Procurement of soft-shell clam spat
Final report

Société de développement de l’industrie maricole (SODIM)
AIMAP-2009-Q08

SUMMARY

The sorting of soft-shell clam spat remains one of the main technological barriers limiting the profitability of the emerging soft-shell clam industry in Quebec. This project aims to equip the industry with an effective and affordable sorting concept that will significantly reduce operating costs. After conducting a critical analysis of the current manual sorting process and previous sorter design trials, the work team designed and manufactured a prototype, which was tested in the Magdalen Islands during the 2010 summer season. Although early tests have gone quite well (i.e. handling of the harvester in the field and adaptation of the Venturi system), they nonetheless highlighted several points that could be improved. However, with a construction cost of around $11,000, the industry could quickly recover this investment because of the considerably reduced labour needed to supply these spat growing operations.

BACKGROUND

For nearly 15 years, many efforts have been made to develop a profitable and competitive clam cultivation industry in Quebec. Given the potential of the Magdalen Islands area, the MIM program (Magdalen Island clam cultivation) that launched in 2000 contributed greatly to the understanding of this bivalve mollusc, thereby offering a solid foundation for setting up a clam cultivation operation in the archipelago. Gérald Noël, who began working in the industry in 1997, has since been operating a small clam cultivation business (Élevage de myes P.G.S. Noël) as a pilot project. Research and development (R&D) support is fundamental to the proper development of this business. One of the main difficulties lies in implementing certain experimental advances on a commercial scale.

For a mariculture business, having a supply of juveniles is an essential part of the production cycle. To ensure a constant supply of marketable resources, the business must see to it that new cohorts of individuals are seeded every year. There are basically two methods for ensuring this supply.

First, there is spat that comes from a hatchery. Although this technique may have its advantages, the production cost per individual may seem relatively high. The other alternative available to producers is collection in the natural environment. This technique allows for better control over production and management of the resource. It is indeed possible to seed clams according to their size, obtain a uniform harvest and, as a result, reduce sorting operations. Although there are various collection methods and, under certain conditions, each method produces excellent yields, they all have certain drawbacks that would make those techniques economically unviable on a commercial scale. At this time, P.G.S. Noël relies on the transfer of individuals rather than spat procurement to ensure its harvests. However, this strategy cannot be relied upon over the long term because, although it may help renew production to a certain extent, it does not necessarily help renew the resource. Although the company is clearly willing to adopt a technique for procuring spat, this change cannot take place without having the appropriate equipment to harvest or process the collected spat. This lack of technical equipment is therefore the essence of the problem.

HISTORY

In the past, Élevage de myes P.G.S. Noël has tested various collection methods to ensure an adequate supply of juvenile clams. For example, it has utilized pelagic collection, a technique that consists of an overlay of AstroTurf mats that are suspended in the water column and to which the clams cling while still in the larval phase.

Although the experiment proved very effective specifically in terms of collection, that technique did have one major drawback. Aside from clams, other species were also found in the collectors, particularly mussel spat. They therefore had to be sorted so that only the clam spat to be used for seeding was kept. The sorting was done manually, which turned out to be a long and laborious task for P.G.S. Noël. The analysis of pilot-scale production costs conducted by the Société de développement de l’industrie maricole (SODIM) revealed that an experienced sorter can only sort an average of 15 clams a minute. This step alone represents 65% of the labour required for collection-based procurement operations. Looking at all production costs combined, the manual sorting step represents 36.5% of those costs.

To resolve this issue, a conventional sorting system was initially considered, similar to what is done in mussel and scallop farming. The major difficulty was in the length-thickness ratios of the species to be sorted—clams and mussels in this case—because they are basically the same. No distinction could be made between the species based solely on their morphology; that avenue was therefore abandoned. In light of that situation, the various stakeholders agreed that the problem should be addressed through mechanization and automation with the support of design and engineering resources. So they began working on developing a mechanized and automated sorting system. Although size ranges for mussels and clams are practically identical, there is one very distinct physical variable that can be used to distinguish them: colour. This is where the idea of developing a machine equipped with an optical reader came from. Although, in principle, it seemed like this could result in a nearly perfect sorting process, this type of method involves a great deal of investment. The issue was then looked at from a behavioural perspective. It was agreed that a concept would be developed for separating the organisms based on their behaviour (settlement on substrate versus burrowing). Aside from colour, another factor that differentiates these two bivalve species is the substrate on which they are found in the natural environment. The clam, as an endobenthic mollusc, needs to burrow in the sand, while the mussel, an epibenthic species, tends to settle on anything found nearby. That is how the idea of a sorting tank came about. The idea was that if removable collectors were installed in a water-filled tank with the sand at the bottom, the species would separate naturally, with the clams burrowing and the mussels attaching themselves to the collectors. To accentuate these behaviors, a light source could be installed above the tank, since mussels are attracted to light and clams flee from it. Trials were therefore conducted, but the results were inconclusive.

In the meantime, however, the company decided to turn to another method to secure its supply of juveniles. Since spring 2008, the MIM-II program has been testing a new collection technique that shows great potential. Similar to the AstroTurf™ mats used for benthic collection, nets are used to collect clams when they were still at the larval stage. This technique operates on the principle that the clam, during its brief settling phase, seeks out substrates on which to settle. When it comes across this net, the clam clings to it and gradually slips through the mesh, becoming trapped but, at the same time, partially protected from predators and natural hazards. Initial results showed a much larger density of clams under these nets then on adjacent control sites. However, the numbers varied, with ten times more clams in 2008 and nearly three times more clams in 2009. Although the potential causes of these variances have yet to be documented, it is realistic to expect an average density of 1 500 individuals/m2.

PROTOTYPE CONSTRUCTION

This change in approach had a major impact on the project, leading it down a different avenue. At this point, it was no longer relevant to focus on sorting idea because the issue now revolved around developing a fast and effective harvesting strategy. Trials were therefore conducted with a clam harvester prototype to determine whether it could resolve the problem. However, because of how it operated, the machine turned out to be ineffective at harvesting clam spat.

Other trials were conducted with a device used by the MIM program team during sampling of experimental seedings and/or sampling under collection nets. That sampler is actually a pipe assembly of different sections that, when fed by a water pump, can be used to suction sand as well as the organisms that are in the sand.

The action by which this suction takes place is called the Venturi effect. The principle states that a fluid flowing through a system at a constant rate produces suction when it passes through a constricted section of the system. This suction is actually a pressure decrease in the pipe caused by an increase in the fluid's velocity.

The sampler introduced above turned out to be highly effective, and it no doubt would have been very useful for harvesting clam spat collected with nets. However, the need was to develop a device that could operate on a commercial scale, where the area to be covered is much greater than it is at an experimental level. The "T" configuration of the sampler would therefore prevent it from being used for continuous harvesting over very large distances. However, because the principle behind it was good, it inspired the design of a suction machine that was better suited to the targeted end-use.

The goal of building this prototype was to determine whether it would be possible to distribute the suction over five pipes in order to give the aspirator a longitudinal shape. Trials confirmed that the suction produced by the system would be sufficient to harvest the sediment at a depth of three inches, where the clam spat to be harvested is found. It was therefore possible to conceive of a harvester that would operate on this principle.

PRESENTATION

Similar to the aspirator that was used in the preliminary trials, the harvester's suction system is driven by a water pump with an output of approximately 400 GPM and that can withstand a pressure of up to 40 psi. It was determined that the ideal pressure at which the pump should operate was approximately 30 psi. That data was obtained during the trials conducted with the previously mentioned sampler. That system produced a pump pressure of 30 psi, and the suction produced by the sampler was excellent. Since the pressure is produced by a restriction in the pipes transporting the fluid, the same restriction was created in the aspirator as was in the sampler, to stay within the same pressure ranges. Looking inside the sampler, one can see that it consists of a two-inch water intake that is reduced down to a ¾-inch pipe, with an end that is slightly flattened. The surface area at the end of this pipe is 0.43 sq. in.

This important data gives a good idea of the area that the water outlets should have in order to produce suction. Since the suction is divided across five pipes, the total value of the sampler's restriction must be divided by five to determine the area of each water outlet.

With an aspirator length of 24 inches, the harvester was designed to harvest a transect of the same width. To use it, first the mouth of the aspirator is placed parallel to the ground. Then, using the lift, the bed on which the aspirator is mounted is lowered and dug down to the desired depth. Since the aspirator is mounted on pivots, its angle of incidence is then adjusted to approximately 60 degrees in relation to the ground.

The clam spat harvester, whose prototype is operational, will be used in the field more intensively over the course of the next year. Ongoing adjustments and improvements will be made so that it meets clam producers' expectations.

CONCLUSION

Although early tests have been quite good (i.e. handling of the harvester in the field and adaptation of the Venturi system), they have nonetheless highlighted several points that could be improved. Firstly, the suction system needs to be optimized. To do this, the pipes that the clams come out of could be shortened because their length greatly affects the degree of suction. Secondly, the length of the aspirator could be shortened because the system may be too big for the capacity of the pump being used. Lastly, a small electric engine could be installed on the harvester to minimize the pull force needed to move the machine. Nonetheless, with a construction cost of around $11,000, the industry could quickly recover this investment because of the considerably reduced labour needed to supply these spat growing operations.