Commercial Algae Production
Many hatcheries opt to grow their own algae to feed their shellfish larvae. Growing algae is labor intensive and can often be its own independent operation within the hatchery. The process is a full time job that takes technical skill and keen attention to detail. The benefit to hatcheries growing their own feed is that they can choose algal species that are high in nutrition, providing the best diet to support their shellfish and typically saving money in the long run.
Algae production starts with small scale cultures; test tubes, small flasks, and carboys (15-30L bottles). These will be used to inoculate large commercial scale bioreactors that hold 100L+ of volume. The bioreactors will be harvested for larval feed daily. Typically these are drained into buckets and carried throughout the hatchery to feed each tank. Hatchery technicians will constantly need to keep up with harvest by inoculating new bioreactors daily. Some will keep their bioreactors semi-continuous. This means that they harvest a majority of the volume and add new media (sterile water with nutrients) for remaining algae to grow back. It could take a week at minimum for the culture to come back up to a harvestable density.
What is a chemostat?
A chemostat is a type of bioreactor that can be programmed to automatically dispense algae and replace media based on either a predefined algal growth rate or a target cell density. Unlike traditional batch cultures where algae are grown in a bioreactor until they are harvested, a chemostat continuously harvests algae and simultaneously refills itself with media, making feeding more efficient with less labor. The algae culture will adapt to this constant harvest and dilution by growing faster, keeping the cell density constant. For example, in an 80L chemostat we can harvest 40L of algae per day and at the same time add in 40L of fresh media and the culture will not need any time to grow back to density. Once properly optimized (e.g. sufficient light and aeration), this process will allow us to continuously feed larvae with a constant and predetermined algal cell density.
We will be testing the efficiency and utility of an open-source, off-the-shelf bioreactor that uses simple and inexpensive turbidity sensors and peristaltic pumps. These electronics are controlled by a simple Arduino microcomputer which costs less than $100. We'll then evaluate this method for feeding our own experimental shellfish production. The goal is to create a product that can be implemented in any hatchery and will increase their efficiency for a low cost.
RECIRCULATING AQUACULTURE SYSTEM (RAS)
There are three traditional systems used in shellfish aquaculture: static, flow-through, and recirculating.
Static culture is when animals are held in a tank that is closed off. Every 48 hours or so, the tank will be drained down so that ammonia levels don't spike. The animals will be collected by a sieve as the water drains out. The tank will then be cleaned, refilled with new water, and restocked with the animals. Food will be added in batches once or twice a day. This is the most common system for larval rearing.
Flow-through culture is when seawater is constantly fed in and out of a tank (picture a hose filling one end and an open drain on the other). For larvae, a “banjo” sieve retains the animals in the tank, while a filter keeps the incoming water clean. Food will be added slowly and continuously. This method isn’t as common with larval culture but is the main method for growing juvenile shellfish in a nursery system. The animals in the nursery stage start at about the size of a grain of sand and can sit on a screen to stay in place within the tank. By this stage, they don’t require filtered water so the tanks are fed directly from the nearby water body.
A Recirculating Aquaculture System(RAS) is designed to have a constant and continuous input and output of water similar to the flow through system. Food will be added slowly and continuously while a "banjo" sieve keeps the larvae in the tank. However, instead of the output being drained, water is directed to a series of filters and treatments to be cleaned and reused. This is then looped back to the inflow.
The major benefit of reusing water is that it's efficient and you can control the environment in the system. During the summer of 2018, the Chesapeake Bay had a wet season that resulted in abnormally fresh water. Hatcheries had difficulty getting larvae to survive these conditions, resulting in reduced seed availability for oyster farmers. With a recirculating system, water can be held for extended periods of time, mitigating exposure to poor water quality from such events. It also allows more freedom to culture animals with temperature and salinity tolerances that differ from our local water.
RAS is most popular with fin fish growers. It is not as common of a practice for shellfish production. At SAIL, we will be testing the efficiency of shellfish aquaculture in our RAS for holding adults, conditioning broodstock, and rearing various species of larvae. Implementing these systems can increase control, efficiency, and survival rates for shellfish growers.
Determining optimal salinity acclimation periods of harvested oysters using recirculating wet storage systems
This project seeks to determine the optimal length of time needed to acclimate harvested oysters from ambient conditions in the Chesapeake Bay (10ppt) to a more market-favored salinity (30ppt) using a recirculating wet storage system. Oysters grown in lower salinity environments have a “sweet” or less salty flavor. The higher the salinity, the saltier the oyster tastes. Understanding how cultured oysters respond to rapid increases in salinity will not only improve the functioning of wet storage for shellfish but will allow growers to increase the market diversity of shellfish products in Chesapeake Bay in a more controlled, efficient manner.
In the summer of 2020, SAIL initiated small-scale oyster production to test the newly refurbished Recirculating Aquaculture System (RAS) and newly assembled nursery equipment. Overcoming initial challenges, the SAIL team achieved a successful spawn of approximately 20 million larvae. These larvae were produced to compare the survival of larvae grown in static culture, such as HPL’s oyster hatchery, with recirculating culture. Larvae that made it to their final stage of development were set in SAIL’s downweller to grow out or used in Assistant Professor Matt Gray's project exploring the use of oysters to enhance living shorelines for coastal resilience.
algal paste vs. live algae
Algal paste such as Reed Mariculture Shellfish Diet 1800® is commonly used in hatcheries to feed broodstock, seed, and larvae when live cultured algae isn’t available. These paste products consist of thick, concentrated, preserved dead algae that can be diluted and fed in lieu of live cultured algae. Though it is convenient and efficient when used in later life stages, little is known about how effectively it can sustain oysters through their sensitive larval stage.
In the summer of 2021, SAIL launched an experiment where growth and survival of oyster larvae fed entirely algal paste (Isochrysis Diet 1800®) was compared to the grpwth and survival of larvae fed entirely live Isochrysis cultured in the hatchery, and larvae fed a 1:1 mix of paste and live cultured algae. Based on the two trials completed, it was found that larvae fed live feed and the paste/live mix followed normal growth trends and both made it to metamorphosis after 20 days. The larvae fed solely algal paste were stunted at only a third of the size comparatively after 20 days and ultimately died off. These preliminary findings suggest that larvae in the hatchery can’t be effectively sustained on algal paste alone but that paste can be used to supplement a live algal diet, however more trials of this experiment must be completed before conclusions can be made.
Charted below are the average size measurements for the two trials of the experiment. “100% Live” (green lines) are larvae fed solely live Isochrysis, “50%/50%” (yellow lines) are larvae fed the 1:1 mix of live algae + algal paste, and “100% Iso” (blue line) are larvae fed solely Reed Mariculture Isochrysis Diet 1800® paste. An additional treatment “50% Live” (red line) was added to the second trial, where larvae were fed half of the “100% Live” ration to identify any significant differences between the full live fed and live + paste fed treatments.
Adult Oyster Feeding and Respiration Rates
Oysters are highly valued for their filtrating ability as they remove particles and nutrients from the water column. These particles and nutrients are either deposited into the sediment or incorporated into the oyster’s shell. This is especially useful for removing nutrients in the water that can cause algal blooms. As a result, filtration and metabolism rates are often used to determine the ecological impact of these organisms on the environment. Previously, these rates for oysters have been determined under laboratory conditions, meaning consistent water quality and single algae culture diets. However, these optimal filtration conditions are most likely overestimating the filtration rates that we would see for oysters in natural water conditions, which vary and incorporate many algal species, both good and bad.
This project aims to create a flow-through system incorporating natural water quality and seston fluctuations. This system is housed within PhytoChop, Horn Point’s new phytoplankton monitoring array, on the HPL Hatchery setting pier. This will allow SAIL to better understand how oyster feeding and respiration change with natural changes in water conditions, and as a result how oysters are impacting their environment. This project will be running throughout the 2022 field season, taking second-by-second measurements over 6+ months to create a long-term, detailed data set to better understand oysters in their natural environment.