[MARMAM] Hydrophones

Greetings, I was asked about hydrophones and its prices for monitoring riverin dolphins in the Amazon. Could someone help me with good equipment for such use and costs of these apparatus. Thanks in advance. Mauricio Carrasquilla Marine Biologist Universidad Jorge Tadeo Lozano, Colombia, SA Masters in Natural Resources and Environment (C) Instituto Politécnico Nacional (MX) mauricio.carrasquilla@gmail.com