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Cite this article as: Moe Føre H, Olsen A, Jackson D, Drumm A, Mendiola D, Gabiña G (2013) Damage to the net cage. In: PREVENT ESCAPE Project Compendium. Chapter 6.2. Commission of the European Communities, 7th Research Framework Program. www.preventescape.eu

authors: Heidi Moe Føre1, Anna Olsen1, Dave Jackson2, Alan Drumm2, Diego Mendiola3 & Gorka Gabiña3

1 SINTEF Fisheries and Aquaculture, Norway, 2 The Marine Institute, Ireland, 3 Azti Technalia, Spain.

Technical standards contain up-to-date information to ensure that fish farms do not fail structurally and that their operations and procedures do not lead to fish escapes. Such standards are well developed in some European countries, but poorly developed, or non-existent, in others. That escapes still occur at a level regarded by many to be detrimental to wild stocks (e.g. Naylor et al. 2005), indicates that technical standards are in constant need of improvement. As shifts in the fish-farming industry occur that result in changes in the type of technology used, the physical environments in which farms are located and practical aspects related to use of the equipment of ever-increasing size, research is constantly required to update the knowledge upon which standards are based. Objective testing and documentation of how fish-farming technology (netting, floaters, mooring systems) interacts with the diverse range of physical (waves, currents) and biological (cultured species, biofouling) environments in Europe is essential to prevent escape.

The net plays an important role in most escape episodes, either directly through holes in the sea-cage from over-loading or contact with other objects, or indirectly by transferring major loads to the fish farm (Jensen 2006, Moe et al. 2007, Norwegian Fisheries Directorate 2007). The mechanical properties of nets and net materials, and how they change over time with use and wear, are poorly understood. Current knowledge and testing, is mainly limited to unused netting materials (Figure 6.2.1; Moe et al. 2007). How net material properties change with damage from wear, UV-radiation, fatigue, abrasion and biting by the caged fish is largely unknown. A wide range of netting materials are available in the marketplace, from simple uncoated and coated nylon nets to Dynema, yet how the strength and durability of these varying materials changes with practical use is poorly understood (Moe et al. 2005).

6.2. DAMAGE TO THE NET CAGE

INTRODUCTION

ISBN: 978-82-14-05565-8

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We identified and assessed the range of critical operations, conditions and circumstances that can lead to escape episodes at sea farms. The study included aquaculture sites from three countries: Norway, Spain, and Ireland. Culture operations for species typical grown in each country were covered, including Atlantic salmon (Salmo salar) and Atlantic cod (Gadus morhua) in Norway and Ireland, and European seabass (Dicentrarchus labrax) and Gilthead seabream (Sparus aurata) in Spain. Farming sites were monitored over time to detect the nature and extent of damages to nets, document general operational procedures, and detect escape and near-escape episodes during a production cycle from smolt to harvest. Findings from the observations, supported by experience from the fish farmers, have been analysed with respect to the level of risk for fish escapes.

Through extensive analysis of past escape incidents and practical experience, the net is known as one of the weakest points in the sea-cage construction (Jensen et al. 2010). Approximately 60% of escapes, measured both in the number of incidents and the numbers of escaped fish, occur due to holes that develop in the net. We inspected cage nets regularly through a production cycle to document: the number of damages to the nets; development of damage (time and size/extent); location of damage; type of damage; and possible cause of damage.

Figure 6.2.1. New undamaged net with antifouling surface treatment.

Fish farmers were also interviewed to determine the critical moments of risk to the cage net integrity in handling operations, such as; splitting and grading, transfer of fish between cages, parasite treatment, and smolt and slaughter-fish delivery. In Norway, data was collected from two salmon and two cod producers, encompassing three farms for each species. Interviews were also conducted with three Irish salmon producers. For seabream and seabass farms, four fish farmers from mainland Spain, one farmer from Gran Canary Island and one from Tenerife, were interviewed.

OBJECTIVE

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Based on the interview results, a risk analysis of critical operations for fish escapes was carried out using qualitative risk analysis (Bahr 1997). The initial results include an overview of the critical damage that can occur in fish-farm nets, which potentially lead to fish escapes, for a number of sea-cage designs. In additional, the interviews provided information on general fish farm operations, plus information to assist in the formulation of guidelines for the design, operation, inspection and repair of nets. These guidelines should help fish-farms to avoid or limit future net damages. Learning from fish farmers about their sea-cage operations, equipment, and previous escape incidents, enables us to better understand the technical/operational causes of fish escapes. This information constitutes an important contribution to current knowledge to the improvements which can be made that will benefit the entire industry.

The questionnaires were completed by direct interviews with the producers. The questions were divided into three sections that specified different causes of fish escapes: technical/mechanical failure, biological factors and human errors. The questions aimed to ascertain the most important issues arising within the three categories, as well as to gather more detailed information concerning day-to-day fish farm management.

The following issues were raised:

Please note that these are the subjective opinions of a selected group of people. We assume that these opinions will represent the majority of fin-fish producers along the coasts of Norway, Ireland and Spain, however, we recognise that individual opinions may differ from those recorded here.

Topics

Net handling

Wear and tear

Feeding regimes

Biofouling

Boat traffic

Predators

Table 6.2.1. Relevant topics in the questionnaire.

QUESTIONNAIRES

The risk of an event can be defined in a number of ways. The common definition of risk is “the prediction of what may happen in the future”, whereas the more academic definition is the following (ISO14121-1:2007):

“Risk is a combination of the probability of occurrence of harm and the severity of that harm”.

Once the interview results were collated and the risks identified, the probability of an escape occurring, and its consequences, were placed under four categories (see Table 6.2.2).

Probability

Category 1 Rare event is defined as event which is expected to occur once in 100 years or not expected to occur, exceptional circumstances only.

Category 2 Possible event is defined as event which is expected to occur once in 10 years.

Category 3 Likely event happens at least once every second year (at least once during the production cycle).

Category 4 Almost certain event happens at least once a month (expected in most circumstances on the daily basis)

Severity of consequences

Category 1 Negligible consequences: Individual escapes less than 100 fish.

Category 2 Minor consequences: Moderate escapes 100 to 10 000 fish.

Category 3 Major consequences: Large escape 10 000 to 100 000 fish.

Category 4 Severe consequences: Catastrophic escape over 100 000 fish.

QUALITATIVE RISK ANALYSIS

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Table 6.2.2. Categories of probability and severity of consequences.

CONSEQUENCES

1 2 3 4

4

3

2

1

PROB

ABIL

ITY

The categories were adapted for this project based on the particulars of the aquaculture industry (Helle 2011). The relevant answers were then assigned within a risk matrix (Table 6.2.3).

Table 6.2.3. Risk matrix.

High risk. Immediate actions should be applied to reduce the risk of escape.

Moderate risk. Correcting actions should be considered.

Low risk. Actions are implemented from other considerations than escape risk.

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We monitored three cages for net damage for each of the following fish species: Atlantic salmon; Atlantic cod; Gilthead seabream, and European seabass, in Norway, Ireland, and Spain.

Relationships were established with fish farmers, their net servicers and their divers. Through this collaboration we gained access to their facilities and equipment documentation.

The following topics were covered:

Description of the cage net, material choices, weight and mooring systems (see Figure 6.2.2), the age and condition of the net, antifouling treatments and the breaking strength of the mesh.

Description of damages to the net (noting the location, size and assumed cause).

Biomass parameters.

Description of environmental loads (where available).

Figure 6.2.2. Illustration of fish farm and net cage components.

NET DAMAGE MONITORING

This data, and supporting information, was gathered at four stages during the production cycle: stocking; during net changes; at approximately half-way between net change and harvest, and at harvest time.

Whenever possible, additional information was gathered from the divers about net conditions between the assigned inspection times. The divers were asked to pay extra attention to the following:

Specify the exact number of damages found and repaired, and the dimensions and position of the damaged areas on the net. The position should be stated as the depth and the proximity to the ropes, either vertical or horizontal.

Specify the position of the damages with respect to the other cage equipment such as chains, weights, cables and mooring lines.

Specify the position of the damages with respect to the main current direction if possible.

If it is possible to recognise wear of the netting and ropes without apparent holes or tear, specify dimensions and position.

Suggest the cause of the damages (wear/fish bites/predators/mechanical damages).

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To ensure consistent terminology, classifications for net and rope damages were used (Tables 6.2.4 & 6.2.5).

Nomenclature Description Illustration

Broken twine A single broken twine in the netting

Hole Several broken twines forming a hole in the netting

Tear Several broken twines in a row

Wear Netting damaged by abrasion against ropes, chains, floater, weight etc

Fraying Singular or small bundles of filaments (fibres) are pulled out of the twine

Torn seam Seam in netting that has been torn

Table 6.2.4. Classification of net damages.

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Nomenclature Description Illustration

Broken rope Not available

Wear Rope damaged by abrasion against other ropes, chains, propeller, floater, weights etc

Torn seam Torn seam (loosened rope)

Table 6.2.5. Classification of rope damages.

The fish-farm managers were also interviewed and a questionnaire completed. The respondents gave their opinions on what they felt were the most critical reasons for fish escapes (Figure 6.2.3). The majority of the Spanish fish farmers interviewed indicated that their main area for concern was technical failure, e.g. inadequate equipment and cage structures that were unable to withstand the harsh marine environment. The Norwegians indicated that both human error and biological factors were the main risks, while the Irish considered human error, net handling and predator attacks to be equally important.

Figure 6.2.3. Most important reason for escapes, sorted by country.

QUESTIONNAIRE

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Figure 6.2.4 shows the same results categorized by fish species (e.g. salmon producers from Norway and Ireland came under the same category). The main causes of escape of seabream and seabass, from Spanish fish farms, were technical. In contrast, cod producers attributed their losses to biological factors, often mentioning fish behaviour. Salmon producers, on the other hand, were most concerned about net failures due to human error.

During the interview the respondents selected the fish farm activities most likely to lead to escapes (Figure 6.2.5 shows activities sorted by countries, and 6.2.6 by species). Irish farmers indicated that equipment wear and tear and predators were their greatest concern, whereas feeding regime and boat traffic did not represent any risk for escapes (Figure 6.2.5). In contrast, the Norwegian and Spanish farmers did not highlight any particular activities as being more likely to lead to escapes.

Figure 6.2.4. Most important reason for escapes, sorted by species.

Figure 6.2.5. Summary of factors which represent a risk of escape, sorted by countries.

Sorting the data by species revealed that feeding behaviour is an important escape risk factor for Atlantic cod (Gadus morhua) and Gilthead seabream (Sparus aurata), but of no concern for Atlantic salmon (Salmo salar) and European seabass (Dicentrarchus labrax) (Figure 6.2.6). The other risk factors were represented more or less uniformly across species.

Figure 6.2.6. Summary of factors which represent a risk of escape, sorted by species.

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The fish farmers also evaluated the frequency of occurrence of critical net-damage events and their consequences. Each answer was assigned to one of the four categories from the lists of ‘probability of occurrence’ and ‘severity of consequences’ (see lists in Table 6.2.2). The questions were also divided into the three groups mentioned before i.e. technical failure, human error and biological factors. Some of the questions could be placed under more than one group. For instance, some of the net handling operations, such as changing and lifting of nets, are regarded both as an event with risk for human error and mechanical failure. This assumption is considered to be valid since several of the respondents explicitly mentioned it in their answers.

In the risk assessment for technical causes, the natural environment lies partly in the red zone, and this represents a high risk of escape (Table 6.2.6 Charts 1-5.a). In Spain, their high energy environment, which is subject to frequent storms, represents a high risk of damage to the critical components of cage structures such as as net pens and the mooring system (Table 6.2.6 Charts 4-5.a). In contrast, Norwegian and Irish sites are mostly semi-exposed and experience less wave loads on the structure. The respondents from these countries gave less weight

Figure 6.2.7. Critical factors which represent a risk of escape, sorted by countries.

RISK ASSESSMENT

to the environment in the risk assesment, moving it to a moderate level (Table 6.2.6 Charts 1-3.a). The issue of cleaning the nets with high pressure cleaning discs was raised on a number of occasions. The respondents reported that the extensive pressure exerted on the nets initiates damage to the netting. Figure 6.2.8, Charts 1-5.b illustrates the risk of human error occurring when net handling operations and boat traffic are included in the analysis. Several Spanish fish farmers mentioned that boat navigation around their facilities led to an increased risk of damage occurring due to the low skill level of the personnel. In addition, many respondents pointed out the risk for human error during net and fish handling operations such as handling of weights, changing of nets and at splitting, grading and stocking of the fish. The issue of poaching was also mentioned. This problem is a high risk factor for Gilthead seabream (Table 6.2.6 Chart 4.b) because they tend to swim near the water surface which makes them easy to catch.

Biofouling, predators and fish behaviour are included as biological risk factors (Table 6.2.6 Charts 1-5.c). Biofouling represents several problems for the fish farmers as it obstructs water flow through the cage, and can also provoke undesirable biting behaviour among fish such as Atlantic cod (Table 6.2.6 Chart 2.c) and Gilthead seabream (Table 6.2.6 Chart 4.c). Net biting is not an issue for European seabass (Table 6.2.6 Chart 5.c) or Atlantic salmon. In Norway, removing biofouling from the nets with a high pressure cleaning disc is regarded as a high risk activity (Table 6.2.6 Chart 1.c). As for predators, the fin-fish farms in Ireland are ocassionally attacked by seals (Table 6.2.6 Chart 3.c), whereas Spanish respondents mentioned several species of wild fish that damage nets as they try to get inside cages, and then scare the farmed fish through the existing holes.

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Table 6.2.6. Graphical representation of risk matrices.

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Norway

Three salmon sites and three cod sites were chosen to monitor net damage in Norway. The seawater production cycle in Norway is approximately 18 months for Atlantic salmon and 22 months for Atlantic cod, and is characterized by two periods. First, the fish are stocked into a smolt net, with small mesh size in the range 26 – 40 mm. After approximately half a year, the smolt net is replaced by a net with a larger mesh size, in the range 44 – 55 mm. Some of the fish farmers grade and sort fish during the net change period to achieve a more uniform biomass distribution in the cage. Others keep fish in one unit for the whole period to avoid stressing the stock.

As already mentioned, the data on net damages was gathered at four stages: stocking; net change; intermediate inspection, and after harvest. Supplementary diver reports were collected where available. The quality of the information provided varied because the net servicer and diver companies often have their own protocols for producing service and repair reports. Figure 6.2.8 gives a summary of the areas of damage of nets monitored in Norway.

RESULTS FROM NET DAMAGE MONITORING

Summary for Salmon cages:

Figure 6.2.8. Summary of areas of net damage - Norway.

I: The area most exposed to environmental conditions is above the waterline and in the top 1 – 3 m of water. The most probable causes of abrasion are; contact with other parts of the structure e.g. floaters, sinker-chains and ropes, exposure to environmental forces (waves, currents) and net handling operations.

II: Significant damage was observed all around the body of the net, both on the vertical walls and in the conical bottom section. The most probable causes are environmental forces (currents) and net handling operations.

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I: The most exposed areas of the net are above the waterline, and the top 1 – 3 m of net showed the greatest number of holes/abrasions. This most likely occurs due to contact with other parts of the cage structure e.g. floaters, chains and ropes. Environmental forces (wind, waves and currents) can be severe, resulting in net abrasion along and above the waterline. Net handling operations are also a contributing factor.

Figure 6.2.9. Summary of areas of net damage - Ireland.

III: The area near the leadline and the central part of the bottom contains damage which is suspected to have been caused by cod biting.

There are approximately 35 current marine salmon farming licenses in Ireland. Organic Salmon production accounts for 83% of the total annual production. The growth cycle period is similar to that of Norway and Scotland. However, the conditions in which Irish salmon are grown differ greatly from the enclosed sea-lochs of Scotland and fjords of Norway. The majority of Irish fin-fish farms are located off the west coast, located in high energy, exposed and semi-exposed areas. This has necessitated the development of strict Standard Operating Procedures (SOPs) for maintenance of all structures e.g. cages, moorings and nets. The introduction of SOPs has resulted in a significant reduction of escape incidents. In conjunction with SOPs, the development of correct weighting systems for pen netting has eliminated the use of predator nets. While the vast majority of companies employ SOPs for net maintenance, the results from this study indicate there is still some room for improvement. Figure 6.2.9 shows a summary of the areas of damage of nets monitored in Ireland.

Ireland

Summary for Cod cages:

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I: As with the Norwegian and Irish nets, the most exposed area is above the waterline and the top water layer down to the 3 m depth. The most probable causes are contact with other parts of the structure (floaters, chains, ropes) and environmental forces (waves, currents).

II: Damage was observed around the whole net enclosure. The most probable causes are environmental forces (currents), predators and biting. Damage to seabream nets is primarily caused by biting of the net by seabream themselves and predators trying to feed on the fish within the cage.

III: The area near the leadline experiences extensive wear and tear due to current pressure and interactions with other parts of the construction, such as the sinker tube and sinker tube chain and rope.

Figure 6.2.10. Summary of the areas of damage of net damage - Spain.

SUMMARY FOR GILTHEAD SEABREAM AND EUROPEAN SEABASS

Spain has developed into one of the primary European aquaculture producers in terms of volume. In 2010, seabream and seabass were the two main marine finfish species being produced in terms of volume and value of production (APROMAR 2010). The total volume of production was approximately 43888 t consisting of 20360 t of seabass and 12495 t of seabream. Another important farmed species is the meagre (Argyrosomus regius); 3250 t was produced in sea-cages in 2010.

Spain

II: Damage was also observed near the leadline, along the net panels and close to the vertical lines in the junction between the net panels. The most probable cause for damage along the leadline was the individual weighting system. The damage along the walls could be attributed to predator attack (seals) and net handling operations.

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This study presents a qualitative evaluation of ‘escape-critical’ damages to nets from a variety of sea-cage designs, operations and fish species. Questionnaires and inspections showed that the causes of net damage are complex and that they vary among countries. Environmental conditions differ between countries; Spanish aquaculture facilities are situated in areas with high energy seas that experience several severe storms per year, whereas Norwegian farms are usually located within more sheltered locations. However, farms in the middle and northern areas of Norway may experience high current loads and ice on the nets and floaters. Irish conditions differ greatly from the enclosed sea-lochs and fjords of Norway and Scotland as the majority of farms are located off the west coast, in high-energy, exposed areas.

The study covered several types of the sea-cage designs, although the dominant structure was a circular design with vertical net walls between 12 – 20 m deep and a conical bottom. Other cage designs were square or octagonal, with flat bottoms, circular nets, inclined walls and conical bottoms. A design that is custom-made, for one of the salmon producers in western Norway, has inclined walls in order to avoid contact of the netting against the chains attached from the floater down to the sinker tube. This new net pen design has been successful so far according to the manager in charge of the fish farm.

In Norway, most of the net damage comes from wear and tear, especially near and above the waterline and near the bottom line, and cod biting behaviour. In Ireland, most of the damage occurs at, or above, the waterline. The farmers reported that leaving the nets loose on the cage during emptying resulted in damage, as wind causes the net to rub with the floaters and stanchions. They also reported predator attack, i.e. seals, as another main cause of small holes and abrasion. However, if the net walls are tensioned correctly, the attacks are minimised. Differing individual weighting systems were observed on the individual sites and it is thought that some improvements to weighting may reduce the damage due to predators. No escape or near-escape incidents occurred in these locations during the monitoring period.

The Spanish situation differed greatly to Norway and Ireland. Over 95% of the holes and damages “routinely” encountered on the aquaculture nets corresponded to cages containing seabream. Their frequency increased during the spring-summer season and relate to fish biting habits. Fish biting was almost negligible in seabass facilities; any damage detected in these farms was likely related to the effects of the environmental loads and/or operational causes.

The survey also showed that, for both fish species, net damages larger than 20 cm occur due to a number of causes such as: wear and tear due to friction between side/weight ropes and the net; environmental pressures and subsequent tearing of weak net mooring points; cannibalistic feeding habits on dead individuals lying on the bottom of the net, poor operational protocols and accumulative damage over time.

CONCLUSIONS

Producers should be encouraged to introduce Standard Operating Procedures (SOPs) for net maintenance.

Current SOPs should be reviewed.

Maintenance data should be kept for all nets.

Cage design should be improved based on the data showing where and how net damages occur.

The level of competence of the farm workers should be raised.

Carry out further investigations into updating the weight systems for nets.

The biting behaviour of seabream and cod should also be further studied.

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RECOMMENDATIONS

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Gabiña G (2011) Damages on sea-cage nets. Internal report 23/03/2011, AZTI Technalia.

ISO 14121-1:2007 Safety of machinery – Risk assessment – Part 1.

Jensen Ø, Dempster T, Thorstad EB, Uglem I, Fredheim A (2010) Escapes of fishes from Norwegian sea-cage aquaculture: causes, consequences and prevention. Aquacult Environ Interact 1:71-83

Jensen Ø, Føre HM, Endresen PC (2011) "6.2.2 REPORT - Mechanical properties of net and ropes". Report on task 6.2.2 Prevent Escape, SINTEF Fisheries and aquaculture.

Helle TA (2011) "Risk". Workshop presentation, 31 May 2011. (in Norwegian) Directorate of Fisheries.

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NS9415:2009. Marine fish farms. Requirements for site survey, risk analysis, design, dimension ring, production, installation and operation. Standard Norway.