Greenwich Bay

The menhaden, or pogy, is a member of the same group that contains herring. It is related to fish such as alewives, blueback herring, and shad.

Menhaden come into Greenwich Bay in the summer, when they are caught commercially. Check out Purse Seining for Pogies.

Growth and development

1. EGG

The menhaden begins its life as one of thousands of eggs released in the summer, beginning in June and continuing sometimes as late as October. Each egg is about 1.5 to 1.8 MILLIMETERS in diameter. It looks like this:

The egg is actually about the size of Abraham Lincoln's bowtie on a penny:

Each egg has a small globule of oil inside. This helps the egg float and provides a source of food for the developing fish.

2. LARVA

If it is not eaten, within 48 hours, the egg has hatched. The fish is now in its larval stage. The new larva is about 4.5 mm long. The larva is nearly transparent.

Look at the word "liberty" on a penny to get an idea of about how big the larva is now:

The larva is a part of what is known as the PLANKTON community. Plankton are plants and animals, usually very small, that are found at or near the surface of the water. They are unable to swim against currents and tides.

3. FRY

As the fish grow, they are called fry.
Gradually, they begin to look more and more like the adult fish.

By their first winter, the fry are 6 to 8 CENTIMETERS (cm) long.

4. ADULT

Menhaden are usually fully mature and ready to reproduce after their third winter. Menhaden spawn mostly offshore, although eggs have also been found in Narragansett Bay.

The average adult menhaden is 18-23 cm long,
and weighs 2/3 to 1 pound.

From dinner to diner: The menhaden's place in the food web

At the very beginning of its life, the menhaden plays only one role in the food web:

It is dinner.

Most menhaden eggs will be eaten by fish and tiny animals (ZOOPLANKTON) before they even hatch. Once the eggs hatch, the tiny larvae continue to provide meals for larger animals. But as the fish grow, the menhaden begins to turn the tables on some of its former predators. It may eat the kind of animals that once ate it when it was smaller.

Unlike many fish, the menhaden doesn't chase individual prey. Instead, it swims through the water with its mouth open and its gill openings spread. As the fish passes through the water, several layers of comb-like gill trap its food. The gill rakers sit in front of the gill filaments, which the fish uses to pull DISSOLVED OXYGEN from the water so that it can breathe.

Because of the way that it feeds, the menhaden doesn't choose what it eats. But it ends up dining on PHYTOPLANKTON, especially diatoms, as well as annelid worms, rotifers, and tiny crustaceans, including crab and lobster larvae. The adult menhaden eats a LOT. One fish can strain 6 to 7 gallons of water per minute!

Even once it becomes the predator, the menhaden will always provide food for other animals. Menhaden are a favorite food of bluefish, who will chase the schools. If you are out on Greenwich Bay in the late summer, you may see the menhaden jumping out of the water, trying deperately to escape from the bluefish below. In open waters, menhaden are eaten by whales, porpoises, sharks, and swordfish.

Coping with salt

Although marine animals spend their lives in salt water, dealing with the effects of salt is a constant challenge. This is especially true in an estuarine environment, where salinity levels may change depending on how much fresh water enters the bay.

Many marine animals have a high tolerance for salinity changes, while others will die if the level of salt changes too much. Some marine animals—including the menhaden—can control the concentration of salts in their bodies. Most cannot do this.

Animals that are able to control salt concentrations in their bodies are called osmoregulators. Animals that cannot control internal salt concentrations are called osmoconformer.

An osmoregulator, the menhaden controls the amount of salt in its body. If you have ever had a cut in your mouth, you know that there is salt in your body because blood tastes salty. But it is not as salty as ocean water. The same is true of the pogy. The fluids in its body contain salts, but are only about one-third as salty as the ocean around them. It is a constant battle for them to make sure their bodies do not get too salty. To do this, they drink a lot of sea water, keeping the water and getting rid of the salt through special cells in their GILLS.

Oxygen and breathing

What do fish breathe? Like humans, they breathe oxygen. Only instead of breathing it from the air, they breathe oxygen that has been dissolved in the water. To think about how oxygen, a gas, is dissolved in water, look at an unopened bottle of soda. How many bubbles do you see? Probably not many. Now open the bottle of soda and pour some into a glass. Now you can see many bubbles. These bubbles are actually carbon dioxide gas that is no longer dissolved in the soda. You can even see that some of the gas is escaping from the soda into the air at the bubbles "jump" out of the soda.

Oxygen dissolves in water in the same way that carbon dioxide is dissolved in soda. The menhaden uses its gills to remove oxygen from the water. The water flows across gill filaments, where the oxygen passes through very thin membranes into the fish's bloodstream.

If concentrations of oxygen are too low, which may happen as a result of pollution, the menhaden may suffocate and die.

The importance of temperature

You know that polar bears don't live on tropical beaches and toucans don't live at the North Pole. One of the reasons for this is temperature. Most animals have a specific range of temperatures within which they are able to survive. They usually have an even narrower range of temperatures in which they can thrive, grow, and reproduce. This is one reason that birds migrate and fish are found in Greenwich Bay and other inshore waters at only certain times of the year.

Menhaden migrate up and down the East Coast from Florida to Maine. Adult menhaden arrive in April, and form schools in May. During the summer, large schools—containing hundreds and sometimes even thousands of fish—move in and out of Narragansett Bay and Greenwich Bay. They move south as water temperatures cool in the fall.

Why is temperature so important to fish? Humans and other mammals are able to maintain a constant core body temperature—98.6 degrees Farenheit in our case. But fish, and also marine invertebrates, cannot do this. When temperatures drop, all the body processes in a fish slow down. A fish may not be able to move quickly enough to get enough food. In some cases, fish will travel to warmer water to follow greater abundances of their food. Fish also require a very specific temperature in which to reproduce and will only spawn when the temperature is right.

On the other hand, if water is too warm, the body processes in a fish's body may speed up so much that is unable to get enough food to meet the increased food needs. If the temperature of water gets too high, the oxygen levels will drop, too, and the fish may not be able to get enough oxygen.