A lightweight fish pen to move farms to deeper seas

The University of Queensland has co-led a project to design a cost-effective but stable pen to expand fish farming to deeper ocean areas to help feed a growing global population. The research was published in Journal of Marine Science and Engineering.

Professor Chien Ming Wang of the School of Civil Engineering said competition for shelter space near water meant industry needed to move further afield, prompting the Blue Economy Cooperative Research Center to commission the project.

“Our design will help fish farms meet the growing demand for seafood in an affordable and sustainable way,” said Professor Wang.

“Moving to deeper and cooler water is great for the ocean environment and for the health and welfare of the fish but if a plume is too thin, the big waves in the ocean during storms can quickly tear it apart .

“This solution—named SeaFisher—combines high-density polyethylene (HDPE) pipes with custom brackets and connectors to create a frame to give us strength in a lightweight structure.

“To keep the pen and the fish safe, water pumps fill the pipes so that it sinks in stormy weather and when the danger has passed, that ballast can be emptied so the pen rises to the surface again .

“A front shield deflects debris around the pen while smooth, reinforced PET plastic netting keeps predators out.”

The pen is estimated to initially cost about $6 million, a fraction of the cost of other, more restrictive offshore fish farming structures used in the northern hemisphere.

Professor Wang said modeling shows the SeaFisher can grow 10 times the fish of other offshore pens on the market, some of which cost up to $180 million dollars.

“It is a modular system of 20 cubic meter cages, linked two side by side to enable easy access for monitoring, feeding, and harvesting the fish stock,” he said.

“We estimate that each cubic cage can comfortably house 24,000 fully grown, 5-kilogram finfish.

“It can be used to farm different types of finfish next to each other and can be adapted to grow seaweed, which uses waste from fish pens to fertilize the plant crop.

“By mooring the pen with a chain attached to a single suction anchor that allows it to move with water currents, the impact on the ocean floor is minimal.”

The design has been patented and scale-models will now be tested before prototypes are made.