| Summary: | The replacement of fishmeal by alternative protein sources in feeds for farmed fish is important in the context of limited global availability and high price of industrial fishmeal. However, little is known about the effects of novel protein sources on the effluents produced by aquaculture operations. Controlled feeding experiments were therefore conducted using Atlantic cod (Gadus morhua) and European sea bass {Dicentrarchus labrax) offered feeds in which up to 35% by weight of the fishmeal was replaced by one of 7 different or differently processed protein sources. All plant proteins were found to be acceptable at the levels tested, without any negative impacts on growth or gastrointestinal tract histo-pathology in either species. Analyses of faecal particle size distribution, zeta potential (C, - particle surface charge characteristics) and rheology (fluid behaviour) were used to determine the physical characteristics of faeces from fish receiving these experimental feeds. Faeces produced by both fish species when fed experimental feeds contained a lower proportion of fine particles compared to fishmeal controls. However, when the actual volume of faeces was accounted for, most experimental feeds resulted in a greater volume of fine particles due to the higher total production of faeces. ζ analysis showed that all faecal samples are negatively charged at system pH, suggesting that there will be electrostatic repulsion under standard rearing conditions. The charge characteristic could be exploited by the addition of oppositely charged flocculants into the feeds; however, the incorporation of blood meal in feeds was not successful in accomplishing this. Rheological analysis demonstrated that the addition of plant proteins into the feeds increased the viscosity and stability of the faeces. Areas of further work are identified, including refinement of rheological methods and the investigation of more suitable flocculant additives, to help reduce the impacts of effluents on rearing systems and the environment.
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