J.W. Hilton

Response of rainbow trout (Salmo gairdneri) to increased levels of available carbohydrate in practical trout diets.

1. The physiological response of rainbow trout (Salmo gairdneri) reared on different levels of available carbohydrate in practical trout diets having the same levels of energy and nitrogen for 16-24 weeks was determined. 2. Weight gain was significantly reduced in trout reared on the highest level of available carbohydrate, 210 g cerelose (alpha-glucose) kg, and there was a significant linear regression (R2 0.88 of dietary carbohydrate on weight gain. 3. Liver: body-weight values and liver glycogen levels increased in relation to increased dietary carbohydrate. 4. Liver glucose-6-phosphate dehydrogenase (EC 1.1.1.49) activity increased and liver phosphoenolpyruvate carboxykinase (EC 4.1.1.32) activity decreased per kg body-weight of fish with increasing dietary carbohydrate. However, no significant effect was noted on the activity of these liver enzymes above a dietary cerelose level of 140 g/kg. 5. Liver fructose diphosphatase (EC 3.1.3.11) activity increased with increasing dietary carbohydrate has been interpreted as meaning a recycling of triosephosphate to glucose-6-phosphate. 6. Dietary carbohydrate level had no significant effect on the liver pyruvate kinase (EC 2.7.1.40) activity, the rate of glucose utilization or the percentage conversion of [14C]alanine to glucose in the plasma of trout. 7. The results indicate that rainbow trout have a limited ability to adapt to increased dietary carbohydrate and a level in excess of 140 g/kg of the diet is not efficiently utilized.

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.

The interaction of vitamins, minerals and diet composition in the diet of fish

The purpose of this paper is to review the available literature on nutrient interaction studies, both dietary and metabolic, that have been conducted in fish nutrition. Four major types of nutrient interactions are considered: 1, vitamin-vitamin interactions (vitamin B12 and folic acid; ascorbic acid-vitamin E); 2, vitamin-mineral interactions (vitamin D-calcium; vitamin E-selenium; ascorbic acid-copper; ascorbic acid-iron); 3, mineral-mineral interactions (calcium-phosphorus; magnesium-calcium; magnesium-phosphorus; copper-zinc; selenium-copper); 4, micronutrient-diet composition interaction (thiamin-carbohydrate; pyridoxine-protein; vitamin E-polyunsaturated fatty acids; zinc-calcium-phytate).

Does oral 3,5,3 ′-triiodo-l-thyronine affect dietary glucose utilization and plasma insulin levels in rainbow trout (Salmo gairdneri)?

A factorial experiment was conducted to determine the effect and interaction of dietary carbohydrate level and triiodo-L-thyronine (T3) supplementation on the growth, physiological response and plasma insulin and cortisol levels of rainbow trout. The oral administration of T3 significantly increased the growth, protein efficiency ratio and feed efficiency of trout, indicating an increased protein and perhaps energy utilization in these fish. However, T, administration did not significantly increase the utilization of dietary glucose as an energy source by the trout. Similarly, the administration of T3 did not significantly affect plasma insulin levels in either the fed or the fasted trout. Plasma insulin levels were significantly higher in fed trout reared on the non-T3 supplemented high carbohydrate diet in comparison to trout reared on the low carbohydrate diets. This indicates that increased dietary carbohydrate stimulates increased insulin secretion in the trout. Therefore, although rainbow trout are not insulin-deficient, they can still be considered a diabetic-like animal due to their poor glucose tolerance. Plasma cortisol levels were not affected by diet composition and altered plasma glucose levels.

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.

Iron catalyzed oxidation of trout diets and its effect on the growth and physiological response of rainbow trout.

Two experiments were conducted to determine 1) the effect of iron supplementation and the quality of fish oils on dietary lipid peroxidation and 2) the concurrent effects of diet rancidity and iron overload on the growth and physiological response of rainbow trout. Semi-purified diets supplemented with graded levels of iron (0–6250 mg/kg diet as ferrous sulphate) were fed to trout for 12–36 weeks. The malonaldehyde (MA) concentration of the test diets increased as the iron levels in the diets increased indicating that iron catalyzed lipid oxidation was occurring. However, when ethoxyquin was added to the oils, the increase in dietary MA level was significantly reduced. Fish oils with an initial high peroxide value were more susceptible to iron-catalyzed lipid oxidation. The concurrent effects of diet rancidity and iron overload (greater than 86 mg/kg) led to the development of unique histopathological signs, poor growth and high mortalities in the trout. In contrast, when diet rancidity was low (less than 10 µg MA/g diet), the toxic level of dietary iron was greater than 1380 mg/kg diet. The concentration of iron in trout tissues, and the hematocrit and hemoglobin concentrations increased as dietary iron levels increased and were not affected by the degree of diet rancidity.

The thiamin deficiency signs and requirement of rainbow trout (Salmo gairdneri, Richardson)

Four growth studies were conducted to determine the signs, biochemical indices and histopathology of a thiamin deficiency and the thiamin requirement of young rainbow trout reared at 15°C on a semi-purified test diet. The major overt signs of a thiamin deficiency in rainbow trout are predominantly neurological: irritability and instability. Other signs include convulsions, feed refusal, dark pigmentation and finally mortalities. Growth reduction in the thiamin deficient trout appear to result from anorexia or feed refusal and not specifically to a thiamin deficiency. Although there were prominant neurological signs in the thiamin deficient trout, there were no histopathological signs in any tissues of the trout, including the brain and central nervous system, examined by light microscopic techniques. The tissue transketolase activity would appear to be a sensitive and specific indicator of the thiamin status in the trout. In addition, the levels of plasma lactate and serum pyruvate are also elevated in thiamin deficient trout. On the basis of the growth parameters, absence of deficiency signs and kidney and liver transketolase activity, the thiamin requirement of rainbow trout reared at 15°C on a semi-purified test diet is 1 mg/kg feed.

Histological observations on intrahepatocytic copper-containing granules in rainbow trout reared on diets containing elevated levels of copper

Abstract The histological nature of hepatic copper deposition in the liver of rainbow trout reared on diets containing elevated levels of copper was investigated. Two growth trials were conducted using practical diets with graded levels of supplemental copper, with determined maximum levels of 3088 mg/kg and 664 mg/kg copper. Histochemical observations using a rhodanine stain with a Harris hematoxylin counterstain revealed the presence of many rhodanine-positive granules in the hepatocytes of trout reared on diets containing the highest levels of copper supplementation. Ultrastructurally, electron-dense granules were visible in the cytoplasm of individual hepatocytes. Electron microprobe analysis confirmed the presence of large amounts of copper in these granules, along with a substantial amount of sulphur. These observations show that rainbow trout have the ability to sequester dietary copper in discrete granules in the cytoplasm of hepatocytes. The pericanalicular arrangement of the granules within the hepatocyte may suggest that these granules function in the excretion of excess copper in the bile.

Effect of dietary tryptophan on plasma and brain tryptophan, brain serotonin, and brain 5-hydroxyindoleacetic acid in rainbow trout

In order to determine the effect of dietary tryptophan level on plasma and brain tryptophan, brain serotonin, and brain 5-hydroxyindoleacetic acid levels, juvenile rainbow trout (Salmo gairdneri) were raised for 16 weeks on semipurified diets containing 0.06%, 0.16%, 0.21%, 0.26%, 0.39%, or 0.59% tryptophan. After 14 weeks, feed intake was depressed in fish fed the diets containing 0.06% or 0.16% tryptophan. No further differences in feed intake were noted between the remaining treatments. In addition, body weight was lower in fish fed diets containing 0.06%, 0.16%, or 0.21% tryptophan compared with fish fed higher levels. After 16 weeks of feeding the test diets, plasma tryptophan levels were found to be directly related to dietary tryptophan levels. Similarly, increased dietary levels of tryptophan resulted in increased brain levels of tryptophan, serotonin, and 5-hydroxyindoleacetic acid. These results demonstrate that in rainbow trout, as in mammals, altered dietary levels of tryptophan result in alterations in plasma and brain tryptophan, brain serotonin, and brain 5-hydroxyindoleacetic acid.