In vitro hydroxylation of vitamin D3 and 25-hydroxy vitamin D3 in tissues of Atlantic salmon Salmo salar, Atlantic mackerel Scomber scombrus, Atlantic halibut Hippoglossus hippoglossus and Atlantic cod Gadus morhua

The in vitro metabolism of ¹⁴CD₃ and ³H25OHD₃ was investigated in different tissues from Atlantic salmon Salmo salar , Atlantic mackerel Scomber scombrus , Atlantic halibut Hippoglossus hippoglossus and Atlantic cod Gadus morhua . The tissues analysed were liver, kidney, head kidney, gills, spleen and intestine. The metabolites were extracted in methanol–chloroform and separated by normal-phase high-pressure liquid chromatography (HPLC) followed by scintillation counting. Identification of the metabolites was by comigration with standards on normal and reversed-phase HPLC systems and by protein-binding assays. All tissues from all species analysed produced hydroxylated derivatives identified as 25OHD₃, 24,25(OH)₂D₃ and 1,25(OH)₂D₃. In addition, some unidentified derivatives were recorded, one probably being 25,26(OH)₂D₃. Organs producing great amounts of one metabolite also produced considerable amounts of the other possible derivatives, suggesting a lower degree of specificity in fish organs than in human organs. The predominating metabolite was 24,25(OH)₂D₃ in all organs from salmon and mackerel during incubation with ¹⁴CD₃ and within most organs from all species during ³H25OHD₃ incubation. The latter observation probably results from the need for decreasing rather than increasing the calcium absorption in these species, which live at least some periods of life in a marine environment.     

Effect of gelatinized wheat and maize in diets for large Atlantic salmon (Salmo salar L.) on glycogen retention, plasma glucose and fish health

This publication reports on analytical data from a large-scale experiment, using 3360 Atlantic salmon, Salmo salar L., distributed into 12 sea cages. Salmon were grown from an average 600 g to an average 3.5 kg. Samples for analyses were taken when fish were in the range of 1.8 and 3.5 kg. Dietary changes between groups were increased starch from 24 to 230 g kg^?1 and balanced with protein. The diets were isolipic. All salmon showed small stores of glycogen in all analysed organs, and only in heart, gills and kidney of large fish (3.5 kg) were the levels correlated with dietary starch. Minor differences between groups were found in liver NADPH production, but with substantially decreased NADPH production per g protein as the fish grew from 1.8 to 3.5 kg, indicating that increasing dietary starch did not lead to induction of liver hexokinase, and that the activity of this enzyme may decline as fish size increases. An increase in plasma glucose concentrations was found as dietary starch increased, but all levels were moderate and ranged within reference values. Plasma total protein concentrations did not, however, vary according to decreased dietary protein, but increased substantially in all groups as the fish grew from 1.8 to 3.5 kg. Dietary treatments had no influence on haematological parameters, except for decreased haemoglobin concentrations as dietary starch increased in large fish (3.5 kg). No impared liver function was detected, evaluated by activities of ASAT, ALAT and LDH, and by histological analyses. Low serum lysozyme activities were recorded in all groups, and were not correlated with plasma glucose or liver glycogen concentrations.     

Growth, feed efficiency and slaughter quality of salmon, Salmo salar L., given feeds with different ratios of carbohydrate and protein

The effects of using increasing levels of carbohydrate to replace protein in diets for Atlantic salmon were studied in a long-term, semicommercial-scale experiment in sea cages, using wheat and corn starch as carbohydrate sources. The fish were fed extruded diets containing 24 g and 602 g kg^?1, 98 g and 521 g kg^?1, 165 g and 454 g kg^?1, 230 g and 384 g kg^?1 of carbohydrate and protein, respectively. The contents of lipid were kept constant at 280 g kg^?1. The experiment lasted for 9 months until the fish had reached about 4 kg. All groups showed good growth in the experimental period. Feed efficiency decreased and feed intake increased linearly with increased level of carbohydrate in the diet. Digestibility of dry matter and energy were seriously decreased with increased carbohydrate level in the diets. This was mainly due to decreased digestibility of carbohydrate, but also to decreased digestibility of lipid. The digestibility of protein or amino acids was not affected by diet. Higher levels of carbohydrate resulted in increased storage of energy as lipid in fillet and abdominal fat. Red coloration showed an optimum at 98 g kg^?1 carbohydrate in the diets. No differences in maturation were found between diets. Overall, based on feed efficiency, growth and slaughter quality, the optimum level of carbohydrate in the diet for Atlantic salmon in sea water is found to be approximately 100 g kg^?1 dry matter.     

Changes in spawning stock structure strengthen the link between climate and recruitment in a heavily fished cod (Gadus morhua) stock

Atlantic cod (Gadus morhua) is one of the commercially most important fish species in the North Atlantic and plays a central role in several ecosystems. Fishing pressure has been heavy over a prolonged period and the recent decades have shown dramatic decline in abundance of many stocks. The Arcto‐Norwegian (or North‐east Arctic) cod stock in the Barents Sea is now the largest stock of Atlantic cod. Recruitment to this stock has varied extensively during the last 60 yr. There is evidence for fluctuations in climate, particularly sea temperature, being a main cause for this variability, higher temperatures being favourable for survival throughout the critical early life stages. Our studies of time series present compelling evidence for a strengthening of the climate–cod recruitment link during the last decades. We suggest this is an effect of the age and length composition of the spawning stock having changed distinctly. The age of the average spawner has decreased by more than 3 yr from between 10 and 11 in the late 1940s to 7–8 in the 1990s, average length from just above 90 cm to around 80 cm. The number of age classes contributing to the spawning stock has also decreased, while the number of length groups present increased slightly. Significant decrease in age of spawners has frequently been described for other heavily fished stocks worldwide. We therefore find it likely that the proposed mechanism of increased influence of climate on recruitment through changes in the spawning stock age and size composition is of a general nature and might be found in other systems.     

Simulated carrying capacities of fish in Norwegian fjords

The influence of oceanographic and meteorological conditions and topography on the carrying capacity of organisms in coastal areas of western Norway is investigated by field studies and dynamic modelling. Published data on Calanus finmarchicus , the dominant species in the zooplankton biomass of west Norwegian coastal waters, demonstrate a strong gradient, with high densities at the outer coastal areas and lower densities at the innermost localities. This gradient spans several orders of magnitude. A similar tendency is found in the data from pelagic trawling and from beach seine catches, and simulation modelling indicates that fish production depends on zooplankton availability. According to the model, the carrying capacity level of fish production is high in the outer areas and decreases with distance from the outer coast according to the decrease in advected biomass of zooplankton. The simulations indicate that while growth prospects for individuals at lower trophic levels vary with the distance from the coast, potential for growth seems largely unaffected by the distance from the coast for the sublittoral piscivores (e.g. cod). This indicates that, for immigration of a new sublittoral piscivore to a habitat, it does not matter whether the habitat is situated close to the outer coast or far from it. A consequence for stock enhancement may be that a small-scale release of sublittoral piscivores will be equally productive in most coastal areas, while the outcome of a large-scale release will be strongly dependent on distances from the coast. The outcome may be high, but variable, in the outer areas and low, but probably more constant in the innermost areas, provided that releases of juvenile, sublittoral, piscivorous fish do not result in a fish stock density in excess of the carrying capacity level.     

Modelling the influence of light, turbulence and ontogeny on ingestion rates in larval cod and herring

Based on existing models of fish vision and turbulence-mediated ingestion rates, we develop a model of ingestion rates in larval fish that combines several physical properties of the environment (turbulence, irradiance, light attenuation) and visual characteristics of predators and prey. The model of visual range was calibrated with observed estimates in larval herring ( Clupea harengus ) and cod ( Gadus morhua ). The improved visual ability with length of larvae was predicted to be the most sensitive part of the model. Both turbulence and light had strong impacts on the ingestion rate of larval fish. The optimal level of turbulence increased with larval length. Also, due to the exponential decay of light with depth, it was predicted that larvae will have higher ingestion rates near the surface, even at high wind velocities. It is also proposed that larvae (particularly the smallest larvae) should concentrate on larger prey in turbulent environments. We suggest that biophysical models of larval growth and survival in field situations should include these factors to account for environmental effects on growth, survival and recruitment processes in the early life stages.