Response of red drum to various dietary levels of menhaden oil

Aquaculture, 70 (1988) 107-120 Elsevier Science Publishers B.V., Amsterdam -

107 Printed in The Netherlands

Response of Red Drum to Various Dietary Levels of Menhaden Oil CHRISTOPHER D. WILLIAMS

and EDWIN H. ROBINSON2,3

Department of Wildlife and Fisheries Sciences, 210 Nagle Hall, Texas A&M University, College Station, TX 77843-2258 (U.S.A.) Present addresses: I7115 Leyte Drive, Oxon Hill, MD 20745 (U.S.A.) and ‘Associate Fishery Biologist, Mississippi Agricultural and Forestry Experiment Station, Delta Branch, P.O. Box 197, Stoneville, MS 38776 (U.S.A.) 3To whom correspondence should be addressed. (Accepted 24 August 1987)

ABSTRACT Williams, C.D. and Robinson, E.H., 1988. Response of red drum to various dietary levels of menhaden oil. Aquaculture, 70: 107-120. A study was conducted to evaluate the effects of feeding, varying levels of menhaden oil on weight gain, feed conversion, and whole-body proximate and fatty acid composition of red drum. The experiment was conducted utilizing a brackishwater (5-6 ppt) recirculating system which supplied water to a series of 40-l aquaria. Fish (1.8 g) were stocked at a rate of 12 fish per aquarium (48 fish/treatment) and fed the appropriate diet twice a day for a period of 6 weeks. All diets contained 40% protein and an estimated digestible energy value of 15.4 kJ/g of diet. Dietary lipid levels ranged from 1.7 to 18.8%. Weight gain, feed conversion, and survival were best at dietary lipid levels of 7.4 and 11.2%. Fish fed the diet containing 18.8% lipid had lower weight gains and higher feed conversion ratios than fish fed lower levels of lipid. Whole-body lipid concentrations increased as dietary lipid increased up to 7.4% then decreased when dietary levels were elevated to 15% or greater. Whole-body fatty acids generally reflected dietary lipid, particularly in respect to the n3 polyunsaturated fatty acids.

INTRODUCTION

Red drum Sciaenops ocellatus, often called redfish or channel bass, is an important commercial species harvested along the Gulf and Atlantic coasts. Concern about declining harvests and overexploitation of indigenous stocks, led Texas to ban the commercial harvest of red drum from native waters in 1981 as well as increase restraints on recreational catches. With the strong possibility that other states may also ban or restrict commercial harvest of red drum, an increased interest in culture has developed. Though red drum possess 0044-8486/88/$03.50

0 1988 Elsevier Science Publishers B.V.

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several characteristics desirable for culture, little is known concerning their nutrition. Early work concerning the nutrition of red drum was limited to studies of food habits in the wild (Boothby and Avault, 1971; Bass and Avault, 1975; Overstreet and Heard, 1978). More recently, studies concerning their dietary protein requirements in salt water (Lin and Arnold, 1983), optimum protein:energy ratio in brackish water (Daniels and Robinson, 1986)) and dietary phosphorus requirement (Davis and Robinson, 1987) have been reported. Several researchers have reported the dietary lipid requirement for various species (Dupree, 1969; Lee and Putnam, 1973; Reinitz and Hitzel, 1980; Gatlin and Stickney, 1982; Millikin, 1983). Perhaps of more importance than dietary lipid concentration is the fatty acid composition of a given lipid source. Various researchers have demonstrated the essentiality of the linolenic family of fatty acids (n3) for fish (Higashi et al., 1964, 1966; Lee et al., 1967; Caste11et al., 1972a,b,c; Watanabe et al., 1974a,b,c; Yone and Fuji, 1975a,b; Gatesoupe et al., 1977; Yamada et al., 1980). Caste11 et al. (1972a) and Watanabe et al. (1974a,b,c) reported that rainbow trout required between 0.8 and 1.6% linolenic acid (18:3 n3 ) to prevent signs of essential fatty acid deficiency. Yone and Fuji (1975a,b) reported that the requirement of red sea bream for n3 fatty acids was satisfied with approximately 0.5% polyunsaturated fatty acid (PUFA) . A mixture of n3 and n6 fatty acids has been shown to be essential for the common carp (Takeuchi and Watanabe, 1977) and the eel (Takeuchi et al., 1980). Excessive levels of certain fatty acids are detrimental to fish performance. Weight gain in channel catfish is suppressed when lipid sources high in linolenic (18:3 n3) are fed (Stickney and Andrews, 1972; Stickney et al., 1983). Takeuchi and Watanabe (1979) found that feeding 4% or more of 18:3 n3 or other n3 PUFA to rainbow trout resulted in poor growth and low feed efficiency. Although lipids are important dietary components which provide a source of concentrated energy and essential fatty acids and can be used to spare dietary protein for growth, there have been no studies concerning lipid utilization by red drum. The present study was designed to define the effects of feeding graded levels of menhaden oil on weight gain, feed conversion, whole-body proximate composition, and whole-body fatty acids of red drum reared in a low-salinity environment. METHODS

Experimental

design and die&

Red drum fry, provided by Texas Parks and Wildlife Department, Palacios, TX, were transported to the Texas A & M Aquacultural Research Center and acclimated to a salinity of 5-6 ppt. All fish were initially fed frozen brine shrimp

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TABLE 1 Composition of experimental diets (% dry matter) Ingredient

Shrimp-head meal Casein Dextrin Carboxymethyl cellulose Vitamins’ Minerals’ Cellulose Menhaden fish oil Ethoxyquin Total lipid’

Diet no. 1

2

3

4

5

6

10.0 38.0 40.0 2.0 0.5 5.5 4.0 0.0 0.00625

10.0 38.0 34.0 2.0 0.5 5.5 7.0 3.0 0.00625

10.0 38.0 26.0 2.0 0.5 5.5 11.0 7.0 0.00625

10.0 38.0 18.0 2.0 0.5 5.5 15.0 11.0 0.00625

10.0 38.0 10.0 2.0 0.5 5.5 19.0 15.0 0.00625

10.0 38.0 2.0 2.0 0.5 5.5 23.0 18.0 0.00625

11.2

15.0

18.8

1.7

4.0

7.4

‘Meets or exceeds NRC recommendations for warmwater fishes (NRC, 1983). ‘By analysis.

and slowly converted to a commercial trout starter (Zeigler Bros., Inc., Gardners, PA). Fish were fed 2-3 times daily and maintained on commercial feed until reaching a desirable experimental size. Before the start of the experiment, all fish were conditioned for a 2-week period on a nutritionally complete diet similar to diet 3 (Table 1) except that it contained only 6% lipid. This preliminary period was used to habituate the fish to environmental conditions and experimental diets. Casein and shrimp-head meal were used as protein sources (3.8:l.O) in the experimental diets. The levels of shrimp-head meal and casein in the basal diet were adjusted to provide approximately 1.7% lipid and 40% crude protein. The basal diet was based on preliminary studies conducted in our laboratory. Menhaden fish oil was added to the basal diet in an attempt to provide dietary lipid levels of 4, 8, 12, 16, and 20% (diets ‘2, 3, 4, 5, and 6, respectively; Table I). Actual lipid levels determined by proximate analysis are given in Table 1. It is not known why actual lipid values varied from those anticipated. All diets were maintained isoenergetic at 15.4 kJ/g of diet by adjusting dextrin levels. Dextrin was substituted for menhaden oil on an isocaloric basis, considering dextrin contained one-half the calories of menhaden oil. As adjustments of dextrin and menhaden oil were made cellulose was added to maintain proper nutrient density. All diets were prepared by mixing the dry ingredients in a twin-shell dry blender (Patterson-Kelly Co., East Stroundsburg, PA) for 40 min. The lipid source was then blended with the mixed, dry ingredients using a Hobart mixer (Hobart Corp., Troy, OH). Water was added at a rate of 40% to facilitate

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pelleting. Each diet was pelleted using a meat grinder. After pelleting, all diets were air dried for 24 h and frozen until needed. Portions, as needed for feeding, were mechanically crumbled and sieved to appropriate sizes and refrigerated. Fingerling red drum were stocked into 40-l aquaria which were part of a closed, brackishwater (5-6 ppt) recirculating system which delivered water at a rate of 0.95 l/min. Supplemental air was supplied to each aquarium by a lowpressure blower. Twelve fish (22.5 + 1.0 g total biomass) were randomly stocked into each aquarium. Four replicates were used for each treatment. All fish were initially fed at 6% of their body weight daily, divided into two equal feedings. This rate was in the range indicated by a preliminary study concerning optimum feeding rates of fingerling red drum. The 6% feeding rate was increased to 10% after 1 week in an attempt to reduce cannibalism. After 2 days, the feeding rate was reduced to 8% (for the remainder of the experiment ) upon observation of uneaten feed in the aquaria. All fish were weighed weekly and feeding rates adjusted accordingly. Every 2 weeks fish were given 90-s freshwater dips after weighing as a prophylactic treatment for saltwater parasites. The experiment was terminated at the end of 6 weeks because of a sudden increase in unionized ammonia. Water quality During the experiment, ammonia, nitrite, and nitrate were monitored weekly by spectrophotometric methods (HACH-DREL/5 spectrophotometer, Hach Chemical Company, Loveland, CO) ; pH was determined weekly with a Digisense pH meter (Cole Parmer, Chicago, IL). Dissolved oxygen (D.O.) and temperature were measured several times a week using a YSI 51B oxygen/ temperature meter (Yellow Springs Instrument Company, Inc., Yellow Springs, OH). Salinity was monitored several times weekly with a salinity refractometer (American Optical, South Bridge, MA) and, if necessary, adjusted to 5-6 ppt by the addition of artificial salts (Instant Ocean, Aquarium Systems, Mento, OH). Sample collection and analysis At the conclusion of the experiment, six fish from each tank were frozen for subsequent chemical analysis. Composite samples of three fish from individual tanks were homogenized using a Virtis “23” homogenizer (Virtis Company, Gardiner, NY). Whole-body composition was determined on triplicate composites from each tank (12 fish/treatment). Dry matter and ash were determined by standard methods and whole-body crude protein was determined by the macro-Kjeldahl method ( AOAC, 1980). Whole-body lipid was determined by the method of Folch et al. (1957). Fish whole-body lipid and dietary lipid were extracted using chloroform and

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methanol ( AOAC, 1980) and frozen for subsequent fatty acid analysis. Fatty acid analyses were performed using a Varian series 2400 gas chromatograph equipped with a flame ionization detector and SP 2330 wide-bore, glass capillary column (Supelco, Inc., Bellefonte, PA). Fatty acids were identified by comparison with standards (Applied Science Laboratories, State College, PA). Quantitative results were determined using a Spectra-Physics integrator (Spectra-Physics Co., Santa Clara, CA). Statistical analysis A one-way analysis of variance was conducted to determine the significance (P < 0.05) of treatment effects, and Duncan’s multiple range test was conducted to determine differences among treatment means (Steel and Torrie, 1960). Lipid percentages and fatty acid data were analyzed as arcsin-transformed values. Statistical analyses were performed using the statistical analysis system, SAS (Helwig and Council, 1979). RESULTS

Water quality Water temperature and dissolved oxygen ranged from 23.0 to 26.5 oC and 6.7 to 7.6 mgjl, respectively. Total ammonia ranged from 0.0 to 0.7 mg/l while the pH remained between 8.0 and 8.4. Nitrite values ranged from a minimum value of 0.005 mg/l at the beginning of the study to a maximum of 0.18 mg/l upon termination of the study. At 6 weeks a sudden unexplained failure in the biofiltration system occurred. Nitrates ranged from 13 to 33 mg/l. Weight gain, feed conversion, and survival Mean percentage weight gain ranged from 108 in fish fed 18.8% total lipid (diet 6 ) to 419 in fish fed 7.4% total lipid ( diet 3 ) over a B-week period (Table 2). Fish fed diets 3 and 4, which contained 7.4 and 11.2% lipid, respectively, had the highest weight gains. Weight gains of fish fed diets containing 1.7 and 15% lipid (diets 1 and 5) were not significantly different from each other. Weight gains of fish fed 18.8% lipid (diet 6) were significantly lower than fish fed diets containing other lipid levels. Feed conversion ratios (FCR) are presented in Table 2. They were lowest for fish fed diets containing 4.0, 7.4, and 11.2% lipid (diets 2,3, and 4, respectively) . Fish fed diet 5 (15% lipid) had a somewhat higher FCR than fish fed diets 2,3, and 4, but it was not significantly different. Fish fed 18.8% lipid (diet 6) had the highest FCR. Survival (Table 2) of fish fed diets 2-5 ranged from 85 to 100%. Fish fed

112 TABLE 2 Weight gain, feed conversion ratio, survival, and whole-body composition ( % dry weight) of red drum fed diets containing varying levels of 1ipidl.a Diet no.

1

2 3 4 5 6

Lipid (%l 1.7 4.0 7.4 11.2 15.0 18.8

Initial weight

Final weight

FCRa

(g)

Weight gain (%o)

(gl 1.8 1.9 1.9 1.8 1.9 1.9

5.1 8.2 9.9 8.9 5.9 3.9

183” 332b 421” 394” 210” 105d

2.9” l.7b 1.5b 1.6b 2.2”b 4.6”

Survival (%l

Whole-body composition ( W) Protein

Lipid

Ash

Dry matter

56” 95b 100b 8gb 85b 54”

64.1” 64.6” 60.6b 63.3ab 60.7b 62.5”b

11.6” 18.1bc 24.2d 23.3d 20.5b 17.2’

19.9” 16.0b 13.8” 14.5bc 16.2s 19.9”

16.6” 19.4b 20.8 21.4” 20.1b” 19.3b

‘Means of three replicates. ‘Feed conversion ratio-dry weight feed/wet weight gain. 3Means with same superscript are not statistically different. Pooled standard errors weight gain, FCR, survival, whole-hody protein, lipid, ash, and dry matter were 9.1,0.10,2.2,0.36,0.36,0.29, and 0.11, respectively.

diets 1 (1.7% lipid) and 6 (18.8% lipid) had survival rates of 56 and 54%, respectively. Whole-body composition Whole-body protein concentrations ranged from approximately 61 to 65% (Table 2). There was no trend observed, though the percentages of protein of fish fed 7.4 and 15% lipid (diets 3 and 5) were lower than that of fish fed 1.7 and 4.0% lipid (diets 1 and 2). There were no other significant differences. Dietary lipid and whole-body protein percentages were not highly correlated (R2=0.21). Whole-body lipid (Table 2) ranged from 11.6 to 24.2%. Whole-body lipid increased in fish as dietary lipid increased to 7.4% then remained essentially the same in fish fed 7.4 and 11.2% dietary lipid. Fish fed diets with lipid levels of 15.0 and 18.8% (diets 5 and 6) had significantly decreased body lipid, compared to fish fed diets 3 and 4. Dry matter ranged from 16.6 to 21.4% (Table 2). Fish fed the lowest lipid level (diet 1) had the lowest dry matter percentage while fish fed diets 3, 4, and 5 had the highest. There was a positive correlation between whole-body dry matter and whole-body lipid concentration ( R2 = 0.70). Fatty acids The fatty acid profiles for the experimental diets and fish whole bodies are given in Tables 3 and 4. In general, tissue lipids contained a predominance of 16-carbon and 18-carbon fatty acids. Average 16-carbon levels (16:0 plus 16:l n7) were approximately 45% of total whole-body fatty acids while 18-carbon

113 TABLE 3 Predominant fatty acids in experimental diets? Fatty acids’

Diet no.

(1.7)3

2 (4.0)

3 (7.4)

4 (11.2)

5 (15.0)

6 (18.8)

Saturates 14:o 16:0 l&O

7.1 ?I 1.2 24.9 i 1.4 7.0 + 0.4

13.6 k 0.2 22.1 If:0.2 5.5LO.2

13.1 f 0.7 19.8 + 0.2 4.OkO.l

14.0 ?I0.5 20.8 + 0.6 5.1+ 0.2

14.0 k 0.5 19.7kO.3 5.0+0.1

9.7kO.4 21.6kO.7 5.3 kO.1

Monoenes 16:l n7 18:l n9

12.8f0.5 17.4kO.7

14.3 + 0.2 14.6fO.l

13.8kO.l 13.2 f 0.4

14.2 + 0.4 13.410.4

14.1+ 0.2 13.410.1

15.OkO.4 13.8f0.2

Dienes l&2 n6

2.9io.2

1.2*0.1

0.6fO.l

0.5fO.l

0.4fO.l

0.3kO.l

Tetraenes 20:4 n3

0.9kO.l

0.9kO.l

2.8? 1.1

1.5kO.5

2.2 k 0.5

1.5kO.5

Pentaenes 20:5 n3 22:5 n3

6.5k0.3 0.8kO.2

12.8kO.l 1.2f0.2

15.8f 1.1 1.2fO.l

14.8 f 1.5 1.2+0.1

16.0 f 0.4 1.2kO.l

14.6 k 0.8 1.2kO.l

Hexaenes 22:6 n3

3.9 f 0.3

3.5kO.4

3.1 Ik0.1

3.4kO.l

3.2 + 0.4

1

3.5 f 0.4

‘Mean percentage of total + standard error. A small percentage of 15:0,17:0,19:0, and l&3 n3 + 2O:O (co-eluted) fatty acids were identified but not reported. ‘Short-hand designation, first number indicates number of carbons, second number the number of double bonds, and the (n+number) designates location of first double bond starting from methyl end of the molecule. 3Percentage dietary lipid.

levels (l&O, 18:l n9, and 18:2 n6) averaged about 31%. The longer-chain fatty acids, 20 carbons and above, averaged about 10% across diets but ranged from 3.7 to 19.5%. Overall levels of n3 fatty acids in the diet increased as menhaden oil increased. These values ranged from about 0.2 to 3.8% of the diet. The level of essential fatty acids (EFA) required by warmwater fish is usually limited to 0.5-2% of the dry diet (NRC, 1983). Saturates All diets had similar saturated fatty acid levels. Whole-body concentrations of saturates were similar, except for fish fed the low-lipid diet (diet 1)) which

114 TABLE 4 Whole-body fatty acid composition of red drum fed diets containing varying levels of menhaden oil1 Fatty acids’

Diet no. 1 (1.7)s

2 (4.0)

3 (7.4)

4 (11.2)

5 (15.0)

6 (18.8)

Saturates 14:o l&O l&O

2.OkO.3 22.4-to.9 8.1 kO.2

5.6kO.4 30.4 + 0.6 9.7kO.2

7.4kO.3 29.8kO.6 8.7kO.l

8.71kO.5 28.1+ 0.2 7.6kO.l

8.9 + 0.5 27.5 &I0.4 7.6kO.l

8.01kO.3 28.3kO.6 7.2 kO.1

Monoenes 16:l n7 181 n9 22:l 7211

22.6kO.7 22.9f0.8 0.8ao.l

15.1 f 0.6 24.2f0.5 0.8fO.l

15.2 I! 0.4 20.3 + 0.4 0.6LO.l

17.2 I! 0.2 19.2 + 0.7 0.6kO.l

16.5 f 0.6 19.1 I! 0.4 0.7t0.1

17.4 kO.6 20.5 k 0.6 0.7IkO.l

Dienes l&2 n6

1.7kO.2

1.2kO.l

0.8 + 0.1

0.7kO.l

0.720.1

0.850.1

Tetraenes 20:4 n3

0.2fO.l

0.6fO.l

0.8+0.1

0.7IkO.l

0.8 k 0.1

0.7kO.l

Pentaenes 20:5 n3 22:5 n3

0.3kO.l 2.020.8

2.920.2 1.2kO.l

5.2 + 0.4 2.0 + 0.1

6.3 f 0.2 1.9kO.l

7.0 ?z0.8 2.OkO.3

5.8kO.9 1.6 + 0.2

Hexaenes 22:6 n3

0.4fO.l

1.2f0.2

1.8fO.l

2.1 I! 0.2

2.6 k 0.4

1.9 & 0.3

IMean percentage of total f standard error. A small percentage of 15:0,17:0,19:0, and 20:0 + 18:3 n3 (co-eluted) were identified but not reported. ‘Short-hand designation, first number indicates number of carbons, second number the number of double bonds, and the (n + number) designates location of first double bond starting from methyl end of the molecule. 3Percentage dietary lipid.

contained lower levels of 14:0 and 16:0. Saturates averaged about 35% of fish total fatty acids. Monoenes Fish fed the low-lipid diet (diet 1) had a significantly higher level of 16:ln7 than fish fed other diets though the diet contained a lower level of 16:l n7. Average percentage of monounsaturated fatty acids across diets was approximately 39% of whole-body fatty acids.

115

Dienes Linoleic (l&2 n6) was the only diene identified in the diet. Dietary levels ranged from a high of 2.9% in diet 1 to 0.3% in diet 6. Tissue 18:2 n6 levels were similar to dietary levels. Trienes A small percentage ( < 0.2% ) of linolenic acid (18:3 n3) was tentatively identified as co-eluting with 20:0 in both diets and tissues. Tetraenes One fatty acid with. four double bonds was identified (20:4 n3) in the diet and tissues. Less than 1.0% 20:4 n3 was found in the tissue fatty acid fraction. Pentaenes Two fatty acids containing five double bonds were found in both the diets and in the tissues. Relatively high levels of 20.5 n3 were found in the diets. tissue levels of 20:5 n3 ranged from 0.3% in fish fed diet 1 (1.7% lipid) to 7.0% in fish fed diet 5 (15% lipid). Smaller percentages of 22:5 n3 were found in the diets and tissue fatty acids. DISCUSSION

Water quality Holt and Arnold (1983) examined the effects of ammonia and nitrites on growth and survival of red drum eggs and larvae. Based on their data it is unlikely that the ammonia or nitrite levels observed in the present study exhibited any significant influence on fish performance during the first 6 weeks. However, the experiment was terminated at 6 weeks due to a sudden rise in environmental ammonia concentration, increased water turbidity, and increased mortalities in two treatments. The increased ammonia was caused by an unexplained die-off of the bacteria in the biofilter. Weight gain, feed conversion ratio, survival and proximate composition The dietary lipid level ( 7-11% ) that appeared to be optimum for growth of red drum is similar to that reported to maximize growth of other species (Dupree, 1969; Lee and Putnam, 1973; Reinitz and Hitzel, 1980; Gatlin and Stickney, 1982; Millikin, 1983). Also, Daniels and Robinson (1986) reported good

116

weight gain of red drum fed diets containing 12% menhaden fish oil. Several studies have indicated that high dietary lipid levels depress weight gain (Dupree, 1969; Andrews et al., 1978; Takeuchi et al., 1978a,b). The reduced weight gain observed in the present study in fish fed high-lipid diets may have been due to a decrease in ability to digest and assimilate high levels of lipid, to a reduction in feed intake, or to an imbalance in dietary fatty acids (discussed under section on fatty acids). Andrews et al. (1978) demonstrated a reduced lipid utilization in channel catfish fed diets containing 15% supplemental animal lipid. Feed intake of fish fed high-lipid diets may have been reduced; however, consumption in farm animals (Maynard et al., 1979) and in fish (Boonyaratpalin, 1978) is related to dietary energy, with consumption being reduced in fish fed diets containing high levels of energy. The diets used in the present study were isocaloric, thus food consumption should not have been altered unless the high-lipid diets were unpalatable. Based on visual inspection of uneaten feed remaining in aquaria and on fish feeding activity, there did not appear to be a difference in feed consumption. Survival was good in all treatments except for fish fed diets containing 1.7 or 18.8% lipid. The cause of the mortalities was not apparent. Increases in tissue lipid deposition have been reported in fish as dietary lipid increases (Lee and Putnam, 1973; Garling and Wilson, 1977; Takeuchi et al., 1978a,b; Bromley, 1980; Millikin, 1983). In the present study, whole-body lipid levels increased in fish fed diets containing up to 11.2% lipid, then declined significantly in fish fed higher dietary lipid levels. The specific reason for the observed decline in fish fed high-lipid diets is not known but may have been related to a decrease in lipid utilization or perhaps to reduced feed intake, though feed consumption appeared to be good for all diets. Andrews et al. (1978) reported substantially reduced digestible energy and apparent lipid absorbability for channel catfish fed diets containing 15% animal lipid (17.5% total lipid) when compared to fish fed diets containing no supplemental animal lipid (2.5% total lipid). Digestible energy and absorbability values were not different for fish fed up to 10% supplemental animal lipid. Whole-body protein did not show a clear trend in relation to dietary lipid. There were some statistical differences, but no definite relationship was evident. Page and Andrews (1973) suggest that whole-body protein is lower in fish fed high-lipid diets as a result of dilution with lipid. Also, dietary lipid has been shown to spare protein for growth and to improve protein utilization (Watanabe, 1977; Takeuchi et al., 1978a,b; Yu and Sinnhuber, 1981; Gatlin and Stickney, 1982). Percentage dry matter increased as lipid increased, which would be expected since, generally, an inverse relationship between body water and lipid is observed as animals fatten (Maynard et al., 1979).

117

Fatty acids Dietary lipid has been shown to affect the composition of tissue lipids in fish (Stickney and Andrews, 1971; Fugi et al., 1976). In the present study the nine major fatty acids found in the diet accounted for 92% of the total fatty acid fraction of red drum whole-body lipids. The increase of tissue 20 carbon n3 fatty acids as the level of dietary menhaden oil increased was of particular importance. The levels the n3 fatty acids (20:4 n3 + 20:5 n3 + 22:6 n3) ranged from 0.2 to 3.8% when expressed as a percentage of the dry diet. Essential fatty acid requirements for rainbow trout have been reported to be 0.8-1.6% ( Caste11 et al., 1972a; Watanabe et al., 1974a,b,c) and about 0.5% n3 fatty acids appear to satisfy the EFA requirements of the red sea bream (Yone et al., 1974). In general, the EFA requirements of warmwater fish is not greater than 2.0% of the diet (NRC, 1983). Takeuchi and Watanabe (1979) reported that feeding 4% or more of n3 fatty acids depressed growth in rainbow trout. Channel catfish growth was depressed when fed high levels of l&3 n3 (Stickney and Andrews, 1972; Stickney et al., 1983). The relative high level (3.8%) of longchain n3 fatty acids found in diet 6 in the present study may have been a factor in the observed growth depression of fish fed this diet. There have been no studies of the EFA requirements for red drum, but it would not be unreasonable to assume that they are similar to the fatty acid requirements of other fish. Saturated fatty acids comprised 31-52% of the total fatty acid fraction of red drum whole-body lipids. Palmitic acid, expressed as a percentage of the total amount of saturated fatty acids, was quite consistent through all treatments, ranging from 60 to 63%. Ackman and Eaton (1966) reported that palmitic acid was nearly constant at 60% of total saturated fatty acids in Atlantic herring. Palmitic acid is an important fatty acid in fish and other animals, and its concentration is relatively independent of diet (Brenner et al., 1963). Stearic acid comprised from 15 to 23% of the total saturated fatty acids and ranged from 7 to 10% of the total fatty acid fraction. Stearic acid levels were consistently higher than previously reported for saltwater species from commercial catch (Gruger et al., 1964). Elevated stearic acid levels may be a response to the lower water salinity (5-6 ppt) maintained during this study, though it is generally PUFA of 20 carbons or greater that are affected by salinity (Gruger et al., 1964; Stansby, 1967; Ackman, 1967). Whole-body lipids contained between 15 and 23% palmitoleic acid. Its contribution to the total monounsaturated level ranged from 37 to 46%. Red drum fed the low-lipid diet (low level of 16:l) exhibited higher levels of 16:l than fish fed other diets. This indicated biosynthesis of 16:l and/or fatty acid interconversion to meet body needs. Polyunsaturated fatty acids comprised 4-13% of the total fatty acid fraction and were all of the linolenic (n3) family except for 18:2 n6. It should be noted

118

that both 20:4 n6 and 225 n6 were tentatively identified to be present in concentrations of less than 1.0%: Previously reported PUFA fractions for marine fish have been reported to range from 21 to 30% of tissue fatty acids (Gruger et al., 1964; Ackman and Eaton, 1966; Stansby, 1967). Since long-chain fatty acids of the n6 family were not identified or only tentatively identified in very small amounts, it may be that the red drum has a limited ability to elongate and desaturate B-carbon fatty acids of the n6 family. Certain marine fish have been shown to have limited ability to desaturate and elongate B-carbon fatty acids (Fuji et al., 1976). The lower levels of PUFA observed in fish in the present study may also be an indication of the effect of the low-salinity environment utilized during this study as well as differences in diet. Several researchers (Gruger et al., 1964; Ackman, 1967; Stansby, 1967) have noted changes in n3 fatty acid of wildcaught fish in response to changes in water salinity. ACKNOWLEDGEMENTS

This research was supported, in part, by Sea Grant and by the Texas Agricultural Experiment Station under project H-6556. Red drum were supplied by the Texas Parks and Wildlife Department. The cooperation of each contributor is greatly appreciated.

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