The ability of juvenile silver perch (Bidyanus bidyanus) to utilize dietary raw wheat meal, raw wheat starch, gelatinized wheat starch and dextrin as energy sources to spare protein for growth was quantified. Energy utilization and protein sparing were assessed by comparing the weight gain, energy retention efficiency, protein retention and body composition of silver perch that had been fed a series of diets in which the basal diet (low carbohydrate) was systematically replaced with graded levels of each carbohydrate ingredient or an inert diluent, diatomaceous earth. The protein content decreased as the carbohydrate content increased, giving four different protein to energy ratios for each of the four carbohydrate sources (except for the 60% inclusion level, at which only three carbohydrate sources were tested). Silver perch were efficient at utilizing carbohydrate for energy to spare protein. Silver perch fed diets containing up to 30% wheat meal, raw wheat starch, gelatinized wheat starch or dextrin exhibited similar growth, protein retention and energy retention efficiency to the fish fed the basal diet. Weight gain of silver perch fed diets containing wheat meal or carbohydrates at 45% inclusion content had significantly reduced weight gain when compared with fish fed the basal diet. However, protein retention and energy retention efficiency were similar or better. Whole-body protein levels of silver perch remained constant regardless of carbohydrate sources, and there was no evidence of increasing whole-body lipid concentrations for fish fed diets with up to 60% dietary carbohydrate. Silver perch were more efficient at utilizing processed starch (either gelatinized starch or dextrin) than wheat meal or raw wheat starch.

Introduction

Protein is the most expensive macronutrient in fish diets and, if oversupplied, amino acids excess to the fish's immediate requirements may be catabolized for energy rather than growth (Phillips 1972). A great deal of research has been conducted on the protein-sparing potential of non-protein ingredients, such as lipids and carbohydrates (Phillips 1972; Watanabe, Takeuchi & Ogino 1979; Cho & Kaushik 1985; Beamish & Medland 1986; Wilson & Poe 1987; Ellis & Reigh 1991; Shiau & Peng 1993; Brauge, Medale & Corraze 1994; Wilson 1994). Lipids, such as fish oil, have been shown to be an excellent alternative energy source to protein and, at the correct ratio to protein, have been demonstrated to exhibit a significant protein-sparing effect (Phillips 1972; Watanabe et al. 1979; Cho & Kaushik 1985). Although lipids are an excellent energy source, they are also expensive relative to plant carbohydrates.

Compared with warm-blooded animals, fish have poor control over their blood glucose levels (Furuichi & Yone 1981, 1982; Wilson 1994). Following carbohydrate digestion, blood glucose concentrations in fish rapidly increase and remain elevated for many hours (Wilson 1994). Absorbed carbohydrate may be used immediately as energy, stored as glycogen in the liver and muscle, synthesized into compounds such as triglycerides and non-essential amino acids or excreted (Furuichi 1983; Lovell 1988). As amino acids are not stored as such, excess amino acids may be deaminated and used for immediate energy requirements, or converted to lipid, carbohydrate or other compounds. Because fish oxidize deaminated amino acids for energy more efficiently and preferentially than glucose, only sufficient quantities of protein to meet anabolic requirements should be supplied in the diet to gain a protein-sparing effect using carbohydrate (Lovell 1988).

Starch and non-starch polysaccharides (NSPs) are the predominant groups of carbohydrates present in plant ingredients used for the production of aquaculture feeds. NSPs have a structural role in plants and seeds and are relatively indigestible. Therefore, they are unavailable to most warm- and cold-water fish as an energy source. Starch comprises predominantly glucose and is potentially a source of energy. Hence, much effort has been directed at investigating the ability of fish to utilize energy from starch in either its raw or processed form (Buhler & Halver 1961; Singh & Nose 1967; Wilson & Poe 1987; Wilson 1994; Shiau 1997). Molecular complexity, physical state and inclusion content of starch are factors that have been reported to have an influence on both digestibility and efficient utilization (Buhler & Halver 1961; Singh & Nose 1967; Anderson, Jackson, Matty & Capper 1984; Wilson & Poe 1987; Wilson 1994; Shiau 1997). While digestibility data are essential in formulating well-balanced diets, digestibility is not the same as utilization and ingredients can be well digested but poorly utilized. For example, Furuichi, Taira & Yone (1986) reported that yellowtail (Seriola quinqueradiata) digested glucose more efficiently (ADC 94%) than gelatinized potato starch (ADC 52%), at the 30% dietary inclusion level, but fish fed the glucose diet grew poorly compared with those fed the potato starch diet.

Silver perch (Bidyanus bidyanus) is an omnivorous freshwater species currently being cultured in Australia on relatively low-protein (≤ 35%) diets containing moderate levels of plant carbohydrates, including starch-rich ingredients such as wheat and field peas (Allan, Rowland, Mifsud, Glendenning, Stone & Ford 2000a). Previous research investigating carbohydrate tolerance, digestibility and utilization by silver perch has indicated that this species has excellent potential to use carbohydrate to spare protein (Stone, Allan & Anderson 2003a, b; Allan, Booth, Stone, Williams & Smith 2000b; Allan, Parkinson, Booth, Stone, Rowland, Frances & Warner-Smith 2000c).

Carbohydrate tolerance tests have indicated that the ability of silver perch to uptake and clear glucose from the bloodstream is comparable to that of carp and tilapia; both of which efficiently utilize carbohydrate (Furuichi & Yone 1981, 1982; Shikata, Iwanaga & Shimeno 1994; Lin, Ho & Shiau 1995; Stone et al. 2003a). Carbohydrate digestibility is influenced by inclusion level and carbohydrate complexity for silver perch. Energy digestibility values were 83%, 91% and 100% for raw wheat starch, gelatinized wheat starch and dextrin, respectively, when included in the diet at 30% (Stone et al. 2003b). Energy digestibility values for wheat starch for silver perch decreased from 71% to 48% when inclusion level increased from 30% to 60% (Stone et al. 2003b). Compared with warm-water omnivorous species currently being cultured such as channel catfish, carp or tilapia, silver perch appear to be relatively efficient at digesting starch and its less complex breakdown products (Wilson & Poe 1987; Lovell 1989; Hernandez, Takeuchi & Watanabe 1994; Shiau & Liang 1995). However, few quantifiable data are available on how well silver perch utilize digested carbohydrates. In this study we measured the utilization of wheat meal, raw wheat starch, gelatinized wheat starch and dextrin as protein-sparing energy sources for silver perch.

Materials and methods

Experimental fish and holding facilities

Silver perch (mean weight 26.5 g, range 23.3–30.1 g) were obtained from the NSW Fisheries Grafton Research Centre, and transported to the NSW Fisheries Port Stephens Fisheries Centre (PSFC). Prior to the experiment fish were held in 10 000-L tanks supplied with recirculating freshwater (bore water; salinity 0.05 g L⁻¹) and fed twice a day with a commercial silver perch diet (95LC2, Allan et al. 2000a) (digestible protein 34%, lipid 9% and digestible energy 14 MJ kg⁻¹) until transferred to the experimental cages. Water temperature was held at 24 ± 2 °C by the use of two 2-kW immersion heaters.

Experimental diets

Four carbohydrates of wheat origin were evaluated in this study: raw wheat meal; raw wheat starch; gelatinized (100%) wheat starch; and dextrin (Table 1). A high-protein, low-carbohydrate basal diet was used. The basal diet contained the following (g kg⁻¹): Chilean fish meal, 801.5; corn gluten, 113.5; cod liver oil, 70; vitamin premix, 7.5; mineral premix, 7.5: The vitamin and mineral premixes were as described by Allan and Rowland (2002) except that 1.0 mg biotin (2%) and 0.33 mg sodium selenite (44%) were added. For all other test diets, part of the basal diet was replaced by one of the carbohydrate test ingredients at 15%, 30%, 45% or 60%, or diatomaceous earth (as an inert filler) at 15%, 30% or 45%. Diets containing 60% dextrin or 60% diatomaceous earth were not included in the experiment as it was not possible to manufacture pellets with this high level of either ingredient. A total of 19 diets were manufactured and evaluated (Tables 2 and 3).

Table 1. Analysed composition of diet ingredients (dry basis)

Diet	Composition (g 100 g⁻¹ or MJ kg⁻¹)
Diet	Dry matter	Crude protein	Fat	Gross energy	Ash	CHO*
Fish meal	90.4	72.6	7.4	19.8	18.0	2.0
Corn gluten	90.2	38.4	2.3	21.1	1.2	58.1
Raw wheat meal	88.8	13.5	1.5	17.9	2.1	82.9
Raw wheat starch†	88.2	0.5	0.0	16.9	0.2	99.3
Gelatinized wheat starch†	94.2	0.3	0.0	16.6	0.2	99.5
Dextrin†	94.7	0.1	0.0	16.6	0.5	99.4

* Total CHO = total carbohydrate (including fibre) g 100 g⁻¹) = (100 – protein + lipid + ash).
† Supplied by Starch Australasia, Summer Hill, Sydney, NSW, Australia.

Table 2. Analysed composition of experimental diets (dry basis)

Diet (test ingredient inclusion content into basal diet)	Composition (g 100 g⁻¹ or MJ kg⁻¹)
Diet (test ingredient inclusion content into basal diet)	Dry matter	Crude protein	Lipid	Gross energy	Ash	Total CHO*†
Basal diet (100)	92.3	63.1	13.4	21.2	15.6	7.9
Wheat meal (15)	93.1	55.6	11.4	20.6	13.8	19.2
Wheat meal (30)	94.8	47.7	10.0	19.9	11.7	30.6
Wheat meal (45)	89.8	40.1	7.9	19.6	9.8	42.2
Wheat meal (60)	91.3	33.5	5.9	18.7	7.9	47.3
Raw wheat starch (15)	94.5	54.2	10.6	20.2	13.3	21.9
Raw wheat starch (30)	92.4	44.7	9.0	19.5	10.8	35.5
Raw wheat starch (45)	90.5	35.1	7.1	18.7	8.8	49.0
Raw wheat starch (60)	92.0	25.4	5.4	18.4	6.6	62.6
Gelatinized wheat starch (15)	94.7	54.0	10.4	20.3	13.2	22.4
Gelatinized wheat starch (30)	92.2	43.9	8.4	20.9	11.0	36.7
Gelatinized wheat starch (45)	91.2	35.2	7.0	18.9	8.9	48.9
Gelatinized wheat starch (60)	91.7	24.8	4.9	18.1	6.5	63.8
Dextrin (15)	94.7	54.8	10.9	20.3	13.3	21.0
Dextrin (30)	90.3	44.7	8.3	19.3	11.3	35.7
Dextrin (45)	89.3	35.2	7.1	19.4	8.9	48.8
Diatomaceous earth (15)	95.5	53.6	11.4	17.7	27.5	7.5
Diatomaceous earth (30)	96.2	44.0	9.0	14.6	40.2	6.8
Diatomaceous earth (45)	95.6	34.5	7.0	11.3	52.5	6.0

*Total CHO = total carbohydrate (including fibre) (g 100 g ⁻¹) = (100 – protein + lipid + ash).
† Basal diet contained > 0.5% of total CHO from starch origin.

Table 3. Digestible nutrient composition of experimental diets (dry basis)

Diet (test ingredient inclusion content percentage)	Digestible dry matter (g 100 g⁻¹)	Digestible protein (DP) (MJ kg⁻¹)	Digestible energy (DE) (g 100 g⁻¹)	DP/DE
Basal diet (100)*	78.0	60.0	20.3	3.0:1
Wheat meal (15)†	76.5	52.0	18.7	2.8:1
Wheat meal (30)†	76.1	44.0	17.0	2.6:1
Wheat meal (45)†	70.6	36.5	15.4	2.4:1
Wheat meal (60)†	70.2	30.1	13.8	2.2:1
Raw wheat starch (15)‡	80.1	50.9	18.3	2.8:1
Raw wheat starch (30)*	78.5	42.0	17.3	2.4:1
Raw wheat starch (45)‡	60.1	33.0	13.9	2.4:1
Raw wheat starch (60)*	49.0	23.9	11.7	2.0:1
Gelatinized wheat starch (15)‡	82.0	50.7	19.1	2.7:1
Gelatinized wheat starch (30)*	81.9	41.6	20.0	2.1:1
Gelatinized wheat starch (45)‡	72.6	33.1	16.4	2.0:1
Gelatinized wheat starch (60)*	67.3	23.2	14.5	1.6:1
Dextrin (15)‡	82.5	51.5	19.4	2.7:1
Dextrin (30)*	81.3	42.4	18.5	2.3:1
Dextrin (45)‡	82.7	33.0	18.7	1.8:1
Diatomaceous earth (15)‡	80.7	51.3	16.9	3.0:1
Diatomaceous earth (30)‡	81.3	41.8	13.9	3.0:1
Diatomaceous earth (45)‡	80.7	32.8	10.8	3.0:1

*Digestible dry matter, protein and energy calculated using apparent digestibility coefficients of diets from Stone et al. (2003b).
†Apparent digestibility coefficients used to calculate digestible energy of wheat meal diets were from Allan et al. (2000c).
‡Apparent digestibility coefficients used to calculate digestible dry matter, protein and energy of diets were estimated using linear regression of apparent digestibility coefficients and ingredient inclusion levels from Stone et al. (2003b).

A single batch of basal diet was prepared by mixing and grinding through a 1.5-mm screen in a hammer mill (Raymond Laboratory Mill, Transfield Technologies Pty Ltd, Rydalmere, NSW, Australia) and then, when required, substituting individual test ingredients. All diets contained the same amount of the vitamin and mineral premixes. After dry mixing, approximately 600 mL of distilled water was added per kilogram of mash, and each batch was mixed using a Hobart mixer (Troy Pty Ltd, OH, USA). The wet mash was then cold pelleted through a meat mincer fitted with a 3-mm pellet die (Barnco Australia Pty Ltd, Leichhardt, NSW, Australia) to produce a 3-mm sinking pellet. After pelleting, diets were dried in a convection drier at < 35 °C for approximately 6 h until all diets had moisture contents of < 10%.

Experimental facilities

The experiment was done in a recirculating system comprising ten 10 000-L fibreglass tanks in a 40 × 15 m plastic greenhouse. Each 10 000-L tank contained six cylindrical floating cages spaced evenly around the perimeter. Floating cages were 200 L in capacity (diameter = 0.6 m; submerged depth = 0.7 m) with walls constructed of 9-mm plastic mesh and the top and bottom were constructed of 1.6-mm plastic mesh (Kinnears Pty Ltd, Footscray, Victoria, Australia). A plastic tray (diameter 0.5 m) was placed on the bottom of each cage to collect uneaten food. Each 10 000-L tank was covered with a black shade cloth lid to reduce algae growth. Fresh water (0.05 g L⁻¹), filtered through a rapid sand filter, was supplied to each experimental tank at a flow-rate of 17 L min⁻¹. Effluent water from each tank flowed out through the bottom of the tank into a 2-m³ biological filter within a common 7000-L reservoir. This water was then returned to the sand filter. Aeration was provided to each tank by two air-stone diffusers and each tank was heated using a 2-kW immersion heater. Fluorescent lighting was used to control photoperiod at 16 h light−8 h dark. Tanks were siphoned every 2 weeks to remove accumulated wastes.

Experimental procedure

At stocking, fish were captured by dip net from each 10 000-L holding tank and anaesthetized in a 200-L container using a bath of clove bud oil (17 mg L⁻¹) (Branson and Jacobs, Sydney, NSW, Australia). The fish were then caught at random, individually weighed, and distributed among the 200-L cages by systematic interspersion.

Each of the 19 diets was allocated to three randomly selected cages (n = 3 cages per diet). Cages were stocked with 10 fish, which were hand fed twice daily (08.30 and 15.00 h) on their restricted ration (85% satiation) for a period of 61 days. Ten minutes following feeding, uneaten food was collected from the tray at the bottom of each cage and the dry weight recorded. The biomass of each cage was weighed fortnightly over the course of the experiment with feed withheld for 24 h prior to weighing. At the end of the growth trial, fish were harvested and weighed individually.

Water quality analysis

During the experiment, water temperature (range 24–28 °C), dissolved oxygen (above 6.0 mg L⁻¹) and pH (between 7.4 and 8.5) were measured daily using a Yeo-Kal 611 water quality analyser (Yeo-Kal Electronics, Brookvale, Sydney, NSW, Australia). Nitrite and ammonia nitrogen (< 60 µg L⁻¹ NO₂-N and < 100 µg L⁻¹ total NH₄-N) were measured weekly using colorimetric methods described by Major, Dal Pont, Kyle & Newell (1972) and Dal Pont, Hogan & Newell (1973).

Plasma glucose and blood sampling

At the completion of the experiment, blood samples were obtained from three fish from each cage to measure plasma glucose. To minimize the disturbance to the fish during the collection of blood samples for glucose analysis at the final weight check, each 10 000-L tank was divided into six segments using black plastic sheeting 2 weeks before the completion of the growth trial. One cage was held within each segment. Feed was withheld from fish for 24 h prior to blood sampling. Blood samples were obtained from fish within 1 min of initial disturbance of each cage to eliminate the confounding effects of stress on blood glucose levels (Stone et al. 2003a).

Blood samples were obtained from the caudal vessel of each fish using a 1-mL syringe and a 27-gauge hypodermic needle. Following collection, blood was transferred immediately into a 1.5-mL microtube containing the anticoagulant EDTA (5 mg mL⁻¹ blood) and the glycolysis inhibitor NaF (2 mg mL⁻¹ blood). Blood samples were immediately centrifuged at 1250 g and the plasma was separated. Samples were stored at −20°C prior to glucose analysis.

Biochemical analyses

Whole-body proximate composition of four fish (freeze dried, ground and homogenized) randomly selected from each cage were determined at the end of the feeding trial. All feed analyses were carried out in duplicate. Values for dry matter, lipid, energy (bomb calorimetry) and ash were determined for fish and diets following procedures described in AOAC (1990). Nitrogen was determined using the Kjeldahl or semimicro-Kjeldahl methods (AOAC 1990), and crude protein content was estimated by multiplying nitrogen by 6.25. Plasma glucose was determined from all blood samples using the glucose oxidase– peroxidase method (Fleming & Pegler 1963) (Sigma 510 A method).

Performance indices

Individual weight gain percentage [(final weight – initial weight)/initial weight] × 100, feed consumption and feed conversion ratio [FCR = dry weight feed consumed/wet weight gain of fish] were determined for each cage to allow comparisons of fish performance among different diets.

Indices for protein retention (PR, %) = [(final body protein – initial body protein)/protein intake] × 100 and energy retention efficiency (ERE, %) = [(final body energy – initial body energy)/energy intake] × 100 were calculated.

Statistical analyses

Statistical evaluation of the data was carried out using the Statsgraphics Plus for Windows 4.1 (1998) software package (Manugistics, Rockville, MD, USA). Homogeneity of variances was assessed using Cochran's C-test (Winer 1991). Initially, the experiment was designed for analysis using two-factor anova with ingredient inclusion content (15%, 30%, 45% and 60%) and ingredient type (wheat, raw wheat starch, gelatinized wheat starch, dextrin and diatomaceous earth) as the two fixed factors. However, owing to the problems associated with the manufacture of the diets containing 60% dextrin and 60% diatomaceous earth, which resulted in an unbalanced two-factor anova, two sets of data that were statistically analysed (all of the diet series containing 15–45% or 15–60%). This analysis allowed investigation of the main effects of both carbohydrate inclusion content and ingredient type as well as any interactions that occurred between the two factors. There were significant interactions found for all indices except plasma glucose and whole-body protein levels, so one-factor anova was used to compare the indices for all treatments (including the basal diet), both at each inclusion level within each diet series and also between ingredient types at each separate inclusion level. Where significant differences were found, comparison between means were made using Student–Newmann– Keuls multiple range test. Means were considered significant at P < 0.05. Unless otherwise stated, all results appear as mean ± standard error of the mean (n = 3).

Results

Performance indices

Weight gain of silver perch was similar for fish fed the basal diet or diets containing wheat meal, raw wheat starch, gelatinized wheat starch or dextrin at inclusion contents of up to 30% (Table 4). For each ingredient, fish weight gain declined at inclusion levels above 30%. Silver perch fed diatomaceous earth diets which contained greater than 15% of the filler exhibited a significant reduction in weight gain when compared with fish fed the basal diet or fish fed any of the carbohydrate test ingredients.

Table 4. Performance indices of silver perch fed a basal diet or the basal diet containing graded levels of wheat meal, raw wheat starch, gelatinized wheat starch, dextrin or diatomaceous earth*†

Performance indices and diet series	Test ingredient inclusion level into basal diet (%)
Performance indices and diet series	0 (Basal)	15	30	45	60
Individual weight gain (%)‡
Basal		107.9 ± 10.9^w	107.9 ± 10.9^x	107.9 ± 10.9^y	107.9 ± 10.9^y
Wheat meal	107.9 ± 10.9^b	104.1 ± 3.9^b ^w	97.9 ± 5.0^b ^x	78.3±1.2^a ^x	65.6 ± 3.0^a ^x
Raw wheat starch	107.9 ± 10.9^c	103.9 ± 8.5^c ^w	89.3 ± 1.1^c ^x	58.8 ± 1.6^b ^x	17.6 ± 1.1^a ^w
Gelatinized wheat starch	107.9 ± 10.9^cd	116.4 ± 5.5^d ^w	93.2 ± 3.6^bc ^x	78.3 ± 2.7^b ^x	51.9 ± 1.9^a ^x
Dextrin (Fieldose 9)	107.9 ± 10.9^ab	119.3±8.1^b ^w	104.6 ± 9.4^ab ^x	76.1 ± 3.5^a ^x	–
Diatomaceous earth	107.9 ± 10.9^c	86.1 ± 5.6^c ^w	52.2 ± 6.1^b ^w	12.2 ± 1.4^a ^w	–
FCR §
Basal		1.7 ± 0.1^w	1.7 ± 0.1^x	1.7 ± 0.1^x	1.7 ± 0.1^y
Wheat meal	1.7 ± 0.1^a	1.7 ± 0.1^a ^w	1.8 ± 0.1^a ^x	2.1 ± 0.1^a ^x	2.7 ± 0.3^b ^x
Raw wheat starch	1.7 ± 0.1^a	1.7 ± 0.1^a ^w	1.9 ± 0.1^a ^x	2.6 ± 0.2^a ^x	7.8 ± 0.4^b ^w
Gelatinized wheat starch	1.7 ± 0.1^bc	1.6 ± 0.1^c ^w	1.8 ± 0.1^bc ^x	2.0 ± 0.1^b ^x	2.9 ± 0.1^a ^x
Dextrin (Fieldose 9)	1.7 ± 0.1^ab	1.6 ± 0.1^b ^w	1.8 ± 0.2^ab ^x	2.1 ± 0.1^a ^x	–
Diatomaceous earth	1.7 ± 0.1^b	2.0 ± 0.1^b ^w	3.0 ± 0.3^b ^w	11.1 ± 1.1^a ^w	–
Energy retention efficiency (%)¶
Basal		28.53 ± 3.7^w	28.53 ± 3.7^x	28.53 ± 3.7^x	28.53 ± 0.37^y
Wheat meal	28.5 ± 3.7^b	26.56 ± 0.9^b ^w	27.08 ± 0.7^b ^wx	21.69 ± 1.4^ab ^x	16.96 ± 2.4^a ^x
Raw wheat starch	28.5 ± 3.7^b	28.49 ± 3.5^b ^w	27.06 ± 1.4^b ^wx	18.72 ± 2.6^b ^x	4.32 ± 1.4^a ^w
Gelatinized wheat starch	28.5 ± 3.7^ab	32.86 ± 2.8^b ^w	25.49 ± 0.8^ab ^wx	26.22 ± 1.3^ab ^x	21.21 ± 0.7^a ^xy
Dextrin (Fieldose 9)	28.5 ± 3.7^a	31.74 ± 2.4^a ^w	30.04 ± 3.5^a ^x	28.03 ± 3.0^a ^x	–
Diatomaceous earth	28.5 ± 3.7^b	26.92 ± 2.7^b ^w	17.28 ± 2.7^b ^w	−3.48 ± 3.8^a ^w	–
Protein retention (%)**
Basal		17.0 ± 2.8^w	17.0 ± 2.8^w	17.0 ± 2.8^x	17.0 ± 2.8^x
Wheat meal	17.0 ± 2.8^a	17.7 ± 0.5^a ^w	18.6 ± 1.3^a ^w	22.9 ±1.9^a ^x	18.8 ± 2.3^a ^x
Raw wheat starch	17.0 ± 2.8^b	17.4 ± 2.0^b ^w	18.7 ±0.7^b ^w	18.7 ± 2.7^b ^x	6.1 ± 1.9^a ^w
Gelatinized wheat starch	17.0 ± 8^a	19.2 ± 0.6^a ^w	20.5 ± 0.4^a ^w	21.1 ± 2.0^a ^x	21.3 ± 0.5^a ^x
Dextrin (Fieldose 9)	17.0 ± 2.8^a	19.5 ± 0.3^a ^w	21.4 ± 3.1^a ^w	24.7 ± 1.9^a ^x	–
Diatomaceous earth	17.0 ± 2.8^b	16.9 ± 1.5^b ^w	13.8 ± 1.3^b ^w	5.8 ± 2.4^a ^w	–

* Values are means ± SE from three groups of fish. Different superscripts ^abcdindicate significant differences between inclusion levels within each diets series;^wxyzindicate differences between diets within each inclusion level (P < 0.05; one-factor anova; Student–Newmann–Keuls).
† Data for the basal diet were included in all anovas.
‡ Individual weight gain percentage = [(final weight − initial weight)/initial weight] × 100.
§ Feed conversion ratio (FCR) = dry weight feed consumed/wet weight increment.
¶ Protein retention (PR, %) = [(final body protein − initial body protein)/protein intake] × 100.
** Energy retention efficiency (ERE, %) = [(final body energy − initial body energy)/energy intake] × 100.

FCR was similar for fish fed the basal diet or diets with up to 45% carbohydrate, but deteriorated when carbohydrate content increased to 60% (Table 4). FCR for diets containing diatomaceous earth decreased with increasing inclusion content (Table 4).

Energy retention efficiency was similar for fish fed the basal diet or diets with any of the carbohydrates at up to 45% inclusion content (Table 4). Beyond this inclusion content, energy retention efficiency declined, with the biggest reduction being for raw wheat starch, wheat meal and gelatinized wheat starch (Table 4). Energy retention efficiency decreased with increasing content of diatomaceous earth (Table 4).

Protein retention was similar for the basal diet and for diets containing up to 60% wheat meal, 60% gelatinized wheat starch and 45% dextrin (Table 4). There was a significant reduction in protein retention for diets containing 60% raw wheat starch or 45% diatomaceous earth (Table 4).

Proximate body composition

Whole-body protein concentrations ranged from 16.8 to 18.6 g 100 g⁻¹ and there were no significant effects of diet, inclusion content or their interactions for any of the diets.

Fish fed the gelatinized starch or dextrin had similar concentrations of body dry matter (DM) to fish fed the basal diet regardless of the inclusion level. There was a significant reduction in body DM of silver perch fed diets containing wheat meal (> 45% inclusion) or raw wheat starch (> 45% inclusion) compared with fish fed the basal diet (Table 5). There was a significant reduction in body DM content of fish fed the diatomaceous earth at > 30% inclusion contents compared with fish fed the basal diet.

Table 5. Proximate body composition of silver perch fed a basal diet or the basal diet containing graded levels of wheat meal, raw wheat starch, gelatinized wheat starch, dextrin or diatomaceous earth*†

Proximate composition index and diet series	Test ingredient inclusion level into basal diet (%)
Proximate composition index and diet series	0 (Basal)	15	30	45	60
Dry matter (g 100 g⁻¹)
Basal		34.5 ± 0.6 ^w	34.5 ± 0.6^x	34.5 ± 0.6^x	34.5 ± 0.6^x
Wheat meal	34.5 ± 0.6^b	33.8 ± 0.5^b ^w	33.4 ± 0.4^b ^x	33.3 ± 0.8^ab ^x	31.0 ± 0.7^a ^w
Raw wheat starch	34.5 ± 0.6^b	33.7 ± 0.8^b ^w	34.4 ± 0.6^b ^x	31.9 ± 0.6^ab ^x	30.8 ± 0.4^a ^w
Gelatinized wheat starch	34.5 ± 0.6^a	34.1 ± 0.6^a ^w	33.6 ± 0.5^a ^x	34.3 ± 0.8^a ^x	34.3 ± 0.1^a ^x
Dextrin (Fieldose 9)	34.5 ± 0.6^a	34.7 ± 0.4^a ^w	33.8 ± 0.4^a ^x	33.8 ± 0.1^a ^x	–
Diatomaceous earth	34.5 ± 0.6^b	33.3 ± 0.4^b ^w	31.5 ± 0.3^ab ^w	29.1 ± 1.3^a ^w	–
Crude protein (g 100 g⁻¹)
Basal		17.5 ± 0.5^w	17.5 ± 0.5^w	17.5 ± 0.5^w	17.5 ± 0.5^w
Wheat meal	17.5 ± 0.5^a	17.6 ± 0.3^a ^w	16.8 ± 0.1^a ^w	18.6 ± 0.6^a ^w	16.8 ± 0.4^a ^w
Raw wheat starch	17.5 ± 0.5^a	17.0 ± 0.4^a ^w	17.2 ± 0.3^a ^w	17.1 ± 0.4^a ^w	17.1 ± 0.3^a ^w
Gelatinized wheat starch	17.5 ± 0.5^a	17.1 ± 0.1^a ^w	17.1 ± 0.1^a ^w	17.0 ± 0.6^a ^w	17.2 ± 0.2^a ^w
Dextrin (Fieldose 9)	17.5 ± 0.5^a	17.4 ± 0.4^a ^w	16.9 ± 0.5^a ^w	17.5 ± 0.1^a ^w	–
Diatomaceous earth	17.5 ± 0.5^a	17.7 ± 0.2^a ^w	18.0 ± 0.1^a ^w	18.5 ± 0.8^a ^w	–
Lipid (g 100 g⁻¹)
Basal		11.8 ± 0.5^w	11.8 ± 0.5^x	11.8 ± 0.5^yz	11.8 ± 0.5^x
Wheat meal	11.8 ± 0.5^b	12.0 ± 0.4^b ^w	11.6 ± 0.4^b ^x	10.3 ± 0.3^ab ^xy	8.7 ± 0.9^a ^w
Raw wheat starch	11.8 ± 0.5^b	11.3 ± 0.5^b ^w	12.2 ± 0.4^b ^x	9.3 ± 0.4^a ^x	8.2 ± 0.4^a ^w
Gelatinized wheat starch	11.8 ± 0.5^a	12.2 ± 0.6^a ^w	12.0 ± 0.3^a ^x	12.5 ± 0.7^a ^z	11.8 ± 0.3^a ^x
Dextrin (Fieldose 9)	11.8 ± 0.5^a	12.2 ± 0.5^a ^w	12.5 ± 0.2^a ^x	11.3 ± 0.4^a ^yz	–
Diatomaceous earth	11.8 ± 0.5^c	10.6 ± 0.6^c ^w	8.1 ± 0.4^b ^w	4.6 ± 0.6^a ^w	–
Gross energy (MJ kg⁻¹)
Basal		8.5 ± 0.2^w	8.5 ± 0.2^x	8.5 ± 0.2^x	8.5 ± 0.2^x
Wheat meal	8.5 ± 0.2^b	8.4 ± 0.2^b ^w	8.4 ± 0.1^b ^x	8.0 ± 0.2^b ^x	7.3 ± 0.3^a ^w
Raw wheat starch	8.5 ± 0.2^b	8.5 ± 0.4^b ^w	8.6 ± 0.2^b ^x	7.6 ± 0.2^a ^x	7.1 ± 0.2^a ^w
Gelatinized wheat starch	8.5 ± 0.2^a	8.9 ± 0.3^a ^w	8.4 ± 0.2^a ^x	8.6 ± 0.1^a ^x	8.6 ± 0.1^a ^x
Dextrin (Fieldose 9)	8.5 ± 0.2^a	8.8 ± 0.2^a ^w	8.4 ± 0.2^a ^x	8.7 ± 0.2^a ^x	–
Diatomaceous earth	8.5 ± 0.^c	8.1 ± 0.2^bc ^w	7.3 ± 0.2^b ^w	6.1 ± 0.4^a ^w	–
Ash (g 100 g⁻¹)
Basal		4.4 ± 0.2^w	4.4 ± 0.2^w	4.4 ± 0.2^w	4.4 ± 0.2^w
Wheat meal	4.4 ± 0.2^a	4.0 ± 0.3^a ^w	4.3 ± 0.1^a ^w	4.0 ± 0.3^a ^w	4.5 ± 0.2^a ^w
Raw wheat starch	4.4 ± 0.2^a	4.2 ± 0.1^a ^w	4.2 ± 0.1^a ^w	4.6 ± 1.3^a ^w	5.2 ± 0.1^b ^x
Gelatinized wheat starch	4.4 ± 0.2^a b	3.9 ± 0.1^a ^w	4.2 ± 0.1^ab ^w	4.3 ± 0.2^ab ^w	4.7 ± 0.1^b ^w
Dextrin (Fieldose 9)	4.4 ± 0.2^a	4.2 ± 0.1^a ^w	3.9 ± 0.1^a ^w	4.3 ± 0.1^a ^w	–
Diatomaceous earth	4.4 ± 0.2^a	4.5 ± 0.1^a ^w	4.8 ± 0.1^a ^x	5.7 ± 0.1^b ^x	–

* Values are means ± SE from three groups of fish. Different superscripts ^abcdindicate significant differences between inclusion levels within each diets series;^wxyzindicate differences between diets within each inclusion level (P < 0.05; one-factor anova; Student–Newmann–Keuls).
† Data for the basal diet were included in all anovas.

Fish fed the gelatinized starch or dextrin series of diets had similar concentrations of whole-body lipid as fish fed the basal diet regardless of the inclusion level. There was a significant reduction in body lipid concentrations of silver perch that were fed diets containing wheat meal (> 45% inclusion), raw wheat starch (> 30% inclusion) or diatomaceous earth (> 15% inclusion level) compared with fish fed the basal diet (Table 5).

Fish fed the gelatinized starch or dextrin series of diets had similar concentrations of whole body gross energy (GE) as fish fed the basal diet regardless of the inclusion level. There was a significant reduction in body GE concentrations of silver perch that were fed diets containing wheat meal (> 45% inclusion) or raw wheat starch (> 30% inclusion) compared with fish fed the basal diet (Table 5). There was a significant reduction of body GE content in fish fed the diatomaceous earth diets at > 15% inclusion contents compared with fish fed the basal diet or any of the carbohydrate test ingredients at > 15% inclusion content.

Silver perch fed the wheat meal or dextrin series of diets had similar concentrations of whole-body ash as fish fed the basal diet regardless of the inclusion level. There was a significant increase in body ash concentrations of silver perch that were fed diets containing raw wheat starch or gelatinized wheat starch at > 45% inclusion content when compared with fish fed the basal diet (Table 5). There was a significant increase in body ash content in fish fed the diatomaceous earth diets at > 30% inclusion contents compared to fish fed the basal diet or any of the diets containing carbohydrate at > 15% inclusion content.

Plasma glucose

Plasma glucose concentrations ranged from 3.99 mmol L⁻¹ to 4.37 mmol L⁻¹. There was no significant effect of diet, inclusion content, or their interaction, on the plasma glucose concentrations of silver perch.

Discussion

With each progressive increase in diatomaceous earth content (reduction in basal diet), there was a progressive reduction in growth performance as indicated by a decrease in weight gain, energy and protein retention and an increase in FCR. There was also a reduction in whole-body lipid and gross energy content with a concomitant increased in whole-body ash content. As the digestible protein–energy ratio of the diatomaceous earth series of diets remained constant (3:1), the observed reduction in growth performance was attributed to a progressive reduction in digestible dietary protein and energy intake. A significant reduction in growth performance was not evident in any of the diet series containing carbohydrate until the dietary inclusion content of the test carbohydrate exceeded 30%. The difference between weight gain (and other indices) of fish for the series of diets containing test carbohydrates and diatomaceous earth is a quantification of carbohydrate utilization in silver perch. The positive relationship between PR and increasing dietary carbohydrate content (up to the 45% inclusion content) for each series of diets demonstrates the protein-sparing effect of the carbohydrate source. Gelatinized starch and dextrin were utilized most efficiently as energy sources to spare protein, although differences between ingredients were not significant except at the 60% inclusion level, at which level raw wheat starch was significantly poorer than other ingredients.

The interpretation above relies on the assumption that diatomaceous earth is a nutrient diluent and does not interfere with feed intake, digestibility or growth of silver perch. This assumption is supported by the lack of evidence of reduced palatability of diets due to the incorporation of diatomaceous earth. Silver perch readily accepted diets containing diatomaceous earth, with no apparent difference in the time taken to consume their daily ration compared with the basal diet (< 2 min). The linear reduction in weight gain of silver perch fed increasing amounts of diatomaceous earth was consistent with previous research in which silver perch (Allan, Frances & Booth 1998) and European sea bass (Dicentrarchus labrax) (Dias, Huelvan, Dinis & Métailler 1998) were fed diets containing inert fillers, indicating that the diatomaceous earth used in this study had minimal negative nutritional effects. Allan et al. (1998) reported that juvenile silver perch fed a series of diets in which α-cellulose or diatomaceous earth replaced similar contents of a control diet exhibited similar weight gain, FCR and protein retention at each inclusion level. Dias et al. (1998) investigated the effects of incorporating silica (similar to diatomaceous earth), cellulose or zeolite at either 10 or 20% inclusion levels as bulking agents (inert fillers) in diets on growth performance of juvenile European sea bass. When feed rates were adjusted in proportion to the per cent dilution of the control diet without the bulking agent, there were no significant effects of filler on protein digestibility, protein retention or weight gain, indicating that the fillers did not affect assimilation of dietary nutrients.

The inclusion of up to 60% carbohydrate in the diets had no effect on plasma glucose concentrations of silver perch 24 h following a meal. Plasma glucose concentrations were within the range of normal concentrations found for similar sized silver perch (∼ 3.44 mm) (Stone et al. 2003a). Stone et al. (2003a) found that silver perch are able to clear glucose from the bloodstream and attain basal plasma glucose levels within 12 h of an intraperitoneal injection of glucose at 0.1% body weight. In contrast, Hemre, Waagbø, Hjeeltnes & Aksnes (1996) recorded a progressive elevation of plasma glucose concentration in Atlantic salmon (Salmo salar L.) fed diets containing from 2.4% to 23% dietary starch over a prolonged period. Hemre, Lie, Lied & Lambertsen (1989) and Bergot (1979) reported similar responses in cod and rainbow trout fed diets with increasing carbohydrate contents. The differences in postprandial plasma glucose concentrations of silver perch and the cold-water and marine species support the theory that warm-water fish are more tolerant to glucose and are more efficient at utilizing higher levels of dietary carbohydrate (Wilson 1994).

The growth of fish is affected by both the inclusion content of dietary starch and level of complexity (Wilson & Poe 1987; Hung, Fynn-Aikins, Lutes & Xu 1989; Shiau & Peng 1993; Hung & Storebakken 1994; Shikata et al. 1994). Based on weight gain and protein and energy retention, it appears that silver perch were more efficient at utilizing carbohydrates that had undergone some form of processing, i.e. gelatinization or dextrinization. These findings support previous studies in which the energy from dextrin and gelatinized wheat starch was more efficiently digested than that from wheat meal or raw wheat starch by silver perch (Stone et al. 2003b).

The protein retention observed for silver perch in this study was comparable to calculated PR values reported for silver perch fed commercial diets at different feed rates (Harpaz, Jiang & Sklan 2001). Weight gain and protein retention of silver perch fed carbohydrates in the current study also exhibited similar trends to tilapia fed diets containing graded levels of raw maize starch or dextrin at up to 40% of the diet (Anderson et al. 1984). As for silver perch, tilapia, a warm-water species, exhibited better growth performance and protein sparing when fed diets containing dextrin as opposed to raw starch at each inclusion content.

Previous research has indicated that, although silver perch digest simple carbohydrates such as glucose extremely efficiently (∼100%) when included in the diet at 30% inclusion content, weight gain was increased by 46% for fish fed gelatinized wheat starch at the same dietary inclusion content (Stone et al. 2003b). This is in contrast to cold-water or marine species, which utilize less complex carbohydrates such as glucose or maltose more efficiently to spare protein (Hung, Fynn-Aikins, Lutes & Xu 1989; Hung & Storebaken 1994).

In the case of raw wheat starch and wheat meal, the relative reduction in growth performance of silver perch would have resulted from the lower digestibility of these ingredients at greater than 30% and 45% inclusion contents respectively. Previous digestibility experiments using silver perch have indicated that the digestibility of wheat starch is negatively correlated with increasing inclusion content and energy digestibility of wheat is relatively low when included at 30% (Energy ADC ∼53%) (Allan et al. 2000c; Stone et al. 2003b). The slightly improved performance of fish fed the wheat meal compared with raw wheat starch may be attributed to the slightly higher protein concentrations in the wheat meal diets compared with the raw wheat starch diets (Table 2).

It has been reported that fish do not have a carbohydrate requirement (NRC 1993). However, the recommended dietary inclusion of digestible carbohydrate is up to 20% for diets fed to salmonids and marine fish and up to 40% for warm-water species (see review, Wilson 1994). Recent research with rainbow trout suggests that carbohydrates play a vital role in fish nutrition and growth. Peragón, Barroso, Garcia-Salguero, de la Higuera & Lupiáñez (1999) fed rainbow trout two diets, one containing no digestible carbohydrate and the other 23% digestible carbohydrate in the form of precooked starch, and reported suppressed daily weight gain for fish fed the diet with no digestible carbohydrate. Feed conversion efficiency, protein retention, white muscle weight gain and the RNA–DNA ratio were also reduced in rainbow trout that were fed the diet with no digestible carbohydrate. Peragón et al. (1999) concluded that the growth suppression resulted from white muscle cell hypotrophy (rather than a reduction in cell numbers) from increased protein degradation and decreased protein synthesis. Effectively, the absence of dietary carbohydrate led to a significant proportion of the amino acids from digested dietary protein and muscle protein breakdown being utilized for gluconeogenic purposes and not for protein synthesis and growth (Peragón et al. 1999). An interesting finding in the current study was that there was a trend for slightly better all-round growth performance in silver perch which were fed diets containing gelatinized starch or dextrin at 15% inclusion content than fish fed the basal diet. Although effects were not significant, the trend suggests that the inclusion of digestible carbohydrate in diets for silver perch may be beneficial. This is worthy of further investigation.

High levels of dietary energy originating from carbohydrate have been reported to produce fatty fish (Anderson et al. 1984; Shiau & Peng 1993; Hemre, Sandes, Lie & Waagbø 1995; Yamamoto & Akiyama 1995; Catacutan & Coloso 1997; Shiau 1997; Nankervis, Matthews & Appleford 2000). However, whole-body lipid levels of silver perch in this study were similar to those of similar sized silver perch (13% lipid) fed a commercial diet containing 32% digestible protein and 13 MJ kg⁻¹ digestible energy (protein to energy ratio of 2.5:1) (Stone, Allan, Parkinson & Rowland 2000) and there was a slight reduction of body lipid in silver perch fed diets containing greater than 30% raw wheat starch or greater than 45% wheat meal. This reduction in body lipid level may have been due to the restricted energy availability resulting from inefficient digestibility, which led to the mobilization of body lipid to meet energy requirements for growth and maintenance. Similar inverse relationships between dietary carbohydrate level and whole-body lipid and energy content for turbot (Scophthalmus maximus) and walking catfish (Clarius batrachus) fed increasing levels of non-protein energy (starch) have been reported (Nijhof & Bult 1994; Erfanullah 1998).

In conclusion, the inclusion of wheat starch or its constituent breakdown products into diets for silver perch has a protein-sparing effect that may increase growth and therefore reduce feed costs. Silver perch are able to utilize the dietary carbohydrates tested in his study at levels of up to 30% inclusion in diets containing ∼42% digestible protein and digestible energy levels of ∼19 MJ kg⁻¹. The efficiency of utilization of each product was positively related to digestibility. Silver perch were more efficient at utilizing dextrin and gelatinized wheat starch as energy sources for growth than wheat meal or raw wheat starch. As it is likely that raw wheat starch or its constituent breakdown products will be supplied from wheat meal in commercial diets for silver perch, it is essential that diets undergo some form of heat processing to increase gelatinization.

Acknowledgments

The authors would like to thank Mr Mark Booth, Mr Luke Cheviott & Ms Rebecca Warner-Smith from NSW Fisheries Port Stephens Fisheries Centre for their technical assistance in conducting this experiment. We would also like to thank Dr Stuart Rowland & Mr Charlie Mifsud from the NSW Fisheries, Grafton Research Centre for supplying the silver perch used in this study. The manuscript was critically reviewed by Dr Neil McMeniman, Dr Wayne O'Connor & Mr Mark Booth & Mrs Helena Heasman provided assistance with manuscript preparation. This study was supported by the Fisheries Research and Development Corporation's Aquaculture Diet Development Sub-program (Project 96/391).

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Carbohydrate utilization by juvenile silver perch, Bidyanus bidyanus (Mitchell). III. The protein-sparing effect of wheat starch-based carbohydrates

Abstract

Introduction

Materials and methods

Experimental fish and holding facilities

Experimental diets

Experimental facilities

Experimental procedure

Water quality analysis

Plasma glucose and blood sampling

Biochemical analyses

Performance indices

Statistical analyses

Results

Performance indices

Proximate body composition

Plasma glucose

Discussion

Acknowledgments

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Carbohydrate utilization by juvenile silver perch, Bidyanus bidyanus (Mitchell). III. The protein-sparing effect of wheat starch-based carbohydrates

Abstract

Introduction

Materials and methods

Experimental fish and holding facilities

Experimental diets

Experimental facilities

Experimental procedure

Water quality analysis

Plasma glucose and blood sampling

Biochemical analyses

Performance indices

Statistical analyses

Results

Performance indices

Proximate body composition

Plasma glucose

Discussion

Acknowledgments

References

Citing Literature

References

Related

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