Influence of genetic adaption of rainbow trout (Oncorhynchus mykiss) fed with alternative protein sources based on Arthrospira platensis and Hermetia illucens on disease resistance against viral haemorrhagic septicaemia virus (VHSV)

Abstract

Regarding feed components, novel protein sources in the diet of carnivorous fish species are of increasing interest in aquaculture, which is caused by the fact that the use of marine resources is still being exhausted to meet consumer demand for fish. However, the renouncement of marine resources represents a decisive step towards a more sustainable aquaculture. Alternative solutions can be found in plant-based fish nutrition. Nonetheless, due to challenges related to food conversion, growth rates, fish health and welfare as well as the consequent weakening of immunocompetence against pathogens, the use of plant-based protein sources in the diet of carnivorous fish remains a subject of justified controversy. Thus, the current study aims at making use of the genetic variability of rainbow trout (Oncorhynchus mykiss) in order to gain new insights into their adaptability to innovative raw materials. Using a viral haemorrhagic septicaemia virus (VHSV) infection model, the particular focus is on the question how infections affect the disease resistance of differently bred and fed populations. For this purpose, genetically distinct trout populations (R3, R7, R8) were bred and fed with differently composed diets, in which 20 % fishmeal of the total composition was totally replaced by Arthospira platensis or Hermetia illucens. The infection experiment included a commercially available trout line (C9) and a diet with fishmeal as control as well. Subsequently, the trout were infected with VHSV, and tissue of the posterior intestine was investigated by means of molecular biology regarding the expression of genes encoding various inflammatory markers, antimicrobial peptides, transmembrane proteins as well as the viral load at days 0, 2 and 4 post- infection. The posterior intestine of infected animals was examined histologically by using AB-PAS and HE- staining. The results showed that the genetics played a decisive role in the disease resistance to VHSV. Diet composition, however, did not negatively affect the susceptibility to VHSV. On the contrary, an upregulation of certain antimicrobial peptides in Arthrospira platensis fed specimens appeared to partially improve immuno- competence. Also, no correlation between the ability of rainbow trout populations to adapt to a novel diet based on proteins of Arthrospira platensis or Hermetia illucens and pathogen resistance were observed. Therefore, Arthrospira platensis and Hermetia illucens appear to be valuable protein sources regarding the replacement of fishmeal for rainbow trout without negatively influencing the immunity and natural resistance of certain rainbow trout populations to important pathogens like VHSV