Copepods (Calanus spp.) are found in enormous quantities in the Nowegian and Barents Seas. The little organisms contain a high proportion of marine oils and are thus of great commercial interest. The problem is that fishing them also captures large amounts of fish fry and jellyfish that choke up the trawl net and are difficult to get rid of.
The tendency of bubbles to rise to the surface has already been exploited in various other contexts, such as removing impurities from process water, but the concept was used for plankton capture for the first time by SINTEF Fisheries and Aquaculture Research a few years ago. At the time, the Norwegian company Calanus AS tested the concept, with somewhat variable results, and now scientists from SINTEF, NTNU and the Norwegian College of Fishery Science have taken the matter further in a project that is being fully financed by the Research Council of Norway.
The concept is based on towing a a pipe at depth, and passing air through it so that tiny bubbles rise to the surface at a certain speed. By getting the bubbles to attach themselves to the copepod plankton, these are raised to the surface, where they can be collected by means of inflatable booms that are towed astern of the vessel. As well as reducing bycatch, the concept also cuts energy consumption since the vertical area that needs to be trawled is reduced.
SINTEF has patented the system, and Calanus holds a contract to exploit the patent commercially. SINTEF's current project is intended to demonstrate whether it actually works. For example, the scientists know that the bubbles attach themselves to the plankton, but are still not certain about the buoyancy of the animals, or about how big the bubbles need to be to attach themselves properly.
In January this year, tests were carried out at SINTEF's SeaLab using microbubbles at the bottom of a tall narrow tank filled with seawater to which live copepods had been added. The idea was to study how bubbles and organisms became attached, how many of the copepods were brought to the surface and what size of bubbles were best and most energy-efficient.
Dr. Ira Leifer of the University of California, Santa Barbara, a specialist in bubble hydrodynamics and marine bubble technology, has also been involved in the project.
Provided by Sintef