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How the Fishable Grades are Calculated

The “Fishable” water grades reflect the quality of the water to support marine life fin fish, shellfish and the many creators that live in our coastal ecosystems. The grading system was created in consultation with scientists who study water quality in Long Island Sound.


Grades for open water regions are created by comparing 4 indicators (dissolved oxygen, chlorophyll a, water clarity and dissolved organic carbon) to scientifically derived goals, or thresholds. These indicators are combined into an overarching Water Quality Index, which is presented as a regional open water grade.


Grades for bays and harbors are created by comparing 5 indicators (dissolved oxygen, chlorophyll a, water clarity, macrophytes and oxygen saturation) to scientifically derived goals, or thresholds. These indicators are combined into an overarching Water Quality Index, which is presented as a section grade in the bay or harbor.


Dissolved Oxygen

The oxygen dissolved in water is critical to the survival of fish and shellfish living in it. All of the living creatures in the water need oxygen to survive but as dissolved oxygen levels decrease, it becomes harder for animals to get the oxygen they need to survive. Low dissolved oxygen is often the result of eutrophication, which occurs when there are too many nutrients (such as nitrogen and phosphorus) in the water which cause dense algal blooms to grow. When the algae die and decompose, the decomposition process uses up dissolved oxygen in the water, reducing the oxygen available for fish, and other organisms, which may become stressed or even die.

Chlorophyll a

Chlorophyll a is the green pigment in tiny marine algae (phytoplankton) that produces food. Measuring chlorophyll is based on the amount of phytoplankton (microalgae), which uses both nitrogen and phosphorus to grow. Too much algae in the water reduces water clarity, and decomposing algae leads to reduced dissolved oxygen. In a balanced ecosystem, phytoplankton provide food for fish, crabs, oysters, and worms. When too many nutrients are available, phytoplankton may grow out of control and form algal blooms that can harm fish, shellfish, mammals, birds, and even people.

Water Clarity

Water clarity is a measure of how much light penetrates though the water column. Water clarity is dependent upon the amount of particles (e.g. suspended sediment and plankton) and colored organic matter present. Clear water is especially critical for seagrasses since, like all plants, they need to be able to absorb the sun’s rays to grow. Excess sediment in the water reduces water clarity by blocking sunlight to seagrasses. Fish and other organisms in the water need aquatic grass habitat to thrive.

Dissolved Organic Carbon

Nutrients (nitrogen, phosphorus, carbon) are the building blocks for life, but too much in a natural system can lead to problems. Our communities contribute excess nutrients to the Sound from wastewater, septic systems, fertilizer, and fossil fuel burning. Nutrients fertilize excessive growth of plantlike organisms, leading to algae blooms. As these organisms and the animals that feed on them respire, die, and decompose, oxygen in the water is depleted.

Dissolved organic carbon is relatively stable, making it a good indicator of human impacts. Most human sources of nutrients are high in dissolved organic carbon.


Seaweeds are common in healthy salt water systems. However, excessive accumulation can be harmful to environmental health and indicate excess nitrogen pollution.

Dissolved Oxygen Saturation

Healthy water should have oxygen levels in equilibrium with the air, termed 100% saturation. Water quality problems are indicated when the highest recorded oxygen values are considerably higher or lower than 100%.