Level of Fouling Scale
Turning a complex biosecurity problem into a single, reliable rating.
Every boat and ship that moves between ports carries the potential to spread marine pests on its submerged surfaces. The long-standing challenge has been to measure that risk quickly and consistently, without specialist equipment or expertise. The Level of Fouling (LOF) scale does exactly that, and it has become a widely used method around the world for rapidly characterising biofouling on the hulls of boats and ships.
Cawthron Institute’s marine biosecurity scientists are at the forefront of the scale’s development and application, with the support of partners, stakeholders and funders including the Ministry of Business, Innovation and Employment, Biosecurity New Zealand and Auckland Council.
What is the Level of Fouling scale?
The LOF scale is a method for rapidly characterising biofouling, the accumulation of marine plants and animals, on the submerged surfaces of boats and ships. It is a nominal rank scale with six categories, running from LOF 0 (completely clean) to LOF 5 (very heavily fouled). Each category is based simply on how much biofouling is visible on a vessel’s underwater surfaces.
The result is a straightforward, repeatable way to score vessel biofouling. There is no laboratory work, no species counts and no specialist sampling gear required.
Tools and Resources
The Scale
Two quick definitions help in applying the scale. A slime layer (or biofilm) is the first, microscopic layer of fouling, not individually visible to the naked eye. Macrofouling describes the larger organisms you can see and tell apart, such as barnacles, seaweeds, shellfish, sponges and sea squirts.
| Rank | Biofouling cover | What it looks like |
| LOF 0 | None | No slime layer and no visible fouling. Only clean surfaces. |
| LOF 1 | Slime only | A slime layer (biofilm) on some or all surfaces, but no visible macrofouling. The amount of slime does not matter. |
| LOF 2 | 1–5% of visible surface | Macrofouling in small patches, or a few isolated individuals or small colonies, often around the waterline and niche areas. |
| LOF 3 | 6–15% of visible surface | Considerable macrofouling, frequently several species, spread across surfaces. |
| LOF 4 | 16–40% of visible surface | Extensive macrofouling on both hull and niche areas, though more than half of surfaces remain clear. |
| LOF 5 | 41–100% of visible surface | Very heavy macrofouling covering substantial portions of the visible surfaces. |
How it works
The LOF scale is built for the real world, and much of its value lies in what it does not require:
- No sampling tools in the field
- No identifying or counting species underwater
- No post-sampling processing of photographic data
- Ranks that can be applied both underwater and on land
Rather than relying on species identification, the scale uses a single key metric: percent cover, the proportion of a visible surface that is covered by biofouling. Users estimate cover for distinct sections of a vessel rather than scoring a whole hull at once, which is both faster and more accurate, particularly in the water where the entire vessel is rarely visible in a single view.
The scale also recognises that fouling is rarely even. Niche areas, the nooks and crannies such as rudders, propellers, struts, shafts, thrusters, grates and keels, tend to accumulate fouling before the flatter hull areas, so they warrant particular attention. A simple sequence of yes or no questions (Is there biofilm? Is there macrofouling? Does it cover more than 5%, 15% or 40% of the surface?) guides the user to the correct rank every time.
This combination of speed, simplicity and consistency is what makes the LOF scale so useful. It retains genuine biological insight while remaining usable by almost anyone, almost anywhere.
Who it is for, and who developed it
The LOF scale is designed for everyone with a stake in keeping our waters free of marine pests, including:
- Regional councils and biosecurity managers running surveillance and vessel-inspection programmes
- Central government agencies setting and applying biosecurity standards
- Marinas, ports and the wider maritime industry managing vessel movements
- Divers, inspectors and vessel owners assessing hull cleanliness in the field
The updated guideline was developed by Cawthron Institute’s marine biosecurity team, Ian Davidson, Oli Floerl, Lauren Fletcher and Patrick Cahill, and was commissioned by Auckland Council. The scale and its application continue to feature in the MBIE-funded Marine Biosecurity Toolbox, a national research programme led by Cawthron.
The LOF scale is a clear example of Cawthron science working in the real world: rigorous research distilled into a practical, accessible tool that helps protect the marine environments of Aotearoa New Zealand, and those further afield.
Why it matters
Biofouling on vessel surfaces is one of the main pathways by which marine non-indigenous species spread around the world. A single fouled hull can carry pest species from one harbour to the next, threatening the ecological, cultural, commercial and recreational values that our coasts depend on.
Managing that risk depends on being able to assess it quickly and consistently, by many different people and in many different places. Characterising vessel biofouling has often been slow, subjective and reliant on specialist knowledge. The LOF scale changes that. It gives councils, biosecurity agencies, industry and vessel owners a common language for hull cleanliness, so that a “clean enough” hull means the same thing whether it is assessed by a diver in the water, an inspector on the hardstand, or an owner checking their own boat.
The scale also helps flag problems with a vessel’s maintenance regime. Because percent cover is what counts, even a single species can push a vessel to LOF 5, a clear signal that antifouling and husbandry practices need attention.
Get in touch
Want to apply the LOF scale in your biosecurity programme, or talk to our team about vessel biofouling and marine pest management? Get in touch by emailing biosecurity@cawthron.org.nz