The main objectives of fish culture are to maximize survival and growth,which accordingly are measured to evaluate the effects of rearing technologies. Survival and growth are usually computed from samples taken at thebeginning and at the end of the experimental period. These determinationsare open to errors which may reduce the reliability of comparisons.
Estimation of survival from the initial and final numbers of fish larvaeassumes a constant rate of mortality during the intervening period. In fact,the survival curve may also be concave (indicating early mortality) orconvex (late mortality).
The growth of young carp larvae shorter than 10 mm reported fromponds (Fig. 6.8, Table 6.4, see also Ostroumova et aI., 1980 - 50-70% d- l )
is unexpectedly high as compared with that reported from indoor tests withnatural food. A possible explanation is that metabolites produced by larvaecultured at high densities and substances lost from starters (Mukhina, 1963;Urban-Jezierska et aI., 1984) may - in spite of water exchange - be partlyresponsible for hampering larval growth. An alternative explanation invokesa stimulating effect of fluctuating temperatures (Galkovskaya and Sushchenya, 1978; Konstantinov and Zdanovich, 1985) on larval growth in thewild, an effect which would be absent under constant temperatures incontrolled conditions. Another alternative is that pond data are more biasedthan culture data. though both in the direction of being too high. That is,greater mortality was experienced by the smaller members of a brood inlaboratory experiments on Salmo trutta larvae (Hansen. 1985), and strongersize-selective mortality of larvae can be expected in natural water bodieswhere slower-growing. smaller individuals are more susceptible to predation. The growth rate computed from field data would therefore be based
to a greater extent on the faster-growing survivors. and consequently.growth in ponds would be more strongly over-estimated. The opposite(under-estimation of growth) is shown by Ricker (1979) for postlarval fishesin which natural and fishery factors cause the faster-growing (larger)individuals to die earlier. The relationship between the population growthrate (Le. that observed in survivors) and the true average growth rate ofindividual larvae. in connection with size variability and selective mortality.remains unsolved in the evaluation of effects of rearing technologies.Another possible source of error is larval cannibalism (Section 6.2; Bergot etaI.. 1986).
7.2 LIVE FOODS V. FORMULATED DIETS
The principal causes of mortality of fish larvae in the wild are fooddeficiency. predation. unfavourable temperature and pathogens. In principle.these are eliminated in indoor cultures under controlled conditions withsufficient exogenous food supplied. However. it is not always possible tosupply natural food - live zooplankton organisms taken from the field - ofadequate size. at appropriate times and in sufficient amounts. The massproduction of living feeds has been extensively used in Japan; their nutritivevalue to fish larvae depends upon the chemical properties of their own food(Watanabe et al.. 1978; Fujita et aI.. 1980; review: Watanabe et aI.. 1983).Zooplankton monocultures take up much space and skilled labour; Artemianauplii are expensive. Another alternative is formulated diets.
During the past decade. studies on application of formulated diets tocyprinid and coregonid larvae have greatly advanced (Ostroumova et aI..1980; D9-browski. 1982b; Charlon and Bergot. 1984; Rosch and Appelbaum. 1985; Charlon et aI.. 1985. 1986; Rosch and D9-browski. 1986;Bergot et aI.. 1986. 1989; Segner et al.• 1988; Kamler et aI.. in press; Table6.4). The turning point was the introduction of yeast (single-cell protein.SCP) as one of the major ingredients of formulated diets (Appelbaum. 1977;Appelbaum and Dor. 1978). Nevertheless. the development of the larvae fedformulated diets can be retarded (Section 6.1). their growth can be impeded(Section 6.3). and the energy and matter transformation efficiencies areoften depressed (Section 6.5).
The inferior results of rearing fish larvae on formulated diets have beenexplained in many ways. The morphological. functional and physiologicalproperties of the larval alimentary tract have been conSidered. as has thenature of the diets.
A short and poorly developed alimentary tract. rapid evacuation of foodfrom it. and low production of digestive enzymes all constrain larval fooddigestion. especially of formulated diets (Section 6.2). However. importantinterspecific differences exist in that respect. D9-browski (1984a) assigned
Live food v. formulated diets 213
larval fish to three categories according to the properties of their alimentarytract. Salmonids have a functional stomach at first feeding. They, and otherstomach-possessing larvae, utilize dry diets efficiently from the very beginning of exogenous feeding. For example, the instantaneous gross efficienciesKli for Salmo trutta of c. 200 mg wet weight fed a dry diet were c. 38% forenergy and c. 45% for protein (Raciborski, 1987), values which arewithin the range for older fishes 5 to 250 times as heavy.
The second group are species in which the stomach develops later inontogenesis. A delayed first intake of formulated diets, in comparison withthat of live food, was shown by Rosch and Appelbaum (1985), Rosch andD1iJ.browski (1986), Segner et al. (1988) and Rosch (1989) for CoregonusIavaretus, which belongs to the second group. Increased mortality during thefirst three weeks of feeding was attributed to the death of larvae that did notaccept dry food (Segner et al., 1988). However, dry food can be used as aninitial food for C. Iavaretus larvae (Rosch and Appelbaum, 1985; Segner etal., 1988) despite some retardation of their growth.
Cyprinid fishes, which have no stomach throughout ontogenesis, form thethird group. Eleven artificial diets were provided to Coregonus sp. larvae andsix diets to Rutilus rutilus larvae during the first 40 d of active feeding (Kockand Hofer, 1989). Better results were reported for Coregonus sp. than for R.rutilus, in which growth was negligible on several diets. High food consumption (in the case of formulated diets) and accelerated gut evacuation rateleads to dilution of digestive enzymes, failure of their reabsorption in thehindgut, losses of proteins (Table 6.7), decreased growth, and increasedmortality of R. rutilus (Kock and Hofer, 1989). Cyprinus carpio is a 'difficult'species to which to feed formulated diets from the very beginning of exogenous feeding (D1iJ.browski, 1984a); various aspects are shown in Figs. 6.3,6.4, 6.8, 6.11 and 6.12, and in Tables 6.3,6.4 and 6.7.
Turning now to the nature of diets, inert particles stimulate the foragingactivity of predator larvae poorly. Low effects of formulated diets have alsobeen attributed to a deficit of essential substances. These may be highlyunsaturated fatty acids, amino acids, vitamins, and minerals, singly or incombination. Lack of some substances in compound diets depresses survivaland impairs growth (Bergot et al., 1986, and many others) and preventsmetamorphosis in fish larvae (Fliichter, 1982). However, fish larvae, withtheir rapid changes of nutrient requirements, are a difficult subject forroutine nutritional studies (D1iJ.browski, 1984a).
A lack of exogenous enzymes is another property distinguishing dry dietsfrom live food. The proteolytic activity of zooplankton enzymes is high(Ostroumova et aI., 1980), and these enzymes may provide a substantialcontribution to the total enzymic activity of the larval alimentary tract (Lauffand Hofer, 1984), but Ilina and Tureckij (1986) did not confirm theimportance of dietary enzymes to early carp larvae at developmental steps Band C1. Attempts were made to imitate the conditions created in the larval
214 Feeding of fish Iarve in aquaculture
alimentary tract by prey proteolytic enzymes. However, pretreated (hydrolysed) diets or diets with added digestive enzymes had little effect onlarval growth (Aoe et aI.. 1974; Dybrowski et aI., 1979; Kamler et al.,1987). Moreover, the hydrolysed starters were less stable in water thannonhydrolysed ones (Urban-Jezierska et a!', 1984).
In general, dry diets for larvae are susceptible to leaching because of theirsmall particle size, c. 0.05-0.70 mm in diameter. Losses of nutrients fromstarter particles begin immediately after contact with water. Dissolvedsubstances and particles smaller than 20 Jim were defined as inaccessible tocyprinid larvae, Le. lost. Two compounds of the losses were supposed to exist(Urban-Jezierska et aI., 1984): first. fine 'dust' surrounding the properparticles of diet - this is lost immediately after immersion of the starter inwater and remains suspended in the water; second, substances slowlyleached from the particles - these comprise vitamins, minerals. carbohydrates and free amino acids which remain dissolved in the water. The lossesof dry matter. total nitrogen and protein from our earlier starters were c.50% after 10 s immersion (Urban-Jezierska et aI., 1984). According to Littaket al. (1980). carp larvae consume at most one-half of the amount of dietoffered. In our later diets (Kamler et aI., 1989), the losses of dry matter werereduced to c. 30-40%; the control diet Ewos C-20 was the most stable. withlosses of 33.8% (range 31.4-36.3). 32.7% (30.9-34.4), and 28.7% (27.430.0), respectively, for the fractions 00. 0 and 1. An adverse effect on fishlarvae of pollution caused by both fine suspended matter and dissolvedsubstances is presumed to exist.
Other advantages of zooplankton over formulated diets are the ability ofthe former to remain suspended in water and their plasticity of shape (Vander Wind, 1979; Rosch and D~browski, 1986). which permits fish larvae toingest them more easily.
It seems. then. that the successful adaptation of fish larvae to formulateddiets will not be achieved by a simple manipulation of diet composition. butthat the improvement of feeding techniques is a vital and complex problem.Recommendations have been made by D~browski (1984a). Szlaminska(1988) and Appelbaum (1989); here a few aspects will be listed.
Diet particle micro-encapsulation seems to be a solution to the loss ofsubstances from diets. Prolonged suspension of food particles in the watercolumn ensures their greater availability to fish larvae. So does frequentfeeding; a rearing system which gave satisfactory growth of carp larvae fedon diets was described by Charlon and Bergot (1984), although Knights(1985) reported high energy transformation efficiency due to low energyexpenditure on feeding metabolism (Ry) in young eels fed once a day. Slowwater motion ensures the mobility of food particles. which attracts thelarvae's attention. The size of food particles administered has to increase aslarval size increases (Fig. 6.5). The need to keep tanks clean is a specificproblem with dry feeds (Rosch and Appelbaum. 1985; Rosch and Dab-
Live food v. formulated diets 215
rowski, 1986); it can be partly resolved by a special design of the rearingsystem (Charlon and Bergot, 1984). A tendency towards rearing fish larvaeat high temperatures - approaching the upper limit for growth - is observed(Lirski, 1985), but the question of how they will adapt to life in the naturalenvironment still remains open. Stress and other disturbances should beavoided, because of the possible depression of feeding and non-optimalpartitioning of assimilated energy, leading to increased activity metabolism(Ra ) and consequently a possible reduction in growth. The optimization ofthe amount of food given in relation to size and temperature still remainsunsolved for many commercial fish larvae.
Although in some laboratory experiments, satisfactory growth of carplarvae was achieved exclusively on dry diets (Ostroumova et aI., 1980;D?-browski, 1982b; Charlon and Bergot, 1984; Charlon et aI., 1986; Bergotet aI., 1989; Kamler et aI., in press), dry diets should not be recommendedas the only food for first feeding of cyprinid larvae in intensive cultures. Thediets can be used after previous feeding with zooplankton, and at thebeginning they should be complemented with live food (Imam and Habashy,1972: Lukowicz and Rutkowski. 1976; Littak et aI., 1980; Bryant andMatty, 1981: D?-browski. 1984a,b; Okoniewska et aI., 1986; KoufH andHamackova, 1989; Table 6.4).
