Reliable operant apparatus for fish: Audio stimulus generator, response button, and pellet-dispensing nipple

As part of ongoing research into the ability of koi to categorize complex a uditory stimuli, we have had to develop novel apparatus. The stimulus gener ator presents sound from two CD drives under computer control through a new underwater speaker. The operant manipulandum is a horizontal button that e liminates spurious triggering by water turbulence and problematic response topographies. The button's design has also been adapted for use by tilapia in an aquacultural food-preference study. The feeder uses a nipple to dispe nse food pellets reliably under water. In this paper, the apparatus is desc ribed in detail. Also discussed are methodological issues related to its de sign, as well as its usage in a pilot study in which koi learned to discrim inate music from silence by using a single manipulandum with food reinforce ment.