PROVIDENCE - Scientists from the Department of Environmental Management are monitoring a large fish kill of adult menhaden in the Seekonk and Providence Rivers. Based on field observations and water quality measurements, the ongoing incident is being caused by low oxygen levels in the bottom waters.
Initial reports indicate the fish kill began in the upper Seekonk River in the evening or early morning hours of July 17-18. A field investigator from DEM's Marine Fisheries section surveyed the area on July 20, measured water quality in the Seekonk River and performed a count of dead fish. Approximately 100 dead menhaden were found during the shoreline count at Bishop's Cove and the Pawtucket boat ramp and pier area. Dead fish were also strewn along inaccessible shoreline areas, and so the estimated total count was in the low hundreds. Oxygen levels below approximately three feet were very low in the channel near the Pawtucket boat ramp and pier on the Seekonk River, and the likely main factor in the fish kill.
DEM has been collaborating with the Narragansett Bay Commission (NBC) to monitor the hypoxic (low oxygen) conditions in the affected waters. NBC field staff performed a regularly-scheduled water quality survey in the Seekonk and Providence Rivers on July 21 and 22. They shared their data with DEM and took fish samples for analysis. The data showed that the low-oxygen water extended all the way to India Point Park, and that almost the entire length of the Seekonk River is experiencing a severe low oxygen event. Menhaden are often pinned in by predators like bluefish that attack them when they attempt to move out of these poor water quality areas, forcing them to remain in these low-oxygen areas.
Reports early this week indicate that dead menhaden are being found along the East Providence shore in the Providence River. This suggests that the low-oxygen water has extended further south and is affecting the lower Providence River this week. A survey of the upper third of Narragansett Bay by Brown University and the University of Rhode Island will be conducted this week. Scientists will have a better picture of the extent of the hypoxia when that data is available.
The hypoxic conditions are brought on by excess nutrients from various sources that cause algae to grow rapidly and often color the water. The Seekonk River is presently a brown color due to a large bloom of nontoxic algae. As algae die (they only live for a short period) they sink to bottom waters, and bacteria in the bottom waters use oxygen to decompose their bodies. Large algae blooms often result in low oxygen levels that are lethal to fish. Major wastewater treatment facilities (WWTFs) in Rhode Island remove a large percent of the nutrients through tertiary treatment before releasing their treated effluent into the state's waters. However, other sources of nutrients such as fertilizers in storm water runoff and discharges from WWTFs that do not provide tertiary treatment are significant sources of nutrients, especially on the Blackstone River. DEM will continue to compile information on the extent of this low-oxygen event. The Department expects that additional fish kills will occur while large schools of menhaden continue to congregate in the Providence and Seekonk River areas, until weather conditions such as a strong wind from a local storm or a cooling weather pattern comes through.