Environmental Engineering Reference
In-Depth Information
4.1.1 Growth and survival
Salinity affects the distribution of a variety of estuarine and marine organisms. Some marine
species, such as shrimps, have life cycles that include an estuarine phase. Since the 1950´s,
there have been reports on congregations of juvenile shrimps at low salinities, whilst adults
reproduced under strict marine conditions. It has also been observed that larval phases
inhabit oceanic waters, where they develop and moult into postlarvae. Through certain
chemo tactile mechanisms and water currents, postlarvae are capable of moving towards
estuaries and coastal lagoons, where they recruit as early juveniles. It is in these habitats,
with strong salinity variations, where some shrimp species are capable of colonizing more
diluted environments than others. For example, studies carried out in the 1960´s by Mac
Farland & Lee, (1963) showed that juvenile
L. setiferus
are better adapted to tolerate low
salinities than juvenile
Farfantepenaeus aztecus.
Similarly, Mair, (1980) observed that amongst
L. stylirostris, F. californiensis,
and
L. vannamei
, the latter is the species that best tolerates
highly diluted environments in estuaries and costal lagoons, and is therefore capable of
inhabiting areas restricted to the other two species. This ecophysiological trait allows
L.
vannamei
to reduce ecological pressures of competition for space and food, both strong
limiting resources in these habitats. In an experimental study, Mair, (1980) observed that
L.
vannamei
postlarvae and early juveniles placed in an experimental salinity gradient
preferred salinities from 3-6 UPS, whereas
L. stylirostris
and
F. californiensis
preferred
salinities from 32-35 UPS and 9-26 UPS, respectively.
The wide tolerance of
L. vannamei
to salinity has also been observed under culture
conditions, both experimentally and within production facilities. Ogle
et al
.
(1992) examined
the effects of salinity on growth in
L. vannamei
postlarvae under different culture conditions.
In that study the authors observed that tolerance to salinity is independent of postlarval (PL)
age, showing that both PL8 and PL22 had better survival and growth at salinities ranging
from 16 to 32 UPS. In that same study, it was demonstrated that tolerance to salinity
increases as temperature declines from 30-16°C, since the highest survival and growth of
postlarvae was obtained at salinities of 8 and 16 UPS at that temperature. Postlarvae can be
acclimated to low salinities for production purposes. Recent studies have shown that a rate
of salinity decline of 25% per hour is appropriate for postlarvae (PL10 to 4 UPS) and early
juveniles (PL 20 to 1 UPS), demonstrating that this species is well adapted to highly diluted
environments (McGraw et al
.
, 2002). From these studies it is possible to conclude that
L.
vannamei
postlarvae can tolerate broad salinity ranges, although shrimp between PL1 and
PL10 are more sensible to low salinities that juveniles with more than 20 days after the last
metamorphic moult (PL20).
In this study juvenile
L. vannamei
(initial mean weight 2.26g) were cultivated at 2, 4 and 8
UPS during 70 days in a semi-closed recirculating experimental system. Density was kept
constant at 28 juveniles per m
2
in all salinity treatments. Results showed high survival rates
in all treatments (98.7-100%). Mean final weight varied slightly between 19.0 -19.28g,
whereas weekly growth rates varied between 1.67-1.7g. No significant differences in growth
or survival amongst treatments were found, suggesting that
L. vannamei
can be cultivated at
low salinities obtaining good survival and growth.
Bray et al
.
, (1994) reported that juvenile
L. vannamei
positive to IHHN virus showed better
growth exposed during 35 days at salinities of 5 and 15 UPS than at 25, 35 and 49 UPS. A
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