By ecoRI News staff
PROVIDENCE — Scientists from the Rhode Island Department of Environmental Management (DEM) 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 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 surveyed the area July 20, measured water quality in the Seekonk River and performed a count of dead fish. About 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, so the estimated total count was in the low hundreds.
Oxygen levels below 3 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, according to DEM.
The state agency 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.
It shared that 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 such as bluefish that attack them when they attempt to move out of these poor water-quality areas, forcing them to remain in these low-oxygen waters.
Reports this week also indicate that dead menhaden are being found along the East Providence shore of the Providence River. This suggests that the low-oxygen water has extended further south and is affecting the lower Providence River, according to DEM. A survey of the upper third of Narragansett Bay by Brown University and the University of Rhode Island will be conducted this week, according to DEM. Scientists will have a better picture of the extent of the hypoxia when that data is available.
Hypoxic conditions are brought on by excess nutrients from various sources, such as lawn fertilizer, 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 there use oxygen to decompose their bodies. Large algae blooms often result in low oxygen levels that are lethal to fish.
Major wastewater treatment facilities in Rhode Island remove a large percent of the nutrients through tertiary treatment before releasing treated effluent into the state’s waters. However, other sources of nutrients such as fertilizers in stormwater runoff and discharges from wastewater facilities that don’t provide tertiary treatment are significant sources of nutrients.
DEM officials said they will continue to compile information on the extent of this low-oxygen event. The agency expects additional fish kills will occur while large schools of menhaden continue to congregate in the Providence and Seekonk rivers, until weather conditions such as a strong wind from a local storm or a cooling weather pattern comes through.