Development of a Pressure-Based Technology for the Purpose of Controlling Biofoulers on Oyster Cages and Oysters

Table of contents

• Introduction
• Summary of results of previous phases
• Phase 1: Preliminary study to mechanize biofouler removal (spring/fall 2009)
• Growth
• Mortality
• Biofouling
• Other observations
• Phase 2: Design and construction of the prototype based on the recommendations of the preliminary study
• Phase 3: On-site prototype testing during the normal operating period and recommendations for its widespread use
• Modifications to the prototype
• Experimental design
• Results
• Effect of pressure washing on oysters
• Results of pressure washing
• Discussion and conclusion
• Acknowledgments

Introduction

King Aquaculture Inc. is an oyster aquaculture company specializing in the off-bottom culture of oysters using OysterGroTM technology. The company currently has an inventory of 4,000 OysterGroTM units containing 24,000 Vexar™ bags, which corresponds to an average production of eight million oysters. 

The conchological industry is facing a number of challenges, one being marine biofouling. The culture structures in the water column provide a surface that traps a number of marine organisms. Biofouling of the structures and the oysters they contain causes damage and economic problems because it accelerates material degradation and reduces oyster growth rate. Almost all of the rearing techniques employed today use structures that float on the surface of the water or are suspended in the water column. One method for addressing this problem is to use an OysterGro™ cage. A number of New Brunswick farms use OysterGroTM cages (www.oystergro.com) for their culture operations. An OysterGro™ unit consists of a vinyl-coated wire mesh cage that can hold six Vexar® bags on two levels. The cage is mounted on two rectangular floats that allow for two different positions in the water: the submerged position – where the Vexar® bags are maintained in the water at a depth of 6 to 12 inches from the surface; and the "flipped" position – where the unit is turned over to expose the Vexar® bags to the sun. In this position, the bags have no contact with the water, which allows the cages and oysters to dry out. This drying process is used to control biofouling (the accumulation of marine organisms on the oyster shells) as well as secondary spat.  

This method of controlling biofouling has been successful, but it has its limitations. As King Aquaculture Inc. has many cages, it must meet all these criteria for controlling biofouling successfully:

• flipping: All King Aquaculture Inc. cages are flipped over five to seven times a year to control biofouling, which is present all season long, depending on the type of biofouling.
  
  
• flipping depends on weather parameters: Conditions are optimal for flipping when the weather is hot, sunny and dry with weak winds (the wind creates little waves, which keep the oysters wet). The cages may be left out of the water for 24 to 52 hours each time they are flipped. This stage is risky.
  
  
• technique involves high labour costs: The technique requires two barges, each with a four-man team (total of eight men) to flip over the 4,000 cages, which costs a total of more than $25,000 every year.
  
  
• slower growth due to emersion: During emersion, oysters cannot feed. This may impact farm production and profitability, extending production time by a few months and affecting how long equipment is used, etc.
  
  
• reduced growth due to fringe breakage: The fringe that forms right around the valves when an oyster grows is fragile. When the cages are flipped over, the oysters are knocked against each other, resulting in fringe breakage.
  
  
• increased stress and mortality: In 2007, overexposure to the sun caused a loss of 11% compared to the usual loss of 5% to 6%.

Mr. Armand King has spent some time searching for a method to reduce labour costs, time and the risks related to cage flipping. A literature review has shown that Europeans have found pressure washing to control biofouling effectively. This project involved adapting this pressure washing technique for use with OysterGro™ cages.

Summary of results of previous phases

The project was divided into the three phases described below.

Phase 1: Preliminary study to mechanize biofouler removal(spring/fall 2009)

The objective of this phase was to optimize the following parameters:

• pressure required to clean the cages and the oysters without damaging them;
• water flow as a function of nozzle pressure and size;
• treatment distance (distance between the cage and the nozzles).

In summary, different pressures were tried with three different nozzles.

1. nozzle size #3.0 at 600 psi
2. nozzle size #3.0 at 800 psi
3. nozzle size #3.0 at 1000 psi
4. nozzle size #4.5 at 400 psi
5. nozzle size #4.5 at 600 psi
6. nozzle size #4.5 at 800 psi
7. nozzle size #4.5 at 1000 psi
8. nozzle size #7.0 at 200 psi
9. nozzle size #7.0 at 400 psi
10. flipping
11. not washed

All the cages were cleaned with a nozzle placed nine inches away from the cage. The cages were cleaned with a hand-operated pressure machine using each of the above parameters. Sampling was done five times during the season. 1- Just after the end of the natural mussel harvesting period (July 7, 2009), 2- one month after the first cleaning or just before the 2nd washing (August 4, 2009), 3- a second washing just after the natural oyster harvesting period (August 11, 2009), 4- one month after the second washing or just before the last washing (September 8, 2009) and 5- one last washing before over-wintering (September 15, 2009). At each sampling, a sample of oysters from each treatment was collected and brought to the laboratory to undergo the following observations: size, survival, weight, broken fringes and biofouling on the oysters (algae, mussels, oysters, barnacles, etc.). Age class T2 and T3 oysters were used for the tests.

Cages at the rest of the site were flipped over <FLIP> five times during the season.

1. June 30, 2009/46 hours
2. July 14, 2009/45 hours
3. August 6, 2009/25 hours
4. August 26, 2009/28 hours
5. September 10, 2009/30 hours, totalling 174 hours out of the water

The results of these efforts showed that:

We observed that pressure washing does not do much damage or cause high oyster mortality. A little care needs to be taken when choosing the nozzle and the pressure. 

Growth:

In these tests, three nozzle sizes and pressures were evaluated. We observed no significant difference with pressure changes, but a few differences were noted when nozzle size was changed. Nozzle #3.0 caused too much fringe damage and slowed down growth in the T2 group. No such observation was noted in the T3 group. Nozzle #3.0 produced a stronger pressure than the others, which might explain the fringe damage. Nozzles #4.5 and 7.0 worked well for both size groups. 

The <flip> method results showed that, unlike the T3 group, size of the T2 group was not affected by the treatment. One difference was noted in the T3 group, which had the worst growth rate during the study period. 

Mortality:

Mortality in both groups was low, but we observed higher mortality with the #4.5 nozzles at 600 and 800 psi in the T2 group. In the T3 group mortality was higher with the <flip> treatment than with any other treatment. 

Biofouling:

Biofouling was present even after cleaning and flipping (<flip>). In the T2 group flipping did a good job in removing barnacles and oyster spat on the oysters compared with the pressure washing treatment, and it was good at removing mussels in the T3 group.

Other observations:

The <no> treatment group did not have more biofouling on the shells than the other treatment groups. This can be explained by the accumulation of algae and because there was less suitable room for the creatures to become attached. Later on in the season no biofouling can become attached to the dirty shells. Nevertheless, this affected the growth of both size groups and especially the T2 group, which had the lowest growth rate.

Based on these results, here are our observations and recommendations:

• Pressure washing can wash oysters and cages without damaging the oysters or the culture structures.
• Nozzle #3.0 should not be used because the jet is too powerful.
• Nozzles #4.5 and 7.0 are the best choice but it should be taken into account that growth may be affected with nozzle #4.5 with a higher pressure. If #4.5 is used, we suggest a pressure of 400 or 600 psi.
• We have observed that the <flip> process seems to affect both growth rate and mortality in the T3 group. This could be caused by fringe breakage and the length of time out of the water without food. With the pressure washing machine, time out of the water is limited, and this method is also easier on the fringes. This method seems to be most advantageous for the T3 group.
• Aquaculturists could use the strategy of machine cleaning throughout the season and then <flip> the cages over at the end of the season to remove any remaining biofouling.
• This pressure washing machine would be useful on large farms to contain costs, which are normally higher with the <flip> method. Pressure washing with the machine would require one or two men compared to the seven or eight men and two barges required for the flipping method.
• This pressure washing machine would also eliminate the hard physical labour required to <flip> the cages over.
• While making these observations, we noted that flipping <flip> may affect the growth and survival of certain size groups of oysters. Smaller farms will still use the <flip> method, and even large farms will continue to use it as an alternative, so attempts should be made to flip the cages over more gently to avoid slowing growth down due to fringe breakage.

Other more technical observations to be taken into account when constructing the prototype:

• With nozzle #7.0 a lot of water is needed to reach the required pressure. A powerful system is required to reach an adequate pressure.
• All the equipment on the barge/platform must be water resistant.
• It must be possible to put the nozzles on and remove them easily.
• The top nozzles must be placed at an angle so that the spray reaches the oysters under the cage floats.
• There must be enough space to clean OysterGro™ cages with bird deterrent systems.
• The platform must be constructed to collect the algae washed out of the cages instead of emptying them back into the sea and to drain the water retained in the algae.
• If feasible, each nozzle should operate separately (valve to stop the water from flowing independently).

Phase 2: Design and construction of the prototype based on the recommendations of the preliminary study

Burke’s Custom Metal was hired to design and construct the prototype. Construction of the prototype started in December 2009 after the phase 1 test results became available. King Aquaculture Inc. personnel visited the construction site three times to document project progress and help with its development. At the first visit, King Aquaculture witnessed the start of prototype construction and, during the February 17 visit, the team brought a line of five cages to manually simulate the mounting and washing of the cages using the prototype. Among other things, they were thus able to specify the location of the nozzles, find the best position for the star wheels and affect a number of small modifications to make the prototype work more effectively. The prototype was delivered to Bedec on April 22, 2010.

Phase 3: On-site prototype testing during the normal operating period and recommendations for its widespread use

Modifications to the prototype

A few modifications had to be made to the prototype before it could be used in the ocean. These included the addition of a safety ramp, a transmission cooler, a filter and a silencer. The speed control was enlarged to make it stronger. A filter for the water entering the system was also added. 

Experimental design

The prototype was used to wash the cages during each of the periods described above (after the mussel harvest, after the oyster harvest and at the end of the season). The only factor in the case studied was the speed at which the cages moved along under the nozzles: 15, 25 and 45 seconds. These were the speeds used for the tests. #4.5 nozzles and a pressure of 400 to 500 psi were used for the washing. All the relevant observations were made during the washing, and the same samples were collected as in phase 1. Washing and sampling were done on July 14, July 28, August 3, September 8 and September 21, 2010.

Results

A nozzle problem was observed, especially at the end of the washing period given the greater biofouling of the cages. Some of the nozzles did not always work. It was difficult to observe at what point in time they stopped working because so much water was used when the prototype was operating. The nozzles stopped working rather frequently and so the cages were not adequately washed. Some places were not washed at all and so some oysters in the cage remained dirty. When the nozzles worked well, the cages were cleaned very well. The problem we observed was that the prototype nozzles had rotating heads. To produce a wide jet, the end of the nozzle had to turn with the pressure of the water. When dirt got into the system, the nozzle head stopped turning and the jet went straight ahead.

Effect of pressure washing on oysters                         

It should be noted that our findings regarding the effects on the oysters only reflect observations and are only suggestive since the prototype did not always work adequately.
Oyster growth with each treatment was measured over time to evaluate the effect of pressure washing with the prototype compared to the traditional flipping method and to cages that were never washed. We observed:

• That, regardless of size group (T2 or T3), oyster growth (fringe breakage) did not seem to be affected by the length of time the oysters spent under the nozzles using the prototype;
• The <no washing> treatment slowed growth down more than any of the other treatments in both size groups, which showed the need for flipping or washing.
• The results this year show the same growth rate with flipping (no significant difference) as with the other washing treatments done with the prototype.  

Results of pressure washing

As the nozzles did not always work adequately, it is difficult to compare this year’s and last year’s results. It was easy to observe whether the nozzles were rotating adequately when a hand-operated pressure washer was used. It was rare for it not to work. Some biofouling certainly remained on the oysters and cages this year due to the nozzle problems. However, there were fewer ascidians and barnacles this year than with the flipping method. Large numbers of mussels and oysters were found on the oysters at the end of the season.

Discussion and conclusion

The nozzle problems affected the final results of the tests in determining the effectiveness of the pressure washing prototype. However, we observed that the system did not have any negative impact on the oysters or the cages. It seems that, with a few modifications, the system could work. 

To use this prototype commercially and effectively, we propose changing the nozzle system to make it always work adequately. Nozzles are available on the market that do not have rotating ends but still produce a wide jet. It would also be important to be able to test the pressure obtained with a hand-operated device, using the prototype and after the nozzles have been modified to be set at a pressure that the oysters in the cage can tolerate during cleaning.

Other modifications will also have to be done to make the system easier to operate and more effective on-site, such as reducing the angle of the barge at which the cages fall into the water in front of and behind the prototype, changing the winch to make it easier for the crew to put the main rope in the <hauler>, adding a bow thruster and modifying the front of the table to deflect the water during navigation and reduce resistance. These modifications will enable the OysterGro™ cages to be washed more efficiently and faster, which will reduce costs.

Acknowledgments

This study was funded by the Aquaculture Innovation and Market Access Program (AIMAP), the National Research Centre, the New Brunswick Department of Agriculture, Aquaculture and Fisheries and Enterprise Kent. A big thank you to the entire King Aquaculture Inc. team as well as the CZRI mollusc research team: Mélanie Degrace, Josée Duguay, Roxanne Thériault and Mathieu Landry for their great work and cooperation.