UAS footage of RNZN Divers surveying the area around HMNZS Manawanui on the Southern Coast of Upulo as part of Op Resolution. Photo: NZDF

Manawanui cleanup resumes – Expert Reaction

The next step in clearing the Manawanui wreck from a reef in Sāmoa is to remove the fuel from the sunken NZ Navy ship.

The NZ Defence Force plans to engage a company to carry out the “complex and technical process” of fuel removal. In the meantime, dive teams are continuing underwater surveillance of the ship.

See previous expert comments on the Manawanui wreck here.

The Science Media Centre asked experts to comment. Feel free to use these comments in your reporting or follow up with the contact details provided.


David Schiel and Chris Battershill, Professors of Marine Science, Canterbury University and Waikato University, comment:

“Three weeks after the sinking of the Manawanui and we still have little idea about the salvage operation and potential environmental effects. The ship is on its side in about 30 m of water, not 150 m as first reported. That is within normal diving limits and offers some hope that salvage is possible without the hazards of deep-sea diving and the limited time divers can be underwater without the need for long decompression times. However, the onset of the cyclone season in the tropical Pacific is cause for concern and speed for recovery of the fuel will be essential.

“Little is known publicly about what fuels may have been removed from the ship, how stable it is in its present position, and what was actually in the several container loads of material taken away, apart from food in at least one of them. Although there are pictures available of the ship underwater, we still have not seen any that would give specific clues about environmental damage to the coral reef so far.

“All shipwrecks are unique because of the types of ships involved, the grades of fuel aboard, and the nature of the ecosystems around them. The Manawanui reportedly did not have heavy fuel oil or bunker oil on board, which is the tar-like goop that can cause extensive damage to birds, mammals and marine species. Those who remember the RV Rena oil spill in New Zealand in 2011 will have seen pictures of sticky black oil on the shores and marine life around the Bay of Plenty. It is hard to clean up such oils from the environment, and they can persist for many years in backwater areas like salt marshes. The fuel oil on the Manawanui is mostly a type of diesel, with short-chain hydrocarbons that tend to evaporate quickly with wave action. It appears so far that at most there are only minor leaks of this fuel.

“It is likely that most of the environmental damage to date will be from the impact of the vessel itself on the coral reef, as it hit the reef and then slid down the slope. It will have scraped off corals and resident invertebrates such as sea fans, sea squirts and big sponges that usually inhabit the steep seaward slopes of coral reefs with high flows of water. Without seeing pictures of that surrounding area, we can only speculate at this stage about the environmental damage. Based on what we saw in our work on the RV Rena, however, we can anticipate seeing scrape marks and damage to the reef slope where the vessel slid down. We need to see some high-resolution photos and videos before we can be more definitive.

“Of relevance here is the type of anti-fouling paint used on the ship. In two instances on the Great Barrier Reef, Tributyl Tin (TBT) anti-fouling paints were infused into the crushed coral under the ship hull and where the coral rubble was left on the reef, little recovery of corals was recorded for many years.

“No salvage operation of a vessel this size is simple. Dive teams need to be working over long periods, which requires a tender vessel or vessels. If fuel is to be removed, barges or tankers need to be on site. These will need to be brought in from overseas, and will take some time to be in place and operable, if that is the plan. They can only operate in benign weather conditions. We can only speculate at this stage whether it is possible to salvage the ship itself, although this seems unlikely, especially if it slides further down the reef into deep water. Then it will be a question of leaving it where it lies or going through the very expensive and time-consuming process of cutting it into pieces and removing it bit by bit.

Conflict of interest statements: Prof. David Schiel: No conflicts of interest. Prof. Chris Battershill: No conflicts of interest. Professor Battershill has dived in this area in Samoa before, and has relevant knowledge regarding the Rena and similar ship groundings on the Great Barrier Reef. 


Nick Ling, Associate Professor in Biodiversity and Ecology, University of Waikato, comments:

“There is clearly considerable urgency to remove the remaining fuel from the wreck as the start of the Pacific cyclone season approaches. I’m somewhat surprised that a contractor is yet to be engaged for this work given that this was always the priority, and that there will obviously be further delay in getting salvage assets to Samoa once a contractor is engaged.

“As to any decision regarding removing the wreck itself, that is very complex decision process given:

  1. the potential cost,
  2. difficulties in achieving its removal considering where it lies adjacent to the reef and in a relatively exposed location,
  3. the wishes of local people and the Samoan Government.

“The Pacific is already littered with the wrecks of warships from past conflicts and nuclear testing, and many of these have become iconic tourist dive sites. There is potential for the same for the Manawanui although that would depend on whether the ship lies in a sufficiently sheltered location to survive long-term exposure to heavy seas. In its current state the wreck itself poses relatively little threat to the reef once the fuel and other potential contaminants are removed, but if it were to break up then it could cause considerable additional physical damage to the structure of the reef.

“While many old warships are purposely sunk as dive sites or for the purposes of forming an artificial reef, they are rigorously decontaminated first. They are also made safe for divers by either sealing passageways to prevent internal access or partially deconstructed to make access safer. If a decision was made to leave the hull in situ then the safety of the wreck and its long-term survival needs to be carefully considered. However, removing the wreck entirely would be extremely challenging given the lack of infrastructure to deal with the salvaged wreck in Samoa or anywhere nearby.

“Finally, with regard to the current or future impact of the wreck or its fuel on the reef and local fish stocks, I’m also surprised that there appears to be no ecological monitoring of the site as yet given concerns of local fishers regarding possible fuel contamination of fish. The NZDF have reported that ongoing monitoring of the wreck and surrounding coastal area is continuing but it is unclear whether this relates simply to the presence of debris and oil. I would have thought the NZ Government owed it to the people of Samoa to undertake whatever work was required to allay fears of the potential or current impacts of the wreck.

“While the fuel carried on the Manawanui is considerably less problematic than heavier forms of oil such as crude or the heavy fuel oil from the Rena container ship, there is still the potential for chemical contamination of the reef and its fish stocks by toxic hydrocarbons. There will also be other forms of oil on board such as lubricating oils. The Rena Recovery Project in Tauranga, following the wreck of the Rena, began ecological monitoring within days of the wreck and was able to not only track the contamination of local wildlife and shellfish but also the rate of recovery from the impacts of the oil spill.”

No conflicts of interest.