Yesterday we freed several hundred illegally caught bluefin tuna, just a week after our ship the Steve Irwin left Malta.

Having recently returned from an eventful anti-whaling campaign in Antarctic waters, I've discovered the Mediterranean brings different challenges. The engine room has turned into a sauna and sleeping is hard with the soaring heat radiating through the deck above. We are in one of the most overfished seas in the world and have been patrolling the area south of Malta for illegal fishing operations.

In summer the waters of the Mediterranean are calm and warm. Warming up from the east, once the temperature near the surface reaches over 20C, it is an ideal spot for bluefin tuna populations to spawn. The bluefin tuna is a highly prized fish, which finds its way into the Asian markets as a sushi delicacy. But this increasing demand for the fish has taken its toll on the Mediterranean populations, of which 85% has disappeared in the last 50 years.

Day and night we encounter fishing vessels. Our radar is dotted with targets but until yesterday all of the French, Italian and Tunisian vessels we have come across had fishing permits and were frequently escorted by French or Maltese patrol ships. It is no surprise they don't take chances, with both Sea Shepherd and Greenpeace currently active in the area.

With European navy and coastguard vessels patrolling the waters around Malta and in the Tyrrhenian Sea, we decided to enter Libyan waters, an area known for illegal and unregulated fishing. The country claimed a 62 mile fishing zone off its coast in 2005 and has since stopped any independent observers or patrol vessels from entering. Inspectors from the International Commission for the Conservation of Atlantic Tunas (ICCAT), which manages the bluefin tuna fishery, have been hindered from doing their work.

Illegal fishing is believed to be widespread in the area, with one important bluefin tuna spawning area in the Libyan zone of the Gulf of Syrte attracting many fishermen. Greenpeace planned to head into Libyan waters a couple of years ago, but shelved the plan. While neither the EU, nor other Mediterranean countries acknowledge Libya's new fishing zone claim, no nation has dared to enter the disputed waters.

The situation is getting desperate for a fish that is set on a course towards extinction in the near future. In recent years, many attempts to give bluefin tuna added protection through the conventional political processes have failed. In this light, the willingness to directly intervene against illegal fishermen in this most lucrative type of fishing operations might be one of the last chances to get attention for the issue and get the species better protection to ensure its survival.

While the atmosphere on the ship is good, there is a slight hint of nervousness, not knowing what lies ahead. We all realise we are in the waters of a country which has facilitated illegal fishing for many years and might not back down from using force to hinder us. However, our crew is defiant and ready to take the risks necessary to see the mission through. As the US senator and environmentalist Gaylord Nelson once said: "The ultimate test of man's conscience may be his willingness to sacrifice something today for future generations whose words of thanks will not be heard".

• Wietse van der Werf works as ship's carpenter and engineer on the Sea Shepherd vessel Steve Irwin