Ecology and Vulnerability

Shortnose Sturgeon

Warmer temperatures, higher salinity, lower dissolved oxygen, increasing ocean acidification, and changing water currents have, and will continue to, affect already declining populations of diadromous fishes (Kerr et al., 2009), from shifts in range, depth, or elevation (e.g., Nye et al., 2009); changes to phenology (e.g., Staudinger et al., 2019; Otero et al., 2014); and increased mortality risk from hypoxia in warming ocean water (e.g., Secor and Gunderson 1998). Secondary effects of climate change may include decreased prey availability (e.g., in American shad [Alosa sapidissima] nurseries; Crecco and Savoy 1984) and increased disease risk (e.g., sea lice [Lepeophtheirus salmonis] in the Gulf of Maine; Bricknell et al., 2021). Climate change has impacted recruitment in some diadromous fish, including Alewife (Tommasi et al., 2015) and American Eels (Miller and Casselman 2014). Overall, because of the complex early life history, migration, and use of multiple habitats (freshwater and marine) of RSGCN diadromous fish, all but one are considered at “Very High” climate change vulnerability risk; American Eels are listed as “High” (Hare et al., 2016 and see Chapter 3). Further, the cumulative responses of shifts towards smaller body size and lower reproductive potential suggest that climate change may lead to a cycle of population decline in the warmer parts of diadromous fishes’ ranges. Contraction of phenophases, as demonstrated in studies (e.g., Atlantic Salmon [Dempson et al., 2016]; Alewife and Blueback Herring [Lombardo et al., 2019]), can increase the likelihood of risk to extreme events from spring storms (e.g., Nor'easters) (Lynch et al., 2015) or anthropogenic actions like water releases from hydropower facilities.

Relevant articles were found for all RSGCN species except Hickory Shad. Despite relatively good coverage of diadromous fish in the literature, most research focused on Atlantic Salmon and, to a lesser extent, Alewife. Only three species were represented in the Bioshifts database (Atlantic Salmon, American Shad, and Alewife; Lenoir et al., 2020, Rubenstein et al., 2023).

Background
Shortnose sturgeon can be identified by four rows of bony plates, called scutes, as well as brownish-olive bodies with white undersides. They can be difficult to distinguish from Atlantic sturgeon, which are similar in appearance; however, shortnose sturgeon are generally smaller (3 feet in length on average, compared to 7 feet for Atlantic sturgeon), and have wider mouths and shorter snouts ^6,9. Shortnose sturgeon are a long-lived (67+ years) migratory fish that live in fresh, brackish, and salt water, and occur from the Saint John River in New Brunswick to the Indian River in Florida ^5,6. They are found year-round in coastal rivers and estuaries, often along large river curves with sand or cobble bottoms ⁸. They are bottom feeders, primarily eating crustaceans, mollusks, and insects.

Migration
Shortnose sturgeon are semi-anadromous, primarily living in brackish and salt water, and spawning in freshwater habitat. Migration varies by latitude and is likely dependent on temperature and flow conditions. Upriver spawning movements occur in late winter in southern rivers (e.g., South Carolina) and in spring in northern rivers (e.g. Massachusetts); however, fall migrations have been observed in the Hudson River, Connecticut River, and St. John River ⁶. Shortnose sturgeon arrive to spawning grounds when water temperatures reach 47-48 °F (8-9 ° C) and spawning typically occurs between 48-54 °F (9-12 °C) ^2,6. They reach sexual maturity at 5 to 15 years depending on location and growth rates ⁵. Juveniles remain in freshwater until they reach approximately 18 in (45 cm) in length, and then migrate into brackish and marine systems ⁵.

Human Induced Impacts
Current threats to shortnose sturgeon include habitat obstruction and degradation, pollution such as nutrient runoff from agriculture, alteration of sediment distribution patterns, and incidental catch ¹³. In Massachusetts, the large river main-stem dams that were constructed in the 1800s significantly reduced sturgeon’s access to spawning habitat ^1,7. Because sturgeon rely on specific hydrological conditions, water flows are an important factor in determining spawning location and success, as has been demonstrated in the Connecticut River ². Additionally, dredging and other sediment-altering activities threaten sturgeon eggs because they are vulnerable to being smothered ¹³. The historic demand for sturgeon caviar and flesh led to overexploitation throughout their range; they were listed under the Endangered Species Act in 1967 ¹⁰. Today they are rarely the target of commercial fishing and only caught incidentally.

Climate Impacts
Shortnose sturgeon populations are already severely stressed due to a range of human activities; climate change is expected to exacerbate poor conditions and have additional negative effects. Shortnose sturgeon rely on specific temperature cues and windows of optimal conditions to initiate important life stages and events such as spawning, which increases their vulnerability to climate change ⁶. Dispersal behavior of juvenile shortnose sturgeon has been shown to change as temperature increases. For example, in Connecticut, juvenile sturgeon had multiple downstream dispersal events when the waters were warmer, and single-event delayed dispersal when waters were cooler ¹¹. In addition, temperature changes and nutrient runoff can alter water oxygen levels, impacting survival. Sea level rise may alter tidally-driven river salinity and change spawning habitat structure through different erosion and deposition patterns ¹⁴. Increased saltwater intrusion threatens optimal salinity ranges, which vary by season and life stages. For shortnose sturgeon in the south Atlantic, optimal salinity ranges have been identified as between 1.75 and 3 ppt during the breeding season, up to 8 ppt during the feeding season for early life stages and up to 20 ppt for older life stages ¹⁵. Increased variability and intensity of precipitation patterns projected under future climate change is expected to alter terrestrial runoff which can lead to suffocation and mortality of eggs. Changes in runoff will also increase the delivery of chemical pollutants into streams and rivers (e.g., PCBs), which may affect juvenile survival by altering the body forms of hatchlings ³. If water velocity events are too high, sturgeon eggs may be swept away into unsurvivable conditions ².