Humans have lived along Long Island Sound for thousands of years, but the growing pressure of more than 20 million people living within 50 miles of its shores has especially in the last century harmed its ecosystem. Pollution reaches the Sound through airborne mercury, storm drains, failing septic systems, farm runoff, Canada geese droppings, fertilizers, and sewage treatment plants, contaminating seafood, making swimming and shellfish unsafe at times, and causing hypoxia that drives away or kills marine life. Development destroyed many tidal marshes as colonists drained them for grazing and hay, later projects filled them for the shoreline railroad and Interstate-95, and mosquito control in the 1930s drained almost all of Connecticut’s remaining marshes, removing natural filtration, habitat, and storm buffers, though restoration efforts had restored tidal flow to 2,025 acres of marshland by 2008. Dams built during the Industrial Revolution blocked migratory fish such as alewives, shad, and eels from reaching spawning grounds, leaving many species in low numbers and eliminating Atlantic salmon from the Sound, leading to restoration through dam removal, fishways, stocking, transplantation, fishery regulation, and habitat protection. Fishing pressure, poor water quality, and disease depleted many fish and shellfish populations, prompting conservation measures, quotas, and surveys. Invasive species such as the Asian shore crab have crowded out native species, and global warming has already warmed Connecticut waters, contributed to a major lobster die-off in 1999, and may raise sea levels enough to threaten the shoreline. Protecting the Sound depends on public actions such as maintaining septic systems, keeping pollutants out of storm drains, washing cars where water can be filtered, disposing of trash properly, and respecting the plants and animals that live there.