Oysters are called bivalves, meaning they have 2 shells that open and close and feed by filtering the plankton they require through their gills from the water around them. Oysters pump through a large amount of water to gain the necessary food for growth. Oysters are not fed any supplementary diet.
By filtering the water the oysters help maintain a healthy marine environment by reducing the nutrient load in waterways. One oyster can filter up to 40 liters of water per day. In the Chesapeake Bay, one of the largest inland saltwater waterways in the world a program is underway to repopulate the bay from hatchery-produced oysters. This has been considered necessary to counter algae blooms that threaten other fish species in the bay by turning the waters acidic.
Deepwater shellfish farms with long lines and cages can replicate a reef habitat and can act as Fish Aggregation Devices where larger fish feed on smaller shrimp-like animals.
Oysters are currently rated as the Number 1 sustainable choice seafood by the UK-based Marine Conservation Society in their publication titled Fish online.
Oysters are also rated as a Better Choice category by the Australian Marine Conservation Society which is the highest rating category by that organization.
All pacific oysters for commercial grow outcome from a hatchery. Most are supplied from 4 to 10mm long and are single seed, (all separated).
The 4mm oysters are placed into seed trays with small plastic mesh and are either suspended just below the water’s surface or on inshore racking.
After a period of 6 to 8 weeks, they are graded and the quickest grown commonly known as (front runners) are placed into 6mm baskets and then the constant job of grading oysters starts. Oysters will be graded between 5 and 7 times by the time they reach the most popular market size (buffet).
Oysters take anywhere from 18 months to 3 years to grow to a marketable size depending on the techniques used.
Constant grading ensures that the oyster shell remains in a good shape, which in turn will maintain good meat quality. Grading also improves the growth rate as the more even they are maintained in the basket the better rate of growth is achieved.
Oysters are grown in 2 distinct areas in the water column. Firstly the most common method used has been the rack and rail method where baskets are placed on wooden rail structures. These oysters are periodically out of the water when the tide is sufficiently low enough to expose the railing and the oysters are therefore not feeding.
The second method used is for the oysters is deep water or sub-tidal, generally, 45-55mm oysters are placed into either baskets or cages which are then suspended clear of the seabed. This method allows the oysters to feed all the time and growth is much quicker, however, they need to be graded and handled on average every 6 weeks to avoid soft and out-of-shape shells.
Oysters eat small microscopic larvae and are not artificially fed therefore good water quality is of huge importance to the oyster farmer.