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Feed for Thought: From carnivores to herbivores

An all-plant diet for fish has been a decade-long aquaculture sustainability agenda. In the past decade, aquaculture has become more receptive to using plant-based ingredients as protein sources or supplements in diet formulations, and produce cost-effective feed. 

Soybean meal is one of the most popular plant-proteins used in aquafeed. Unfortunately, some species of fish, especially carnivores like salmonids, do not perform very well on diets with high soybean meal content. 

In the process of producing soybean meal, soybeans undergo roasting prior to the grinding step. The main purpose of this is to eliminate anti-nutritional factors that might have adverse effects on fish welfare. Despite the roasting step, some of the anti-nutritional factors (including but not limited to trypsin inhibitors, phytates, glycinin, and oligosaccharides, among others) remain in the beans. 

These factors were shown to cause enteritis in fish, and inflammation of the small intestine that is quite similar to Crohn’s disease in humans. Enteritis deteriorates fish health and results in poor growth. Luckily, habits can impact us on a genetic level in the long run. Having been exposed to soybean meal repetitively and for a long time in experimental setups, some fish exhibit genetic changes that help them cope with the change in their diet. 

A research team from the University of Idaho, in collaboration with the U.S. Department of Agriculture-Agricultural Research Service (USDA-ARS), was able to develop a “vegetarian” (herbivorous) strain of rainbow trout, employing genetic selection rather than gene modification. Using fish from the eighth generation, the team compared the expression of inflammatory genes in trout offered a soybean meal free diet and a 40 per cent soybean meal diet.

The approach
The laboratory was able to develop a strain of rainbow trout – named Hagerman strain (after the laboratory) that is selected for growth on a 100 per cent plant-based diet comprised of around 20 per cent soybean meal and close to 26 per cent soy protein concentrate. The Hagerman strain is currently in its ninth generation and is characterized by good growth and resistance to developing distal gut enteritis.

Two diets were formulated to meet the dietary requirements of rainbow trout at 40 per cent protein and 20 per cent lipid. The first diet did not contain any soybean meal, whereas the second contained 40 per cent soybean meal. Then, the diets were randomly assigned to 12 tanks, each stocked with 30 fish (either Hagerman strain from the eight generation or a commercial strain). 

Fish were offered feed twice a day to apparent satiation, and feed was quantified. The experiment was performed over 12 weeks, during which the fish were weighed and counted every four weeks. 

Additionally, five fish were euthanized from each tank at each weighing in order to collect distal gut samples for gene expression analysis and histology. At termination, fish were weighed, counted, and aside from the fish euthanized for distal gut collection, three fish per tank were euthanized for whole-body protein, lipid, energy, and moisture analysis. 

The remaining fish were returned to the tanks and offered feed in order to collect feces for nutrient digestibility analysis and microbiome analysis, for which feces were collected 16 hours post feeding. 

Growth was expressed in terms of final biomass, weight gain, and specific growth rate (SGR). Survival was also assessed, and the condition factor (K) was calculated in order to evaluate fish body condition. Feed intake was reported, the feed conversion ratio (FCR) was calculated, and fish metabolism and body status were evaluated using hepatosomatic (HSI) and viscerosomatic indices (VSI), in addition to fillet yield. 

Symptoms of enteritis were evaluated histologically by measuring the thickness of the lamina propria and the submucosa – those layers of the gut that become thicker when inflamed, hence hampering nutrient absorption. Inflammatory genes and genes involved with immune cell regulation were also studied. 

The outcome
Results of the present work indicated that the Hagerman strain grew better than the commercial strain, regardless of the diet offered. Also, the vegetarian strain had a smaller FCR than the commercial strain. 

That means the new rainbow trout strain not only can grow well on cost-effective plant-based diets, but doesn’t need much feed to grow, hence is cheaper to culture. 

Results of the present trial also suggested a reduction in the inflammatory response of Hagerman strain rainbow trout even when offered a diet rich in soybean meal.

Taking into consideration that aquaculture today covers more than half of our seafood needs, it comes as no surprise that sustainability is an important aspect of the industry. But when we think of sustainability, our thoughts often jump to the more conspicuous aspects of it, such as feed, waste, and energy, and we tend to forget to look “deeper” into the fish itself. 

And yet the genes are there, and genetic selection might very well be the way forward to increased production of naturally carnivorous fish such as rainbow trout. The future of sustainable aquaculture production lies in combining genetic selection with cost-effective feed, sustainable ingredients, and smart formulations. 

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References
Full scientific article citation: Blaufuss, P. C., Bledsoe, J. W., Gaylord, T. G., Sealey, W. M., Overturf, K. E., & Powell, M. S. (2020). Selection on a plant-based diet reveals changes in oral tolerance, microbiota and growth in rainbow trout (Oncorhynchus mykiss) when fed a high soy diet. Aquaculture, 525, 735287. https://doi.org/10.1016/j.aquaculture.2020.735287

[1] The State of World Fisheries and Aquaculture 2022. (2022). FAO. 

[2] Marandel, L., Seiliez, I., Véron, V., Skiba-Cassy, S., & Panserat, S. (2015). New insights into the nutritional regulation of gluconeogenesis in carnivorous rainbow trout (Oncorhynchus mykiss): A gene duplication trail. Physiological Genomics, 47(7), 253–263. https://doi.org/10.1152/physiolgenomics.00026.2015