Eel Lake Oyster Farm Anti-fouling Device

AIMAP Project - 2008/09

Final Report

Table of Contents

• Background
• Project Objectives
• Technology & Process
• Phase #1 - Technology
• Phase #2 – Process
• Results
• Conclusions
• Summary

Background:

Invasive species (e.g. mussel, tunicates, ship worms, etc), compete with oysters for nutrients and plankton found in the water column. In addition, mussel spawn in June and spat attach themselves to floating bags. During the winter months, when oysters are dormant, the mussel spat continue to feed and grow. This results in increased mortality rates for young oysters (they can not open in spring to feed) and decreased growth rates for adult oysters as they must compete for space and food with invasive populations that have grown over the winter months. If left unchecked, this can lead to a substantive reduction in overall production.

In the past, the method used to reduce invasive species on the Eel Lake sites has been to turn over the floating bags, by hand, to expose them to the sun. This was done to all 3000 bags on the site, twice during a 3 day period, every 2 weeks during the 4 months of summer – an extremely labour intensive exercise.

This project was aimed at designing, building and testing an anti-fouling device that could decrease the labour required (reduce the number of times floating bags turned by hand) to combat invasive species by increasing the mortality rate of mussels attached to the oyster floating bags.

Project Objectives:

To create a device and environmentally friendly process to clean oysters that is 1) more effective in reducing invasive species (mainly mussels); and 2) is less labour intensive.

Technology & Process:

Phase #1 - Technology:

The first phase of this project was the design and construction of the antifouling device by the staff at the Eel Lake Oyster Farm. The device was made up of two rolling conveyor arms, one that dipped into the water and hauled the floating bags up the side of the boat and into a stainless steel tank, and the other that returned the floating bags of oysters to the waters of Eel Lake. The 150 gallon tank was outfitted with a furnace and generator that heated water, through a set of pipes at the bottom of the tank, to a temperature of 60°C. 

The construction of this device was completed in June/08 and installation onto the boat completed in early July, in preparation for testing.

Modifications to the antifouling device were required during the testing phase when it was realized that samples needed to be immersed deeper into the water, and a dipping tool was made to accomplish this task.

Phase #2 – Process:

The second phase of the project saw the field testing of the device and process. The initial process was as follows:

• Field testing conducted in July (after mussels had spawned and spat has attached to the oysters / floating bags)
• Eel Lake water was used, pumped into the tank and heated to 60°C
• Antifouling device brought the floating bags from the water and immersed them in the tank’s heated water for 5 seconds to kill mussel spat (oyster shells are thicker and more durable, enabling them to withstand this temperature for short periods
•  Two sample types of oysters were tested:
• One year olds
• Adults (two and three year olds)
• Initial sample size of 10 bags (200 oysters/bag). Sample size was increased based on success rates
• Success was determined visually, based on an assessment that the process resulted in 90-95% mussel spat mortality on the oysters sample tested.

Results:

Following are the results of the field testing for effectiveness in reducing invasive species (mussels) and labour requirements.

During this test mussel spat had accumulated at the bottom of the tank (died and fallen off the oysters/floating bags) from the previous two tests, covering the pipes that heated the tank water. This was an ongoing challenge and while the spat was removed as it accumulated, water temperature would fluctuate between 56°C and 60°C. Labour costs associated with waiting for the water to heat was weighed against continuing to work but dipping only adult oysters (more durable shells that 1-yr olds) for a longer period (10 seconds). The latter was chosen as more cost effective, if it proved to be successful in killing mussel spat without killing the oyster. This was the case after a test of 10 bags of adult oyster (see test #4).

Based on the results from the successful tests (#1, 2, 4 & 5), and using the modifications to the antifouling device and process (5 or 10 sec immersion for one year olds / adults respectively), this antifouling method was applied to the Farm’s entire oyster stock during the reminder of July and August.

2. Labour reductions: When comparing labour used for the old method and the new method during the same time period, a reduction of 16 hours (6.25%) was realized using the new technology and process.

Old process for reducing invasive species:

• 3000 floating bags turned by hand for exposure to sun 8 times (both sides)
• Turned twice (in 3 days), every 2 weeks over the 4 months of summer (June-Sept)
• Labour = (2 staff x 8 hours) x 2 times [both sides] x 8 sun exposures [2 times/month x  4 months] = 256 hrs (32 days)
• This method does not kill mussels located in the core (centre) of the floating bag

New process and equipment:

• 3000 floating bags raised and immersed by equipment in heated water at site for 5-10 seconds and returned to water plus 7 exposures to sun (both sides)
• Completed once in July (after mussels have spawned and attached to oysters)
• Labour = Antifouling dipping process (mussels) requires 2 staff x 8 hours  +  sun exposures (other invasives) requires 2 staff x 8 hrs x 2 times x 7 exposures = 240 hrs (30 days)
• Bath method also kills mussels located in the bags core

Conclusions:

In summarizing the results of this project the following conclusions are made:

• By using the new “dipping process” for the anti-fouling of mussel spat, and based on visual observations, up 90-95% of mussel spat were killed, including mussels at the core of the floating bags, increasing the effectiveness (mortality rate) of mussel spat and therefore invasive species overall.

• Reductions in the mortality of other invasive species using the “dipping process” were inconclusive during field testing. As a result, continuation of the “sun exposure method” was required during the summer months to ensure other invasive species were controlled. Ongoing use of the “dipping process” may prove to be somewhat effective in killing other invasives, but this was beyond the timeframe of this project.

• An application of the “dipping process” and it’s technology for antifouling mussels used 50% less labour then an application of the “sun exposure method” (no requirement to turn bags), reducing overall labour by a minimum of 6.25% (2 days). Additional labour reductions may be realized in the future if this method also proves to increase the mortality rates of other invasive species, as the “sun exposure method” may not be needed as often.

Summary:

Based on the results of this project, and its success in reducing mussel spat populations on oyster aquaculture sites, this process and its technology will continue to be used at the Eel Lake Oyster Farms operations.

Information on this project and its results will be shared with the industry and plans are being made to post the project and its results on the Eel Lake Oyster Farm website by the fall, 2009.