Investigating the Gizzard Shad Die-off in the Upper Thames River
A recent die-off of gizzard shad (Dorosoma cepedianum) in the Thames River within the City of London sparked concern among residents and conservationists. First reported in mid-November 2024, the die-off occurred near the Blackfriars Bridge, where a small tributary carrying warmer stormwater empties into the cooler Thames River. This confluence has been a known congregation site for shad in recent years.
The die-off involved over 100 fish exhibiting lethargy, circular swimming, and visible hemorrhaging. University of Guelph graduate student James Clare is investigating the cause, with support from the Upper Thames River Conservation Authority (UTRCA). While the exact reasons remain unclear, researchers are exploring several possibilities and further testing is planned.
Field investigations have suggested that water quality issues, such as elevated sediment or nutrient levels, could act as stressors, though they alone seem unlikely to explain the event. Natural factors, such as seasonal changes and temperature fluctuations, have likely also contributed. Gizzard shad are sensitive to colder temperatures, and significant mortality is known to occur when water temperatures drop below 4掳C. James Clare also noted the possibility that the die-off may not have been a sudden occurrence but, rather, part of a gradual trend, with recent low water levels making dead fish more visible.
These findings underscore the complexity of determining exact causes in aquatic ecosystems. Laboratory tests at the University of Guelph have ruled out viral hemorrhagic septicemia and bacterial kidney infections鈥攃ommon culprits in fish mortality events鈥攂ut the investigation is ongoing.
Gizzard shad play a critical role in the Thames River ecosystem. They provide an essential food source for piscivorous (fish-eating) fish, birds, and mammals, and their feeding behaviours contribute to nutrient cycling. A significant decline in this species could disrupt food webs and exacerbate ecological imbalances. On the other hand, milder winters鈥攕uch as those London has experienced in recent years鈥攎ay reduce natural population controls for this species, allowing their populations to grow unchecked and further affect ecosystem dynamics.
This die-off highlights the importance of considering both natural and human-induced factors when investigating aquatic mortality events. Ongoing monitoring and collaboration are essential to protect the health of the Thames River watershed. With over 90 fish species dependent on this freshwater system, understanding the cumulative effects of environmental and ecological stressors is critical. Continued investigations will provide valuable insights into such incidents and help ensure the long-term sustainability of the river's diverse ecosystem.
