Soon, the Korean portion of our Ocean Defenders Tour 2012 will end.

It was sad to lose the chance to talk to the people of Ulsan because of the typhoon that forced us to cancel our open boat activities there, but it also reflects the reality of seafaring life – weather determines everything.

And in this tour, we have tried to explain the overfishing crisis to the Korean public and the problems associated with Korea's scientific whaling plans. At the same time we have called for support to manage ocean resources sustainably.

When Greenpeace launched its canned tuna ranking in Korea, we wanted to shine a spotlight on the tuna brand that is the least transparent about its sourcing and fishing practices: Dongwon. Just last week we unfurled a large banner at Dongwon's headquarters in Seoul demanding accountability for its ocean destroying practices.

And as the Greenpeace ship Esperanza was leaving Korea, we encountered a Dongwon purse seine fishing vessel – exactly the type of boat that uses one of the most destructive fishing methods.

So … what to do with an ocean plunderer? Yes, expose it again! The Korean public needs to know how destructive their most popular canned tuna brand is.

The vessel MV Granada is part of Dongwon's 16-strong purse seining fleet – the biggest in Korea (the company also has 22 longliners and four reefers). The ship was built in 1981 and has been plundering the oceans for more than 30 years.

Dongwon's fleet is known to use fish aggregating devices (FAD), a very harmful way of fishing because of the high levels of by-catch of sharks, whales, turtles and juvenile tuna.

Sadly, 45% of Dongwon's purse seiner tuna catch is caught with this deadly device. And with FADs, the by-catch is worse. In fact, the by-catch rate is 20% higher compared with non-FAD seining.

Dongwon's FAD purse seine ships catch tuna in the Pacific and then send most of the tuna products to the US and Korea.

The Granada is being repaired so it can go back to fish in the Pacific, where it has a fishing license for Papua New Guinea waters.

Greenpeace activists put a huge banner at the mouth of the dry dock, where the vessel would normally be offloaded into the water. The banner said: "Dongwon's Destructive Fishing Starts Here".

But unfurling the banner was no easy task. At 25 meters in length, the banner is extremely heavy, and with a little wind it is like setting a sail. This was the most difficult action our office in Korea has done.

Prior to today's peaceful protest, Greenpeace has held discussions with Dongwon following the launch of the tuna ranking report two weeks ago. However, the company has made no solid commitments to improve the sustainability of its catch and canned tuna products.

As the biggest tuna company in Korea with more than 50% market share of canned tuna, Dongwon should abandon destructive fishing practices and start fishing more responsibly. It should care about passing healthy oceans to future generations so they too can catch and eat fish.

Dongwon should show leadership by becoming a champion of oceans protection and encouraging the government to support conservation measures at global fisheries summits.

Greenpeace is campaigning for tuna brands to improve their sustainability policies. Tuna companies should end destructive fishing methods, including FADs set on purse seine nets, and support the creation of a global network of marine reserves covering 40% of the world’s oceans.

These are necessary steps to help restore our oceans to health and to maintain living oceans with ample fish for future generations.

Yuen Ping Chow is a Greenpeace East Asia senior oceans campaigner based in Seoul