The amazing Amazon River

It’s not often that you can combine the very best in sports fishing with field work and be on the most iconic of the world’s rivers at the same time. That’s what I experienced on the Amazon River in January 2011. Not only was it a trip of a lifetime, it was a life changing experience that confirmed my decision to make a late life career change from veterinary practice to ecology-based field work. I had the good fortune to spend two weeks on the 12 berth river boat Dorinha, with owner and captain Mo Soares at the helm, exploring the Rio Negro and the Rio Solimoes.

Dorinha	Riparian jungle along the Rio Negro banks

On board were a diverse bunch of ecologists and conservation biologists from the US, Australia and Brazil, all keen to do a little sampling of this mighty waterway and its tributaries and oxbow lakes (and of course a little Peacock Bass fishing on the side.) The Rio Negro had dropped to a historically low level the preceding dry season and the wild rice (Oryza glumaepatula) that would normally grow close to the river banks had consequently experienced an explosion of growth. This species of rice only germinates when the river bed becomes parched after a long dry season. When the river starts to rise the rice enters an aquatic phase, the plants elongating their internodes, and then their brittle culms easily separate forming free-floating rafts. These large rafts of plant material provide food, shelter and places to breed for large numbers and species of fish. It was here that I had the chance to see a technique for presence sampling that I hadn’t seen before.

Gymnotiform fish, also known as Knife Fish, are a South American order of fish characterized by their ability to generate an electrical field. For most species the field is weak, only a few millivolts, and is used for navigation, signaling and sexual selection (females show a preference for males that produce energy expensive pulses of lower frequency and longer duration).

Knife Fish	Knife Fish habtiat

The Electric Eel is an exception; their electrical discharge is very capable of stunning prey. Each species has a unique signal that varies in frequency, amplitude and waveform. The difficulty in sampling for knife fish is the enormous bulk of wild rice in which they spend their time. Running a seine net through this material is hard work. Instead we were able to detect presence and assemblage structure simply with the aid of a set of electrodes and a small loudspeaker. Experienced ears were able to distinguish the signal to species level when the electrode tips were dipped into the water and the signal converted to an acoustic one. It certainly was more enjoyable than seine netting!

Tony Pusey