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RESEARCH PRODUCT

Complementary approaches for assessing protein sources in fish feed

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Fluctuating fishmeal (FM) prices and declining marine fish stocks have led to the current trend of replacing dietary FM with plant proteins (PP) in fish feed. PPs offer challenges both with respect to nutritional quality and the occurrence of antinutrients. The assessment of utilisation efficiency of PPs for growth is important for the sustainable production of fish feed. The overall aim of this thesis was to investigate the applicability of stable isotope analysis in combination with respirometry in the evaluation of dietary PPs for juvenile rainbow trout (Oncorhynchus mykiss). The dietary PPs were corn gluten meal (CGM), rapeseed protein concentrate (RPC) and potato protein (PoP). Complete replacement of FM with CGM was feasible without adversely affecting the metabolic oxygen consumption (MO2) of the trout. However, the growth of fish and apparent digestibility coefficient (ADC) of CGM and/or RPC decreased with their increasing dietary proportion through which the nutritional quality of the diets was compromised. The growth reduction may be caused by lower nutritional quality and antinutrients of the PPs. ADC of CGM and/or RPC was estimated from the faecal nitrogen isotope ratio (δ15N). There was a strong negative correlation between diet–tissue shift (∆15N) of δ15N values and dietary protein quality, and these correlations observed in muscle, liver, mucus and faeces were similar. Carbon and nitrogen turnover in muscle and liver of fish fed with corn and potato diet were estimated using stable isotope mixing model. This thesis only suggests the potential applications of stable isotope analysis and respirometry in fish nutrition research instead of providing an exact replacement level of FM. Stable isotope analysis cannot replace conventional assessment of feed ingredients and feeds, but it will bring new insight into the field of fish nutrition research provided the preconditions for using stable isotope analysis are met. In addition, development of non-lethal approach is possible by measuring the isotope ratio of faeces and mucus.