Assessment of Biophysical Properties of Faecal Pellets from Channel Catfish ( Ictalurus punctatus ) and Bighead Carp ( Aristichthys nobilis )

Fish faeces are a crucial component of solid wastes from cage culture systems. In order to investigate the environmental impacts of faeces from channel catfish ( Ictalurus punctatus ) and bighead carp ( Aristichthys nobilis ), certain biophysical characteristics during faecal sinking at three temperatures (10, 20 and 30 °C for winter, spring-autumn and summer conditions, respectively) were assessed in the present study. Settling velocities of faeces from channel catfish (1.72–13.33 cm/s) and bighead carp (4.16–13.83 cm/s) accelerated with an increase in water temperature. For channel catfish faeces, there were positive correlations between settling velocity and physical properties, i.e., weight, volume, length and diameter; however, for bighead carp faeces, no linear relationship between settling velocity and length was found. The main faecal water absorption period for these two species occurred after 2.5 min of immersion. The main leaching period of faecal carbon and nitrogen was 0–2.5 min, and the leaching period of faecal phosphorus was 0–10 min. The nutrient contents in channel catfish faeces were significantly higher than those in bighead carp faeces. These results suggest that co-culturing channel catfish with bighead carp can effectively reduce the discharge of nutrients from aquaculture. The biophysical properties of these two types of fish faeces can also provide guidance in particle waste collection.