Sturgeon are ancient, ray-finned fish, distinguished by their cartilaginous skeletons and rows of bony plates, called scutes. Whether these creatures are saltwater fish is complex, as their biology allows them to inhabit a range of aquatic environments. This flexibility results from unique biological adaptations and a life cycle that often requires movement between water types. Understanding this requires examining the habitats they occupy, the mechanisms allowing them to survive salinity changes, and their migratory patterns.
The Sturgeon Family: An Overview of Habitat Diversity
Sturgeon are not exclusively saltwater fish, nor are they purely freshwater dwellers; instead, their twenty-seven species exhibit a spectrum of habitat tolerances. The majority of sturgeon species are considered diadromous, meaning they migrate between fresh and salt water during their life cycle. These species, such as the Atlantic Sturgeon, spend their adult lives in coastal marine waters or estuaries before returning to freshwater rivers to reproduce.
Within the family, some species are entirely confined to freshwater environments, known as potamodromous fish. The Lake Sturgeon, for example, is found exclusively in large North American river systems and lakes, like the Great Lakes, without ever entering the sea. Many species spend a significant portion of their adult lives foraging in nutrient-rich, brackish estuaries and nearshore ocean areas. Classifying the entire family as either saltwater or freshwater is inaccurate, as their environmental residence depends on the specific species and its life stage.
The Mechanism of Adaptability: Surviving Salinity Changes
The ability of many sturgeon species to transition between freshwater and saltwater is managed by a physiological process called osmoregulation. These euryhaline fish must constantly regulate the internal concentration of salts and water in their bodies to maintain a stable internal environment.
When sturgeon are in saltwater (a hypertonic environment), they face the challenge of losing water and gaining excess salt from the surrounding high-salinity water. To counteract this, they actively drink the seawater, absorbing the water and salt through their digestive tract. Their gills, which contain specialized ionocytes or mitochondria-rich cells, work to excrete the excess sodium and chloride ions back into the ocean.
Conversely, when they move into freshwater (a hypotonic environment), the challenge is reversed: their body fluids are saltier than the surrounding water, causing water to constantly diffuse in and salts to diffuse out. In this environment, the sturgeon cease drinking water, and their kidneys produce large volumes of dilute urine to expel the excess water.
The gill cells then reverse function, actively taking up ions like sodium and chloride from the dilute freshwater to prevent internal salt depletion. This reversible physiological shift allows species like the Green Sturgeon to tolerate a wide range of salinities, from zero parts per thousand (ppt) in rivers to full-strength seawater (around 32 ppt).
Migration Patterns: The Anadromous Life Cycle
The movement between water types is not random but is driven by the anadromous life cycle of many sturgeon species. This cycle dictates that the fish are born in freshwater, migrate to the sea to grow, and then return to the same freshwater system to reproduce.
Spawning occurs in the freshwater habitats, typically in fast-flowing sections of rivers over rocky or gravel substrates, which provides the ideal conditions for eggs to adhere and develop. Juvenile sturgeon remain in their natal river for an extended period, often several months to years, feeding and growing before they begin their journey downstream toward brackish estuaries and the open ocean.
The migration to the ocean is primarily a feeding and maturation migration, allowing sub-adults to exploit the rich food sources of coastal marine environments for rapid growth. Environmental factors such as water temperature and river flow act as cues, triggering the movement of juveniles heading to the sea and mature adults returning to the river. Adults often make their upstream spawning run when water temperatures warm in the spring and summer, sometimes traveling hundreds of miles to reach their specific spawning grounds.