MICROBIAL AND MUSSEL PROTEIN MEALS AS NOVEL INGREDIENTS IN FEED FOR FARMED ARCTIC CHARR (SALVELINUS ALPINUS)
Introduction
Current production of finfish and crustaceans is largely dependent on fish meal as a protein source in compound aqua-feeds (Tacon and Metian, 2008). Fungal organisms, such as yeasts (Saccharomyces cerevisiae) and filamentous fungi (Rhizopus oryzae) could be suitable alternatives to fish meal as they can utilize a variety of substrates and have a high reproductive rate (Kiessling, 2009). The blue mussel, Mytilus edulis, is a filter feeder with a high capacity to remove nitrogen from aquatic environments, while also creating a high quality protein (Lindahl et al., 2005). The objectives of this study were to evaluate growth performance and nutrient utilization of Arctic charr (Salvelinus alpinus) fed intact and extracted S. cerevisiae, R. oryzae and M. edulis and to evaluate the digestibility of these feed components.
Material and methods
A 99-day feeding experiment was performed to evaluate four alternative protein sources for Arctic charr: intact yeast S. cerevisiae (ISC diet), extracted yeast S. cerevisiae (ESC diet), filamentous fungi R. oryzae (RHO diet) and blue mussel M. edulis (MYE diet). All diets were iso-nitrogenous, iso-energetic and produced by extrusion. The reference diet (REF) contained fish meal as the main protein source while 40% of the fish meal derived crude protein (CP) was exchanged with CP from the test ingredient in each test diet. The experiment was performed in 15 tanks (750L volume) on triplicate groups of 50 fish with average start weight 48.1 ± 8.9 g that were fed a restricted ration of 1% of their body weight. Body weight and length were recorded for each fish at the start, middle and the end of the experiment for calculations of growth. Surgical faeces striping was performed at the end of the experiment (five fish per tank) for determination of the apparent digestibility of the test diets. An additional five fish from each tank were sampled for determination of viscerosomatic index (VSI) and hepatosomatic index (HSI).
Results
Growth performance was not significantly affected (P>0.05) by substituting 40% of the fish meal with ISC or MYE. Fish fed diets RHO and ESC showed significantly lower growth performance both in terms of weight gain and specific growth rate than fish fed the REF diet. No differences in nitrogen and protein retention were observed between diets REF, ISC and MYE. Apparent digestibility coefficient (ADC) for crude protein was significantly lower for fish fed diets ISC (83.8%) and RHO (80.4%) than fish fed diets ESC (89.5%), REF (87.0%) and MYE (88.2%). The ADC for dry matter was similar for diets REF, ISC and ESC, but that for the MYE diet was significantly higher than for diets REF and ISC. There were no differences in ADC for gross energy between any of the treatments.
Discussion and conclusion
Unlike in similar studies up to date, the diets in our study were produced by extrusion, which may have caused partial disruption of rigid cell walls and consequently an increase in availability and CP retention. The results also demonstrated that extracted S. cerevisiae and R. oryzae reduce the growth performance of Arctic charr when used at current inclusion levels. This indicates that lower, graded levels of these ingredients should be tested in the future. In addition, the REF diet contained higher amounts of methionine than the other diets, which suggests that methionine supplementation may improve the growth performance of Arctic charr fed these test ingredients. In conclusion, 40% of CP derived from fish meal in diets for Arctic charr can be replaced with M. edulis or intact S. cerevisiae yeast without negatively affecting growth performance and nutrient utilisation.
References
Kiessling, A. (2009) Feed-the key to sustainable fish farming In KSLA- Fish, Trade and Development (Ackefors, H., Cullberg, M., Wramner, P. Sundberg ed.), pp. 303-322.
Lindahl, O., Hart, R., Hernroth, B., Kollberg, S., Loo, L.-O., Olrog, L., Rehnstam-Holm, A.-S., Svensson, J., Svensson, S. & Syversen, U. (2005) Improving Marine Water Quality by Mussel Farming: A Profitable Solution for Swedish Society. AMBIO: A Journal of the Human Environment, 34, 131-138.
Tacon, A.G.J. & Metian, M. (2008) Global overview on the use of fish meal and fish oil in industrially compounded aquafeeds: Trends and future prospects. Aquaculture, 285, 146-158.
