An evaluation of the complete replacement of both fishmeal and fish oil in diets for juvenile Asian seabass, Lates calcarifer
Introduction
The reliance of aquaculture on fishmeal as a protein source and fish oil as a lipid source has been recognised for a long time as a significant risk for the industry (Tacon and Metian, 2008). Over the recent past decades there have been a multitude of studies examining a range of different raw materials that have potential application in reducing reliance on these resources for aquaculture (reviewed by Gatlin et al., 2007; reviewed by Glencross, 2009). In assessing new and different raw materials, a series of key knowledge elements is required to enable their effective utilisation by the feed production sector. Those being the characterisation of the raw material, the determination of its digestible nutrient and energy value, before assessing palatability and utilisation value parameters (Glencross et al., 2007).
For barramundi (Lates calcarifer), there has been a significant volume of work examining elements of the raw material assessment process. Much of this work has focussed on either rendered animal meals (Glencross, 2011, Glencross et al., 2011, Williams et al., 2001, Williams et al., 2003a, Williams et al., 2003b) or feed grains (Glencross, 2011, Glencross et al., 2011, Glencross et al., 2012, Irvin et al., 2015, Ngo et al., 2015). In both cases it has been demonstrated that either rendered animal meals or feed grains can replace substantial amounts of fishmeal in diets for this species. However, it has also been determined that a critical threshold of 15% fishmeal was pertinent to barramundi, based on a diet balanced for digestible protein, energy and amino acids using a plant protein concentrate as the alternative (Glencross et al., 2011).
There has been less work on establishing the boundaries of fish oil replacement in feeds for barramundi, though there has been much work done on other fish species (Borlongan and Parazo, 1991). It has recently been demonstrated that the requirement for the long-chain polyunsaturated fatty acids (LC-PUFA) for barramundi is around 1% of the diet (Glencross and Rutherford, 2011, Salini et al., 2015). Other recent studies have demonstrated that it has been possible to replace all the fish oil in barramundi diets, so long as high inclusions of fishmeal were maintained and this level of LC-PUFA maintained (Alhazzaa et al., 2011).
In this study, it was hypothesised that the combined replacement of fishmeal and fish oil will be problematic. But it was not known if the thresholds for this were singular or interactive. Therefore, to address this question a factorial study of replacement in fishmeal (FM) and fish oil (FO) was undertaken. To push the boundaries on this question FM inclusion was reduced from 30% to 0% at 10% increments, while FO has one inclusion level at 100%, but the other treatments were at 30%, 15% and 0% of the added oil components. Importantly, each of the dietary formulations was based on the measured digestible value of each of the key raw materials that were used.
Section snippets
Experiment concepts
An experiment was conducted to examine the application of the complete replacement of fish meal and fish oil in diets for barramundi. Digestibility data from earlier studies using the same batches of raw materials were used to formulate the diets in this experiment (Glencross et al. unpublished), with the nutrient composition of those ingredients shown in Table 1. The experiment examined the productivity of fish grown when fed diets formulated with fishmeal levels ranging from 0 g/kg to 300 g/kg
Fish growth and feed utilisation
All treatments showed substantial increases in weight during the trial (Table 4), and all treatments more than doubled their initial weight (Table 4; Fig. 1). Weight gain was poorest by those fish fed the FM:20–FO:0 diet (gain of 167.1 g/fish) and best by those fish fed the FM:30–FO:15 diet (gain of 221.0 g/fish). However, the weight gain by the poorest performing fish was not significantly different from that of each of the FM:0 diets, which all performed sub-optimally relative to the remainder
Discussion
This study examined the co-replacement of both fishmeal and fish oil in diets for juvenile barramundi (L. calcarifer). The diets were all formulated on an equal digestible nutrient and energy basis, which has previously been shown to enable effective use of a wide range of alternatives (Glencross et al., 2011). Based on the diets being formulated on an equivalent digestible energy and nutrient basis, it was hypothesised that the main response by the juvenile barramundi would be towards the
Statement of relevance
This study demonstrates clear potential to replace almost all the fishmeal in barramundi diets without loss of performance. Replacement of fishoil was more successful with the complete replacement of all fishoil demonstrated at all but the lowest inclusion levels of fishmeal. Results demonstrate that the near complete replacement of both fishmeal and fishoil in barramundi diets is a technical reality.
Acknowledgements
The amino acid analyses work by Malcolm McGrath of the West Australian Animal Health Laboratories is gratefully acknowledged. Thanks also to Nick Polymeris, Natalie Habilay, Kiname Salee and Jake Goodall for technical assistance. We also thank Mat Cook for reviewing a draft version of this manuscript.
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