VESSEL REVIEW | Egget – Norwegian aquaculture company introduces durable, self-contained fish pen concept

Norwegian aquaculture technology company Hauge Aqua will soon deploy a new type of enclosed salmon pen that was developed to address some of the issues that adversely affect the output of fish farming, particularly farming using traditional open pen systems.

Built by local company Herde Kompositt, the 21-metre-high pen has been given the appropriate name Egget (“Egg”) due to its shape. It is a fully contained system within a hull made from a sandwich of composite and FRP.

Hauge Aqua said the shape provides a complete double-curved surface. Ninety per cent of the tank is submerged and not visible during operation, whereas ten percent is above the water and filled with ventilated air. Inside the tank, a central tube may be placed and may stiffen off the structure vertically.

The shape diverts external forces and makes a robust geometrical structure that is fully enclosed with a dome protecting against flushing of waves into the unit. Also, it provides a sheltered and safe working area for personnel.

Egget was developed as a sustainable replacement for the large circular net pens used in today’s typical fish farm site setup. It is also designed to be supported by existing infrastructure such as feed barges and electrical power barges.

The pen is towed into the anchoring frame while still empty of water. Once inside the frame and plugged to a power supply, valves in the bottom are opened, and the pen will gradually sink until it reaches its operating position. The tank is then moored horizontally into an existing mooring system in a farm.

The water intake is located at the bottom of the unit at a depth where lice are not normally found. Also, due to the level-separated inflow and outflow of water, the risk of infection via site-to-site transfer is significantly reduced.

The unique water flow enables the system to draw water coming into the structure separately from water being released. The inlets are located in the bottom of the pen, and water enters via two main pumps that draw water from depths of 17 to 38 metres.

The water is then put in a circular movement with the aid of specialised equipment and eventually brought to the top of the pen where it is then released three to six metres beneath the surface. Water quality and flow rate can be controlled, ensuring steady oxygen levels for the fish within as well as sufficient release of CO2. The water inlets and outlets are double-secured so that the fish cannot escape and that they are protected from predators.

The pen also enables feeding using precise volumes to prevent over- and undersupply of feed to the fish within. The interior is designed so that the horizontal and upwards movement of the water keeps feed pellets accessible to fish for a longer period compared to open systems where feed that is not eaten by the fish drifts along with the current and out of the pen, resulting in losses.

The shape of the pen is designed to increase the speed of the water and of the vertical and centripetal forces on particles as the water flows to the top of the tank. These forces may be utilised to trap particles and prevent these from being discharged. While most of the water is discharged through main valves, the technology will filter solid fish faeces in the bottom part of the pen through the constant extraction of water brought to particle filters. Faeces, uneaten feed, and other organic solids are then collected in the tank and can either be recycled or repurposed.

The effective collection of faeces translates into a reduced risk of nutrient discharge. Hauge Aqua said this improves efficiency by making it possible for a greater volume of fish to be produced per site.

Egget’s shape and durability make it suitable for a broad range of sites.