Delta Journal

by Bob Thomas

One of the strangest looking fish in coastal Louisiana is the paddlefish (or, spoonbill), Polyodon spathula.

It lives in the Mississippi River and its tributaries and distributaries. It is almost always found in rather murky water associated with river flood plains.

There is only one other member of its family (Polyodontidae), and that species occurs in the Yangtze River of China.

Folks say the white meat of paddlefish tastes like chicken (just kidding; they say it is like salmon or swordfish). Its eggs (roe) have been an important source of caviar. The species is protected in all states, and cannot be caught at all in Louisiana (must be immediately released if accidentally caught). Since it is a filter feeder, it does not take a baited hook, so, in states with a legal season, it must be snagged. They are also caught in gill nets, though this practice is illegal in most (if not all) states.

The paddlefish has smooth skin like a catfish; a cartilaginous skeleton like sharks; a long, flat snout; and an absolutely huge mouth. Though all these features are weird, some are adaptations for the paddlefish’s peculiar way of life - that of a filter feeder. Yes, the paddle fish, which grows to over 6 ft long and 150 lb, feeds on plankton, especially the water flea (Daphnia).

How do the characters listed above help the paddlefish survive? The smooth skin that is covered with a slimy mucus helps in swimming efficiency. The cartilaginous skeleton indicates that it is a rather primitive species.

The long snout is covered with little electroreceptors that can detect tiny electric fields (less than 1/1,000,000,000 volt per centimeter). This is important because it allows the fish to find a single water flea, a necessity for young fish that feed on plankton one at a time. For the adults, the snout works like a plankton radar dish as the fish swims through its murky water habitat. Many think the flattened snout also helps stabilize the fish as it swims about with its large mouth open.

As mentioned above, small paddlefish catch their prey one at a time. But as the fish grows, it needs more food to sustain its activities. The gills of paddlefish have long, thin gill rakers that look like combs. When the fish opens its mouth wide and swims forward, large quantities of water flow into the mouth, through the gills and their rakers, and out through the gill covers (operculum). All plankton in the fish’s path are strained out on the comb-like gill rakers and swallowed by the fish. A paddlefish spends most of its time swimming with its mouth open, continually swallowing plankton. Only in this way can such a large fish live on such small individual food items.

A Note on Habitat: Paddlefish live in rivers that have flood plains (large areas that routinely flood after heavy rains or snow melts). The reason this habitat is important for a filter feeder is that during the dry period, leaves and other organic materials build up on the ground, then wash into the river when the floods occur. This abundance of organic matter feeds the lower end of the food chain (the plankton) and makes it easy for paddle fish to make a living.

As we dam and levee our rivers, the flood plains disappear, so the rivers do not have amounts of organic material required to sustain a large population of plankton. Of course, this leads to the demise of the paddlefish. In many areas, they have become scarce.

Simply protecting flood plains enhances the lives of many species of animals that are very interesting to behold.