Stanley J. Slinger

Effect of extrusion processing and steam pelleting diets on pellet durability, pellet water absorption, and the physiological response of rainbow trout (Salmo gairdneri R.)

Abstract The effect of extrusion processing and steam pelleting on pellet durability, pellet water absorption and the physiological response of rainbow trout reared upon these diets was investigated. Extruded pellets were observed to be more durable, had superior water stability and absorbed more water than steam pellets. Trout reared on extruded pellets had prolonged gastric emptying in comparison with those reared on steam pellets; this may have been responsible for the reduction in daily feed consumption. Trout reared on extruded pellets also showed significantly lower weight gain but higher feed efficiency than those reared on steam pellets. Liver: body weight ratio and percent liver glycogen were significantly higher in trout reared on the estruded pellets as compared with those reared on steam pellets. Thus, extrusion processing may increase the bioavailability of carbohydrate in the diet and the enlarged livers and increased liver glycogen content could impair liver function.

Maximum tolerable and toxicity levels of dietary copper in rainbow trout (Salmo gairdneri Richardson)

Two feeding trials were conducted to determine the maximum tolerable and toxicity levels of dietary copper, supplemented as CuSO4·5H2O, in rainbow trout. Dietary copper toxicity occurred at 730 mg Cu/kg diet, and was characterized by reduced growth, increased feed:gain ratios, food refusal and elevated liver copper levels. The maximum tolerable level of dietary copper was approximately 665 mg Cu/kg diet; however, there was some adaptation by the trout to this level of dietary copper. Increased levels of dietary copper had no significant effect on the ascorbic acid content of the head kidney and liver, plasma copper, plasma glucose, hemoglobin or hematocrit levels, and the hepatosomatic index of the trout. Copper, iron and zinc levels in the kidney, and zinc levels in the liver showed no apparent trend in relation to increased dietary copper. However, both copper and iron levels in the liver increased in relation to increased levels of dietary copper. This may indicate an interaction between copper and iron metabolism in the trout.

Absorption, distribution, half-life and possible routes of elimination of dietary selenium in juvenile rainbow trout (Salmo gairdneri)

1. The influence of different levels of dietary selenium on the metabolism of selenium in rainbow trout was studied using 75Se as an indicator. 2. Gastric absorption of selenium by the trout appeared to be very efficient. 3. Highest tissue concentrations of selenium were noted in the liver and kidney. 4. Blood did not concentrate selenium and the plasma was the major transport medium. 5. The liver and kidney appeared to be involved in selenium excretion based on high tissue concentrations and variations in half-lives with selenium loading. 6. The biological half-life of selenium in the tissues decreased with increased selenium loading except in the liver, which at toxic dietary selenium concentrations became longer, suggesting a rate-limiting metabolic transformation of selenium for excretion in this organ.

Dietary carbohydrate and growth, body composition and heat increment in rainbow trout (Salmo gairdneri)

A study was conducted to determine growth, body composition and heat increment (HI) of rainbow trout reared on isonitrogenous high digestible carbohydrate (HC) and high fat (HL) diets at 15°C. Trout reared on the HC diet had a significantly lower final body weight and carcass lipid content and a significantly higher feed:gain ratio and carcass protein content than trout reared on the HL diet after 12 weeks on the test diets. The lower carcass lipid composition indicates that trout do not readily convert dietary carbohydrates into fat. HI of trout reared on the HC diet was significantly higher than that of fish reared on the HL diet. Considering the poor utilization of dietary carbohydrates as an energy source and the apparently low conversion of dietary glucose into fat, the cause of the higher HI associated with the HC diet is not completely understood. However, it is possible the elimination of excess dietary glucose is an energy dependent process resulting in an increased heat production which would partly account for the increased HI. Furthermore, if the HC diet is a low net energy diet, then the metabolism and utilization of dietary and tissue proteins (amino acids) for energy by the trout may also be partly responsible for the increased HI of the trout reared on the HC diet.

The essentiality of cholecalciferol in the diets of rainbow trout (Salmo gairdneri)

Abstract 1. 1. Vitamin D3 was found to be essential in the diet of rainbow trout. 2. 2. Vitamin D3 deficiency in rainbow trout was characterized by a marked decrease in weight gain and feed efficiency, lethargy and anorexia, the clinical manifestations of tetany without hypocalcaemia. an increase in the lipid content of carcass, white muscle and liver, and increased carcass dry matter. 3. 3. Unlike mammals, bone and blood mineral stores appeared unaltered in vitamin D3 deficient rainbow trout. 4. 4. With trout maintained in water containing 30ppm of calcium, increasing the level of dietary calcium from 0.36% to 1.1% increased the amount of white muscle lipid and decreased the kidney magnesium levels, but exerted no effect on weight gain or feed efficiency, regardless of vitamin D3, status. 5. 5. Rainbow trout receiving feed containing 1000 i.u./kg of vitamin D3 showed no detectable amounts of 25-hydroxycholecalciferol or 24R. 25-dihydroxycholecalciferol in their blood plasma.

The synthesis, half-life and distribution of ascorbic acid in rainbow trout

Abstract 1. 1. Synthesis of ascorbic acid from [1- 14 C] glucose in yearling and mature rainbow trout is not conclusive at this time but may occur in some fish. 2. 2. The biological half-life of [1- 14 C]ascorbic acid in the head kidney of yearling rainbow trout on the basis of TLC- 14 C radioactivity analysis was approximately 20–21 days. 3. 3. The ascorbic acid levels in the tissues of rainbow on a replete diet showed that the highest concentrations were found in the female gonads, with the brain, head kidney, testes, spleen, liver, eyes, red muscle, white muscle and heart tissues having lower concentrations in that order. 4. 4. The ascorbic acid concentration in the ripe ovary would represent a considerable store of the vitamin, which would also suggest a critical function of this vitamin in the reproduction of this animal and the feeding of an ascorbic deficient diet to the brood stock could therefore impair the reproductive performance of the trout.

Effect of ascorbic acid on dietary copper toxicity in rainbow trout (Salmo gairdneri Richardson)

Abstract Triplicate groups of juvenile rainbow trout (Salmo gairdneri) were reared for 16 weeks on practical trout diets containing graded levels of ascorbic acid (0, 100, 1000 or 10 000 mg/kg) with or without a copper supplement of 800 mg/kg. At the end of 16 weeks, trout reared on copper-supplemented diets had significantly lower final body weights than trout reared on diets without supplemental copper. Supplementation with ascorbic acid up to and including 10 000 mg/kg diet did not appear to affect markedly either the absorption or metabolism of copper. However, ascorbic acid supplementation at a level of 10 000 mg/kg did result in a small but statistically significant increase in final body weight of trout reared on 800 mg Cu/kg diet when compared with trout reared on the three lower levels of ascorbic acid. Ascorbic acid concentrations in the head kidney and liver increased with dietary ascorbic acid supplementation, but were not affected by dietary copper supplementation. Copper levels in the kidney, liver and whole body increased with dietary copper supplementation but were not affected by dietary ascorbic acid level. Zinc levels in the liver and whole body increased with dietary copper supplementation while zinc levels of the kidney were not affected. Ascorbic acid supplementation had no effect on the zinc content of the kidney or liver. Iron levels in the kidney were variable, but showed no relation to dietary treatment; however, iron levels in the liver were affected and indicate some interaction in the metabolism of copper and iron in the trout. It was concluded that dietary ascorbic acid supplementation up to 10 000 mg/kg does not significantly alleviate dietary copper toxicosis in juvenile rainbow trout.

A semi-purified test diet for Macrobrachium rosenbergii and the lack of need for supplemental lecithin

Abstract Post-larval Macrobrachium rosenbergii were reared for 12 weeks at 27°C in a recirculated aquatic system on semi-purified diet supplemented with between 0 and 10% lecithin. No significant differences were detected in either the weight gain or mortality rates of the prawn reared on the different test diets and no sign of ‘molt death syndrome’ was observed in any of the prawn. It was concluded that supplemental lecithin is not required in the semi-purified diet developed for Macrobrachium rosenbergii when cultured under the experimental conditions of this study. The semi-purified diet formulated in this study would appear to be an adequate test diet to determine many of the qualitative and quantitative nutrient requirements of Macrobrachium rosenbergii. However, the best weight gain of the prawn was on a practical-type diet indicating that further refinement of the test diet formulation may be required.

Effects of thyroid hormone supplementation of canola meal-based diets on growth, and interrenal and thyroid gland physiology of rainbow trout (Salmo gairdneri)

Rainbow trout fed a 26% canola meal-based (CM) diet for 12 weeks at 15°C exhibited reduced growth, lower feed conversion, enlarged thyroid glands and lower plasma thyroid hormone (TH) levels than comparable fish fed equinitrogenous, equicaloric soybean meal-based (SB) diets. Supplementation of the SB diets with either T4 (20 mg/kg) or T3 (10 or 20 mg/kg) had no effect on the growth rate, feed conversion and thyroid histology of the trout. However, plasma T4 levels weredepressed in trout fed the T4- and high T3-supplemented SB diets. In trout fed T4- and T3-supplemented CM diets the growth rates and feed conversion were not significantly different from those of the SB-fed groups. Moreover, in the T4-supplemented group, plasma T4 levels were in the normal range. However, thyroid enlargement was evident in all the CM-fed fish, and plasma T3 levels were markedly elevated in groups fed the T3-supplemented CM diets. The data suggest that antithyroid components in the CM diets inhibited TH synthesis (but not their release), and impaired TH clearance from the circulation. There were no significant differences in plasma cortisol levels in the 8 treatment groups, nor were there differences in the histological appearance of the interrenal gland. However, when the data from SB- and CM-fed fish were pooled, plasma cortisol levels in the SB-fed fish were significantly lower than in the CM-fed animals. Glucosinolates at a level of 164 mg/kg diet were toxic to young trout, but the effect was ameliorated by dietary TH supplementation.

Effect of wheat bran replacement of wheat middlings in extrusion processed (floating) diets on the growth of juvenile rainbow trout (Salmo gairdneri)

Abstract The effect of partial replacement of wheat middlings with wheat bran in extruded diets on the growth and physiological response of juvenile rainbow trout was determined in a 16-week study. Trout reared on an extruded diet containing a normally accepted level of wheat middlings showed a depressed growth rate, increased liver weight, increased liver glycogen content and elevated and prolonged plasma glucose levels compared with trout reared on extruded diets with wheat bran partially replacing the wheat middlings. The results suggest that the higher level of available carbohydrate in the diet containing the highest (but normally accepted) level of wheat middlings was, at least, partially responsible for the growth depression of the trout reared on this diet. The elevated and prolonged plasma glucose levels of these trout may have depressed appetite, feeding response and feed consumption which would appear to be the major cause of the growth depression.