I’ve just returned from a dive beneath a giant floating catastrophe, an ugly lump of death-dealing metal floating in the high seas. No, it wasn’t a warship. This particular lump of metal was a fish aggregating device, or FAD, that we happened upon on the high seas by a mix of luck, good eyes and some sketchy old data.

Along with seven of its foul fetid friends it now sits stinking and dripping on the poop-deck (that’s the bit at the back) of the Esperanza, Greenpeace’s largest ship and my home for the last few hot weeks. My usual home is in Sydney; and my usual work-day is spent negotiating with Australian tuna brands and retailers mostly about dropping FADs from their suppliers and making sure they know where the tuna in every tin comes from.

Being out on the ship in the waters of the Pacific (where nearly all our tuna comes from) adds a whole new perspective to what I do.

I’ve seen the destruction caused by fishing vessels called longliners catching the tuna that ends up in Japanese sashimi markets. But the biggest catches come from a different style of fishing vessel: Purse seine boats use giant nets to fish for skipjack tuna – the main species that ends up in the 40,000 tonnes of tinned tuna we gobble up in Australia each year.

This is what the FADs are used for. They can be great missile-like metal hunks anchored to the seafloor or free-floating islands of flotsam and jetsam lashed together, with sonar devices and radio beacons attached making them easy for fishing boats to find.

When they do find them, and set their mammoth net around them, every creature larger than a tadpole is caught up and crushed by the sheer weight of marine life that makes up the average catch. The biggest haul I’ve witnessed was 82 tonnes. Imagine being at the bottom of that. Very little survives.

The reality of this monstrous exercise is truly brought home when you dive beneath one of these FADs, as I’ve been lucky enough to do. Charles Clover’s quip ‘killed alongside your tinned skipjack is almost the entire cast of finding Nemo’ is proved dead-right.

Sharks, barracuda; trigger, bat and flying fish are inevitably schooling around all the FADs we find. Even the odd turtle popped up for some of the dives, but – sod’s law – never when I happened to be wearing my togs. Seeing FADs in action, brings home the enormity of the changes the tuna industry has made in recent months in Australia.

When we released our tuna ranking in August we were happy to announce the first major and widely available brand in Australia to drop the destructive use of FADs in favour of pole and line tuna – Safcol. Tuna caught by pole and line is the most sustainable option available when it’s done well. Pole and line fishers rarely catch anything but tuna, and because they’re caught one at a time, with special straight hooks, any juvenile fish can be thrown back alive.

Most tuna is still caught using FADs. So every tin of regular tuna, swapped for a pole and line option is saving marine life.

It’s tremendous then that there are now numerous pole and line options available to us. Once someone shows that it can be done, as Safcol did, then industry starts to change. All the major supermarket chains in Australia now have their own brands of pole and line tuna. Coles were first to follow Safcol with a range of pole and line caught tuna, then Aldi and Woolworths. IGA will bring out its own version before the year is out. I expect more will follow. Perhaps the biggest news was when Greenseas announced all of its tuna would be FAD-free by 2015. 2015 is too far away and we hope it happens sooner. But it’s a start. Now it’s up to the remaining brands, like John West, to better Greenseas’ commitment.

It’s incredibly satisfying to have been a part of the changes and to have witnessed the power of consumers who don’t want their tuna to be wreaking havoc on Pacific marine life. Tuna companies are listening. Now it’s up to these companies to continue their reform and make ALL their tuna sustainable. And to do it urgently.

Every tin of tuna caught the old way is most certainly responsible for killing the types of marine life I saw swimming over recent days. So please, choose a sustainable option. And write, phone and harass the brands still selling tuna caught using these dirty oceanic death traps.

TAKE ACTION: Change your tuna!