Non-indigenous species and Invasive Aquatic Species

The oceans are home to a large variety of species (plants, algae, fish, microorganisms, etc.) that have evolved in their habitats, separated by natural barriers. But some species have been moved, intentionally or not, as a result of human activity. When the adopting habitat has similar characteristics, the introduced non-indigenous species have a good opportunity to adapt and thrive. Due to some competitive advantage such as the absence of natural predators, some non-indigenous species have become dominant and disrupted the biodiversity of their newly adopted habitat.  These species are generally referred to as Invasive Aquatic Species (IAS).


The introduction and establishment of Invasive Aquatic Species (IAS) is considered to be one of the greatest threats to the world’s freshwater, coastal and marine ecosystems. The global economic impacts of IAS, including through disruption to fisheries, biofouling of coastal industry and infrastructure and interference with human amenity, have been estimated at several hundred million dollars per year.  The main vectors for unintentional transfer of non-indigenous species are ships' ballast water, biofouling of mobile marine structures and aquaculture

Biofouling and ballast water

In the past, common belief concurred that ships’ ballast water was primarily responsible for the introduction of non-indigenous species. Significant progress towards managing this transportation pathway has been achieved in the last ten years through the GEF-UNDP- IMO GloBallast Partnerships Project and the entry into force on 8 September 2017 of the International Convention for the Control and Management of Ships' Ballast Water and Sediments (BWM Convention). However, despite new measures to manage the transfer of invasive species through ballast water, recent research suggests that biofouling has been underestimated as a possible vector and may in fact represent the most common mechanism for the introduction of non-indigenous species. For example, some research estimates that up to 69% of introductions may have occurred via biofouling (ref. footnotes 1 and 2,).


Additionally, domestic spread of non-indigenous species has been observed within their introduced range via ship biofouling, particularly recreational craft, and serves to distribute new species from port regions which serve as central “hubs” for invasion (ref. footnote 3) into broader coastal ecosystems. Indeed, there is now an understanding that, in some cases, biofouling may have contributed to more introductions of invasive aquatic species in many parts around the world than ballast water and other dispersal mechanisms.


Considering that in excess of 50% of non-indigenous species may have been transported via biofouling, the global impacts invasive aquatic species transferred through this vector is likely to be significant.  However, it is poorly documented and difficult to quantify (ref. footnote 4). To give an example of a single invasive aquatic specie, the economic impacts of zebra mussels in the Great Lakes (initially introduced through ballast water, but further expanded through biofouling) are one of the better documented introductions. Although they have not been fully estimated but are far in excess of $100 million for the last 20 years (ref. 5) with other estimates as high as US$6.5 billion dollars per decade (ref. 6).

Biofouling pathways

Where merchant trading vessels are the key mechanism for IAS transfer in ballast water due to the required use of ballast water to compensate for the loading and offloading or cargo, the risk associated with biofouling may be more unevenly distributed between different shipping sectors and between regions. In particular, non-trading vessels such as offshore oil and gas infrastructure, fishing vessels and recreational craft may in some circumstances present a higher risk of IAS transfer through biofouling due to slower transit speeds, complex niche areas and greater periods of time spent in coastal waters, often stationary, where they are subject to biofouling recruitment. For example, offshore oil and gas structures such as Mobile Offshore Drilling Units (MODUs), Floating Production, Storage and Offloading units (FPSOs) and Floating Liquefied Natural Gas facilities (FLNGs), are large complex artificial surfaces that are frequently coated with coatings compliant with the AFS Convention and are generally towed slowly between locations. When not managed to control biofouling, such facilities represent a very high risk of transferring IAS into new regions when towed into coastal waters. Furthermore, once in position at an offshore location such structures can also serve as a source of infection for domestic conveyances which may interact with the structure and subsequently transfer IAS to adjacent coastal regions.


Many oil and gas structures are designed to remain on-station for extended periods (often decades) and, as such, very limited opportunities exist to effect management of biofouling once on-site. Thus, despite merchant vessels representing the largest proportion of vessels afloat on the world’s oceans, measures targeted towards managing biofouling in specific high-risk sectors may provide significant benefits. This could be the case for developing countries in particular, including LDCs and SIDS, while they may have little leverage to change practices in the global shipping sector, countries could be in a position to enforce specific management requirements for industries operating in their waters where permits or licensing approval may be necessary.

In parallel with the increased demand for seafood, technology has made possible the production of food in coastal marine waters and even in open oceans. Aquaculture breeds or harvests fish, shellfish, algae and other organisms in the sea. But farm structures and networks are becoming ever larger and more prone to colonization by biofouling organisms that can reduce the growth and survival of cultured animals and increase the risk of containment failure. Research is needed on technologies and biofouling management approaches, including improvement of non-toxic materials and coatings. In-water cleaning with ROVs is also a potential tool for the control of biofouing in aquaculture, but only if capture of fouling materials is considered to prevent further spreading of any non-indigenous species or damaging stocks.


Biofouling also affects the emerging marine renewable energy structures, such as tidal, wave and wind-based.  Biofouling can impair the performance of these structures and increase corrosion increasing the risk of structural damage .

Root causes and barriers that need to be addressed

A root cause of the difficulty in fully and effectively stemming the spread of invasive species by biofouling is the complex, multi-sectoral nature of biofouling sources, which makes it essential to tackle biofouling across the full range of anthropogenic structures in the marine environment. In addition to the problem of biofouling on ships resulting in the introduction of IAS, there are a growing number and variety of fixed surfaces in marine waters (e.g. oil and gas platforms, aquaculture nets, wave energy equipment, etc.) that can provide the substrate for potentially invasive species to settle and grow in proximity to ships. These anthropogenic structures thus can serve as a source for biofouling organisms which can attach to a previously “clean” ship, with the biofouling organisms then transported to a location where they can become invasive. Furthermore, such structures are also capable of translocation between regions, with structures like Mobile Offshore Drilling Units (MODUs) regularly being moved across ocean basins and LMEs, and structures like aquaculture nets or cages being regularly being moved domestically and regionally, resulting in the potential for transboundary introductions of IAS.  


Environmental threats and socioeconomic impacts 

Because of the technical, scientific, environmental and economic implications, the biofouling issue is more complex than most other marine pollution threats faced by countries and the global marine ecosystem. The environmental threats and socioeconomic impacts that could be derived from the transfer of invasive aquatic species detail above can be summarised in the table below. This provides a useful starting point from which to identify and discuss the overall root causes of these threats and impacts.

Environmental threats
Socioeconomic impacts
Damage to commercial and recreational fishery and aquaculture
Removal of native species from recreational fishing areas (e.g. parasite or viral infections). Threat to aquaculture operations by fouling structures, equipment and the shellfish themselves, requiring cleaning and increasing operational costs. Reduced establishment and growth of farmed species through predation and competition (for example, tunicates) as well as parasitism with IAS as vectors. Biofouling of fishing gear
Modification of physical structures
Damage to coastal infrastructure (tourism, water cooling intakes and heat exchangers for power plants, desalination plant intakes etc.). Reduced value of waterfront properties. Biofouling of commercial craft and standing/fixed structures (shipping, fishing, oil and gas, tourism) reducing cost-efficiency, blocking physical inspection and threatening operations. Increased fuel consumption levels leading to increased GHG emissions with consequent global effects
Alteration of overall habitat dynamics and fundamental changes in ecosystems
Reduction in amenity value. Loss of tourist attractions (beach deterioration/change). Restricted access for coastal recreation. Bioturbation and erosion from burrowing fauna
Predation on and competition with native species
Removal of traditional food and recreational species. Collapse in biodiversity toward a monoculture habitat. Over-exploitation of primary productivity collapsing native food chains


1.  Hewitt, C., Campbell, M., Thresher, R; Martin, R. 1999. Marine Biological Invasions in Port Phillip Bay, Victoria. CSIRO Centre For Research on Introduced Marine Pests (Ed.). Technical Report No. 20.

2.   Hewitt, C. Campbell, M., 2010. The relative contribution of vectors to the introduction and translocation of invasive marine species, Canberra City, The Department of Agriculture, Fisheries and Forestry.

3.  Floerl O., Inglis G. J. 2005. Starting the invasion pathway: the interaction between source populations and human transport vectors, Biological Invasions. 7: 589-606.

4.  Lovell, J., Stone, F. and Fernandez, L. 2006. The Economic Impacts of Aquatic Invasive Species: A Review of the Literatures. Agricultural and Resource Economics Review 35/1 (April 2006) 195-208.

5.  Strayer, D.L. 2009. Twenty years of zebra mussels: lessons from the mollusc that made headlines. Frontier in Ecology 7(3): 135-141.

6.  Sun, J.F. 1994. "The Evaluation of Impacts of Colonization of Zebra Mussels on the Recreational Demand in Lake Erie". In Proceedings of the Fourth International Zebra Mussel Conference. Madison, Wisconsin (March).