State DEC investigating fish kill in Peconic River

State environmental officials are investigating the death of more than 100 Atlantic Menhaden — known as bunker — in the Peconic River downtown. Thousands more bunker could be seen congregated in the river from Peconic Avenue east to the Cross River Drive bridge this morning. Thick schools of bunker were crowded into Meeting House Creek also.

Staff from the DEC’s marine resources division were in Riverhead this morning testing the river’s dissolved oxygen levels after the agency received calls about the large number of fish present and visible die-off.

The exact cause of the fish kill has not yet been determined but water quality measurements show low dissolved oxygen in some pockets, a State DEC spokesperson said this afternoon.

A small number of fish were also observed gulping for air at the surface which is indicative of low oxygen levels, the DEC said. Substantially larger aggregations of bunker were seen in deeper water where dissolved oxygen levels were higher, the DEC said. Samples of fish were collected for additional analysis.

Fish like bunker that swim in large schools are particularly vulnerable to low dissolved oxygen, or hypoxia, which can often be fatal to large schools — though other fishes and marine organisms are stressed but will ultimately survive, according to DEC.

Hypoxia can be triggered by large numbers of fish in confined bodies of water, excessive algal growth and warm water temperatures. In addition to these factors, warm daytime temperatures can further exacerbate low dissolved oxygen levels in shallow bodies of water.

Riverhead experienced a massive bunker kill in May 2015, when more than 300,000 bunker fish washed up dead in the Peconic River.

State, county and academic researchers subsequently determined that a spike in algal blooms, fueled by increased levels of nitrogen in the water, deprived the water of oxygen and caused “mass asphyxiation” of the bunker.

A large population of predatory blue fish at the mouth of the river made matters worse, blocking the bunker from escaping the oxygen-drained river, the study found. The algal blooms also damaged the gills of the bunker which may have exposed them to a virus, the researchers said.