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In a surprising discovery, researchers from the Academy of Natural Sciences of Drexel University have found a substantial number of anadromous blueback herring in New Jersey's Cooper River. The fish, similar to salmon, swim upriver to spawn, and had been considered all but extinct in the region by the 1970s. But the finding suggests they have made a robust return in the river, which is somewhat unexpected considering the area along it remains highly urbanized. The finding suggests that the herring's return corresponds with improvements in water quality and fish passage over the last 50 years.

Recently in the journal Marine and Costal Fisheries the research reports that highly urbanized rivers can鈥攁nd in some cases, do鈥攕upport substantial runs of anadromous fishes.

Rivers on the east coast of North America, like the Cooper, used to teem with fish migrating upstream from salt water to freshwater to reproduce鈥攂ut historically viable stocks have weakened as waterways were dammed and water quality deteriorated well through the adoption of The Clean Water Act in 1972.

"In the '50s, '60s and '70s, water quality was very poor and when the Clean Water Act was passed, it took time for the legislation to influence infrastructure and divert discharge from water treatment plants that were discharging directly to the Cooper River," said David Keller, Ph.D., head of the Fisheries Department at the Academy of Natural Sciences of Drexel and lead author on the study.

"By the '90s, these fish were able to be observed again, and a fish ladder was installed鈥攏ow as water quality improves and migratory fish can reach historical spawning grounds, we are seeing the return of several species, like American shad, but most remarkably, a robust return of blueback herring."

The study took place within the Cooper River watershed and Delaware River basin located in the mid-Atlantic region of North America鈥攊n tidal portions above and below the Kaighn Avenue Dam. The research team surveyed the area in the spring and summer of 2022 and 2023, overall collecting six anadromous species including blueback herring, hickory shad, alewife, American shad, striped bass, and sea lamprey.

"As a general pattern, our results indicated that densities of these fishes, mostly blueback herring and striped bass, were lower above the dam in both years, suggesting that the dam may be impeding runs of those species, or that the fish ladder may be inadequate," said Keller.

Collection results suggest that Cooper River Lake may provide suitable spawning habitat for these species or access to spawning grounds upstream鈥攍eading the team to emphasize the importance of considering urbanized watersheds on the east coast of North America as candidates for restoration鈥攄espite having high urban land uses typically associated with poor habitat quality.

Researchers aim to better understand anadromous fish relationships with urbanization as it relates to critical factors, such as low dissolved oxygen and high temperatures that negatively affect habitat suitability for the many life stages of these fishes.

"Continued efforts to address the urban syndrome through management practices has improved water quality, as evidenced by the return of blueback herring, a species that requires good dissolved oxygen levels in the Cooper River and Cooper River Lake," said Keller.

Researchers concluded that continued monitoring of urbanized streams is needed to better describe the long-term response of these species to efforts that ameliorate ecological disturbances caused by urbanization.

The team conducted weekly sampling of the Cooper River in 2024 and additional collection is planned for spring 2025 to further understand the spatial and seasonal patterns of anadromous fish recovery near the dam.