Menhaden Schools Stressed by Suffocating Algae

By ecoRI News staff

Scientists from the Rhode Island Department of Environmental Management (DEM) are investigating reports of stressed schools of adult and juvenile menhaden in the Seekonk River and Pawtuxet Cove near the Pawtuxet Falls.

Based on seine data and menhaden school spotter flights, this has been a good year for menhaden in Narragansett Bay, according to DEM. As such, the state agency expect menhaden schools to continue to be found throughout the bay, including in areas that may be stressful to the fish in terms of water temperature and oxygen levels.

A large school of adult and juvenile menhaden remains just below the Pawtuxet Falls. Some of these fish appear to be stressed, sometimes exhibiting odd swimming behaviors, according to DEM. The fish likely are congregating in this area looking for lower water temperatures and possibly higher oxygen levels.

Cooler water flowing from the Pawtuxet River and the falls carry more oxygen. The schools may also be congregating in small coves like this in an attempt to escape from predators such as bluefish and striped bass, according to DEM.

The Seekonk River area is known to experience low-oxygen conditions (hypoxia) that form when excess nutrients from various sources cause algae to grow rapidly. As algae die — they only live for a short period — they sink to bottom, where decomposition by bacteria in the bottom waters depletes oxygen. Large algae blooms can result in low-oxygen levels that are lethal to fish.

While progress has been made reducing nutrients discharged from wastewater treatment facilities in Rhode Island and Massachusetts, more work remains to be done to address these and other sources of nutrients, such as fertilizers in stormwater runoff and nutrients from poorly functioning onsite wastewater treatment systems.

The potential for fish kills from excess nutrients and low dissolved oxygen are greater during periods of sustained high temperatures and low-water river flows. Because of the high water temperatures in recent days, DEM expects the threat to schooling menhaden to continue. The threat will dissipate once temperatures cool, algal blooms subside and oxygen levels are restored. Greater mixing of the water column due to stronger winds can also help restore oxygen levels.