8. Lipids and Essential Fatty Acids in Aquatic Food Webs
173
patterns in ORA metabolism specified above characterize two groups of animals
with different evolutionary histories. The ORA-conservative species are presumably evolutionarily adapted to low temperatures (e.g., salmon, halibut, turbot,
cod, red feed, and other coastal copepods at high latitudes), whereas the ORAcatabolizing species are adapted to high temperatures (e.g., B. plicatilis, Artemia
sp.).
8.2.4. Methods for Evaluation of EFA Requirements
The requirements of essential components of animals can theoretically be determined by feeding the animal food with variable contents of the critical component
while all other factors are kept constant. A typical result of a successful experiment is that the animal growth rate is positively related to the concentration of the
critical component below a certain critical concentration and constant at concentrations above this level (i.e., saturation kinetics). The inflection point can be
interpreted as the actual requirement for the specific essential component. This
method is not always suitable for estimation of EFA requirements of larval stages
for the following reasons:
• It is not straightforward, or perhaps even possible, to establish a gradient of a
single EFA in live feed, and synergetic effects of other fatty acids within the
essential family cannot be excluded.
• The composition of the live feed organism varies with time (i.e., lipids, fatty
acid composition, and protein contents), and specific countermeasures must be
implemented to reduce the effect of this (Reitan, 1994).
• Pure fatty acid extracts are not easily available, and in any event, they are
expensive.
• Metabolic conversion of dietary EFA takes place in animals, and the conversion
rates presumably depend on the composition of the dietary lipids.
• The exposure time of the test experiments affects the magnitude of the response,
because some time will always pass before the ultimate state variables (e.g.,
variation in larval survival and growth rate) are manifested. General and unified
criteria for early assessment of malnutrition in larvae are not well established.
• Sudden mortality caused by non-nutritional boundary conditions, such as inadequate physical and microbial cultivation conditions, is still common in larval
cultures. This makes reproducibility of replicate experiments low, and small
amounts of material still often restrict our analytical capability.
• Boundary conditions may affect the critical level of the component under study
(i.e., temperature, bacteria associated with the larvae, or maternal supply to
early larval stages).
Alternatives of the above approach have been applied in several studies of EFA
requirements with some success (Koven et aI., 1993, 1990; Mourente et aI., 1993;
Watanabe, 1993; Izquierdo et aI., 1989; Watanabe et aI., 1989). The typical range
for marine fish larvae/juveniles is 5-40 mg 003 RUFA per gram dry matter of
food. Another conclusion is that ORA seems to be more important than EPA
173
patterns in ORA metabolism specified above characterize two groups of animals
with different evolutionary histories. The ORA-conservative species are presumably evolutionarily adapted to low temperatures (e.g., salmon, halibut, turbot,
cod, red feed, and other coastal copepods at high latitudes), whereas the ORAcatabolizing species are adapted to high temperatures (e.g., B. plicatilis, Artemia
sp.).
8.2.4. Methods for Evaluation of EFA Requirements
The requirements of essential components of animals can theoretically be determined by feeding the animal food with variable contents of the critical component
while all other factors are kept constant. A typical result of a successful experiment is that the animal growth rate is positively related to the concentration of the
critical component below a certain critical concentration and constant at concentrations above this level (i.e., saturation kinetics). The inflection point can be
interpreted as the actual requirement for the specific essential component. This
method is not always suitable for estimation of EFA requirements of larval stages
for the following reasons:
• It is not straightforward, or perhaps even possible, to establish a gradient of a
single EFA in live feed, and synergetic effects of other fatty acids within the
essential family cannot be excluded.
• The composition of the live feed organism varies with time (i.e., lipids, fatty
acid composition, and protein contents), and specific countermeasures must be
implemented to reduce the effect of this (Reitan, 1994).
• Pure fatty acid extracts are not easily available, and in any event, they are
expensive.
• Metabolic conversion of dietary EFA takes place in animals, and the conversion
rates presumably depend on the composition of the dietary lipids.
• The exposure time of the test experiments affects the magnitude of the response,
because some time will always pass before the ultimate state variables (e.g.,
variation in larval survival and growth rate) are manifested. General and unified
criteria for early assessment of malnutrition in larvae are not well established.
• Sudden mortality caused by non-nutritional boundary conditions, such as inadequate physical and microbial cultivation conditions, is still common in larval
cultures. This makes reproducibility of replicate experiments low, and small
amounts of material still often restrict our analytical capability.
• Boundary conditions may affect the critical level of the component under study
(i.e., temperature, bacteria associated with the larvae, or maternal supply to
early larval stages).
Alternatives of the above approach have been applied in several studies of EFA
requirements with some success (Koven et aI., 1993, 1990; Mourente et aI., 1993;
Watanabe, 1993; Izquierdo et aI., 1989; Watanabe et aI., 1989). The typical range
for marine fish larvae/juveniles is 5-40 mg 003 RUFA per gram dry matter of
food. Another conclusion is that ORA seems to be more important than EPA