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2 Fishes INTRODUCTION The farming of fish is a comparatively recent develop- ment in animal agriculture, and present feeding prac- tices reflect this fact. Traditionally, the amount of feed offered fish has been based on a percentage of body weight adjusted by the length of the fish, the water tem- perature, and an anticipated feed conversion ratio (Bu- terbaugh and Willoughby, 1967; National Research Council [NRC], 1981~. It was difficult to feed fish ad libitum until the more recent development of demand feeders excess feed was either carried away by the flowing water or it disintegrated to the point that it was unavailable to the fish while it simultaneously competed for the available dissolved oxygen. Ad libitum or de- mand feeding has now become more practicable be- cause of the development of demand feeders that can dispense small volumes of feed when activated by the fish. These feeders have found wide acceptance in the fish farming industry because they reduce the labor re- quired for feeding and result in improved growth (Boyd- stun and Patterson, 1980~. Also, as more data on digestibility and metabolizable energy of feeds become available (NRC, 1981, 1983), feeding levels can be more accurately predicted. In this chapter, voluntary feed in- take refers to feed consumed by fish when it is available at all times or when fish are fed to satiation at frequent intervals. Generalizations are difficult when one attempts to de- scribe the factors that affect feed intake in fishes; they are an extremely numerous group, representing more than half of all vertebrate species and are, accordingly, a group with great latitude for variation. Carp and gold- fish (both cyprinid fishes) are as anatomically and physi- ologically different from trout and salmon (both salmonid fishes) as the pig is from the cow. Fishes have also adapted to an enormous variety of environments. Their feed intake, within a genus, is determined largely 16 by metabolic rate, which is directly proportional to the temperature of the water in which they live. The phylo- genetic system of classifying fish according to anatomi- cal development is also misleading especially if one is considering the same species raised in the wild as op- posed to a controlled environment. Another system of classifying fishes that is more appropriate for the present review is one that separates them into groups according to the degree of control that man exerts over their rearing and propagation. Wild fish reproduce, grow, and die in natural rivers, lakes, and streams in fresh or salt water and are never removed from this environment or restricted in any manner; their feeding and survival depend on the environment and their own instincts. The second group, which lies between the wild and cultured groups, includes fishes used for the restoration or maintenance of a sport fishery. These are partly dependent on external factors in their environ- ment, as they are spawned, hatched, and grown to stackable size in a controlled environment (hatchery) and then are released into a natural environment where they must again become dependent on their instincts for survival. Cultured fishes grow in a controlled environ- ment and are completely dependent on external factors for survival. This group may be subdivided into aquar- ium fishes, which are cultured specifically for the fish hobbyist or medical-biological researcher, and food fishes (e.g., bouts, catfishes, eels, and carps), which are intensively grown to a desired market size by applying husbandry techniques not unlike those used with do- mestic farm animals. Little basic research into the environmental, physio- logical, and dietary factors that affect feed intake in fishes has been accomplished, despite the modest amount that has been published on the nutritional re- quirements of fishes (NRC, 1981, 1983~. Also surprising is the lack of scientific objectivity in the published re- search on estimates of food consumption of wild fish.

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Fishes 17 Because the worldwide populations of wild fishes are diminishing, it is important to determine whether the density and age structure of a population are in proper balance with available food resources. If a fish popula- tion is to be effectively managed for optimum produc- tion, its food consumption must be accurately estimated. Two basic approaches have been used in this type of research. In one, field data on stomach contents are converted into estimates of daily food consumption; the second involves data developed from laboratory ex- periments on feeding or metabolism extrapolated to field conditions. Both approaches have been severely criticized since the usefulness of the data they yield is limited with respect to size and age classes, tempera- tures, and seasons. Methods based on stomach con- tents, besides being laborious, do not indicate how food consumption might be expected to change with chang- ing environmental conditions or food availability. Fur- thermore, it can be argued that extrapolations from laboratory studies on food intake and growth are not valid because the feed intake of fish in nature is directly dependent on food availability. The amount of energy expended on foraging activity is also a significant item in determining feed intake and feed utilization effi- ciency. The review in this chapter encompasses the published research as it relates to factors that affect feed intake in cultured fishes. Such a review is reasonable, since results extrapolated from studies with wild fishes are of doubtful validity; it is justifiable because many of the valuable natural fisheries have been overexploited (NRC, 1981~. The harvesting of larger amounts of fish from the seas and inland waters does not appear promis- ing because the costs of equipment and effort far exceed prospective additional fishery yields. Furthermore, the supply and demand situation for desirable fish is rapidly changing, and production by fish farming or aquacul- ture has become progressively more feasible. Most factors known to modify feed intake in fish can be characterized as environmental, physiological, or di- etary. It should be remembered, however, that many of the environmental factors that modify feed intake can- not be isolated from the physiological and dietary fac- tors and vice versa. ENVIRONMENTAL FACTORS Water temperature, chemistry (including pH), veloc- ity, as well as photoperiod are all ecological determi- nants of feed intake in fishes. Brocksen and Bugge (1974) investigated the influence of temperature on feed intake in rainbow trout (Salmo gairdnera) and reported that intake increased with temperature from 5C to 20C. Below 5C, feeding activity was very little or nil. Wallace (19743 found that the tropical blenny (Blennius pholis) ate 1.8 times as much food at 25C as at 10C. It is clear that different species of fish have different phys- iological optimum temperatures and that temperature changes do not equally affect the voluntary feed intake. In rainbow trout, brown trout (Salmo tr?,llta), and sock- eye salmon (Oncorhynch?`s nerka) these optimum tem- peratures are 17C, 13C, and 15C to 17C, re- spectively (Storebakken et al., 19811. Some species of fish live and grow in arctic and antarctic waters where body temperatures are at or below the freezing temper- ature of freshwater, while other species thrive in hot springs with body temperatures as high as 40C (Sum- ner and Lanham, 19421. By contrast, the tolerable body temperature range for homeotherms is only a few de- grees (Brody,1945~. Adaptations have been made over a wide range of temperatures by fishes, with each species having a preferred temperature at which voluntary feed intake is highest. Studies on the effects of water chemistry on feed in- take seem to be limited to situations with various water salinities. Most are confounded with such variables as temperature and food availability. One study in which conditions were standardized and food intake was mea- sured was reported by MacLeod (1977~. Working with rainbow trout acclimated to various salinities and tem- peratures, MacLeod showed that feed intake was high- est at the intermediate water salinities from 15.0 %o to 28 %o, lower in freshwater and 7.5 ~oo salinity, and low- est (by a statistically significant margin) at 32.5 %o salin- ity. Food intake was adversely affected by sudden changes in salinity in either direction. MacLeod specu- lated that "the increased feed intake recorded in salini- ties from 7.5 to 28 %o could be exploited to increase the throughput of rainbow trout production in fish farming operations." Frequency of feeding is important; small trout are fed as often as 10 to 24 times per day (NRC, 19811. The frequency is gradually reduced to one to three times per day as the size of the fish increases. Murai and Andrews (1976) showed that the growth of channel catfish (Icta- I?vr?vs punctat?'s) weighing less than 1.5 g was fastest when the fish were fed eight times daily. Feed intake decreased from about 10 to 5 percent of body weight as the fish grew, suggesting that the high frequency of feeding requirement by small catfish is related to their high feed intake rate. Luquet et al. (1981), in a series of experiments with rainbow trout, showed that both fre- quency of feeding and length of fasting periods mark- edly affected feed intake and weight gain. Weight gains were significantly lower in groups of trout fed once a day than in those fed more than once a day. Total feed in- takes on the day of refeeding after fasts of varying

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1~3 Predicting Feed Intake lengths (1 day to 1 week) did not differ significantly. A compensatory increase in feed intake after a fast was limited to the first meal on the day that feeding resumed; thereafter the level of intake was low. Feed intake, as well as growth, can sometimes be accelerated by increased photoperiod (Komourdjian et al., 1976~. Daylight affected the feed intake of Dover sole (Solea solea) and dab (Limanda limanda), whereas tidal cycles were more important in a blenny (Krunk, 1963; Tobling, 1974; Crawford, 19771. Food consump- tion of green sunfish (Lepomis cyanellus) was highest after exposure to 8 to 16 h of light per day (Brett and Groves, 19791. The above findings support the conjec- ture that photoperiod effects are probably mediated via the hypothalamic pituitary axis and are expressed as changes in the production or release (or both) of growth hormone. Factors such as oxygen and ammonia concentrations in water have a marked and predictable effect on feed intake. Lowered dissolved oxygen and elevated ammo- nia in water lead to a diminution in feed intake in cul- tured fishes under otherwise identical environmental conditions (Haskell, 1959~. Fish have pronounced effects on each other when they feed in groups, even when unlimited feed is avail- able. When among fish of the same species, each fish tends to consume less feed than when fed alone (Mann, 1967~. It has been speculated that the proximity of two or more fish of the same species stimulates each to in- crease its rate of feeding, probably due to competition. Antithetically, Kinne (1960) found a reduction in feed intake due to increased stocking density with highly territorial fishes. He postulated that the accumulation of metabolic ammonia and lower dissolved oxygen de- pressed metabolism and the resultant feeding activity. PHYSIOLOGICAL FACTORS Although physiological stimuli are among the most important factors governing feed intake of higher verte- brates, they have received only scant attention in fishes. According to studies by Thorpe and Morgan (1978), different populations of Atlantic salmon (Salmo salary demonstrated that genetics are not a determinant in feed intake. They showed that feed intake and growth response occurred at the same ratio of food particle size to fish length, irrespective of family group. Since the general relationship between growth and this ratio was common to all groups tested, they concluded that "it was unlikely the relative effect of feed size was influ- enced by differences in the genetic background of the fish." The amount of food eaten in a day is often expressed as a percentage of the body weight of the fish. This is interesting when one considers the wide range of weights encountered during the feeding period. Trout weigh about 150 ma when they start feeding and usually are grown to a weight of about 500 g. This 3,000-fold increase in weight is considerably more than the poten- tial weight increases in most mammals. The percentage of food eaten in relation to body weight decreases as weight increases, because small fish have a higher metabolic rate and thus require more food per unit of weight than do larger fish (NRC, 1981~. Brett (1971) found that the daily feed intake of young sockeye salmon held at 15C decreased from an average of 16.9 percent of the dry body weight for a 4-g fish to 4.5 per- cent for a 216-g fish. Young chinook salmon (Oncorhyn- ch?~s tschawytscha), weighing 0.6 g, consumed up to 20 percent of their dry body weight per day (Davis and Warren, 1968) at 10C. Elliott (1975) reported a value of 16.4 percent for a 1-g brown trout held at 18C. On a dry food to wet body weight basis, feed as a percentage of body weight can vary between 0.5 and 10.0 percent for growing fishes, depending on water temperature and chemistry (NRC, 1981, 19831. The causal effect of in- creasing size or age to decreased feed intake can only be pondered at this time. These factors are probably re- lated to changes in the endocrine and metabolic func- tions of the fish. Peter (1979) showed that the neural aspects of feed intake control in fishes are similar to those of higher vertebrates. Even though lesion and electrical stimulation techniques have been success- fully used to locate the brain regions involved with feed- ing behavior (e.g., hypothalamus and other brain glucoreceptors), clear interpretations of such studies have not been possible. Fletcher (1984) suggested that an important consideration is the fluctuations of plasma metabolite levels that have been associated with changes in feed intake and the role of select neural re- gions in their regulation. Several hormones have been shown to affect feed in- take in fishes. A number of fish species show periodic depletion and repletion of carcass energy reserves that can be linked with seasonal changes in hormone secre- tion (Higgs et al., 1982~. It is widely recognized that a number of fish species cease feeding during the repro- ductive season. Such periods are linked to changes in feeding behavior and occur in connection with migra- tion as well as with reproductive cycles. Higgs et al. (1982) and Fletcher (1984) have suggested that hor- mones may influence hunger indirectly by affecting the secretion of other hormones or by inducing changes in the levels of various plasma nutrients. Donaldson et al. (1979) and Higgs et al. (1982) summa- rized the published literature concerning the effects of

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Fishes 19 thyroid hormones on fish growth and, indirectly, feed intake. There is some evidence that thyroid hormones increase the rate of absorption of some nutrients across the gut, thereby enhancing feed conversion efficiency and fish growth. The maintenance of adequate thyroid status in fishes is regarded as a prerequisite for normal growth. The major thyroid hormone effect may lie in its potentiating action on other anabolic hormones, most notably growth hormone. Steroid hormones (both androgens and estrogens) have been shown to either suppress or enhance appetite in fishes while simultaneously altering plasma nutrient levels (Yu et al., 1979; Ince et al., 19821. Donaldson et al. (1979) showed that growth of fish may be stimulated by at least 14 of the known androgen-anabolic steroids that are effective in mammals. Most, if not all, of these ste- roid hormones increased feed consumption while they improved feed conversion efficiency and dietary protein utilization. Markert et al. (1977) and Chua and Teng (1980) showed that the use of bovine growth hormone in fish substantially improved feed intake and increased growth, while it concurrently lowered production costs. Hormones such as prolactin have been shown to affect diurnal variations of plasma fatty acids in Pacific salmon (Peter, 1979~. High blood levels of free fatty acids may suppress feed intake, as they do in higher vertebrates. The activity of prolactin may be entrained by either thyroxine or cortisol; and circadian rhythms in the plasma levels of a number of hormones, including pro- lactin, thyroxine, and growth hormone, have been ob- served in some species (Meter, 1970; de Vlaming and Sage, 1972~. It seems highly likely that feed intake is affected by such endocrine activity. Murat et al. (1981) observed that a variety of fish spe- cies showed appreciable changes in plasma insulin lev- els in response to fluctuations in plasma nutrients. However, the possible influences of insulin and other hormones (i.e., cholecystokinin, cerebrospinal fluid in- sulin) associated with feed intake and nutrient assimila- tion remain to be investigated. Stomach fullness and systemic factors such as circu- lating nutrients and respiratory rate have been shown to be closely related to feed intake (Muir and Niimi, 1972; Ware, 1972; Lee and Putnam, 1973; Toates, 1981~. Grove et al. (1978) found that when food of decreased energy content per unit weight is presented to trout, the compensation which follows to increase daily intake is an increase in the rate of feeding along with a demon- strable decrease in gastric emptying time. Even though it is reasonable to assume that feeding activity ceases when the stomach is full, the total feed intake by some fish species has been observed to be limited by the spe- cific gravity of the feed (Lovell, 1979~. ~, , ~ DIETARY FACTORS Nutrient Deficiency Many examples in the literature relate reduced feed intake to a nutrient deficiency in the diet (NRC, 1981, 1983~. For instance, a deficiency of thiamin or zinc can cause anorexia. It is not known whether these depres- sions of intake are primary or secondary, but it is under- stood that the lack of an essential nutrient causes a deterioration in general health, which in turn would af- fect feeding behavior. Jobling (1983) speculated, with- out supporting data, that dietary energy concentrations (total, digestible energy [DEi, or metabolizable energy tME]) are more important than specific nutrients in the regulation of food consumption. Lee and Putnam (1973) showed that the daily feed intake increased when the energy content of the available food decreased. It is generally held that fishes eat to satisfy their energy re- quirements and that metabolizable energy is the main factor limiting feed intake (NRC, 1981, 1983; Bromley and Adkins, 19841. Food Particle Size and Composition Feed intake and growth rate have been shown to be closely related to food particle size (Wankowski, 1977; Wankowski and Thorpe, 1979~. Optimum particle size in Atlantic salmon increased in direct proportion to fish length. Growth was fastest in Atlantic salmon finger- lings given feed with particle diameters equal to 2.2 to 2.6 percent of body length. At particle sizes correspond- ing to 6.8 percent of body length, feed ingestion ceased. Furthermore, it was demonstrated that fishes select against feed particles with diameters less than 2.2 per- cent of body length. Wankowski speculated (based on morphometric measurements of mouth breadth and gill raker spacing) that these results could be extrapolated to marine fishes. The use of pelleted (compressed) and extruded (ex- panded) diets in fish farming has led to the establish- ment of empirical rules relating to the size of pellet to be fed to fish of a particular size class. Recommendations have been based largely on experience rather than on experimentation (Table 2-1~. Although particle size plays a major role in acceptance or rejection of a diet, it is much more critical for dry diets than for semimoist or moist diets (NRC, 19811. Poston (1974) found that brown trout fed diets con- taining 10 to 55 percent moisture at similar rates, when adjusted for dietary moisture, had comparable dry mat- ter intakes. Likewise, Bromley (1980) fed turbot identi- cal amounts of dry matter in a ration ranging from 0 to

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20 Predicting Feed Intake TABLE 2-1 Food Particle Size Recommendations for Trout U.S. Series (Sieves)a Screen Effect Opening on Feedb Size Through 595,um Over 420 am Through 841 am Over 595,um Through 1.19 mm Over 841 am Through 1.68 mm Over 1.19 mm Through 2.83 mm Over 1.68 mm Granule Pellet Size Standard Fish Size (g) No. Begin End -up to 0.2 Starter #1 Granule #2 Granule #3 Granule #4 Granule 3.18-mm pellet 4.76-mm pellet 6.35-mm pellet 30 40 20 30 16 20 12 16 7 12 Diameter (mm) Length (mm) 3.18 x 3.18 4.76 x 4.76 6.35 x 6.35 0.3 0.5 0.6 1.8 1.9 4.6 15 46 151 4.5 15.0 45 150 aFrom the National Bureau of Standards. bFor example, starter particles will sift through a 595 am (30) screen; the particles will rest on top of a 420 am (40) screen. SOURCE: NRC (1978). 74 percent water. Dry matter intake was the same and he, like Poston (1974), concluded that water content was immaterial as long as the basic nutrient requirements were met. It has been reported that fry of chinook salmon apparently accept moist diets better than air-dry feeds (NRC, 19831. Although the reason for this prefer- ence is unknown, it has been suggested that the ability to incorporate water into the ingesta differs among dif- ferent fishes. Feeding Stimulants In terms of control of feed intake in fishes, olfactory and gustatory stimuli have received the most attention by researchers. Although food may be detected at a distance either visually or chemically by fish, the final decision about whether to swallow or reject potential food material is based on gustation (Aaron and Mackie, 1978~. The feeding behavior of several species of ma- rine and freshwater fishes was found to be mediated by mixtures of chemicals, implying that a number of differ- ent chemosensory cells must be stimulated to induce a feeding response (Mackie et al., 1980~. The mixtures that cause the greatest behavioral responses are com- posed of amino acids, nucleotides, and quaternary amines. Among the single compounds that show the highest effectiveness in these mixtures are betaine, gly- cine, alanine, and taurine. Feed intake in Dover sole was shown by Mackie et al. (1980) to be markedly stimulated by betaine alone in larger ~ > 50 g) fish and by betaine plus glycine or alanine in smaller (about 2.5 g) fish. Betaine plus mixtures of amino acids are feeding stim- ulants for the pinfish (Lagodon rhomboides), pigfish (Orthopristis chrysopter?vs), and puffer fish (F?,lgn perda- lis) (Carr and Chaney, 1976; Carr et al., 1977; Ohsugi et al., 19781. Glycine was a particularly strong attractant for flounder (Pseudlopleuronectes americanus) (Sutterlin, 1975~. Two dipeptides, hypotauryl-2-carboxyglycine and C-methylimino-diacetic acid, were active feeding stimulants for grunt (Bathystoma rimator) (Sangster et al., 1975~. Mackie and Adron (1978) demonstrated that the turbot (Scophthalm?`s maximus) showed a strong gustatory sensitivity to specific nucleotides such as in- osine and inosine 5'-monophosphate. Mixtures of amino acids were effective feeding stimulants for rainbow trout (Aaron and Mackie, 1978~. Rainbow trout that were trained to use demand feeders showed a marked preference for a diet that contained all synthetic amino acids and that simulated the amino acid profile of squid muscle over a plain casein diet with an amino acid pro- file based on known requirements. When amino acid D- forms were substituted for the ~-forms, the resulting diet was repellent, thus indicating a possible stereospe- cificity at the receptor surface. Individual amino acids were either without effect or repellent. When the amino acid mixture was arbitrarily divided into several frac- tions, feeding activity was greatest in fish fed the aro- matic and basic amino acid fractions. Two aromatic amino acids, tyrosine and phenylalanine plus either ly- sine or histidine, were highly stimulatory. No other com- bination of these amino acids was effective, and in fact, phenylalanine, lysine, and histidine were repellent. When certain individual amino acids or combinations were shown to be repellent, fish usually took the pellet into the mouth and then rejected it. This behavior sug- gests that if olfaction were the main sense involved, the repellent diet would be rejected before it was mouthed. Goh and Tamura (1980), working with bream (Chry- sophyrys major), found the rank of feeding behavior re- sponse to the best 7 of the 15 compounds tested was as follows: alanine-betaine > glycine-betaine > t~-alanine > -valise > anserine > ~-arginine > ~-glutamine. The behavioral responses were consonant with electrical physiological measurements of the gustatory system. Gob et al. (1979) speculated that taste responses to amino acids are species specific while olfactory re- sponses are not. Feed Quality Feed quality, including the presence of adventitious toxins, is readily detectable by fish (NRC, 1981, 1983~.

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Fishes 2 1 . The inclusion of oxidized fish and vegetable oils in fish 2. The length-weight relationships for salmonid fish diets results in reduced feed intake. Substances like published by Piper et al. (1982) are correct. gossypol in cottonseed and protease inhibitors in soy- 3. A growth rate of 1 in. (2.54 cm) per month is ac ceptable growth for rainbow trout at 15C (Piper et al., 1982~. bean meal cause anorexia when they are present in feed- stuffs used in fish feed manufacturing. Aflatoxins and T-2 toxins, which are produced by different molds grow- ing on feedstuffs, depress feed intake (NRC, 1981; Pos- ton et al., 1982~. Finally, there is much evidence that manmade contaminants-as typified by industrial chemicals, pesticides, and herbicides sometimes lead to depressed appetite as well as a myriad of metabolic disorders (NRC, 19831. PREDICTING FEED INTAKE Quantitatively, the most important factors regulating the amount of feed consumed by fishes are species, body temperature (which is close to water temperature), body weight, feed DE content, feed palatability, and particle size. Table 2-2 was developed by Hilton and Slinger (1981) and used by the NRC (1981) in its report on cold- water fish nutrient requirements. Table 2-2, the feeding guide publication of Buterbaugh and Willoughby (1967), and the feeding guide recommendations of sev- eral feed manufacturers (unpublished) have been used to derive some feeding equations for rainbow trout. The mathematical derivation of these equations required that the following assumptions be made: 1. The metabolic body size and the feed energy re- quirement of fish is proportional to the 0.75 power of body weight (BW075) (Smith et al., 1978~. TABLE 2-2 Fish Feeding Guide Number of Fish/kg 2,600 1,300 700 400 200 130 90 40 30 20 15 10 Granule and Pellet Size, in. (mm) #1 #1 #2 #2 #3 #3-4 #4 3/32 (2.4) 3/32 (2.4) 1/8 (3.4) 1/8 (3.4) 3/16 (4.8) 3/16 (4.8) 1/4 (6.4) 4. Feed containing 3,000 kcal of DE/kg will produce fish at a conversion efficiency of 1.5 g of feed/g of growth (NRC, 1981~. 5. The finding by Haskell (1959) that "the normal growth of trout under conditions of constant water tem- perature and adequate food supply is such that the rate of increase in body length is constant" is correct. 6. The effect of temperature on metabolism and the feed (energy) requirement of trout is linear in the range of temperatures usually encountered in salmonid pro- duction (6C to 18C) (Iwama and Toutz, 1981~. Feeding Equation Derivation A 100-g fish in 15C water growing at the rate of 1.4 percent per day will gain 1.4 g/day. At a feed conversion ratio (feed/gain) of 1.5 g of feed/g of gain, this requires 1.4 x 1.5 = 2.1 g of feed/fish/day, which is equal to 2.1 percent BW. However, metabolic body size and feed requirement is proportional to BW075, e.g., Amount of feed/fish = K 201 75 = 0.0664. This procedure was repeated for several sizes of fish ranging in weight from 12 to 180 g, and a constant (K) with a (rounded-off) mean value of 0.066 was obtained and used to calculate feed intake using the following simplified equation: Amount to feed ('ho BW) = 0.066BW -0 25100. (1) % BW Consumed per Day at the Following Water Temperatures (C) 6 2.9 2.8 2.7 2.6 2.3 2.1 1.9 1.6 1.5 1.3 1.2 1.1 1.0 0.8 3.4 3.3 3.0 2.8 2.6 2.3 2.0 1.7 1.6 1.4 1.3 1.2 1.1 0.9 3.7 3.6 3.3 3.0 2.8 2.5 2.1 1.8 1.7 1.5 1.4 1.3 1.2 1.0 9 3.9 3.8 3.6 3.2 3.0 2.8 2.4 1.9 1.8 1.6 1.5 1.4 1.3 1.0 10 11 12 4.6 4.8 5.2 4.4 4.7 4.9 4.1 4.5 4.8 3.9 4.0 4.6 3.6 3.8 4.3 3.3 3.6 3.7 2.7 2.9 3.0 2.0 2.1 2.4 1.8 1.9 2.0 1.7 1.8 1.9 1.6 1.7 1.8 1.5 1.6 1.7 1.4 1.5 1.6 1.1 1.1 1.2 13 5.8 5.6 5.1 4.9 4.5 3.9 3.2 2.6 2.2 2.1 2.0 1.8 1.7 1.3 14 6.0 5.9 5.6 5.0 4.6 4.0 3.6 3.0 2.8 2.4 2.3 1.9 1.8 1.5 NOTE: Feeding rates are based on a single strain of rainbow trout fed dry diets containing digestible energy of approximately 3,000 kcal per kg (J. W. Hilton and S. J. Slinger, University of Guelph, Guelph, Ontario, Canada, personal communication, 1979). SOURCE: NRC (1981). 15 6.4 6.1 5.8 5.1 4.7 4.1 3.8 3.2 2.9 2.5 2.4 2.0 1.9 1.6

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Temperature (%)Fish Weight (g) H & sa Rangenb MurrayC B & We DE (3,200) DE (3,000) 61.4 2.7 3.9 2.8 1.3 2.6 2.7 65.0 2.3 3.1 2.0 0.8 1.9 2.0 625.0 1.6 1.9 1.2 0.4 1.2 1.3 650.0 1.3 1.5 0.9 0.3 1.1 1.1 6100.0 1.1 1.2 0.3 0.3 0.9 0.9 101.4 4.1 4.6 4.3 3.3 3.7 3.9 105.0 3.6 4.0 3.6 2.1 2.7 2.8 1025.0 2.0 2.6 2.2 1.3 1.8 1.9 1050.0 1.7 2.0 1.7 1.0 1.5 1.6 10100.0 1.5 1.6 1.4 0.8 1.3 1.3 151.4 5.8 5.7 7.8 7.8 5.8 6.1 155.0 4.7 4.7 6.0 5.0 4.2 4.5 1525.0 3.2 3.0 4.0 3.2 2.8 3.0 1550.0 2.5 2.4 3.1 2.7 2.4 2.5 15100.0 2.0 2.0 2.5 2.4 2.0 2.1 22 Predicting Feed Intake TABLE 2-3 Comparison of Recommended Feeding Levels (Percent of BW to Feed/Day) for Rainbow Trout at 15C Fish Size Calculate de Length (in., cm approx.) Weight (g) H & sa Rangenb MurrayC B & We DE (3,200) DE (3,000) 0.250 - - - - 9.1 - 0.385 6.4 6.5 - 11.8 8.1 8.7 0.5 - - - - 7.6 - 0.770 6.1 6.0 - 9.4 6.8 7.3 2 (5.1) 1.43 5.8 5.8 7.8 7.4 5.9 6.3 2.50 5.1 5.2 6.5 6.2 5.1 5.4 3 (7.7) 5.00 4.7 5.0 6.0 5.0 4.3 4.6 7.70 4.1 4.3 5.2 4.3 3.9 4.1 4 (10.2) 11.1 3.8 3.9 4.9 3.8 3.5 3.7 5 (12.7) 25.0 3.2 3.3 4.0 3.0 2.9 3.1 6 (15.2) 33.3 2.9 3.0 3.5 2.6 2.7 2.8 50 2.5 2.7 3.1 2.4 2.4 2.6 7 (17.8) 67 2.4 2.6 2.7 2.2 2.2 2.4 8 (20.3) 100 2.0 2.2 2.4 1.8 2.0 2.2 10 (25.4) 200 1.9 1.8 1.8 1.6 1.7 1.8 500 1.6 1.6 1.3 - 1.4 1.5 eHilton and Slinger (1981). bRangen, Inc. CKlontz et al. (1985). dButerbaugh and Willoughby (1967). eCalculated by Equation 4. Numbers in parentheses are DE kcal/kg. It is generally held that, within the range of the physi- cal capacity of the digestive system, fish will eat to meet their energy needs. It has been determined by many years of hatchery experience and research studies that diets which contain 3,000 kcal of DE/kg will allow trout to convert feed to flesh at about 1.5 feed conversion value. To introduce the energy value of feeds, Equation 1 is multiplied by 3,000 and divided by the DE of the diet being fed: 0.066BW-" 253,000 200BW-" 25 Amount to feed (% BW) = DE DE (2) TABLE 2-4 Comparison of Recommended Feeding Levels (Percent of BW to Feed/Day) for Rainbow Trout at Three Water Temperatures and Five Different Fish Sizes Calculated aHilton and Slinger (1981). bRangen, Inc. CKlontz et al. (1985). ~Buterbaugh and Willoughby (1967). eCalculated by Equation 4. Numbers in parentheses are DE Kcal/kg.

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Fishes 23 The constant 200 in Equation 2 applies to rainbow trout growing in 15C water at the rate of about 1 in. (2.54 cm) per month. The constant should be considered a guide and can be modified to fit various rearing condi- tions. By adjusting this factor up or down, more or less growth can be obtained with some loss of efficiency. A constant that would apply to other species and other growth rates would need to be determined experimen- tally. Table 2-3 shows a comparison of suggested amounts of feed to be fed according to Hilton and Slinger (1981), Buterbaugh and Willoughby (1967), and two leading feed manufacturers versus those calculated using Equation 2. It is worth mentioning that some newer feeds contain upwards of 4,000 kcal of DE/kg. These higher-caloric-density feeds would be wasted if fed according to existing feeding tables. Estimates on the quantitative change in feeding due to changes in water temperature range from 5 to 10 percent/C. However, over the range of temperatures usually experienced in most trout farming operations, the response is nearly linear. For this exercise we have used a rate of [ceding change of 9 percent/C and a modification of the compound growth equation Al = Aced, where Al is the amount of feed at some future temperature, Ao is the amount of feed at the present temperature, r is rate, t is time, and e is the base of natural logarithms. The effect of temperature (Tin C) on fish feed intake can be calculated lay modifying the equation as follows: A ~ = AOe009~7'-151- (3) By combining Equation 2 with Equation 3, we can determine the amount to feed (percent BW) at any tem- perature using the following equation: 200 BW-025eo o9~7-~5' Amount to feed = DE (4) Table 2-4 shows a comparison of the amounts to feed at three temperatures and five fish body weights from several different sources with those calculated by Equa- tion 4. REFERENCES Adron, J. W., and A. M. Mackie.1978. 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