Closed fish-farming "bags" must withstand nature's forces

Jan 14, 2014 by Kjell Emil Naas
Closed fish-farming “bags” must withstand nature’s forces
Ida Marlen Strand says that reinforcing the fish-farming bags yields some positive results but also new surprises and challenges. Credit: Zsolt Volent

New kinds of aquaculture net cages that physically separate the farmed salmon from the open waters are already in the testing phase. The idea is to prevent the dreaded salmon louse from ever reaching its intended victim by enclosing the fish inside a closed system and pumping in seawater from the depths where salmon lice do not live.

A closed system has the added benefit of enabling fish farmers to better control the salmon's environment and any emissions from production. This is good for the fish and the environment alike.

The flexible, closed net cages currently undergoing testing are basically large of GORE-TEX-like material with conventional mooring systems. The researchers are gathering much-needed knowledge about how this new production system will stand up to the forces of the open sea. Graduate engineer Ida Marlen Strand has made this the topic of her doctoral degree at the Norwegian University of Science and Technology (NTNU).

"Currents and waves will have a different impact on these closed-system bags since they are less permeable than conventional, open net cages," she says.

Unexpected findings

The design of a submerged, flexible structure is critical to prevent its collapse from the ocean's strong forces. One tactic tested was to fill the bag only partially, based on the thinking that its greater flexibility would reduce the current's effect. But experiments towing a partially filled plastic-bag model through water surprised the researchers.

"The experiments showed precisely the opposite of what we had expected," says Ms Strand. "The current's forces can actually be multiplied several times over when we reduce the filling level."

Closed fish-farming “bags” must withstand nature’s forces
Testing the impact of currents and waves on closed-system bags for fish farming. Credit: Zsolt Volent

Testing alternatives

The bag's geometry turns out to be crucial. Up to now its shape has basically been that of a conventional aquaculture net cage. But a current will deform such a bag, creating even more hydrodynamic drag.

"To try and counteract this deformation," continues the engineer, "we have carried out trials with reinforced bags. This yielded some positive results, but also new surprises and challenges. We still have a long way to go until we have a reliable enough model for calculating all the forces on these new bags."

Explore further: Simple test for resistance in lice

add to favorites email to friend print save as pdf

Related Stories

Simple test for resistance in lice

Jan 13, 2014

The Norwegian Food Safety Authority adopted a field test developed by Kari Olli Helgesen for testing salmon louse resistance to the most common treatments. The method is now being used in Chile as well.

"Pressured" for a solution

Jan 13, 2014

A simple five-minute pressure treatment makes farmed salmon sterile. Florian Sambraus may have found the solution to a highly controversial issue in Norwegian salmon production.

Raising efficiency, sustainability in salmon farming

Jan 13, 2014

Increasingly, plant-based ingredients are being substituted for marine ingredients in fish feed. Is there a limit to how much of a vegetarian diet salmon can tolerate? Marta Bou Mira is seeking answers.

Good vision for a good appetite

Jan 13, 2014

The incidence of cataracts in farmed salmon is on the rise due to vegetable-based feeds, a strong focus on fish growth and warm waters. "This is a condition we can do something about," asserts Sofie Charlotte ...

Recommended for you

Research helps identify memory molecules

1 hour ago

A newly discovered method of identifying the creation of proteins in the body could lead to new insights into how learning and memories are impaired in Alzheimer's disease.

Sorghum and biodiversity

1 hour ago

It is difficult to distinguish the human impact on the effects of natural factors on the evolution of crop plants. A Franco-Kenyan research team has managed to do just that for sorghum, one of the main cereals ...

Robotics to combat slimy pest

1 hour ago

One hundred years after they arrived in a sack of grain, white Italian snails are the target of beleaguered South Australian farmers who have joined forces with University of Sydney robotics experts to eradicate ...

User comments : 0