Pre-slaughter oxygen

Marine Harvest secures the fish quality with automatic oxygenation. With a yearly production of 85 000 tonnes of salmon, the Eggesbønes plant is Marine Harvests largest processing unit in Norway. 

To buffer the transport of fish from the farms, an arrangement of four holding pens with an automatic system for oxygenation has been installed next to the slaughtery.The oxygen secures the fish against physical stress. At the slightest drop in oxygen, the systems will deliver sufficient oxygen to stabilise the condition. Oxygen measurements are recorded.   

Holding pens
Eggesbønes is the largest of Marine Harvests(MH) four regional processing plants in Norway. All fish from Marine Harvest´s fish farms between the Sognefjord in the south to Hustadvika in the north is slaughtered here. Recently a filleting factory has been joined  to the plant.
Previously, well boats were engaged in the transport and keeping of fish before slaughter. The fish was held in the well boat and pumped directly into the slaugthery. However, this practice proved very costly and demanded very close coordination.

Today four holding pens instead serve as a buffer, as a result only a single well boat is necessary for bringing all the fish from the farms to the slaughtery.  
- We commenced this new practice two years ago, and the system serves us well. First of all it has saved us a lot of expenses, since we engage only a single well boat. In the holding pens we can also observe the fish and be sure of its condition, states Mr. Cato Ryste, chief coordinator at Marine Harvest Eggesbønes, responsible for the transport and pumping of fish into the slaughtery. The buffering effect makes everything more smooth for the slaughtery, which today work in two shifts.

Cato Ryste of Marine Harvest

Oxygen diffusers
The four square holding pens are of galvanized steel type, measuring 24 by 24 meters, and with a depth of 15 meters. Each pen is equipped with a network of microdiffuser hoses, called NetOx, made by Oxyvision. The diffusers are supplied with pure oxygen in the form om microscopic bubbles. The diffusors are supplied through a battery of valves, controlled by a PLS-unit. 
In each of the holding pens there are separate optical probes for continouous measurement of oxygen content. If the oxygen drops below 100 % sat., the system will respond by adding oxygen in sufficient quantity, until 100 %  is restored. 
- The addition of oxygen is dynamic, the amount being added is relative to the deficit of oxygen in the water. The system is self regulating, Mr. Ryste explains. 
The water quality of the sea is excellent, with no local pollution or disturbing effluents. However, the cages being placed inside a small bay, the surface temperatures rise during summer, causing not only less oxygen in the water, but also a larger oxygen need from the fish. Measurements show that oxygen drops during nighttime, when algae and seaweed also consume oxygen.
- Fish constricted are often stressed if there is insufficient amount of oxygen available, however we can observe that adding oxygen calms the fish down, explains mr. Ryste. The complete installation of the oxygenation system has been carried out by Storvik Aqua.  

A quality issue
The well boats brings in 400 tonnes of fish at each delivery. After the fish is transferred to the holding pens it is kept for minimum 12 - maximum 72 hours before slaughtering. The fish is pumped from the holding pens on to live cooling tanks. At this stage extra oxygen is added to prevent stress.  During pumping the fish is closely observed to make sure that the fish is not stressed unneccessarily. 
- Fish that experience oxygen deficiency becomes stressed and exhausted, which may lead to oxygen debt in the flesh and subsequent loss of quality. This may lead to earlier onset of rigor mortis etc. Oxygen helps to prevent this from happening, Mr. Ryste explains.
Recently a new filleting plant has been established in conjunction with the slaughtery. A late as possible onset of rigor is of importance for succesful filleting.
- Oxygen management is our most important tool in our task of securing quality, says Mr. Ryste. 
The daily liquid oxygen consumption is 350 kg in average, supplying both the holding pens and the live cooling tanks.
- The oxygenation is self driven and does not require electric power to function. Which means that in any given situation the fish will not experience oxygen deficiency, says Cato Ryste.

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