If you’ve looked in the waters around Greenwich lately, you may have noticed massive schools of fish swimming in slow circles near the surface of the water. Commonly seen near bridges and in harbors around this time of year, these foot-long fish are called Atlantic menhaden, or bunker, and they are a very important part of Long Island Sound ecology.

Menhaden are an abundant fish that range from the cold waters of Nova Scotia to as far south as Florida. During fall, they migrate south to spawn, and as the weather warms, they return north to feed. Their schools can number in the thousands, and sometimes millions of menhaden will form giant moving shadows beneath the waves. These huge schools attract many predators, such as dolphins, sharks, birds, seals, and even whales. They are such an abundant food source that they are believed to be the most common prey of the osprey, a local favorite around the Bruce Museum.

And what do menhaden eat? When you see them swirling around a harbor, you might see their mouths gaping wide. That is because they are filter feeders, using specialized gill rakers to harvest plankton and other tiny organisms. They eat some of the smallest organisms in the ocean, and become food for some of the largest. This forms an integral energy transfer system in many marine food webs.

The menhaden’s nature as prey fish is the reason why they end up in shallow harbors so often – They get chased there by bluefish or other predators, and will remain schooling in the harbor until the threat clears.

Menhaden are still sometimes used for fertilizer today.

Menhaden might not get along with bluefish, but what about humans?

As far as fish go, menhaden don’t make for good eating. They are bony, oily, and have a strong fishy flavor. They can be eaten in a similar manner to sardines and are sometimes fried, but for the most part, they stay off human plates. However, we still have found ways to exploit this bountiful natural resource.

The name “menhaden” is believed to derive from a word in the Narragansett tribal language, “munnawhatteaug.” This roughly translates to “fertilizer,” and that’s what Native Americans used them for. In the modern day, menhaden have acquired numerous other uses, and are processed into omega-3 fish oil, ground into fish meal, and used as bait by fishermen.

Before regulations and catch limits were established, menhaden populations were overharvested into a steep decline off the coast of the United States. Fortunately, their populations have rebounded, and now the massive schools of menhaden are back. This is a positive development, but under just the wrong circumstances, you might find yourself cursing this abundant fish…

Circled in blue are two places where I have seen menhaden near the Bruce Museum. Have you seen them anywhere else?

Fish Kills

Humans need oxygen to survive, and fish do too. When you breathe, your lungs filter oxygen out of the air and release carbon dioxide. Fish use their gills to draw oxygen out of the water in a similar way, and the temperature of that water plays a big role. Cooler water is capable of holding much more oxygen than warmer water, and fish that find themselves in warm water can end up in big trouble.

When water temperature rises above 70 degrees Fahrenheit, it becomes hard for menhaden to receive enough oxygen to survive. Shallow waters warm more quickly than deeper, so if a school of menhaden are trapped in the shallows as the water begins to heat, the fish will start to suffer from hypoxia. This causes fish to act more erratically, which in turn, further depletes the oxygen from the water. Often a school will escape into more hospitable waters, but if not, it can become what is called a fish kill.

A fish kill is when a localized area is marked by large numbers of fish deaths. Low oxygen can cause menhaden to die in this manner, but poor water quality is another potential trigger. Though fish kills in themselves are not harmful to humans, the rotting bodies and fetid stench of thousands of dead fish are a great nuisance for those who live or work near the water. We are fortunate to not have massive fish kills very often in the Greenwich area, but fish kills on a smaller scale sometimes happen in our local Bruce Pond, where menhaden sometimes swim into the freshwater pond at high tide, and become trapped as the tide goes out.

So, next time you take a walk, whether it is around Bruce Park or elsewhere in Connecticut, consider strolling by the harbor or over a bridge and peering into the water. You may see swirling schools of menhaden, and with a little luck, it won’t be long until they’re back out to sea and continuing on their way south for the winter.

- Kate Dzikiewicz, Science Curatorial Associate and Seaside Center Manager