Tracking the Ecosystem Interactions of Shellfish Aquaculture
To fill the growing gap between consumer demand and seafood production from traditional fisheries, aquaculture has expanded internationally to become the fastest-growing, food-producing sector in the world. As the demand for aquaculture products continues to grow, it is important that the industry be managed in an environmentally sustainable way.
For more than two decades, Fisheries and Oceans Canada research scientist Dr. Peter Cranford of the Bedford Institute of Oceanography has been studying ecological interactions at aquaculture sites in Canada and Europe. His research explores how aquaculture—particularly shellfish farming —interacts with coastal ecosystems and vice versa.
"The major issue we've been exploring is 'ecological carrying capacity'—how much aquaculture production a bay or other area can naturally support while maintaining wild species, communities and the ecosystem for future generations," says Cranford.
Assessing mussel colonies
Much of his research has focussed on the farming of mussels, which live in dense colonies and filter feed on phytoplankton and other suspended particulate matter in ocean water. Since just one mussel can filter up to four litres of water every hour, environmental concerns mainly relate to how cultured mussels interact with their food supply.
"Some bays contain millions of cultured mussels, and they can filter the entire volume of a bay every day," says Cranford.
The depletion of phytoplankton and other food resources in the water is only a concern if they are consumed faster than tidal exchange and primary production can replace them. In that case, the food available to the mussels will become limited and production for that site will be less than maximal. This is referred to as exceeding 'production carrying capacity.'
To study the interactions between aquaculture and coastal ecosystems, Cranford equipped a commercially available "light tow vehicle" called the Acrobat with electronic sensors. Towed behind a boat, the computer-controlled Acrobat undulates up and down through the water while continuously gathering extensive data on ocean properties that influence aquaculture.
"Coastal systems have a lot of natural variability, so it requires a lot of data to detect the potential effects of aquaculture within that natural variability," says Cranford.
Two key measurements for his research—the amount of chlorophyll in the water (an indication of the amount of phytoplankton present), and total particle concentration—provide information on the mussel food supply. Since phytoplankton is at the very foundation of the marine food web, it is important to determine whether levels are adequate to support both wild filter-feeder populations and the cultured mussels.
Acrobat also collects data on the physical properties of the water (salinity, temperature and depth), and other sensors are sometimes used. For example, an oxygen sensor can be added to assess whether mussels farming is affecting the amount of oxygen in the water. An optical plankton counter was attached in one study to examine aquaculture interactions with zooplankton.
"Acrobat has enabled us to rapidly assess how farmed mussels and other shellfish are interacting with their environment and whether an aquaculture operation is within the carrying capacity of the ecosystem," says Cranford. "There are so many issues to explore and so many ways we can use this tool. It's given us a capability that no one had tried before, and it keeps opening up new avenues of research."
Cranford first used the technology in Tracadie Bay, P.E.I., one of the first mussel farming sites in Canada; aquaculture leases now occupy half of the volume of the bay. Phytoplankton measurements were gathered throughout Tracadie Bay at different stages of the tidal cycle to document the daily inflow of food particles from the offshore and removal by mussels. The extensive geo-referenced data enabled Cranford to produce maps that clearly identify the processes that supply phytoplankton to this coastal ecosystem and the considerable effect of grazing by mussels.
"That was the first time anyone was able to measure the bay-scale effect of mussel culture and confirm the predictions of theoretical carrying capacity models," says Cranford. Subsequent research in other locations has enabled him to document how regional oceanographic conditions interact with mussel aquaculture. Scientists in other countries have asked Dr. Cranford to study their aquaculture systems using the Acrobat, so they can compare the data with their own research results.
"The carrying capacity of mussel culture is site specific and depends on a range of factors including the intensity of culture activities and how currents flow through a bay," says Cranford. "That means we need to study as many areas as possible to develop a general model of how aquaculture affects phytoplankton under a wide range of conditions."
In addition to the more than 10 sites he has studied in Canada, Cranford has also assessed aquaculture operations in Spain, the Netherlands, Denmark and Norway.
"Our research reveals that some areas might have exceeded the carrying capacity for growing mussels, causing a reduction in mussel production, and possibly bay-scale ecological changes," says Cranford.
Developing "carrying capacity" indicators
The data collected by Acrobat are being used to develop ecosystem models and indicators of ecological carrying capacity. Due to the size of their gills, mussels filter particles that range in size from three micrometres (microns) to a few hundred microns. They leave behind very small species (from 0.2 to 3 microns) known as picoplankton, which don’t typically dominate coastal waters. One research project in collaboration with the Institute of Marine Research in Bergen, Norway, tested the theory that picoplankton will be dominant in areas where mussels control phytoplankton biomass—a theory that proved to be correct.
Based on this knowledge, Dr. Cranford is determining picoplankton levels in coastal systems to develop an easily measurable index for routinely assessing the bay-scale impacts of mussel culture. This has led to research on the interactions between mussels and tunicates, which compete for the same food resources with one difference: some tunicate species can also graze on picoplankton.
Always looking to apply his research tools and expertise, Dr. Cranford recently collaborated with scientists in Canada and Denmark to document how efficient mussels can be at improving coastal water quality. Since shellfish are essentially biofilters, they can help to clean up eutrophic waters, which are systems clogged with excessive phytoplankton caused by too many nutrients in the water. They are also being used to capture and extract waste particles produced by fish aquaculture.
Knowledge in support of sustainability
"The ultimate goal of my research is to gain a broad perspective of environmental interactions with mussel culture and use the knowledge to provide advice on aquaculture regulatory issues as they arise," says Cranford. "The development of ecosystem indicators of particular relevance to mussel aquaculture, and the wealth of knowledge we've gained about the sustainability of the industry, will aid in the development of effective strategies to promote its sustainable growth and management in the future."
Dr. Cranford continues to lead and collaborate on numerous research projects under Fisheries and Oceans Canada's Program for Aquaculture Regulatory Research (PARR).
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