What is marine biofouling?

Biofouling (or fouling) is the accumulation of aquatic microorganisms, algae, plants and animals on vessel hulls and submerged surfaces. 

How does biofouling form on vessels?

Even though the accumulation process is continuous, the terms primary, secondary and tertiary can be used to describe the levels of biofouling present on a vessel. The images below illustrate each of these levels.

Primary biofouling begins as soon as the surface of a vessel is submerged in seawater with the formation of a slimy surface film consisting of bacteria and microscopic algae.

As the vessel remains submerged in seawater, the secondary biofouling process occurs as organisms settle on top of the primary biofouling layer. Secondary biofouling usually includes hard encrusting animals such as acorn barnacles, bryozoans and serpulid worms, but may also include soft algal tufts and mobile amphipods.

Tertiary biofouling generally consists of larger organisms, such as sponges, sea squirts, mussels, oysters and seaweeds that build up on the secondary biofouling layer, and mobile animals such as crabs and seastars that can live in this growth.

Levels of biofouling increase the longer a vessel or structure remains submerged in seawater.

Marine pests

Pest species are more likely to be found on vessels that have accumulated high levels of secondary or tertiary biofouling.

To help prevent the spread of marine pests, it is very important that vessels are well maintained and that biofouling is minimised. For more information on managing biofouling see the national biofouling management guidelines. These are available for a range of specific vessel categories.


primary biofouling on vessel bowThis vessel has primary biofouling in the form of slime (including microalgae and filamentous algae) on the right side of its bow. The vessel is undergoing cleaning prior to a new application of antifouling.

secondary biofouling on vessel hullThis vessel hull shows secondary biofouling in the form of algal tufts that have settled on top of the primary biofouling layer.

tertiary biofouling on vessel hullTertiary biofouling generally consists of larger organisms that build up on and amongst the secondary biofouling layer. This image includes clumps of mussels amongst algal tufts attached to a vessel hull.