Atlantic salmon in salt water weighing around 640 grammes were given either a feed containing mainly fish meal as protein source or a feed containing a high level of a plant protein mixture (85 per cent of the protein) with soy, wheat and corn gluten.
The plant protein feed also contained some fish meal, fish protein concentrate and squid, corresponding to totally 15 per cent marine protein. The trial period was three months. Squid was added to the plant protein feed in order to enhance taste, but the salmon still ate less of this feed.
- The Atlantic salmon in this study seemed to adapt to the plant protein diet by an alteration in the growth hormones, says researcher Ernst Morten Hevrøy at NIFES.
- We measured the gene expression of several growth hormones in the brain, liver and muscle. The growth hormones are naturally present in the fish` organs. There was a clear tendency towards down-regulation of the growth hormone IGF-I in white muscle, but not in the other organs.
- It is therefore very likely that less of this hormone is produced when the salmon is fed a high proportion of plant protein. In our study, this resulted in 31 per cent less protein deposited in the salmon, says Hevrøy.
Previous research, where Atlantic salmon has been given plant-based feed for a whole year, has shown a satisfactory growth, but still a lower growth than when Atlantic salmon was fed a feed based on fish oil and fish meal. The slightly lower growth found in previous studies with high levels of plant proteins (80 per cent) in the diet has been mainly been explained by a lower feed intake.
- The present study shows that farmed salmon does not achieve maximum growth or feed utilisation when the level of plant protein in the feed exceeds a certain limit. This can be explained by the slightly lower feed intake and the change in the hormones which control growth, says Hevrøy.
- Salmon which were given feed with a high level of plant protein achieved 25 per cent less growth than salmon which were fed traditional feed with a higher level of fish meal. The composition and quality of the plant protein are factors which may affect feed intake and growth. Future studies will show whether it is the plant-based diet or the reduced feed intake that contributes most to the reduced growth.
First study of its kind
This study differs from previous ones in that both the control feed and the plant protein feed contained the same amount of the different essential amino acids and the same balance of fat, carbohydrates, proteins, trace elements and minerals, even if the raw plant material sources were different.
- The Atlantic salmon in both groups thereby had access to the same quantity of nutrients. This kind of feeding trial with a link to endocrine growth regulation has not previously been published on Atlantic salmon, says Hevrøy.
Important to ensure good utilisation of marine raw materials Analysis of the amino acid composition in the salmons’ organs showed that more of the amino acids were used for maintenance than growth in Atlantic salmon fed a high level of plant protein.
- The study shows that it is important to compose a fish feed which gives the best possible utilisation of the marine proteins. This is especially important in the light of current developments where fish meal to a less or greater extent is being replaced by plant protein in grow-out feed for Atlantic salmon, says Hevrøy.