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cultures. The elimination of bacteria from live prey may favour a more rapid colonization by
opportunistic bacteria with high growth rates (
r
-strategists) when the live feed is introduced
into the larval rearing system. Moreover, the use of disinfectants or antibiotics may enhance
the growth and proliferation of
Vibrio
spp. (Høj
et al.
2009), which could indicate a higher
resistance of
Vibrio
to some treatments.
16.4 ENRICHMENT OF LIVE FEED AND MICROBIAL
IMPLICATIONS
Before delivery to fish larvae, live prey are usually enriched with essential fatty acids (PUFAs)
by the addition of microalgae (e.g.
Tetraselmis
sp.,
Isochrysis
sp.,
Rhinomonas
sp. and
Rhodomonas
sp.) or emulsions rich in HUFAs such as EPA and DHA. Enrichment greatly
improves the viability of first feeding larvae (Reitan
et al.
1993; 1998; Bergh
et al.
1994).
Addition of microalgae into the rearing tanks (green water technique) results in reduced
proliferation of opportunistic bacteria (Salvesen
et al.
1999). When the microalgae are added
together with their growth medium, large amounts of bacteria associated with the microalgae
cultures are also added (Makridis
et al.
2009). As described in Section 16.2.1, algal cultures
have been associated with specific bacterial populations (Nicolas
et al.
1989; 2004; Sandaa
et al.
2003; Schulze
et al.
2006), which might influence bacterial numbers and the microbiota
in live feeds and fish larvae (Olsen
etal.
2000). Algae can also influence the grazing of bacteria
by live feeds (Nicolas
et al.
1989).
There are several mechanisms by which microalgae influence the microbiota associated
with live prey: (1) the effect of the microbiota associated with microalgae, (2) the effect
of oxygen radicals produced during the process of photosynthesis, and (3) the effect of the
microalgal cells. As mentioned earlier in this chapter, bacteria isolated from microalgae cul-
tures have shown inhibitory abilities against fish pathogens (Makridis
et al.
2006), indicating
that these bacteria may play a significant role in the modulation of bacterial communities
present in a rearing system. During photosynthesis, oxygen radicals with a general antibac-
terial activity (Marshall
et al.
2005) are produced. Oxygen radicals may have a modulatory
effect on the species composition of bacterial communities in the system. Under certain con-
ditions, some microalgae cells show antibacterial activity against specific groups such as
Vibrio
spp. by production of antibacterial compounds such as superoxide (Marshall
et al.
2005).
This has been demonstrated in microalgae cultures, thereby excluding the effect of associated
microbiotas (Kokou
et al.
2012). The presence of axenic algae (
Tetraselmis chui
,
Chlorella
minutissima
,
Nannochloropsis
sp.,
Arthrospira platensis
sp., and
Isochrysis
sp.) inhibited the
growth of all
Vibrio
species (
V. parahaemolyticus
,
V. anguillarum
,
V. splendidus
,
V. scoph-
thalmi
,
V. alginolyticus and V. lentus
) tested compared with the control treatments, in which
all
Vibrio
bacteria tested were able to utilize the microalgae medium to a high degree.
16.5 PROBIOTICS IN LIVE FEED PRODUCTION
Probiotics constitute a potential tool in the reduction of mortalities in the rearing of aquatic
organisms (Gatesoupe 1999; Verschuere
et al.
2000a; Gram and Ringø 2005; Vine
et al.
2006;
Kesarcodi-Watson
etal.
2008; Dimitroglou
etal.
2011). In the rearing systems, where host and
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