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Recent important developments in sole aquaculture have increased the availability of the farmed product on the market. The aim of this research was to characterize and compare for the first time in common sole (Solea solea) morphometric parameters, nutrients and sensory traits of cultured and wild specimens. Farmed sole, while maintaining the characteristics of a lean fish species (2% of lipid content), displayed a EPA+DHA fillet content more than twice as high as its wild counterpart. Sensory traits of “potato”, “boiled fish”, sweet, firmness, astringency, chewiness, were correlated to farmed origin while whiteness, briny, “octopus”, “crab”, salty, acid, bitter, umami, intensity and juiciness were linked to wild origin. Intensive farming conditions improved the nutritional value of common sole in terms of lipid content and fatty acid profile and led to sweeter sensory traits particularly associated with a terrestrial vegetable perception.

An experiment was conducted to evaluate the use of soybean protein concentrate (SPC) meal as a partial replacement for fishmeal (FM) protein at levels of 0%, 25%, 50%, 75%, and 100% (SPC0, SPC25, SPC50, SPC75, and SPC100 groups, respectively) in weaning diets (~55% crude protein) for post-larvae of common sole (Solea solea). During the weaning period, larvae were fed on the experimental diets simultaneously with the life food for 15 days. After completing the weaning period, post-larvae (5.5 mg) were fed with the experimental diets for further 30 days in triplicates. No significant (P > 0.05) variations were recorded in the growth performance and survival rate up to the 50% SPC level, after which the larvae growth and survival rates were significantly (P

The effects of early weaning on the performances and molecular ontogeny of digestive enzyme precursors in common sole (Solea solea) larvae were studied. Four experimental weaning time protocols (WPs) were performed in triplicate. One protocol was used for the control group where larvae were fed live feed until 27. days post-hatching (dph) (WP-27) and weaned with dry feed. The other groups provided only with dry feed (WP-4), live feed until 13. dph (WP-13) and live feed until 18. dph (WP-18), followed by weaning on to the same commercial microdiet (MD) used in the WP-27 group (AgloNorse K/S Tromsø Fiskeindustry A/S & Co., Norway). The trial lasted until 33. dph. The larvae of the WP-4 group displayed a significantly lower survival rate, lower growth increase and a delay in the onset of metamorphosis when compared with the other groups. The WP-13 and WP-18 groups displayed significantly lower weight increases when compared with the WP-27 group, whereas metamorphosis development and survival were not influenced. Partial transcripts of trypsinogen 1 (tryp1), trypsinogen 3 (tryp3), trypsinogen Y (trypY), chymotrypsinogen (chymt), amylase (amy) and hepatic lipase (HL) were cloned and analysed by quantitative real-time PCR (qPCR). The tryp1 and chymt mRNA levels displayed a rapid increase in all the treatments between 4 and 8. dph and thereafter displayed a homogeneous fluctuating pattern with a higher increasing tendency for chymt at the end of the trial. The amount of tryp3 and trypY mRNA remained stable or slightly increased until 27. dph in all WP groups and thereafter displayed a general up-regulation trend. The amy transcript levels displayed a gradual increase throughout the study in all WP groups. The HL mRNA levels increased until 13. dph in the WP-27, WP-18 and WP-13 groups and thereafter remained generally stable until 27. dph, whereas in the WP-4 group, there was a linear time-related increasing trend until the end of the trial. This study supports previous results on the feasibility of early weaning of common sole larvae using a MD before the end of metamorphosis. The molecular ontogeny of some important digestive enzyme precursors is described for the first time in this species. The transcripts analysed were not greatly influenced by the WPs. © 2013 Elsevier B.V., The study was financed by the Istituto Zooprofilattico Sperimentale della Lombardia e dell'Emilia Romagna (IZLER, Italy) and by the Spanish Ministry of Economic Affairs and Competitiveness (MINECO), by “Ridigest” (AGL2011-23722) and “AQUAGENOMICS” (Consolider-Ingenio Program; CDS2007-0002) project grants to MY. We also wish to thank Larvanet-COST ACTION FA0801 (EU RTD framework Programme) (www.larvanet.org).

Supplementation of dietary arachidonic acid (ARA) is known to cause hypopigmentation in common sole larvae ( Solea solea L.). This study examined a possible link between dietary ARA supplementation — light intensity and tank colour on pigment defects in common sole larvae. Larval tissue ARA and prostaglandin PGE 2 content increased significantly when fed Artemia enriched by a fish oil emulsion supplemented with 24% dietary ARA during premetamorphosis (until 11 days post hatch, dph) as compared to larvae fed on Artemia enriched by a fish oil based emulsion. More than 90% of all larvae in groups treated with the ARA supplemented emulsion during premetamorphosis showed partly or complete dorsal hypopigmentation. There were no significant effects of light intensity or tank background colour in combination with ARA on malpigmentation. Larval hypopigmentation was below 10% in the groups not treated with ARA supplemented diets. In these groups, however, the proportion of hypopigmented larvae increased significantly by a combination of a high visual light intensity (4000 lx) and transparent tanks. A high light intensity of 4000 lx increased growth as compared to low intensity of 100 lx, suggested to be related to a higher feed intake. Early pigment cell (chromatophor) development until 11 dph (i.e. start of metamorphosis) was not significantly related to dietary treatment, but during metamorphosis (from 16 dph) total chromatophore concentration (cells larvae − 1 ) was significantly lower for larvae treated with ARA and a possible lack of pigment cell differentiation or degeneration/cytolysis continued for this group during post metamorphosis.

The effects of dietary administration of two probiotics, Shewanella putrefaciens Pdp11 and Shewanella baltica Pdp13, on growth, respiratory burst activity of phagocytes of Senegalese sole (Solea senegalensis), and survival of fish challenged with Photobacterium damselae subsp. piscicida have been studied. Fish were fed for 60 days with three different diets: one control, and two diets supplemented with 109 cfu g− 1of probiotics Pdp11 (Pdp11 diet) and Pdp13 (Pdp13 diet). Respiratory burst activity of phagocytes from fish fed with diet Pdp11 significantly increased after 60 days of feeding, whilst this significant increase was not detected in phagocytes from fish fed control and Pdp13 diets. On the other hand, the cumulative percentage of mortality after the challenges with P. damselae subsp. piscicida, was 100% in the groups fed with control diet, whereas mortality rates observed in the groups fed with diets supplemented with Pdp11 and Pdp13 ranged from 75–100% and 65–80%, respectively. These results suggest that an increased respiratory burst activity of the phagocytes is not essential to increase the protection against the P. damselae subsp. piscicida. On the other hand, the use of both probiotics improved the growth, and the survival against the pathogen, in comparison with those fish receiving the control diet.

Dietary manipulations of arachidonic acid, ARA and eicosapentaenoic acid, EPA may have an influence on pigmentation in common sole larvae ( Solea solea L., Linnaeus 1758) which may be related to a “pigmentation window”. This is a specific period in the larval ontogeny where nutritional factors determine pigmentation. Malpigmentation defined as hypomelanosis was significantly related to elevated dietary and larval ARA contents, but not to EPA. The study reports evidence for a pigmentation window, as larval sensitivity to ARA or its derivatives was much higher during premetamorphosis, than during metamorphosis. Initiation of metamorphosis (i.e. start of eye migration) was related to the size of larvae and not related to ARA or EPA content. Dietary EPA or DHA did not retard the advance of eye migration. More than 90% of highly malpigmented juveniles, (i.e. “albinos”) had a permanent aberrant eye migration, which was not related to dietary treatments. This malformation was not observed in normal or slightly malpigmented juveniles. Mean larval ARA tissue contents, malpigmentation and prostaglandin, PGE 2 tissue contents were significantly positively correlated. PGE 2 levels and pigmentation were not affected by dietary or larval EPA contents. Consequently ARA induced prostaglandins are suggested to be involved in malpigmentation in common sole.

Evidence confirms that polyunsaturated fatty acids (PUFAs), arachidonic acid (ARA), eicosapentaenoic acid (EPA) and docosahexaenoic acid, DHA are involved in growth as well in pigmentation of marine fish larvae. In the present study we examined the performance of common sole larvae reared on Artemia enriched with 10 formulated emulsions, differing in inclusions of ARA, EPA, and DHA. The specific growth rate of the sole larvae until late metamorphosis, 21 days after hatching (dah) was 20 to 27% d − 1 . Even though the relative tissue essential fatty acid (EFA) concentrations significantly reflected dietary composition, neither standard growth nor larval survival were significantly related to the absolute concentrations of ARA, EPA and DHA or their ratios. This suggests low requirements for essential polyunsaturated fatty acids (PUFAs) in common sole. Malpigmentation was significantly related to increased dietary ARA content. However, pigmentation was not affected by inclusion levels of EPA or DHA when ARA was high. This, and no relation between DHA: EPA or ARA: EPA ratios and pigmentation and only a weak relation to ARA: DHA ratio, advocate for that it is the absolute concentration of ARA in larval tissues, that is responsible for malpigmentation rather than the relative concentration to other PUFAs. Within malpigmentation, the trait “albinism” was characterised by an abnormal incomplete eye migration, but this trait is suggested not to be related to dietary ARA. Furthermore, albinism resulted in a lower growth rate, which suggests that visual aberrations affected prey capture.

The present study was undertaken in order to determine the effect of a dietary incorporation of soybean meal (SBM) on growth performance, nutrient utilization and gut histology of Egyptian sole Solea aegyptiaca. This species, which is genetically and morphologically close to Solea solea, is currently present in the Mediterranean Sea and reared in some Italian farms, using the same techniques for S. solea production and it could represent a promising new species for Mediterranean aquaculture. Three isoproteic and isolipidic extruded diets (crude protein, 47%, crude fat, 20%) were formulated containing 0 (diet 1), 18 (diet 2) and 30% SBM (diet 3). 540 juveniles (initial body weight of 6.9 ± 0.2 g) were equally distributed into nine 150 l squared tanks (bottom surface: 520 cm2) connected with a recirculation system (temperature 20 ± 1 °C and dissolved oxygen above 6 ppm). Animals have been hand-fed twice a day (at 9.00 a.m. and 5.00 p.m.) at a daily ratio of 1.5% body weight− 1. Diets were tested in triplicate for 87 days. The fish were weighed at days 28, 57 and 87, and they were sampled at days 0, 57 and 87 for nutrient retention determination. One-way ANOVA with Newman–Keuls' post-test were used to analyze data (P ≤ 0.05). No differences in terms of palatability were observed and any diet has been consumed completely all throughout the experiment. At day 87, fish fed diet 1, 2 and 3 reached weights of 25.8 ± 0.8 g, 27.5 ± 0.9 g and 26.1 ± 1.5 g, respectively and no differences in performances or nutrient retention were found. Histopathological gut examinations have revealed no noticeable differences in the appearances of the intestines between any of diet groups. Intestinal mucosal cells presented well vacuolated upper, with well defined microvillar brush borders. On the basis of the results of this trial, SBM seems to be a good protein source for Egyptian sole and can be added in the diet up to 30% without any reduction in growth rate and no adverse effect on gut histology.

Research on the rearing cycle of the common sole, Solea solea , was done at an experimental hatchery in the Lagoon of Venice (Italy). The aim was to develop rearing schedules suitable for market production and document any technical problems. Larval metamorphosis studied on 9 groups of larvae reared at 18 °C demonstrated high temporal variability. Caudal metamorphosis and eye migration occurred between 9 days after hatching (DAH) and 24 DAH, and between 13 DAH and 25 DAH, respectively. A larval rearing schedule based on live-food feeding was set at 18–19 °C, which achieved an average survival rate of 40% at 28 DAH. Three weaning trials comparing two commercial feeds were carried out on larvae about 30 DAH. One of these feeds was sufficient in itself to complete juvenile weaning, reaching average survival rates of 85%, which are comparable to those obtained in the control groups fed with live Artemia . Average survival rates of 43% were obtained with the second commercial feed. Both commercial feeds enabled superior juvenile growth on average to that in the control groups. An on-growing trial in extensive conditions was done in an earthen pond of 370 m 2 , stocking juveniles with an average weight of 3.6 g at a density of 1.5 juveniles/m 2 . The trial started in mid-September and lasted until the following August, when it was stopped because of high mortality due to viral encephalopathy and retinopathy infections. Growth was negligible during winter and began again in spring, reaching the maximum incremental rate between May and June, at temperatures of between 20 and 25 °C. The specific growth rate never exceeded a daily value of 2%, while the average final size reached after 10 months rearing was 12 g. A double replicate trial of intensive rearing was carried out starting with soles of 7 g, reared at a density of 150 juveniles/m 2 in circular fibreglass tanks of 10 m 2 surface. This trial was also stopped during the following summer because of the considerable mortality due to viral infection. The soles had reached an average size of 54 g in August, after 300-day rearing. Sole can be bred and reared with good efficiency related to its survival rate, but the results of the growth trials, both intensive and extensive, do not allow conclusions to be made on the growth performance in the experimented conditions. The health problems compromised the growth trials towards the middle of the favourable growing season. The trials highlight both the high susceptibility of sole to viral encephalopathy and retinopathy infections, and the scarce tolerance of this species to temperatures of above 25 °C, which caused the onset of frequent bacterial infections.

The eggs of the polychaete worm Nereis virens were used as a vehicle for the delivery of oxytetracycline (OTC) in Dover sole, Solea solea larvae. The eggs were osmotically loaded, and the level of antibiotic was estimated by HPLC and a microbiological bioassay. The amount of OTC in the N. virens eggs was over 1 μg per egg, which was considered more than sufficient for a therapeutic dose for larval fish. The loaded eggs were tested on the sole larvae, and the results of the experiment (9 μg OTC per fish larva) indicated that the eggs of the Nereis worm can be used as biocarriers of water soluble drugs.

Renewed interest in the farming of sole has been stimulated largely by a desire, if not a need, for the existing marine fish farming industry to diversify. Despite the extensive studies of the 1960s and 1970s, commercially viable cultivation techniques for this valuable species were not realised. This species proved relatively easy to rear through the larval stages but the juvenile stages performed poorly on formulated feeds, displaying low growth rates and an apparently high susceptibility to disease. Consequently, effort was directed to other species which did not present such difficulties. Recent research has demonstrated that the technical problems may not be as intractable as was once thought. This paper reviews the results of these studies, which have focused particularly on the problems of providing adequate nourishment to the larvae and juvenile stages. They have demonstrated (a) the ease with which larvae may be reared to the juvenile stages and a link between larval diet quality and juvenile performance, (b) that the reputedly virulent disease, black patch necrosis, may be associated with nutritional deprivation and that juveniles may be grown successfully without a sand substrate, (c) that juveniles can attain a growth rate of over 3 cm per month on natural prey and (d) that diets may be formulated which support survival rates in excess of 90% during weaning and subsequent growth rates approaching those on natural prey. The implications of this work on the prospects for developing commercially viable culture methods is discussed with reference to other important biological characteristics of the species, such as their response to crowding. It is concluded that there are no insuperable problems to the development of cultivation methods for the sole, but further work is required in certain critical areas before economic feasibility can be demonstrated unequivocally.

Minimum inhibition concentrations (MICs) were determined for Furanace (Dainippon Pharmaceutical Co. Ltd.) against 17 strains of pathogenic vibrios isolated from marine fish and salmonidae. All the strains tested were sensitive to Furanace, the highest MIC being 3.1 μg/ml for one strain and the remainder being < 1.6 μg/ml. Plaice (Pleuronectes platessa L.), Dover sole (Solea solea (L.)) and brill (Scophthalmus rhombus (L.)) rapidly took up Furanace from low-concentration baths (5–10 μg/ml) at 9.5–11.5°C. Uptake varied between species at these temperatures and in most cases blood levels above 1 μg/ml were achieved after 1 hour's exposure. Two hours after exposure over 70% of the Furanace had been eliminated from the blood.

Sole were weaned from live Artemia onto a compound diet between days 10 and 15 after hatching. By day 62, between 28 and 61 sole survived per 100 hatched larvae, and their mean weight at day 70 was between 189 and 414 mg. These results were comparable with those obtained by other authors who weaned sole between days 25 and 35, except for a mean weight of 1070 mg on day 70 obtained by Fuchs (1979). The interest of such an early weaning lies in its saving of live food. Approximately 3 × 10 3 nauplii were required per sole surviving at the end of these experiments, i.e. a saving of 90% compared with Fuchs' experiments.

Two species of sole with potential for aquaculture, found along the Portuguese continental coast, were examined for the presence of helminth parasites. The prevalence and intensity of infection were relatively low, and no lesions were observed. Some of the helminths were found to be present in both sole species.

Trials with soles of age 0 to 1 month carried out in 1977 and 1978 are described. The aim was to improve and simplify the rearing method proposed by Girin (1978). The quantity of alevins produced was about 15 000 to 20 000 per 2-m3 tank and survival and growth were comparable with the best results obtained thus far in small volumes (70–80% for survival, 50–80 mg for average weight). Precise quantification of conditions of feeding (quality and quantity), rearing conditions and environmental factors (aeration, photoperiod, type and shape of the tanks) for larvae (0–15 days) and juveniles (15–30 days) allow us to consider the rearing of this species to be well controllable up to the end of the first month. On the basis of the results it is possible to envisage the extension of this rearing method to the production of many hundreds of thousands of individuals without the strict conditions of the laboratory.

The disease condition “Black Patch Necrosis” which causes serious mortalities of cultivated Dover sole through the development of areas of fin and skin necrosis, was shown to be infectious by co-habitation of diseased and healthy stocks. Spread of the disease throughout an affected tank was rapid and high pathogenicity was evident, with most affected sole dying 1–2 days after the first appearance of clinical signs. Addition of a sand substrate to tanks both prevented occurrence of the disease and cured an established disease situation. No aetiological agent has yet been identified.

Spawning was induced in the common sole ( S. solea L.) by a synthetic analogue of LH-RH administered at doses of 10 μg/kg of body weight. The results suggest that LH-RH analogue accelerated maturation of the ovaries in females. The significance of this work in piscicultural practice and the mechanism of hormonal action are briefly discussed.

Three bacterial diseases (tailrot, furunculosis and red-spot disease) are described. A Pseudomonas was associated with the first two and a Vibrio with the third. Tailrot and furunculosis could be cured by treating the fish with antibiotics, but red-spot disease could not because of its very rapid progression and late on-set of signs. Ectoparasites probably facilitate the entry of disease organisms.

The feeding efficiency of larval, post larval and juvenile Dover sole ( Solea solea L.) was studied by using Artemia stained with five different food colourings — brilliant blue, pink, lemon yellow, red “C” and black. The results show that the feeding efficiency is markedly improved by this treatment. The maximum feeding efficiency was found using black stained Artemia and minimum for the unstained control. This clearly suggests that the contrast perception of the food in relation to the illumination of the background is of major importance especially during the critical weaning period of the larval stage when feeding is being established.

Sole eggs have been naturally spawned and fertilized in captivity for the last 12 years. Spawners, submitted to natural temperature and photoperiod, produced 17 300 000 eggs in 229 different batches. Those for which the spawning season was shifted by artificial combinations of temperature and light produced 1 900 000 eggs in 55 different spawnings. Each batch of eggs was closely observed in order to study the optimal conditions required for gametogenesis and spawning. This article gives data on egg weight, diameter, biochemical composition and viability rate. The embryonic salinity and temperature tolerance is also discussed on the basis of experiments run in very efficient automatic open water circulation incubators.

The response of the developmental stages tested was examined under constant flow conditions using direct electrolysis of sea water as a source of chlorine. From LC 50 determinations, the eggs of plaice were found to be more tolerant than the newly-hatched larvae, and for both plaice and Dover sole the tolerance of the larvae increased as their development proceeded up to metamophosis. Less change in tolerance was evident with increasing size of fish after metamorphosis. Determinations of time to kill 50% of a test population showed that at chlorine concentrations only slightly higher than the LC 50 level the time for survival was considerably reduced. Growth of the metamorphosed fish of both species at sub-lethal levels only showed a very marked depression at concentrations very close to the LC 50 level. There was some evidence of acclimation to chlorine during long exposure. The growth of fish in power station effluents should not be affected by the use of low level chlorination as an anti-fouling measure, since the chlorine is normally rapidly absorbed by reactions taking place in the sea water. Large fluctuations in the chlorine dose could however present a hazard.

A growth trial was conducted to assess the usefulness of two unfed Artemia nauplii regimes for ‘0’ group Dover sole cultivation. San Francisco strain Artemia salina, harvested at either 24 h (30°C) and/or 48 h (23°C), were investigated with sole between day 20 and day 85 post spawning. At day 60, a proportion of the fish from each trial was weaned onto Lumbricillus spp., an oligochaete used as a second stage foodstuff at Hunterston, Scotland. The results showed a pronounced growth increase in fish fed with 24-h Artemia compared to those reared on 48-h Artemia. Lumbricillus weaning produced a superior growth rate compared to both Artemia regimes, but had little effect on the growth differences established during Artemia feeding.

Sole larvae were given live Artemia nauplii only during their first few days of life. They were then fed on artificial diet, semi-moist and crumbly in form. Good results were obtained without supply of frozen nauplii (15 and 28% survivors from hatching up to day 70, respectively for two lots abruptly weaned from day 10 or 11, with 50 and 90 mg as the final mean weight, and a supply of 1980 or 2660 live nauplii per 70-day-old fish). Gradual weaning was also tried, supplying frozen nauplii for the first 5 days of inert feeding. The best weaning was found to be the latter one, i.e. with frozen nauplii being given from day 11 to day 15. This produced 49% survivors from hatching up to day 50. A total of 2800 live nauplii were required per sole at this stage. But the growth rate, 27 mg as the mean weight by day 50, was unsatisfactory. In addition, the pigmentation of sole from the three tested batches of eggs was observed. The proportion of normally pigmented sole at the end of the experiments was between 13 and 70%. The differences between lots from the same spawning were found to be significant. This would indicate that unchecked rearing conditions might produce the very high level of pigmentary anomalies generally observed in the rearing of flatfish.

The growth of sole and turbot was followed in groups of fish exposed to constant unionised ammonia concentrations at pH levels both at and below that of normal sea water. Low pH levels were produced by the addition of carbon dioxide. Results for both species showed evidence for a threshold level of unionised ammonia below which little or no effect on growth was evident. This level was determined to be 0.066 mg N/l for sole and 0.11 mg N/l for turbot at 16°C, 34‰ salinity. The different pH conditions did not have any marked effect on these threshold levels. Above the threshold levels growth appeared to be depressed in a linear manner with increased unionised ammonia. The rate at which growth was depressed was greatest at the low pH levels for both species. The level at which growth ceased thus decreased from 0.77 to 0.38 mg N/l for sole and from 0.9 to 0.3 mg N/l for turbot, over the pH range from 7.9 to 6.9. The absence of disease and gill damage in the experimental populations is discussed. It is suggested that this and the tolerance shown by the fish, which is higher than is generally accepted for fish in high density culture, may be due to the clean environment in which the fish were held while under test.

Dover sole (Solea solea (L.)) of 1.3–2.8 cm in length (18–190 mg) were weaned from live Artemia salina nauplii to prepared diest ranging from extruded pastes to dry powders. A variety of techniques was used, including the abrupt change to the new diet, the use of a series of intermediary diets, and the addition of limited quantities of live Artemia during the early stages of weaning. Survival rates of 41–76% were achieved and growth averaged from 0.04 to 0.33 mm/day.

Spawning was induced in the common sole ( Solea solea L.) by injecting HCG (human chorionic gonadotropin) intramuscularly in single doses ranging from 250 to 1000 IU/kg fish. Low doses of HCG (250–500 IU/kg fish) injected into females with oocytes in the final stages of vitellogenesis induced spawning with the highest fertilization percentage, number of eggs and number of spawnings. HCG treatment is important in pisciculture practice.

An examination of the changes in distribution of exo- and endoproteases has been made in Dover sole intestinal extracts from 24-day larvae to adult fish. Pepsin-like activity was not detectable in 24-day-old animals but was first measured in appreciable amounts at 200 days. All other proteases were present at 24 days and increased in value as the fish matured. The significance of these results to the release of nutrients from fish diets is discussed.

The effect of photoperiod on survival and growth of cultured young soles ( Solea solea ) was investigated from hatching to the age of three months, at 18° C. Two experiments were performed, comparing days with 12, 18 and 24 h light. The fishes were reared in 60 1 tanks, conical ones before metamorphosis, square ones with a sandy bottom from metamorphosis to the age of three months. They were fed on live food organisms for 1 month, on frozen flesh of the bivalve Laevicardium crassum for the two other ones. During the larval period, no significant difference was recorded in the survival rate but a better growth was obtained with 18 and 24 h light, suggesting a predominant diurnal feeding. From metamorphosis to the age of three months (mean weight: 1 g) no significant effect of the photoperiod on the survival and growth of the juveniles could be demonstrated.

In an experiment in which fish were held in static water, ammonia accumulated only in those tanks which did not contain a population of the unicellular alga Dunaliella tertiolecta. The poorer growth of the fish held in the tanks without algae is suggested as being related to this result.

The density dependent growth was investigated for sole. The Von Bertalanffy equation was adopted with constant coefficient K of catabolism. Two techniques were used, one based on the integrated expression l t = L ∞ (l−e −K (t−t 0 ) ) where density dependence is referred to L ∞ . The other technique analyzes the incremental expression dw dt = h 1 w 2 3 − kw n with n = 1 and n = 5 6 . No relation between stock density and growth could be demonstrated.

It is well known that in rearing marine fish there is generally a high mortality observed at the end of the yolksac stage, when the larvae should start feeding. The reasons for this mortality are largely unknown. The success of the process is unpredictable and rearing attempts usually do not bear repeating. Observations made in the breeding season 1972, may throw new light on the reasons for this unpredictability.