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An Innovative Approach to Nursery Technology for the Production of Large Geoduck Clam Seed: The Missing Link for Geoduck Aquaculture in BC

Final Report
Nova Harvest Ltd
AIMAP 2012-P09

Executive Summary

Geoduck seed for British Columbia's (BC) aquaculture industry have been available during limited seasonal periods, on a semi continuous basis with 5-8 mm shell lengths (SL) and these seed experience high mortality relative to their cost. The goal of this project was to address the industries lack of supply of geoduck seed and create a product with year round out-planting opportunities.

The primary objective of the project was to develop innovative technology capable of supplying grower's high quality geoduck seed with a shell length of  ≥ 30 mm SL. The longer-term objective of the project is to supply consistent geoduck seed to farmers and tenure owners. The high quality seed we produced was grown using a land based nursery and a suspended, off-bottom ocean based nursery at Nova Harvest Ltd in Bamfield, BC. We are currently incorporating the innovative land based nursery results into operations at the Nova Harvest Ltd. hatchery.

Due to significant loss during larval rearing, we completed nursery trials with lower seed numbers than originally planned. Regardless, the land based nursery seed experienced 13% higher survivorship, a maximum SL growth rate of 64 µm/day (December 4, 2012 to May 4, 2013), and produced final SL's of 18.1-18.3 mm compared to the ocean nursery seed SL growth rate of 48 µm/day and final SL's of 15.9 mm. Seed grown in the land nursery did have higher survivorship, better overall growth rates and longer SL's in than seed grown in the ocean nursery. Given the promising initial results for the land based nursery, Nova Harvest Ltd. will continue testing this technology with a goal of optimizing growth rates and out planted seed survival.

Technology developed with the Aquaculture Innovation and Market Access Program (AIMAP) has directly increased productivity and the focus of our geoduck seeding supply system at Nova Harvest Ltd in comparison to the existing operations. This project also diversified business opportunities for the company at multiple levels of the geoduck aquaculture industry.

Furthermore, this project fits AIMAP priorities of environmental (re-use of algal feed), social (interaction with geoduck aquaculture industry and Bamfield Marine Science Centre) and economic (creating employment opportunity in rural communities) sustainability.

General Introduction

Geoduck clams (Panope abrupta ) are an emerging, high value aquaculture species in British Columbia, having significant potential for the stimulation of rural coastal economies. The industry has been in the emerging stage for over a decade due to a number of issues, one being the inconsistent supply of seed stock. Geoduck seed is available on a semi continuous basis of 5-8 mm shell length (SL) which experience high mortality relative to their cost and are limited to a narrow planting season. Given the high investment required for the purchase and planting of geoduck seed, growers are taking on a high risk planting small seed at current market price. To establish a level of confidence in growers that will support investment and growth in the geoduck aquaculture industry, Nova Harvest Ltd, has developed a land based nursery for the production of quality geoduck seed produced in a system capable of matching out planting conditions, pre-planting thermal conditioning as well as predator avoidance training.

The purpose of a shellfish nursery is to grow hatchery seed to an appropriate size for planting or grow-out. There are two main types of shellfish nurseries, primary for the setting and metamorphose stage of larvae and secondary nurseries for the later grow-out once the shell length has reached at least 1 mm. This project focused on innovations targeting the secondary style nursery system for the efficient growth and high survival of geoduck seed over the winter to a target size of ≥ 30 mm SL.

The unique burrowing behaviour and morphology of the geoduck clam prevents the application of existing shellfish nursery technology for the continued grow out of seed. It is important that a geoduck clam be put on sand early in seed production cycle to ensure proper development, health and later digging ability when out planted on a tenure. Previous attempts to incorporate sand trays into the Floating Upweller System (FLUPSY) design were met with obstacles such as high initial investment cost, bio-fouling and the low stocking density prevented commercial adoptions of this technique. Another nursery design referred to as “bag-on-bottom” consists of a mesh bag with a sand bottom layer, which is stocked with seed and placed on the sea floor. Shortcomings of this design are the high cost of dive time required for maintenance, exposure to bottom predators, the highly variable survival and relatively low numbers each system is capable of holding. To improve on previous nursery designs, one innovative test we performed utilized stackable sand tanks for high density rearing. This design enables us to overcome stocking density issues in the nursery and economies of scale using an increase in production capabilities.

The land based nursery consists of 39 sand tanks with 30 m2 of sand substrate capable of rearing ~500,000 juvenile geoduck seed to 30 mm shell length. Stocked sand tanks are fed microalgae produced in two greenhouse buildings that hold 40 x 1500 L tanks. These algae tanks are originally inoculated from 2 x 1000 L photo bioreactors and multiple 450 L bag cultures. Alternatively, microalgae is bloomed from raw seawater and nursery effluent water containing uneaten microalgae and metabolic waste which acts as a nutrient. One of the greatest challenges of operating a nursery is supplying sufficient quantities of algae to support proper allometric growth rates. The operational efficiency of the land based nursery was greatly improved by re-blooming recycled algae feed using a water reuse loop.

An ocean based floating nursery was also explored to contrast the advantages and economic efficiency of two different nursery systems. Advantages of a floating nursery are the availability of natural algae blooms, reduced predation due to seed stock being suspended in the water column, as well as, reduced overhead and operating costs. Our floating nursery comprises modified stackable oyster trays stocked with sand and hung from a raft. We discuss the proposed innovative design of adding sand to the oyster trays and hanging a fibreglass tank to contain the oyster trays for feeding purposes.

Our land and ocean based nursery incorporated existing nursery technology with innovative adaptations to improve the operational efficiency of the nursery design allowing for the cost effective production of large geoduck seed. 

AIMAP priorities accomplished by the completion of this project include:

Land Based Nursery

Introduction

The advantage of a land-based nursery is control over environmental conditions, food availability, accessibility and reduced predators. The nursery consisted of multiple tanks located in a greenhouse style building linked in parallel to outdoor algae tanks where waste products and uneaten algae are mixed with fresh seawater to be re-bloomed. The algae culture tanks operated as a bio-filter for the system by converting nitrogenous waste into algae feed and allowing water and uneaten algae to pass through the system multiple times before discharge. The waste generated by the clams from the conversion of algae into biomass is reused for the re-growth of algae with the addition of sunlight. The re-use loop will also aid in the capture of radiant energy from the sun maintaining a higher than ambient temperature providing better conditions for growth. The re-use of water and heat within the system will improve the operational efficiency through higher growth rates, efficient feed use as well as reducing the energy requirements. The goal of using a land based nursery is to optimize the system for high growth and survival, taking full advantage of the control the land based system provides.

Methodology

We modified indoor nursery tanks to create a semi-recirculation system. Round tanks with a sloping bottom (total number of tanks = 39) were fitted with a removable standpipe that retained sand at a 10 cm depth in each tank. Drain holes (3; 2 mm diameter) in the bottom of the standpipe allowed the entire tank to drain, thus delivering oxygenated water into the sand substrate to help maintain aerobic conditions. We inserted a second standpipe into the top of the sand retaining standpipe, which held a water depth at 15 cm above the sand. Tanks were stacked in 3 discrete rows of 13 columns with water flowing via gravity through each tank starting from the top tank. Each sand tank column is plumbed into a single 3” drainpipe which terminates at a 600L sump thank fitted with two 2 Hp pumps. A portion of the sump tank water was re-circulated back through the nursery tanks, while the rest of the sump water was sent to the greenhouse and re-bloomed or directed to wastewater. Water flowed into the green house and distributed amongst 40 rectangular static tanks each having a 1500 L capacity. These static water tanks functioned to enhance algae growth and were re-feed back into the nursery as food for juveniles geoduck. The algae re-use loop was a key component to the operating efficiency of the land based nursery.

Due to significant loss during the larval rearing, trials were completed with lower seed numbers than originally planned. The land nursery was tested with an initial stocking rate of 50,000 seed into 4 tanks at a density of 1.6 seed/cm2. Given the low stocking density in the nursery tanks, an excess capacity existed for feed production. When running at full capacity the nursery will require anywhere from 3-8 lpm of algae with a minimum cell density of 1.5 million cell/ml. Algae used as feed in the nursery is replaced in the greenhouse at the same rate with nursery overflow water which contains metabolites, uneaten food and unsettled faeces. The greenhouse and algae re-blooming protocols are discussed in the algae feed production section below.

For ongoing maintenance a 100% (5000L) water change was completed once/day to prevent sediment collecting in the water lines. Raw seawater was supplied to the nursery tanks at a rate of 15 lpm. Algae supplied to the nursery originated in one of two greenhouses with a holding capacity of 60,000 L in 40 static rectangular tanks and capable of producing up to 12000L/day of algae feed. When possible, we integrated additional algae produced by two photo bioreactors to maintain a circulating density of 80,000 cells/ml.

A control subset of geoduck seed at 8.7 mm shell length was stocked into a modified oyster tray (60 cm x 60 cm x 25 cm) containing 15 cm of sand during this same time period. We held the control seed at ambient water temperatures in flow through raw seawater for the duration of the land and ocean nursery testing trial. Data on growth and environmental parameters was collected each month from December 2012 to May 2013 with survival data being collected in May 2013.

Results

Land nursery P. generosa had larger shells, greater mass, higher survivorship, better burrowing abilities and normal length-weight relationships compared to control seed. Shell lengths in the land nursery grew to an averaged length of 18.3 mm compared to the control shell length of 11.2 mm. Total wet weight increased to 3.1 g compared to a final averaged weight of 0.42 g in the control juveniles. Furthermore, seed survivorship was 53 % greater in the land nursery compared to controls, as well as approximately 71 % of land nursery seed burrowed to a depth of 7-9 cm while controls were found at depths of 3-7 cm. Finally, seed grown in our land nursery had length-weight ratios comparable to other bivalves living in their natural habitat. In general, our land based nursery promoted growth, low mortality and behaviours (digging depth) that likely improve juvenile survivorship during out-planting stages.

Ocean Based Nursery

Methodology

We modified oyster trays to stock juvenile P. generosa in sand substrate. Sand was shifted (2 mm mesh) and added to oyster trays lined with 70 % shade cloth to a depth of 15cm. Before stocking seed was measured for total length and weight. Two days were allowed for acclimation and burrowing before ocean deployment. A single control nursery tray stocked with seed remained in the hatchery and was fed by raw seawater at the ambient temperature.

Ocean deployment of the stocked nursery trays occurred on December 5, 2012. We secured a raft in 10 meters of water on the south-west side of Tzardus Island (Latitude: 48.902288; Longitude: -125.083133). The nursery trays hung from the raft at a depth of 2 meters and remained as an open system to the environment. We determined average shell length and overall weight from monthly subsamples and collected the ocean trays on May 4, 2013 to calculate overall growth and survivorship.

Initially we planned to hang a feeding tank from the ocean nursery that was to be drawn to the surface to enclose the hanging trays, thus allowing for the addition of feed periodically throughout the growing cycle. Initial testing of tanks and bag style containment for the ocean nursery was unfruitful. The greatest challenge with the ocean nursery tank enclosure was the destabilizing effect of having a large tank directly attached to the oyster style raft which also carried the weight of the sand trays. A potential solution to solve this issue is selecting a site that would allow the feeding tank to settle on the seafloor while remaining attached to the floating structure- this was not possible at our location. Resting the feeding tank on the seafloor would correct the anchoring effect caused by the submerged tank when the floating structure is raised during swell events. Alternatively, increased flotation devices or a permanent structure would enable us to contain and supplement feed during winter months to an ocean based nursery.

Significant loss during the larvae rearing cycle resulted in a shortage of seed available for the testing of the ocean nursery. Given the limited number of seed being tested, expense of transporting feed to the ocean nursery and design problems with hanging the submerged tank, we decided to focus our efforts on the land based nursery justified by immediate promising results.

Results

Ocean nursery P. generosa had larger shells, greater mass, higher survivorship, better burrowing abilities and normal length-weight ratios compared to control seed in the hatchery sea-tank. Ocean conditions at the ocean nursery site remained within expected ranges throughout the testing period. Shell lengths grew to 15.9 mm compared to final control shells of 11.2 mm and similarly, weights increased to 1.3 g compared to final control weights of 0.42 g. Ocean seed survivorship was approximately 40 % higher and on average they burrowed to a depth of 8-11 cm compared control seed found at shallower depths of 3-7 cm. Similarly to land nursery seed, juveniles grown in the ocean nursery had length-weight relationships comparable to other bivalves living in their natural habitat. Overall, the ocean based nursery produced normal growth, high survivorship but issues surrounding scale-up to commercial quantities require more attention.

Going Forward

Additional food supplements would increase the growth rate of juveniles over winter in our ocean tray system, however, the effort and cost to produce microalgae and contain the food within the hanging trays would be too great to endure for these trials. For example, the floatation devices needed to keep stackable sand filled trays and additional feed tanks buoyant was not manageable. Furthermore, additional algae feed would need to be transported weekly to the floating nursery site which was unreasonable due to ongoing hatchery production .
Incredibly, biofouling was minimal and only small clumps of macro-algae were attached to the outside of the trays. The winter deployment and tray depth are likely contributing factors to the low biofouling. We deployed the ocean trays on December 5, 2012, thus most biofouling organisms would have already settled out of the water column and on to their respective substrates. As well, the minimal biofouling could also be the outcome of our fortuitously set tray depth of 2 m.

Conclusions

The ocean nursery produced high quality juvenile geoducks seed with a minimal amount of inputs. Regardless of the limited food supply overwinter, clams grown in the field were larger, and better adapted for eventual out-planting in marine tenures. Although higher growth rates would likely occur from April to late September, the decreased plankton densities at the time of nursery stocking minimized problematic settlement of biofouling organisms on trays. Future efforts will focus on improving the scalability of the ocean nursery as it is a promising tool for overwintering seed with minimal loss to predations as well as adding growth to seed if stocked during natural bloom periods.

Algae feed production

Methods

The algae diet required for the land nursery was grown during winter conditions in 40 x 1400 L tanks in two greenhouses located onsite. Algal species Tetraselmis suecica, and Skeletonema costatum were grown from stock cultures to approximately 1 to 1.5 million cells/mL. All tanks were contained in an outdoor greenhouse under ambient conditions (lighting and temperature). To inoculate starter tanks approximately 25 -50L of each species was transferred into 1400 L of raw sea water. This process was completed for five tanks per species, once every four to seven days depending on time required to achieve harvestable densities. In an attempt to select for local algae species (diatoms) raw seawater was fertilized with f/2 + silicate and aerated until a stable culture developed. The most predominant algae that grew from the raw seawater trials was a Skeletonema like diatom. 

Algal tanks were fed appropriate F/2 and silicate amounts weekly and monitored for algal growth. We did not actively regulate pH because no artificial lights were used, thus allowing the cultures to under go natural light and dark dependent pH fluctuations. 

Algae Bioreactors

As of 2013, a new technology has become available for the production of microalgae by way of an automated bioreactor designed and built by a BC based company in Victoria; Industrial Plankton. Nova Harvest Ltd. has leased the first two production units available to greatly improving algae production capacity. Each unit is capable of producing  ≥300 litres of high density microalgae feed per day. Once inoculated scale-up, cell density management and harvesting is fully automated using customizable set points. The reactors were predominantly utilized for the production of Tetraselmis sp. and Isochrysis sp. (T-Iso) microalgae which was fed directly to the land nursery on a continuous basis. Harvested algae from the reactors were also used as inoculum for open culture algae tanks in the greenhouse described above.

Results and Going Forward

Cultures of Tetraselmis sp. and Skeletonema costatum grew to harvestable concentrations within 7-14 days under natural winter conditions. Approximately 1400 L/tank (5 tanks/species) of each species was grown to over 1,500,000 cells/mL, while being supplemented with F/2 and silicate feed during initial inoculation. The wide rage in time require to produce a harvestable density of algae was largely dependent on reduced evening temperatures effectively delaying the growth cycle. In the future, methods for daytime heat capture to store heat that would be transferred to algae tanks during the evening period will be explored. Increased light concentration to the culture tanks would improve growth rates and harvest yields but our experienced this far is an acceptable photoperiod exists for diatom production provided temperatures can be maintained within a range of tolerance.

Algae bio reactors appear to be a cost effective means of producing high volumes of dense microalgae for direct feeding as well as inoculum of larger culture volumes. The full scale automation allowed for us to add two units without additional labour requirements, effectively increasing our daily algae production volume by 600 L at 6 million cells/ml. The consistent density of harvested cells from the reactors will allow for continued automation of downstream hatchery processes not easily accomplished by the conventional procedure for algae production. The most significant advantage enabled by Industrial Planktons bioreactors is the labour saving automation built into each step of the production cycle post inoculation. As algae production is one of the most significant challenges in both quality and quantity for a shellfish hatchery, the bioreactors developed by Industrial Plankton have the capability to improve the operational efficiency of hatcheries as this technology is adopted by the industry.

General Conclusion

Due to significant loss during larval rearing, the land and ocean based nursery trials were stocked with 50,000 and 5,000 seed, respectively, which were lower numbers than originally planned. As a consequence, we completed trials at 10% of the total seed capacity for the land based nursery and less effort was delegated to the ocean nursery. Despite these set backs, the land based nursery seed experienced a 53 % higher survivorship, compared to control seed, a maximum SL growth rate of 64 µm/day and produced final SL's of 18.1-18.3 mm . Incredibly, the ocean nursery seed SL growth averaged 48 µm/day with final SL's of 15.9 mm and had higher survivorships (40%) than control seed. Nonetheless, the seed grown in the land nursery did have better overall growth rates and longer SL's than seed grown in the ocean nursery. 

The growth rate observed in the land based nursery is by no means and optimum and can be greatly improved. Evaluation of, and modifications to the land nurseries re-use system is currently in progress as we prepare for the 2014 geoduck seeding season. Our micro-algae growth re-use system, the use of the highly efficient photo bioreactors, seed stocking densities and seed rearing temperatures will also be evaluated to maximize seed growth rates in the land nursery.

These promising results from the land based nursery will potentially benefit producers of large shellfish seed such as horseclams, geoduck or softshell clam on the east coast. Farmers will experience higher clam survivorship with the production of consistently larger seed during peak outplanting periods. Improving seed survivorship will help to establish the BC geoduck aquaculture industry at the same time as increasing potential returns on the capital intensive cost of establishing geoduck aquaculture operations. Therefore, results and continued improvement of the technology will increase the value of BC's shellfish industry and its ability to compete internationally, and at the same time on a local level aid in stimulating an industry that will provide employment and investment opportunities in BC's rural communities.

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