174
Y. Olsen
(Watanabe, 1993). It is, however, again emphasized that the biochemical instability of the live feed makes values derived for larvae only approximate (Reitan, 1994).
An alternative, but admittedly less quantitative, way to assess EFA requirements for larval growth and survival is to compare the fatty acid composition of
cultured larvae with that of naturally occurring larvae of the same species raised
on natural food. Comparisons of fatty acid profiles of larvae of Atlantic halibut
(H. hippog!ossus), which were cultured with rotifers and Artemia, with the profile
of larvae feeding on copepods in natural systems, have clearly demonstrated the
problem of providing sufficient amounts of DHA for this species through rotifers
and Artemia, both typical warm water-adapted zooplankton species. Halibut
larvae cultured on a diet of rotifers and Artemia typically contain", I 0% DHA of
their total fatty acids, whereas larvae cultured with copepods contain ",40% during
the early feeding stages (J.O. Evjemo, A. Olsen, and others, unpublished results).
This may indicate DHA deficiency of the former larvae, and it certainly demonstrates that the DHA supplied with cultured live feed is much lower than in nature.
The above approach cannot unequivocally prove DHA deficiency, but we should
strive to enhance the DHA level of cultured live feed. It is also important to
recognize that when DHA values are equal in both larval groups, any hypothesis
of DHA deficiency is likely to be rejected.
Analysis of the percentage fatty acid distribution of fish larvae in the late part of
the larval stage and its live food has been useful in evaluating larval EFA requirements. This method is quick compared with most other methods, and it is also
reliable provided that a comprehensive database for the EFA-sufficient conditions
is available as a reference for comparison.
The hypothesis that DHA is a critical component for many species during the
larval and later stages has been independently supported by starvation experiments, which represent a third method for evaluation of EFA requirements. Most
marine fish larvae and copepods, and an uncommon strain of Artemia as well
(Artemia sinica; Evjemo et aI., 1997). retain DHA with higher efficiency than all
other fatty acids during starvation (Rainuzzo, 1993). Some other strains of Artemia and B. plicatilis tend to catabolize DHA more efficiently than other fatty
acids (Evjemo et aI., 1997; Olsen et aI., 1993a). When an animal tends to retain a
specific EFA level during starvation, this is interpreted as a high requirement of
that specific EFA, and the percentage value of total fatty acids may yield an
estimate of the quantitative requirements.
Efficient DHA retention, as shown by an enhanced percentage DHA of total
fatty acids during starvation, is typical for starving, wild Atlantic salmon that
migrate upstream for spawning. Their DHA retention efficiency during starvation
is found to be higher at low than at high temperatures, whereas most other fatty
acids are only moderately affected by temperature (Olsen and Skjervold, 1991).
Figure 8.6 shows percentage DHA in the salmon flesh as a function of the latitude
of the river outlet in the sea. The regression analysis (see legend of Fig. 8.6)
showed that the percentage DHA of total fatty acids increases significantly (P
<.05) by on average 0.76% per latitude. The water temperature of the northern
rivers is generally 3-6°C lower than that of the southern rivers, but mountains and
Y. Olsen
(Watanabe, 1993). It is, however, again emphasized that the biochemical instability of the live feed makes values derived for larvae only approximate (Reitan, 1994).
An alternative, but admittedly less quantitative, way to assess EFA requirements for larval growth and survival is to compare the fatty acid composition of
cultured larvae with that of naturally occurring larvae of the same species raised
on natural food. Comparisons of fatty acid profiles of larvae of Atlantic halibut
(H. hippog!ossus), which were cultured with rotifers and Artemia, with the profile
of larvae feeding on copepods in natural systems, have clearly demonstrated the
problem of providing sufficient amounts of DHA for this species through rotifers
and Artemia, both typical warm water-adapted zooplankton species. Halibut
larvae cultured on a diet of rotifers and Artemia typically contain", I 0% DHA of
their total fatty acids, whereas larvae cultured with copepods contain ",40% during
the early feeding stages (J.O. Evjemo, A. Olsen, and others, unpublished results).
This may indicate DHA deficiency of the former larvae, and it certainly demonstrates that the DHA supplied with cultured live feed is much lower than in nature.
The above approach cannot unequivocally prove DHA deficiency, but we should
strive to enhance the DHA level of cultured live feed. It is also important to
recognize that when DHA values are equal in both larval groups, any hypothesis
of DHA deficiency is likely to be rejected.
Analysis of the percentage fatty acid distribution of fish larvae in the late part of
the larval stage and its live food has been useful in evaluating larval EFA requirements. This method is quick compared with most other methods, and it is also
reliable provided that a comprehensive database for the EFA-sufficient conditions
is available as a reference for comparison.
The hypothesis that DHA is a critical component for many species during the
larval and later stages has been independently supported by starvation experiments, which represent a third method for evaluation of EFA requirements. Most
marine fish larvae and copepods, and an uncommon strain of Artemia as well
(Artemia sinica; Evjemo et aI., 1997). retain DHA with higher efficiency than all
other fatty acids during starvation (Rainuzzo, 1993). Some other strains of Artemia and B. plicatilis tend to catabolize DHA more efficiently than other fatty
acids (Evjemo et aI., 1997; Olsen et aI., 1993a). When an animal tends to retain a
specific EFA level during starvation, this is interpreted as a high requirement of
that specific EFA, and the percentage value of total fatty acids may yield an
estimate of the quantitative requirements.
Efficient DHA retention, as shown by an enhanced percentage DHA of total
fatty acids during starvation, is typical for starving, wild Atlantic salmon that
migrate upstream for spawning. Their DHA retention efficiency during starvation
is found to be higher at low than at high temperatures, whereas most other fatty
acids are only moderately affected by temperature (Olsen and Skjervold, 1991).
Figure 8.6 shows percentage DHA in the salmon flesh as a function of the latitude
of the river outlet in the sea. The regression analysis (see legend of Fig. 8.6)
showed that the percentage DHA of total fatty acids increases significantly (P
<.05) by on average 0.76% per latitude. The water temperature of the northern
rivers is generally 3-6°C lower than that of the southern rivers, but mountains and