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Why do fish live around shipwrecks?
Much of the seafloor is sand or mud, which offers few places for attached organisms to settle. Steel hulls, artillery shells and other debris create firm surfaces where microbes, algae, corals, sponges and other invertebrates can attach. Openings and collapsed compartments provide shelter for fish and mobile animals, while the structure can concentrate feeding opportunities.
NOAA’s synthesis describes wreck communities extending from microbes and algae to corals, sponges, invertebrates, fish and larger predators. NOAA also estimates that about three million shipwrecks lie across oceans, rivers and lakes; that is an estimate, not a current census.
What the Gulf of Mexico wreck study found
A government multidisciplinary study examined seven World War II-era wrecks in the north-central Gulf of Mexico. Six of the wrecks resulted directly from wartime activity in 1942. Researchers investigated six sites over 18 days with a remotely operated vehicle at depths from 87 to 1,964 metres.
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At every depth studied, the abundance and species richness of hard-substrate epifaunal invertebrates decreased as sampling locations moved farther from the wrecks. The pattern shows a localized association between wreck structure and attached invertebrate communities at those sites. It does not establish that every wreck produces the same effect, and other organisms in the study also varied with depth and local habitat.
Ordnance Reef off Oahu: refuge alongside contamination questions
NOAA’s June 2006 survey covered a five-square-nautical-mile area off Pokai Bay, Oahu, known as Ordnance Reef. The munitions included small-arms projectiles, large-caliber artillery projectiles and naval gun ammunition.
Most observed munitions had coral growth. NOAA reported that they provided some of the only refuge for fish on otherwise uncolonized hard bottom. The survey combined seafloor mapping and imaging with remotely operated vehicle work and sampling by specially trained divers, and was requested and funded by the U.S. Department of Defense.
The biological observation is not a safety certification. In the sampled material, NOAA detected no RDX, TNT or tetryl. DNT was detected in four sediment samples—three near munitions and one not associated with munitions—and no explosives or related compounds were detected in sampled fish. Those results apply to the locations, samples and methods used in that survey; they do not prove that unexploded ordnance is harmless or beneficial in general.
Do shipwrecks become artificial reefs?
They can function as artificial reefs, but the terms are not interchangeable. NOAA defines an artificial reef as a man-made structure that may mimic characteristics of a natural reef. Submerged shipwrecks are the most common type, yet many wrecks were accidental wartime losses rather than structures deliberately placed for habitat.
| Feature | Shipwreck or discarded munition | Purpose-built artificial reef | Natural rocky reef |
|---|---|---|---|
| How structure arrived | Usually accidental loss, disposal or wartime sinking | Intentionally deployed for a management or habitat objective | Geological formation |
| Substrate and relief | Steel, concrete, shells and debris can create hard surfaces, cavities and vertical relief | Designed or selected structures provide hard surface and relief | Bedrock, boulders and ledges formed by natural processes |
| Setting | Depends on the wreck’s location, depth, seabed and currents | Placement site is selected, but local currents and seabed still control use | Determined by geology, depth, currents and water conditions |
| Ecological function | Can shelter fish and support attached communities; effects are site-specific | Can attract high fish abundance or biomass in some regions | Provides naturally developed habitat and food webs |
| Extra management issue | Cultural remains, corrosion, oil, fuel, explosives or invasive species may be present | Materials, permitting and long-term monitoring must be managed | Protection from fishing, anchoring and other human impacts may be needed |
NOAA research on the southeastern U.S. shelf found ecological value in shipwrecks, natural rocky reefs and intentionally placed artificial reefs, while also finding differences in how they function. In that regional work, artificial reefs hosted high abundance and biomass of reef fish and elevated abundance of top predators. That result should not be generalized into a claim that every wreck increases fish stocks or regional biodiversity.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Are sunken WWII munitions dangerous to marine life?
Potential risk depends on what a site contains, how well the structure is preserved, corrosion, nearby habitat and what monitoring has actually found. NOAA identifies several possible hazards:
- Oil or fuel can escape, especially if a deteriorating wreck collapses.
- Unexploded munitions can present explosive and chemical hazards.
- A wreck can damage surrounding habitat or alter local conditions.
- Wreck surfaces and cavities can provide footholds for invasive species.
- Harmful cargo may remain inside a vessel long after sinking.
Consequently, a site may provide fish refuge while still requiring restrictions, contamination testing and structural monitoring. The presence of coral or fish is evidence of use of habitat, not proof that the wreck has a positive net effect or is safe to approach.
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How scientists assess a wreck site
Conservation surveys establish a baseline before managers decide whether to monitor, restrict access or intervene. Teams may combine:
- Historical records to identify a vessel’s cargo, sinking and wartime history.
- Divers, remotely operated vehicles or autonomous underwater vehicles to document structure and exposed artifacts.
- Oceanographic instruments to measure depth, currents, water conditions and sediment.
- Biological observations and samples to identify plants, invertebrates, fish and other organisms.
- Corrosion and threat assessments covering structural failure, pollution and human disturbance.
NOAA describes in-situ preservation as the preferred approach where appropriate under UNESCO’s underwater cultural heritage framework. That means leaving a site undisturbed when monitoring and protection can manage the risks. Divers and recreational visitors should never handle, move or disturb munitions or wreck artifacts.
What “thrives” really means
“Thrives” is best read as an observation that organisms colonize and use hard structure. Gulf surveys found denser and richer attached-invertebrate communities near particular wrecks, and the Oahu survey found coral-covered munitions serving as fish refuge on otherwise uncolonized bottom. Neither example shows that ordnance itself is good for ecosystems, that every wreck improves biodiversity, or that ecological value cancels contamination and cultural-heritage concerns.
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