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 Proposed criteria for safe in-water hull husbandry

practices with respect to the risk of transferring 

Non-Indigenous Species 

 

 with particular attention to 

 Surface Treated Coatings 

Subsea Industries Technical Report No. 2009.4 Revision 0 

Maxim Candries 

April 2009
 

2

Table of contents 

Executive Summary............................................................................................... 3 

Objective ............................................................................................................... 6 

Abbreviations and terminology .............................................................................. 7 

1 Introduction........................................................................................... 8 

1.1 Non-indigenous species (NIS) ................................................................ 8 

1.2 Shipping vectors..................................................................................... 9 

1.3 NIS management strategy: hierarchical approach.................................. 10 

2 Hull fouling management .................................................................... 11 

2.1 Fouling settlement ................................................................................ 11 

2.2 Antifouling coatings ............................................................................. 11 

2.3 Fouling removal ................................................................................... 12 

3 Surface treated coatings....................................................................... 14 

3.1 The incorporation of coating and maintenance technology.................... 14 

3.2 Effluent analysis and evaluation of non-toxicity during the in-water 

cleaning of STCs. ................................................................................ 17 

4 Assessment of hull fouling extent ........................................................ 17 

5 Risk factors in the translocation of NIS by hull fouling........................ 20 

5.1 Risk assessment with regards to vessel type.......................................... 21 

5.2 Risk assessment with regards to geographic location and environmental 

conditions............................................................................................ 22 

5.3 Risk assessment with regards to antifouling system .............................. 24 

5.4 Niche areas........................................................................................... 24 

6 Safe hull husbandry............................................................................. 25 

6.1 The need for in-water cleaning ............................................................. 25 

6.2 Criterion 1: Non-toxic coating .............................................................. 27 

6.3 Criterion 2: Lightly fouled.................................................................... 30 

6.4 Criterion 3: Active vessels.................................................................... 31 

6.5 Total risk factor and procedures............................................................ 31 

6.6 Suggestions for future research............................................................. 32 

References ........................................................................................................... 34 

Appendix I. Figures of increasing FR................................................................... 38 
 

3

Executive Summary 

Several regulatory bodies worldwide have put programs in place to compile 

guidelines or regulations aimed at combating the introduction of non-indigenous 

species (NIS) associated with shipping. NIS are translocated in a number of ways. 

Over the last decade, the possible transfer of species by hull fouling has been given 

due attention and it is now recognized as one of the most important pathways of NIS 

translocation. 

 

Published literature on the subject has mainly focused on reporting the spread of NIS 

in various regions and on identifying the different factors associated with hull fouling 

that may have caused that spread. Relatively little information is available on 

management issues, although research efforts are underway and being published. 

The management issues that are generally considered are in-water cleaning or out-ofwater removal. Since drydock or haul-out facilities offer more possibilities for 

containment of solid debris and filtering of effluents, thus minimizing the risk of toxic 

paint waste or viable NIS entering port waters, out-of-water removal is generally 

recommended or even required by authorities. 

In terms of preventing the transfer NIS, due attention has been given to the efficacy of 

antifouling technology. The overviews in literature almost invariably discuss coating 

technology with an emphasis on biocidal and fouling release coatings. Novel 

technologies are often discussed only in terms of prospective efficacy. 

Virtually no attention has been given to Surface Treated Coatings (STCs) and it is the 

main objective of this report to discuss this concept in view of the risk of transferring 

NIS. 

STCs essentially involve both coating technology and regular in-water treatment. This 

treatment consists of a unique conditioning aspect and a cleaning aspect. Whereas 

cleaning removes any marine fouling in an early stage of development, conditioning 

is carried out with specially designed tools to bring the smoothness of the paint 

surface to an optimum condition and thereby reduce frictional resistance. 
 

4

STCs are entirely biocide-free and the analysis of effluent samples has shown that no 

toxins are released at any stage, either during curing, conditioning or the in-water 

cleaning. In addition, it was found that during in-water cleaning any fouling 

organisms were crushed to such an extent that the organic matter was considered 

unviable. 

This report reviews the main risk factors involved with hull fouling as a pathway of 

NIS in order to identify the circumstances in which the overall risk is minimized. The 

literature survey reveals that the risk of transferring NIS mainly depends on the 

following factors: 

1. vessel type; including time spent in port or stationary, voyage speed, 

duration and route 

2. geographic location and environmental conditions; 

3. antifouling system; quality of its application; age, type, suitability and 

condition of the antifouling system; 

4. presence of niche areas and application of antifouling to them; 

5. defouling activities, i.e. in-water cleaning or out-of-water removal. 

In-water defouling activities have come under environmental scrutiny mainly because 

of concerns of pulse releases of toxins and ineffective removal. It is argued that these 

concerns do not hold for in-water cleaning on STCs. No biocides are released at any 

stage and because STSs are so durable, there is no concern for preserving the paint 

system so that maintenance technology can be optimized to be as effective as 

possible. This involves several parameters of the specially designed tools such as 

materials, speed of rotation and operational pressure. 

Several publications have pointed out that in-water cleaning as a remedial measure 

under controlled circumstances is preferable to an outright ban in order to reduce the 

risk on NIS incursions. 

It is emphasized, however, that the in-water cleaning of STCs is not a remedial 

measure against NIS and therefore on a scale down from preventive measures in the 

hierarchical approach to NIS management. Regular in-water cleaning keeps the 
 

5

surface conditions of STCs optimal and lightly fouled and should be regarded as a 

truly preventive measure against NIS. 

Based on the identified risk factors, criteria for safe hull husbandry with respect to the 

risk of transferring NIS are proposed. These criteria are: 

1. The ships are coated with an entirely non-toxic paint system 

2. The hull of the arriving vessel is only lightly fouled, which implies that 

the hull is only covered by diatom and bacterial slimes or fine green 

and brown algae (classified as < 5mm in length) 

3. The in-water cleaning is carried out on low-risk type of ships. 

Criterion 1 is self-evident (e.g. by the IAFS Certificate) and criterion 2 can be 

checked by underwater inspection using a fouling rate or index. Criterion 3 depends 

on the type of vessel, the level of activity (i.e. restricted port residency time) and the 

service speed. Port authorities should determine locally what exactly constitutes a 

low-risk type of vessel. 

The total risk of transferring NIS is minimal if criterion 1 is met simultaneously with 

either criterion 2 or 3. The second criterion is preferable because it implies a direct 

evaluation of the hull fouling extent, but it is proposed that criterion 3 is a valid 

indirect indication that the hull is lightly fouled. In-water cleaning should be allowed 

when criteria 1 and 3 are met and should be accompanied by visual inspection to 

validate criterion 2, with special attention to niche areas. 

In this way, it is argued that regular in-water cleaning coupled with fouling 

assessment by divers minimizes the risk of a hull becoming (heavily) fouled at all. 

The report concludes with some recommendations for further research to support the 

proposed criteria. In particular, it is suggested that the viability of defouled material 

should be further studied and that a database with the hull fouling extent on STCs is 

compiled. 
 

6

Objective 

This report is submitted to regulatory bodies that are reviewing the relative risks of 

hull fouling for the introduction of non-indigenous species (NIS) from commercial 

vessels. 

It is the main objective of this report to present the concept of Surface Treated 

Coatings and to discuss it in view of the risk of transferring NIS. An STC is a 

preventive measure against NIS that partly relies on regular in-water cleaning. 

The second objective of this report is to review the risk factors associated with hull 

fouling and in-water cleaning and to propose criteria for safe in-water hull husbandry 

practices with respect to NIS. 
 

7

Abbreviations and terminology 

Antifouling system A coating, paint, surface treatment, or device that is used on a 

ship to control or prevent attachment of unwanted organisms, as 

defined by the IMO. 

Fouling removal Generic term to denote any defouling activities to remove 

marine growth from a vessel, either by out-of-water removal 

(e.g. in drydock or haul-out facilities) or by in-water cleaning 

FR Fouling Rate (cf. Section 4) 

HHRF Hull Husbandry Reporting Form 

IAFS Certificate International Anti-fouling System Certificate as stipulated by 

the IMO Marine Environment Protection Committee 

IMO International Maritime Organization 

Lightly fouled Hull condition whereby the hull is covered only by diatom and 

bacterial slimes or fine green and brown algae (classified as < 

5mm in length) 

Niche areas Submerged parts of a vessel that are at higher risk of being 

fouled 

NIS Non-indigenous species, referring to aquatic species in this 

report 

SPC Self-Polishing Co-polymer 

STC Surface Treated Coating that incorporates both coating and inwater maintenance technology 

TBT Tributyltin 

VOC Volatile Organic Compounds 
 

8

1 Introduction 

1.1 Non-indigenous species (NIS) 

A non-indigenous species (NIS) is a species which has been introduced by human 

activity to a new geographic area or ecosystem outside of its natural distribution 

range, and which has then established and spread, threatening ecosystems, habitats 

and/or other species, and potentially causing economic and/or environmental damage, 

or harm to human health (GISP, 2008). 

NIS in the marine environment are called by a variety of names including Invasive 

Alien Species (IAS), Introduced Marine Pests (IMP), Aquatic Nuisance Species 

(ANS), Aquatic Invasive Species (AIS) and Harmful Aquatic Organisms (GISP, 

2008). The term ‘invasive’ is not always used consistently and may refer to 

introductions of different extent (Ricciardi and Cohen, 2007). Since this report is 

concerned with preventing that a non-indigenous species transported by hull fouling 

reaches invasive status to any extent, the choice is made for non-indigenous species 

(NIS). 

NIS are now generally recognized as one of the greatest threats to biodiversity 

globally. In marine and coastal environments, invasive species have been identified as 

one of the four greatest threats to the world’s oceans (Bax et al., 2003; GISP, 2008). 

NIS affect biodiversity by displacing native species, by altering community structure, 

food webs and ecological processes, and have now been documented in the majority 

(84%) of the world’s 232 marine ecoregions, with particularly high levels of invasion 

in Northern California, the Hawaiian Islands, the North Sea, and the Eastern 

Mediterranean (Molnar et al., 2008): 

- more than 250 introduced species have been reported in San Francisco Bay 

alone with approximately 350 non-native marine invertebrates and algae being 

considered established in the USA (Ruiz et al., 2000; GISP, 2008) 

- 343 NIS have been found in Hawaii (Godwin et al., 2004); 
 

9

- in the North Sea, Gollasch (2002) reported established populations of 80 

introduced species and 61 NIS were reported for Belgian marine and brackish 

waters (Kerckhof et al., 2007). 

- in the Mediterranean, more than 500 NIS were reported, the majority 

introduced via the Suez canal, although there are distinct differences between 

the western and eastern Mediterranean, with far fewer introductions in the 

west (Galil, 2008). 

NIS also impact on commercial fisheries, including mariculture with serious 

economic implications. In addition, fouling of physical structures by introduced 

species has major impacts on other industries by, for example, decreasing the speed of 

vessels and clogging water intake pipes (GISP, 2008). 

1.2 Shipping vectors 

The means and routes by which species are introduced into new environments are 

called pathways or vectors. A number of studies have looked at the relative 

importance of shipping in relation to other pathways. Such studies are complicated by 

the facts that the provided information is often incomplete and that numerous species 

have probably been introduced by several vectors at different times and in different 

geographic areas (GISP, 2008). In any case, it is generally agreed that the rate, and 

thus the risk, of species invasions has increased significantly during recent decades. 

One of the primary factors leading to this increase has been the vast expansion of 

global trade during the past years, which in turn has lead to significantly more ballast 

water, fouled hulls, and associated organisms moving around the world (Minchin and 

Gollasch, 2003). 

A recent analysis by Molnar et al. (2008) drawing on information from over 350 

databases and other sources, showed that for the 329 marine NIS considered, shipping 

was the most common pathway (69%), with others being aquaculture (41%), canals 

(17%), the aquarium trade (6%), and the live seafood trade (2%) whereby, on average, 

2 pathways per species were identified. Of the 205 species introduced via shipping – 

and for which sufficient information was available – 39% were introduced by hullfouling, 31% via ballast water, and the remainder by both. 
 

10

A study on ship-related vectors of introduction into the North Sea between 1992 and 

1996 found that samples of hull-fouling, taken from 186 commercial vessels visiting 

German ports, had more than twice the number of NIS as those from ballast water – 

74% of the species identified. The study concluded that, despite inevitable differences 

in sampling methodology, hull fouling was an important – if not the most important – 

vector for introductions (Gollasch, 2002). 

1.3 NIS management strategy: hierarchical approach 

The first step in a biological invasion is the introduction of the species concerned into 

the new environment. Once there, the species must be able to survive in the ambient 

conditions (temperature, salinity, etc.), and the survivors must persist and reproduce 

successfully until they establish a self-sustaining founder population. Once 

established, the species begins to multiply and spread, sometimes after a substantial 

time lag – maybe a few years, or even decades. The period of spreading – sometimes 

known as the explosion phase – is when the species/population becomes invasive 

(GISP, 2008). 

 

Management measures may be applied at various points in the process of invasion, 

starting from prevention of introductions, to early detection of introduced species or 

founder populations and rapid response thereto – in the form of eradication or 

containment - and long-term control of invasive populations. The further along in the 

process of invasion that the measure is applied the more costly and less effective it 

generally is. In other words, although prevention measures may be costly, an analysis 

of the long-term costs and benefits (environmental, economic and social) will 

invariably show that they are less than the losses and costs which are incurred if the 

NIS are allowed to establish, acquire invasive status, and then require ongoing 

control. 

 

The starting point for the management of NIS – as for most environmental problems - 

is that prevention is better than cure. In other words, prevention is the most costeffective and environmentally desirable option, and should be given priority in any 

management strategy. This is commonly known as the hierarchical approach to 
 

11

management of biological invasions. Prevention is especially important for marine 

environments given the difficulties of detecting introductions in marine habitats, of 

eradicating species that have become established, and of functional control and 

mitigation programs (GISP, 2008). 

2 Hull fouling management 

2.1 Fouling settlement 

Preventing the introduction of NIS and spread via fouling requires adherence to 

effective antifouling technology aimed at one of the following three approaches: 

i) preventing the settlement of fouling organisms by repelling or killing 

them; 

ii) preventing growth of settled organisms by killing them, inhibiting their 

growth or reducing their adhesion ability, 

iii) removing and disposing of biofouling organisms that have established. 

Antifouling coatings are commonly applied for the first two approaches, whereas the 

third approach.is concerned with fouling removal, either by underwater cleaning or by 

out-of-water removal (e.g. in drydock.or haul-out facilities). 

2.2 Antifouling coatings 

Antifouling coatings broadly function in one of two ways: 

i) The coating contains biocides or non-toxic active agents which gradually 

leach from the coating, thereby deterring the settlement of marine 

organisms; 

ii) The coating aims to prevent attachment of marine organisms – mainly 

fouling-release or metallic coatings. 

The most widely applied type of biocidal antifoulings are self-polishing copolymers 

(SPCs), which allow a gradual release of the biocide from the polymer. Other types of 

biocidal coatings leach their toxins faster and do not last as long as SPCs. 
 

12

The biocide tributyltin (TBT) dominated the market because of its wide spectrum 

efficacy and but its use has been banned completely as of September 17, 2008. As it 

became clear that the use of TBT would be banned sooner or later, and particularly 

during the phase-out period, it has often been mentioned in the literature that the risk 

posed by hull fouling as a transport mechanism for NIS may increase as new coatings 

are developed and vessels shift to different antifouling coatings with potentially lower 

efficacies (Drake and Lodge, 2007). 

 

Most SPCs now use copper compounds as biocides, though they are generally less 

effective and their longevity is shorter than TBT. In addition, bans and restrictions on 

copper-based paints are being considered in a number of places so there has been 

increasing emphasis on the development of environmentally acceptable, biocide-free, 

alternatives (Lewis, 2002b). 

While several biocide-free coatings are in development to prevent the settlement of 

organisms, the majority of biocide-free coatings that are presently on the market are 

formulated to prevent firm attachment and development of fouling organisms. These 

coatings are known as foul(ing) release coatings (Chambers et al., 2006). They are 

more costly to apply than biocidal antifoulings and are currently only practically 

effective for active, swift vessels (Falkner et al., 2009). 

2.3 Fouling removal 

The frequency with which most vessels remove fouling is usually based on the 

maintenance rules of the classification society (i.e. organization that establishes and 

applies technical standards for ship design, construction and survey). Vessel-specific 

programs may include a five-year cycle of annual in-water surveys and special out-ofwater (i.e. drydock or slipway) surveys. Most vessel owners commonly take 

advantage of required drydockings to remove fouling organisms and apply new 

antifouling coatings. 

Because fouling continues to accumulate between required drydockings, vessel 

owners also conduct interim in-water cleanings of the vessel hull. In-water cleaning 

techniques generally vary depending on the size of the vessel as well as the 
 

13

composition of the hull and the type of antifouling paint applied thereon. Small 

vessels (i.e., less than 25m) are typically cleaned by divers using hand-held scrapers, 

while commercial vessels generally require diver-operated devices with rotating 

brushes. 

With the shift to TBT-free antifoulings, there has been a surge in antifoulings losing 

their efficacy more rapidly than the previously common 5-year inter-drydocking 

period (Drake and Lodge, 2007). As a consequence in-water cleaning practices have 

increased in recent years to extend the time between an antifouling losing 

effectiveness and drydocking. For commercial vessels, the diver operated machines 

with rotating brushes can clean the hull while the ship is loading or unloading in port. 

Such practices, however, have become increasingly restricted because of concerns 

about the pulse release of antifouling biocides during the scrubbing of the paint 

(Conway and Locke, 1994; Valkirs et al., 1994). 

Out-of-water fouling removal during drydock allow for the containment of toxic 

materials, including fouling organisms that are removed from the vessel hull. Even 

though out-of-water removal is not 100% effective at containing debris and depends 

on environmental-, facility- and biotic-related variables, the debris produced during 

water-blasting carried out at drydock or haul-out facilities generally contains fewer 

viable organisms than in-water cleaning (Woods et al., 2007). Especially mobile 

organisms such as byssal mussels and other molluscs, dislodged during cleaning can 

be transported by currents to nearby substrates such as pilings, rocks, or rubble and 

sand patches where they can settle and re-attach. Moreover, it has been reported that 

the crushing of invertebrate species can release eggs and sperm, while fragments of 

some macroalgae - such as Caulerpa - bryozoans, ascideans, sponges and worms such 

as Sabella can regenerate (Lewis, 2002a; Hopkins and Forrest, 2008). In-water 

cleaning, therefore, has increasingly come under scrutiny not only due to concerns 

about toxins and water quality, but also because of NIS introductions (Falkner et al., 

2009). 

To address this issue, experimental in-water cleaning technology is being researched 

and developed to in order to retain all defouled material and released substances. At 

present, these systems are not yet fully effective. Hopkins and Forrest (2008) tested 
 

14

two diver-operated rotating brush systems which were coupled with a purpose-built 

suction and collection capability and found that even where brushes were combined 

with a suction system to collect the material, up to 12% was still lost. 

As an alternative to remove fouling by in-water cleaning, in-water encapsulation 

techniques have recently been developed. Although these techniques appear 

promising, the development of anoxic conditions may be too slow to cause mortality 

where the time frames of visiting vessels are short and they are therefore not yet a 

viable option at present for commercial vessels (Hopkins and Forrest, 2008). 

3 Surface treated coatings 

3.1 The incorporation of coating and maintenance technology 

As mentioned in Section 2.1, antifouling technologies may be distinguished based on 

the sequence of settlement. A consequence of this classification, however, is that 

coating technology is commonly regarded as the only preventive measure with respect 

to NIS, whereas cleaning technology is regarded as a remedial measure (i.e. on a scale 

down in the hierarchical approach to management of NIS, cf. Section 1.3). 

Figure 1 gives an overview of antifouling systems that rely on coating technology and 

those that do not. An antifouling system is hereby defined by the International 

Convention on the Control of Harmful Antifouling Systems on Ships, 2001 (AFS 

Convention) as “a coating, paint, surface treatment, or device that is used on a ship to 

control or prevent attachment of unwanted organisms”. 

This Section deals with Surface Treated Coatings (STCs), which is a concept that is 

often overlooked and that was originally not included in Figure 1. The edit in red on 

Figure 1 shows that STCs involve both coating and non-coating technology. 
 

15

Figure 1. Technologies and strategies to combat biofouling on submerged surfaces (Willemsen, 

2005) (Edited). 

Surface treated coatings are by definition subjected to regular in-water treatment. The 

treatment consists of a conditioning aspect that improves the surface characteristics of 

the coating and a cleaning aspect that removes any marine fouling in an early stage of 

development (Van Rompay, 2008). 
 

16

One STC that is commercially available under the brand name Ecospeed, is 

formulated as a vinyl ester with a high concentration of embedded glass platelets. This 

coating has a low amount of volatile organic compounds (VOC) and is typically 

applied in two coats of 500μm, which is considerably higher than the total dry film 

thickness of traditional antifouling schemes. The high film thickness and the presence 

of glass flakes which act as an impermeable barrier contribute to the anti-corrosive 

properties and the high durability explain why the coating is so well suited for regular 

in-water treatment. 

The conditioning aspect is unique to STCs because this in-water treatment results in a 

smooth but wavy surface to which fouling organisms have difficulty re-attaching. In 

effect, for the commercial STC example given above, the conditioning results in a 

surface layer of aligned glass platelets that are suspended in a leveled matrix of vinyl 

ester. It is assumed that the number of crevices around the impermeable glass flakes is 

drastically reduced which would explain why good antifouling properties are obtained 

for a coating which was originally developed as an anti-corrosive barrier coating. Ongoing research is evaluating this assumption by studying the roughness characteristics 

over short cut-off lengths (Van Rompay, 2008). 

Figure 2. In-water treatment by rotating brush technology of a hull coated with STC. 
 

17

By means of specially developed tools, the conditioning and cleaning operations of an 

STC can be carried out simultaneously, as shown in Figure 2. This significantly 

reduces the required amount of time and consequently saves on costs. 

3.2 Effluent analysis and evaluation of non-toxicity during the inwater cleaning of STCs. 

Wijga et al. (2008) studied the effluents released at various stages in the lifecycle of a 

commercially available STC. By chemical analysis they found that in the curing phase 

only aromatic solvents were leached (xylene, styrene). During the conditioning 168 

hours after exposure in artificial seawater, non-toxic polymer was also released. The 

absence of biocidal substances was validated by toxicity and oestrogenic tests on a 

number of organisms. 

During the in-water cleaning of a test plate which had been exposed in seawater for 1 

year and which was moderately fouled, the composition of the released substances 

was significantly different: aromatic solvents and non-toxic polymer were only 

present in modest quantities as the released substances consisted mainly of crushed 

organic matter. A taxonomic analysis of the fouling organisms present prior to inwater cleaning was not carried out. 

The reported concentrations of released matter were analyzed in a volume of water 

that was restricted by the dimensions of the test basin. In practice, the concentrations 

of released effluent, either during cleaning or conditioning, will be diluted by 

naturally occurring current and flow effects (Wijga et al., 2008). 

4 Assessment of hull fouling extent 

No matter how effective the antifouling system, fouling of submerged surfaces will 

occur sooner or later. The process of biological fouling is often grouped in the 

literature into key growth stages which include an initial accumulation of adsorbed 

organics, the settlement and growth of pioneering bacteria creating a biofilm matrix 

within a matter of hours upon immersion and the subsequent succession of micro and 

macrofoulers (Chambers et al., 2006). Settlement of the latter almost invariably takes 
 

18

place in port regions when ship speeds are low or when ships are anchored or berthed 

since few species attach to substrates at speeds of > 2m/s (Minchin and Gollasch, 

2003). 

Compared to the ballast water vector, there is very little recent data directly from 

commercial ships on which assessments can be made regarding the movements of 

species at various scales and the factors that influence biofouling accumulation, 

survival and species incursions (Davidson et al., 2006). 

Hull sampling of commercial vessels is traditionally achieved by divers when the ship 

is in water or during drydocking events when the ship is more readily accessible. A 

major constraint of drydock surveys is that vessels tend to be skewed toward the most 

fouled vessels that have reached the end of their inter-drydocking period. In addition, 

accessibility to all locations on the hull is not guaranteed, usually rendering much of 

the hull’s area (beyond the reach of a step ladder) inaccessible. The unconstrained 

field-of-view of the vessel is beneficial, however, as is the controlled environment in 

which the survey takes place, the collection of specimens for species level 

identification, and the low cost of data collection. 

Underwater inspection allows for a more representative subset of vessels to be 

sampled with access to all underwater locations and species level data collection but 

may be constrained by visibility (Davidson et al., 2006). 

Methodologies have been proposed to assess the level of fouling on a hull based on 

the change in fouling communities over time. 

Floerl et al. (2005) developed an index to assess levels of fouling on small craft. Table 

1 shows that the index goes from 0 – 5, with, for example, 0 indicating no biofilm or 

macro-fouling; 2 indicating light fouling - hull covered in biofilm, with 1 – 5% of 

the surface with small patches of macro-fouling; and 5 indicating very heavy fouling 

with 41-100% coverage of the hull surface by diverse assemblages of fouling species. 
 

19

Table 1. Fouling index used to quantify hull fouling on private yachts arriving in New Zealand 

(Floerl et al., 2005a). 

Floerl et al. (2005) carried out a visual inspection of 189 yachts in situ using a remoteoperated video camera. 55% of the vessels surveyed were allocated a fouling index of 

0 or 1, with an average macrofouling coverage of 0.1 ± 0.1%. 12% of vessels were 

allocated an index of 2, with average macrofouling coverage of 4.35 ± 2.39%. None 

of the remaining third of vessels surveyed with an allocated index of 3-5 were devoid 

of macrofouling. 

Because of the need for longer inter-drydocking intervals, hull husbandry practices 

have always been common on navy vessels, even in the era of antifoulings containing 

tributyltin (Preiser et al., 1975). The US Naval Ships’ Technical Manual contains 

several criteria for evaluating fouling severity and coating condition. The fouling 

rating scale given in Table 2 describes the ten most frequently encountered fouling 

patterns in order of increasing severity. The types of fouling are separated into soft, 

hard, and composite categories and the fouling rating is then further refined by the 

percentage of covered area. For illustrative purposes, 22 figures directly taken from 

the Manual, are included in Appendix I. 

It seems logical to use either of these methods or a similar method in the hull fouling 

assessment of commercial vessels but at present this is rarely done during underwater 

inspections. 
 

20

Table 2. US Navy Fouling Ratings (FR) in order of increasing severity (Naval Ships' Technical 

Manual, 2006) 

5 Risk factors in the translocation of NIS by hull fouling 

The threat of possible NIS transferred by hull fouling depends on a variety of factors 

which influence the likelihood of any particular vessel carrying potentially harmful 

species. The most important factors are: 

1. vessel type; including time spent in port or stationary, voyage speed, 

duration and route 

2. antifouling system; quality of its application; age, type, suitability and 

condition of the anti-fouling system; 

3. geographic location and environmental conditions; 

4. presence of niche areas and application of antifouling to them; 

5. defouling activities, either by out-of-water removal or by in-water 

cleaning. 

The concerns about defouling activities have been discussed in Section 2.3, while the 

other factors are discussed below. 

 
 

21

5.1 Risk assessment with regards to vessel type 

It has long been known that ships on active duty tend to be less fouled than those 

which spend much of their time stationary. This is mainly because fouling organisms 

will only attach to slowly moving surfaces (cf. Section 4) but also because vessels 

with short port residency times generally invest in better antifouling measures. 

Biofouling on a ship’s hull will increase the hydrodynamic drag and lower the 

maneuverability of the vessel, leading to increased fuel consumption (Chambers et al., 

2006). Depending on the type of vessel, fuel may make up about 50% of the 

operational costs of a ship, and it has been estimated that fouling increases the annual 

fuel consumption of the commercial shipping fleet by 40%, or 120 million tonnes of 

fuel at a cost of about $ 7.5 billion in 2000 per year (GISP, 2008). 

For commercial vessels, the increased drag and associated fuel consumption as a 

result of biofouling alone make it economically inevitable for shipping companies to 

invest in antifouling technology, i.e. coatings and/or cleaning measures. Other 

benefits include the improved maneuverability, which can be impaired by propeller 

fouling and better control of microbially induced corrosion (Chambers et al., 2006). 

Trading vessels include chemical tankers, container ships, crude oil, gas and dry bulk 

carriers, general cargo ships, livestock carriers, product tankers, vehicle carriers, 

various roll-on roll-off ferries and cargo ships (ro-ro vessels). 

It is clear that low activity vessels present a high risk. These include barges, drilling 

platforms and dredgers. With the decline in fish catches in many parts of the world, 

many commercial fishing vessels are underutilized and poorly maintained with 

vessels being laid up and/or sold off for fisheries or other purposes in other areas. 

They therefore also represent a considerable risk in terms of hull fouling (Godwin, 

2003; Minchin and Gollasch, 2003; GISP, 2008). 

Small craft have long been known to play an important role in the spread of NIS and 

their role in the spread via hull fouling has been investigated in depth. The emerging 
 

22

picture suggests that these vessels in fact pose a significant threat for various reasons 

(Floerl and Inglis, 2003; Mineur et al., 2008) : 

- many are stationary for long periods of time, 

- they are often poorly maintained, 

- the probabilities of yacht-mediated dispersal are multiplied by the relatively 

high numbers of recreational yachts sailing along shores 

- most are moored in marinas or in sheltered bays or estuaries which provide 

ideal conditions for the establishment and spread of fouling organisms. 

Based on a survey of the voyage characteristics of 400 vessels (through the collection 

of hull husbandry reporting forms), Falkner et al. (2009) suggest a gradient of risk 

associated with different vessel types, with auto, container, and passenger vessels 

posing the lowest amount of risk and barges, bulk, and tank vessels posing the 

greatest amount of risk. The lower risk vessel classes all share short average port 

residency times, elevated traveling speeds and freshwater port calls (Falkner et al., 

2009). 

5.2 Risk assessment with regards to geographic location and 

environmental conditions 

The ability of NIS to invade a particular port will vary according to the degree of 

biophysical similarity between its native range and that available in or close to the 

port of introduction. Critical parameters include extreme maximum and minimum 

temperatures, salinities, dissolved oxygen levels and sediment stability (Hutchings et 

al., 2002). It is long known (e.g. by the pioneering work of Visscher) that fouling 

organisms are capable of surviving on vessels remaining at similar latitudes because 

of the relatively consistent ambient water temperatures (and sometimes salinity levels) 

at similar latitudes, and short voyages have little influence on the ability of biofouling 

organisms to feed and grow rapidly (Woods Hole Oceanographic Institution, 1952). 

Other factors include the composition, biodiversity, health and stress status of the 

organisms that are present in the port of arrival. Healthy communities will be able to 

offer stronger resistance to invasive alien species and diverse communities will offer 

fewer opportunities of introduction. 
 

23

5.2.1 Example: Northern Australia vs. Hawaii 

Whether or not a NIS may become invasive depends on a number of factors and a 

good example of this is a comparison of the well-documented cases of Northern 

Australia versus Hawaii. 

Hutchings et al. (2002) report a number of surveys looking for introduced species in 

tropical ports across northern Australia. The surveys supported the conclusions from a 

1997 risk assessment examining introductions to 12 tropical ports in Queensland 

(Northern Australia) which concluded that far fewer marine species appeared to have 

been introduced, even at major bulk export ports where the number of ship visits and 

volume of discharged ballast water are more than at most of Australia’s cooler water 

ports. The 1997 report also concluded that, apart from pathogens and parasites of 

warm-water species, the potential for marine pest invasions in Queensland tropical 

ports appeared to be low, and not only because the majority of arriving vessels 

originated from temperate ports in North Asia, increasing the likelihood that NIS will 

not survive the changes in temperature, salinity etc. (Hutchings et al., 2002). 

In contrast, surveys of harbors in Hawaii have found over 110 introduced species 

(including 23 cryptogenic species, i.e. opportunistic cosmopolitan species). Hutchings 

et al. (2002) suggest that the biogeographically isolated and less diverse marine 

communities of Hawaiian ports have been more susceptible to introductions than 

those of tropical Australia for several reasons, including the proximity of Australia to 

the central Indo-Pacific triangle of megadiversity (Indonesia-Philippines-Papua New 

Guinea) and consequent high biodiversity and low endemicity, hence offering fewer 

niches for non-indigenous species to become established. 

These considerations should be combined with the risk factor described in the 

previous Section. With regards to arriving vessel types, the isolated central Pacific 

position of Hawaii has a long history of receiving a relatively large share of naval 

shipping, which pose a markedly greater risk of being heavily fouled than highactivity bulkers that predominantly arrive in northern Australian ports (Hutchings et 

al., 2002). 
 

24

Baseline surveys of several of these ports have been carried out to further monitor the 

risk of NIS incursions (Hoedt et al., 2001; Neil et al., 2008). 

5.3 Risk assessment with regards to antifouling system 

Several studies have reported that paint age is the most important factor governing the 

effectiveness of antifouling coatings on both recreational (Floerl and Inglis, 2005; 

Floerl et al., 2005b) and commercial vessels (Coutts and Taylor, 2004; Otani et al., 

2007). This is simply because the majority of applied coatings are still biocidal, and 

with time there is an increased likelihood that the coatings are depleted of biocides. 

Mechanical integrity and quality of application is another important factor. Piola and 

Johnston (2008) studied fouling taxa recruitment on experimental scrapes of paint 

surfaces and concluded that slight disruptions to newly antifouled surfaces may be all 

that is necessary for the establishment of fouling organisms and the translocation of a 

wide range of invasive taxa (Piola and Johnston, 2008). 

It should be noted that long stationary periods reduce the effectiveness of both 

biocidal antifoulings and fouling release coatings. 

5.4 Niche areas 

Fouling organisms tend to concentrate in sheltered areas of the hull, such as sea chest 

intakes, the rudderpost and other crevices which are protected from turbulent flow 

(Coutts et al., 2003). Other areas which require specific attention are drydock support 

strips that are often left uncoated. These have become known as ‘niche areas’ and 

should be inspected with special care. 
 

25

6 Safe hull husbandry 

6.1 The need for in-water cleaning 

In-water cleaning should remain a viable option for hull fouling management. From 

an economic point of view, there is great demand from ship operators to sail with 

optimum hull condition. From an environmental point of view, avoiding the added 

drag of a fouled hull will save on fuel consumption and reduce greenhouse gas 

emissions. Drydocking or out-of-water removal is an expensive option that is not 

always available or feasible. With potentially less effective biocidal antifoulings after 

the TBT ban, in-water cleaning is therefore often the only option to extend the interdrydocking period. 

 

Leaving a fouled hull totally unmanaged poses a great risk of introducing NIS. Once 

heavily fouled vessels arrive in a new port with already mature species, cues within 

enclosed harbor basins or estuaries, such as changes in temperature or salinity, can 

stimulate these adults to spawn within a few hours of arrival, with nearby wharf piles, 

jetties, breakwaters and other hard surfaces offering convenient settlement areas 

(Minchin and Gollasch, 2003; GISP, 2008). Fouling organisms may also be dislodged 

during mooring operations (Takata et al., 2006). Moreover, mobile organisms, such as 

crabs or seastars, which are typically nestled amongst encrusting species on heavily 

fouled vessels, may find ways to escape from the hull and settle within the vicinity of 

port waters (Lewis, 2002a). 

It is clear that heavily fouled vessels should be treated with care and only after this 

has been given due attention by the authorities, e.g. by deciding which is the most 

viable management strategy and by issuing permits on a case-by-case basis. 

A heavily fouled hull implies that the antifouling system has failed as a preventive 

measure in the transfer of NIS. Too often, in-water cleaning is only considered at this 

stage, i.e. when heavy fouling is either clearly visible or evident through markedly 

increased ship fuel consumption. In-water cleaning is then an option as a remedial 

measure against NIS, which has come under increased environmental scrutiny. 
 

26

Several researchers have nevertheless pointed out that in-water cleaning as a remedial 

measure under controlled circumstances is preferable to an outright ban in order to 

reduce the risk on NIS incursions (Lewis, 2002b; Hopkins and Forrest, 2008). For 

example, it may be advisable that a vessel that has been stationary for a long time and 

has acquired substantial fouling is cleaned in-water before it leaves port and sails 

elsewhere. Figure 3 illustrates the risks of in-water cleaning as a measure against NIS 

versus the risks of no management measure whatsoever. 

Figure 3. Conceptual diagram of the risks posed by a fouled vessel if left unmanaged compared 

with the risks of in-water cleaning (Edited from Hopkins and Forrest, 2008). 

It should not be forgotten, however, that in-water cleaning may also be a preventive 

measure. or a precautionary approach to managing hull-fouling NIS (Woods et al., 

2007). This is indicated in red in Figure 3, which is edited from Hopkins and Forrest 
 

27

(2008). Hull cleaning as a preventive measure occurs when theoretically no fouling is 

present and in practice when the hull is ‘lightly fouled’ (which is further discussed in 

Section 6.3 below). This is the design scenario for STCs where regular in-water 

cleaning to keep fouling to a minimum is essential to the whole concept. STCs 

combine maintenance and coating technology to provide an effective preventive 

measure against NIS. As the fouling extent magnifies, however, the baseline risk with 

potential release of planktonic propagules or dispersed fragments increases and inwater cleaning becomes a remedial measure against NIS. 

The main premise of this report is to argue that the in-water cleaning of hulls of ships 

exceeding 25m in length can be an acceptable preventive measure and will not 

exacerbate the risk of spreading NIS if a number of criteria are satisfied. These 

criteria are: 

1. the hull is coated with an entirely non-toxic paint system 

2. cleaning is only carried out when the hull is lightly fouled 

3. cleaning is only carried out on ships which have a sufficiently high level of 

activity and which have not been stationary for long periods 

6.2 Criterion 1: Non-toxic coating 

In-water cleaning is only truly acceptable when the release of toxic matter is entirely 

avoided and when it leaves the paint system undamaged. As mentioned in Section 2.3, 

the main concern of cleaning hull surfaces is that it may result in a pulse release of 

biocides around the vessel and gradual buildup of toxicity levels within port waters 

(Valkirs et al., 1994). Another aspect of in-water cleaning is that it is likely to remove 

at least part of the existing anti-fouling, reducing he scheduled lifetime of the paint 

system. Since no new paint is applied, it therefore increases the chance of the surface 

being recolonized. Boats that had been cleaned in Australia, for example, were found 

to have up to six times more recruitment than those that had been sterilized or left 

uncleaned (Floerl et al., 2005b; Hopkins and Forrest, 2008). 

The only types of biocide-free antifoulings which are currently in use on a 

commercial basis are fouling release and surface treated coatings. Fouling release 

coatings, however, pose the disadvantage that they are mechanically soft and 
 

28

therefore easily damaged (Meyer et al., 1995; Eiss, 1999; Holm et al., 2003). In 

addition, Nendza (2007) states that the silicone oils which are present in a number of 

fouling release coatings formulated around chains of polydimethylsiloxane may leach 

out from the coatings. These oils are highly persistent and have low solubility in 

water. Consequently, they may clog surface sediments and cause anoxic conditions 

(Nendza, 2007). 

It is proposed that cleaning on foul release coatings is accompanied by visual 

inspection using a paint deterioration rating (cf. Section 4) to ensure that the coating is 

in acceptable condition. 

STCs, however, are specifically formulated to be subjected to regular in-water 

cleaning by specially designed tools. Because of their durability, higher forces can 

therefore be applied by the tools than the rotating brushes traditionally used for other 

antifoulings and preserving the life span of an STC is not an issue. In-house research 

and development has determined the best combination of design parameters such as 

for surface optimization (i.e. conditioning) and organism removal (i.e. in-water 

cleaning). With respect to removal, various configurations and materials can be 

selected, depending on the inter- and intra-specific variation in adhesion strength 

among organisms. In addition, smaller and more flexible tools have specially been 

designed to address niche areas. To illustrate this, Figure 4 (Hopkins and Forrest, 

2008) shows that in general, removal by brushes is not 100% effective and becomes 

increasingly ineffective as the fouling extent prior to treatment is greater. In contrast, 

Figure 5 shows the hull of a cruise vessel coated with STC which had spent nine 

months stationary in tropical waters. The upper left part of the picture shows the 

fouling extent whereas the lower right part of the picture shows the effective removal 

where the in-water cleaning tools have passed. 
 

29

Figure 4. Photographs of settlement plates (350 by 350 mm) before and after treatment by 

rotating brushes after (a) 3 months and (b) 12 months of fouling (Hopkins and Forrest, 2008). 

Figure 5. Photograph taken underwater of a hull coated with STC that is partially covered by 

fouling after spending 9 months stationary in tropical waters (upper left) and that has partially 

undergone in-water cleaning (lower right). 
 

30

In addition, as mentioned in Section 3.2, effluent analysis has shown that any 

substances released during in-water cleaning of STCs are non-toxic and that all 

defouled material is crushed to the extent that it is not considered viable (Geens, 

2008; Wijga et al., 2008). 

Woods et al (2007) compared the viability of organisms in solid debris and found 

fewer viable organisms after water blasting in drydock and haul-out facilities than 

after in-water cleaning by manual scraping, with less damage done to soft-bodied taxa 

than to brittle hard-bodied taxa by either cleaning method. It may be expected, 

however, that the pressure put on organisms by rotating brushes used for the in-water 

cleaning of commercial vessels strongly reduces the overall viability compared to 

manual scraping. Preiser et al (1975) compared several methods of in-water cleaning 

and concluded that brush machines were the most effective removal method. The 

advantage of STCs over other coatings in this respect is that significantly more 

pressure can be transmitted by the specially designed in-water cleaning tools to 

effectively remove and crush fouling organisms without damaging the surface. 

6.3 Criterion 2: Lightly fouled 

It is proposed to automatically allow the in-water cleaning of ships arriving in port 

areas when the hull is ‘lightly fouled’ only. This includes the presence of diatom and 

bacterial slimes which are not distinguished from bare metal and anti-fouling paint 

with any certainty and are therefore generally not included as separate taxa in surveys 

of fouling extent (Coutts and Taylor, 2004). It is generally agreed that the adhesion of 

slime films cannot be prevented by any type of antifouling, including biocidal and 

foul release coatings (Anderson et al., 2003; Holland et al., 2004). A ‘lightly fouled’ 

hull may also be covered by fine green and brown algae (classified as < 5mm in 

length). In terms of Floerl et al.’s fouling index, this proposition would effectively 

imply a fouling index of less than three. 

It may be argued to also allow moderate levels of filamentous algae (i.e. longer than 5 

mm in length) since these are generally cryptogenic and pose no risk as NIS. This 

would imply to allow a fouling index of 3 or a Navy fouling rate of 30 with limited 

cover (cf. Section 4). Port authorities should decide whether this is locally acceptable 
 

31

since there is some discussion as to which species are regarded as cryptogenic 

(Gollasch and Nehring, 2006; Kerckhof et al., 2007; Minchin, 2007). 

The US Navy combines the Fouling Rating with a Paint Deterioration Rating in order 

to determine when hull husbandry should be performed and to what extent. The 

decision to initiate an in-water cleaning operation is based on the results of precleaning hull inspections performed on regularly scheduled intervals (Naval Ships' 

Technical Manual, 2006). The difference here is that it is proposed to allow in-water 

cleaning below a certain fouling rate, whereas the Navy performs cleaning above a 

certain fouling rate. 

6.4 Criterion 3: Active vessels 

There is general agreement that high activity vessels with short stays in port represent 

a low risk with respect to transferring NIS (cf. Section 5.1). Based on the type of 

vessel, the service speed and the documented activity level (i.e. latest port entries), it 

is proposed that criterion 3 could be an alternative to criterion 2. 

Port authorities should determine locally what exactly constitutes a low-risk type of 

vessel. Ports in tropical waters, for example, may require higher service speeds, 

shorter port residency times or, in the case of STCs, shorter intervals between inwater cleanings. 

6.5 Total risk factor and procedures 

The total risk of transferring NIS is minimal if criterion 1 is met simultaneously with 

either criterion 2 or 3. The second criterion is preferable because it implies a direct 

evaluation of the hull fouling extent, but it is proposed that criterion 3 is a valid 

indirect indication that the hull is lightly fouled. 

The advantage of initially validating criteria 1 and 3 is that they can readily be 

demonstrated by providing the appropriate documents (e.g. IAFS Certificate in 

combination with latest port entries) so that in-water cleaning can be allowed for a 

ship arriving in a port without the lengthy procedure of applying for a permit, now 
 

32

often given on a case-by-case basis. During in-water cleaning, visual documentation 

should then be collected to validate criterion 2. This documentation, preferably in the 

form of a fouling rating and paint deterioration rating accompanied by archival video 

material and diver report forms that pay special attention to niche areas, should be 

kept onboard. The visual documentation and monitoring will also provide evidence 

that regular in-water treatment is a justified preventive measure. This will contribute 

to the evaluation of hull fouling as a NIS vector and can serve as an appropriate 

monitoring program to inform adopted management approaches in general (Neil et al., 

2008). 

 In this way, it may be argued that regular in-water treatment coupled with fouling 

assessment by divers minimizes the risk of a hull becoming (heavily) fouled at all. 

Over the last couple of years a lot of new paint formulations were launched in order to 

find the best response to the TBT ban. Not all of these formulations have been as 

successful as hoped and it may be argued that the last decade has seen more heavily 

fouled vessels than the decade before (Drake and Lodge, 2007). Allowing regular inwater cleaning when the criteria proposed above are satisfied would avoid this. 

The in-water cleaning and inspection frequency will depend on the type of vessel (i.e. 

service speed and port residence time) and the voyage route. As a first indication, a 

frequency of twice a year may be assumed for vessels plying only temperate waters 

and every three months for vessels plying tropical waters. 

6.6 Suggestions for future research 

6.6.1 Viability of defouled material 

This literature survey indicates that the viability of organisms removed by rotating 

brush technology or by the in-water cleaning tools used with STCs have not been 

studied by quantitative assessments for different taxa. This could be researched by 

following a similar methodology to Woods et al. (2007) whereby fouling organisms 

removed from hulls are collected using catch bags made from fine nylon mesh 

(0.2 mm). The fouling organisms are consequently placed in sorting trays filled 
 

33

with clean filtered (60 μm) seawater, and the viability of organisms can be 

assessed using the procedures described by Woods et a. (2007). 

6.6.2 Hull fouling assessment 

The extent of hull fouling on non-toxic coatings should be further evaluated in order 

to find further support that criterion 3 is a valid alternative to criterion 2. This may be 

done by taking a number of photographs (20 by 20 cm) during underwater inspections 

at selected areas around the hull (including rudder and keel). The average coverage 

per digital image can be calculated by superimposing 60 random dots and twenty 

randomly placed images may be used for the extrapolation formula to estimate 

coverage for the total wetted surface area. 

These figures may be used to establish a database with the fouling rate/ fouling index 

per vessel. This database can then be used to study the correlation between vessel 

activity and port residency time with increasing fouling rate. 
 

34

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34pp. 

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38

Appendix I. Figures of increasing FR 

The figures have been taken directly from the Naval Ships’ Technical Manual for 

illustrative and contemplative purposes (Naval Ships' Technical Manual, 2006). 

FR 10. Over 30 Percent Of Area 

FR 10. Over 100 Percent Of Area 
 

39

FR 20. Over 80 Percent Of Area 

FR 30. Over 40 Percent Of Area 
 

40

FR 40. Over 20 Percent Of Area 

FR 40. Over 30 Percent Of Area 
 

41

FR 40. Over 90 Percent Of Area 

FR 50. Over 20 Percent Of Area 
 

42

FR 50. Over 40 Percent Of Area 

FR 50. Over 100 Percent Of Area 
 

43

FR 60. Over 15 Percent Of Area 

FR 60. Over 20 Percent Of Area 
 

44

FR 60. Over 90 Percent Of Area 

FR 70. Over 20 Percent Of Area 
 

45

FR 70. Over 80 Percent Of Area 

FR 80. Over 60 Percent Of Area 
 

46

FR 80. Over 80 Percent Of Area 

FR 80. Over 90 Percent Of Area 
 

47

FR 90. Over 90 Percent Of Area 

FR 90. Over 90 Percent Of Area 
 

48

FR 100. Over 50 Percent Of Area 

FR 100. Over 100 Percent Of Area