Biochemical impacts of salinity on the catfish,Heteropneustes fossilis (Bloch, 1794), and possibility of their farming at low saline water

Although a stenohaline freshwater fish, the stinging catfish Heteropneustes fossilis, is also available in the freshwater fringes of the coastal areas of Bangladesh, the tolerance of this species to variable environmental salinity has not been thoroughly investigated. Based on median lethal salinity (MLS‐50 96 h), three sublethal salinity levels (3 ppt, 6 ppt and 9 ppt) and a control (0 ppt), each with three replications were selected to observe the effects of mildly brackish conditions on the fish for a period of 90‐day exposure. Better growth and survival were found up to 6 ppt compared with control. Salinity more than 6 ppt appeared unsuitable for H. fossilis fingerling due to increased mortality and reduced growth. To determine biochemical alterations, a few important physiological parameters were observed after 90 days of exposure. Glycogen level of liver and muscle in the fish reared at 9 ppt salinity decreased significantly (P < 0.05) as compared to the control. Glucose level in blood and liver was also found to be increased in fish with increase in salinity. ALP and ATPase activities were reduced significantly in both muscle and liver tissues at higher salinity, indicating the stress mitigation effect. However, all the biochemical parameters were found in normal condition up to 6 ppt compared with control. This evidence suggests that H. fossilis can sustain and grow well below 6 ppt and can be a potential candidate for culture in coastal areas after heavy downpour when the salinities level falls to 6 ppt or lower.     

Effect of Salinity on Larval Rearing of Pacu,Piaractus mesopotamicus,a Freshwater Species

The aim of this study was to evaluate the growth and survival of pacu, Piaractus mesopotamicus, larvae reared in different salinities and to determine the Artemia nauplii life span in freshwater and in saline water. First feeding 5‐d‐old pacu larvae were reared in freshwater or at 2, 4, 6, 8, 10, 12, and 14 ppt salinities. The larvae were reared in 1.5‐L aquaria at a density of 10 larvae/L with three replicates per treatment. After 10 d of rearing, significant differences (P < 0.05) were observed for growth and survival. Larval growth was higher at 2 and 4 ppt, and survival at 2 ppt was 100%. In freshwater and at 4, 6 and 8 ppt, the survival was 91.1, 93.3, 73.3, and 39.9%, respectively. At higher salinities, there was 100% mortality after 2 h (12 and 14 ppt) and 8 h (10 ppt) of exposure. The slightly saline water of at least 2 ppt increased the Artemia nauplii life span compared to the life span in freshwater. Later, in a second trial, 5‐d‐old pacu larvae were reared in freshwater and at 2 and 4 ppt salinities during the first 5 or 10 d of active feeding, and then the fish were transferred to freshwater. At the end of 15 d, larval growth was lower in freshwater (42 mg) than in treatments 2 and 4 ppt (59–63 mg). The abrupt transfer of fish from freshwater to slightly saline water and the return to freshwater did not affect the survival rates (89–97%). The larvae were able to adapt to these saline environments and handle abrupt changes in salt concentration. We concluded that salinity concentration of 2 ppt can be used for pacu larval rearing, allowing the Artemia nauplii lifetime to last longer and cause faster fish growth.     

Effect of salinity at constant 10 °C on grow-out of anadromous Arctic charr from Labrador

The goal is to determine the requirements allowing cultured Salvelinus alpinus to thrive in seawater, as they do in the wild. In late-June, eight families of individually identified 1+ year-old charr (mean wt: 427 g) of a domesticated strain derived from the Fraser River population were directly transferred from freshwater (9 °C) to salinities of either 0, 10, 20 and 30 ppt at 10 °C, then on-grown in tanks until December. Cumulative mortality was 16% in 30 ppt salinity, and < 4% in the lower salinities. Repeated measures analysis revealed somatic growth was inhibited by both elevated salinity and sexual maturation. Among immature fish, final mean weight and condition factor in 30 ppt salinity was 490 g and 1.2, compared to nearly 1 kg and > 1.7 in 0 and 10 ppt. In 20 ppt salinity, growth was initially similar to that in ≤ 10 ppt salinity but deteriorated from September onwards. Sexually maturing fish in ≤ 10 ppt salinity attained a final mean weight about 35% less than immatures. Plasma osmolality was only slightly elevated in the 30 ppt salinity, remaining < 340 mOsm kg^− 1. Food intake and conversion were affected by the interaction between salinity and time, being optimum in 0 and 10 ppt treatments. Family effects on final body size were large, but the effect of salinity on growth was independent of family. In conclusion, despite their large body size, direct transfer of this strain from freshwater to seawater does not appear viable for commercial aquaculture.     

Effect of piscine GH/IGF-I on final oocyte maturation in vitro in Heteropneustes fossilis

Growth hormone (GH) has recently been identified as co-gonadotropin regulating fish reproduction, hitherto, no effort has been made to see its effect on oocyte maturation in fishes, though some reports demonstrate the role of insulin like growth factor-I (IGF-I) in oocyte maturation in teleosts. Hence, effect of GH on oocyte maturation in post-vitellogenic H. fossilis has been worked out in the present study. Post-vitellogenic follicles in the ovarian tissue were challenged in vitro with H. fossilis pituitary homogenate (fPH), Clarias batrachus GH and GtH, barramundi IGF-I (IGF-I), 17α, 20β-dihydroxy-4-pregnen-3-one (DHP) and testosterone alone, or in combination with IGF-I for 18 h at 26±1°C. Incubation of ovarian tissue with GH in the presence of actinomycin d or cycloheximide or barramundi IGF-I antiserum was also made separately. In general, oocyte maturation was induced by fPH, barramundi IGF-I, GtH, GH and DHP, which was augmented further by addition of barramundi IGF-I. Testosterone had no effect on GVBD. Actinomycin d, cycloheximide and anti barramundi IGF-I abolished the GH induced oocyte maturation. Present study suggests for the first time that GH has a role in egg maturation in fish.     

Ovarian histology of stunted pond-reared Macrobrachium rosenbergii females

The present study describes the ovarian histology of stunted freshwater prawn Macrobrachium rosenbergii. The ovarian maturation of stunted animals was examined and compared with similar‐aged normal females. Ten animals of the stunted group and each maturation stage of the normal group were sampled from the same pond and had their ovaries removed for histological analysis. Body weight, body length, ovarian weight and gonadosomatic index (GSI) were recorded for each female. The diameters of the different oocyte types were compared among groups through histological assessments. The ovarian histology of stunted M. rosenbergii females indicated that although the somatic growth is severely affected (7.6 g), some energy has been placed on the vitellogensis. Stunted females showed the simultaneous occurrence of previtellogenic, vitellogenic and mature oocytes in their ovarian tissue, but overall oocyte diameter and GSI (1.02%) were significantly affected when compared with normal females.     

Effects of Nitrite Exposure on Growth and Survival of Sea Bass,Lates calcarifer,Fingerlings in Various Salinities

Sea bass, Lates calcarifer, fingerlings were acclimated to 0. 15, and 32 ppt, and the toxic effects of nitrite exposure were assessed. The 96-hour median lethal concentrations (96-hour LC₅₀ for nitrite were estimated to be 14.5 mg/L at 0 ppt, 105 mg/L at 15 ppt and 93 mg/L at 32 ppt salinity. Chronic exposure to a nitrite concentration equivalent to 10% of the respective 96 hour LC₅₀ resulted in marked growth reduction: growth being reduced in the order of 0 ppt > 32 ppt > 15 ppt. In nitrite-free water, growth rate for fish raised at a salinity of 15 ppt was higher compared to fish raised at salinities of 0 ppt and 32 ppt, a phenomenon which probably reflected the advantage of a reduction in osmoregulatory work in an iso-osmotic environment.     

Diurnal rhythm of serum steroid hormone levels in the Japanese whiting,Sillago japonica,a daily-spawning teleost

Ovarian developmental stages and serum steroid hormone levels were examined at six different times of day (0100, 0600, 1000, 1300, 1600, 2000 h) in a marine teleost, the Japanese whiting Sillago japonica, which has an asynchronous-type ovary containing oocytes at various stages of development and spawns every day during a period ranging up to three months. The largest oocytes in the ovaries at the active vitellogenic or post-vitellogenic stages were found between 0100 and 1300 h. Oocyte maturation indicated by germinal vesicle breakdown (GVBD) occurred at 1600 h, and ovulated oocytes were observed in the ovaries collected at 2000 h. These processes were accompanied by a significant daily change in serum steroid hormone levels. The serum level of estradiol-17β showed a peak in fish with mature oocytes sampled at 1600 h. In these fish, the second-largest oocytes in the ovaries were at the initial stage of vigorous vitellogenesis, the secondary yolk stage. Therefore the highest level of serum estradiol-17β was considered to be due to the second-largest oocytes. Testosterone levels remained low and constant throughout the experimental period. The serum levels of 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-diOHprog) peaked at 1600 h at which time all fish had mature oocytes. These results indicate that the Japanese whiting possesses a diurnal rhythm of oocyte development including vitellogenesis, oocyte maturation and ovulation, and further suggest that daily cycles in oocyte growth and maturation which simultaneously take place in an ovary are regulated by diurnal secretions of estradiol-17β and the maturation-inducing steroid, 17α,20β-diOHprog.     

Effects of Salinity on Growth, Survival, and Selected Hematological Parameters of Juvenile Cobia Rachycentron canadum

Cobia Rachycentron canadum juveniles (119.7 mm TL, weight 8.5 g) were reared for 10 wk at three salinity levels: 5 ppt, 15 ppt. and 30 ppt. Growth and survival were determined through biweekly sampling. Blood samples obtained at termination of the study were analyzed to determine hematocrit, blood osmolality, and total protein. Results indicated that the overall growth of fish was significantly affected by salinity. Mean (± SE) total length (TL) and weight of fish reared at a salinity of 30 ppt were 201.7 ± 2.6 mm and 47.6 ± 1.9 g, respectively, followed by fish reared at 15 ppt (182.2 ± 1.7 mm, 34.1 ± 1.6 g). and 5 ppt (168.3 ± 5.8 mm TL, 28.3 ± 2.3 g). Differences in specific growth rates among treatments for the 10-wk period were also significant. No differences were detected in mean survival among fish reared at salinities of 5, 15, and 30 ppt (84, 94, and 94%, respectively). However, fish reared at salinity 5 ppt appeared to be in poor health as skin lesions, fin erosion, and discoloration were evident. Analysis of blood revealed that, while no differences existed among treatments with respect to plasma total protein, fish reared at a salinity of 5 ppt exhibited significantly reduced hematocrit (25% vs. > 30%) and plasma osmolality values (318 vs. > 353 mmolkg) relative to fish reared at higher salinities. Cobia can tolerate exposure to low salinity environments for short periods of time without mortality; however, moderate to high salinities are required for sustained growth and health of this species.     

Nutrient uptakes and biochemical composition of Lemna minor in brackish water

The freshwater plant species Lemna minor suffers significantly in nutrient uptake and biomass accumulation performance when grown in saline effluents, such as those produced in brackish aquaculture operations. To determine the exact impact of salinity to these plant traits, this study measured the nutrient uptakes and biochemical composition of L. minor grown in synthetic aquaculture medium of increasing salinity levels. The overall trend for biomass growth showed that higher salinity levels resulted in lower growth with a mass gain of about 13% in 12.5 ppt medium compared with 96% in the control. However, the NO₃^– uptake appeared to be unaffected by differences in salinity. NH₄⁺ uptake was significantly affected only at salinity concentrations of 10 ppt and 12.5 ppt. The relationship between PO₄^3– uptake and salinity was less clear, as PO₄^3– levels appeared to decrease linearly for all test groups and were shown to be statistically insignificant. At the end of the experiment period, the control medium showed the lowest measured COD levels, 17 mg/L while the 12.5 ppt solution has the highest COD level, 61 mg/L. Protein content showed a decline with increasing salinity of growth medium, while carbohydrate content was shown to be increasing. These preliminary data identify the general relationship between salinity and the measured criteria of L. minor and will subsequently serve as the basis for further remediation studies and the development of salinity mitigation methods.     

Influence of water salinity on the survival of embryos and growth of the sichel larvae Pelecus cultratus (L.) under controlled conditions

The purpose of this study was to analyse the influence of water salinity on the incubation of eggs and growth of the larvae of sichel Pelecus cultratus (L.) under controlled conditions. The following water salinity was considered: 3, 6, 9, 12, 15 and 0 ppt for the control. The eggs incubation time depended on the water salinity, lasting from 56.3°D [9 ppt] to 107.7°D [0 ppt]. The higher water salinity, shorter the incubation time. The highest survivability of larvae was obtained in water of the salinity equal 3 ppt and in freshwater, where respectively 87% and 84% of the larvae survived. No hatched larvae were obtained in water of the salinity of 12 and 15 ppt, as all embryos died during the experiment. Larvae were reared up for 21 days. The final body weight, depending on the water salinity, reached from 43.5 mg [9 ppt] to 74.6 mg [3 ppt], at the respective body lengths of 19.3 mm and 23.2 mm. The highest body gains and lengths of larvae were observed in water of the salinity of 3 ppt. The lowest survivability of sichel larvae occurred in water of the salinity 9 ppt, where it was only 53.2%. The survivability of larvae in freshwater and water of the salinity of 3 ppt was quite similar: 96.2% and 95.6% respectively. Water salinity degree of 12 and 15 ppt appeared to be lethal for both embryos and larvae of the sichel.     

The reproductive physiology of three age classes of adult female diploid and triploid brook trout (Salvelinus fontinalis)

Triploid female fish show impaired gametogenesis and are unable to produce viable offspring. The reproductive physiology of artificially-induced triploid female salmonids has been well described up until the time of first sexual maturation in diploids, but few reports exist for older triploids. This study reports the influence of triploidy on growth, ovarian development and reproductive endocrinology among three age classes of female brook trout (Salvelinus fontinalis) in comparison to sibling diploids. Triploids were larger than diploids for most of the study period, but the difference was statistically significant only during maturation and spawning of 2⁺ diploids. Plasma estradiol-17 (E₂), testosterone (T) and vitellogenin (VTG) levels in triploids were generally lower than in diploids, and VTG was the only parameter to show seasonal fluctuations resembling those of diploids. Triploids showed significantly lower GSI and total oocyte number than diploids of similar age, and only half of all triploids sacrificed during the study (n=56) had developing oocytes in their ovaries. At age 3⁺, 13 of 19 triploid females had oocytes at various stages of development, including perinucleolar, yolk vesicle and yolk globule stages. In addition, three of these fish had collectively produced 72 mature stage oocytes. Thus, whereas diploid brook trout can produce mature oocytes as two-year-olds, triploids cannot do so until four years of age, with the number of mature oocytes being greatly reduced.     

Histological examination of final oocyte maturation and atresia in wild and domesticated Penaeus monodon (Fabricius) broodstock

Oocyte maturation and gonadosomatic index (GSI) of eyestalk ablated Penaeus monodon females collected from the wild and from two first‐generation domesticated lines were assessed. Frequency and diameter of the different oocytes, and the intensity of oocyte atresia, were compared among groups through histological assessments of the sections of the middle ovarian lobe. Digitized images from ovary sections were used to record the frequency and diameter of different oocyte types. Spawning performance of the three groups were expressed in terms of the percentage of females that spawned at least once (productive females), time from eyestalk ablation to first spawning (latency period) and the number of spawnings per female stocked. Final ovarian maturation was attained in all groups, as indicated by the presence of mature oocytes with cortical rods (cortical oocytes), dark‐green ovarian colour and high GSI values (5.83–6.86%). However, domesticated groups presented significant larger immature oocyte types (previtellogenic and yolky oocytes) and smaller cortical oocytes compared with wild females, indicating a reduced vitellogenic activity during final oocyte maturation. Additionally, the frequency of atresia was comparatively higher for both domesticated groups, which could be related to their inferior spawning performance. The implications of these results on the reproductive potential and development of domesticated P. monodon are discussed.     

A Preliminary Study on the Effects of Salinity on Growth and Survival of Mulloway Argyrosomus japonicus Larvae and Juveniles

Abstract.— Tko experiments were conducted to determine the effects of salinity on growth and survival of mulloway Argyrosomus japonicus larvae and juveniles. First, 6-d-old larvae were stocked into different salinities (5, 12.5, 20, 27.5 and 35 ppt) for 14 d. Larvae grew at all salinities, but based on results for growth and survival, the optimum range of salinity for 6-d-old to 20-d-old larvae is 5–12.5 ppt. During this experiment larvae held in all experimental salinities were infested by a dinoflagellate ectoparasite, Amyloodinium sp. Degree of infestation was affected by salinity. There were very low infestation rates at 5 ppt (0.2 parasites/larva). Infestation increased with salinity to 20 ppt (33.1 parasites/larva), then declined with salinity to 35 ppt (1.5 parasites/larva). For the second experiment, juveniles (6.1 ± 0.1 g/fish) were stocked into different salinities (0.6, 5, 10, 20 and 35 ppt) for 28 d. Juveniles were removed from freshwater 3 d after transfer as they did not feed, several fish died and many fish had lost equilibrium. However, when transferred directly to 5 ppt. these stressed fish recovered and behaved normally. Trends in final mean weight and food conversion ratio of juvenile mulloway suggest that fish performed best at 5 ppt. Although salinity (5 to 35 ppt) had no significant ( P > 0.05) effect on growth, survival, or food conversion ratio of juveniles, statistical power of the experiment was low (0.22). Based on these results we recommend that mulloway larvae older than 6 d be cultured at 5 to 12.5 ppt. Optimum growth of juveniles may also be achieved at low salinities.     

半滑舌鳎卵巢发育的组织学和形态数量特征研究

采用组织学方法系统研究了人工养殖条件下半滑舌鳎亲鱼卵巢的组织发育周年变化特征.结果表明,卵母细胞发育可分为6个时相,卵巢发育分为6期.卵巢不同发育时期都由不同类型的卵母细胞组成,半滑舌鳎为非同步分批多次产卵类型.周年发育过程中,6月龄前的鱼卵巢发育处于第Ⅰ期;9～12月龄卵巢可发育至第Ⅱ期;12月龄半滑舌鳎性腺可发育达到Ⅲ期,并一直保持至24月龄.3龄雌鱼达到性成熟,卵巢可发育至第Ⅳ期;随着水温和光周期的调控,卵巢发育进入成熟期(Ⅴ期),发生水合现象是卵母细胞成熟并准备排卵的信号;产卵结束后卵巢退化至第Ⅵ期,排出的卵母细胞退化吸收,以Ⅲ期卵巢越冬后,卵巢退化至Ⅱ期并持续至下一个繁殖周期.人工养殖条件下,卵巢内存在卵母细胞闭锁现象.性腺周年发育过程中,性腺指数(GSI)在性腺达到成熟期时达到峰值,产卵结束后性腺快速退化,GSI值各月份差异显著(P<0.05),亲鱼性腺重量在卵巢发育成熟过程中一直处于生长状态.亲鱼肝脏指数(HSI)在产卵前7～8月份(性腺达到成熟期前30 d左右)达到最大,表明7～8月份是性腺进行卵子发育、卵黄能量储备的重要时期,亲鱼在产卵前通过积极摄食储备体内能量,肝脏在卵母细胞卵黄积累方面具有重要的生理作用.肥满度(CF)值在繁殖盛期10～11月份达到最大,表现出与GSI类似的变化趋势.     

Size dependent early salinity tolerance in two sizes of juvenile white sturgeon,Acipenser transmontanus

The objective of this study was to investigate the influence of size on salinity tolerance in 1 year old juvenile white sturgeon. Two sizes of sturgeon (10 and 30 g) from the same spawning event (thus reducing confounding effects of genetic make-up and size) and reared in the same environment were exposed to a salinity of 0, 8, 16, 24, or 32 ppt for up to 120 h. Both 10 and 30 g fish exhibited > 93% mortality within 24 h after transfer to 24 or 32 ppt, regardless of whether they were transferred directly from freshwater (FW) or following a 48 h pre-treatment period at 16 ppt. Direct transfer from FW to 16 ppt was associated with 25 to 30% mortality, indicating that these fish have some ability to tolerate large changes in salinity for up to 5 days at this stage. Following exposure to 8 and 16 ppt, an elevation in plasma osmolarity, [Na⁺], and [Cl^?] was observed between 24 and 72 h in both 10 and 30 g sturgeon, but plasma ions and osmolarity in surviving fish at 120 h were not significantly different between groups held at 0, 8, and 16 ppt. Despite being unprepared for either direct or stepwise transfer to salinities of 24 ppt or greater, size confers some ionoregulatory advantage, as mortality occurred more slowly and the degree of ionoregulatory perturbation was less in 30 g than 10 g fish over the course of the exposures. It is not known whether the apparent advantage of size is related to a size-dependent development of ionoregulatory capacity or due to social status which can also influence ionoregulatory capacity, but age and genetic differences did not likely contribute to this size effect.     

The effect of salinity on growth and survival of juvenile black bream (Acanthopagrus butcheri)

Growth and survival of juvenile black bream (Acanthopagrus butcheri) were determined at salinities from 0 to 60 ppt (in 12-ppt increments) and from 0 to 12 ppt (in 4-ppt increments) in two separate trials of 6 and 4 months duration, respectively. Juvenile black bream were able to survive and grow at salinities ranging from freshwater (0 ppt) to 48 ppt. Osmotic stress was evident at 60 ppt, however, survival was not significantly affected. Fish reared at 24 ppt in trial 1 had a specific growth rate of 2.34±0.03%/day, a rate significantly higher only to those fish reared at 60 ppt (2.16±0.04%/day). Growth was greater at 24 ppt in association with the highest food intake and most efficient FCR. Although both food intake and FCR were not significantly higher than those obtained with fish reared at 12, 36 and 48 ppt, the combination of the two factors being optimised at 24 ppt lead to the greatest growth. Analysis of data from the second trial found no significant difference in the growth rate of black bream reared at salinities ranging from freshwater to 12 ppt, with SGR ranging from 1.92±0.05%/day to 2.05±0.02%/day. Variable results in freshwater between the two trials suggested that total hardness of freshwater may influence survival and/or an ontogenetic change in salinity tolerance may occur.     

Growth of Tiger Puffer, Takifugu rubripes, at Different Salinities

Two 12‐wk rearing experiments were conducted to examine the effect of rearing salinities of 10–35 ppt on the growth of 3‐ and 170‐g‐size tiger puffer, Takifugu rubripes. Fish were reared in a closed recirculation system without introducing fresh culture water at 23 C and were fed commercial pellet diet for tiger puffer twice or three times daily to apparent satiation each, almost everyday. Growth of 3‐g‐size fish seemed to increase with decreasing salinity; however, there were no significant differences in the specific growth rate and weight gain among treatments because of differences in initial body weight. Final body weight and length of fish reared at 10 ppt were significantly higher than those for fish reared at 30 ppt although initial sizes were similar. Differences were not found for the feed efficiency (FE) and daily feed consumption. Apparent relationships were not observed between salinity and blood characteristics or proximate compositions of muscle of the cultured fish. Differing from smaller fish, growth of 170‐g‐size fish tended to decrease with decreasing salinity from 30 to 10 ppt and with increasing salinity from 30 to 35 ppt. Similar trends for FE were observed.     

Patterns of oocyte development in natural habitat and captive <Emphasis Type="Italic">Salminus hilarii</Emphasis> Valenciennes, 1850 (Teleostei: Characidae)

Fecundity and oocyte development in Salminus hilarii female brood stock were analyzed with the aim of investigating the impact of migration impediment on oogenesis. Histological analyses of the ovaries were performed in adult females caught in two different environments—the Tietê River (natural) and captivity—and the gonadossomatic index, oocyte diameter and fecundity determined. Five germ cell development stages (oogonium, perinucleolar, cortical alveoli, vitellogenic, ripe) and two other structures (postovulatory follicles and atretic oocytes) were observed in females caught in the river. Captive animals lacked the ripe oocytes and postovulatory follicles and had a relatively higher number of atretic oocytes. Females in captivity are known to produce larger oocytes, and they release fewer eggs in each spawn (absolute fecundity) when compared with animals that are able to migrate. Our results suggest that the Tietê River is undergoing alterations which are being reflected in the reproductive performance of S. hilarii, mainly due to the presence of atretic oocytes in females caught in the river. The lack of postovulatory follicles and ripe oocytes in captive animals reveals that migratory impediment negatively impacts final oocyte maturation. However, the stage of maturation reached is adequate for ovulation induction with hormone manipulation.     

Salinity of the growth medium is important for production potential and nutritional value of white worms (Enchytraeus albidus Henle)

Enchytraeus albidus is a small euryhaline earthworm that is abundant in the supralittoral zone where it can be found in decaying seaweed. This species is readily mass‐cultured, and due to its relatively high contents of protein and polyunsaturated fatty acids (PUFAs), there has been a renewed interest in this species as an alternative source of valuable nutrients for fish feed. Here, we have investigated the influence of substrate salinity (0, 1, 2, 8, 15, 30 and 40 ppt) on reproduction, growth and nutrient content of worms. We found that intermediate salinities (8–15 ppt) of the substrate maximize biomass production as well as protein and PUFA contents of the worm tissue. In particular, we observed that the often used culturing of E. albidus in non‐saline growth medium resulted in low reproduction and biomass production. Increasing the salinity from 0 to 15 ppt resulted in a higher proportion of unsaturated fatty acids both in storage lipid and in membrane phospholipid fractions.     

Oocyte maturation and ovulation in the Atlantic halibut, Hippoglossus hippoglossus (L.), examined using ultrasonography

High resolution ultrasound scanning was tested as a non-invasive technique for monitoring oocyte maturation and ovulalion in the Atlantic halibut Hippoglossus hippoglossus (L.). Female broodstock halibut were examined using a 7.5-Mhz linear ultrasound transducer, prior to and during spawning. Representative images of halibut ovaries are presented and discussed. Individual oocytes of spawning females were discernible during oocyte final maturation, due to the large increase in volume caused by water uptake. The yolky (vitellogenic) cocytes of pre-spawning fish were more reflective to ultrasound than hydrating oocytes. The resulting differences in depth of ultrasound penetration permitted easy distinction of pre-spawning from spawning females. In addition, short-term changes in the depth of ultrasound penetration were observed during repeated scanning of a spawning female, owing to progressive hydration of the oocyte batch destined for ovulation. Although of similar diameter, hydrating oocytes in the ovarian tissue could be discerned from eggs in the ovarian lumen because of the different acoustic properties of the surrounding media. The findings of the present study are considered promising for the future routine use of ultrasonography in halibut broodstock management.