Alternative Fish Cage Nets for Improved Biofouling, Durability, and Fish Growth

Marine Harvest Canada

Table of contents

• Executive Summary
• Project Description
• Introduction
• Project Objectives
• Methods and Solutions
• Project Activities and Deliverables
• Performance Conclusions
• Changes and Revisions during the Project
• Finances
• Project Success
• Future Activities

Executive Summary

Marine Harvest Canada tested the effectiveness of three alternative salmon cage nets for six months at a high current and wave area near Port Hardy, British Columbia. The three objectives of the testing were

1. to improve the environmental sustainability of salmon aquaculture by reducing the use of copper treated nets
2. to increase farm productivity by maximizing water flow, hence dissolved oxygen flow, through the net cages
3. to develop a proven system of cages and nets suitable for use in high current and wave offshore locations

Marine Harvest installed the best available offshore cages at a fully operational commercial salmon farm site at Shelter Bay in September 2010. With the AIMAP contribution of $196,000 two alternative nets that could meet the above objectives were purchased in November and June in 2010. The Dyneema and Thorn-D nets were tested in a controlled study using traditional nylon nets as the control. Monthly sampling generated data on biofouling community composition, biomass, and percent net occlusion. The nets were also evaluated for cost, fish growth, predator interaction, washing cost, handling, and physical performance.

This project provided the company with detailed information on the actual performance of nets that are marketed as being resistant to biofouling. The project compared the level of biofouling with the cost of the nets and treatments with the purpose of developing a program to ensure the best culture environment for the salmon at the best cost. The results showed that none of the materials tested performed well enough at preventing biofouling to meet the industry standards. 

This project was successful in proving that the net manufacturer’s claims for reduced biofouling (either inherent in the net material or through treatment) were not supported and regular in situ net washing is the only option to ensure nets remain free of fouling if copper treatment is not used. Based on the information from this project, the company has prepared production plans for the next 2-5 years that includes in situ net washing.

A further success of the project was that University of Victoria Master’s student Courtenay Edwards used this project as part of her research. This enabled a student to work collaboratively with an industry leader and build a strong research relationship for the benefit of both parties. 

Ongoing work will include tests for net fatigue from continuous washing on currently used nets and on new rigid net materials. Communication of project information is ongoing and preparation of this report is the final requirement for the AIMAP funding.  

Project Description

Introduction

Biofouling in salmon aquaculture is a costly problem, both environmentally and economically. The accumulation of fouling organisms on a fish cage negatively impacts cultured stock by occluding net openings, which reduces water flow and associated nutrient and oxygen exchange. It also physically damages nets and infrastructure. Biofouling is typically managed by the use of toxic copper-based coatings and time consuming manual cleaning.

In accordance with the Marine Harvest corporate objective of eliminating copper treated nets, during 2009 Marine Harvest began converting from copper dipped nets to untreated nets. To reduce biofouling, the untreated nets have bi-weekly net maintenance using manually operated high-pressure washers. This prevents the initial growth of mussels and other marine life. Net maintenance is a labour intensive operation and one which cannot be done safely in high seas, especially on farms with circle cages. Marine Harvest is upgrading one of its highest energy sites at Shelter Bay, near Port Hardy, British Columbia.

The overall goal of this upgrade and testing is developing a proven system of environmentally sustainable cages and nets capable of use at offshore, high energy sites. Marine Harvest purchased eight of the best offshore cages available, Aqualine 120m circles, for Shelter Bay and these were installed in September 2010.  This project was the first opportunity for Marine Harvest to test cages and nets suitable for further offshore use.

Project Objectives

Over six months, alternative nets were tested in a controlled study using traditional nylon nets as the control. The study took place on a fully operational commercial salmon farm site (Shelter Bay, operated by Marine Harvest Canada). 

• The first objective the net testing was to reduce the trace copper sedimentation under salmon farms. Trace copper concentrations in the seabed under salmon farms require a period of time to remediate. Reducing or eliminating copper anti-foulants would increase the environmental sustainability of salmon farming.
• A second objective was to maximize water flow through the nets. Any reduction of water flow through nets reduces the oxygen available for fish growth and, at times, can result in fish mortality.
• The third objective was to develop a proven system of cages and nets suitable for use in high current and wave offshore locations. Having a proven offshore cage and net system will enable expansion, or migration to, high energy offshore sites. The salmon aquaculture industry will be moving to more offshore sites since these optimize salmon growth for biological and operational reasons.

Methods and Solutions

For the duration of the project the site had seven 120m diameter polar circle cages (made by Aqualine®) set up in a double array. The untreated Dyneema®, Dyneema+Sancure® and untreated nylon cages were deployed in January 2011. The ThornD® net was late in arriving from the manufacturer in Europe, and proved to be structurally unsound. To be able to include this net in the study, adjustments were made and the net was deployed in February but did not have any fish in it. Panel samples were entered in January 2011. Atlantic salmon (Salmo salar) were entered on February 14, with up to 75 thousand in each polar circle. 

Dyneema is an ultra-high-molecular-weight polyethylene (UHMwPE) fibre created by the Dutch chemical company DSM®. The material is 15 times stronger than steel, chemically inert; UV, abrasion and moisture resistant; and very durable (Dyneema 2011). Nets made with Dyneema do not absorb water, retain their knot strength and have limited stretch. The Dyneema net has a somewhat rectangular twine thickness of 2mm wide and 1mm thick.

Nylon is a name that encompasses various synthetic, thermoplastic polymers which are considered to be fairly tough, lightweight and resistant to heat and chemicals. Nylon netting absorbs water and can lose 10-20% of knot strength when submerged (Badinotti 2011). Nylon netting has a round twine thickness of ~3mm.

ThornD is a coating designed specifically to prevent biofouling. The coating is applied by flocking short fibres onto the netting material. This "fuzzy" surface is thought to deter settling organisms by damaging planktonic cell structure and by swaying with water movement thus dislodging spores (Micanti 2011). Nylon net with a ThornD coating has a round twine thickness of ~5mm ±1 due to the varying length of the fibres.

Sancure 1511 (manufactured by Lubrizol Advanced Materials Inc.) is an aromatic waterborne urethane polymer. It has a high gloss, is abrasion resistant and flexible. The coating was supposed to cause the fibers to adhere together, and solve the issue of loose fibers that occurred on a large section of the Dyneema net. Sancure does not alter the initial twine thickness.

The monthly biofouling evaluation was conducted by video monitoring of the salmon cages and a controlled, statistically viable panel-study which was monitored with still images.

Project Activities and Deliverables

1. Document whether Dyneema and Thorn-D claims about reduced bio-fouling are accurate
   
   The Dyneema and Thorn-D nets claimed to have less biofouling than untreated nets.  The results for biomass show that there is a significant difference at 18m. The Dyneema+Sancure had significantly less biomass than the untreated nylon, and the ThornD had significantly more. At 5m there continues to be significantly less biomass on the untreated Dyneema. At 1m there is no significant difference between any of the four combinations tested in this study. 
   
   Mesh size and twine thickness change the total surface area of a net (Hellio & Yebra 2009). As described by De Nys and Guenther (2009), an increase in surface area equates to an increase in available area for fouling organism to settle. Consequently, small-mesh cages tend to have higher levels of biofouling than large mesh cages. Likewise, a thick twine mesh has more surface area than a thinner twine mesh of the same size which means that thinner twine thickness creates less drag from currents and less biofouling accumulation.
   
   
2. Cost/benefit analysis of net maintenance compared to net capital cost
   
   The total cost of nets annually fluctuates based on age of nets and the number of new nets required. 2010-2011 almost $3.5 million (approximate) was spent on new nets and an additional cost to repair existing nets. Since at 1m there was no significant difference between any of the four combinations tested in this study it is expected that the cost of net maintenance compared to capital cost will remain the same previous years.
   
   
3. Analysis of bio-fouling species and colonization
   
   The results from the ANOSIM analysis showed that there was a significant difference between the community composition at the three depths (ANOSIM: R= 0.81, p = < 0.001) which is to be expected. There was no significant difference found in the biofouling community composition of the four materials when the samples were separated by depth (ANOSIM: R= 0.486, p = 0.005).
   
   There was no overall significant difference between untreated nylon and untreated Dyneema.  Both did well in terms of net occlusion, biomass and species composition. Copper wax had an accumulation of diatoms early in the season and minor hydroid fouling by the end of the study. Environet maintained very low fouling of just diatoms until June, but then became heavily fouled with barnacles. The thick diatom may have prevented other organisms from settling during the early part of the season, but was disturbed when there was a barnacle spatfall. ThornD maintained low fouling until May, and then became heavily fouled with Hydroids for the rest of the study. There was no overall significant difference between the untreated nylon and untreated Dyneema. Both performed reasonably well in terms of net occlusion, biomass and species composition.
   
   
4. A ranking method that relates bio-fouling growth to oxygen flux, and hence fish growth
   
   Since there were only 2m transects on the cages that were left to foul changes in environmental conditions, (oxygen and nutrient levels) inside the cages could not be related to the accumulation of biofouling. Any variation in fish growth between the cages would be due to other external factors. The ThornD® net was late in arriving from the manufacturer in Europe, and proved to be structurally unsound. To be able to include this net in the study, adjustments were made and the net was deployed in February but did not have any fish in it. 
   
   
5. Analyse predator hits to determine if the Dyneema net or Thorn-D net is more or less susceptible to predators
   
   Predator hits were likely based on location, (e.g. the first net they came to.) after a few attempts the predator would likely choose the net that set with the least tension, in order to push the inwards in a big billow to be able to bite a fish. That being said, the tightest net in this study was the Dyneema net and it still suffered from predators. More research would be needed to determine if there are any patterns in their behavior.The net coatings appeared to have no effect on predator hits.
   
   
6. Lifecycle net washing cost
   
   The results show that the Dyneema+Sancure had significantly less biomass than the untreated nylon, and the ThornD had significantly more. Therefore, the reduced biofouling biomass on the Dyneema net would require less frequent net washing. This would reduce the labour required to handle the nets and reduce the wear on washing equipment.
   
   On average it costs $.05/sq m to wash a net, approximately $ 3750 per year per net. (120 meter) A net washer costs $75000 and is depreciated over 10 years.  A net is designed to last 10 years but sometimes fails the break strength testing after less time. Typically after every generation we service the smolt and grower nets at a cost ranging from $2-6000/net depending on the age.
   
   
7. Evaluation of practical handling issues with Dyneema and Thorn-D nets
   
   A net coated in wax (ie Copper and Environet) is more difficult to handle. The nets are much heavier and stick together during shipping. Also, rough handling can damage the coating, leaving foothold exposed for fouling organisms to settle. The ThornD net was very rigid and heavy from the spiky coating making it difficult to handle. The Dyneema is the lightest and supplest of the nets, (also substantially stronger) making is easier to handle.
   
   
8. Conclusions about fish growth in each net type
   
   See 3 above.
   
   
9. Analysis of the physical performance of nets
   
   The Dyneema net is substantially stronger and has a smaller twine thickness. The Copper and Environet may be superior if the manufacturer's claims of protection from UV damage, abrasion and wear are accurate. The ThornD net has a much larger twine thickness, resulting in a larger surface area for fouling and drag.
   
   Far Superior: The Dyneema net is substantially stronger and doesn't lose any of the strength when it gets wet. It also has a smaller twine thickness, meaning it has a less overall surface area. This results in less space for fouling to accumulate and experiences less drag from currents.
   
   Possibly Superior: If the coating manufacturer's claims are true about the coating providing protection from UV damage, abrasion and wear then the nylons nets with coatings may have a longer lifespan and maintain their breaking strength for a longer period of time. However, the improved durability may by a margin so small it's not operationally relevant.
   
   Inferior: The ThornD net has a much larger twine thickness due to the spiky coating. This increases the surface area for both fouling and drag, (making it a poor choice for rough seas and high currents.) Increase drag would mean an increased dynamic loading of all the infrastructure and higher rates of net deformation.
   
   
10. Communications
    
    Project information is available to the industry stakeholders such as the Aquaculture Coordinator Office and the regional and federal offices of DFO AIMAP upon request.  This report is the final requirement for the AIMAP funding agreement.

Performance Conclusions

Although some statistically significant results were found throughout this study none of the materials tested performed well enough at preventing biofouling to be considered effective by industry standards.

Despite the combinations having slightly different succession and disturbance patterns, species composition in September (which is considered to be the initial stages of the climax community) showed no significant difference between the combinations. This is supported by the percent net occlusion results which showed that all four combinations in this study finished at the roughly the same levels. The biomass results showed that the Dyneema net has potential to somewhat reduce biomass; and the ThornD net generally showed higher levels of biomass. There was no difference in biofouling between the untreated and treated Dyneema.  Dyneema treated and untreated nets were considerably more costly than other options with a non-significant difference in biofouling results.

Although a combination‘s ability to prevent the accumulation biofouling remains an important factor in choosing a netting material and coating, the current state of environmentally benign antifouling technology means that other factors need also be considered. These factors include: the cost effectiveness of a new material or coating, the strength and lifespan of a material and its associated repair and maintenance costs, and a reduction in overall surface area by having larger mesh and finer twine. This study exemplifies the diversity and tenacity of fouling organisms and the challenge all marine ventures face when working in this biologically diverse environment.

The Dyneema net performed the best in other criteria. It is easiest to use, is substantially stronger and does not lose strength when wet. It also has a smaller twine thickness reducing surface area for biofouling. Relative to other nets, the reduced biofouling creates less drag and reduces the need for net washing.  

No difference was observed in predator reactions and there was inadequate data to compare fish growth between the nets. The Dyneema net has a greater capital cost than the other options but was rated as “Far Superior” in durability and “superior” for handling.

Changes and Revisions during the Project

The project was extended to October 31, 2011 with the final report submitted by December 31, 2011. This extension was necessary due to severe winter weather that delayed installing nets and fish at Shelter Bay until late January 2011. 

The ThornD® net was late in arriving from the manufacturer in Europe, and proved to be structurally unsound. It failed mesh break testing and since the net could not be relied on to hold fish it was decided not to spend more money completing the net bottom. To be able to include this net in the study it was deployed in February but did not have any fish in it. 

Due to the delay in the ThornD net arrival, the biofouling evaluation and net performance evaluation was completed during the summer of 2011.

Finances

The project total cost was estimated at $1.8M at the time of funding application. At completion, the project expenses totaled $2.8M (cash $1.5M and inkind $1.3M) with the additional costs covered by Marine Harvest. AIMAP funding contribution of $196,000 was used for purchase of nets in the study. All purchases were complete prior to March 31, 2011. 

Project Success

This project successfully provided Marine Harvest with definitive evidence that the claims made by the various net manufacturers either for the biofouling resistant materials or for specific net treatments were not supported and that another method of keeping the nets clean was required. This valuable information has provided the basis of long term production plans (2-5 years) that now include regular net washing as the method of ensuring the nets are kept free of fouling.

Future Activities

All evaluation of biofouling and net performance is complete. Project information will be provided to stakeholders upon request. Future research on resilience of various net materials to continuous washing is planned.