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Flowing fresh

Salmon and scallop farms rely on strong currents to keep them clean.

Wouldn't it be great at the end of a dive if you could end up swimming with thousands of wild salmon? But this, of course, is a salmon farm. Current is crucial to salmon farms. Salmon themselves need moving water to bring them oxygen and keep them healthy. But also, as you can imagine, 10,000 salmon produce an awful lot of waste. The salmon are fed little and often, so that most of the pellets are eaten. But inevitably some falls through, so below nets that aren't washed by currents, it all accumulates in a foul decomposing mass. It's this that conservationists object to. Currents keep the sea and its life healthy by providing food, oxygen and waste disposal. Scallop farms exploit these natural benefits without any great impact on the environment. All you need is to be in the right place at the right time - with onion bags, hundreds of them. These submarine sieves collect the natural 'fall' of colonising scallop larvae, called spat. But it is not only the spat that set up home in the bags. This is where the hard work starts. Out go the hopeful settlers such as sea squirts and scallop predators such as young starfish. Still to be winkled out - a crab or two. Two different types of scallop - kings and queens - are separated at this stage. A handful will be kept in their uncrowded pearl net to grow to maturity in a plankton-rich current.

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3 minutes