Requirements include:

a low disturbance location.
adequate water quality.
dense enough populations to ensure egg fertilization by sperm released into the water column.

Threats Include:

toxic pollution.
nutrient loading of estuaries by rivers draining upland sources (agriculture, septic fields, sewage treatment plants etc.). This can cause massive algae blooms that strip oxygen from the water at night or produce toxins that accumlate in shellfish.
poaching.
competition or predation from introduced species.

Install the best available sewage treatment technology.
Ensure that runoff from pavement adequately treated in new developments through the subdivision and development permitting processes.
Retrofit storm sewers with oil/grease separators, sediment traps, and/or treatment wetlands during other maintenance and upgrading projects.
Prevent direct dumping of toxic materials into storm or sanitary sewers through public education and a bylaw.
Ban insecticide and herbicide use for cosmetic purposes on lands within jurisdiction (municipalities only).
Eliminate insecticide and herbicide use on local government lands.
Keep high-risk industries away from habitats through zoning.
Require large setbacks from important habitats through the zoning, subdivision and development permit processes.
Enforce stringent anti-pollution regulations for boats and port facilities (port authorities).

Target lands adjacent to important habitats for acquisition by purchase or dedication via the rezoning, subdivision and development permit processes.
Restrict access on local government lands adjacent to key habitats.
Restrict public access to sensitive habitats by water (port authorities).
Include information on listed marine shellfish in public education materials and programs.

Educate the public on the risks posed by non-native species and on precautions to avoid their introduction.
Enforce strict ballast change protocols for ships to reduce the risk of invasive species introductions (port authorities).