Sea Lice Control II

ABSTRACT

Sea lice, Lepeophtheirus salmonis, mobile stages were caught using a light trap, in cages holding Atlantic salmon, Salmo salar, and their abundance determined through calculation. Seasonal changes were shown with an increase in abundance as the temperature increased toward the end of the season. Analysis of manual lice-count data indicated variations in lice abundance between cages with and without deployed light traps. Analysis indicated that these variations were in larval presence (P = 0.006) and presence of gravid females (P = 0.015), but not in abundance of in pre-adults (P = 0.172). Cage net condition also affected the abundance and infection intensity of sea lice on Atlantic salmon within aquaculture cages. Abundance of larvae were affected by net condition (P = 0.01), as were abundance of pre-adults on salmon (P = 0.04). The infection intensity of gravid females, however, did not vary with net condition (P = 0.74). Other species of zooplankton such as, American lobster, Homarus americanus, larvae and various Brachi-pod larvae, were also collected with the light trap. The light trap, as presented in this research, could provide an alternate method of lice control in aquaculture cages.     

Survival and quality of halibut larvae (Hippoglossus hippoglossus L.) in intensive farming: Possible impact of the intestinal bacterial community

The high mortality commonly observed during the early life stages of intensively reared halibut (Hippoglossus hippoglossus L) is believed to be caused by e.g. opportunistic bacteria. However, the impact of particular bacterial species is poorly defined and still remains disputable. The study describes the bacterial diversity in the gastrointestinal tract of halibut larvae in a large number of incubators at a commercial production site. The overall success of larvae was found to be highly variable and analysis of the gut microbiota revealed high variation of the cultivable part as well as the bacterial community of surface sterilised larvae analysed by denaturing gradient gel electrophoresis (DGGE) of PCR amplified 16s rDNA products. Analysis of the bacterial community of unfed yolk sac larvae revealed higher diversity than previously reported, with Marinomonas, Marinobacter, Aeromonas and Shewanella dominating the community structure. There are indications that Marinomonas is found only in the overall most successful first feeding larvae of the period where the Vibrio group dominated the bacterial community together with Shewanella. Vibrio wodanis was identified as a part of the bacterial community of feeding larvae that yielded the poorest overall success of the period. α-Proteobacteria, not previously reported in halibut, were also found as a part of the bacterial community of first feeding larvae. The diverse bacterial community was only partly reflected in the cultivable part which, however, may reflect the dominating bacterial groups of the highly heterogeneous bacterial community of larvae in the production system as a whole. The bacterial community of the Artemia was found to be highly variable in different samples collected through the period. Only a small part of the different groups observed in the bacterial community of surface sterilised larvae was reflected in the cultivable part which was dominated by highly variable groups in different samples of Artemia. Also, the numbers of cultivable bacteria were found to positively correlate with jaw deformation of unfed yolk sac larvae as well as incomplete metamorphosis of feeding larvae.     

黄颡鱼IgM基因的个体发生和抗体的代间传递

为了解黄颡鱼IgM基因表达的个体发生和IgM抗体代间传递机制,实验利用Real-time PCR和ELISA等技术研究了黄颡鱼IgM重链基因在卵巢、胚胎和仔鱼的表达变化,以及黄颡鱼IgM抗体在卵巢、胚胎和仔鱼中的含量变化。结果显示,黄颡鱼3~7 d仔鱼中,没有检测到IgM mRNA;14 d仔鱼IgM基因开始表达。用细菌免疫亲鱼后,对仔鱼IgM mRNA表达没有影响。黄颡鱼IgM抗体在卵巢、胚胎和仔鱼中都有分布,且呈现下降趋势,至9 d仔鱼中抗体水平最低(是卵抗体含量的0.31倍)。14 d仔鱼中IgM抗体水平上升(是9 d仔鱼抗体含量的1.6倍)。用细菌免疫亲鱼后,能显著提高胚胎、仔鱼中的IgM抗体水平,在卵中抗体含量提高了2.3倍,9 d仔鱼中提高了1.8倍。研究表明,黄颡鱼IgM抗体可以在母本和后代之间传递,早期仔鱼IgM抗体主要来自于亲本;因此,免疫亲鱼能显著增加子代的抗体水平。     

Lipid as an index of growth and viability in three species of bivalve larvae

The larvae of three species of bivalve molluscs, Crassostrea virginica (Gmelin), Ostrea edulis (L.) and Mercenaria mercenaria (L.) were cultured under a variety of temperature and food species regimes in the laboratory. Data were collected on growth, survival, total lipid content and lipid class composition of both healthy and starved larvae. Subsamples were stained specifically for lipid content with the lipid-specific stain Oil Red O. Newly formed straight hinge larvae of all three species exhibited small lipid droplets dispersed throughout the tissues. These were resorbed within 4 days of the initiation of feeding while the digestive gland filled with exogenously supplied lipid. Embryogenesis took place at the expense of 69 and 71% of parentally derived total lipid, 92.8 and 68.3% of which was composed of triacylglycerols (M. mercenaria and C. virginica, respectively). After 8 days of feeding on Isochrysis aff. galbana (clone T-ISO), total lipid had increased 2.7-fold in larvae of C. virginica grown at 25°C, 37.6-fold in larvae of M. mercenaria grown at 22°C, and 1.7-fold in larvae of O. edulis grown at 25°C. Shell growth in larvae of C. virginica grown at 30°C was initially higher than that at 25°C, but lipid was accumulated 2.5 times slower and survival of pediveliger larvae through metamorphosis was considerably reduced at this elevated temperature. The lipid class composition of healthy larvae of C. virginica and M. mercenaria was dominated by triacylglycerols and phospholipids in approximately equal proportions; however, triacylglycerols were preferentially catabolized during starvation and were responsible for greater than 80% of the total lipid loss. Color photographs of larvae stained with Oil Red O are presented that illustrate lipid levels in developing larvae in both healthy and poor condition. From these, a visual index of lipid content was developed to allow comparisons between larval survival and lipid levels. In general, high survival was always accompanied by a high lipid index but a high lipid index did not guarantee good survival. We suggest that the lipid index may be used to document visually the lipid content in individual bivalve larvae as an indicator of physiological condition and potential for successful metamorphosis.     

The importance of live‐feed traps – farming marine fish species

This article analyses the challenges of different live‐feed regimes for the rearing of marine finfish larvae and discusses the potential alternative live feeds to avert a future live‐feed trap. Live feeds are indispensable for the successful rearing of larvae of most marine fish species. Brine shrimps (Artemia) and rotifers comprise the live feeds of choice in marine aquaculture today. However, their nutritional composition is deficient in especially essential fatty acids, and enrichment with fish oil is needed. Fish oil is considered a limited resource owing to its origin in fully exploited wild fish stocks. Moreover, fluctuations of the natural population of Artemia will, most likely, influence future availability and prices. This emphasizes the need for optimal exploitation of available live‐feed resources and development of new sustainable alternatives, such as copepods. An array of solutions to these problems are presented to avoid a future live‐feed trap and to reduce dependence on limited resources that influence future production possibilities, species diversification, price volatility and productivity in the aquaculture sector.     

Larval rearing of an Asian catfish Pangasius hypophthalmus (Siluroidei,Pangasiidae): Analysis of precocious mortality and proposition of appropriate treatments

Both in Indonesia and Vietnam, larval rearing of Pangasius hypophthalmus remains problematic due to the variable and generally low survival rates obtained. The first week represents the most critical period for these larvae and up to now, cannibalistic behaviour was considered as the main cause of mortality. In the present study, two experiments were carried out in order to better understand the evolution and causes of mortality of P. hypophthalmus larvae from hatching up to 8 d of age and to find measures to improve survival rates. The first experiment was designed to evaluate the importance of cannibalism and differences in mortality when larvae from two different females were reared either in groups of 30 individuals or in isolated condition (30 larvae reared separately). In both cases, the culture was carried out either with or without antibiotic. The aim of the second experiment was to test oxytetracycline and different disinfectants (chloramine-T, formalin and ‘formalin + malachite green oxalate’) at different dosages in order to prevent bacterial outbreaks in the culture. The results indicated that the survival rates of P. hypophthalmus larvae was dependent on the initial quality of larvae or eggs and that larval mortality was more a consequence of pathogenic infection than a direct effect of cannibalism. The present study demonstrated that the survival rates of larvae were systematically improved when rearing was carried out in water containing antibiotic (oxytetracycline at a dose of 5 to 20 mg·L¹). Survival rate and final mean body weight of larvae as high as those obtained using antibiotic were also reached with applications of disinfectants such as chloramine-T and formalin. The use of these disinfectants is recommended for commercial P. hypophthalmus hatcheries.     

High-intensity light of full-spectrum LED promotes survival rate but not development of the larval swimming crab Portunus trituberculatus

In this study, a 25-day experiment was performed to evaluate the survival and development of Portunus trituberculatus larvae under different light intensities (0, 61.55, 93.57, 124.31, 195.31, 248.51, and 310.95 μmol m⁻² s^-1). The newly hatched larvae were cultured in 1-L beakers at a density of 100 individuals/L, with four replicates per treatment. Full-spectrum light-emitting diode (LED) strip lamps were used as the light source. The larvae were fed with Artemia nauplii until all the larvae metamorphosed to young crabs or died, and survival and development of the larvae were recorded. The results showed that P. trituberculatus zoeae were not able to complete the entire larval cycle in the absence of light. In the dark treatment, the survival rate of the larvae decreased sharply in Z3 and reached 0.8 % in Z4, and all the larvae failed to develop to the megalopa stage. In the light treatment, no significant differences were observed in the survival rates of the Z1, Z2, and Z3 larvae under different light intensities (P > 0.05). However, the highest survival rates were detected for Z4 larvae and megalopae at 195.31 and 124.31 μmol m⁻² s^-1, respectively, indicating a decline in the light requirement. Although the Z2 and Z4 larvae were found to develop faster at a light intensity of 195.31 μmol m⁻² s^-1 (P < 0.05), no significant differences were found among the treatments at the end of the experiment (P> 0.05). In conclusion, the results show that P. trituberculatus needs more lighting than other crab species, and the light requirement declines as the zoeal and megalopal larvae develop into young crabs.     

Decreasing dietary lipids improves larval survival and growth of Japanese eel Anguilla japonica

Shark eggs-based diet is the only diet by which eel larvae can grow to glass eels in captivity. However, the high level of lipids in the diet is suggested to negatively affect eel larvae. This paper examines the effect of defatted shark eggs (DSE) and hen egg yolk (HY) on growth and survival of larvae of Japanese eel Anguilla japonica. Lyophilized shark egg and commercial HY were defatted with n-hexane, and four experimental diets were prepared using both defatted and untreated shark eggs and HY. Larvae were reared for 3 weeks by feeding the experimental diets. The highest survival rate was observed in the larvae fed DSE, and larvae fed HY showed the lowest survival rate. The best growth was found in larvae fed DSE, followed by shark eggs and defatted HY, and the worst growth was in HY-fed larvae. These results show that decreasing dietary lipids improves the nutritional value of both shark eggs and HY for eel larvae. Regulation of the dietary lipid level may positively affect the larval performance of eels by combination of ingredients with a low lipid content.     

Experiments of the rearing of larval turbot, Scophthalmus maximus L.

Experiments on the live food requirements of cultured turbot larvae are described. A mixture of six species of unicellular algae did not sustain early larvae, but the growth and survival of larvae fed with rotifers were considerably improved when the rotifers were feeding on Isochrysis galbana rather than on Dunaliella tertiolecta. Oyster (Crassostrea gigas) larvae offered alone or with rotifers, did not produce higher larval growth or survival than rotifers alone. Black tanks were found to be more suitable than white tanks for rearing larval turbot.     

Effect of dietary taurine levels on growth performance and taurine content of Nibea albiflora larvae

The effect of taurine supplementation to a microencapsulated diet at 0 % (T-0 %), 1 % (T-1 %) and 2 % (T-2 %) level on growth performance and taurine content of Nibea albiflora larvae was evaluated. The microencapsulated diet was prepared using wet granulation and fluidized bed coating process. Scanning electron microscopy microphotography of the microencapsulated diet showed the appearance with a dense film around the core. More than 50 % of diet particles were between 250 and 590 μm. A 30-day feeding experiment was conducted with larvae at 15 days after hatching (DAH), in which larvae were weaned at 20 DAH from copepods to the experimental diets or fed copepods throughout the experimental period. The survival and total length of larvae were significantly higher in T-1 % group compared with T-0 % group (P < 0.05). The survival, total length and wet weight of larvae were significantly higher in T-2 % group compared with T-0 % group (P < 0.05). Fish larvae fed copepods had the best growth performance in survival, total length and wet weight, which were significantly higher than those in other treatments (P < 0.05). The taurine content of larvae fed with supplemental taurine (T-2 %, T-1 %) was significantly higher than that of larvae fed without supplemental taurine (T-0 %) (P < 0.05). These results confirmed that taurine is an essential nutrient for N. albiflora larvae.     

The effect of dietary n-3 highly unsaturated fatty acids on growth,survival and biochemical composition of the coral reef damselfish Acanthochromis polyacanthus

Larvae of the coral reef damselfish Acanthochromis polyacanthus (Bleeker) were fed either unenriched Artemia nauplii or nauplii which had been enriched by pre-feeding with microcapsules containing either squid oil (SQO) or cod liver oil (CLO). Enriched nauplii contained elevated levels of the n-3 highly unsaturated fatty acids (HUFA) eicosapentaenoic acid (EPA; 20:5n-3) and docosahexaenoic acid (DHA; 22:6n-3) which made up 5.22 ± 0.34 and 2.62 ± 0.28 %, respectively, of total fatty acids in nauplii enriched with CLO, and 10.48 ± 0.36 and 3.43 ± 0.33 %, respectively, in nauplii enriched with SQO. In contrast, unenriched nauplii contained EPA (5.03 ± 1.04 %) but did not contain DHA. Survival differed significantly between treatments over the 18-d study; larvae receiving CLO enriched nauplii showed 100 % survival and those receiving SQO enriched nauplii showed 93.3 ± 6.6 % survival. In contrast, only 46 ± 6.7 % of larvae receiving unenriched nauplii survived to the end of the 18-d study. Wet weight, dry weight and proximate biochemical composition did not differ significantly between treatments at the end of the study. Mean standard length of larvae fed CLO enriched nauplii was significantly smaller than that of larvae fed SQO enriched nauplii; however, neither differed significantly from larvae fed unenriched nauplii. The fatty acid composition of A. polyacanthus larvae was significantly influenced by the fatty acid composition of the diet. The results indicate that A. polyacanthus larvae are unable to synthesise DHA from available dietary precursors and, as such, dietary DHA is required to maximise survival. Development of appropriate culture techniques for the larvae of coral reef fishes will allow controlled laboratory studies with these species and may eventually reduce pressure on wild populations exploited for the aquarium trade.     

Survival and growth of turbot larvae Scophthalmus maximus L. Reared on different food organisms with special regard to long-chain polyunsaturated fatty acids

Turbot larvae reared on rotifers (Brachionus plicatilis) and Artemia (San Francisco brand) nauplii (A) were compared with those fed on different copepod (Eurytemora affinis) stages (B). In one trial, growth and survival of larvae from hatching to day 21, their fatty acid pattern and that of their food organisms were examined. At the age of 21 days larvae of group A measured 11·1 ± 1·6 mm total length (TL) compared with 14·8 ± 1·1 mm TL for group B. From day 3 to 8 survival was 29% (A) and 38% (B). Long-chain polyunsaturated fatty acid composition of turbots resembled that of their corresponding diets. Artemia and Artemia fed larvae were lacking 22 : 6 n-3 fatty acid.In a further study, survival of larvae from day 11 to 21 fed on Artemia nauplii (C) or copepods (D) was found to be greater with the latter diet (73 versus 93%) whereas growth during this experiment was similar in both groups. Before the start of this trial they had been reared on a mixture of copepod nauplii and Brachionus. No influence on weaning success was observed. 22 : 6 n-3 fatty acid is seen to be essential for turbot larvae: elongations of fatty acids by the larvae were not found.     

Variations on development and stress defences in Solea senegalensis larvae fed on live and microencapsulated diets

Growth, development, antioxidant enzymes, stress proteins (HSP70 and HSP60), lipid peroxidation (LP) and histology in Solea senegalensis larvae were followed from 8 to 30 days post hatching (dph). Larvae were fed on three different diets: (1) live Artemia nauplii, (2) microcapsules elaborated by internal gelation, (MA) and (3) these same microcapsules but 10-fold supplemented with vitamin A (MAV). The Artemia fed group showed higher growth and a faster metamorphosis than the ones fed with microencapsulated diets, although all had similarly high survival rates of 80%. Vitamin A (VA) supplementation improved growth and development from 15 dph in relation to the strictly inert diet (MA). Larvae fed with Artemia showed organs and tissues with a normal pattern of development, whereas histological alterations were seen in larvae fed with both inert diets. The antioxidant enzymes: catalase (KAT), superoxide dismutase (SOD), and total glutathione peroxidase (t-GPX) as well as LP levels and stress proteins (HSP70 but not HSP60), measured in whole larvae, showed diet and age dependence in their response. Larvae fed with both inert diets showed similar biomarker activities, but these activities were different (p < 0.05) from larvae fed with Artemia. That is, KAT and HSP70 were lower in larvae fed with live prey and t-GPX and LP levels were lower in larvae fed with the inert food. Among the factors responsible for increased antioxidant defenses were the initiation of metamorphosis and the use of inert food. This study suggests the usefulness of the biomarkers selected as tools to evaluate the effects of compound diets on larvae.     

A technical solution to the mass-culturing of larval turbot

A 1000 litre recirculation system of eight cylindrical rearing tanks has been tested in three 40-day periods to determine its capacity for rearing larval turbot (Scophthalmus maximus L.).The larvae were fed on rotifers and Artemia nauplii, as well as mixed marine algae. Continuous artificial light of 1500–2000 lux was applied at the surface. The applied algae served a dual function; keeping rotifers and Artemia at a high nutritional level as well as effectively removing the released ammonia. The combination of upwelling water and light at the surface maximized the contact surface between larvae, food items and algae.With this system, using a stocking density of 16 larvae litre^?1 in the rearing tanks, a survival total of 40% at Day 40 was achieved, giving a production of 6·4 larvae litre^?1 or 3000 larvae metre^?2 of the surface of the rearing tanks.     

Variability of preferred environmental conditions for Atlantic bluefin tuna (Thunnus thynnus) larvae in the Gulf of Mexico during 1993–2011

The Gulf of Mexico (GOM) is the primary spawning ground for western Atlantic bluefin tuna (Thunnus thynnus). In this work, information reported by previous studies about the preferred environmental conditions for the occurrence of bluefin tuna larvae in the GOM is integrated into a dimensionless index, the BFT_Index. This index is used to evaluate the spatial and temporal variability of areas with favorable environmental conditions for larvae within the GOM during 1993–2011. The main findings of this work are that: (i) the proposed index successfully captures the spatial and temporal variability in the in situ occurrence of bluefin tuna larvae; (ii) areas with favorable environmental conditions for larvae in the GOM exhibit year‐to‐year spatial and temporal variability linked with mesoscale ocean features and sea surface temperature; and (iii) comparison of the BFT_Index‐derived variability with recruitment of age‐0 fish estimated from recent stock assessment indicates that changes in environmental conditions may drive a relevant component (~58%) of the recruitment variability. The comparison with the recruitment dataset further revealed the existence of key regions linked with recruitment in the central/northern GOM, and that the Loop Current may function as a trap for larvae, possibly leading to low survival rates. Above (below) average conditions for occurrence of larvae in the GOM during spring were observed in 2000, 2001, 2002, 2006–2008, and 2011 (1994, 1996, 1998, 1999, 2003 and 2010). Results reported here have potential applications to assessment of bluefin tuna.     

Evaluating light trap efficiency by application of mesh to prevent inter- and intra-specific in situ predation on fish larvae and juveniles

Amongst passive collecting gear types, light traps have been used extensively to sample the larvae and juveniles of fish in both marine and freshwater environments, and especially so in structurally complex habitats. Although a number of modifications have been proposed to increase efficiency, no account has hitherto been made of the possible loss of trapped larvae and juveniles as a result of within-trap predation by piscivorous fish. To address this issue, we evaluated the efficiency of modified light traps by the addition of mesh (3 mm knot-to-knot wrapped around the entrance chambers), by combining results from a long-term monitoring study with those from a designed field experiment. Abundance of larvae and juveniles collected during the monitoring program was higher in the years following mesh application (hence, beyond monthly variation), and this was supported by the experiment. Based on the latter, light/mesh traps consistently caught only larvae/juveniles, hence excluding adults; whereas the opposite was true of the light/no-mesh trap combination. Also, mesh/no-light traps did not capture any fish, unlike no-mesh/no-light traps, which sampled only adults. Our findings suggest that in situ predation on larvae and juveniles within light traps should not be overlooked. The occurrence of piscivorous and cannibalistic behaviour in Hypseleotris spp. is also discussed.     

Ultrastructure of the anterior intestinal epithelia of the orange-spotted grouper Epinephelus coioides larvae under different feeding regimes

Enterocytes of the anterior to midsection of the intestine in grouper Epinephelus coioides larvae were compared among different treatments: unfed to the point-of-no-return (PNR), fed natural food only, and co-fed natural food and artificial diet. On day 3, the nutritional condition of unfed grouper larvae regressed with its reduced enterocyte heights which were further degraded on day 4, the PNR, when all the enterocytes were in advanced stages of apoptosis. The apoptosis appeared to be internally directed via the mitochondria. Among day 3 fed larvae, enterocyte heights of those fed artificial diet did not differ from those fed natural food only. Dietary phospholipid deficiency was indicated in larvae co-fed artificial diet on day 3 with an unusually large chylomicron opening into the inter-enterocyte space, and on days 6 and 33 by intestinal steatosis. On day 19, scant to absent lipid droplets in enterocytes of larvae disclosed heightened nutritional requirement preparatory to metamorphosis. As observed in unfed day 3 and premetamorphic day 19 E. coioides, larvae undergoing critical periods and starvation during development employ apoptosis to dispose of degenerated enterocytes that are phagocytosed by adjacent healthy enterocytes without causing inflammatory distress. Upon metamorphosis, grouper larval gut develops better immunity fitness with eosinophilic granule cells observed in the intestinal epithelia of day 33 larvae. Future studies on grouper larval nutrition may consider the appropriate dietary phospholipid levels and larval competence to biosynthesize highly unsaturated fatty acid from linoleic acid vis-à-vis the use of plant ingredients in artificial diet formulations. In vivo challenge tests may validate appropriate dietary nutrient supplementation and lead to better feed formulation, matching the varying energetic demands and digestive capacities of developing E. coioides larvae.     

Effects of n-3 highly unsaturated fatty acid content in Artemia on survival and development of laboratory-reared horsehair crab Erimacrus isenbeckii larvae

The effect of varying levels of dietary n-3 highly unsaturated fatty acid (HUFA) content in Artemia sp. on survival, the number of days required to reach each larval stage, and the growth and morphogenesis of larvae of the horsehair crab Erimacrus isenbeckii were examined. Artemia enriched with materials of six types [condensed Nannochloropsis sp., ethyl oleate, two enrichment levels of eicosapentaenoic acid (EPA), and those of docosahexaenoic acid], and unenriched Artemia were fed to larvae. Newly hatched larvae were reared in groups of 50 individuals in two 2-l beakers until they reached the first crab stage. No significant difference was found in survival rate, growth, or morphogenesis of larvae, but fewer days tended to be required to reach the first crab stage in the group enriched with high EPA compared to the other groups. Results show that, although the amount of n-3 HUFA in unenriched Artemia might meet the demands of horsehair crab larvae for their survival, growth, and morphogenesis, Artemia should be enriched with high EPA to improve the rate of larval development.     

Swimbladder inflation associated with body density change and larval survival in southern bluefin tuna Thunnus maccoyii

High mortality of southern bluefin tuna (SBT) Thunnus maccoyii larvae in captivity is a major problem hindering culture of this species. The relationships between body density of SBT larvae, swimbladder inflation and survival were investigated in this study. Swimbladder inflation and changes in volume have a direct effect on body density in larval SBT. Swimbladder inflation was first observed at 3 days post hatch, and larvae with successful swimbladder inflation were able to maintain their body density within a narrow range (?ρ = 0.0006 g cm^?3). Although swimbladder volume increased with larval growth, it could not compensate for the increase in body density and did not prevent nocturnal sinking. The increase in body density was greater for larvae that did not inflate their swimbladder. Low percentages of swimbladder inflation (27.5 ± 3.5 %) coupled with negative body buoyancy of SBT might contribute to mortality as larvae sank in the dark phase and made contact with the tank bottom. Management strategies that maintain larvae within the water column and enhance swimbladder inflation are required to improve survival of SBT larvae reared in hatcheries.     

Review of the present knowledge of rearing whitefish (Coregonidae) larvae

Intensive fishing of whitefish (Coregonus lavaretus) and a survival rate of the early stages of the fry that is generally too low lead, at present, to the whitefish stocks not being able to utilize fully the natural food resources now produced many times in excess of the requirements by the eutrophication of lakes.Feeding experiments in aquaria have, for several years, shown very impressively, that there are two main reasons for the high mortality of the larvae in the natural environment: high sensitivity to even a short-term lack of food and specific food requirements — the necessity for the larvae to feed on the juvenile stages of certain zooplanktonic crustaceans.Artificial breeding of whitefish eggs in cold water at 1°C caused a delay in hatching of about 8 weeks compared with natural hatching, thus allowing the release of the larvae at a time when the density of zooplankton is about ten times higher and mainly juvenile stages of crustaceans needed by the whitefish larvae are present. Hence, this is one proven way of improving the survival rate of whitefish larvae in the natural environment.Young fish, after metamorphosis, have less specialized food requirements than the larval stage and can even be fed adequately with dry food. In rearing the larvae, natural zooplankton can be replaced by the nauplii of the brine shrimp, Artemia salina. Larvae have not yet been reared on dry food or even on slowly frozen zooplankton or Artemia, but Artemia nauplii shock-frozen in liquid nitrogen (at ?196°C) were found to be as acceptable as living ones and allowed metamorphosis of the larvae to take place. The substance, assumed to be present in the living Artemia, which was lost on slow-freezing but retained after shock-freezing and was shown to be essential to the whitefish larvae, was insoluble in water. Biochemical investigation and identification of this substance are urgently required in order to synthesize an artificial complete food for whitefish larvae.As long as this artificial food is not available, attention will have to be focussed on rearing the larvae in natural ponds where the specific and essential zooplankton can be encouraged.