A project carried out at Marine Harvest by Nofima and the Institute of Marine Research has found that gentle and short-lived crowding and pumping of salmon from the sea cages to processing vessels is best in order to achieve good quality and a long shelf life of fillets.
One of the aims of the project, which was carried out between 2010-2011, was to find out if salmon harvested straight from the cage is more economically viable and of better quality than salmon harvested via a holding cage or straight from a well boat. The issue of high temperatures during the summer months was also studied as it can create major challenges in relation to disease and weaker fish, and at times leads to high mortality when the fish are stored in a holding cage or transported by well boat.
The well boat B/B Tauranga was reconstructed just before the project, becoming the first processing vessel in Norway.
The scientists ascertained that salmon harvested on the boat straight from the cage had extremely long ‘pre-rigor’ time, sometimes more than 35 hours. The results also showed that the onset of rigor mortis was slower in salmon stored in refrigerated sea water (RSW) in tanks onboard than salmon that was stored on ice in containers. This is of major significance, as salmon that has gone into rigor mortis when it is pumped is at major risk of developing gaping.
Stunning
The onset of rigor mortis is more rapid for salmon subjected to tough handling. This is also the case when electric stunning is used, so consequently only percussive stunning may be used during harvesting straight from the cage. It is difficult to follow the development of the salmon in the RSW tanks and the fish becomes soft regardless of how long it is stored onboard.
Different muscle groups in salmon go into rigor mortis at different times. Dark muscle (just under the skin), muscles in the neck and near the bone in the ear go into rigor mortis first. As a result, it is easy to believe that the whole fish has gone into rigor mortis, but if the salmon is provoked out of rigor mortis, other muscle groups will go into rigor mortis if it is stored on ice.
“In order to test this, experiments were carried out with salmon from the storage tanks on B/B Tauranga. We ascertained that salmon that had gone into rigor mortis were pumped out of rigor mortis and the further they had entered into rigor mortis, the greater the damage to the fillet,” said project manager and Nofima senior scientist Kjell Ø. Midling.
“We bent various groups of salmon around a 10-inch steel pipe 6, 14, 20 and 32 hours after being killed. The first three groups became stiff again, but the salmon subjected to this handling after 32 hours remained soft. The amount and degree of gaping increased in accordance with the time the salmon had been in rigor mortis.”
In all likelihood, storage in RSW onboard is gentler for the salmon than storage in containers, and salmon that are pumped while in rigor mortis develop gaping. Salmon stored and filleted 20 hours after killing had extremely little gaping, but the consistency of the fillet was softer the longer that elapsed from the time of killing until processing.
