Unlike all other forms of marine aquaculture, commercially grown bivalve shellfish have been identified as the only sustainable form of aquaculture that has no negative impact on the environment.
As the demand for seafood continues to surpass supplies of wild-caught fish and shellfish, marine aquaculture is becoming recognized as the only serious solution for feeding a future global population of 9 billion. However, critics contend conventional aquaculture has numerous challenges from caging fish in farms that generate waste from feces and unconsumed commercial feed. These wastes can carry disease and the phosphate and nitrates in the mix can cause Harmful Algal Blooms (HABs) that suck oxygen from the water, leaving it uninhabitable. Conventional near-shore cages have become excessively dependent upon pesticides and antibiotics to combat diseases that are rampant in highly concentrated farming conditions—not unlike industrial-scale hog, poultry, and cattle farming on land.
Shellfish aquaculture operations actually improve water quality by filtering out pollutants, sediments, and nutrients from the water column. Furthermore, “open ocean” farming is a new paradigm challenging traditional shellfish cultivation in bays and estuaries. The swift currents and upwelling supply ample food to promote faster growth rates and suspended open ocean long-lines prevent predation and parasites that impact shellfish in calmer shore waters. Low salinity and heavy siltation following torrential rains cause mortalities for shellfish harvested from congested and contaminated bays and estuaries leading to quality problems.
Shellfish ecosystem services
Filter-feeding bivalve shellfish – oysters, mussels, clams and scallops – are successfully farmed across the globe as a sustainable food source while also enhancing the marine environment. Incredibly, an adult shellfish can filter up to 50 gallons of water a day removing suspended solids from surrounding waters, thereby increasing water clarity allowing sunlight to penetrate for enabling sea grass growth and the foundations of the marine food chain to flourish. Beds of bivalve shellfish provide ecosystem services by naturally filtering silt and also removing bacteria, viruses and nutrients from the water.
The Nature Conservancy released a report Shellfish Reefs at Risk stating: “Centuries of intensive fisheries extraction exacerbated by more recent coastal degradation have put oyster reefs near or past the point of functional extinction worldwide. Globally, 85 percent of reefs have been lost, making oyster reefs the most severely impacted marine habitat on the planet”.
Bivalve shellfish are no longer the global ecosystem engineers and enablers of prosperous habitats for other species. Enormous oyster beds in the United States’ largest estuary, the Chesapeake Bay, could once filter its entire volume of water (about 19 trillion gallons) in a week. Today, it would take the nearly decimated remaining bay oysters more than a year to perform such services.
Harmful algae blooms
Several decades ago relatively few countries appeared to be affected by HABs but now most coastal countries are threatened. The causes of this global expansion are debated with possible explanations ranging from natural mechanisms of species dispersal to a host of human-related phenomena such as pollution in the form of nitrogen and phosphorus. These blooms form massive blankets of slime on the water’s surface that precipitate bacteria growth, deplete oxygen, and kill much of the life in the water below.
HABs have been spreading and increasing along the coast of China in the past two decades, causing damage to the marine environment and posing a threat to human health. Chinese government and concerned scientists started to pay greater attention to HABs after the especially devastating bloom in the Bohai Sea, in August 1977. This event covered an area of 560 square kilometers and lasted 20 days, causing a mass mortality of fish and resulting in great losses to the local fishery. With increased awareness of the issue and economic development along the coast of China, HABs numbers have increased dramatically each year. It is clear that HABs have been increasing rapidly along the Chinese coast since the 1970s, and that they occur more frequently along the south coast than along the north coast. Thus, at least 322 documented HAB events have occurred from 1952-1998 in Mainland China.
A specific program for HAB monitoring along the Chinese coast, sponsored by the State Oceanic Administration, was initiated in 2001. At the present time, however, Mainland China, Hong Kong and Taiwan each utilize different systems for monitoring HABs. Since China has the largest mariculture operation in the world, future research efforts on the relationship between shellfish mariculture activity and HAB occurrences in key coastal areas, would foster and maintain the sustainable development of both shellfish mariculture operations and healthy marine ecosystems along the Chinese coast.
Scientists calculate that bacteria in sediment around bivalve beds biologically remove at least 20 percent of the nitrogen in wastes through the same process used in modern wastewater treatment plants. While most of the nutrients filtered from the water by shellfish are recycled back into the water column, the flux of undigested plant matter into the sediments stimulates bacterial processes known as denitrification. This process of turning fertilizer ammonia into nitrate and then into harmless nitrogen gas allows its escape into the atmosphere instead of stimulating phytoplankton blooms. These findings suggest that if large shellfish populations could be restored, these beds could play an important role in helping to achieve nitrogen reductions for mitigating the propagation of HABs across the globe.
Shellfish farms are delectable and nutritious nutrient sinks. Consider that a weekly harvest of 10,000 bivalves removes about 200 pounds of nitrogen per year; thus, a shellfish farm of about 1,000 acres would compensate for the nitrogenous wastes of about 50,000 coastal inhabitants.
Shellfish are a healthful addition to a balanced low-fat diet and are especially good sources of iron, zinc, copper, and vitamin B12. Furthermore, shellfish are much lower in saturated fat than pork or beef and provide high quality protein. Moreover, shellfish contain significant amounts of heart-healthy omega-3’s, which are undetectable in chicken, beef and pork. Although salmon are one of the richest sources of omega-3 fatty acids at 1.17 grams per 100-gram edible portion, shellfish are close behind with mussels at .84 and oysters .6 grams per portion.
From a “harvest” perspective, sustainable shellfish aquaculture would provide greater global food security and when farmed locally, mitigate the carbon transportation footprint. From a “habitat” perspective, shellfish farms would provide enormous and valuable benefits to the global environment.