Halibut Larvae Research Articles

Ascorbic acid and α-tocopherol levels in larvae of Atlantic halibut before and after exogenous feeding

Ascorbic acid and α-tocopherol levels in larvae of Atlantic halibut before and after exogenous feeding

Thyroxine as a Mediator of Metamorphosis of Atlantic Halibut, Hippoglossus Hippoglossus

The response of morphological, histological and endocrinological development to exogenous 1-thyroxine (T4) and to water depth during metamorphosis in Atlantic halibut, Hippoglossus hippoglossus, was investigated. Exogenous T4 was given in daily doses of 0.1, 0.05 ppm or a control treatment to halibut larvae at 550 daydegrees (posthatch, premetamorphic) for 14 days. Water depths of 40 cm, 10 cm or 1.5 cm were used to rear halibut larvae from 590 daydegrees for 21 days. Halibut larvae given exogenous T4 at 0.1 ppm had accelerated eye migration relative to MH in fish given 0.05 ppm and in control fish. Pigmentation was correlated with dosage after 14 days. The volume of thyroid tissue was expressed in a dose-dependent manner and exhibited a size-dependency within each treatment. However, the follicles were atypical with reduced colloid, increased lumen and low epithelial cells even in the control group. The results indicate that T4 is a mediator in halibut metamorphosis. In the water depth experiment, only cortisol levels of larvae reared in 1.5 cm water were significantly affected after 21 days, but this was not correlated with metamorphic rate. Hormone profiles, morphological changes and size suggest the existence of a ‘window of opportunity’ for metamorphosis in halibut extending from about 16 mm and tapering off about 21 mm SL. The pooled hormone profiles indicate the commencement of a hormonal cascade similar to that of other flatfishes during metamorphosis. The results indicate that growth, neural and skeletal transformation, and pigmentation are biochemically separate processes in the metamorphosis of Atlantic halibut.

Weaning of Atlantic halibut Hippoglossus hippoglossus L. using formulated diets with various levels of ascorbic acid

Atlantic halibut larvae (120 mg) were weaned to formulated diets with different supplementations of ascorbate- poly-phosphate, ApP (300, 2000 and 3000 mg ascorbic acid (AA) equivalents kg−1 diet). The experiment lasted for 50 days with cofeeding of enriched Artemia and formulated diets during the first 30 days. During the last 20 days, only formulated diets were offered to the fish. One control group was fed only Artemia (770 mg AA kg−1 dry weight) during the entire experimental period. The specific growth rate during the 50 days was ≈ 4.5% day−1 and the mean weights in all dietary groups were ≈ 1 g when the experiment was terminated. No differences in mean weight and mortality were observed between the groups fed formulated diets and that fed Artemia during the experiment. The fish size in the groups fed formulated diets ranged between 0.10 and 3.05 g and this differed from the Artemia group where the size ranged between 0.35 and 1.35 g. Dietary levels of ApP had no positive effect on growth and survival. The retention of AA was significantly higher in the groups fed high dietary levels of ApP. Apparently, the bioavailability of high dietary levels of ApP appeared to be low for young halibut. After stressing the fish using a high-salinity challenge test, no significant difference in survival occurred among the dietary groups. Cortisol levels in plasma recorded 3 h post stress was significant lower in the Artemia group compared with the groups fed the formulated diets.

Influence of size and nutritional value of Artemia franciscana on growth and quality of halibut larvae ( Hippoglossus hippoglossus) during the live feed period

In a first feeding experiment, halibut larvae were offered either short term enriched (ST) 1-day old Artemia franciscana or A. franciscana of successively increasing size (ST, 2-, 3- and 4-day old juveniles), from day 0 to day 60. No differences, either in growth (approximately 6.5% daily weight increase, DWI) or in survival (ca. 25%) between the two treatments were observed, but both were satisfactory. However, feeding increasing sizes of A. franciscana increased the number of completely pigmented and metamorphosed larvae from 4 to 20%. Protein/lipid ratio in the feed seems to be important for pigmentation and metamorphosis, provided the levels of essential fatty acids (esp. DHA) are sufficient. Juvenile A. franciscana may therefore be a better live feed than ST A. franciscana for halibut larvae. The fact that a relatively high load of bacteria was introduced by 4-day old A. franciscana, may have had a negative influence on the growth of the larvae. Tests indicated that a great part of the bacteria exhibited hemolytic activity, and many of the bacteria were apparently Vibrios. Strict microbial control in the production of live feed is very likely just as important as control of the nutritional composition of the larval feed. This is a challenge for further work.

Recent developments in the essential fatty acid nutrition of fish

Because of competitive interactions in the metabolism of polyunsaturated fatty acids, tissue and bodily requirements for each of the three dietary essential fatty acids in marine fish, 22:6 n−3, 20:5 n−3 and 20:4 n−6, cannot be meaningfully considered in isolation. Rather, it is necessary to consider requirements in relative as well as absolute amounts, i.e., in terms of the ratio of 22:6 n−3:20:5 n−3:20:4 n−6. This is illustrated by recent research in our laboratories which has suggested that the optimal dietary ratio of 22:6 n−3:20:5 n−3 in sea bass larvae is circa 2:1 with the optimal dietary ratio of 20:5 n−3:20:4 n−6 being circa 1:1. The optimal dietary ratio of 22:6 n−3:20:5 n−3 in turbot and halibut larvae is similarly circa 2:1 but the optimal dietary ratio of 20:5 n−3:20:4 n−6 in these species is 10:1 or greater. In addition, studies with salmon parr point to dietary 18:3 n−3 and 18:2 n−6 being important in determining the optimal tissue ratio of 20:5 n−3:20:4 n−6 for successful parr–smolt transition. We deduce that differences in essential fatty acid requirements for different species of fish reflect different dietary and metabolic adaptations to different habitats and consider how such knowledge can be exploited to develop improved diets for fish, especially in their early stages of development.

Natural Copepods Are Superior to Enriched Artemia Nauplii as Feed for Halibut Larvae (Hippoglossus hippoglossus) in Terms of Survival, Pigmentation and Retinal Morphology: Relation to Dietary Essential Fatty Acids

Replicate groups of halibut larvae were fed to d 71 post-first feeding (PFF) either the marine copepod, Eurytemora velox, or Artemia nauplii doubly enriched with the marine chromist or golden algae, Schizochytrium sp., (Algamac 2000) and a commercial oil emulsion (SuperSelco). The fatty acid compositions of eyes, brains and livers from larvae fed the two diets were measured, and indices of growth, eye migration and skin pigmentation were recorded along with histological examinations of eye and liver. The docosahexaenoic acid [22:6(n-3); DHA]/eicosapentaenoic acid [20:5(n-3); EPA] ratios in Artemia nauplii enriched with the SuperSelco and Algamac 2000 were 0.4 and 1.0, respectively. The E. velox copepods were divided into two size ranges (125–250 and 250–400 μm) with the smaller size range containing the highest level of (n-3) highly unsaturated fatty acids (HUFA). The DHA/EPA ratios for the two size ranges of copepods were 2.0 and 0.9, respectively. The total lipids of eyes, brains and livers of larvae fed copepods had higher levels of DHA and lower levels of EPA than those of larvae fed enriched Artemia. The percentage of survival of the halibut larvae was significantly higher when copepods rather than enriched Artemia nauplii were fed, but larval specific growth rates did not differ. The indices of eye migration were high and not significantly different in larvae fed the two diets, but the percentage of larvae undergoing successful metamorphosis (complete eye migration and dorsal pigmentation) was higher in larvae fed copepods (40%) than in larvae fed enriched Artemia (4%). The rod/cone ratios in histological sections of the retina were 2.5 ± 0.7 in larvae fed copepods and 1.3 ± 0.6 in larvae fed enriched Artemia (P < 0.01). Histological examination of the livers and intestines of the larvae were consistent with better assimilation of lipid from copepods than lipid from Artemia nauplii up to 46 d post-first feeding. Thus, marine copepods are superior to enriched Artemia as food for halibut larvae in terms of survival, eye development and pigmentation, and this superiority can be related to the level of DHA in the feed.

Forced settlement of metamorphosing halibut ( Hippoglossus hippoglossus L.) in shallow raceways: growth pattern, survival, and behaviour

Pelagic metamorphosing halibut ( Hippoglossus hippoglossus L.) larvae with wet weights between 50 and 150 mg were transferred into shallow raceways with a water depth of 7 mm; they were the smallest-sized 10% among 6000 mostly metamorphosed halibut. After transfer they settled to the bottom within few hours. Three treatment groups with two replicates were established with 100 halibut larvae each (initial mean wet weight of 77 mg). Each group was offered one of three diets: live yolk-sac larvae of cod ( Gadus morhua L.), newly hatched Artemia salina nauplii, and a combination of the two diets. The water current in the raceways was maintained at 1 cm s −1. The system was self-cleaning throughout the 19-day experiment; the temperature was kept at 13°C throughout the study. Halibut larvae offered a mixture of the two live organisms and those offered only A. salina had 98 and 99% survival. Halibut larvae offered only cod larvae resulted in an overall survival of 78% at termination. The smallest-sized fraction did not ingest cod larvae on day 7 explaining why almost all of these small halibut larvae died. Percent coverage of the bottom by halibut increased from 5% to 8–10%. Final mean wet weight ranged from 217 to 257 mg for the three groups. The specific growth rate (SGR) was almost the same in all three groups, 5.8 to 6.0% after correcting for the biased mortality observed among larvae offered only cod larvae. The specific growth rate pattern was examined from ranking of wet weights as was the daily ingestion of cod larvae and Artemia nauplii based on a gross growth efficiency, K 1, of 40%. Aggression among halibut larvae was observed only in the group offered cod larvae; the larger larvae were concentrated close to the front screen through which the cod larvae entered. They frequently attacked smaller conspecifics as they tried to establish themselves in the front screen area.

Further description of the development of the digestive organs in Atlantic halibut (Hippoglossus hippoglossus) larvae, with notes on differential absorption of copepod and Artemia prey

Despite the scientific and commercial interest that Atlantic halibut has received over the past decade, little is known of the ontogeny of its digestive tract during larval development. The provision of enriched Artemia as prey has been associated with incomplete metamorphosis while improved development has been achieved using wild plankton e.g., copepods. The aim of the present study was to investigate whether there was any variation during development in the ability of the larvae to digest and incorporate Artemia or copepods. Groups of larvae fed either Artemia enriched with an oil emulsion (Super Selco™) and with a dried protist Schizochytrium (Algamac 2000™) or with a marine calanoid copepod ( Eurytemora velox) were sampled weekly from the end of yolksac resorption to the completion of metamorphosis. The sites of absorption were visualised by the presence of vacuoles in the epithelial cells of the digestive tract's mucosa. During the first 30 days of exogenous feeding, before the stomach had formed, there were small inclusions on the apical side of the rectal epithelial cells in both groups. Between 30 and 50 days post first-feeding, at the time when the stomach was differentiating with the appearance of gastric glands, and the four pyloric caecae were starting to elongate, numerous supranuclear vacuoles were observed in the mid- and hind-gut epithelial cells of the copepod-fed group. The rectal epithelial cells showed lipid vacuoles that filled their apical ends suggesting significant absorptive activity. By contrast, few vacuoles were present in the rectum of the Artemia-fed larvae. By the end of metamorphosis (up to 83 days post first-feeding), the copepod-fed larvae had an adult-like digestive tract with functional stomach and highly absorptive intestine. At the same age, the Artemia-fed larvae showed that the rectum was still the major site of absorption; this is a typical larval feature. Furthermore, observations of the gut's content confirmed the poor ability of the halibut larvae to digest Artemia in comparison to copepods. Indeed, while no individual copepod could be identified in the digestive chyme past the mid-gut of these larvae, partially digested Artemia could be seen as far as the anus in those fed on Artemia. Although the rate of development and of differentiation of the digestive tract of first-feeding halibut larvae appeared unaffected by the nature of their prey items, our results show a difference in the absorption sites of copepods vs. Artemia. These findings are of considerable significance to the rearing techniques used in the commercial production of halibut juveniles.

Status of the cultivation technology for production of Atlantic halibut ( Hippoglossus hippoglossus) juveniles in Norway/Europe

Comprehensive research programs form the basis for the commercial juvenile production technology of Atlantic halibut ( Hippoglossus hippoglossus) in Europe, and commercial production is currently being tested in Iceland, Scotland, and Norway. Broodstocks of halibut are normally maintained in cylindrical tanks (diameter 5–15 m, 1-m depth, light-protected, 34‰ salinity, temperature <8°C). Each female releases several batches of eggs (4–16 per season, 3–4-day intervals), and manipulation of the photoperiod may be used to have continuous egg production. Eggs are obtained by stripping; at least 90% of the eggs are normally fertilised, and 75–80% of the fertilised eggs hatch. The eggs must be disinfected before hatching, and appropriate treatment by glutaric aldehyde is most efficient. The disinfected eggs are normally hatched in large flow through systems in which the larvae are maintained during their prolonged critical yolk sac stage. The fragile yolk sac larvae are very sensitive to physical and microbial conditions, and must be handled with care. Survival during the yolk sac stage may be as high as 50–70% of the initial population, however, jaw deformation is a main problem. Density during first feeding is normally 0.5–5 larvae l −1. The feeding regime used in commercial production in Norway consists of harvested marine copepods and Artemia nauplii. International feeding technology with cultivated live feed is being adapted to commercial scale. The dietary components used are microalgae, rotifers, short-term enriched Artemia nauplii, and larger Artemia juveniles. The larval feeding period lasts for ca. 60 days. Light conditions are critical for survival in the initial stage of feeding, and a component of UV-light and appropriate shading is needed for adequate larval behaviour. Nutritional and microbial conditions are more important for older stages. The results with rotifers and Artemia have gradually improved over the last few years. Halibut larvae have high requirements for DHA (docosahexaenoic acid, 22:6 n−3), and the supply of DHA through Artemia has turned out to be a major problem, probably because of the high DHA-catabolism of Artemia. Poor pigmentation of the fry and the low larval resistance to stress are frequently observed. The numbers of weaned halibut fry transferred to the growout phase in Europe have been quite variable; approximately 370,000 in 1997. The price for halibut fry is on the order of US$12 per fry.

A tank design for first feeding of Atlantic halibut, Hippoglossus hippoglossus L., larvae

An intensive method of juvenile Atlantic halibut, Hippoglossus hippoglossus (L.), production has been under development over the last decade because of the problems associated with the extensive method. The lack of initiation of feeding behaviour has been the main obstacle for successful indoor rearing under artificial light and feeding conditions. In the present paper, an intensive method for the first feeding of halibut larvae is described and verified by practical feeding trials. The method involves circular 1.5-m3 indoor tanks with a peripheral ring-shaped cover, the use of continuous light, UV-A radiation during the first 24 h, central up-welling of water made by aeration and the use of microalgae in the rearing water. The single most important factor in such systems is to maintain a current pattern which allows the larvae to orientate and position themselves to face the water current for easy capture of prey.

Feeding Artemia to larvae of Atlantic halibut ( Hippoglossus hippoglossus L.) results in lower larval vitamin A content compared with feeding copepods

Halibut ( Hippoglossus hippoglossus) larvae were fed excess DHA Selco enriched Artemia or natural copepods (mainly Temora longicornis). Fourteen days after onset of first feeding, a time that is critical for normal pigmentation of Atlantic halibut, the content of retinol and retinal were from 50 to 80% lower in halibut larvae fed Artemia compared with larvae fed zooplankton. At day 28 post first feeding, the differences between larvae fed Artemia and zooplankton were few although the content of retinol in larvae fed only Artemia was still only about 50% of the other groups. There were major differences in the vitamin A and carotenoid composition of Artemia and Temora. Temora did not contain any form of vitamin A: all- trans retinal and all- trans retinol. In Artemia, however, large amounts of an unknown retinoid component were present. This component was not seen in Temora. Lutein and astaxanthin, the quantitatively most important carotenoids in Temora, were not detected in Artemia. Likewise, cryptoxanthin/cantaxanthin, the major carotenoid in Artemia, was not observed in Temora. This suggests that halibut larvae are not able to efficiently convert the available carotenoids or the unknown retinoid component into retinal and retinol during the first period after onset of exogenous feeding.

Lipid and fatty acid composition of normal and malpigmented Atlantic halibut ( Hippoglossus hippoglossus) fed enriched Artemia: a comparison with fry fed wild copepods

Atlantic halibut larvae originating from a single spawn of one female and one male were fed either Super Selco™ enriched, enrichment-grade Artemia nauplii or extensively grown zooplankton. The feeding experiment was conducted in 1.5-m 3 outdoor tanks to evaluate if diet affected pigmentation. Pigmentation rates were found to be higher in the zooplankton-fed fish: 99.2% compared to 66.4% in the Artemia-fed halibut. Lipid and fatty acid analyses were performed on five malpigmented Artemia-fed fry, five normally pigmented Artemia-fed fry and 10 copepod-fed fry after 43 days feeding. Lipid class analysis was carried out on the eyes and carcasses of the halibut fry by HPTLC and densitometry. Fatty acid analyses of the same tissues were performed by gas liquid chromatography. Copepod-fed fry showed significantly higher levels of docosahexaenoic acid (22:6 n−3; DHA) than their Artemia-fed counterparts (54.9% and 15.8% DHA in eye phosphatidylethanolamine (PE), respectively). Ratios of DHA: eicosapentaenoic acid (20:5 n−3; EPA) were also higher in the copepod-fed fish: 11.1, 4.1 and 2.3 in, respectively, PE, phosphatidylcholine (PC) and triglyceride (TAG) in the eye lipid fractions of copepod-fed fry compared to 0.7, 0.3 and 0.1 in the same lipid of eyes of Artemia-fed fry. Within the Artemia-fed population, DHA and EPA levels were slightly, but significantly, raised in the eye PC fractions of normally pigmented fish compared to malpigmented ones. However, there was no significant difference in DHA:EPA ratios between normal and malpigmented fish fed Artemia. EPA: arachidonic acid (ARA) ratios were higher in zooplankton-fed fish but there was no significant difference in these ratios between normal and malpigmented Artemia-fed halibut.

The influence of salinity on the morphological development of yolk sac larvae of Atlantic halibut, Hippoglossus hippoglossus (L.)

The effects of incubation salinity on survival and morphological development of yolk sac larvae of Atlantic halibut, Hippoglossus hippoglossus (L.), were studied at two salinities, 35.5%o and 32.3%o. Yolk sac larvae incubated at 35.5%o developed early tail deformities before 50 day-degrees (9 days post-hatch), and necrosis, oedema and calculus in the urinary bladder at about 150 day-degrees (28 days post-hatch). However, the incidence and severity of mouth deformity was not influenced by salinity. The cumulative mortality up to first feeding was 17% when the larvae were incubated at 3 5.5%o compared to 7% at 32.3%o. A broad variation between the different larval groups was noted in the extent of injuries induced by the higher salinity. Therefore, salinity must be considered as a critical environmental parameter for the yolk sac larvae of Atlantic halibut in aquaculture.

The influence of salinity on the morphological development of yolk sac larvae of Atlantic halibut, Hippoglossus hippoglossus (L.)

The influence of salinity on the morphological development of yolk sac larvae of Atlantic halibut, Hippoglossus hippoglossus (L.)

Ammonia dynamics in relation to hatching in Atlantic halibut (Hippoglossus hippoglossus L.)

Total ammonia content (TAmm) and excretion (JAmm), and ionic content (K+, Na+, Cl-) have been studied in embryos and yolk-sac larvae of Atlantic halibut (Hippoglossus hippoglossus L.) maintained in darkness (6 °C and 34% S) or in light (light-arrested, nonon-hatching embryos). The TAmm of the eggs increased from 250-300 nmol ind-1 during the first 5-10 days post fertilization (dPF) to 400-500 nmol ind-1 at the time of natural hatching in darkness. TAmm did not start to decrease before 2-5 days after hatch, and yolk-sac larvae had a higher TAmm compared to the light-arrested embryos of the same age and batch. JAmm increased exponentially rom about 0.05 nmol ind-1 h-1 at 10 dPF to about 3 nmol ind-1 h-1 at hatch. Embryos undergoing hatching showed a significantly higher JAmm than non-hatching, light-arrested embryos of the same age. The content of K+ in distilled water rinsed eggs declined from about 0.85 µmol ind-1 at 2-4 dPF to about 0.15 µmol ind-1 at hatch, while that of Cl- remained constant at about t 0.85 µmol in-1, and that of Na+ increased slowly from 0.10 to 0.20 µmol ind-1 . Based on the measured ions, there was an anionic deficit in the egg, especially during the first week of development. When exposed to increased ambient total ammonia (0-27 mM NH4Cl), no mortality occurred, and no significant increase was found in the TAmm of the Atlantic halibut egg before 9.2 dPF despite an estimated large inward diffusion gradient for the un-ionized ammonia species (NH3). The delayed release of TAmm comp ared to embryos and yolk-sac larvae of other marine teleosts, may relate to the deep-water spawning of the Atlantic halibut, and the buoyancy regulation of the yolk-sac larvae.

Combined effects of temperature and salinity on development and survival of Atlantic halibut larvae

The effects of different combinations of temperature and salinity on development and survival of Atlantic halibut, Hippoglossus hippoglossus (L.), larvae were studied in two experiments. In the period from 57 to 120 d° post hatching, approximately 38% of the larvae died: in both experiments, mortality was significantly related to temperature, but there was no relationship to salinity. Cumulative, chronic mortality during the yolk-sac stage (until 210 d°) was related to salinity. High mortality was observed in both experiments when high salinity (>34%) was combined with incubation at either high (9 °C) or low (3 °C) temperature. The development of head lesions was significantly related to temperature, and for larvae kept at high salinity, increased mortality may have been a result of salinity stress in the injured larvae. Abnormal development of the caudal notochord, sometimes resulting in a 90-degree bend of the tail, occurred during incubation of early yolk-sac larvae in high-salinity water. The occurrence of larvae with locked jaws, and larvae with oedema in the yolk-sac sinus and pericardium, was significantly related to temperature. © Rapid Science Ltd. 1998

Vitamin B 6 in Atlantic halibut, Hippoglossus hippoglossus—endogenous utilization and retention in larvae fed natural zooplankton

A single batch of Atlantic halibut larvae was reared in floating plastic enclosures, offered natural plankton collected from a lagoon and sampled regularly for measurements of vitamin B 6. At hatching individual whole larvae contained 9.0 ± 0.2 ng of vitamin B 6, of which 95% was present in the yolk. At the onset of first feeding 50% of the vitamin B 6 present in the yolk at hatching had been transferred to the larval body. There was a relatively constant weight specific content of vitamin B 6 in the larval body from hatching until first feeding at 206 Day degrees post hatch (D°PH) at 3.5 ± 0.3 μg g −1 DW. During this period there was a net decline of 2.3 ng vitamin B 6 per larvae. After onset of first feeding the vitamin B 6 content in the larvae continued to decline and reached its lowest level at 300 D°PH with 5.0 ng individual −1. With continued development the feeding larvae increased their content of vitamin B 6. However, in weight specific terms the content of vitamin B 6 in halibut larvae above a DW weight of 5 mg stabilized at 5.1 ± 0.2 μg g −1 DW, about the same value as the plankton feed offered. The data in this study represents vitamin B 6 status in halibut larvae and planktonic prey in semi-intensive culture systems with a high outcome of metamorphosed larvae and may therefore serve as a basis for feed formulation in intensive farming of Atlantic halibut.

Effects of salinity on yolk sac larvae of Atlantic halibut ( Hippoglossus hippoglossus L.)

Larvae of Atlantic halibut ( Hippoglossus hippoglossus L.) were transferred to nine different salinities (range 20–42 ppt) immediately after hatching, or to five different salinities (range 24–39 ppt) at day 30 post-hatching (p.h.) The experiments were terminated 49 days p.h. (at 5 °C); i.e. at first feeding. When newly-hatched larvae were introduced to different salinities, the highest survival rates (69–90%) were obtained at 27–32 ppt. A high yield of normally developed larvae was only obtained within a limited range of salinities (29–34 ppt). Thirty-day-old larvae tolerated a wider range of salinities than newly-hatched larvae, and only larvae transferred to the highest salinity (39 ppt) exhibited significantly reduced survival and yield of functional larvae. Myotomal height, larval dry weight and yolk volume were little influenced by the rearing salinity in either experiment. The buoyancy of halibut larvae was influenced both by the rearing salinity and by larval age.

Effects of temperature on yolk sac larvae of Atlantic halibut ( Hippoglossus hippoglossus L.)

Larvae of Atlantic halibut were incubated at five different temperatures (4, 6, 8, 10 and 12 °C) from hatching onwards (Experiment 1), and at 4 °C for a period of 0, 10, 20, 30 or 40 days before the temperature was increased to 6 °C or 8 °C (Experiment 2). Both experiments were terminated at time of first feeding, 230 day-degrees (d °) after hatching. In Experiment 1, larval survival was significantly lower at 12 °C, and the yield of normally developed larvae dropped significantly when larvae were incubated at 6 °C or higher. When larvae were incubated for different periods at 4 °C (Experiment 2), the number of days at 4 °C had a significant effect on larval survival and the frequencies of functional larvae, jaw deformations and yolk sac oedema, while the temperatures in the second period (6 °C or 8 °C) had no significant effect on the same traits. Frequency of normally developed larvae increased with increasing number of days at 4 °C while the survival rate tended to decrease with increasing number of days at 4 °C. Myotomal height, larval dry weight, yolk sac volume and feeding incidence were less influenced by the incubation temperature in both experiments. The results demonstrated that the development of abnormalities like gaping jaws and yolk sac oedema in halibut yolk sac larvae can be associated with high temperatures and presence of different temperature tolerances at different developmental stages.

Feeding marine copepods or enriched Artemia affects physiological parameters and lipid composition in halibut larvae : R.J. Shields., J.G. Bell, F. Luizi and B. M. Gara. Sea Fish Industry Authority, Ardtoe, Argyll PH36 4LD. NERC Unit of Aquatic Biochemistry and Institute of Aquaculture, University of Stirling, Stirling FK9 4LA, Scotland

Feeding marine copepods or enriched Artemia affects physiological parameters and lipid composition in halibut larvae : R.J. Shields., J.G. Bell, F. Luizi and B. M. Gara. Sea Fish Industry Authority, Ardtoe, Argyll PH36 4LD. NERC Unit of Aquatic Biochemistry and Institute of Aquaculture, University of Stirling, Stirling FK9 4LA, Scotland