Biology Reference
In-Depth Information
drastically reduced the use of chemotherapeutics in aquaculture, increasing
the survival of farmed fi sh. However, fi sh larvae have an undeveloped
immune system, relying on maternal antibodies and non-specifi c immune
response, which prevent the use of vaccination (Vadstein, 1997).
Usual approaches to control bacterial growth in intensive rearing
of marine fi sh larvae are based on preventive measures, leading to the
development of a clean environment by water treatment processes (e.g.,
fi ltration, UV-irradiation, ozonization and disinfectants). Disinfection
of fi sh eggs (Salvesen and Vadstein, 1995), and measures for control of
bacterial microbiota in live feed such as, disinfection of rotifer eggs for
the production of axenic cultures (Douillet, 1998; Dhert et al., 2001),
treatment with hydrogen peroxide (Giménez et al., 2006), or ultraviolet
radiation for partial decontamination (Munro et al., 1993, 1999) have
been proposed, but would be diffi cult to implement at the industrial
scale. Complete elimination of bacteria from the organisms and culture
system is not possible, while disinfection implies in most cases a loss of
a stable microbial population, with dominance of slow-growing bacteria
(
K
-strategists) (Pianka, 1970; Andrews and Harris, 1986). Disturbance
of these stable bacterial populations may promote, in a nutrient-rich
environment (due to feed and excretions) as are larval culture systems,
the rapid colonization by opportunistic bacteria (
r
-strategists), with
high growth rates and potentially harmful to fi sh larvae (Skjermo and
Vadstein, 1993).
Therefore, at present, efforts have been made towards new strategies
for the control of bacterial microbiota in the rearing systems that, by
avoiding the use of antibiotics and disinfectants, would lead to a more
environmentally friendly and sustainable aquaculture. This approach
would be in accordance with the Ecosystem Approach for the sustainable
growth and expansion of aquaculture, promoted by FAO in The State of
the World Fisheries and Aquaculture (FAO, 2007).
This chapter discusses these new strategies. Most of them have been
proposed or developed at the laboratory scale and some of them in pilot-
plant trials. Only a few have been applied at an industrial scale.
BACTERIA IN FISH LARVAE
Fish larvae at hatching are commonly colonized by very few bacteria,
which may originate from the egg surface, or from the surrounding
water. During the yolk-sac stage, marine fi sh larvae drink seawater to
osmoregulate. They thus accumulate bacteria and microalgae in their gut
(Reitan et al., 1998). The numbers of bacteria during this phase of rearing
are low [<100 CFU (Colony Forming Units) per larva], whereas the
biodiversity of microbiota can be quite high (Hansen and Olafsen, 1999).
