Aquaculture Cage Technology Review

We have written about aquaculture cages on a number of occasions in the past. If you would be interested in reviewing the array or articles on this topic please click here.

Marine finfish aquaculture uses cages to contain fish and to prevent access by predators. A typical cage consists of a net suspended at the surface by a floating structure (e.g., a circular plastic pipe or collar). The walls of the net hang vertically from the water’s surface to a depth of around 6 to 20 meters. The cage is also closed across the bottom of the cylinder.

Since the 1980s, the size of cages has increased markedly. Early on cages were 40 meters in circumference (or approx 130 square meters in area across the top). Today’s standard containment pen is 100 meters in circumference, (800 square metres across the top) with volumes of 6,000 to 20,000 cubic meters at depths between 7.5 and 25 metres. A single aquaculture farm operation can have 30 of these circular pens.

Consequently, the nets used in such cages have become are huge. Dry, they weigh several tons and at the end of a growing cycle they can weigh 20 tons or more owing to biofouling from marine organisms such as algae and mussels.

Practically speaking, the handling of these huge nets for repair and maintenance has become a logistical nightmare as well as a significant economic problem for fish farmers.

Too bulky and heavy to handle, nets must be cut off the pens, often ending up on the ocean bottom where, according to newly issued environmental regulations, they must be recovered. An inconvenient and costly exercise… even when things go smoothly!

Aquaculture nets are subject to a number of challenges.

Sea currents deform the nets, causing sagging, and bagging. This can result in pockets where predators such as seals can push in to bite fish, with resulting tears to the net.

The standard solution to this is yet more weight and inconvenience – a predator net. Typically the predator net is a secondary, coarser net entirely surrounding the containment net. It is normally suspended from outside the floating circular collar, and the containment net from the inside of the collar, to maintain adequate separation between predator and containment nets. This is generally believed to be sufficient to keep predators from pushing into the containment net.

Often a third net has to be incorporated into the current configuration – a bird net is suspended above the surface to prevent access to the fish by predator birds, e.g., osprey, eagles, herons, and gulls.

To keep all this secure, adequate mooring technology must be deployed, taking into account both normal and exceptional sea states.  A complex business when zero defects is the aim!

Currently, the largest aquaculture companies on the planet are producing in the order of 300,000 tonnes per annum.  The FAO tells us that there is likely to be a 40 million tonnes per annum fish supply gap by 2030. Clearly, the fish containment technologies we are currently using are a major limiter on mankind’s ability to meet the FAO challenge.

What are the alternatives? Where must we go to achieve the scale of production to fill the gap??

Tags: cage
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