Increased dietary phosphorous prevents vertebral deformities in triploid Atlantic salmon (Salmo salar L.)

Triplicate groups of triploid and diploid Atlantic salmon were fed diets with a low (LP, total P: 7.1 g kg^?1), medium (MP, total P: 9.4 g kg^?1) or high (HP, total P: 16.3 g kg^?1) phosphorous (P) level from first feeding (0.18 g) to transfer to sea water (~50 g, duration: 203 days) and subsequently fed a commercial diet in sea water for 426 days (~3 kg). This study examined the short‐ and long‐term effects of dietary P on freshwater performance (mortality, growth), vertebral deformities (radiology), bone cell activity (ALP and TRACP enzyme activity in vertebrae and scales, and fgf23, bgp and igf‐I relative gene expression in vertebrae), bone mineralization (ash content) and some parameters related to fish condition (heart and liver size). Irrespective of ploidy, at seawater transfer, fish fed the MP diet had significantly highest length and weight and those fed the LP diet significantly lowest length and weight, while those fed the HP diet had intermediate lengths and weights. Increased dietary phosphorus reduced deformities in both ploidies at seawater transfer; however, triploids fed the LP and MP diets had more deformities than diploids fed the respective diets, while there was no ploidy effect observed for fish fed the HP diet. The vertebral bone ash content at seawater transfer was significantly higher in diploids than in triploids when fed the MP diet only. Alkaline phosphatase (ALP) and tartrate‐resistant acid phosphatase (TRACP) enzyme activities and relative gene expression of bone hormones involved in metabolism of plasma phosphate (fgf23) and bone growth (bgp) were not affected by ploidy at seawater transfer, but by dietary P level; LP increased ALP activity and reduced TRACP activity and fgf23 and bgp expression levels in vertebral bone. In scales, LP increased both ALP and TRACP activity. At the termination of the seawater period, the group‐wise pattern in occurrence of vertebral deformities was the same as at seawater transfer. The present results on mortality, growth, bone mineralization and development of skeletal deformities all demonstrate that triploids have a higher P requirement than diploids in fresh water. This study shows that an optimalization of P nutrition for triploid Atlantic salmon can improve health and welfare and reduce down‐grading of triploid salmon.     

Allozyme heterozygosity and development in Atlantic salmon, Salmo salar

The development of Atlantic salmon (Salmo salar) parr was monitored during the first year in a fresh-water hatchery environment to test whether multilocus heterozygosity (computed for six allozyme loci: _s AAT-4^*, IDDH-1^*, IDDH-2^*, IDPH-3^*, _s MDH-3,4^* and_m MEP-2^*) is associated with growth (body mass, fork length) and maturity status (mature male parr). The length frequency distribution of population was unimodal in September, but was clearly bimodal by December. UMG fish tended to be more heterozygous and had higher growth rates than LMG fish. The heterozygosity of UMG fish and mature male parr (MMP) did not differ significantly but MMP showed lower growth rates than UMG. There were positive correlations between the number of heterozygotic loci and the fork length, weight or growth rates of individuals.     

Growth performance and organ development in Atlantic salmon, Salmo salar L. parr fed genetically modified (GM) soybean and maize

Although the inclusion of genetically modified (GM) plants in diets fed to fish is a contentious issue, there are few empirical data. The present study addressed nutritional value and potential risks of four maize types (two traditional and two GM maize varieties) and two soy types (one traditional and one Roundup Ready^® soy) included at moderate levels in diets fed to Atlantic salmon parr (initial mean weight ± SD; 0.21 ± 0.02 g) during the first 8 months of feeding (March to October), which included the parr‐smolt transformation. The GM‐maize varieties [Dekalb 1 (D1) and Pioneer 1 (P1)] were hybrids of traditional maize variants [Dekalb 2 (D2) and Pioneer 2 (P2)] and the GM maize MON810^®. Four maize diets, two soy diets and one Standard fishmeal‐based diet were formulated and fed to fish in triplicate. The maize diets were formulated with 121 g kg^?1 GM maize (P1 and D1) or 121 g kg^?1 of the traditional untransformed line (nGM‐maize, P2 and D2) and the soy diets were formulated with 125 g kg^?1 GM soy or 125 g kg^?1 of the traditional untransformed line (nGM‐soy), all of equivalent nutrient composition. All diets supported good growth and showed no evidence of diet‐related mortality. Based on samplings every 6th week, growth was within the normal range and at conclusion of the study body weight did not differ among any of the treatments (range 101–116 g). Besides minor differences on heptatosomatic index (HSI), plasma triacylglycerol (TAG) values and thermal growth coefficient (TGC), body composition, relative organ weights, plasma nutrient concentrations and enzyme activities did not vary among treatments at any sampling. The present findings indicate that the inclusion of GM plants at the given level in salmonid diets poses little, or no, adverse risk to the health of first feeding Atlantic salmon parr and promote normal growth. The paper presents the production related data of this feeding study. Results regarding structure and function of intestinal segments and intestinal organs are presented in Sanden et al.     

Maximum limits of organic and inorganic mercury in fish feed

The relatively high levels of mercury found in fish feeds might form a fish health and food safety risk. The present study aims to establish sublethal toxic threshold levels in fish and assess feed‐fillet transfer of dietary mercury. Atlantic salmon (Salmo salar L.) parr were fed for 4 months on fish meal‐based diets supplemented with mercuric chloride (0, 0.1, 1, 10 or 100 mg Hg kg^?1 dry weight (DW)) or methylmercuric chloride (0, 0.1, 0.5, 5 or 10 mg MeHg kg^?1 DW). At the end of the experiment, dietary inorganic mercury mainly accumulated in intestine (80% of body burden) and assimilation was low (6%). In contrast, methylmercury readily accumulated in internal organs and muscle (80% of body burden) and had a relatively high assimilation (23%). Highest accumulation of dietary inorganic mercury was observed in the gut and kidney. Fish fed 10 mg Hg kg^?1 had an early (after 2 months) significant increase in renal metallothionein (MT) level and intestinal cell proliferation, followed by intestinal pathological conditions after 4 months of exposure. At 100 mg Hg kg^?1, intestinal and renal function were reduced as seen from the significantly reduced protein and glycogen digestibility and increased plasma creatinine levels. For dietary methylmercury (MeHg), highest accumulation was found in blood and muscle. Intestinal cell proliferation and liver MT significantly increased at 5 mg MeHg kg^?1 after 2 months of exposure. At the end of the experiment, blood haematology was significantly affected in fish fed 5 mg MeHg kg^?1 and these fish exceeded the current food safety limit for mercury. Tissue MT induction and intestinal cell proliferation appeared to be useful and quantifiable early indicators of toxic mercury exposures. Based on the absence of induction of these early biological markers such as MT and cell proliferation, nonobserved effect levels (NOELs) could be set to 0.5 mg Hg kg^?1 for dietary methylmercury and 1 mg Hg kg^?1 for inorganic mercury. Lowest observed effect levels (LOELs) levels could be set to 5 mg kg^?1 for methylmercury and 10 mg Hg kg^?1 for inorganic mercury.     

Polyunsaturated fatty acid metabolism in Atlantic salmon (Salmo salar) undergoing parr-smolt transformation and the effects of dietary linseed and rapeseed oils

Duplicate groups of Atlantic salmon parr were fed diets containing either fish oil (FO), rapeseed oil (RO), linseed oil (LO) or linseed oil supplemented with arachidonic acid (20:4n-6; AA) (LOA) from October (week 0) to seawater transfer in March (week 19). From March to July (weeks 20–34) all fish were fed a fish oil-containing diet. Fatty acyl desaturation and elongation activity in isolated hepatocytes incubated with [1-¹⁴C]18:3n-3 increased in all dietary groups, peaking in early March about one month prior to seawater transfer. Desaturation activities at their peak were significantly greater in fish fed the vegetable oils, particularly RO, compared to fish fed FO. Docosahexaenoic acid (22:6n-3:DHA) and AA in liver and gill polar lipids (PL) increased in all dietary groups during the freshwater phase whereas eicosapentaenoic acid (20:5n-3; EPA) increased greatly in all groups after seawater transfer. The AA/EPA ratio in tissue PL increased up to seawater transfer and then decreased after transfer. AA levels and the AA/EPA ratio in gill PL were generally higher in the LOA group. The levels of 18:3n-3 in muscle total lipid were increased significantly in the LO, LOA and, to a lesser extent, RO groups prior to transfer but were reduced to initial levels by the termination of the experiment (week 34). In contrast, 18:2n-6 in muscle total lipid was significantly increased after 18 weeks in fish fed the diets supplemented with RO and LO, and was significantly greater in the FO and RO groups at the termination of the experiment. Gill PGF production showed a large peak about two months after transfer to seawater. The production of total PGF post-transfer was significantly lower in fish previously fed the LOA diet. However, plasma chloride concentrations in fish subjected to a seawater challenge at 18 weeks were all lower in fish fed the diets with vegetable oils. This effect was significant in the case of fish receiving the diet with LOA, compared to those fed the diet containing FO. The present study showed that during parr-smolt transformation in Atlantic salmon there is a pre-adaptive increase in hepatocyte fatty acyl desaturation/elongation activities that is controlled primarily by environmental factors such as photoperiod and temperature but that can also be significantly modulated by diet. Feeding salmon parr diets supplemented with rapeseed or linseed oils prevented inhibition of the desaturase activities that is induced by feeding parr diets with fish oils and thus influenced the smoltification process by altering tissue PL fatty acid compositions and eicosanoid production. These effects, in turn, had a beneficial effect on the ability of the fish to osmoregulate and thus adapt to salinity changes.     

Natural food and artificial,dry starter diets: effects on growth and survival in intensively reared European grayling

Four artificial, dry feeds and two diets consisting of natural, live food were evaluated for use as starter diets in the intensive rearing of European grayling (Thymallus thymallus). Long-term effects of the different diets were also studied. The investigation was divided into two parts: a 14 day starter period and the subsequent 115 days, during which the long-term effects were studied. Diets differed between the different treatment groups during the initial 2 weeks, whereafter all fish were offered a single diet. At the end of the starter period, significant differences in weight (p<0.001) were obtained between natural (control) feed groups and three of the artificial feed groups. The highest growth rate (9.6% day^–1), and length increase (9 mm) as well as the lowest mortality rate (6.0%) were obtained with one of the artificial diets. Growth rate, length increase and mortality of grayling in the other diet groups varied from 3.6 to 7.7% day^–1, 5 to 7 mm and 7.0 to 9.1%, respectively. The switchover to a different dry diet resulted in an increase in mean daily mortality for all groups previously fed a dry diet. For the fish previously fed live food the mortality rate remained about the same. Groups of fish growing slowly during one period seemed to compensate for their initially low growth by increasing their growth rates during the next period. The highest mean daily mortality for all groups during the experiment was observed during the period with the highest water temperature. It was concluded that European grayling can be intensively reared during the start-feeding phase on artificial as well as natural diets, and that artificial dry diets can be used exclusively throughout the first summer.     

Evaluation of microalgae and industrial cheese whey as diets for Tapes decussatus (L.) seed: effects on water quality, growth, survival, condition and filtration rate

As reference diets for bivalve molluscs are missing, this study first evaluated the best mixed algal diet (Tetraselmis suecica Butch/Phaeodactylum tricornutum Bohlin) concentration for 4‐mm‐length Tapes decussatus (L.) clams. It also assessed the performance of an industrial by‐product (cheese whey) as a single diet or supplement for the best algal ration previously determined. Growth, survival, condition index and filtration rate were used to compare the different diets. Implications of rations tested on rearing water quality were also investigated, especially ammonium and heterotrophic bacteria levels. With a food concentration as algal dry weight per total seed live weight of 0.7% day^?1, seed for grow‐out (7 mm) could be obtained within 1 month. A mixture of 25% algal ration and 75% cheese whey gave the best performances. Artificial diets resulted in lower growth rates than live food (20 % to 40% of the best algal ration), but could be considered good maintenance rations in energetic terms. Using cheese whey, and without any algae, hatchery produced seed can be kept economically indoors for at least 30 days without loss in condition.     

Effects of dietary vitamin C on growth and parr-smolt transformation in Atlantic salmon, Salmo salar L.

Atlantic salmon (Salmo salar) parr were fed fish meal based pelleted diets supplemented with graded levels of ascorbate-2-monophosphate (AP), equivalent to 0, 20, 60 and 1000 mg ascorbic acid (AA) kg”¹ throughout smoltification on a continuous light regime from February to June. No differences were observed in growth rate and body length distribution between the dietary regimes. The condition factor and the hepatosomatic index were somewhat elevated in fish fed no vitamin C throughout the smoltification, which can reflect changes in lipid metabolism in fish with suboptimal vitamin C nutrition. Sea water challenge tests (exposure to sea water with salinity of 34 gL^?1 and ambient temperature for 24 h) performed monthly did not reveal differences attributed to the vitamin C status, as measured by mortality, serum chloride and cortisol concentrations, haematological parameters and liver and head kidney ascorbate concentrations after 24 h. Elevated serum cortisol concentrations most probably reflected stress in the challenge tests, and some lower concentrations in fish fed high vitamin C levels may indicate a certain stress-ameliorating effect. The present results do not, however, support the anticipation of increased requirement of dietary vitamin C above the minimum requirement during smoltification in Atlantic salmon.     

Cortisol-induced changes in oxygen consumption and ionic regulation in coastal cutthroat trout (Oncorhynchus clarki clarki) parr

The influence of cortisol on oxygen consumption and osmoregulatory variables was examined in coastal cutthroat trout (Oncorhynchus clarki clarki) parr kept in fresh water (FW) and transferred to seawater (SW). Intraperitoneal implants containing cortisol (50 g g^–1) in vegetable oil resulted in elevated plasma cortisol titres similar to those observed in fish following a 24h SW exposure. Cortisol treatment significantly increased the oxygen consumption and plasma glucose levels of trout in FW, consistent with the glucocorticoid role of cortisol. Cortisol treatment did not cause any changes in plasma ion concentrations or gill Na⁺,K⁺-ATPase activity in FW after 10 days. Cortisol-implanted fish exposed to SW for 24h showed slightly improved ion regulatory ability compare to non-implanted controls. The results of this study suggest that during SW transfer in juvenile salmonids, increases in cortisol may act as both a mineralocorticoid and a glucocorticoid, depending on the developmental state of the fish (e.g., smolt versus parr). Furthermore, the relative energetic costs of osmoregulation and that of the stress associated SW transfer cannot be discerned using whole-animal oxygen consumption rates.     

Morphological variability of Atlantic salmon Salmo salar and brown trout Salmo trutta in different river environments

Abstract – The morphologies of sympatric juvenile Atlantic salmon Salmo salar parr and brown trout Salmo trutta parr were compared between Irish rivers with contrasting hydraulic environments – a high‐gradient and a low‐gradient tributary from the River Barrow catchment, south‐east Ireland and a high‐gradient river from the Burrishoole catchment, west of Ireland. The two catchments differ markedly in mean annual precipitation (849.0 mm year^?1 and 1370.3 mm year^?1 for the Barrow and Burrishoole catchments, respectively). Parr of both species demonstrated morphological variation between and within catchments. Changes in metrics such as pectoral fin length, body depth and body length between and within catchments suggest that hydraulic forces were a major determinant of morphological variation. Both species from the Burrishoole catchment had relatively larger pectoral fins, longer heads, larger eyes and shallower bodies than conspecifics from the Barrow catchment. In rivers subject to frequent rainfall‐driven high discharges, such as those in the Burrishoole catchment, a more fusiform body and head shape coupled with larger pectoral fins may reduce the energetic cost of maintaining position in the water column, as well as increase stability and manoeuvrability. The larger eyes in both parr species in the Burrishoole catchment could further be a response to the lower visibility of the more turbid and peat‐stained waters or to the reduced prey availability. The results of this study demonstrate that local adaptation to flow conditions can yield morphologically distinct populations and that multiple species can exhibit parallel phenotypic responses to changing environmental conditions.     

Water temperature and dietary histidine affect cataract formation in Atlantic salmon (Salmo salar L.) diploid and triploid yearling smolt

The aim of the present study was to investigate cataract development in diploid (2N) and triploid (3N) Atlantic salmon smolts and post‐smolts at two water temperatures (10 and 16 °C) given diets with different histidine supplementation (LH, 10.4 and HH, 13.1 g kg^?1) before and after seawater transfer. In freshwater, a severe cataract outbreak was recorded in both ploidies reared at 16 °C. The cataract score was significantly higher in triploids compared to diploids, and the severity was lower in both ploidies fed the HH diet. The cataract development at 10 °C was minor. Low gill Na⁺, K⁺‐ATPase activity in fish reared at 16 °C before seawater transfer was followed by osmoregulatory stress with elevated plasma electrolyte concentrations and high mortality in sea water. Both diploids and triploids reared at 10 °C developed cataracts during the seawater period, with higher severities in triploids than diploids and a reduced severity in the fish fed the HH diet. The findings of this study demonstrate the importance of environmental conditions in the husbandry of Atlantic salmon, and particularly triploids, with regard to smoltification and adjusted diets to mitigate cataract development in fresh and sea water.     

Effects of dietary thyroid hormones on the red drum (Sciaenops ocellatus)

Four separate 8-week feeding trials were conducted to assess the effects of supplementing semipurified diets with either triiodothyronine (T₃) or thyroxine (T₄) at 0, 2, 10, and 50 mg/kg on growth and body composition of juvenile red drum (Sciaenops ocellatus) held in artificial brackish water (6‰) and artificial seawater (32‰). At both levels of salinity, increasing doses of T₃ resulted in fish with reduced weight gain, feed efficiency, condition factor (weight × 100/length³), and muscle ratio (muscle weight × 100/body weight), as well as a lighter body color. Significant (p < 0.05) effects of T₃ on the proximate composition of whole body, liver, and muscle were variable, generally reflecting decreased lipid and protein storage in liver and muscle, respectively. The two highest doses of T₃ given to seawater adapted fish increased survival. Dietary T₄ supplementation had no distinctive effects on appearance, growth or proximate body composition. These results indicate that whereas T₃ may function to regulate protein and lipid metabolism in red drum, dietary supplementation with T₃ leads to a hyperthyroidism-induced catabolic state. The elevated endogenous thyroid hormone levels found in fish fed optimal diets may thus adequately supply tissue needs during juvenile growth.     

Diets supplemented with microalgal biomass: effects on growth,survival and colouration of ornamental fish Hyphessobrycon eques (Steindacher 1882)

This study evaluated the effects of the addition of microalgae (Ankistrodesmus gracilis and Haematococcus pluvialis) to the fish diet in improving the growth and optimal pigmentation (red carotenoid) of Hyphessobrycon eques. The basal mixed diets consisted of a formulated diet, supplemented with dried microalgae biomass of A. gracilis (1.5 g kg^?1) and H. pluvialis (1.5 g kg^?1). The live food diets contained zooplankton was cultured in open ponds, associated with microalgae. All the microalgae were cultured in the laboratory. No mortality was observed with any experimental diets. Fish performance results showed significant differences (P < 0.05) between the basal diet (BD) and the live food diet. The higher weight and total length were observed with mixed diets (BD + H. pluvialis and BD + A. gracilis). The mixed diets promoted more intense values of chroma (Cab*), lightness (L*) and redness (a*) to H. eques. Diaphanosoma birgei (Cladocera) represented more than 32% of zooplankton ingested by ornamental fish in live food (zooplankton and zooplankton + microalgae), and Argyrodiaptomus furcatus (Copepoda) was the species most ingested by H. eques in live food dietary treatment zooplankton. The feeding behaviour observed in the laboratory as well as the food preferences of H. eques was dependent on the zooplankton composition present in the used open ponds. This study showed that diets with microalgae and zooplankton were able to enhance the pigmentation of H. eques, being a good tool to benefit the culture management of this species.     

The effect of protein levels on growth,postprandial excretion and tryptic activity of juvenile mullet Mugil platanus (Günther)

The objective of the present work was to determine the optimum dietary protein level for juvenile mullets. Five isocaloric diets were formulated to contain increasing levels (300, 350, 400, 450 and 500 g kg^?1) of crude protein (CP) corresponding to 18.7 MJ metabolizable energy kg^?1. All diets were tested in triplicate. Each experimental unit was composed of a 50 L tank with 50 juveniles (mean ± SE initial weight and length equal to 1.17 ± 0.02 g and 4.34 ± 0.03 cm respectively). Diets were offered five times a day until apparent satiation for 35 days. No significant difference (P>0.05) was observed in survival rate, feed efficiency and body composition between treatments. However, weight gain, feed consumption and specific growth rate were higher in fish fed the 350 g kg^?1 CP level than those fed the highest protein content diet (500 g kg^?1 CP). The amount of postprandial ammonia excreted by mullet was linearly related to protein intake. Intestinal tryptic activity was inversely proportional to the percentage of dietary CP. It is likely that diets containing <350 g kg^?1 CP will be needed for on‐growing mullet, especially when reared in ponds with abundant natural food.     

Feeding and growth of exercised and unexercised juvenile Atlantic salmon in freshwater,and performance after transfer to seawater

Juvenile Atlantic salmon (Salmo salar), selected from the upper modal group, were first held for 122 days (from December to May) in freshwater, and then for 49 days in seawater (34% salinity). In freshwater, the fish were exposed to either standing water or water currents corresponding to a swimming speed of 1 Bl s^–1. Natural light conditions prevailed throughout the whole experiment.In the freshwater phase, food intake, growth rate and condition factor of the fish increased with increasing daylength. Condition factors, however, decreased slightly during the last month of the freshwater period, suggesting that the fish underwent parr-smolt transformation. The growth rate and food intake of the fish subjected to standing water were approximately 15% lower than those of fish exposed to water currents.Transient suppression of appetite and growth, and an increase in plasma concentration of Cl^–, were seen after exposure of the fish to seawater. Within 28 days, all these parameters had been restored to pre-transfer levels. Previous exercise did not appear to have influenced the hypoosmoregulatory capacity of the fish. Growth rate and food intake observed during the seawater phase were similar, irrespective of the treatment experienced by the fish during freshwater rearing.     

Some behavioural and ecological factors affecting distribution,biomass and production of juvenile Atlantic salmon

Observations were made in an experimental stream tank (total area 14.7 m²) on juvenile Atlantic salmon, Salmo salar L (parr), relating experimental observations to field observations, including the reported diurnal fasting behaviour of juvenile salmon at water temperatures <10 °C. Densities in the tank ranged from five to twenty parr, at water temperatures ranging from 4.6 °C to 15.8 °C. The wide channel of the stream tank, with mean water velocity of 18.8 cm·s^?1, was the preferred section, where territorial behaviour was observed. Biomass was regulated in the wide channel by territorial mosaics or by dominance hierarchies. Dominance hierarchies were reflected in coloration of the fish. Dominant salmon were generally in the wide channel. Densities of salmon parr (of mean fork length 10.2 cm) in the channel ranged from 0.84 m^?2 to 1.73 m^?2, with an average biomass of 14.2 g·m^?2. Growth was least at the 5.9 °C temperature treatment. In experiments at temperatures below 10 °C, feeding, dominance hierarchy and territorial behaviour were observed in daylight hours, contrary to the published literature. Interactions with other species may affect behaviour. Some observations were made on a closely related species, brown trout (Salmo trutta L.), a commonly cohabiting species in many systems. Trout displaced salmon from their preferred locations in the tank and were more aggressive than the salmon, reducing agonistic behaviour by the salmon. The commonest agonistic act shown by salmon was ‘charge’ and that by the trout was ‘approach’. Some field observations affecting behaviour and production are discussed.     

Dietary calcium requirements of juvenile tilapia, Oreochromis niloticus × O. aureus, reared in fresh water

A feeding experiment was conducted to determine the dietary calcium (Ca) requirement for juvenile hybrid tilapia, Oreochromis niloticus × O. aureus reared in nature water. Purified diet supplemented with 0, 1, 2, 3, 4, 5, 7 and 10 g Ca kg⁻¹ diet providing of 0.6, 1.6, 2.6, 3.7, 4.7, 5.5, 7.5 and 10.7 g Ca kg⁻¹ diet, respectively, were fed to tilapia (mean initial weight: 0.52 ± 0.01 g, n = 3) for 8 weeks. Each diet was fed to three replicate groups of fish in a closed, recirculating fresh water rearing system. The rearing water contained 27.1–33.3 mg L⁻¹ Ca. The tilapia fed the diets supplemented with ≥3.7 g Ca kg⁻¹ had significantly (P < 0.05) higher weight gain, when compared with fish fed the diet with ≤1.6 g Ca kg⁻¹. Fish fed the unsupplemented control showed significantly lower weight gain when compared with the other groups (P < 0.05). Bone Ca concentration was highest in fish fed the diets with ≥4.7 g Ca kg⁻¹, intermediate in fish fed the diet with 2.6 g Ca kg⁻¹ and lowest in fish fed the control diet. Scale Ca concentration was higher in fish fed the diets with ≥3.7 g Ca kg⁻¹ than in fish fed the diets with ≤2.6 g Ca kg⁻¹. Serum alkaline phosphatase activity was 36% increased in fish fed the diets with ≥2.6 g Ca kg⁻¹ than fish fed the diets with <1.6 g Ca kg⁻¹. Analysis by broken‐line regression of weight gain, bone and scale Ca concentrations indicated that the adequate dietary Ca concentration for tilapia in water containing 27.1–33.3 mg Ca L⁻¹ was 3.5, 4.3 and 4.2 g Ca kg⁻¹ diet, respectively, supplied as Ca‐lactate.     

Response of Nile tilapia (Oreochromis niloticus) to diet acidification: Effect on growth performance and feed utilization

A 12-week feeding trial was undertaken to assess growth, nutrient utilization, some hematological parameters, and proximate composition of Nile tilapia (Oreochromis niloticus) fed diets supplemented with graded levels (0.5%, 1.0%, and 1.5%) of two blends of organic acids or organic salts. Seven isonitrogenous (300 g CP kg^?1) and isocaloric (19.0 MJ gross energy kg^?1) diets were formulated. The control diet (D1) contained neither added blend of organic acids nor organic salts. Three diets (D2, D3, and D4) were supplemented with 0.5%, 1.0%, and 1.5% of malic acid + oxalic acid blend (OAB), respectively. The other three diets (D5, D6, and D7) were supplemented with 0.5%, 1.5%, and 1.5% of calcium lactate + sodium acetate blend (OSB), respectively. Fingerlings (7.05 ± 0.02 g) were randomly distributed into 21 glass aquaria (160 liter), with each aquarium holding 15 fish. At the end of the feeding trial, O. niloticus offered the control diet showed lower growth and feed utilization than all those fed the organic acids or organic salts blends supplemented diets. Fish fed the diet D3 showed the highest body weight (BW), body length (BL), weight gain (WG), specific growth rate (SGR), feed intake (FI), and the best feed conversion ratio (FCR) and protein efficiency ratio (PER). Fish fed diet supplemented by 1.5% OAB (D4) showed the highest serum protein content, while control group (D1) showed the lower level of total lipids compared with the other treatments. Fish group fed the diet supplemented with 0.5% of OSB (D5) showed the highest protein and ash contents and the lowest lipid content of the whole fish body, while the control group showed the lowest protein and the highest fat content. The present observations suggest that the growth, feed utilization, and health status of juvenile O. niloticus can be enhanced with a supplementation of 1% of OAB or OSB in the diet.     

Plasma levels of insulin and liver glycogen contents in one- and two-year old Atlantic salmon (Salmo salar L.) during the period of parr-smolt transformation

Plasma levels of insulin were measured by specific radioimmunoassay in 1-year and 2-year old Atlantic salmon (Salmo salar) parr during the period of parr-smolt transformation. The two-year old fish were of two different categories; silvering pre-smolts and previously mature male parr. If insulin plays an important role in parr-smolt transformation and/or subsequent osmoregulatory changes it was expected that the pre-smolts would show a different insulin profile compared to the mature male parr and one-year old parr, both of which show impaired hypoosmoregulatory ability compared to smolts. Measurements were taken during two separate years. Between January and April both categories of two-year old fish had generally higher plasma levels of insulin compared to the non-smolting one-year old parr. In the pre-smolts insulin levels ranged from 4.0 to 7.9 ng ml⁻¹, and from 7.8 to 16.7 ng ml⁻¹ in 1990 and 1992 respectively, while in the previously mature males the same respective values were from 4.3 to 10.0 ng ml⁻¹, and from 6.6 to 24.1 ng ml⁻¹. In the two-year old fish, whether pre-smolts or mature males, plasma insulin levels peaked between 1–2 months before final smoltification, after which insulin titers declined sharply. In 1990, the 1-year old parr showed a dual peak in plasma insulin. Insulin first peaked in February (7.8 ng ml⁻¹), and then again in April–May (7.7 ng ml⁻¹), while in 1992 the 1-year old parr showed a number of smaller transient peaks (5–7 ng ml⁻¹) between March–May, followed by sharp elevation of insulin levels in June. Liver glycogen contents were at their highest (3.5–5.0 g 100 g⁻¹ I liver wet weight) in March in both 1-year and 2-year old fish. Glycogen levels were low during the later stages of parr-smolt transformation, before rising again in June in both the 1-year old and precociously mature parr, but not in the smolts.     

Effects of dietary protein and lipid levels on growth and energy productive value of pacific white shrimp,Litopenaeus vannamei,at different salinities

A 8‐week feeding experiment was conducted to evaluate the effect of different dietary protein and lipid levels on growth and energy productive value of juvenile Litopenaeus vannamei, at 30 and 2 ppt, respectively. Nine practical diets were formulated to contain three protein levels (380, 410 and 440 g kg^?1) and three lipid levels (60, 80 and 100 g kg^?1). Each diet was randomly fed to triplicate groups of 30 shrimps per tank (260 L). The effects of salinity and an interaction between dietary protein level and lipid level on growth and energy productive value of shrimp were observed under the experimental conditions of this study. At 30 ppt seawater, shrimp fed with 440 g kg^?1protein diets had significantly higher weight gain (WG) than those fed with 380 g kg^?1 protein diets at the same dietary lipid level, and the 60 g kg^?1 lipid group showed higher growth than 80 g kg^?1and 100 g kg^?1 lipid groups at the same dietary protein level. At 2 ppt seawater, the growth of shrimp was little affected by dietary protein treatments when shrimp fed the 80 and 100 g kg^?1 lipid, shrimp fed the 80 g kg^?1 lipid diets had only slightly higher growth than that fed 60and 100 g kg^?1 lipid diets when fed 380 and 410 g kg^?1 dietary protein diets. A significant effect of salinity on growth of shrimp was detected with the growth responses at 30 ppt > 2ppt (P < 0.05). Final body lipid content, body protein content and energy productive value of shrimp was significantly higher in animals exposed to 30 ppt than in shrimp held at 2 ppt.