International Journal of Aquaculture, 2013, Vol.3, No.22, 126
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The predominance of PUFAs and SFAs as when
compared to MUFAs across the test diets in this study,
is in accordance with previous findings (Wiegand, 1996).
Preferential catabolism of MUFAs along with
preferential retention of DHA, EPA and AA, and
specific SFAs, usually 16:0 or 18:0 by embryos of a
variety of species has been recorded (Wiegand, 1996).
This reflects the essential structural role of DHA in
membranes, the importance of AA and EPA in
eicosanoid production and specific roles of SFAs
in the sn-1 position of structural phospholipids
(
Rainuzzo, 1993; Tocher, 2003).
In this study, total replacement of
Artemia
with
Cyclopoid copepods conferred quantitatively higher
composition of three (LNA, AA, DHA) of the five
recorded essential fatty acids. Interestingly, partial
replacement of
Artemia
with Cyclopoid copepods
even yielded better results on all the five essential
fatty acids. The ability of the Cyclopoid copepods like
other herbivores, to convert linolenic acid (18:3n3) to
eicosapentaenoic (EPA, 20:5n3) and consequently to
docosahexaenoic (DHA, 22:6n3) acids (Norsker and
Støttrup, 1994; Nanton and Castell, 1998; Desvilettes
et al., 1997; Veloza et al., 2006) is a true reflection on
the nutritional superiority of Cyclopoid copepods to
Artemia
.
This observation is in accordance with other
researchers who have indicated that copepods clearly
outperform enriched rotifers and enriched
Artemia
in
terms of meeting fish larval HUFA requirements
(
Bell et al., 2003; Evjemo et al
.
,
2003;
McKinnon et
al
.
,
2003;
Olivotto et al
.
, 2006;
Conceica
et al
.
, 2010).
Conceica
et al
.
(2010)
further argues that biovailability
of HUFA in copepods is better than in
Artemia
because of the location of HUFAs in the phospholipid
fraction in the former and in the neutral lipid fraction
in the latter.
Docosahexaenoic acid (DHA) has important structural
and functional roles in all membranes, but especially
neural membranes (Feller, 2008; Wassell and Stillwell, 2008;
Tocher, 2010) and a quite specific role in visual cell
membranes, as in young developing mammals (Brett
and Muller-Navarra, 1997). Fish larvae are thought to
be visual feeders, adapted to attacking moving prey in
nature (Conceica
et al
.
, 2010).
Deprivation of DHA in
larval herring (
Clupea harengus
L.) caused impaired
visual performance, particularly at low light intensities
when rod cells are active (Bell et al
.
, 1995;
Shields
et al
.
, 1999).
Higher DHA content in cyclpoid
copepod-fed larvae in this study could explain better
growth due to improved visual performance of the
larva leading to more larval feeding responses.
The high composition of AA in Cyclopoid
copepod-fed catfish larvae in this study may further
explain improved growth performance of catfish
larvae as compared to when
Artemia
was used.
Arachidonic acid (20:4n6; AA) is believed to be the
chief source of eicosanoids in fish (Tocher, 2010).
Eicosanoids produce highly bioactive molecules
following regulated dioxygenase enzyme-catalysed
oxidation of HUFA 20:4n6 (AA) and 20:5n3 (EPA). In
fish, these molecules are involved in a great variety of
physiological functions including blood clotting,
immune and inflammatory responses, cardiovascular
tone, renal and neural functions (Schmitz and Ecker, 2008).
A diet combination of
Artemia
with Cyclopoid
copepods as a starter feed for African catfish in this
study culminated into the best larval growth. The high
composition of DHA and AA, the C22 and C20
metabolites of 18:3n-3 (LNA) and18:2n-6 (LA),
respectively in Cyclopoid copepods is viewed as a
booster that led to an increase in all the five essential
fatty acids in the combination diet that led to overall
improved growth. Of the essential fatty acids, LA and
LNA are designated as true essential fatty acids for
freshwater fish, because freshwater fish, are known to
have an innate capacity to desaturate and elongate LA
to 20:4n−6 (AA) and LNA to 20:5n−3 (EPA) and
ultimately to 22:6n−3 (DHA) (Sargent et al., 2002).
According to Sargent et al. (2002), a dietary source
containing a combination of LNA, EPA, and DHA
caused significant weight gain to milkfish (
Chanos
chanos
).
Although it is generally believed that
freshwater/diadromous fish species require more of
LA and LNA, while marine fish have a strict
requirement for DHA, EPA and AA, larval organisms
are thought to be more dependent on dietary HUFA
than adults (Brett and Muller-Navarra, 1997;
Vengadeshperumal et al., 2010) because the rate at
which desaturation and elongation of LA and LNA
may be too low to satisfy the growth demands of
fast growing larval fish. Accordingly, Brett and
Muller-Navarra (1997) recorded that various organisms
with the ability to convert linolenic (LNA) acid to
EPA and DHA grow better when provided with direct