Design and testing of a faeces-collecting device for fish digestibility studies using demand or automatic feeding

A system suitable for conducting fish digestibility studies, where animals are fed using demand and automatic feeders was designed. The system consists of a cone-shaped, bottom-tapered tank with two lower outlets, both fitted with solenoid valves that allow or prevent water passage into either a faecal or an uneaten food reservoir. Solenoid valves are electrically connected to the sensor of a self-demand feeding device. Sensor activation by a fish closes the solenoid valve controlling access to the faeces reservoir, while opening that of the uneaten food reservoir, for the time it takes for pellets to be removed from the bottom of the tank plus an additional 15-s safety time. This “activation time” has to be experimentally determined and adjusted based on the characteristics of the feed. After the activation time has elapsed, the system automatically closes the solenoid valve of the uneaten food reservoir while opening that of the faeces reservoir. The device was first tested using direct manual activation of the sensor, and then in the presence of goldfish (Carassius auratus) of about 100 g in weight. Two different tests were performed: a digestibility study with demand feeding, and a performance test with automatic feeding in light and dark conditions. From our results, it can be concluded that this system is an efficient tool for digestibility studies, where fish are fed on demand or with automatic feeding.