a milder form.

Occurrence of PD peaked a couple of years ago. Even though last year was not so bad, we are talking about great financial losses, says Lerfall

The virus first infects the pancreas and progresses to the heart. The heart is weakened and becomes unable to pump the blood properly into the muscles and tissues. This leads to muscle damage and inflammation of the muscle tissue.

Muscle damage and quality loss is really a secondary damage. (???)Fish that dies from this illness dies due to a weak heart or a damaged pancreas and inability to digest food.

Cooperation

The project was led by Nofima as a cooperative effort with Norwegian Veterinary Institute (VI) and the Technical University of Denmark (DTU). DTU was responsible for the microbiological investigations, and VI conducted the follow-up of the disease. Lerfall’s focus has been on the consequences of the disease on product quality.

This means that the fish used in this project is well documented as relates to the disease, says Lerfall.

Optimised salting

Lerfall has also looked at optimising salting in order to achieve the best possible fillet colour, both in fish filleted straight after slaughter and fish slaughtered 4-5 days before filleting. In this study, Lerfall examined healthy fish.

The results show that salting must be done in different ways according to whether the fish has been through the rigor phase while the fillet is on the bone or not, says Lerfall.

This means that it is possible to influence quality in the preferred direction through the choice of salting method.

Source and financing

Jørgen Lerfall defended his PhD thesis in September 2011 at the Norwegian University of Life Sciences. The title of the thesis was “The effect of pancreas disease and salting conditions on the quality of raw and cold-smoked Atlantic salmon (Salmo salar L.)”. The studies included in the thesis were financed by the Research Council of Norway, The Fishery and Aquaculture Industry Research Fund (FHF), The Norwegian Seafood Association (NSL) and Sør-Trøndelag University College (HiST).

Updated: 16.11.11
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