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New strategies for the control of bacterial infections in marine fish larval rearing 3
1. INTRODUCTION
New strategies for the control of bacterial infections in
marine fish larval rearing
One of the limiting steps of marine fish aquaculture expansion is the supply of
juvenile fish. Intensive fish larvae production is highly susceptible to the proliferation of
bacteria, which may cause poor growth or mass mortality of the larvae. In most cases,
mortality cannot be attributed to a specific pathogen, but to the proliferation of
opportunistic bacteria (Olafsen, 1993, 2001). The rearing environment, with high larvae
densities and high load of organic matter (from faeces, dead larvae debris or from live
feed) is highly susceptible for bacterial growth. The control of bacteria in rearing
systems and live feed production is a determinant factor for survival of fish larvae
(Dhert et al., 2001; Planas and Cunha, 1999; Skjermo and Vadstein, 1999) and the
increment in the survival of larvae treated with antibiotics supports this fact (Gatesoupe,
1982, 1989; Pérez-Benavente and Gatesoupe, 1988).
In aquatic environment, the use of antibiotics induces the development of
resistances, which can be transferred to other bacteria, including pathogenic strains to
fish or even humans (Cabello, 2006). Therefore, the use of antibiotics should be kept to
a minimum. Vaccination of juveniles has drastically reduced the use of
chemotherapeutics in aquaculture, increasing the survival of farmed fish. However, fish
larvae have an undeveloped immune system, relying on maternal antibodies and non
specific immune response (Vadstein, 1997), which prevents the use of vaccination.
Usual approaches to control bacterial growth in intensive rearing of marine fish
larvae are based on preventive hygienic measures, leading to the development of a clean
environment, by seawater treatment processes (e.g. filtration, UV-irradiation,
ozonization, disinfectants). Disinfection of fish 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 (Dhert et al., 2001; Douillet, 1998), 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 difficult
to implement at industrial scale. Complete elimination of bacteria from the organisms
and culture system is not possible, while disinfection implies in most cases the loss of a