Effects of short-term feed restriction on production, processing and body shape traits in market-weight channel catfish, Ictalurus punctatus (Rafinesque)

The effects of feed restriction on channel catfish production, processing yield (carcass and fillet), visceral composition and body shape traits were determined. Channel catfish (initial mean weight =0.77 kg) were stocked into six 0.04‐ha ponds at ～5775 kg ha^?1. Two ponds were assigned to each of three feeding regimes for a 4‐week trial: fed daily to satiation, fed once weekly to satiation and not fed. Fish were measured for weight, processing yield and visceral components after 2 and 4 weeks, and for body shape after 4 weeks. Growth was fastest in fish fed daily, intermediate in fish fed weekly and slowest in unfed fish. There were no differences in survival among feeding regimes. After covariate adjustment for weight differences, fish fed daily had shorter, thicker bodies, and smaller heads than fish from feed‐restricted treatments. Carcass yield was higher for females than males and higher at week 2 than at week 4, but was not affected by feeding regime. Fillet yield was higher for females than males, higher at week 2 than at week 4, and higher for fish fed daily than for feed‐restricted fish (fed once weekly and not fed). Viscerosomatic index (VSI), visceral fat‐somatic index (VFI) and hepatosomatic index were higher at week 2 than at week 4, and highest for fish fed daily, intermediate for fish fed weekly and lowest for unfed fish. Hepatosomatic index and VSI were higher for females than males, but VFI was not different between genders. The female gonadosomatic index increased over time but was not affected by feeding regime. Short‐term feed restriction had negative impacts on growth and fillet yield. Processors could benefit by marketing severely feed‐restricted channel catfish as carcasses rather than fillets since fillet yield declined but carcass yield was unchanged by feed restriction.     

Comparative challenge model of Flavobacterium columnare using abraded and unabraded channel catfish, Ictalurus punctatus (Rafinesque)

The early entry of the fish pathogen Flavobacterium columnare and enhancement by abrasion was studied in channel catfish, Ictalurus punctatus (Rafinesque), using the polymerase chain reaction and a species-specific primer set for a bacterial 16S rRNA gene product. Evaluations were conducted following an abrasion bath immersion challenge with F. columnare. Abrasion, a practice which has historically been used prior to bacterial challenge, had significant effects on the early entry of the pathogen and on cumulative percent survival (CPS). The FvpF1-FvpR1 primer set was useful in detecting the early entry of F. columnare in mucus, skin, gill, blood, liver and trunk kidney tissues in both abraded and unabraded fish following immersion challenge at 29 +/- 2 degrees C. Bacteria were detected earlier in all tissues in abraded fish, except in the trunk kidney. These differences were not significant, except in the case of blood. Mucus, skin and gill tissues were positive for F. columnare earliest regardless of treatment (after 5 min in abraded fish and after 15 min in unabraded fish). CPS following challenge with F. columnare was significantly affected by abrasion, which supports the use of abrasion for the F. columnare challenge model for channel catfish.     

Effect of Dietary Vitamin C on Weight Gain,Tissue Ascorbate Concentration,Stress Response,and Disease Resistance of Channel Catfish Ictalurus punctatus1

Juvenile channel catfish Ictalurus punctatus (average initial weight, 6.5 g/fish) were fed twice daily to apparent satiation with practical-type diets containing 0, 50, 150, or 250 mg supplemental vitamin C/kg from L-ascorbyl-2-polyphosphate for 10 wk under laboratory conditions. At the end of the feeding period, one half of the fish were stressed for 2 h by confinement and both stressed and nonstressed fish were exposed to a virulent strain of Edwardsiella ictaluri. Weight gain and feed conversion efficiency were lower for fish fed the basal diet than those fed diets containing supplemental vitamin C. No differences were observed in weight gain and feed conversion among fish fed diets containing supplemental vitamin C. There were no differences in feed consumption and survival (prior to experimental infection) among treatments. No vitamin C deficiency signs except reduced weight gain were observed in fish fed the basal diet. Serum cortisol concentrations were higher in stressed fish than in non-stressed fish. Dietary vitamin C level had no effect on serum cortisol concentration. As dietary vitamin C increased, ascorbate concentration in serum and liver increased. Confinement stress had no effect on serum and liver ascorbate concentrations. Cumulative mortality of channel catfish 21 d subsequent to experimental infection with E. ictaluri was higher for stressed fish than for nonstressed fish. Regardless of stress or nonstress, overall mortality for fish fed the basal diet was lower than the fish fed diets containing supplemental vitamin C. There were no differences in post-infection antibody levels among treatments or between stressed and nonstressed fish. Results from this study indicate that channel catfish require no more than 50 mg/kg dietary vitamin C for normal growth, stress response, and disease resistance.     

Re-evaluation of vitamin E supplementation of practical diets for channel catfish, Ictalurus punctatus, production

A 10-month feeding trial was performed with channel catfish, Ictalurus punctatus , to re-evaluate the minimum level of dietary vitamin E supplementation required under conditions of commercial production. Four levels (0, 15, 30 and 60 mg kg⁻¹ diet) of supplemental vitamin E were added as DL -α-tocopherol acetate to a typical commercial catfish diet. Sixteen 0.04-ha ponds were stocked in July, at a rate of 25 000 fish ha⁻¹, with two size classes (averaging 18 and 265 g fish⁻¹) of fingerling channel catfish, and each diet was fed to fish in four replicate ponds once daily to satiation. After 10 months of feeding, a total harvest was performed to obtain final production data, and samples were obtained from fish in all ponds. Liver and plasma samples were obtained for α-tocopherol determination, and liver, heart and muscle samples were processed for histological evaluation.
No discernible differences occurred among fish fed the different diets with respect to weight gain or survival. Mean production rate was 9734 kg ha⁻¹, with a final average fish weight of 0.53 kg. A significant ( P < 0.05) effect of diet was noted in plasma and liver α-tocopherol levels, which increased with dietary supplementation. An effect of size also was apparent, with plasma and liver α-tocopherol levels being higher in larger fish compared with smaller fish within a dietary treatment. Histological evaluation revealed no differences among fish fed the different dietary treatments. Based on the lack of overt histological signs of deficiency, it appears that the current level of vitamin E supplementation of commercial catfish diets may be reduced considerably with no detriment to channel catfish health or production.     

Growth performance,vitamin E status,and proximate and fatty acid composition of channel catfish, <Emphasis Type="Italic">Ictalurus punctatus</Emphasis>, fed diets containing various levels of fish oil and vitamin E

A study was conducted to determine the effect of increasing dietary levels of fish oil on vitamin E requirement and their effect on growth performance, liver vitamin E status, and tissue proximate and fatty acid compositions of channel catfish. Basal purified diets (42% protein and 3,800 kcal DE/kg) supplemented with 6, 10, and 14% menhaden fish oil were each supplemented with 50, 100, and 200 mg vitamin E/kg (3 × 3 factorial experiment). Each diet was fed to juvenile channel catfish in three random aquaria to apparent satiation twice daily for 12 weeks. Weight gain, feed intake, and feed efficiency ratio were not affected by dietary levels of fish oil, vitamin E, or their interaction. Survival rate at the end of week 12 was significantly lower for fish fed diets containing 14% fish oil, regardless of vitamin E content. Whole-body moisture significantly decreased and lipid increased when dietary lipid levels were increased to 10 or 14%. Dietary vitamin E levels had no effect on body proximate composition. Lipid content of liver was not influenced by dietary levels of fish oil and vitamin E or their interaction. Hepatosomatic index significantly decreased with increasing lipid levels but was not affected by dietary levels of vitamin E. Liver vitamin E increased with increasing dietary vitamin E but decreased with increasing fish oil levels. Fatty acid composition of whole body and liver reflected that of dietary lipid but was not influenced by dietary levels of vitamin E. Whole-body saturates increased, whereas MUFA decreased with increasing dietary levels of fish oil. Liver saturates were not affected by fish oil levels, but MUFA and n-6 decreased and increased, respectively, with increasing fish oil levels. Total n-3 and n-3 HUFA in both tissues increased with increasing fish oil levels in diets, but liver stored much higher levels of these fatty acids.     

Growth,feed conversion,body composition and nutrient retention efficiencies in fingerling catfish, Heteropneustes fossilis (Bloch), fed different sources of dietary carbohydrate

Growth, conversion efficiency, body composition, nutrient retention and plasma glucose concentration were evaluated in fingerling catfish, Heteropneustes fossilis (Bloch), (3.25 ± 0.1 g) fed iso-nitrogenous (40% crude protein) and iso-caloric (4.7 kcal g^–1 gross energy) test diets containing different sources of carbohydrates (i.e. glucose, fructose, maltose, sucrose, dextrin, pre-cooked corn-starch or α- cellulose) at the 20% level of inclusion. Each dietary treatment had three replicates of 20 fish each. The growth trial was conducted in 70-L high-density polyvinyl flow-through (1–1.5 L min^–1) indoor circular troughs (water volume = 55 L). The catfish were fed to apparent satiation twice daily at 0800 and 1600 h for 6 days a week over ≈ 8 weeks. No significant (P > 0.05) differences were noted in the growth and feed conversion efficiencies when fish were fed the sucrose, dextrin and corn-starch diets. Similarly, growth and conversion efficiencies were similar (P > 0.05) in fish fed the glucose, fructose and maltose diets. The minimum growth and conversion efficiencies were found in fish fed the α-cellulose diet. Post-feeding glucose or maltose resulted in the maximum increase in plasma glucose, followed by sucrose, dextrin, fructose or corn-starch diets, over the 8-h sampling period. Post-feeding α-cellulose produced a relatively low (P > 0.05) variation in the plasma glucose level. Compared to the initial values, higher values of dry matter, crude protein, lipid and body energy, and lower percentages of ash were recorded in catfish fed different sources of carbohydrate. The maximum protein retention rate was seen in fish fed the dextrin diet, while a higher energy retention rate was observed in fish fed the sucrose-based diet. Dietary α-cellulose produced significantly (P < 0.05) lower values of protein and energy retention in fish. The present study indicates that utilization of complex carbohydrates in catfish is more successful with di- and monosaccharides, while dietary α-cellulose is poorly utilized.     

Dietary protein concentration and daily feed allowance influence response of channel catfish, Ictalurus punctatus (Rafinesque), to ractopamine

Year-1 channel catfish, Ictalurus punctatus (Rafinesque) (average initial weight, 48 g) were fed diets containing 240 g kg^?1, 300 g kg^?1, or 360 g kg^?1 dietary protein with 0 or 20 mg kg^?1 ractopamine to satiation and diets containing 240 g kg^?1 and 360 g kg^?1 protein with 0 or 20 mg kg^?1 ractopamine at a restricted rate (60% of satiation) in aquaria at 29 ± 2°C for 8 weeks. Fish fed ractopamine at the 360 g kg^?1 protein level gained significantly more weight than controls when fed to satiation but not when the ration was restricted. Fish fed ractopamine at the 240 g kg^?1 protein level did not gain more weight than the controls at satiation or restricted feeding. Fish fed ractopamine in the diet containing 300 g kg^?1 protein gained significantly more weight than controls when fed to satiation. Ractopamine caused no significant differences in feed conversion at the 240 g kg^?1 or 360 g kg^?1 protein levels with restricted or satiation feeding. Muscle fat decreased when ractopamine was fed at all dietary protein levels and at both feeding rates. Muscle protein increased with ractopamine feeding in all treatments except the high-protein, satiation treatment. Weight gain response of year-1 channel catfish to ractopamine is dependent upon nutrient concentration of the diet and ration size; however, the effect of ractopamine on muscle composition of channel catfish is less dependent on nutrient concentration or amount of diet fed.     

Effect of Time of Feeding on Growth of Channel Catfish1

Channel catfish fingerlings were stocked into 16 0.04-ha ponds at a rate of 24,700 fish/ha on 5 May 1992. Four replicate ponds were used for each of the following treatments: 1) fed once daily at 0830h; 2) fed once daily at 1600h; 3) fed once daily at 2000h; and 4) fed on demand using demand feeders. Fish on the first three treatments were fed to satiation. All fish were harvested 145d after stocking. Results from this study showed that when channel catfish raised in ponds were fed once daily to satiation, time of feeding had no significant impact on water quality, feed consumption, feed conversion, weight gain, or body proximate composition. Fish fed on demand consumed more feed than fish fed once daily to satiation, but difference in weight gain was not significant. These data indicated that feeding time may not be critical for channel catfish production as long as fish are fed when dissolved oxygen is sufficient. Although feeding at night was not detrimental in this study, night feeding is not recommended on large ponds unless sufficient aeration is available to quickly provide oxygen in an emergency—and even then it would be problematic.     

Comparative growth and yield of channel catfish and channel × blue hybrid catfish fed a full or restricted ration

Growth and yield (kg ha^?1) of the channel catfish (Ictalurus punctatus, Rafinesque, 1818) and the channel × blue hybrid catfish [I. punctatus female ×I. furcatus (Lesueur, 1840) male], which shared the Jubilee strain of channel catfish as the maternal parent, were compared in sixteen 0.1 ha earthen ponds (14 852 fish ha^?1) during the April to November growing season. Each fish genetic group was fed a commercially formulated 32% protein feed daily to apparent satiation or at 80% of the mean daily satiation ration. Net yield and individual weight were higher for channel × blue hybrid catfish compared with channel catfish and for fish fed a full ration compared with a restricted ration. When fed a full ration, the channel × blue hybrid catfish grew faster from May to September than did the purebred channel catfish because the hybrid catfish consumed a greater percentage of its body weight at each feeding. Net yield within each fish genetic group was lower when feed ration was restricted. The per cent reduction in net yield in response to feed restriction was similar for each fish genetic group.     

The Effects of Soybean Lecithin Supplementation to a Practical Diet Formulation on Juvenile Channel Catfish,Ictalurus punctatus: Growth,Survival, Hematology,Innate Immune Activity,and Lipid Biochemistry

A feeding trial was conducted to determine the effects of soy lecithin supplementation on production performance of juvenile channel catfish, Ictalurus punctatus (mean ± SE; 5.8 ± 0 g). The basal diet consisted of a practical dietary formulation for channel catfish, containing 4.3% endogenous phospholipids (PL) from dietary ingredients, to which supplemental PL from soybean lecithin were added. The study diets were 1 control and 2 experimental diets to which 0, 2, or 4% supplemental lecithin was added, respectively. Soy lecithin inclusion did not affect survival, growth, feed consumption, whole‐body total lipid, innate immune response, plasma cholesterol or triglyceride concentrations, or hepatosomatic index. Feed conversion (gain/intake) improved in fish fed 4% supplemental lecithin compared with 0% lecithin. Whole‐body crude protein was greater in fish fed 2% supplemental lecithin compared with 0% lecithin, while 4% supplemental lecithin was intermediate. Phosphatidylcholine (PC) content was greater in fish fed 2 or 4% lecithin than 0% lecithin. Plasma concentrations of PC were inversely proportional to dietary concentrations. Liver glycogen was greater in fish fed 0% lecithin compared with 2 or 4% lecithin. Liver lipid and phospholipid were lower in fish fed 0% lecithin than 2 or 4% lecithin. The dietary phospholipid requirement, if any, of juvenile channel catfish for growth and survival is less than or equal to 4.3% (1.5% PC) of the diet. Feed conversion is improved in channel catfish fed diets supplemented with 4% soy lecithin (7.2% phospholipid; 5.1% PC), which might offset additional costs due to phospholipid supplementation. Dietary soy lecithin inclusion altered plasma and liver lipid composition, but it is unknown whether these effects can alter the ability of juvenile catfish to survive and grow under various conditions.     

Palatability of Sarafloxacin HCl-Medicated Feed to Channel Catfish

Two experiments were conducted to evaluate the palatability of sarafloxacin HCl to channel catfish, Ictalurus punctatus. Fingerling channel catfish were raised in 110-1 glass aquaria supplied with flowing well water at a rate of 1 l/min. Fish were fed to satiation once daily either a purified or a practical diet formulated to contain 0, 500, 1,000, 2,000, or 4,000 mg active sarafloxin HCl/kg of diet. Feed consumption was quantified daily for 5 days. All fish were weighed at the beginning and at the end of each experiment. There was a significant decrease in feed consumption of fish fed purified diets with increasing levels of sarafloxacin HCl. However, feed consumption was adequate to provide the proposed dosage regardless of drug concentration. Fingerling channel catfish readily consumed practical diets containing sarafloxacin HCl; feed consumption and weight gain of fish fed medicated and non-medicated practical diets did not differ. There were no apparent palatability problems that would negate use of sarafloxacin HCl in commercial channel catfish feeds.     

Effect of natural free gossypol and gossypol‐acetic acid on growth performance and resistance of channel catfish (Ictalurus punctatus) to Edwardsiella ictaluri challenge

A study was conducted to evaluate the effect of free gossypol from glanded‐cottonseed meal (G‐CSM) (natural free gossypol) or gossypol‐acetic acid on growth performance, body composition, haematology, immune response and resistance of channel catfish (Ictalurus punctatus) to Edwardsiella ictaluri challenge. Soya bean meal‐based diets supplemented with 0, 100, 200, 400, and 800 mg kg^?1 free gossypol from G‐CSM or gossypol‐acetic acid were fed to juvenile channel catfish in triplicate aquaria to apparent satiation twice daily for 12 weeks. Neither sources nor levels of dietary gossypol significantly influenced the final weight gain, feed intake, feed efficiency and survival of channel catfish. Similarly, whole‐body proximate composition, haematological parameters (red blood cell, white blood cell counts, haemoglobin and haematocrit), serum protein concentration, macrophage chemotaxis ratio, phagocytic activity and antibody production against E. ictaluri 21‐day postinfection were not significantly affected at either dietary sources or levels of gossypol. Gossypol concentrations of liver were linearly related to dietary level of gossypol but the retention rate varied dependent on sources of the dietary gossypol. At dietary gossypol levels of 400 or 800 mg kg^?1, total gossypol concentrations in liver of fish fed dietary gossypol from G‐CSM were significantly higher than those of fish fed the corresponding levels of gossypol from gossypol‐acetic acid. The (+)‐isomer of gossypol was predominantly retained in liver regardless of dietary sources of gossypol. The ratio of (+) to (?) gossypol isomers in liver decreased with increasing dietary concentrations of gossypol. Serum lysozyme activity of fish fed dietary gossypol levels of 200 mg kg^?1 or higher, either from G‐CSM or gossypol‐acetic acid, was significantly higher than that of the control. At a level of 800 mg kg^?1 diet, gossypol from G‐CSM stimulated significantly higher lysozyme activity than gossypol from gossypol‐acetic acid. Fish fed diets containing 400 mg kg^?1 gossypol or higher from G‐CSM or 800 mg kg^?1 gossypol from gossypol‐acetic acid had significantly increased superoxide anion (O) production. However, neither the sources nor the levels of dietary free gossypol influenced the resistance of juvenile channel catfish to E. ictaluri challenge.     

Effects of Graded Levels of Carbohydrate on Growth and Survival of Largemouth Bass, Micropterus salmoides

Excessive carbohydrates (CHO) in diets for largemouth bass (LMB), Micropterus salmoides, are suspected of accumulating glycogen in hepatocytes, which may result in liver dysfunction. This study evaluated the effect of graded levels of dietary CHO on growth, survival, and liver histology of LMB. One hundred feed‐trained advanced fingerling LMB (128.5 ± 21.5 g) were stocked into each of nine 3400 ‐ L polyethylene tanks. Tanks were randomly assigned one of three experimental diets containing different CHO levels (13, 19, or 25% of diet). The extruded diets were approximately isonitrogenous (42% crude protein) and isocaloric (3 kcal/g energy). There were three replicate tanks per dietary treatment. Bass were fed to apparent satiation twice daily for 148 d. Survival was significantly higher (P ≤ 0.05) for fish fed the 13 and 19% CHO diets (89 and 90%, respectively) compared to those fed the 25% CHO diets (82%). Average harvest weight of fish fed the 13% CHO diet (380 g) was significantly greater (P ≤ 0.05) than for fish fed other diets. Average harvest weight of fish fed the 19% CHO diet (347 g) was significantly greater (P ≤ 0.05) than for fish fed the 25% CHO diet (310 g). Specific growth rates (%/d) were significantly higher (P ≤ 0.05) in fish fed the 13 and 19% CHO diets than in fish fed 25% CHO diet. Feed conversion ratios for fish fed the 13 and 19% CHO diets (2.3 and 2.4, respectively) were both significantly lower (P ≤ 0.05) than in fish fed the 25% CHO diet (3.6). There were no significant differences (P > 0.05) in condition factor, protein efficiency ratio, hepatosomatic index, or liver glycogen concentration among fish fed the different experimental diets. Overall, mean blood glucose levels in fish fed the 13 and 19% CHO diets (61.0 and 71.2 mg/dL, respectively) were significantly lower (P ≤ 0.05) than in fish fed the 25% CHO diet (87 mg/dL). Histopathological examination of livers from fish fed the three diets was used to score the degree of vacuolization of hepatic tissues (0 = normal, 1 = slight, 2 = mild, 3 = moderate, and 4 = severe). Regression of vacuolization scores on dietary CHO levels was statistically significant (P ≤ 0.05) and indicated a direct positive relationship between liver vacuolization and dietary CHO level (R² = 0.57). These data indicate that LMB grow faster and use feeds more efficiently when CHO are maintained at <20% of diet. CHO levels >20% negatively impacted liver histology, but a liver tissue analyses did not document glycogen accumulation.     

Effects of Dietary Protein Level and Feeding Regimen on Growth and on Fattiness of Channel Catfish,Ictalurus punctatus

Two studies were conducted to evaluate the effects of (I) high-protein (38%) finisher feed fed to satiation for 30,45,60, or 90 days prior to harvest and (2) dietary protein (32 vs. 38%) and feeding regimen (satiation or restricted) on growth and fattiness of channel catfish. Each study was conducted for two years in earthen ponds stocked with channel catfish at a rate of 13,590 fish/ha (35 to 40 g initial weight) for year 1 and 6,800 fish/ha (0.45 to 0.6 kg initial weight) for year 2. Years refer to year of experiment and not fish age class. There were no significant differences in total yield, dressed yield, or muscle fat, regardless of diet or feeding regimen within year. Year-one fish (study 2) fed to satiation tended to gain more weight and appeared to convert feed better than fish fed at a restricted rate. In study 2 there were significant interactions between year and feed, and between year and feeding regimen. Percentage visceral fat was reduced by feeding a high-protein feed during year 1 (study 1). Females also generally had a higher percentage visceral fat than males, regardless of diet. Year-two-fish generally contained a higher percentage of visceral fat as compared to year-one fish. In study 2, there was a significant interaction effect between year, feed, feeding regimen, and sex on visceral fat. Increasing dietary protein or restricting feed appeared to have only marginal effects on fattiness in channel catfish. Fish size and/or age appeared to influence fattiness more than diet or feeding regimen.     

Effects of Periodic Feed Deprivation on Growth, Feed Efficiency, Processing Yield, and Body Composition of Channel Catfish Ictalurus punctatus

Two studies were conducted in 110‐L flow‐through aquaria and 0.4‐ha ponds to evaluate effects of periodic feed deprivation on the growth performance of channel catfish Ictalurus punctatus. Fish were deprived of feed 0, 1, 2, or 3 consecutive d/wk, l d per 5‐d period, or 3 consecutive d per 10‐d period and fed to satiation on days fish were fed. In Experiment 1, fish fed less frequently than daily consumed significantly less feed (over the experimental period) and gained significantly less weight than fish fed daily, except that feed consumption of fish deprived of feed 1 d/wk was not significantly different from that of fish fed daily. Compared with fish fed daily, fish deprived of feed 2 d/wk had significantly lower feed conversion ratio (FCR). Visceral fat of fish deprived of feed 1 or 2 d/wk was similar to that of fish fed daily, but fish deprived of feed for longer periods had significantly lower visceral fat than fish fed daily. Regression analysis indicated that feed consumption, weight gain, and visceral fat increased linearly as the number of days that fish were fed increased. In Experiment 2, there were no significant differences in the amount of feed fed between fish deprived of feed 1 d/wk and those fed daily. Net production of fish deprived of feed 1 or 2 d/wk or 1 d per 5‐d period was not significantly different from that of fish fed daily, but fish deprived of feed for longer periods had significantly lower net production than fish fed daily. Visceral fat of fish deprived of feed 1 d/wk or 1 d per 5‐d period was similar to that of fish fed daily, but fish on other treatments had significantly lower visceral fat than fish fed daily. Regression analysis showed that as the number of days fed increased the amount of feed fed and net production increased quadratically. Feed conversion ratio, carcass yield, visceral fat, and fillet fat increased, while fillet moisture decreased linearly as the number of days fed increased. Although feeding less frequently than daily may improve feed efficiency, and fish deprived of feed may demonstrate compensatory growth when a full feeding regime is resumed, it may be difficult to provide enough feed to satiate all size‐classes of fish under a multiple‐batch cropping system without causing water quality problems. Under normal economic conditions, fish should be fed daily to apparent satiation without waste and without causing water quality problems. However, during periods of unfavorable economic conditions, channel catfish raised from advanced fingerlings to market size may be fed less frequently than daily to reduce production cost. Results from the present study indicated that feeding channel catfish to satiation 5 or 6 d/wk (not feeding on one or two weekend days) could provide some benefits in reducing production cost through reduced feed and labor costs for food‐sized channel catfish during periods of low fish prices and high feed prices.     

Growth performance and metabolic utilization of diets including starch,dextrin, maltose or glucose as carbohydrate source by gilthead sea bream (<Emphasis Type="Italic">Sparus aurata</Emphasis>) juveniles

The effect of dietary carbohydrate complexity on growth, feed utilization and activity of selected key liver enzymes of intermediary metabolism were studied in gilthead sea bream juveniles. Four isonitrogenous (50% crude protein) and isolipidic (16% crude lipids) diets were formulated to contain 20% of pregelatinized maize starch, dextrin, maltose or glucose. Triplicate groups of fish (117 g initial weight) were fed each diet to near satiation during 6 weeks. No effect of dietary carbohydrate on growth was noticed. Feed efficiency was lower in fish fed the glucose diet than the maltose and dextrin diets. The lowest protein efficiency ratio was observed in fish fed the glucose diet. Six hours after feeding, glycemia was higher in fish fed the glucose diet than the maltose and starch diets. Liver glycogen content was unaffected by dietary carbohydrate complexity. Hepatic glucokinase (GK) activity was higher in fish fed the glucose and the maltose diets, while higher pyruvate kinase (PK) activity was recorded in fish fed the glucose diet than in fish fed the starch diet. Fructose-1,6-bisphosphatase (FBPase) and glucose-6-phosphate dehydrogenase (G6PD) activities were higher in fish fed the starch diet compared to dextrin and glucose diets. Data suggest that dietary glucose and maltose are more effective than complex carbohydrates in enhancing liver glycolytic activity. Dietary glucose also seems to be more effective than starch in depressing liver gluconeogenic and lipogenic activities. Overall, dietary maltose, dextrin or starch was better utilized than glucose as energy source by gilthead sea bream juveniles.     

Morphological and genetic characteristics of Flavobacterium columnare isolates: correlations with virulence in fish

Variability in pathogenicity of Flavobacterium columnare makes disease treatment difficult because there is currently no way to easily recognize those strains that warrant aggressive treatments. In order to identify suitable virulence markers, 17 isolates of F. columnare were cultured from six different fish species. The DNA from all isolates was analysed using random amplified polymorphic DNA (RAPD). Bootstrap analysis of the RAPD data produced a tree with three major groups supported by bootstrap scores of 80-100%. Virulence of the isolates was determined by bath exposure of channel catfish, Ictaluruspunctatus (Rafinesque), and golden shiners, Notemigonus crysoleucas (Mitchill), to broth cultures of F. columnare. In channel catfish, 13 of 17 isolates produced 100% mortality within 48 h post-exposure. All isolates of cyprinid fish origin clustered in a single RAPD group. At least two of the four isolates that were not virulent in channel catfish were of cyprinid fish origin. There was a wide variation in cell morphology between isolates with lengths of cells or cell chains ranging from 3 to 11 microm, even under identical culture conditions. Most of the shorter or single cell isolates fell into a single RAPD group. No clear association was identified between virulence and any other characteristic, including RAPD group.     

Histologic changes in channel catfish,Ictalurus punctatus Rafinesque,fed diets containing graded levels of gossypol–acetic acid

A study was performed to evaluate the histologic changes among fingerling channel catfish, Ictalurus punctatus Rafinesque, fed purified diets containing gossypol from gossypol–acetic acid. The catfish were maintained on diets with 0, 300, 600, 900, 1200 or 1500 mg gossypol kg^?1 diet for 12 weeks, and histologic samples from the stomach, proximal and distal intestines, pancreas, liver, and spleen were obtained from fish in all groups. Stomach sections exhibited significant gastric gland necrosis in fish fed 600 mg gossypol kg^?1 diet or higher. Non‐existent‐to‐mild enterocyte vacuolization loss, inflammatory cell infiltration, and hyperplastic lamina propria were noted in the intestinal sections from gossypol‐fed fish, but no significant differences in severity scores were noted. The pancreas from fish fed doses of gossypol above 600 mg gossypol kg^?1 diet exhibited significant mild‐to‐severe necrosis, and livers from fish fed 900 and 1500 mg gossypol kg^?1 diet exhibited significantly higher pigment deposition. No other significant histologic differences were observed in the fish fed diets containing gossypol–acetic acid. The data in this study indicates that at least 600–900 mg gossypol kg^?1 purified diet can cause statistically significant histologic changes in fingerling channel catfish, suggesting that gossypol should remain at concentrations below 600 mg gossypol kg^?1 diet.     

Evaluation of mixed feeding schedules with respect to compensatory growth and body composition in African catfish Clarias gariepinus

An 8‐week feeding trial was conducted in a recycling water system at 28 ± 1 °C to investigate compensatory growth and body composition in African catfish Clarias gariepinus (13.05 ± 0.05 g). A fishmeal‐based diet containing 350 g kg^?1 protein and 17.5 kJ g^?1 gross energy was fed to triplicate groups of 20 fish (per 30‐L tank). Fish were fed the diet either to satiation or feed restricted in six feeding schedules as satiation 56 days; restricted 28 days + satiation 28 days; restricted 14 days + satiation 14 days; restricted 7 days +satiation 7 days; restricted 3 days + satiation 4 days; and restricted 2 days + satiation 2 days. The restricted regime was achieved by offering fish 1% (maintenance ration) of their body weight per day adjusted after fortnightly weighing. African catfish showed partial compensatory growth under alternating periods of feed restriction and satiation feeding with significantly different values (P < 0.05) from fish fed in satiation throughout. However, significantly indifferent (P > 0.05) values of feed, protein, lipid and energy utilization were found under alternating periods of feed restriction and satiation feeding. Significantly higher (P < 0.05) feed intake was observed in treatment with satiation throughout than those in other treatments. All the feeding schedules showed no significant differences (P > 0.05) on body composition, organ indices, eviscerated carcass composition, viscera lipid and liver lipid. These studies reveals that C. gariepinus showed partial compensatory growth responses at alternating periods of restricted and satiation feeding.     

Effect of Dietary Protein Concentration and Feeding Rate on Weight Gain,Feed Efficiency,and Body Composition of Pond-Raised Channel Catfish Ictalurus punctatus1

Two experiments were conducted in earthen ponds to evaluate the effect of dietary protein concentration and feeding rate on weight gain, feed efficiency, and body composition of channel catfish. In Experiment 1, two dietary protein concentrations (28% or 32%) and four feeding rates (≤ 90. ≤ 112, ≤ 135 kg/ha per d, or satiation) were used in a factorial arrangement. Channel catfish Ictalurus punctatus fingerlings (average size: 27 g/fish) were stocked into 0.04-ha ponds at a rate of 24,700 fish/ha. Fish were fed once daily at the predetermined maximum feeding rates for 282 d (two growing seasons). In Experiment 2, three dietary protein concentrations (24, 28, or 32%) and two feeding rates (≤ 135 kg/ha per d or satiation) were used. Channel catfish (average size: 373 g/fish) were stocked into 0.04-ha ponds at a rate of 17,300 fish/ha. Fish were fed once daily for 155 d. In both experiments, five ponds were used for each dietary treatment. Results from Experiment 1 showed no differences in total feed fed, feed consumption per fish, weight gain, feed conversion ratio (FCR), or survival between fish fed diets containing 28% and 32% protein diets. As maximum feeding rate increased, total feed fed, feed consumption per fish, and weight gain increased. There were no differences in total feed fed, feed consumption per fish, or weight gain between fish fed at ≤ 135 kg/ha per d and those fed to satiation. Fish fed the 28% protein diet had a lower percentage carcass dressout and higher percentage visceral fat than fish fed the 32% protein diet. Dietary protein concentrations of 28% or 32% had no effect on fillet protein, fat, moisture, and ash. Feeding rate did not affect FCR, survival, percentage carcass dressout, or fillet composition, except fillet fat. As feeding rate increased, percentage visceral fat increased. Fish fed at ≤ 90 kg/ha per d had a lower percentage fillet fat than fish fed at higher feeding rates. In Experiment 2, dietary protein concentration or maximum feeding rate did not affect total feed fed, feed consumption per fish, weight gain, FCR, or survival of channel catfish. Feeding rate had no effect on percentage carcass dressout and visceral fat, or fillet composition. This was due to the similar feed consumption by the fish fed at the two feeding rates. Fish fed the 24% protein diet had lower carcass dressout, higher visceral fat and fillet fat than those fed the 28% or 32% protein diet. Results from the present study indicate that both 28% and 32% protein diets provide satisfactory fish production, dressed yield, and body composition characteristics for pond-raised channel catfish fed a maximum rate of 90 kg/ha per d or ahove.