Probiotic Applications in Cold Water Fish Species - Aquaculture Nutrition: Gut Health, Probiotics and Prebiotics - page 225

9.2 SALMONIDAE
Salmonids are one of the most important farmed fish groups; in 2011 salmonid aquaculture
production exceeded 2.7 million tonnes and was valued at over $14 billion. As such, con-
siderable information is available on the applications of probiotics for the main aquacultured
salmonid species (refer to Tables 9.1 and 9.2). Readers with a specific interest in salmonids,
beyond the information presented here, are referred to the review on probiotics in salmonids
of Merrifield et al . (2010a).
9.2.1 Rainbow trout ( Oncorhynchus mykiss )
The production of rainbow trout has grown exponentially since the 1950s, especially in
Europe and more recently in Chile. The largest producers are currently Chile, Norway, and to
a lesser extent Turkey, Denmark, Iran, France, Italy, USA, Spain, Germany, Poland, Finland
and the UK (Stevenson 2007). However, diseases have become a major limiting factor for
trout production. Traditional control strategies are based on the use of chemotherapeutic
agents and vaccination; however, the use of antibiotics should be limited due to the emergence
of antibiotic-resistant bacteria (Balcázar et al . 2006; Cabello 2006; Romero et al . 2012).
Although the prevention of disease by vaccination is increasing, the process is impractical and
often ineffective when applied to juvenile fish because they are not fully immunocompetent
and do not always respond to vaccination (Gram et al . 1999).
Probiotics have been considered a more environmentally friendly approach to disease man-
agement in these species, as they could compete with pathogenic bacteria. Different probiotic
bacteria have been tested for the control of the most important cold water bacterial pathogens,
including Flavobacterium psychrophilum (Ström-Bestor and Wiklund 2011; Burbank et al .
2011; 2012), Lactococcus garvieae (Brunt and Austin 2005; Vendrell et al . 2008), Aeromonas
salmonicida (Irianto and Austin 2002; Balcázar et al . 2007a), Vibrio ( Listonella ) anguillarum
(Brunt et al . 2007; Harper et al . 2011) and Yersinia ruckeri (Raida et al . 2003; Brunt et al .
2007). A summary of probiotic studies assessing disease resistance in rainbow trout is shown
in Table 9.2.
The most common mechanisms by which probiotics may prevent bacterial diseases are the
creation of a hostile environment for pathogens by the production of inhibitory compounds,
by competing for essential nutrients and adhesion sites, or by modulating the host immune
responses (Balcázar et al . 2006; Merrifield et al . 2010a). Previous studies have suggested that
competition for nutrients and adhesion receptors may provide protection against A. salmoni-
cida and Lc. garvieae after the administration of Leuconostoc mesenteroides in rainbow trout
(Balcázar et al . 2007b; Vendrell et al . 2008). The ability to colonize mucosal surfaces is an
important criterion when selecting potential probiotics (Balcázar etal . 2006; Vine etal . 2006).
Burbank et al . (2012) isolated two potential probiotics able to provide an alternative strategy
for managing F. psychrophilum infections, mainly mediated through mechanisms based on
colonization of fish skin and competitive exclusion at the site of infection. Similarly, Lacto-
coccuslactis subsp .lactis , Lactococcuslactis subsp .cremoris and Lactobacillussakei reduced
the adhesion of A. salmonicida and Y. ruckeri to intestinal mucus from rainbow trout (Bal-
cázar et al . 2007c). Brunt et al . (2007) demonstrated that Aeromonas sobria GC2 and Bacillus