Class II includes the subcategories of designated uses described in this section. Shellfish Harvesting. This subcategory includes waters where there are: Actual or potential areas for the harvesting of oysters, soft-shell clams, hard-shell clams, or brackish water clams; or Actual or potential areas suitable for the propagation or storage of oysters, hard-shell clams, soft-shell clams, and brackish water clams for marketing purposes, except areas excluded by the Department. Seasonal Migratory Fish Spawning and Nursery Subcategory. This subcategory includes waters of the Chesapeake Bay and its tidal tributaries that have the potential for or are supporting the survival, growth, and propagation of balanced indigenous populations of ecologically, recreationally, and commercially important anadromous, semi-anadromous and tidal-fresh resident fish species inhabiting spawning and nursery grounds from February 1 through May 31. Seasonal Shallow-Water Submerged Aquatic Vegetation Subcategory. This subcategory includes: Tidal fresh waters of the Chesapeake Bay and its tidal tributaries that have the potential for or are supporting the survival, growth, and propagation of rooted, underwater bay grasses in tidally influenced waters from April 1 through October 1; and Low salinity (oligohaline and mesohaline) waters of the Chesapeake Bay and its tidal tributaries that have the potential for or are supporting the survival, growth, and propagation of rooted, underwater bay grasses in tidally influenced waters from April 1 through October 1. Open-Water Fish and Shellfish Subcategory. This subcategory includes waters of the Chesapeake Bay and its tidal tributaries that have the potential for or are supporting the survival, growth, and propagation of balanced, indigenous populations of ecologically, recreationally, and commercially important fish and shellfish species inhabiting open-water habitats. This subcategory applies from June 1 to September 30 in tidally influenced waters from the shoreline to the adjacent shoreline (and from the shoreline to the opposite shoreline), and from the surface to the measured boundary of the pycnocline, if the pycnocline prevents oxygen replenishment, otherwise the subcategory is applied from the surface to the bottom. If a pycnocline exists but other physical circulation patterns, such as the inflow of oxygen-rich oceanic bottom waters, provide oxygen replenishment to the deep waters, the open-water fish and shellfish designated use extends to the bottom. From October 1 through May 31, the boundaries of the open-water designated use include all tidally influenced waters from the shoreline measured from the shoreline to the adjacent or opposite shoreline and down to the bottom. Seasonal Deep-Water Fish and Shellfish Subcategory. This subcategory includes waters of the Chesapeake Bay and its tidal tributaries that have the potential for or are supporting the survival, growth, and propagation of balanced, indigenous populations of important fish and shellfish species inhabiting deep-water habitats as described as follows: One of the following applies: From June 1 through September 30 in tidally influenced waters located between the measured depths of the upper and lower boundaries of the pycnocline, where a pycnocline is present and presents a barrier to oxygen replenishment; or From June 1 through September 30 from the upper boundary of the pycnocline down to the sediment/water interface at the bottom, where a lower boundary of the pycnocline cannot be calculated due to the depth of the water column; and From October 1 to May 31, the open water fish and shellfish subcategory designated use applies. Seasonal Deep-Channel Refuge Use. This subcategory includes waters of the Chesapeake Bay and its tidal tributaries that have the potential for or are supporting the survival of balanced, indigenous populations of ecologically important benthic infaunal and epifaunal worms and clams, which provide food for bottom-feeding fish and crabs. This subcategory applies from June 1 through September 30 in tidally influenced waters located below the measured lower boundary of the pycnocline to the bottom where a measured pycnocline is present and presents a barrier to oxygen replenishment. From October 1 to May 31, the open water fish and shellfish subcategory designated use applies.