Pilot test on pre-growout of common clams in suspended sandboxes - year 2009

Table of contents

• 1. Context
• 2. Methodology
• 3. Results
• 4. Conclusions and Recommendations
• 5. References

1. Context

The clams collected in the fall by spat collection are 8 to 10 mm on average. Small clams of this size have not yet reached their refuge size, often resulting in significant losses after seeding (B.F. Beal, University of Maine at Machias; J.K. Buttner, Salem State College, J. Grundstrom, Ipswich Bay Cultured Clam Cooperative; pers. comm.). The losses turned out to be much too great in our case (Chevarie et al. 2009). Among other things, losses occur due to passive displacement caused by strong winds. Predation (crabs and others) also plays an important role (Beal, 1993, Zaklan and Ydenberg, 1997, Beal and Vencile, 2001), but the extent is difficult to quantify. In order to increase the size of the clams before seeding, developing pre-growout techniques for clam spat has become a priority for the MIM program in recent years.

Experiments have been conducted using several pre-growout techniques: floating oyster bags, FLUPSY (Floating Upweller System) and sand baskets. For each of these techniques, work was performed on several variables such as density, period or others. However, the majority of the growth rates obtained using the various methods were quite variable and often insufficient (Chevarie et al, 2010, data not published). The recovery rates are also quite variable depending on the year of study and method used.

Pre-growout in sand baskets is the most recently tested method in the Havre-aux-Maisons Lagoon. It is also, without a doubt, the technique that has yielded the best growth rates so far. 

The local company “Élevage de myes PGS Noël Inc.” conducted initial testing with a variant of this technique several years ago. Bread baskets covered with plastic and filled with sand had yielded very good results (Leblanc, 2007).

The small sand baskets that showed very good potential in recent years were modified in the summer of 2008 to more closely resemble the system tested at Martha’s Vineyard (Massachusetts, USA Once again, the results were very encouraging This resulted in the motivation to test the sandboxes in use at Martha’s Vineyard in 2009. These boxes have been used commercially for some years already (M. David M. Grunden, Oak Bluffs; pers. comm.).

2. Methodology

Before the first tests, a trip was organized to Martha's Vineyard to observe the sandbox technique up close. This technique has been used successfully for several years, and the method has been mastered. Unfortunately, contingencies prevented our colleagues from immersing the bins on the scheduled date. Therefore, we were not able to conduct observations in the natural environment during our visit. However, we were able to observe the rafts up close and collect a lot of relevant information (Chevarie and Werstink, 2009). Similar structures were built on our return from the trip.   

Bins measuring 4 x 8 ft. (1.22 m x 2.44 m) were constructed of wood (2 x 6 in. plywood and sides), and an antifouling paint applied to the exterior of the bins. Styrofoam floats were used to provide flotation and stability to the bins. The bins were filled with sand and a density of almost 5,600 clams/m2, or a little over 7,100 clams in each half of the bin. A concrete block tied to the raft by a cord served as an anchor. Each bin had its own anchor and was secured on one end so that it could move with the direction of the current.

Three bins were put in the water in early July. Getting them in the water required a catamaran boat equipped with a hydraulic puller. The sand and clams were dispersed in the rafts using a hose for uniform dispersion. The task was rather strenuous and required several people to complete. The first launching occurred on July 8, 2009. 

After two weeks, protective nets were added to the bins to reduce potential losses from turnover. On July 21, divers placed 4-mm mesh nets over the bins. However, biofouling (primarily algae) quickly took over the nets, and they had to be removed just under a month later (August 17). The nets, completely covered in biofouling, might have slowed clam growth. Unfortunately, around August 25, a short time after the nets were removed, waves produced by the tail end of Hurricane Danny did some damage and the rafts were emptied of their entire contents (sand and test clams).

A second test was conducted in September after modifications were made to the bin system to reduce the effects of turnover. The bins were anchored using the same method used in Massachusetts, i.e. the bins were anchored two by two lengthwise, with anchors placed at both ends. Furthermore, rims were added to the edges of some of the bins in the hopes of reducing sand loss when eddies were present, while additional divisions were added to the inside of other bins. Two bins were also anchored at a greater depth in the hopes that the amplitude of turnover would be less.

3. Results

Unfortunately, during the first test in July, the tail end of Hurricanes Bill and Danny, which occurred one week apart right after the nets were removed, produced rather significant turnover, emptying the bins of the sand and clams. Therefore, almost two months after immersion, there was nothing left in the rafts. Therefore, it was not possible to obtain growth data with these tests since loss was 100%. It would have been relevant to conduct follow-up a few weeks later to obtain growth data. However, handling the heavy rafts (launching and removal) is a strenuous task that could negatively affect the growth and survival of individuals, given the high risk of sand loss. If other tests are conducted using this technique, a method will have to be found to periodically sample the clams submerged in the rafts anchored one metre under the surface.

After a number of technical modifications, a second test was conducted in early September. This time, only sand was placed in the bins in order to test the stability of the system in the water column. Although the performance of the bins was much improved, it was not enough to stand up to fall storms. By the end of October, the bins were again emptied of their contents. However, the information collected on the behaviour of the sandboxes in our specific climate conditions will help us to make other modifications to the technique next season. 

No data was obtained on the growth or survival of the pre-growout clams placed in the bins during the first pilot test conducted in 2009. The second test was conducted only to try to improve the technique.

Since it was not possible to go directly to the Martha's Vineyard culture site during our visit in the summer of 2009 (Chevarie and Werstink, 2009), it was difficult to realistically compare their site to our experimental site. Given that the Havre-aux-Maisons Lagoon is shallow and relatively well sheltered, we think that it would be sufficient for the sand rafts. After obtaining our very poor results, we checked a satellite image of the Martha’s Vineyard site. The image showed us that the site where the rafts have been set up in Massachusetts is a much smaller and much better sheltered body of water than our experimental site in the Havre-aux-Maisons Lagoon. Although data is available on the dominant currents and other hydrodynamic conditions of our lagoon, there was no comparison with the site of our neighbours to the south.

4. Conclusions and Recommendations

The tests have clearly demonstrated that the technique as tested in 2009 was not well suited to the severe weather conditions that occasionally prevail in the lagoons of the Magdalen Islands. A way will have to be found to reduce the effect of strong winds on the tossing of bins. Turnover caused a complete loss of sand and clams in 2009. The method for securing the bins in the body of water could be modified, for example by securing the bins to the bottom in shallow areas.

Selecting a better sheltered site is also a possible solution to be considered. However, a spacious enough site will have to be found to allow for the immersion of a sufficient number of bins for a commercial business. The environment used for the immersion of buckets at Martha’s Vineyard is a smaller body of water that is much better sheltered than the Havre-aux-Maisons Lagoon. It would be interesting to compare the hydrodynamic conditions of both sites (U.S. and here) to help select a new, more appropriate site.

The significant size of the sandboxes made handling quite strenuous. They also become very heavy once full of sand and clams. No doubt it would be possible to reduce their size by half, since the bins are divided in two already. They would be less cumbersome to carry and the risk of injury (primarily back injuries) would be significantly reduced. It will be important to take this into account in our future endeavours.

5. References

Beal, B.F. 1993. Effects of initial clam size and type of protective mesh netting on the survival and growth of hatchery-reared individuals of Mya arenaria in eastern Maine. J. Shellfi sh Res., 12: 138-139.

Beal, B.F. et K.W. Vencile. 2001. Short-term effects of commercial clam (Mya arenaria L.) and worm (Glycera dibranchiata Ehlers) harvesting on survival and growth of juveniles of the soft-shell clam. J. Shellfi sh Res., 20 : 1145-1157.

Chevarie L., B. Myrand, R. Tremblay. 2010. Prégrossissement des myes avant ensemencement. In Chevarie et Al. (eds). Programme de recherche-développement en myiculture aux Îles-de-la-Madeleine (Programme MIM-II) 2007-2009. Rapport en rédaction.

Chevarie L., B. Myrand, R. Tremblay. 2009. Suivis des ensemencements 2003 à 2004 et nouveaux ensemencements de 2005 et 2006 au site aquacole de la lagune du Havre aux Maisons. P29-43. In Chevarie et Al. (eds). Programme de recherche-développement en myiculture aux Îles-de-la-Madeleine (Programme MIM-II) 2005-2006. MAPAQ, DIT, Compte rendu no 36, 8 pp.

Chevarie, L. et G. Werstink. 2009. Techniques de préélevage en paniers de sable et autres techniques d’élevage de la mye commune. Mission exploratoire réalisée à Martha’s Vineyard, Rowley et Cat Cove Marine Laboratory (Massachussetts). 8 au 11 juin 2009. Rapport interne.

Leblanc, M-A. 2007. Rapport final des activités pour l’entreprise « Élevage de myes PGS Noël inc. » aux Îles-de-la-Madeleine de mai à octobre 2007. Rapport interne. 8 pp.

Zaklan, S.D., et R. Ydenberg. 1997. The body size-burial depth relationship in the infaunal clam Mya arenaria. J. Exp. Mar. Biol. Ecol., 215: 1-17.