# Feed quality and feeding strategies for sustained aquaculture development.

S.J. Kaushik

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S.J. Kaushik. Feed quality and feeding strategies for sustained aquaculture development.. Suisan-zoshoku, 1996, 44 (2), pp.231-234. hal-02684290

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# Feed Quality and Feeding Strategies for Sustained Aquaculture Development

Sadasivam J. KAUSHIK

Fish Nutrition Laboratory

Unité Mixte INRA-IFREMER, Station d'Hydrobiologie
64310 Saint-Pée-sur-Nivelle, France

tel (33) 59 51 59 90; fax: (33) 59 54 51 52; e-mail: kaushik @ st-pee. inra. fr

## Introduction

Since feed cost represents the major proportion of the overall production cost in aquaculture, optimisation of feed utilisation bears economic significance. This optimisation depends upon not only the quality of the diet but also the quantity of supply in close correspondence to the growing animal's requirements. For aquatic animals such as finfish and crustacea under cultivation, even with nutritionally adequate diets, much variation might be encountered in terms of feed efficiency. A significant portion of this variability probably reflects inadequate form of supply and acceptance of the diets as well as to ill-adapted feeding practices, most of them anthropomorphic, taking little into account the inherent behavioural constraints of the species under cultivation.

Some of the basic questions which arise are: what to feed? what is the ideal form of supply? which are the factors that govern voluntary feed intake? From a practical point of view, the following questions are of importance: when to feed? how much and how frequently should this feed distribution be made? how should this feed be distributed for optimal performance?

## Constraints for a good feed

The maintenance of vital physiological functions, growth and health of the animal depends upon the availability of all the forty or so essential nutrients. Then, suffice it to say that a diet should supply all essential nutrients and energy in tune with the growing animal's needs. This depends mainly upon the quantita-

tively adequate supply of essential nutrients and energy in the form of diets which are easily accepted by cultivated organisms. Despite considerable advances made over the past five decades, the quantitative needs for all essential nutrients for the variety of species under aquaculture are not known today¹). In the fast developing global aquaculture scene, quantification of essential nutrient requirements for all the new species remains a major task.

Even when the diets are properly formulated to contain adequate amounts of all essential nutrients, the form of supply of a diet has an impact not only in the acceptance of the diet but also on the aquatic environment. The past practices of the use of "moist" or "semi-moist" diets for the culture of salmonids have been abandoned because of the difficulties involved in maintaining quality of the diets as well as its effects on the environment. In the case of salmonids, since nearly a decade, the development of high-energy extruded diets or "nutrient-dense" diets has contributed significantly to the reduction of aquaculture wastes. For several marine species, it is still common practice to use moist diets, despite considerabel progress which had been made in obtaining stabel quality "soft-dry" pellets²). In this context, the possible role of feed attractants in improving nutrient intake is also a field which requires careful analysis.

## Factors that affect voluntary feed intake VFI

A number of environmental as well as intrinsic factors affect VFI. Of the biological factors, fish size, physiolog-
ical stage and genotype are known to influence voluntary
feed intake. Among the environmental factors, tempera-
ture and photoperiod both seem to control and govern
feed intake. Feed intake is also under close metabolic,
endocrine and neuronal control, all of which are
intimately related to the nutritional status of the animal
(Fig. 1). Consequently, the dietary nutrient / energy
balance are factors that have to be considered when
trying to understand the mechanisms involved in the
control of feed intake.

<figure><img src="image_2.png" /><figcaption>Fig. 1. CNS, nutritional status and feeding behaviour.</figcaption></figure>

The voluntary feed intake (VFI) is also known to be
modulated by the adequacy of supply of energy in all
animals including man. Very much like in the case of
higher vertebrates, whenever there is an increase in the
energy density of the feed, the voluntary feed intake is
reduced at any given production level. There is more and
more evidence to the fact that fish are capable of
adjusting their voluntary feed intake depending upon the
quality of the diets and especially upon the dietary
digestible energy (DE) levels³,⁴,⁵. More precise and
reliable studies are necessary to evaluate the effects of
DE on voluntary feed intake in all species of fish. But, it
should be borne in mind that, an inconsiderate increase
in DE might have adverse consequences of deficiencies in
other essential nutrients (Fig. 2). This might in some
cases result also in hyperphagia, although no clear
demonstration of such a phenomenon has not been made
in fish. Other dietary factors that bear significance
especially in aquaculture are the palatability and
attractiveness of the diets offered. There is also some
indication that factors such as fat body mass might
influence feed intake.

How much to feed ?

In order to know how much to feed, it is then
necessary to know the potential of the animal in terms of
VFI. A number of invasive and non-invasive techniques
are available for monitoring VFI, some of them valid
only for quantifying this *a posteriori*. Gut content analy-
ses are invasive methods which can hardly be applied
under aquacultural conditions. Some other techniques
are available to monitor individual feed intakes through
radiography from time to time⁶,⁷, which are extremely
useful for experimental purposes but do not hold much
promise under practical culture conditions.

<figure><img src="image_3.png" /><figcaption>Fig. 2. Relationship between dietary energy density, feed intake and growth.</figcaption></figure>

Any excess feed supply will definitely decrease feed
efficiency. Part of this is also because inconsiderate
feeding in fact leads more to waste of feed ("feeding the
tank, cage or the pond") than the animals living therein.
It is and easy common practice for the fish farmers to use
feeding tables, provided by the feed manufacturers and
to adapt them more or less depending upon technical
proficiency and skills. One should recognise that the
allocation of feed (ration size) should be adapted to
nutrient/energy needs and growth rate which are
affected by a number of factors. It follows then that the
feeding tables should take into account the biological as
well as environmental factors such as temperature, fish
size, genotype, growth rate, nutrient and energy
requirements for a given growth performance (or
fattening), dietary energy density, nutrient concentra-
tion etc., Some guidelines have been provided for
drawing feeding charts for salmonids based on nutrition-
al bioenergetic principles⁸,⁹). The first requisite is to
have reliable data on the energy requirements of fish for
a given growth rate as influenced by body size and
temperature. Together with data on the digestible energy
density of the diet, it has been possible to draw feeding
charts for salmonids. For validating such an approach to
other finfish species, more knowledge is required to
precisely predict growth and to estimate the total energy
needs per unit growth. This will also have to take into
account the specific production characteristics (water
quality, genotype, husbandry...) in order to have tables
adapted to each specific site.

When to feed?

It is now well established that there are specific
feeding rhythms in almost all species of fish<sup>10)</sup>.
Salmonids such as rainbow trout or Atlantic salmon<sup>11)</sup>
and cichlids such as tilapia appear to show high feeding
activity both at dawn and dusk the former being
quantitatively more important than the latter. Many of
the catfishes show a high feeding activity only during the
dark phases of the daily cycle<sup>12,13)</sup>. Some species such as
the sea bass or the common carp appear to be more
capable of phase shifting their feeding rhythms. This
implies that the feeding practices should be in tune with
such biological rhythms (endogenous or environmental-
ly entrained) to avoid any decrease in feed efficiency.

How frequent should the feed distribution be?

Besides the quantity of feed to be supplied, there is a
need to know the frequency with which the same amount
be supplied for optimal utilisation. The frequency of
feeding depends upon two major factors: body size and
water temperature. Both these factors affect the rate of
passage of foodstuffs through the digestive tract<sup>14)</sup>. A
relation between stomach fullness and return of appetite
has been shown in several fish. For instance, in small
larvae, with a very short digestive tract, the frequency of
feeding can be several times an hour, whereas in
broodstock fish, a single meal a day might be sufficient.

Decrease competition for feed

Aquatic animals are generally grown in groups and
not as individuals. One might encounter considerable

day to day variation in voluntary feed intake in any
group of animals. It is difficult to follow the individual
feed intake, even if individual growth rates can be
monitored if needed. Using radiographic techniques<sup>7)</sup>,
have been able to show that when feed supply is
sufficient, there is decreased inter-individual variabil-
ity. Optimal feed supply decreases competition for feed
and hence size heterogeneity<sup>15)</sup>. Hence, the ideal feeding
practice is the one which should take into account and
more so try to avoid day-to-day and inter-individual
variability. By providing feed to satiety at all areas of
the growing pond / cage / raceway, we can decrease
variability in feed intake and consequently produce more
homogenous batches.

Proper feeding strategy

Feeding to satiety can be achieved by a number of
means provided there is close monitoring of the trophic
activity and behaviour of fish. Three major strategies
are available to distribute feed: hand-feeding, automatic
/mechanical feed distribution and self-feeders. A
proper feeding strategy is the one which increases feed
efficiency and decreases the environmental load. The
bottom line is that both behavioural and nutritional
bioenergetic principles are taken into account<sup>9)</sup>. There
are also a number of more or less sophisticated devices
to automatise feed distribution for experimental
purposes<sup>16)</sup> as well as under fish farm conditions<sup>17,18)</sup>.
The choice of the adequate system will have to take into
account the relative merits and inconveniences in terms
of the potential benefit gained on feed utilisation and on
the overall production costs. In many species of finfish,
the demand feeding capacity has been demon-
strated<sup>19,20,21)</sup> while for crustacea this might not be
applicable. The use of demand feeders well adapted to
the species and size of fish and to the culture site has the
advantages of meeting the behavioural (rhythms) as
well as the metabolic demands in relation with the
nutritional quality of the diets. By exerting a self-control
over their feed intake, such systems contribute also
towards the reduction of feed costs, optimisation of feed
intake and utilisation and in the decrease in environmen-
tal load of aquaculture.
References

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