Improve Production Efficiency of Scallop Aquaculture by Integrating HDPE Salmon Cage Technology with Existing Lantern Net Culture Methods and the Addition of Communal species

Table of contents

• 1. Executive Summary
• 2. Introduction
• 3. Project Overview
• a. Project Objectives
• b. Prototype Design
• i) Cage
• ii) Containment Net
• c. Deployment and Stocking
• i) Deployment
• ii) Stocking
• iii) Sampling
• 4. Results
• a. Prototype
• b. Stability
• c. Bio-fouling
• d. Biological Performance
• e. Cost of Production
• 5. Conclusions
• 6. Communications
• 7. Acknowledgements

1. Executive Summary

Magellan Aqua Farms utilizes lantern nets suspended from a buoyed long line.  This technology was originally developed for suspended oyster culture and has not significantly changed since its original inception.  In productive environments these culture methods suffer from high operational and maintenance cost.  Magellan Aqua Farms is looking for innovative ways to improve efficiencies by adapting salmon cage technology with traditional lantern methods in combination with the addition of beneficial communal species. This project utilized modified Salmon Cage bird net stands constructed from High Density Polyethylene (HDPE) pipe to create larger more cost efficient rearing units.

This AIMAP project had three objectives: 1) Design and build a prototype cage; 2) Access and develop the operational needs associated with their use; and 3) To assess the biologic advantages of using the larger volume system in conjunction with the addition of communal species.

The first objective was highly successful. The bird stand design currently used by salmon farms required only slight modification with the addition of a hub and spoke reinforcement to the top and bottom. This resulted in a structure which was strong and resilient, manageable in size and an excellent platform for the scallop containment. 

The second objective was successful in the sense that a number of unanticipated buoyancy control and stability issues were identified during this phase of the project. Through trial and error, slight modifications to the cages were made. In addition significant changes to the mooring design were necessary. The changes to the mooring system resulted in a simpler, less costly, and more user friendly design.

The third objective was also successful. The design performed well with lower mortality relative to the lantern nets and the introduction of urchins significantly reduced fouling. This cage system once deployed demonstrated reduced fouling relative to the cohort lantern nets and therefore less required maintenance. Subjective assessment of net material suggested the Aquagrid tm was easier to clean, less affected by fouling and provided a more stable substrate for the scallops.

This project is an important step in Magellan Aqua Farms goal to develop shellfish culture in the Bay of Fundy through the adoption of Integrated Multitropic Aquaculture principles in conjunction with larger more cost efficient rearing units. This will help Magellan Aqua Farms to continue to grow as the premium high quality sustainable shellfish producer in Southern New Brunswick.

2. Introduction

Passamaquoddy Bay is a highly productive nutrient rich ecosystem. It is an environment which provides a plentiful supply of spat along with abundant nourishment on which the scallops to feed. Unfortunately, such a productive environment means that shortly after an object is placed in the water, it rapidly becomes colonized by fouling organisms. The greater the surface area, the greater the bio-fouling is. The primary fouling organisms at MS-1077 are: tunicates, hydra and mussels. The mesh size on traditional lantern systems available in our region is 9.0mm. This presents a high surface area for colonizing and mesh openings are quickly reduced by the colonizing organisms reducing water flow. The problem is compounded by the fact that the fouling organisms, also being filter feeders, partially strip the food and oxygen supply from the already reduced inflow. With reduced inflow there is concurrent reduced outflow and removal of waste, creating a stressful environment for the scallops.

Magellan Aqua Farms has also identified two other significant issues in Passamaquoddy Bay: 1) Sea Star larvae “snowflakes” are abundant and once settled become a predatory concern for scallops in housed lantern nets; and 2) polydora, a parasitic annelid worm colonizing the shells of scallops. The sea stars are removed by bringing the lantern nets to the surface and transferring the scallops into clean lanterns. This is generally performed in early fall following the settlement in mid August.  Polydora are removed by physical debridement using a scrub brush during grading. These issues constitute the majority of labour inputs during grow out.

For this project Magellan Aqua was looking for innovative ways to mitigate the negative aspects of such a productive ecosystem while preserving the benefits. Based on our observations described above we postulated that by increasing the size of the rearing unit would create an environment which more closely resembles their natural habitat and allow more freedom of movement. The small mesh size encouraged fouling so for this system we increased it to 26mm to impede fouling and allow more unrestricted water flow. It has been reported that urchins routinely feed on many of the common fouling organisms observed in Passamaquoddy Bay as well as polydora. The addition of urchins to the rearing environment would thus provide a natural means of controlling this problem while at the same time provide additional market opportunities. These became the basis for the design elements. As many of these same issues were in common with the local salmon industry we looked at ways to adapt current salmon technologies to shellfish production.

3. Project Overview

a. Project Objectives

This AIMAP project had three objectives:

1. Design and build a prototype cage;     
2. Access and develop the operational needs associated with their use;
3. Assess the biologic advantages of using the larger volume system in conjunction with the addition of communal species.

Project success was measured using unit cost of production calculations on marketed product.

b. Prototype Design

i) Cage

The basic bird-net support stand design currently utilized by salmon farms served as the basis for the scallop cage. These units were constructed of HDPE welded pipe. They are 20 m in circumference and 2.2 meters in height, giving a floor surface area of 29 square meters. In September of 2009, 3 surplus bird stands were obtained from Chris Saulnier at Aqua Fish Farms Ltd. It was hoped these would provide the basic structure to support the fully enclosed containment net. This design is suitable for supporting a bird net within a circular salmon cage. When tension simulating that resulting from an internal net was applied, the top rails began to bend toward the center. In Addition there was insufficient support on the bottom so the test net displayed significant conation toward the center. If used in this configuration the scallops would pile up in center resulting in significant mortality due to scissoring. In consultation with Dr. Shawn Robinson DFO and Darren Cheney from MSI, a modified design was developed and three prototype cages were ordered on January 30, 2010. The modification consisted of reducing the height of the cage to 1.5 meters and the addition of a center hub with radiant spokes. Three of these cages were constructed and delivered by Marine Systems International Inc. (MSI) on November 5, 2010.

ii) Containment Net

The containment net was constructed out of both Aquagrid tm and traditional knotted polyester mesh. The completed net was reinforced with 3/8” Polysteel tm rope in vertical and horizontal planes as well as the upper and lower circumference. A zipper to allow entry by divers was placed in the top panel. The containment net was secured to the cage using 3/8” Polysteel tm rope ties. It took 25 minutes to complete the attachment process. The zipper used for the prototype provided access for delivering the animals to the cage, but was too short to allow a dressed diver to safely enter the cage without risk of entrapment at depth, and therefore these cages can only be entered safely on the surface.

c. Deployment and Stocking

i) Deployment

A modified submerged long line system was placed on site in preparation for deployment. The barge for deploying the mooring system belonged to a neighboring site, the plan called for deployment of the moorings in spring of 2010, however due to the massive problems with sea lice in Passamaquoddy Bay we were not able to utilize the barge as planned.  The barge owner required it for carrying out sea lice treatments on their salmon farm.  We managed to get access to the barge on Sept 9, 2010 and deployed the mooring system.  With no mooring system in place we were unable to take delivery of the cages in April as planned. The prepared cages were transported to the site and moored on November 26th until the cages could be submerged and stocked.  The cages had more buoyancy than could be overcome by the mooring pulley system. Weather conditions and cold temperatures ensued and stocking was deferred until spring. During a subsequent storm on April 7, 2011, the site barge, one prototype cage and the surplus bird stands broke away from their mooring. The barge was located several days later and towed to safety. A subsequent search for the cages was unsuccessful, leaving only two cages for the remainder of the study.  With plugs removed and air within the lower ring released the cages partially settled in the water.

ii) Stocking

On June 11, 2011, yearling Scallops were harvested from Lantern nets. A subsample was measured for Shell Diameter and Shell Height in mm. The animals were then divided and stocked at rate of 30 scallops per square meter in 4 clean lantern nets and both test cages. In addition, 150 Urchins were also placed in each test cage.

iii) Sampling

Initial sampling took place on June 11, 2011, the day of stocking. One scallop from each layer was collected and measured (mm) for shell diameter and shell thickness until a total of 50 scallops were measured. The plan was to measure these two parameters monthly through the remaining trial period. The difficulty in raising and lowering the cage and risk of diver entanglement in small opening prevented this schedule. The second sampling was conducted at the conclusion of the trial and included the additional parameter of meat weight measured in grams.

4) Results

a. Prototype

The cage system provided a rigid sturdy and robust platform for which to attach the containment net. The containment net was attached and readied for deployment in 25 minutes. This was significantly faster than our anticipated 2 hour preparation time. Following rigging the cages were temporarily moored in preparation for transport to the site. Unfortunately during this time a hurricane passed through the area and cages were lost and not recovered. Only two cages remained for transport to the site for the remainder of the project. During stocking on June 11, 2011 it became apparent that the zipper installed for diver access was too short; when open it did not provide sufficient access to allow a fully equipped diver to safely enter the cage without risk of entanglement. Given this risk we decided not to permit divers entry at depth. For sample collection the diver used a long handled dip net to pass through the zipper and collect scallops within reach. The scallops were stocked by pouring them through the opened zipper of the partially submerged cage. Within a very short time they distributed themselves within the cage floor and began exhibiting feeding behaviour. Structurally the prototype and containment net met all of our expectations with the exception of diver entry. This can be easily remedied by installing a longer zipper on future units.

b. Stability

On the surface and partially submerged the cage was very stable and easy to work around. It was during the attempts to submerge the cage where a number of unexpected issues came to light. Unresolved, these issues would call into question the cost effectiveness of the cages. The mooring system, although simple in design, proved to be practically very difficult for several reasons. Firstly the intrinsic buoyancy of the cage structure prevented us from sinking the cage with our existing equipment on site. To reduce the buoyancy we tied 6 – 60 lb. weight balls to the top rail equally distant around the circumference and drilled holes in the lower ring to allow water to enter the structure and reduce overall buoyancy. Secondly, the addition of the weight balls to the top rail made the structure very unstable. In response to current, the unit took on a Ferris wheel position in the water column and it was very difficult to right the cage from this position. Finally the poor visibility of the water, caused by the silt stirred up from the activity of attempting to sink the cage, made it impossible for the divers to find the submerged long line with which to anchor the cage. It took multiple attempts over several days with additional contract divers in order to fully submerge the cage and with that they were not fully secured as initially planned.

In order to complete the biological component of this trial the cages were left in place once submerged and progress samplings were reduced. Although frustrated by the difficulty in making this work, we learned a great deal about handling these cages on site and have developed a modified mooring system which should solve the issues at hand. This new mooring system also has the advantage of using the existing long-lines on-site currently in use with the lantern nets. The lateral lines can be tied between existing lanterns and therefore do not reduce the usable space on the long line. They require no modification and because the cages are positioned between the lines, there is no restriction on the use of the lines for the lanterns or spat collection. The cost of a separate mooring grid is eliminated. With the redesigned mooring system, the 4 mooring lines are attached to the long line while the cage is floating on the surface, and where the diver is in shallow water with good visibility. The 4-point harness originally used to pull the cage down via a mooring block and pulley system can be used to tie several weight balls until the cage becomes slightly negatively buoyant. All downward forces are from bottom, evenly applied and balanced against the upward tension in the lateral lines. A preliminary test with cage #2 while on the surface for final data collection showed great promise. This system will be implemented this spring

c. Bio-fouling

Once stocked the cages and lanterns were left in situ throughout the remainder of the study at which time a subjective assessment was made on the level of fouling.

At this stage both appear to be significantly fouled with sessile organisms however on closer examination the test cage shows significant advantages over the Lantern net. The much smaller mesh with the level of fouling is restricting the water flow through the unit and at this point it needs to be removed from the water and cleaned.  The cage with the larger mesh still has an open path for water flow through the mesh.

Observations within cage indicate that the environment closely resembles the typical benthic habitat for scallops. The scallops exhibit active feeding behavior and are very clean. They appear very effective at removing fouling material.

Divers placed the urchins in the cage in a manner similar to the scallops due to the restricted access. Consequently, the urchins were slow to move away from the entry point and concentrated their feeding activity where they were placed. Over time the urchins started to spread out and some began to migrate up the sides of the cage.  No polydora was observed on the scallops in the lanterns or the cages and therefore we were not able to assess this effect.

The cage containment nets were constructed out of both Aquagrid tm and traditional knotless poly mesh. The Aquagrid proved superior to the traditional mesh in that much of the light fouling could easily be wiped off and other sessile organisms found it less appealing. The traditional mesh with its many crevices and higher relative surface area provided a surface more prone to bio-fouling.

There were numerous sea star observed within the test cages, however they seemed to be more focused on the mussel set which was present on the mesh. In this sense they were acting as a biologic antifoulant. Because of the large volume of the cage the scallops are able to move away from advancing sea star. This was not possible in the Lantern Nets. The confined space of the Lantern Nets allowed the sea stars to develop an ingenious method for attacking the scallops.  The sea stars were observed hanging down from the ceiling of the compartment with 2 – 3 arms and then placing the remaining arms on the top of the scallop. The scallop was not aware of the attack until the sea star had a firm grip. This was not a viable option in the cage due to the high ceiling and large volume. There were several urchin remains in the cage and these are likely the result of sea star predation. There is a concern that once the sea stars consume all of the mussels and reach a larger size, that the scallops will be at risk.  Based on our observations, it is obvious the scallops are avoiding this area of the cage. There were large amount of open area in the cage side which allows for good flow and negating the need for servicing this cage. This was not apparent with the Lantern Nets.

d. Biological Performance

The results indicate a slight improvement in growth performance that is likely related to the improved water flow characteristics in the test cage resulting from the larger mesh and reduced fouling. There was also lower mortality associated with starfish predation despite there being a higher density of starfish in the test cage. No mortalities were observed in the test cages, however 5 dead scallops were found in the lantern net which appeared to be the result of starfish predation as 3 large starfish were recovered from the lantern.

e. Cost of Production

The ultimate goal for switching gear types and operational systems is to be able to lower the overall cost of production. Based on the results of this project the cage system resulted in a higher unit cost than the traditional method ($0.99 for cage versus $ 0.72 for the lantern net). The higher capital cost is a contributing factor, however the most significant impediment to efficiency is the high labour costs associated with deployment and necessity for separate mooring systems. We also noted there was ample space in the cage at the current stocking density and this could easily be doubled. Based on the version 2 Mooring system which eliminated the costs associated with a separate mooring system and a greatly simplified method for deployment, a revised calculation was made. For his calculation a stocking density of 44 scallops per square meter was used, representing a more conservative 50% increase in stocking density. This change resulted in a more favourable unit cost of production of $ 0.63 per scallop.

5)Conclusions

The first objective was highly successful. The bird stand design currently used by salmon farms required only slight modification with the addition of a hub and spoke reinforcement to the top and bottom and reduction in overall height. This resulted in a structure which was strong and resilient, manageable in size and an excellent platform for the scallop containment. 

The second objective highlighted the risks associated with prototype development. There were several obstacles encountered with this phase of the project. These were largely due to the increased buoyancy associated with the reinforcement of the cage structure. This was then complicated by attempts to offset this with counter weights and flooding chambers. Through trial and error we were able to assess the causes and develop new methods to make the system functional. Slight modifications to the cages were made. Significant changes to the mooring design were also necessary. The resulting modifications led to a simplified, less costly mooring system and deployment procedures requiring less diver involvement with friendlier working conditions.

Passamaquoddy Bay is a highly productive ecosystem where Bio-fouling organisms flourish. The integrated multitropic aquaculture approach with employment of urchins significantly reduced the need for cleaning by providing a nature driven antifouling mechanism. The large open cage area provided a less stressful environment for the scallops and afforded them an opportunity to escape from the inevitable settlement of sea stars in their containment system.

The third objective was also successful. The design performed well with lower mortality relative to the lantern nets and the introduction of urchins significantly reduced fouling. This cage system once deployed demonstrated reduced fouling relative to the cohort lantern nets and therefore less required maintenance. Subjective assessment of net material suggested the Aquagrid tm was easier to clean, less affected by fouling and provided a more stable substrate for the scallops. The increased cost of production associated with the use of these cages was a concern, however with future refinement this system will prove more costs efficient in the long run. Magellan Aqua Farms plans to continue to refine the mechanics of their use and replace the current lantern nets with updated versions of these prototypes for scallop grow-out.

6) Communications

Magellan Aqua Farms has given an interview with Portico Magazine which included information on this project. Magellan Aqua Farms will continue work cooperatively with industry associations to communicate the results of the study.

7) Acknowledgements

Magellan Aqua Farms sincerely thanks the following Agencies and Individuals for their contributions and support of the project.

1. Fisheries and Oceans Canada (AIMAP)  Carla Dale, Craig Reynolds, Cindy Webster
2. St. Andrews Biological Station (SABS) Dr. Shawn Robinson
3. Aqua Fish Farms Ltd.  Chris Saulnier
4. Future Nets Inc.  Clarence Blanchard    
5. MSI  Darren Cheney
6. Admiral Fish Farms Inc.
7. Jeremy Rattray and Amy Robinson who volunteered countless hours in order to make this project successful.