Bunker Die-Off Leads to Investigation

The Atlantic menhaden is no stranger to our part of the river. Also known as bunker, they are a species of schooling fish that swim in large, (really large) groups. This important species is arguably one of the most vital parts of the aquatic food chain model bridging the gap between microscopic organisms and larger predators. These abundant filter-feeding fish convert plankton into a much sought-after source of protein for birds, mammals and larger fish. Seals, birds, striped bass and even whales rely on this abundant food source. The Atlantic menhaden fish stock is very healthy and so abundant that they are still harvested for use as fertilizer and animal feed.

As abundant as this species can be, seeing large quantities of any dead fish on the side of our river can be concerning. During early and mid-December, the NYDEC and other agencies began to receive reports of large amounts of dead fish found along the river banks from as far south as Newark Bay to as far north as Tarrytown. The case of the most recent die-off has grabbed the attention of environmentalists and state agencies because of the time of year the die-offs occurred. Bunker are fragile fish and are very sensitive to changes in temperature and oxygen levels. Mass die-offs are common in summer months when algal blooms and large storm runoff can cause the oxygen levels in the river to fall causing entire schools of these fish to perish. For a die-off to occur in the middle of December when the cold river water temperatures should have prevented algal blooms and when water contains more dissolved oxygen was a mystery. Water quality testing was performed as well as testing samples of the washed-up fish. However, nothing conclusive was reported at the time of this writing.

A possible cause for the die-off was a sudden temperature change in the river. Storms or melt-offs can release large amounts of colder runoff into the river causing local sudden temperature changes in areas the schooling fish are swimming through. A 2015 mass die-off in Long Island Sound was attributed to a sudden and drastic local water temperature change. Stonybrook University’s School of Marine and Atmospheric Sciences has also noted that bunker are also staying around much later into the year than usual as a result of warmer summer temperatures and consequently the fish put themselves at more of a risk to these sudden fluctuations in water temperature.

As concerning as a fish die-off can appear, it can actually be interpreted as a sign of a healthy marine ecosystem with more fish in our rivers.