THE invisibility of CO2 is a real problem because it means it’s easy to ignore and so we can avoid the responsibility of doing something about it. It’s the same with fish. They are also generally invisible in the wide and deep oceans that are hidden over the horizon, and so what happens to them is not really our fault.

While we have made a half-hearted effort to revise the way we treat the land, with partially successful attempts to deal with waste and rubbish, both personally and communally, we have almost completely ignored the oceans which cover more than 70 per cent of the planet.

This was made abundantly clear by the experiences of a Plymouth long distance swimmer who recently completed seven long distance swims in a range of seas and oceans. Most were heavily polluted with waste fuel and plastic debris, almost as bad as the debris swirl that spoils the central Pacific. More worryingly he saw very few fish during any of his swims. We never seem to learn. In 1914, when my parents were children, the last passenger pigeon died, in the United States. A century earlier the skies were full, hundreds of millions darkening the horizon, but shooting and forest clearance destroyed the species, and they are now no more, gone for ever, and it’s our fault.

We’re now repeating this mistake with marine life – another example of the way that we abuse something we share with others, and as far as fish are concerned it seems that anything goes. Deep sea fishing and factory fleets, with remarkable modern technology, wouldn’t be able to destroy fish populations so efficiently if they didn’t receive subsidies, worth millions, to pay for over a billion litres of fuel each year.

It’s not surprising, then, that about 85 per cent of the world’s oceans are overexploited, becoming marine deserts, with declining catches, and it’s not helped that fish consumption has risen four fold since the 1950s at the same time that population has trebled. Then it was four kg each per year, and now, with many more of us, it’s 17 kg.

And that’s not the worst of it, as over one third of the fish that are caught are made into fish meal for farmed fish and farm animals to eat. Fishy tasting chicken shouldn’t be too much of a surprise.

There are some attempts to improve matters, with marine reserves that ban fishing, but we are clearly waging a war on fish and we are winning.