Biology Reference
In-Depth Information
larvae. In the fi rst case, microalgae are cultured in large tanks and when
optimal conditions are reached these tanks are stocked with fi sh eggs or
larvae (green water technique). In this case, production of microalgae takes
place in the same tank as the rearing of the larvae. In a more common
approach, production of microalgae takes place is separate culture tanks,
the microalgae are harvested regularly and added to the rearing tanks of
the larvae (pseudogreen water technique).
Use of microalgae has several implications for different aspects of
rearing of the larvae. Marine fi sh larvae drink seawater to osmoregulate
and ingest microalgae particles, which are accumulated in the gut. The
phenomenon of “green gut” has been shown in larval rearing of many
marine fi sh species (Cahu et al., 1998). Filling of the gut seems to have an
effect on the stimulation of production of digestive enzymes in the larval
gut (Hjelmeland et al., 1988).
In the case when fi sh larvae are stocked at a low density, average
residence time of live food organisms can be up to several days. Microalgal
cells are available for the live food organisms present in the tank and may
prevent the incidence of starved live food organisms (Makridis and Olsen,
1999).
Addition of microalgae changes the physicochemical parameters such
as light, colour of the tank water, the concentration of ammonia and other
harmful compounds, and reduces light intensity in the water mass and the
stress caused by direct light on the larvae.
Another aspect of addition of microalgae is that microalgae are normally
produced in non-axenic cultures and high numbers of bacteria are present
in the microalgae cultures (Salvesen et al., 2000). Microalgae are usually
added together with their growth medium, so bacteria from microalgae
cultures are added in the rearing tanks as well and may have an effect on
the bacterial communities in the rearing tanks. Bacterial strains isolated
from microalgae have shown
in vitro
antibacterial activity against fi sh
pathogens (Makridis et al., 2006). The antibacterial effect of microalgae can
be involved in a strategy to reduce the bacterial load in live feed organisms,
where rotifers or
Artemia
are incubated in microalgae cultures (Makridis
et al., 2006). It has been shown that an excessive bacterial content in live
feed may overload the bacterial load in the rearing system, and that larvae
avoid prey with high bacterial content (Pérez-Benavente and Gatesoupe,
1988).
Prebiotics
Prebiotics are generally non-digestible diet compounds which cause a
benefi cial effect on the host by selecting growth or activating metabolism
of one or a limited number of health-promoting bacteria (Gibson and
Roberfroid, 1995). Research on application of prebiotics to aquaculture
