Preface
P.
Sorgeloos, A. Tandler, G. Van Stappen
Aquaculture, 227(1-4): 3-7
The trends that are apparent in the "larvi"
symposia are a very vivid reflection of the history of larval research in
the last four decades or so. It was only in the 1960s that the rotifer was
first used for the mass larval rearing of marine fish. The use of rotifers
was pioneered by Ito (1960); Hirata and Mori (1967) developed the principles
of mass production of this food organism. The mass production of rotifers
was the topic of a couple of papers in the present symposium. The
development of the rotifer, followed by the brine shrimp (Artemia),
and lately the use of copepods as "vehicles", which carry feed
ingredients, fatty acids in particular, to the tiny marine fish larvae was
introduced by Watanabe and his co-workers in their pioneering experiments in
the 1970s and early 1980s, but is still part of the larval research and was
the topic of a few papers in the present symposium. This area of research
and the development of novel enrichment is presently a fertile ground for
cooperation between the feed industry and researchers.
Watanabe et al. (1983) laid the foundations and the
initial tools for marine larval nutritional research. Very soon it was
demonstrated that the research of larval nutrient requirements based on live
feeds was limited to lipid requirements, as the enrichment with protein,
free amino acids and minerals was limited. The importance of highly
unsaturated fatty acids (HUFA), DHA and EPA of the n-3 HUFA family in
particular, and the ratio between them, for marine fish larvae was
demonstrated to be of particular importance with resulting effects on larval
growth, survival, neural development and resistance to handling stress (Léger
et al., 1986, and Sorgeloos and Léger, 1992). Later on it was demonstrated
that the essentiality of these HUFA stems from the fact that most marine
fish lack an active 5' desaturase (Tocher and Ghioni, 1999) and as a result
require long chain fatty acids in their diet to satisfy the need for these
fatty acids (FA) for biomembrane construction and the maintenance of its
liquidity. The importance of arachidonic acid for larval welfare was
demonstrated only lately. This n-6 HUFA was shown to be associated
with fish larval growth (Bessonart et al., 1999), the moderation of the
stress response in fish as well as the osmoregulatory capacity (Koven et
al., 2001). It is interesting to note that the requirement for this FA
varies from species to species as well as with the life stage. The area of
larval FA requirement remains the focus of research of many investigations.
Studies on the involvement of FA in the control of gene expression in the
area of digestive physiology and the resulting growth and larval survival
were presented.
As a result of the need to improve the nutritional
profile of the live feeds for marine fish larvae, an array of live feed
enrichment products was developed. Initially in the 1970s and early 1980s,
HUFA-rich live algae became a staple component of the enrichment of the
larval live feed. This was followed by an array of industrial products:
initially HUFA-enriched yeast that was followed with direct enrichment
products in the form of HUFA-rich emulsions (Dhert et al., 2001, and
Sorgeloos et al., 2001). Lately there is a return to algae-based enrichment
in the form of HUFA-rich dried algae, mostly grown heterotrophically. The
latter products proved their efficiency as they provide also a protein
source to the larvae via the live feed chain.
The free amino acid (FAA) pool in tissues provides
the essential building blocks for the synthesis of structural and functional
proteins. However, in the developing egg and larvae of marine fish, the role
of FAA appears multifaceted and changing. FAA and their role in the
developing larvae was the focus of research by Fyhn and his students (Fyhn,
1989, and Rřnnestad et al., 2000) in the late 1980s. We had two papers,
which reviewed the role of FAA in the developing marine larvae. Briefly, FAA
serve as osmotic metabolites during oocyte hydration, which leads to egg
buoyancy. During egg development, they serve as an energy substrate where in
the first hours after fertilization and up to 60-h post-fertilization 69% of
the metabolic fuel utilized by the embryo is FAA (Rřnnestad et al., 1994).
Two presentations on this topic were given at "larvi '01".
Lately it was demonstrated in herring larvae that FAA
are signals of a cascade of endocrine events that control digestion in fish
larvae (Koven et al., 2002). Cholecystokinin (CCK) is a digestive hormone
which, when present in the circulating blood, stimulates the release of
pancreatic enzymes and the emptying of the gall bladder as well as
initiating the motility of the digestive tract. In these experiments, FAA
injected into the stomach of herring larvae not only initiated the secretion
of CCK but also started a more vigorous digestive process. Moreover, when
these larvae were given FAA in the presence of a protein like BSA, their CCK
synthesis and digestive tryptic activity increased fivefold!
The accumulating evidence clearly suggests a
multifunctional role for FAA not only in egg and larval development, but
also in the ingestion and digestion of nutrients by larvae. As demonstrated
in the present symposium, more research is needed to understand the
biological underpinning of FAA as physiological modulators and not just
simple nutrients. This will allow the development of larval microdiets that
will be effective growth promoters and viable alternatives to the provision
of live feed.
The interest in maximizing larval performance took a
turn in the 1980s when a series of studies by Lam (1994) hypothesized that
beyond the nutrient requirements of larvae, their capacity to catch (ingest)
and digest the feed and absorb it is limited. This limitation could be
alleviated as a result of speeding up their metamorphosis using hormonal
treatment such as thyroid hormones and cortisol, with a resulting
"maturation" of the digestive tract as seen in its anatomical
development as well as its digestive enzyme response tract. A different
approach of reinforcement of the digestive response was taken by aiding the
digestive process by the supplementation of microdiets with digestive
enzymes or substances, which elicit a digestive endocrine response. The
potential of the various latter strategies in aiding the larval digestive
process could be quantified using various new sophisticated tools. These
include mRNA expression vectors of various digestive enzymes whose magnitude
could be detected and measured. Furthermore, these expression vectors can
help localize the digestive processes by using in situ hybridization
methods.
Transfer of nutrients to the developing larva can
very effectively use the maternal route. In fact, very soon after the early
successes in marine larval rearing it became evident that the quality of the
brood is a direct reflection of the quality of the diet offered to the
broodstock. Studies presented in the past and in the present symposium
elucidated the fact that diets or treatments given to broodstock affected
egg quantity and quality in terms of survival and resistance to stress. In
fact even endocrine agents such as thyroid hormones given to broodstock had
a strong effect on larval performance. Moreover, findings with seabream
demonstrated that the nutritional history of the broodstock had a
significant effect on the concentration of vitellogenin receptors on the
developing oocyte as well as the concentration of circulating blood
vitellogenin with resulting larval growth and swimbladder performance. This
further stresses the importance of a continuing research effort in the area
of broodstock larvae quality interaction for maximal industrial output.
Finally, the integration of the broad understanding
that we presently have of the ontogeny of larval nutritional requirements,
digestive capacity, diet attraction, and endocrine events associated with
larval feeding resulted in impressive progress in microdiet development.
This is particularly true with seabass. Generally, though, microdiet
research has yet to produce an artificial diet for larval marine fish, which
will completely replace the requirements for live feed. This results from
the fact that microdiet research still has to resolve the problems
associated with changes in the feeding, digestive capacity and nutrient
requirements in the developing larva. Aquaculture and the feed industry are
still waiting for these developments. Papers presented in the present
symposium clearly suggest that we are approaching the stage at which the
question of microdiet development will not be their biological feasibility,
but their economical feasibility as compared to available live feeds.
We believe that larval research will keep on
developing new tools and adopting new tools from other areas of science.
Beyond the excellent research in the past few decades, which was
instrumental in keeping the aquaculture growth at an annual rate of about
10%, the future calls for the adoption of novel techniques and concepts in
the area of biotechnology that will help speed up the development of new
candidate species and management techniques for broader environmental
challenges, so that aquaculture can be efficiently performed around the
globe independently of the environment.
These "larvi 01" Proceedings are dedicated
to the memory of Niall Bromage, who passed away on 8 May 2003, and who was a
member of the scientific committee of the "larvi" Symposia in 1995
and 2001. He will be remembered by the "larvi" participants as an
outstanding scientist, a brilliant debater and a most sociable colleague.
(Laboratory of Aquaculture & Artemia Reference Center, Ghent University,
Rozier 44, Ghent 9000, Belgium, e-mail of P. Sorgeloos: Patrick.sorgeloos@Ugent.be,
e-mail of A. Tandler: tandler@agri.huji.ac.il,
e-mail of G. Van Stappen: gilbert.vanstappen@Ugent.be)