Jens Chr. Holm

The effect of fish density and feeding regimes on individual growth rate and mortality in rainbow trout (Oncorhynchus mykiss)

Abstract Feeding frequency and fish density were varied in a 3 2 matrix rearing experiment with rainbow trout of initial size between 130 and 250 mm fork length. All fish groups were given similar daily rations. Initial densities were 107–219 kg m −3 and final densities 240–450 kg m −3 . The parameters of daily fork length increase, specific growth rate and mortality were recorded. Mean individual growth rate, irrespective of feeding regime, was highest in the lowest density and lowest in the highest density. Growth rate increased with increased feed availability. High feed availability (high frequency of feeding) was especially important when densities were extremely high.

The effect of artificial light treatment and depth on the infestation of the sea louse Lepeophtheirus salmonis on Atlantic salmon (Salmo salar L.) culture

Abstract Two field studies were carried out with farmed Atlantic salmon (Salmo salar L.) in sea cages to examine various effects of artificial light (AL) and the vertical distribution of salmon on lice infestation. The use of AL light caused an overall increase in lice infestation in both experiments. The first study showed that salmon held at 0–4 m depth in cages developed higher infestation than salmon held at greater depths (4–8 and 8–12 m) under both natural light (NL) and AL. In the second study, salmon maintained in 14-m deep sea cages that were exposed to AL with different light intensities. The AL treatments resulted directly in different diel and seasonal patterns of vertical distribution of the salmon and also different temporal patterns in lice infestations. So indirectly the infestation pattern appeared to be correlated with median day-time swimming depth of the salmon.

Macronutrient composition of formulated diets for Atlantic halibut (Hippoglossus hippoglossus, L.) juveniles

Abstract A three component mixture design was used to investigate the optimum composition of macronutrients for Atlantic halibut juveniles (0.5 g startweight). Protein, lipid and carbohydrate were varied between 530–830, 50–300 and 0–150 g kg−1 dry wt., respectively. Of the 21 diets, 20 produced were fed to fish in single tanks, while 1 diet was fed to fish in four replicate tanks to estimate inter-tank variation. The experiment lasted for 2 months. Carbohydrate levels above 50 g kg−1 were suboptimal for the halibut as seen from lowered specific growth rates (SGR), increased hepatosomatic indices and accumulation of carbohydrate in the liver. At low carbohydrate levels (0 and 50 g kg−1), the specific growth rates appeared similar with lipid levels between 50 and 250 g kg−1, while 300 g lipid kg−1 resulted in growth depression. At higher carbohydrate levels, fish growth was reduced with lipid levels increasing above 100 g kg−1. It can be concluded that Atlantic halibut juveniles have low tolerance to dietary carbohydrate, whereas dietary lipid can be varied over a wide range without affecting growth performance. Based on the limits of 50 g kg−1 for carbohydrate and 250 g kg−1 for lipids, the protein requirement appears to be at least 630 g kg−1.

Atlantic salmon in duoculture with Arctic charr: decreased aggression enhances growth and stocking density potential

Abstract Reduced aggression for Atlantic salmon in duoculture with Arctic charr, compared to monoculture, was previously reported. It was then not clarified whether the reduction of aggression was a result of reduced salmon stocking density itself, or alternatively, if the physical presence of Arctic charr reduced the probability for visual and physical contact between salmon additionally (termed shading). To test these hypotheses, Atlantic salmon and Arctic charr were reared in mono- and duoculture under two densities; 0.6 and 1.2 individuals 1 −1 . Variations in Atlantic salmon length, weight and condition factor were significantly affected by culture type and salmon density, but not by total fish density. Culture type and charr density did not significantly affect charr size parameters, except for variation in condition factor. For a given salmon density, addition of charr reduced the aggression received by each salmon and increased mean salmon size and growth rate significantly. Duoculture improved growth rate in salmon due to shading between conspecifics. A practical application of the results could imply increased total stocking density potential of salmonids.

Growth and prey selection by Atlantic salmon yearlings reared on live freshwater zooplankton

Abstract The experiment was carried out in a coastal lake near Bergen, Norway. Two experimental groups of Atlantic salmon ( Salmo salar ) yearlings were fed with live zooplankton in an artificial current. A third group, given commercial dry food pellets, was used as a control. The offered zooplankton biomass reached a maximum in the second week of July. The caladoceran Daphnia longispina dominated the zooplankton at this moment. A stock breakdown for this species was soon observed. The salmon yearlings preferred cladocerans. At the height of the zooplankton occurrence, Daphnia became the only prey species observed in the stomachs — and a prey selection for larger individuals was observed when food was in surplus. Earlier in the period the salmon selected the largest individuals of two cladoceran species of different shape, pigmentation and size at the same time. In the experiment live zooplankton was recognised as a satisfactory food when it occurred in sufficient amounts, compared with commercial fry food pellets. Best mean growth rate of zooplankton-fed fish was 0.92 mm/day. Negative growth rate was observed when low zooplankton densities occurred.

Yolk sac absorption and early food selection in Atlantic salmon feeding on live prey

Abstract Six groups of Atlantic salmon ( Salmo salar ) fry were startfed with frozen Daphnia longispina , standard Ewos startfood or unfiltered live zooplankton drained from the littoral zone of a coastal lake. Through the last 5 days of the experiment, those fed live zooplankton were given additional food in the form of frozen Daphnia . Dry-fed groups were the only ones to have an overall weight gain but they had the lowest activity. Fry fed live zooplankton had the highest activity but the natural species composition of this food was not optimal for startfeeding. Survival was highest in the group fed frozen Daphnia and kept in a tank with bottom drainage. High yolk absorption rates, implying early termination of the yolk-sac phase, as found in groups fed live zooplankton, may be favourable if a net energy gain can be achieved by early feeding on a live prey.

Free amino acids in live freshwater zooplankton and dry feed: Possible importance for first feeding in Atlantic salmon fry (Salmo salar)

Abstract Feeding preference and free amino acids (FAA) contents were determined in live and dry food, and also correlated with salmon growth rate. A possible signalling effect of FAA during feeding is supported by the rapid leakage of FAA from available food. Feeding response experiments indicated that salmon fry show a preference for daphnids of high rather than low nutritional value. It is proposed that the observed FAA leakage may explain this observation. The data are consistent with FAA being sensed when the fish make olfactory and gustatory choices, and thus the salmon fry may economize its efforts at prey capture as well as prey digestion. Certain FAA may have nutritional and pharmacological effects on the salmon fry, independent of the metabolic and protein synthetic roles of amino acids derived from food proteins. Hence the FAA content of food should be varied independently of general amino acid nutritional requirements in order to evaluate the suitability of a given food for salmon fry growth.

Atlantic salmon start-feeding with live zooplankton: Pressure shock treatment to increase prey availability

Abstract The experiment was carried out in 1984 in a coastal lake near Bergen, Norway. A total of four (later eight) groups of Atlantic salmon ( Salmo salar ) fry were start-fed with live zooplankton delivered by a pump. Small fine-meshed net pens were used. Supplemental feed promoted additional growth after the start-feeding phase. The fish in zooplankton-fed groups increased their average length by 0·15 mm/day during the first 30 days compared with 0·08 mm/day in fish given both zooplankton and dry feed. Initial food lack (low density of cladocerans) is the main explanation for the low growth rates. Food selection and pump-increased zooplankton capture probabilities are also discussed. An unexpected avoidance of calanoid copepods, even when debilitated, was observed at the end of the experiment when other potential prey species were scarce. Even Daphnia were not preferred as expected. Live zooplankton is recommended for initial start-feeding, but dry food should be available to salmon when the yolksac has been absorbed.

Growth depression and acclimatization of protease activity in Atlantic salmon first-feeding fry responding to a diet supplemented with zooplankton

Abstract Groups of Atlantic salmon fry were given dry feed supplemented with frozen zooplankton, or dry feed only. A tendency toward decreased endogenous digestive protease activity and a highly significant depression in growth rate were observed in groups given supplemental feed.

Review on experiments on use of zooplankton as food in salmonid smolt production

Abstract Nine experiments carried out in the period 1981–1983 in a coastal lake near Bergen, Norway, are the basis for this discussion of feeding of Atlantic salmon on live freshwater zooplankton. Factors discussed are different enclosure types, diurnal variation and current velocity, including a review of selected literature. The aim was to evaluate to what degree zooplankton may be of certain importance as start- and supplement-food for salmonids in the freshwater stage.