Ammonia and pH effects on some metabolic parameters and gill histology of silver catfish, Rhamdia quelen (Heptapteridae)

The aim of the present study was to assess the effect of ammonia exposure at different pH on survivorship and metabolic parameters in the liver, muscle and gill histology of silver catfish (Rhamdia quelen). The 96 h-LC50 of un-ionized ammonia (mg L^− 1) at pH 6.0, 7.5 and 8.2 were: 0.44 (C.I. 0.38–0.49), 1.45 (C.I. 1.25–1.65) and 2.09 (C.I. 1.85–2.36), respectively. Survival of juveniles exposed to different ammonia levels was altered by pH, and fish exposed to all ammonia levels and different pH showed muscle glucose, muscle and liver glycogen reduction. Liver glucose and muscle and liver lactate levels increased in all fish exposed to ammonia as compared to the control. Exposure to waterborne ammonia increased total ammonia levels in both tissues and also induced gill epithelium damages such as lamellar fusion and edema as compared with controls at different pH. Silver catfish exposed to pH 6.0 and different NH₃ levels presented significantly higher hepatic glucose and protein levels when compared to those maintained at low NH₃ levels. Juveniles exposed to NH₃ levels at pH 7.5 and 8.2 showed lower hepatic protein levels compared to those maintained at low NH₃ levels. These parameters are indicative of pH dependence on ammonia toxicity in silver catfish. The metabolic parameters and gill histology may be used as early indicators of ammonia toxicity in silver catfish.     

Dopamine modulates the physiological response of the tiger shrimp Penaeus monodon

Levels of glucose, lactate, pO₂, pCO₂, HCO₃⁻, TCO₂, Na⁺, K⁺, Cl⁻, protein, and oxyhemocyanin in the hemolymph and its osmolality and pH were measured when tiger shrimp, Penaeus monodon (13.5 ± 1.5 g body weight), were individually injected with saline or dopamine at 10^− 8, 10^− 7, or 10^− 6 mol shrimp^− 1. Results showed that hemolymph glucose, lactate, pCO₂, HCO₃⁻, and TCO₂ values increased from 2 to 4 h; hemolymph osmolality, Na⁺, and total protein had increased at 2 h; and hemolymph K⁺ decreased from 2 to 8 h after the dopamine injection. All physiological parameters returned to the control values 4–16 h after receiving dopamine. The dopamine injection also significantly decreased the oxyhemocyanin/protein ratio of P. monodon which occurred at 2 h, resulting from an elevation of hemolymph protein and a slight decrease of oxyhemocyanin. These results suggest that stress-inducing dopamine caused a transient period of modulation of energy metabolism, osmoregulation, respiration, and the acid–base balance in P. monodon in adapting to this environmental stress.     

Heritability estimates for processing and quality traits in common carp (Cyprinus carpio L.) using a molecular pedigree

Heritability estimates with a microsatellite parentage assignment based pedigree are of special interest in common carp cultured under traditional pond conditions. This method reduces common environment effects as all families can be grown immediately after hatching mixed in the same pond. We applied this method to study genetics of growth and processing traits in common carp at market size (1.5 kg and more). The experimental progeny was established by crossing 147 two-year old males and 8 females (six to eight-year old) of Hungarian synthetic mirror carp population. The fish grew up through three vegetation seasons and at the end a sample of 331 fish was examined for biometrical traits (standard length, body weight, relative head length, relative body height, relative body width), percent fat and processing traits (% processed body, % fillets with skin, % fillets without skin). It was shown that sex had a significant effect on most traits: females were larger and fatter than males, and they had both higher percent processed body and percent fillet with skin. Standard length, body weight, percent fat and relative head length had a high heritability (> 0.5), while relative body height, relative body width, percent processed body and fillet yields had a medium heritability (0.2–0.5). We found relatively high positive genetic correlations between body size (standard length and body weight) and percent fat (0.71 and 0.59, respectively), favourable genetic correlations between body size and percent processed body (0.69 for standard length and 0.74 for body weight) and between body size and fillet yields (0.50–0.77). Genetic correlations between body size and body shape (relative head length, relative body height and relative body width) were weak to moderate, thus selection for better growth should have little impact towards more rotund shape. Relative head length had strong negative correlation (− 0.7 to − 0.9) with percent fat, percent processed body and percent fillet yields. This means that indirect selection for reduced relative head length should be effective in improving of fillet yield.     

Hydroacoustic in situ target strength of smelt (Osmerus eperlanus (L.))

Dependence of the in situ acoustic target strength (TS) of smelt (Osmerus eperlanus (L.)) on the total fish length (L) was estimated from TS measurements made with a split-beam 120 kHz echo sounder with reference to fish length–frequency distributions in trawl catches. The studies were conducted in the eutrophic and turbid Lake Hiidenvesi in southern Finland. Smelt dominated the catches-in-number with proportions at different sampling stations ranging from 91.8 to 100%. A TS–length equation of the form TS = alog L − b was fitted through the modes in the TS-distributions and in the respective fish length distributions. The estimated model (±parameter standard error) was TS = 23.4 (±2.24)log₁₀ L − 68.7 (±1.82). With a pre-defined slope of 20, the fitted equation was TS = 20 log₁₀ L − 65.9 (±0.23). The estimated day–night difference in TS–length relationship was not significant (t-test, p = 0.37).     

Spawning of ocean pout (Macrozoarces americanus L.): evidence in favour of internal fertilization of eggs

Sperm physiology, in vivo artificial insemination and spawning of the ocean pout (Macrozoarces americanus L.), a marine bottom fish, were studied. Milt was collected from the reproductive tract of mature males by suction using a catheter. The uncontaminated milt, having a very low sperm concentration, contains highly motile spermatozoa and sperm motility was retained in vitro at 4 °C for at least 24 h in both seminal plasma and ovarian slime collected from the oviduct of pre-spawning females. Instead of activating sperm, dilution in sea water instantly immobilized the spermatozoa of ocean pout. Osmolarity and pH of ocean pout seminal plasma were in the ranges 365–406 mOsM and 7.2–7.5, respectively. A study of the ionic composition of ocean pout seminal plasma demonstrated the presence of various ions including Na⁺, K⁺, Ca²⁺, Mg²⁺, and Cl⁻, with a remarkably lower K⁺ concentration compared to that from other fish species. Since injections of milt containing motile sperm into the ovaries of pre-spawning females, which spawned in the absence of males, yielded fertilized ocean pout eggs, it is concluded that the ocean pout exhibits internal fertilization. The larvae hatched after 3 months of egg incubation in ambient sea water (9–10 °C). With proper timing of in vivo artificial insemination of mature females, fertilized ocean pout eggs can be obtained from fish reared in captivity.     

Safety and efficacy of emamectin benzoate administered in-feed to Atlantic salmon, Salmo salar L., smolts in freshwater, as a preventative treatment against infestations of sea lice, Lepeophtheirus salmonis (Krøyer)

The safety and efficacy of emamectin benzoate, administered in-feed to Atlantic salmon smolts, Salmo salar L., held in freshwater, was evaluated as a preventative treatment against sea lice, Lepeophtheirus salmonis, following transfer of fish to seawater.

In the safety study, salmon smolts held in freshwater were fed with diets containing emamectin benzoate at nominal doses of 0 (control), 50 (recommended dose) and 250 (5× recommended dose) μg kg⁻¹ fish day⁻¹ for 7 days (days 0–6). Actual dose rates, based on measured concentrations of emamectin benzoate in feed, differences in fish weight, and feed consumed, were 0, 54, and 272 μg kg⁻¹ day⁻¹, respectively. On day 9, fish were transferred to seawater and observed for 14 days. No differences in feeding response, coordination, behaviour, gross and histological appearance were observed between control fish and those that received 54 μg kg⁻¹ day⁻¹. Among smolts that received 272 μg kg⁻¹ day⁻¹, approximately 50% exhibited darker coloration, and one fish (1%) exhibited uncoordinated swimming behaviour. No pathognomonic signs of emamectin benzoate toxicity were identified.

In the efficacy study, smolts held in freshwater were fed an unmedicated ration (control group) or emamectin benzoate at 50 μg kg⁻¹ day⁻¹ (treated group) for 7 days (days 0–6). On day 9, fish were re-distributed to eight seawater tanks, each holding 30 control and 30 treated fish. On days 28, 56, 77 and 109, respectively, control and treated fish in two tanks were challenged with L. salmonis copepodites. When lice in each group reached chalimus stage IV, fish were sampled and the numbers of lice were recorded. Fish challenged at day 109 were sampled for the second time when lice were at the adult stage. Efficacy was calculated as the reduction in the mean number of lice on treated fish relative to the mean on control fish. Treatment with emamectin benzoate resulted in an efficacy of 85.0–99.8% in fish challenged at days 28–77, from the start of treatment, and lice counts were significantly lower (P<0.001) on treated fish than on controls. When fish challenged at day 109 were sampled at day 128, efficacy was 44.3%, but survival of chalimus to adult lice on treated fish was lower, and at day 159, efficacy had increased to 73%. These results demonstrate that treatment of salmon smolts with emamectin benzoate in freshwater was well tolerated and highly effective in preventing sea lice infestation following transfer of fish to seawater.     

Intensive polyculture of common carp (Cyprinus carpio L.) and herbivorous fish [silver carp, Hypophthalmichthys molitrix (Val.), and grass carp, Ctenopharyngodon idella (Val.)]

Intensive polyculture of common carp and herbivorous fish (silver carp and grass carp) at high stocking densities and with intensive feeding with fodder and mineral and organic fertilization was carried out without mechanized aeration and automated feeding.Two stocking patterns were tested. The first, in Pond 1 (with an area of 2.5 ha), had the following stocking rates: one-year carp (C₁), 8000 fish/ha; one-year silver carp (S₁), 1500 fish/ha; and one-year grass carp (G₁), 200 fish/ha. The yield was 6292 kg/ha of standard consumer fish (C₁₊ 743 g, S₁₊ 944 g, and G₁₊ 1000 g) at a relatively low food conversion ratio (2.24) and high survival of all fish. Within the total yield, the two-year carp contributed 76%, the silver carp 21%, and the grass carp 3%. The second stocking pattern, in Pond 2 (0.25 ha), had the following rates: C₁, 8500 fish/ha; S₁, 2000 fish/ha; and G₁, 200 fish/ha, and produced a still higher total yield (6629 kg/ha) at a relatively low food conversion ratio (2.10). The two-year carp in Pond 2 contributed 88% of the total yield, and the herbivorous fish only 12% even though they accounted for 20% of the stock.Our results (Pond 2) are compared with other record yield experiments under similar conditions but including tilapia. At a total stocking rate of 10 700 fish/ha and without participation of tilapia, over a period of 186 days (as against 126 days in the other experiment), our experiments gave a total yield which was 347 kg/ha higher at a food coefficient 15% lower. The growth period was 60 days longer in our experiments but it included April and October when water temperature is much below the optimum and only 4% of the total amount of fodder was assimilated.     

Appetite, gut transit, oxygen uptake and nitrogen excretion in captive Atlantic halibut, Hippoglossus hippoglossus L., and lemon sole, Microstomus kitt (Walbaum)

Atlantic halibut, Hippoglossus hippoglossus L., eat larger satiation meals (mean 11.7% body weight) than lemon sole, Microstomus kitt (Walbaum), (2.6% body weight). Total gut clearance time was about 120 h for halibut and 72 h for lemon sole. There are marked differences in feeding behaviour between the two species; halibut feed in midwater and require several body lengths of approach swimming before taking large items of food, while lemon sole eat only off the bottom. In shared tanks, no aggressive interaction was observed. A duoculture system holding small numbers of lemon sole with the more valuable halibut is recommended as a means of minimizing food waste and tank fouling. Oxygen uptakes of 0.07-0.11 ml O₂ g fish wt⁻¹ h⁻¹ (depending on nutritional state) were recorded for the two species. Ammonia nitrogen outputs were also similar. Starved halibut excreted 2.32 μg N g⁻¹ h⁻¹, fed animals 5.08 μg N g⁻¹ h⁻¹. The corresponding values for lemon sole were 3.26 μg N g⁻¹ h⁻¹ and 6.37 μg N g⁻¹ h⁻¹, respectively.     

Intensive polyculture of common carp,Cyprinus carpio L., silver carp,Hypophthalmichthys molitrix (Val.) and black buffalo,Ictiobus niger (Raf.)

Intensive polyculture was carried out with common carp, silver carp and black buffalo fed a mixture of 60% pellets (with protein level of 25%) and 40% barley, along with mineral and organic fertilization. Two stocking patterns were tested. From pattern I, in pond 1 (total stocking density 4600 fish/ha), the yield of fish after 180 days was 3538 kg/ha, with standard weight (above 500 g), at a food conversion ratio of 2.36. From pattern II, in pond 2 (total stocking density 9200 fish/ha), the respective figures were 5022 kg/ha and 2.56.An economic comparison of several polyculture trials showed that buffalo fish (bigmouth and black) at a stocking rate of 2000 fish/ha, grown with common carp (2500 fish/ha) and silver carp (1200–1500 fish/ha), gave the best breeding and economic results (a profit of 962 levs/ha or 0.27 lev/kg). The buffalo fish have excellent organoleptic properties and are considered a delicacy favoured on the Bulgarian market.     

Furunculosis in Atlantic salmon (Salmo salar L.) is not readily controllable by bacteriophage therapy

The potential of bacteriophage therapy to control bacterial disease in farmed fish was tested using, as an example, furunculosis of Atlantic salmon, caused by Aeromonas salmonicida subsp. salmonicida.

In vivo testing with Atlantic salmon and rainbow trout (Oncorhynchus mykiss Walbaum) showed no adverse effects, with bacteriophage generally cleared within 96 h of administration by either intraperitoneal (i.p.) injection or oral in-feed.

Juvenile Atlantic salmon were administered a combination of bacteriophage O, R and B (1.9 × 10⁸ pfu fish^− 1) by i.p. injection, after they had been challenged with A. salmonicida subsp. salmonicida 78027, also by i.p. injection. The fish that were injected with bacteriophage immediately after challenge died at a significantly slower rate then those that were either not treated with bacteriophage, or treated 24 h post-challenge. However, the end result (100% mortality) was not affected.

In further experiments the effects of oral (1.88 × 10⁵ pfu g^− 1 fish^− 1 daily for 30 days), bath (1.04 × 10⁵ ml^− 1 daily for 30 days) and i.p. (6.25 × 10⁷ pfu fish^− 1) phage treatment to control furunculosis in experimentally infected Atlantic salmon were compared with antibiotherapy (treatment with 10 mg kg^− 1 bw^− 1 day^− 1 oxolinic acid for 10 days), using an indirect cohabitation challenge. No protection was offered by any of the bacteriophage treatments, compared to the positive challenge group, although significant protection was offered by the oxolinic acid treatment. Analysis of samples taken from the trials demonstrated that bacteriophage were correctly administered to the fish and, on occasion, were isolated from fish that had succumbed to furunculosis. It was also shown that bacteriophage resistant A. salmonicida subsp. salmonicida isolates could be recovered from mortalities in all the treatment groups.

The results suggest that, although there were no safety problems associated with the approach, furunculosis in Atlantic salmon is not readily controllable by application of bacteriophage.     

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

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

The influence of feeding history on the acute stress response of common carp (Cyprinus carpio)

The effect of an 80-day maintenance-feeding period on the acute stress response of common carp, Cyprinus carpio, to net confinement was determined. Fish were raised on an optimal feeding level of 20 g food/kg of metabolic fish weight per day (g/kg^0.8 day⁻¹) until 124 days post-hatch (dph). Feeding in group one (L>H) was then reduced to maintenance levels, i.e. 5 g/kg^0.8 day⁻¹ until 204 dph, when the feeding was again increased to 20 g/kg^0.8 day⁻¹. In group two (H>L), the feeding level was reduced from 20 to 5 g/kg^0.8 day⁻¹ on day 146. All fish were sampled at 226 dph. Food ration had significant effects on the growth rate and food conversion values with fish fed the high ration performing better than those on a lower level. Prior to the application of the stressor, only plasma levels of triglycerides were lower in fish fed a low food ration (H>L). Feeding history influenced the onset of the stress response with stressor-induced elevations of plasma cortisol, glucose and free fatty acids being higher in fish fed a high ration compared with those fed a low ration prior to sampling. These results suggest that feeding history through modification of the energy reserves can influence the onset of the acute stress response.     

Cyclical fluctuations in ammonia and nitrite-nitrogen resulting from the feeding of turbot, scophthalmus maximus (L.), in recirculating systems

Daily and weekly cyclical fluctuations in ammonia (NH₄---N μg litre⁻¹) and nitrite---nitrogen (NO₂---N μg litre⁻¹) were shown to occur in marine recirculating systems containing juvenile turbot, Scophthalmus maximus (L.). A stepwise multivariant regression analysis, computed for the prefeeding morning base levels, showed that ammonia---nitrogen concentration in the fish tanks was related to inflow ammonia---nitrogen concentration, water flow rate, water temperature, mean fish weight and fish biomass. Influent ammonia concentration and water flow rate were by far the most important variables accounting for 68·0% and 16·7% of the variation in the regression respectively.

The fluctuations, which occurred at concentrations thought to be tolerated by the fish, were attributed to the amount and timing of feeding. The significance of the existence of such fluctuations was discussed together with the importance of their amplitude and frequency.     

Gut contents of silver carp, Hypophthalmichthys molitrix, and the disruption of a centric diatom, Cyclotella, on passage through the esophagus and intestine

The digestibility of algae by stomachless filter-feeding fish has been debated for decades. Results from gut contents and digestive enzyme analysis suggest poor utilization, while the measurement of food assimilation using radiolabeled isotope techniques indicate reasonable assimilation efficiency. Phytoplankton in the gut contents of the planktivorous filter-feeding silver carp were studied during March–May. The fish were cultured in a large net cage in a shallow, hypertrophic subtropical Chinese lake, Lake Donghu. In terms of biomass, the dominant phytoplankton in the fore-gut contents were Cyclotella (average 77.8%, range 69.7–93.5%) and Cryptomonas (average 9.57%, range 0–20.4%). The Ivlev's electivity index E of silver carp was much higher for Cyclotella (1.54) than for Cryptomonas (0.56). The majority of the phytoplankton found in the intestines of silver carp were 8–20 μm, but they were also able to collect particles as small as 4.5–10 μm, smaller than their filtering net meshes, suggesting that the secretion of mucus may play an important role in collecting such small particles. We conclude that disruption of cell walls is principally by the pharyngeal teeth, explaining previous contradictory conclusions.     

Lipid absorption in different segments of the gastrointestinal tract of Atlantic salmon (Salmo salar L.)

Gastrointestinal lipid absorption in Atlantic salmon was studied following force-feeding with a standard grower diet to which isotope-labelled free fatty acid (FFA) [1-¹⁴C]-decanoic acid (¹⁴C-10:0) and triacylglycerol (TAG) [9,10-³H(N)]-triolein (trioleic acid; ³H-18:1) were added. Following force-feeding, the 15 fish, with an average weight of 704 g, were kept in five circular tanks, three fish per tank, supplied with seawater (salinity, 32 g l⁻¹). At 3, 6, 12, 18 and 48 h, the fish from one tank were killed, and the gastrointestinal (GI) tract was removed and divided into six segments: stomach (ST), anterior, mid, and distal pyloric ceca (PCA, PCM, and PCD, respectively), mid intestine (MI), and distal intestine (DI). The levels of radioactivity (dpm [disintegrations per minute] μg⁻¹ tissue) from the ¹⁴C-10:0 and the ³H-18:1 substrates were measured in tissue samples from each segment, allowing an estimation of the amounts of fatty acid substrates absorbed from the lumen into the intestinal cells. The highest levels of radioactivity from both fatty acid substrates were measured in tissue from pyloric ceca (PC) 12 to 18 h after force-feeding. The level of radioactivity from the ¹⁴C-10:0 substrate was significantly higher in the PCA than in the PCM and PCD, while there was no clear difference in the level of radioactivity from the ³H-18:1 substrate measured among the different PC tissue segments and the level measured in MI. The level of radioactivity in tissue from DI was negligible to low for both fatty acid substrates. Total lipid accumulation in tissue from PCM, MI and DI was quantified morphometrically at each sampling point using light microscopy and image analysis. These results confirmed the findings from the isotope study, where the highest levels of fat were present at 12 and 18 h following force-feeding. In conclusion, PC was the most important site for fatty acid absorption in the GI tract of Atlantic salmon.     

Dietary impact of algal and artificial diets, fed at different feeding rations, on the growth and fatty acid composition of Tapes philippinarum (L.) spat

The effect of lipid supplementation and algal ration on growth and fatty acid composition of juvenile clams, Tapes philippinarum was investigated. A diet of Tetraselmis suecica was supplemented with a lipid emulsion and fed at a daily ration of 0.5, 1.0 and 1.5% WW DW⁻¹ day⁻¹. A mixed algal diet of Isochrysis galbana (clone T-Iso) and T. suecica (1:1 on DW basis) and starvation functioned as positive and negative control treatments, respectively. T. suecica, which contains eicosapentaenoic acid (20:5n−3) but no docosahexaenoic acid (22:6n−3), was supplemented with 50% (% of algal dry weight) of a 22:6n−3-rich emulsion. Higher growth rates of T. philippinarum were associated with higher T. suecica feeding rations. Lipid supplementation resulted in significantly better growth rates of T. philippinarum at a T. suecica ration of 0.5, 1.0 and 1.5% WW DW⁻¹ day⁻¹. The growth promoting effect of lipid supplementation was significantly reduced during 4 consecutive weeks or by increasing the algal feeding ration. Increasing T. suecica rations were associated with a decrease in the % composition of 22:6n−3 while the 20:5n−3% composition was hardly affected in the total lipids of T. philippinarum. On the contrary, the absolute concentration of 22:6n−3 (μg per gram dry weight, μg g DW⁻¹) in the total lipids of T. philippinarum was similar at all T. suecica rations while the 20:5n−3 concentration significantly increased. The major effect of lipid supplementation on the fatty acid composition of the clams, was a significant increase of the 22:6n−3% composition in the total lipids, polar lipids and triglycerides. In contrast with 20:5n−3, the effect of lipid supplementation on the absolute 22:6n−3 content (μg g DW⁻¹) was more pronounced at a higher T. suecica ration. After a starvation period of 4 weeks, the concentration (μg g DW⁻¹) of all fatty acids was significantly reduced, except the n−7 monoenoic fatty acids (MUFA) and the non-methylene interrupted dienoic fatty acids (NMID). The concentration (μg g DW⁻¹) of the NMID in starved clams and clams fed different diets was similar to the concentration in the initial sample. This indicated that growing spat was actively synthesising NMID and they were selectively retained in unfed animals. The results indicated that either dietary lipid or 22:6n−3 was a growth-limiting factor when T. suecica was fed at a ration of 0.5, 1 and 1.5% WW DW⁻¹ day⁻¹. A selective retention in starved clams and a preferential accumulation in fed clams was observed for 22:6n−3 but not for 20:5n−3 which indicated a greater importance of 22:6n−3 compared to 20:5n−3.     

Effects of dietary medium-chain triacylglycerols (tricaprylin and tricaproin) and phospholipid supply on survival, growth and lipid metabolism in common carp (Cyprinus carpio L.) larvae

The present study investigated the interaction of dietary medium-chain fatty acids (MCFA) and phospholipids (PL) on survival, growth and lipid metabolism in common carp larvae. Nine diets based on casein and dextrin and with a variable lipid part were tested in triplicate for 22 days post first feeding. The 3×3 design consisted of three triacylglycerols (3% of diet) combined with three different lipid supplements. Tested triacylglycerols were triolein (TOL), tricaprylin (TC8) and tricaproin (TC6), and lipid supplements were 2% soybean oil (low-fat diets without PL), 2% soybean lecithin (low-fat diets with 2% PL) or both 2% soybean lecithin and 6% TOL (high-fat diets with 2% PL).

In the first step, both TC6 and TC8 resulted in improved survival and growth rates compared to TOL, irrespective of the PL supply. In the second step, TC8 decreased survival and growth rates, whereas the difference between TC6 and TOL became less. Histological signs of impaired intestinal absorption of neutral lipids were evidenced in larvae fed TOL without PL and also in high-fat diets with 2% PL. The latter diets also resulted in poorer growth rates compared to low-fat diets with 2% PL. These results suggest that the quantitative PL requirement of larvae increases as the dietary level of long-chain triacylglycerols increases. Larvae fed TC6 or TC8 showed enlarged liver and hepatocyte volume and a decreased level of body neutral lipids. Based on β-hydroxybutyrate (β-HBA) measurements in whole larvae, TC8 was found to be more ketogenic than TC6. TC6 and TC8 affected differently the fatty acid profile of larval body neutral lipids. TC6 did not induce the appearance of MCFA, whereas TC8 feeding resulted in a low level of 8:0 and relatively high levels of 10:0 (3.8% of total fatty acids). Neither 8:0 nor 10:0 were found in larval polar lipids.

This study confirmed the essentiality of PL in common carp larval diets and underlines differences in the utilization of TC6 and TC8, which both initially stimulate growth during the first week, but only temporarily in the case of TC8.     

Influence of broodstock gilthead seabream (Sparus aurata L.) dietary fatty acids on egg quality and egg fatty acid composition throughout the spawning season

The influence of broodstock dietary lipids on egg quality and egg fatty acid composition throughout the spawning season of gilthead seabream was investigated. For this purpose, the fish were fed for 7 months either a control diet (diet C) or a diet deficient in n−3 highly unsaturated fatty acids (n−3 HUFA) but rich in both oleic (18:1n−9) and linolenic (18:3n−3) acids (diet D). Eggs spawned by both groups of fish were sampled at the beginning, middle and end of the spawning season and the fatty acid composition of their neutral (NL) and polar lipids (PL) determined. In the early season, percentages of fertilized and hatched eggs, relative proportions of NL and PL as well as their fatty acid compositions, were not affected by the lipid composition of the broodstock diet. However, the eggs spawned during the middle and late seasons showed marked differences among the two groups of fish, clearly reflecting the influence of dietary fatty acids. This influence was more evident in the neutral lipid fraction than in the polar lipids. No correlation was found between the number of buoyant eggs and eicosapentaenoic (20:5n−3, EPA), docosahexaenoic (22:6n−3, DHA) fatty acids or total n−3 HUFA contents in egg phospholipids. However, a negative correlation was detected when percentages of fertilized eggs were compared with the levels of 18:1n−9, 18:3n−3 and with the ratio 18:1n−9/n−3 HUFA present in the phospholipids. Our results indicate the importance of maintaining not only the level of n−3 HUFA in egg membrane phospholipids, but also the balance between n−3 HUFA and other fatty acids such as 18:1n−9 and 18:3n−3, in order to obtain a high spawning quality.     

Age, growth, and reproduction of silky sharks, Carcharhinus falciformis, in northeastern Taiwan waters

To examine the age, growth and reproduction of silky sharks, Carcharhinus falciformis, in the waters off northeastern Taiwan, 469 specimens (213 females and 256 males) were collected from August 2000 to January 2002 at the Nanfanao fish market, northeastern Taiwan. The relationship between body weight (W) and total length (TL) for both sexes combined was expressed as: W = 2.92 × 10⁻⁶ TL^3.15 (n = 469, p < 0.01). The relationship between TL and vertebral centrum radius (R) for both sexes combined data was estimated as: TL = 25.979 + 18.197R (n = 250, p < 0.01). Growth bands (including translucent and opaque zones) in precaudal vertebrae formed once a year between December and January and were counted up to 11 and 14 for females and males, respectively. The von Bertalanffy growth function (VBGF) was used to model the observed length at age data. The sexes combined VBGF predicted an asymptotic length (L_∞) = 332.0 cm TL, growth coefficient (k) = 0.0838 year⁻¹, age at zero length (t₀) = −2.761 year (n = 250, p < 0.01). Size at 50% maturity for males was estimated to be 212.5 cm based on the logistic curve, which corresponded to 9.3 years. Females matured at 210–220 cm, which correspond to 9.2–10.2 years. The length at birth was estimated to be 63.5–75.5 cm TL. The number of embryos per litter was 8–10 and sex ratio of embryos was 1:1.     

A simple test to estimate the salinity resistance of fish with specific application to O. niloticus and S. melanotheron

In order to develop a simple and accurate index of the salinity resistance of tilapia, batches of 10 juveniles (5 to 20 g) of two different species Oreochromis niloticus and Sarotherodon melanotheron reared in freshwater were subjected to gradual increases in salinity until 100% mortality. Seven daily increments of salinity were tested with 4 replicates: 2, 4, 6, 8, 10, 12 and 14 g l⁻¹ day⁻¹, while control batches were kept in fresh water. The temperature was maintained at 27 °C. The concentration of oxygen, ammonia and the pH were not limiting factors. The mortality, monitored on a daily basis, appeared after 2–51 days and was spread out over 1–20 days, depending on the increment of salinity. The higher the daily rate in salinity increase, then the shorter the time lapse before total mortality occurred. The cumulative mortality as a function of salinity fit well with simple linear regressions. The criterion of the resistance to salinity was the index MLS (median lethal salinity) defined at each daily rate as the salinity at which 50% of fish died. For S. melanotheron, the mean MLS was 123.7±3.5 g l⁻¹ whatever the daily rate in salinity. For O. niloticus, the MLS was 46.3±3.4 g l⁻¹ for daily increases in salinity ranging from 2 to 8 g l⁻¹ day⁻¹ and decreased significantly (P<0.05) above this level. The MLS-8 g l⁻¹ day⁻¹ ,which takes into account the full capacity of the fish to adapt to the increasing salinity, appeared to be a simple, optimized and efficient criterion for assessing the resistance to salinity for O. niloticus and S. melanotheron. This criterion can be a useful tool for ranking the different parental strains and hybrids of different genus and species of tilapia used in programmes of genetic selection for growth and salinity tolerance.