Rainbow trout fed diets with varying content of marine and plant origin; how does that influence the outcome of experimental infections of the fry with Flavobacterium psychrophilum and Yersinia ruckeri?

Summary

Feed for rainbow trout aquaculture has traditionally been based on marine resources such as fish meal and fish oil. Because of a shortage of marine resources as well as the growing production of farmed fish, the feed industry has been forced to partially exchange fish meal protein with proteins derived from plants, like soy bean meal. This has been shown to affect the salmonid intestinal mucosa, and in addition, plant-based dietary proteins have been associated with changes in disease susceptibility in salmon and it has been suggested that these special diet types weakens the immune status of the fish.
One major cause for losses in Danish freshwater fish farms is the fry disease rainbow trout fry syndrome (RTFS), caused by the bacterium Flavobacterium psychrophilum, and experiences of the fish farmers suggest that the diet type is an important factor for disease development. Enteric redmouth disease caused by Yersinia ruckeri is also an economically important disease which causes problems in rainbow trout fry as well as larger fish.
Rainbow trout were fed from first-feeding with five different diets; diet A with marine fish oil (conventional fry diet), diet B (an organic version of A), diet C with rape seed oil (like B but with rape seed oil exchanging marine fish oil), diet C with pea protein (like B but added pea protein) and diet E with rape seed oil and pea protein. When the fry had reached sizes 1.5 g and 4 g, groups of fish from the five diet groups were infected with Flavobacterium psychrophilum and Yersinia ruckeri, respectively. An intraperitoneal injection model was used for F. psychrophilum, whereas a bath challenge was used for Y. ruckeri. Before and after infection, samples were taken from internal organs including the intestine for traditional bacteriology and only intestinal samples for next generation sequencing. The cumulative mortalities among the diet groups did not differ between groups in either of the two infection trials, suggesting that the diets did not have different effects on the immune status of the fish, when it comes to survival after infection, meaning that plant content did not seem to weaken the immune status of the rainbow trout fry. Results of the trial as well as the bacteriological examinations and the next generation sequencing results will be presented.