When the spring breeding season nears, the river rumba begins.
It is an eerie melody, heard from the Great Lakes to the Gulf of Mexico.
An angler on the water at night will often hear the peculiar combination of purring and grunting welling up from the depths, and wonder about its origin.
I remember when I first heard it. I was 14. A friend and I were trotlining in east Arkansas' L'Anguille River. It was near midnight.
We had baited our lines and laid back in our boat to catch some shut-eye until it was time to check the lines again.
I felt the sound before I heard it. One hand rested on the boat's metal bottom, and I was awakened by a curious vibration coursing through my fingertips.
The vibration stopped, then started, then stopped again. I awoke my companion.
"What's that?" I asked, pressing his hand to the bottom of the boat. My friend was as bewildered as I.
The sound grew louder. It was distinctly audible now, and, in minutes, similar sounds were emanating from the water all around us.
The bottom of the boat hummed as the deep notes passed through the metal molecules and set them atwitter.
We were not frightened by the noise, only curious. It was oddly soothing.
Neither of us had ever heard such a sound, and though we knew it must originate from some type of aquatic animal, we did not know what kind.
We listened throughout the night, intrigued by the strange notes carried in the current.
"What you heard is drum fish," my uncle told me the next day.
"When drum get ready to spawn, the males make that sound to attract their sweethearts. It may not sound pretty to you and me, but to the drum, it's a love song."
My uncle, who spouted many tall tales about wildlife, was correct in this instance. The sound comes from the male gaspergou, or freshwater drum, croaking away as part of its mating ritual.
A unique set of muscles contracts around the fish's swim bladder, causing the air-filled organ to boom like a balloon rubbed with the fingertips.
Fisheries scientists speculate that female drum, ready to spawn, swim toward the males they hear calling from a distance.
Some of the gaspergou's common names thunderpumper, croaker and bubbler are derived from this exercise of voice.
James Gowanloch commented in "Fish and Fishing in Louisiana" that "The members of this family are peculiarly able to produce quite vigorous sounds, so vigorous indeed that a school of drums, swimming past an anchored boat, can awaken a sound sleeper."
On Wisconsin's Lake Winnebago, the noise produced by big drum in June resembles "a motorcycle gang racing in the distance," according to one fisheries biologist there.
While fishing on the Rio Negro in Brazil, I learned that drum aren't the only fish that "sing."
Once again, I was catfishing, this time with Walter Delazari, a friend from Sao Paulo. The sun was setting.
As Walter and I waited for a catfish to find our bait, we laid back in our boat and watched hundreds of scarlet macaws flocking to a roost on the edge of the rainforest. The din created by these beautiful birds was almost deafening, but it was not loud enough to hide the sounds we soon heard coming from the river.
They began abruptly, weird little chirps and warbles almost like singing frogs. But these sounds originated deep in the water beneath us. The volume rose as more and more of the creatures began to sing their strange paean, and soon the metal hull of the boat began vibrating.
"Que é isso?" I asked Walter in Brazilian Portuguese. What's that?
A look of feigned fright crossed his face. "Espírito!" he said. Spirits!
It was only by coincidence that I learned the true origin of those sounds. While doing research on the Internet, I stumbled across a site about the 1993 Calhamazon Expedition, a scientific foray to study fish diversity in the Amazon River.
By clicking on a link, I was able to hear the sounds of South America's talking catfish recorded by expedition scientists. These fish create their loud croaking noises by grinding the base of their pectoral fin bones on their shoulder bones.
Most, if not all, of the 80 to 100 species in this genus can "talk" in this manner. Amongst these are some true giants (four feet plus), which must be mesmerizing to hear. You can hear them yourself by accessing the website. Scroll down the opening page and click on the link to "catfish sounds."
On another website devoted to the subject of fish acoustics, I listened to sounds of the gaspergou's saltwater relatives the red drum, Atlantic croaker and black drum and other fish such as the spotted seatrout, weakfish, silver perch and oyster toadfish.
I learned, too, that listening to the singing of these fish using special underwater microphones, biologists can determine changes in their abundance from season to season and year to year.
The scientific aspects of fish songs are important, no doubt.
But, for some of us, the songs are nothing more than lullabies.
Recently, while catfishing on the Mississippi River, I lay in the bottom of my boat, my ear pressed against the hull, listening to the love songs of 1,000 gaspergous.
The river rumba lulled me to sleep.
To contact Keith Sutton, email him at firstname.lastname@example.org