Backyard Biology

Nature stories from my backyard and beyond

Nature Stories


Monday, March 17, 2014

As a child swimming in the murky waters of the Fore River in Weymouth, that one word was enough to send half us kids scurrying to shore. And the sight of one emerging from the depths would keep many a young snorkeler on shore for the rest of the day.
As with so many things in nature, that which seems so scary when young proves to be merely fascinating as an adult. And the Eel is certainly one fascinating fish.
Many people are surprised to learn that the eel is a fish! It looks much more like a snake. But like other fish, it has fins for swimming, gills for breathing and slime covering its body. Lots of slime. Unlike most fish, however, eels either lack scales or they are so small as to be unnoticeable.
Almost all fish come in three main varieties: saltwater fish, freshwater fish and anadromous fish. Anadromous fish are saltwater fish that run upstream into freshwater to spawn. In our watershed they include herring, shad, salmon and smelt. Alone of all our fish, the eel is catadromous, a freshwater fish that spawns in the ocean.

Eels live in the rivers and ponds of our watershed. Most of these are females. (the majority of the males live south of the Chesapeake Bay in Virginia). All of the eels found in the watershed are young, sexually immature eels. They may spend anywhere from ten to twenty years growing before they head out to sea. They have voracious appetites, considering anything they can swallow as legitimate food. This includes animals both living and dead. They are true scavengers.
Eels are primarily gulpers. They open their mouths quickly, sucking in both water and food. However, since their mouth is rather small, it limits the size of the food they can gulp. To eat larger items, an eel will grab the food in its jaws and shake, much like a dog shaking a rag doll. If this doesn’t tear off a bite-sized piece, the eel might resort to spinning. For example, an eel might grab the claw of a crayfish and roll over until the claw breaks off. One researcher counted up to fourteen spins per second!

Eels are nocturnal fish, feeding mostly at night and burying themselves in the bottom during the day. Being pointed at both ends, the eels can bury themselves both tail first or headfirst.
As they mature, they stop eating and change from their normal greenish-yellow color to a silvery bronze. Now it is time to head downstream and out to sea for the big event of their lives - mating. At this point they may be 2-3 feet long, although larger ones have been found. This migration to the sea takes place in the autumn and the eels will travel at night and hide during the day. During this migration they must change from a freshwater fish to a saltwater fish. If you catch a silvery looking eel in the autumn in the brackish or salt water of the North or South Rivers, you have caught an adult eel returning to its spawning grounds in the ocean. Observe it closely before you release it, for that eel will never be seen again!

Once the eels reach the ocean, they disappear. Until recently no one knew where he or she went to spawn. What was known is that after spawning, they die. No large eels have ever been seen migrating back upriver.
The eels spawning ground was finally located in the 1920’s by a Danish fisheries biologist named Johannes Schmidt. After twenty-five years of study, he noticed that the eel young or larva were smallest in the Sargasso Sea, southeast of Bermuda. Today, even though it is known that the eel’s spawn in the Sargasso Sea, no one has ever witnessed this event!
The baby eels, called Leptocephalus, are flat and colorless, shaped like a leaf. They slowly migrate north with the currents and by the time they reach the mouth of the river in the autumn, they are about 3 inches long. At this point, they shrink somewhat and round out. Still clear at this stage, these “glass eels” will eventually turn a dark color into the “elver” stage and begin their migration upstream in the spring. Once in freshwater, they will settle down as a freshwater fish for the next 10-20 years until it is their turn to make the trip to the Sargasso Sea.
girl with bucketIt is possible to view the elver migration back upstream using an easy-to-construct underwater viewer. Cut the bottom out of a plastic bucket and glue a piece of clear plexiglass in its place, making sure of a watertight seal between the bucket and plexiglass. Submerge the bucket halfway and look through to the riverbottom below for the migrating elvers.

Although much is known about eels, many questions remain. Why do males and females live in different parts of the country? How do they find their way to the breeding grounds? What are their mating rituals and why do they die after mating? Are the elvers migrating upstream descendants of eels that came from our watershed, or did they pick the mouth of the North River strictly by chance? As with most things in nature, what is not known is just as fascinating as what is known.