The Development of a Technology to Recover Mussel Seed from the Synthetic Mesh Used to Protect Against Predation from Sea Ducks

Indian Point Marine Farms

Table of contents

• 1. Introduction
• 2. Engineering – design and construction
• 3. Field testing
• 3.1. May 7-10 2009
• 3.2. May 27 2009
• 3.3. June 27-29 2009
• 3.4. Mussel seed trials – Fall 2009
• 3.5. Mussel seed performance
• 4. Discussion
• 5. Conclusions

1. Introduction

Mussel seed deployed in the fall in biodegradable cotton sleeving becomes vulnerable to sea duck predation after 2-3 weeks. In the case of Indian Point Marine Farms (IPMF), located in Mahone Bay, Nova Scotia, which imports mussel seed from outside sources every fall, this predation can constitute an immediate loss of $15,000 and the potential loss of a crop worth $250,000. In a preliminary AIMAP project (2007-2008), it was found that mussel seed deployed in non-biodegradable synthetic mesh is effectively protected until the following spring when the threat from sea ducks has passed. For IPMF, the process of deploying mussel seed in the fall is relatively efficient because of their highly mechanized continuous sleeving approach (New Zealand technology). However, the process of recovering the seed from inside the synthetic mesh can be laborious and needed to be mechanized. In the spring 2008 it was estimated that 150 L of seed could be harvested manually per person per hour, a commercially-unacceptable rate. This project aimed at designing, constructing and field testing a technology to recover mussel seed deployed inside protective synthetic mesh. Our goal was to achieve a harvest rate of 1500 L of seed per hour, or one full longline, which would represent a 1-fold increase over the manual harvesting approach.

The Work was divided into two phases with the following specific objectives:
Phase 1: Engineering

Task 1: Design and fabricate the technology with an engineer at Atlantic Systems Manufacturing (ASM) in Charlottetown, PEI. This company has considerable experience in the design of new technology for the shellfish industry.

Phase 2: Field testing
Task 2: Assess the commercial value of the technology with commercial scale deployment of mussel seed. Field test different mesh sizes of the synthetic sleeving material to determine their relative effectiveness.

2. Engineering – design and construction

This phase of the project involved several visits to the Atlantic Systems Manufacturing (ASM) plant in Prince Edward Island by Andre Mallet and Claire Carver to develop the concept as well as a visit by the company engineer, Mike McKenna, to IPMF to evaluate the constraints imposed by the boat design. The first preliminary drawing of the mussel seed stripper was made by André Mallet following these initial meetings.

The first trial conducted by Mike McKenna at ASM showed that the mesh was too resistant to be cut by a sharp knife and that a rotating blade would likely be required to provide effective cutting. It was also concluded that the harvesting of the seed would require a hydraulic- powered conveyor to direct the continuous mussel sock into the stripping device where the socking material would be cut and the seed would be stripped away from the central rope.  The following set of plans were proposed and approved by Peter Darnell and André Mallet in November 2008.

On December 16 2008, the ASM facility was visited by Andre Mallet and Claire Carver to discuss the progress made on the fabrication of the two components of the harvesting system. Basically, the conveyor was mostly completed and the seed stripping device was in the process of being completed.

3. Field testing

3.1. May 7-10 2009

In the fall 2008, mussel seed was deployed in the protective synthetic mesh in anticipation of the trials which were to take place when the seed stripping technology became available. From January to mid-April 2009, the construction of the system was completed, delivered to Mahone Bay and installed on the IPMF harvesting barge. Various changes had to be made to the existing hydraulic system on the barge in order to integrate the mussel seed recovery technology.

3.2. May 27 2009

Another field trial was planned for the end of May 2009 in order to test another configuration for harvesting the continuous sleeves of mussel seed. In this instance, we tested the configuration used for the harvesting market-size mussels where the sleeve was brought onboard using the conveyor and directed into the main harvesting hopper. In this case the protective mesh was cut open on the conveyor belt by a staff member using a XACTO knife. In the final stage the central rope is reeled in by the hauler and the mussels are scraped off automatically by the rubber guards. Meanwhile another staff member pulled away the mussel mesh manually. In general, this configuration worked well but a few issues needed to be addressed. First, the mesh size of the conveyor belt was too coarse allowing the mussel seed to fall through as they were hauled from the water and the protective mesh was cut open.  Second, this configuration required that someone be installed on a high platform at the top of the conveyor to pull away the protective mesh from the rope as it entered the rubber guard.  It was concluded that in order for this configuration to work properly, the conveyor should have to be waist high with one person cutting open the protective mesh and the other person pulling the old mesh away as the central rope is reeled in.

3.3. June 27-29 2009

Given the degradation of the protective mesh as well as the clogging of the cutting wheel, an alternative approach was used at the end of June 2009 to accelerate the harvesting speed. Note that the water temperature had increased substantially and near 35-40% mortality was observed in the mussel seed remaining within the protective mesh. It will be important in the future to complete the seed harvesting by mid-May. In the alternative configuration a hauler was suspended over a XACTICs tank by means of the hydraulic arm. The hauler reeled in the central rope bringing the mussel sleeve on board where one worker cut open the mesh using a XACTO knife and another stripped the mussels off the rope and the protective mesh by hand. Despite the disintegrating mesh an acceptable harvest rate of 1500 L of seed per hour could be achieved, as long as the sleeves were heavily stocked with mussel seed. This became a research objective for the fall 2009 trials.

3.4. Mussel seed trials – Fall 2009

The trials in the fall 2009 were aimed at studying the effect of mussel seed stocking density on survival. Density was gauged according to the diameter of the mussel sleeve: 5 cm, 7 cm or 9 cm. While the mussels were being deployed, we marked sections of 5 loops for each density treatment (2 lines x 3 density treatments per line). This was carried out on October 28 and December 1 2009 with two different seed groups (New Brunswick seed and Prince Edward Island seed).. A sampling survey in February 2010 revealed that the seed survival was greater than 95% in all density treatments. We counted approximately 100 sea ducks in the area, half scoters and the other half Old Squaws. However, there was no indication that the ducks had damaged the mussels or the protective mesh, rather they appeared to be feeding on the older mussels. The smaller mussels deployed in October had tended to migrate through the mesh and attach on the outside where they become vulnerable to sea duck predation.

3.5. Mussel seed performance

In general mussel survival was excellent in both years even when the mussels accumulated in big bulges in the upper sections of the sleeves. Only in late June when water temperatures exceeded 10-15oC was there any evidence of significant mortality. Growth over the winter months was negligible in 2009 and results for 2010 look to be similar based on the February survey. It would appear that the protective mesh provides adequate conditions for seed survival but there is probably insufficient access to food resources to support significant growth. In order to provide access to better growing conditions, seed should be removed from the protective mesh and transferred to new sleeves as soon as the threat of sea duck predation has passed.

4. Discussion

Sea duck predation on seed mussels can cause significant financial losses to the revenue of a mussel farm. In many instances, Atlantic Canadian growers who suffer heavy seed losses over the winter can simply re-sock in the spring with easily-accessible seed sources. However, IPMF depends on outside seed sources to sustain their production in Mahone Bay, in particular seed with a high <<edulis>>, as opposed to <<trossulus>>, component which originates from the Gulf of St. Lawrence (northern NB and Prince Edward Island). In the past, leases with early ice coverage offered the necessary protection against sea duck predation, but the general absence of winter ice has substantially increased the risk of seed losses. Purchasing seed every year is very expensive for IPMF, especially in terms of the transport costs to deliver the seed to the site. The fall is typically the preferred time to obtain the seed as it is abundant, cheaper and cleaner than in the spring, and the temperature is ideal for deployment. For IPMF, the approach of securing the seed stock in the fall and protecting it against sea duck predation during the winter has become an important avenue to secure the long-term viability of their operation. This project has provided the means to both validate the concept of using protective synthetic mesh and evaluate various techniques for recovering that seed at a pilot-scale level.

The large-scale deployment of the mussel seed in the fall 2009 was efficiently executed and the survey conducted in February 2010 suggested that the outcome will be a success. The recovery of this seed and their re-deployment in biodegradable mesh will be carried out when the threat of sea duck predation has subsided, probably in early April 2010. We are proposing to provide an addendum to this report in terms of assessing the various stocking densities of seed deployed in the fall 2009, and the impact of seed density on survival and the economies of recovery. The total volume of seed deployed relative to the total volume of seed recovered will provide important statistics for the commercial operation.

Two components of the technology developed as part of this project were assessed in May-June 2009: the conveyor and the mussel seed stripper. The conveyor was designed to transfer continuous mussel sleeving with either seed or market-size mussels into the barge with minimal losses due to fall-off. This conveyor has proved to be extremely useful in the harvest of market-size mussels, in particular when the mussels are covered with tunicates. Clumps of mussels which were previously lost to the bottom as they were being hauled on board are now trapped by the flights on the conveyor. The reduction in the loss of market-size mussels during harvesting will certain translate into higher revenues and lower production costs for the company. In terms of recovering mussel seed from the continuous sleeving, conveyor technology is an important element in the design. However, in the current configuration, the conveyor is too high and the mesh openings of the belt are too wide thereby allowing mussel seed to fall through as they are transferred to the drop-off point. A workable concept for a mussel seed conveyor would be a lower structure (approx 1.5 m high) with a solid belt or one with very small openings. This would facilitate the smooth transition from the water into a XACTICs tank or a large seed storage bag.

The second component of the technology, the mussel stripper head, did not work as well as expected. There were two major reasons for this failure: the lower tensile strength of the mesh after being soaked in seawater for several months, and the clogging of the cutting wheel with the mesh. Until the properties of this protective mesh are better understood, it will likely be difficult to fully mechanize the recovery of the seed from the material. From a production perspective, it is estimated that a commercial harvest rate of 1500 L per hour can be achieved as long as the continuous rope is densely-packed with mussel seed and these seed show a high survival rate. In April 2010, the harvest will proceed initially using the basic protocol developed in June 2009. One person will be assigned to slit open the protective mesh with an XACTO knife, while a second person will pull away the mesh and removed any remaining seed. A hauler will be used to reel in the central rope and rubber scrapers will remove any remaining mussels. The recovery of seed per hour for the three seed stocking densities will be used to calculate the cost efficiency of this process as well as to evaluate the need to further mechanize this aspect of seed harvesting.

5. Conclusions

The conveyor developed in this project has proven to be very useful in the harvesting of market-size mussels, especially when the tunicate biomass is high. Previously, mussels would slip off the rope and be lost to the bottom as they were being loaded into the boat, whereas now they are retained by the flights on the conveyor. This design will likely be useful for other farms that rely on the continuous rope system for growing mussels. However, in its present configuration, the conveyor is too high and the mesh of the belt is too coarse to retain the mussel seed. A lower structure with a tighter mesh belt would be more effective for hauling the protective mesh sleeves into the barge for processing. The present configuration of the mussel stripper assembly also needs to be redesigned, particularly the cutting mechanism which tends to bind up the mesh. In order to develop the proper specifications for this sleeve processing system, the properties of the protective mesh under commercial harvesting conditions will need to be further evaluated. Indian Point Marine Farms is interested in the complete mechanization of the mussel seed recovery process and will be pursuing this development in future years. Future activities related to this project will involve the assessment of the recovery rates for the different stocking densities in mid-to-late April, followed by data analysis and preparation of the addendum report which will then be forwarded to the various funding agencies. We would expect this report to be completed by May 31 2010. Peter Darnell of Indian Point Marine Farms will present the results of this study to Scotia Pride 2011 using a PowerPoint Presentation prepared by Claire Carver of Carver Marine Consulting.