Biology Reference
In-Depth Information
relation to its relatively small volume. These surfaces offer a new substrate
for bacteria present in the water and thereby enhance the proliferation
of opportunistic bacteria. Immediate use of this non-mature water has
often led to mass mortalities during the rearing of marine fi sh larvae. The
fi nal aim of matured water is to reduce the percentage of opportunistic
bacteria in the infl ow water and thereby in the rearing system. A strategy
has been developed where seawater was retained in a large container
where material with high surface to volume ratio was placed under strong
aeration for a long time period (optimally several weeks). Seawater is
fi ltered through a membrane and the total numbers of bacteria are thus
heavily reduced. The infl ow rate of seawater and thereby the supply of
nutrients is kept quite low inducing the proliferation of slow-growing,
K- strategist bacterial species.
This approach involves two steps. At fi rst, a biofi lm is formed on the
surface of the material, at the interface with water, where bacteria are
established at a high density, thereby favouring K -strategists species. At
the same time, a high bacterial load should be avoided in seawater in the
rearing tanks. The second step of this strategy is the transfer of biofi lm
members from the biofi lm to the water. A residence time of at least 24 h
is considered necessary prior to the use of matured water in the rearing
of marine fi sh larvae. A high residence time is accomplished by the use of
large containers in combination with low infl ow/outfl ow rates.
The basic principle involved in the use of matured water is that, as non-
opportunistic and less harmful bacterial species are at an increased level in
infl ow water, they will prevent the proliferation of harmful opportunistic
species. Slow-growing bacteria will thereby be present in high numbers
during the fi rst days after hatching and may have a competitive advantage
over opportunistic bacteria, which proliferate at later stages of rearing.
Matured water has been successfully applied in several experiments
with marine fi sh larvae, as in the case of incubation of Atlantic halibut
yolk-sac larvae (Skjermo et al., 1997) and with fi rst feeding of turbot larvae
(Salvesen et al., 1999). It should be noted that in recirculation systems, there
is an analogy with the matured water system, as there is material with high
surface to volume ratio in the biofi lter, which offers the necessary support
for the establishment of a biofi lm with bacterial populations at a high
density. Stable bacterial conditions are normally observed in recirculation
systems (Attramadal et al., 2005).
Green Water
Use of microalgae has often resulted in successful rearing of marine fi sh
larvae, as determined by low mortalities, high growth and viable fry
(Howell, 1979; Naas et al., 1991; Reitan et al., 1993). There are two different
variants related to the application of microalgae in rearing of marine fi sh
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