8. Lipids and Essential Fatty Acids in Aquatic Food Webs
169
and hatch early, the mother fish invests little in a single egg, and the larvae are
relatively immature at the time of hatching. The newly hatched larvae have to rely
on its yolk sac material before they start feeding (i.e., normally for a few days to a
month, depending on the species). The composition of the maternal yolk sac
material is dependent on the nutritional state of the mother. Tissue differentiation,
including eye, brain, and neural development in general, takes place just after
hatching. Neural tissues, including eyes, contain very high amounts of DHA
(Sargent et aI., 1993a; Mourente et aI., 1991), implying that this fatty acid is very
important for very young stages of marine fish larvae. The expected high DHA
requirements during the early stages, which may be deduced based on the rapid
growth and differentiation of neural tissues of brain and eyes, are supported by the
finding that eggs of marine species tend to be very rich in DHA (Rainuzzo et aI.,
1992; Sargent et aI., 1993b). From an evolutionary point of view, we may assume
that essential components that are found in high quantities in the eggs are important for normal growth and development during the early stages of life. It is easy to
imagine that inadequate brain function and vision, demonstrated for DHAdeficient herring larvae (Sargent et aI., 1993a), will be fatal for marine larvae,
which are visual feeders.
The EFA requirements of marine fishes are also substantial in the later juvenile
stages, when the rate of increase in body biomass is still high. The contents of
DHA, and also shorter 003 fatty acids such as EPA, are high in juveniles as well as
in adult stages of many marine species (see below). The specific growth rates of
fish larvae and older stages of juveniles are, however, normally much higher than
that of the adult fish. Larvae and juveniles of cold water species, for example, cod
(Gadus morhua) and Atlantic halibut (Hippoglossus hippoglossus), will double
their biomass within 1-2 weeks, and some fast-growing species such as turbot
(Scophthalmus maximus) may have a biomass turnover time of <3 d (Reitan et aI.,
1993). Their rapid growth, along with their inefficient digestion of food in the
very early stages (0ie et aI., 1997), suggests that the EFA requirements for body
growth are relatively high. The requirements will gradually become reduced with
age, because older fish have a lower specific growth rate. Even adult stages of
marine fish exhibit high EFA requirements compared with, for example, common
species in agriculture.
There are pronounced differences in dietary EFA requirements of marine fish
species (Reitan et aI., 1994a; Koven et aI., 1993, 1990; Mourente et al., 1993;
Watanabe, 1993; Izquierdo et aI., 1989; Watanabe et aI., 1989). Temperature is
normally believed to be decisive. More specifically, it is assumed that fish living
at high latitudes and species that reside permanently in deep water will normally
have higher EFA requirements than fish residing in warmer water. Associated
with these trends is the relative ability of the different species to elongate and
desaturate shorter EFA to long-chain PUFA such as DHA. High metabolic flexibility to modify EFA within the essential families will probably make the species
more flexible with regard to the composition of the dietary lipids. High metabolic
flexibility is apparently characteristic for many salmonids, including rainbow
trout (Sargent et aI., 1993b; Owen et aI., 1975).
169
and hatch early, the mother fish invests little in a single egg, and the larvae are
relatively immature at the time of hatching. The newly hatched larvae have to rely
on its yolk sac material before they start feeding (i.e., normally for a few days to a
month, depending on the species). The composition of the maternal yolk sac
material is dependent on the nutritional state of the mother. Tissue differentiation,
including eye, brain, and neural development in general, takes place just after
hatching. Neural tissues, including eyes, contain very high amounts of DHA
(Sargent et aI., 1993a; Mourente et aI., 1991), implying that this fatty acid is very
important for very young stages of marine fish larvae. The expected high DHA
requirements during the early stages, which may be deduced based on the rapid
growth and differentiation of neural tissues of brain and eyes, are supported by the
finding that eggs of marine species tend to be very rich in DHA (Rainuzzo et aI.,
1992; Sargent et aI., 1993b). From an evolutionary point of view, we may assume
that essential components that are found in high quantities in the eggs are important for normal growth and development during the early stages of life. It is easy to
imagine that inadequate brain function and vision, demonstrated for DHAdeficient herring larvae (Sargent et aI., 1993a), will be fatal for marine larvae,
which are visual feeders.
The EFA requirements of marine fishes are also substantial in the later juvenile
stages, when the rate of increase in body biomass is still high. The contents of
DHA, and also shorter 003 fatty acids such as EPA, are high in juveniles as well as
in adult stages of many marine species (see below). The specific growth rates of
fish larvae and older stages of juveniles are, however, normally much higher than
that of the adult fish. Larvae and juveniles of cold water species, for example, cod
(Gadus morhua) and Atlantic halibut (Hippoglossus hippoglossus), will double
their biomass within 1-2 weeks, and some fast-growing species such as turbot
(Scophthalmus maximus) may have a biomass turnover time of <3 d (Reitan et aI.,
1993). Their rapid growth, along with their inefficient digestion of food in the
very early stages (0ie et aI., 1997), suggests that the EFA requirements for body
growth are relatively high. The requirements will gradually become reduced with
age, because older fish have a lower specific growth rate. Even adult stages of
marine fish exhibit high EFA requirements compared with, for example, common
species in agriculture.
There are pronounced differences in dietary EFA requirements of marine fish
species (Reitan et aI., 1994a; Koven et aI., 1993, 1990; Mourente et al., 1993;
Watanabe, 1993; Izquierdo et aI., 1989; Watanabe et aI., 1989). Temperature is
normally believed to be decisive. More specifically, it is assumed that fish living
at high latitudes and species that reside permanently in deep water will normally
have higher EFA requirements than fish residing in warmer water. Associated
with these trends is the relative ability of the different species to elongate and
desaturate shorter EFA to long-chain PUFA such as DHA. High metabolic flexibility to modify EFA within the essential families will probably make the species
more flexible with regard to the composition of the dietary lipids. High metabolic
flexibility is apparently characteristic for many salmonids, including rainbow
trout (Sargent et aI., 1993b; Owen et aI., 1975).