Is the bay clean enough to support commercial shellfish farming?

Yes, in active harvesting areas.

Shellfish Farming in Morro Bay

Indicator symbol: Very Good/Good

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Morro Bay is fortunate to be one of the few areas on the California coast with our own source of local oysters. The Morro Bay Oyster Company and Grassy Bar Oyster Company grow Pacific oysters in the bay’s waters for sale in markets and restaurants. There are approximately 73 acres available for oyster farming, which is only 3% of the bay’s total acreage. The aquaculture industry depends on clean water, and the California Department of Public Health (CDPH) is responsible for determining if the water quality in the bay is suitable for growing oysters. One of the measures they consider are the levels of bacteria in the water at the oyster farming sites.

Header photo courtesy of Grassy Bar Oyster Company.

Bacteria (Fecal Coliform) Status in Morro Bay Shellfish Growing Areas

Morro Bay Shellfish Growing Areas

The shaded areas on the map represent potential shellfish farming lease areas. The green sections of each lease area have clean water where shellfish harvesting is permitted. Shellfish harvesting is not permitted in the orange areas either because of poor water quality or a lack of data.

A Closer Look at Water Quality

The oyster farms in Morro Bay are required to conduct regular water quality testing to ensure the shellfish are not exposed to high concentrations of pathogens or biotoxins. Samples are collected monthly and after rainfall events, since runoff is known to bring bacteria and other pathogens with it when draining into the bay. The farms are required to close seasonally for most of the winter, when rainfall is typically the highest. After a storm has passed, the water quality samples must meet all regulatory requirements before the farms can reopen.

Each year, CDPH compiles the results from water quality sampling into an Annual Sanitary Survey Update Report for the shellfish growing areas. These reports are used to reevaluate pollution sources and management practices to ensure the oysters are safe to bring to market. In the most recent report, covering the 2021-2022 season, no biotoxin concentrations were reported above closure limits and seasonal closures have proven effective at keeping bacteria concentrations below exceedance thresholds. In addition, there were no bacteria concentration violations from the local wastewater treatment plants and there were no reports of illicit discharge from vessels docked in the bay.

Updates from the Farms

Pacific oysters arrive in Morro Bay from hatcheries. They startout at around 3 mm in size and are grown in tanks until they are large enough to be placed in mesh bags on the lease areas designated for shellfish farming. The bags must be flipped regularly to prevent the growth of fouling organisms like barnacles and to ensure that the oysters done grow together into clumps. Traditionally the mesh bags were flipped by hand, but both oyster companies have been working on converting their lease areas to tumble bags. These bags have floats attached to one end which rise and fall with the tides. This system is not only more efficient but also results in an oyster with a more desirable shape for easy shucking. Photo courtesy of Grassy Bar Oyster Company and Phil Sammet.

In November 2020, the Grassy Bar Oyster Company completed construction of a depuration facility approved by CDPH. This facility cleans incoming bay water using an array of pumps and filters, allowing oysters to be harvested during seasonal closures. Depuration systems are particularly effective at removing bacterial contaminants, ensuring the oysters remain safe to eat even if water quality in the estuary doesn’t meet regulatory standards.

The facility can hold thousands of oysters at a time, depending on their size. The oysters are kept in blue bins arranged like shelves, where water continuously passes through them, ensuring there are no “dead zones” of inadequate oxygen content. Morro Bay Oyster Company also operates a depuration facility, which was featured in the 2020 State of the Bay Report.

History of Oyster Farming in Morro Bay

The oyster industry in California began with the Gold Rush. This influx of easterners wanted familiar foods, including the eastern oyster. Native oysters, with their darker meat and strong coppery flavor, did not appeal to eastern palettes. Initially oysters were shipped to California by boat from Washington state. With the completion of the transcontinental railroad in 1869, fresh eastern oysters could reach the west coast in three weeks. Oyster seed was also sent west, where it was raised to harvestable size, often in San Francisco Bay.

In 1910, the industry began declining in San Francisco, likely due to increasing industrial pollution and changing economics. In 1930, the California Department of Fish and Wildlife introduced the Pacific oyster from Japan. Pacific oysters far out-competed both the native Olympia oyster and the eastern oyster due to their lower production costs and mortality rates.

This historical map from a California Department of Fish & Wildlife report shows the areas available for oyster farming and clamming in the 1930s. By the mid-1930s, 1,677 acres of the 2,300-acre bay were allotted for oyster farming, making it the top oyster-producing area in the state. Farmers scattered seed oysters from boats when the beds were underwater and harvested them by hand at low tide after about two years of growth.

When trade with Japan was cut off during World War II, Morro Bay obtained seed from Washington state and became the leading oyster producing area in California. In 1964, Morro Bay’s production peaked at nearly 250,000 pounds of oysters harvested during the year. At that time, oysters were sold shucked in jars. Today, consumers prefer their oysters whole in the shell. There are currently 75 acres available for oyster farming, which is only 3% of the bay’s acreage.

Monitoring Partnerships

Olympia Oysters: Morro Bay's Only Native Oyster

The Olympia oyster is the only oyster native to the Pacific coast of North America, including Morro Bay. They are a keystone species in the intertidal zone due to their ability to improve water quality and provide important habitat for other species. Oyster filter feeding reduces the risk of harmful algal blooms and improves water clarity, allowing more light to penetrate through the water, which benefits marine plants like eelgrass.

Olympia oysters are found in bays and estuaries from Southern Alaska to Baja California. Their lifespan is thought to be about ten years. They can live in waters that are up to 71 meters (233 feet) deep and within a temperature range of 6 to 20° Celsius (42.8 to 60° Fahrenheit).

Prior to the arrival of European settlers, the Olympia oyster was so abundant in Morro Bay that the Chumash people relied on them as a major food source. Unfortunately, Olympia oysters have been threatened throughout their range by habitat loss, disease, pollution, and overharvesting. The decline began with the California Gold Rush in the late 1840s and has continued to the present day. Olympia oysters and other shellfish are currently impacted by excess carbon emissions that lead to ocean acidification, which can inhibit shell formation.

Cal Poly researchers began a study of Morro Bay Olympia oysters in the summer of 2022 by counting oysters along fifty-meter transects in the intertidal zone during low tides. While a study conducted in 2009 found no Olympia oysters in Morro Bay, the recent Cal Poly survey revealed that they are making a comeback. Cal Poly researchers found 482 Olympia oysters and 393 Olympia oyster scars (spots where oysters had been previously) in the bay.

Just like mature oysters, oyster larvae need to attach to hard substrates like rocks, pilings, piers, or other shells. Cal Poly researchers placed larval recruitment tiles in various sections of the bay and monitored them periodically under a microscope to check for attachment. Five juvenile Olympia oysters were found on the tiles at Tidelands Park, the first recruitment to be observed in two years of monitoring.

These monitoring efforts are useful because they show us which areas are preferred habitat for Olympia oysters as well as when and where they reproduce. This data informs future oyster restoration and habitat protection projects in Morro Bay.

Harmful Algal Bloom Monitoring

The Estuary Program worked with Dr. Alexis Pasulka of Cal Poly to develop a harmful algal bloom monitoring project. Dr. Pasulka and her students track the abundance of over 200 phytoplankton species from two sites in Morro Bay. Some phytoplankton produce biotoxins that can be harmful to marine mammals or humans. Oysters and other shellfish consume phytoplankton as they filter water through their gills. If large numbers of toxic phytoplankton are present in the estuary, harmful levels of biotoxins may accumulate in shellfish tissue, making them dangerous for humans to eat. This project seeks to develop a better understanding of the driving factors behind these toxic blooms and develop ways to predict their occurrence. Pictured here are phytoplankton under the microscope.

Cal Poly students prepare microscope slides by settling a fixed amount of water in a small plastic cylinder called a Utermohl chamber, that sits atop the slide. The chamber is left to sit for 24 hours so that the plankton suspended in the water can settle onto the microscope slide below. Then the Utermohl chamber is removed and the plankton on the slide can be counted.

Data Notes

Lease areas displayed in the map, data from the 2021-2022 Annual Sanitary Survey Update Report, and regulations outlined in the 2022 Management Plan for Commerical Shellfishing in Morro Bay, California were provided by CDPH.

The bacteria data from 2017 through 2021 was analyzed for the geomean of the fecal coliform concentration with data provided by CDPH.

The historical map and information on farming is from a 1963 report by CDFW.

Is the bay clean enough to support commercial shellfish farming?