A comparison of retinol, retinal and retinyl ester concentrations in larvae of Atlantic halibut (Hippoglossus hippoglossus L.) fed Artemia or zooplankton

Atlantic halibut (Hippoglossus hippoglossus L.) larvae were fed enriched Artemia or zooplankton in duplicate tanks from 0 to 60 days after first‐feeding. Both diets and the larvae were analysed for vitamin A (VA) in order to confirm earlier findings, in which Artemia fed larvae had lower levels of VA compared with larvae fed zooplankton. Furthermore, we wanted to investigate the composition of the retinoids in the larvae. The results showed that Artemia and zooplankton contains low levels of VA, probably too low to sustain the assumed requirement. Nevertheless, larvae fed Artemia had the same level of retinal and retinol as larvae fed zooplankton. We found a significant lower level of retinyl esters in larvae fed Artemia. The total VA level was lower in larvae fed Artemia only at the end of the feeding trial after the onset of metamorphosis. Our conclusion is that feeding Artemia to Atlantic halibut larvae is not likely to cause VA deficiency.     

Element concentrations in meals from krill and amphipods, — Possible alternative protein sources in complete diets for farmed fish

In the marine environment, organisms from lower trophic levels seem as a good alternative to the traditional meal and oil sources. In the present study, meals were produced from Antarctic krill (Euphausia superba), Arctic krill (Thysanoessa inermis) and the Arctic amphipod Themisto libellula. Diets were then prepared for Atlantic salmon and Atlantic cod where up to 100% of the fish meal protein was replaced by protein from these organisms. Concentrations of copper (Cu), zinc (Zn), arsenic (As), cadmium (Cd), mercury (Hg) and lead (Pb) were determined by ICP-MS in the various krill and amphipod meals, complete diets and muscle samples from fish fed these diets. The element concentrations were related to growth and general fish health as well as present EU legislations on feed ingredients and complete diets. The cod showed no difference in growth during the trial, while salmon fed diets where 40% of the fish meal protein was replaced with Arctic krill or amphipod meal shoved improved SGR during the first period of feeding (first 100 days). No adverse effects on growth rate or fish health were observed in any fish species or treatment. Nevertheless, high levels of Cu were found in the meal from Antarctic krill (46 mg kg^− 1 dry matter (dm)) resulting in a dietary level of Cu exceeding the upper limit for complete feedingstuff set by EU. Furthermore, the Cd level found in the meal from amphipod (12 mg kg^− 1 dm) was 6 times higher than EU's upper limit. This indicates limitations for the use of certain zooplanktons as alternative protein sources in feed for farmed fish, unless future processing methods yield lower levels of these unwanted elements.     

The impact of nutrition on metamorphosis in Atlantic halibut (Hippoglossus hippoglossus L.)

Fatty acids, vitamin A and thyroid hormone have all been shown to affect development of flatfish larvae and they are ligands to nuclear receptors that participate in the control of development. Our hypothesis was that one of these factors or an interaction between them may be the cause of abnormal development of flatfish larvae. Atlantic halibut larvae were fed either DHA-selco-enriched Artemia or copepods from first feeding. In fish that had been fed Artemia, only 7% had normal pigmentation and 10% normal eye migration. The numbers for fish fed copepods were 68% and 88%, respectively. Malpigmented fish fed Artemia were depigmented, while those fed copepods had ambicoloration. The differences in development were probably nutrient dependent, since all other conditions were similar for the two groups. Larvae fed copepods had markedly higher body levels of docosahexanoic acid (DHA, 22:6n−3) and eicosapentaenoic acid (EPA, 20:5n−3) and lower levels of arachidonic acid (ARA, 20:4 n−6) than larvae fed Artemia. The DHA/EPA ratio was similar in the two groups, but the EPA/ARA ratio was more than four times higher in larvae fed copepods than in larvae fed Artemia. Larvae fed copepods had higher body levels of total retinol than larvae fed Artemia, but the difference was due to higher levels of the storage forms, retinyl esters, whereas the levels of free retinol and retinal were similar in the two groups. The level of iodine was 700 times higher in copepods than in Artemia and 3–4 times higher in larvae fed copepods than in larvae fed Artemia. There was a significantly higher level of T₄ in larvae fed copepods during the “window of opportunity”, 15–30 days after first feeding. In an experiment where Atlantic halibut larvae were fed Artemia enriched in iodine up to the levels found in copepods, there was a significant effect on the body level of iodine and a non-significant tendency of higher levels of thyroid hormone, but no effect on pigmentation or eye migration. It is concluded that Artemia probably offers a sufficient access to vitamin A precursors to meet the larval requirement. More research should be done to elucidate possible effects of iodine on development of Atlantic halibut larvae. Fatty acid composition is still the most likely candidate for causing abnormal development in Atlantic halibut larvae.     

Dietary taurine supplementation in plant protein based diets do not affect growth and reproductive performance of zebrafish

Taurine (Tau) has been regarded as a conditional essential nutrient for some fish species. Although its role has been extensively studied in higher vertebrates, limited results are reported with fish especially its role on reproductive performance and the ontogenic changes on Tau levels throughout the life cycle. Therefore, we designed a feeding trial using zebrafish as a model species to test whether Tau supplementation to plant protein diets would have a positive effect on growth and reproductive performance. Zebrafish were fed plant protein diets containing graded levels of Tau (0.2, 4.6, 5.9 and 13.7 g/kg diet) from 10 days post fertilization (dpf) to sexual maturity. An additional commercial diet was used as a positive control for performance. The trial followed a completely randomized design with five treatments (diets) and three replications. After 60 days of feeding, growth, Tau concentration in the body, redox status, lipid body composition, reproductive and offspring performances were analysed. Tau supplementation did not affect growth and/or reproductive performance; however, zebrafish seems to differently modulate Tau concentration according to the growth stage. Tau seemed to induce a hypolipidemic effect in zebrafish by reducing lipid accumulation in their bodies (p < .05). A trend to a more pro‐oxidant effect of Tau supplementation was observed by the decreased reduced glutathione levels. In sum, Tau does not affect growth and reproductive performance of zebrafish but it is important for normal lipid utilization and redox status.     

Conversion of β-carotene, canthaxanthin and astaxanthin to vitamin A in Atlantic halibut (Hippoglossus hippoglossus L.) juveniles

Larval Atlantic halibut fed Artemia has previously been shown to contain lower levels of Vitamin A compared to larvae fed zooplankton. The two types of live prey contain small or no amounts of vitamin A, but high levels of carotenoids that can be converted to vitamin A in other fish species. The purpose of this study was to investigate the ability of Atlantic halibut juveniles to convert β-carotene, astaxanthin and canthaxanthin to vitamin A. Three levels of each carotenoid and retinyl acetate were fed to Atlantic halibut juveniles for 60 days. A vitamin A and carotenoid deficient diet was fed in triplicate as control. A HPLC method modified from Nöll (1996) and validated for fish matrix was used to quantify both all-trans-retinol and 3,4-didehydro retinol. By comparing regression coefficients we observed that the increasing levels of carotenoids in the diets were reflected in increasing levels of vitamin A in both whole fish and liver samples. All carotenoids were converted to vitamin A, but to different degrees. Retinyl acetate and β-carotene resulted in whole fish vitamin A levels significantly higher than canthaxanthin and astaxanthin. 3,4-didehydro retinol was not detected when the overall level of all-trans-retinol was low. When 3,4-didehydro retinol appeared, it was always in lower levels than all-trans-retinol.     

Control of light condition affects the feeding regime and enables successful eye migration in Atlantic halibut juveniles

Incomplete eye migration is one of the major problems in intensive production of juvenile Atlantic halibut. More than 60% of an average juvenile population reared according to best practice suffers from this abnormality. In commercial production, these fish are discharged and represent a substantial economic loss and a large welfare problem. In the present investigation it is demonstrated that by controlling diurnal light and darkness periods together with a meal based feeding regime, incomplete eye migration can be dramatically reduced in production systems for Atlantic halibut.Control groups were reared under continuous light conditions, whereas the experimental groups were given 7 h of darkness and 17 h of light during a 24 hour cycle, in a period lasting from 12 to 35 days post first-feeding. Otherwise both groups were reared under continuous light conditions. All larvae were fed short time enriched Artemia supplied two times daily.The experimental conditions did not affect the overall growth or survival up to day 85 after first feeding. However, 27 ± 3% of the fry reared under continuous light conditions had complete eye migration, whereas in juveniles reared under shifting light and darkness conditions, complete eye migration was 85 ± 7%. These results represent a major improvement in production systems for Atlantic halibut juveniles.     

Iodine and selenium supplementation increased survival and changed thyroid hormone status in Senegalese sole (Solea senegalensis) larvae reared in a recirculation system

To test how iodine and both iodine and selenium supplementation affected the thyroid status as well as growth and survival in Senegalese sole, larvae were reared in a recirculation system from 15 to 34 DAH. Sets of three tanks were assigned to each of the following three diets: control (C), iodine (I) and iodine and selenium (I?+?Se). Samples were collected at 15, 27 and 34 DAH to determine dry weight, iodine and selenium levels, GPx and ORD activities, thyroid hormone levels and thyroid follicles histology. At 34 DAH, fish from the control (C) treatment suffered from hyperplasia of the thyroid follicles (goitre), whereas iodine-treated larvae did not (I and I?+?Se). Lower survival rates in the C groups were probably a consequence of the hyperplasia. Moreover, there was an improvement in thyroid hormone status in I- and I?+?Se-treated larvae, showing that further supplementation of live feed with iodine can be crucial for fish at early life stages, as it seems to sustain normal larval development, when reared in a recirculation system. Selenium did not affect the results. Together with previous results, this indicates selenium supplement is more important at younger life stages.