Sea lice – the scourge of the intensive salmon farming industry – are having a field day. These tiny tadpole-like parasites eat away at the skin and scales of affected fish. The effect around the head can be so bad that the bone of the living fishes’ skull is exposed – a condition known as the ‘death crown’.

Sea lice represent the single biggest problem facing industrial fish farms. So much so that companies in Scotland are reaching for ever more desperate measures to counteract them.

Wild salmon rid themselves of the lice naturally as they drop off when the fish enter freshwater on migration. For farmed salmon, caged in their tens of thousands, parasites have to be controlled using various methods from chemicals to mechanical devices.

I recently wrote about how nearly a hundred thousand salmon are reported to have been killed after the use of a new device, the ‘thermolicer’. This contraption sucks salmon out of cold seawater and pumps them through heated water before dumping them back into the sea. The temperature shock is enough to dislodge the sea lice and clearly doesn’t do much either for the health and welfare of the farmed fish themselves.

Now, according to the BBC, Scottish fish farmers have taken to using hydrogen peroxide ‘bleach’ in a bid to control sea lice. The use of hydrogen peroxide rose by 15 times between 2011 and 2015, reaching 42 litres of bleach per tonne of fish produced.

Hydrogen peroxide is bubbled through porous pipes into the cage containing crowded fish. It has been shown to be neither fully effective nor welfare-friendly. Hydrogen peroxide is a well-known irritant. Fish find it very stressful, and its application can cause significant mortality. As well as causing the fish to suffer, it is also not fully effective at removing lice. It works by stunning the lice rather than killing them. Successful treatment relies on the crowded fish knocking against each other or rubbing against the nets to dislodge the stunned lice. Any lice that are not removed simply recover.

As I recently told the Sunday Times, Compassion in World Farming has long condemned the use of Hydrogen peroxide because it is known to cause suffering to farmed fish.

It seems the industrial salmon farming industry is swimming in ever-decreasing circles trying to get on top of the parasite problems of its own making. For how much longer will we continue to see these factory farms in the sea?

Scottish salmon farms pour chemicals on parasites