Sofronios E. Papoutsoglou

Effects of stocking density on behavior and growth rate of European sea bass (Dicentrarchus labrax) juveniles reared in a closed circulated system

Four duplicated groups of sea bass juveniles (mean initial body wet weight 6.6±1.1 g and mean total length 8.56±0.52 cm) were reared under closed circulated water system conditions at different stocking densities (80, 165, 325 and 650 specimens/m3) for 168 days. The aim of the experiment was to determine the general behavior and the effect of fish stocking density on fish growth, feeding and metabolism during the critical period of the first 6 months of the commercial life of sea bass. Statistically significant differences (P<0.05) for final fish weight were observed between groups of the two highest and the lowest stocking densities. The highest specific growth rate and the lowest food conversion ratio were observed for fish of the highest stocking density. No statistically significant differences (P<0.05) were found in haematocrit values, while no meaningful statistical differences were found in hepatosomatic indices in the groups of fish held at any stocking density. Significant differences in water quality among treatments was observed, with relatively poorer water quality occurring at the higher fish densities. The results suggest that schooling behavior of the sea bass may have been a factor in the observed differences in fish performance. However, no formation of clear dominance hierarchies was observed. It is concluded that sea bass, within the range of stocking densities tested and at least at its early stages, does not belong to that fish category which is characterised by an inverse correlation between stocking density and growth rate.

Effect of density on growth rate and production of rainbow trout (Salmo gairdneri Rich.) over a full rearing period

Abstract Five groups of raibow trout (mean initial weight 0.84 g and standard length 3.75 cm) were reared in raceways, under commercial conditions at different densities (31, 94, 156, 250 and 312 Specimens/m3) for 365 days. The aim of the experiment was to investigate the most suitable, effective and economical rearing density during a full rearing period under conditions widely found in Greece. Differences in food consumption, mean body weights and metabolic and physiological responses of the groups were noted throughout the experimental period. The lowest growth rate appeared in fish at the highest density; the differences observed between most of the other densities were not significant. It is concluded that although higher densities depressed the growth rate, the proper choice of density depends on the required final live body weight.

Comparative studies on body composition of rainbow trout (Salmo gairdneri R.) in relation to type of diet and growth rate

Abstract Three populations of rainbow trout (mean initial live weight 40 g) were fed experimentally under almost the same conditions for 49 weeks. The aim of the experiment was the examination of the changes in the relative proportions of the major whole body constituents (water, protein, fat and ash) in relation to type and amount of diet and growth rate. Two pelleted dry diets (A and B) and one mixed diet (C) were used. The amount of food given daily to the fish in the case of diets A and B was continuously based on fish body weight and water temperature. The population which received diet C (raw material and pelleted dry food) was fed on maximum ratio. Sampling was carried out approximately every two months. After 25 weeks, and besides the regular sampling, fish were sampled from each population and analyses were made on their fillets. The same analyses were also carried out. on fillets and whole body material on a sample of wild fish, of almost the same age, fed on a variety of food organisms in a stream near the experimental tanks. When the results are expressed in terms of dry weight, the analyses of whole body material showed that ash content remained fairly constant in all populations throughout the experimental time. Further, with increasing body weight and age the percentages of water and protein decreased and the percentage of fat content increased in all populations and especially in fish fed on diet C which appeared to have the maximum growth rate and final mean body weight. Similarly, fish fed on diet B, which showed the lowest growth rate and final mean body weight, had low changes in the major body constituents throughout the experimental period. The lowest and the most regular rate in the changes of these parameters appeared in fish fed on diet A which also had a sufficient growth rate and mean final body weight. Analyses of the fillets for the three different types of food used have different results. The analysed wild fish had the highest percentage of water content and, expressing the results in terms of dry weight, the highest percentage of ash and protein content, while they appeared to have the lowest percentage of fat content, on both whole body material and fillets. Since the three types of diets used in this experiment gave three different growth rates, as well as three different body compositions of the reared trout, it is suggested from the present results that the relative proportions and changes of the four body constituents and effects on the growth rate of trout are strongly affected by the type of food.

Polyculture rearing of Cyprinus carpio (L.) and Oreochromis aureus (St.) using a closed circulated system

Two monoculture (100% Cyprinus carpio and 100% Oreochromis aureus) and two polyculture (60% carp — 40% tilapia and 60% tilapia — 40% carp) groups were reared using a closed circulated system, for 291 days. The mean initial weight of carp was 9.3 ± 0.3 g and that of tilapia 7.1 ± 0.1 g. The aim of the experiment was to investigate the interactions between the species and their influence on fish growth characteristics, under polyculture conditions, using an intensive-superintensive production system. Polyculture groups (C 60% — T 40% and T 60% — C 40%) showed the highest final yield (126.1 kg/m3 and 132.S kg/m3, respectively) compared to that of the monoculture populations (C 100%: 116 kg/m3 and T 100%: 98.8 kg/m3). Also, the polyculture populations of both species showed the highest specific growth rates. It is concluded that the achievement of the well known advantages of polyculture fish rearing, in the case of an intensive production system, is mainly based on fish ethology, food habits and the percentage combination of species.

Blue tilapia (Oreochromis aureus) growth rate in relation to dissolved oxygen concentration under recirculated water conditions

The growth rate of Oreochromis aureus in relation to dissolved oxygen concentrations (2·63±0·12, 3·75±0·12, 6·51±0·13 ppm or 31·3, 44·6 and 77·5% saturation, respectively) was investigated. Three duplicated populations of 29 specimens (mean initial body weight ∼27·3 g) were reared in 100-litre tanks for 200 days under recirculated water conditions. Fish were offered an artificial diet three times per day, 6 days per week. The obtained results showed statistically significant final body weight differences (P < 0·05) between oxygen groups and actual differences regarding their specific growth rate and food conversion ratios. However, although mean body weight increased according to the dissolved oxygen concentration, the best food conversion ratio was shown by fish of the intermediate dissolved oxygen group. It is concluded that the lowest feeding cost involved in tilapia controlled mass production, could be achieved with relatively low dissolved oxygen concentration, under simple recirculated water system conditions.

Studies on stress responses and recovery from removal in gilthead sea bream Sparus aurata (L.) using recirculated seawater system

The effects of serial cohort-removal stress on physiological responses in immature specimens of the euryhaline species, the gilthead sea bream Sparus aurata, were investigated (trial 1). In addition, physiological responses during recovery in tanks of 175 l (10.5 kg/m3) after a single (trial 2) or serial (trial 3) cohort-removal from a holding tank of 1 m3 (14 kg/m3) were examined. All trials were performed in a recirculated seawater system. Serial cohort-removal for 90 min, combined with a continuous pursuing (trial 1), caused a significant increase of plasma cortisol, glucose, osmolality, Na+, Cl− and pCO2, and a significant reduction of pH, K+ and HCO3− in the remaining fish of the holding tank. These alterations in most of the plasma parameters studied were progressive for 45–60 min and then remained almost constant. When undisturbed fish were netted and transferred (trial 2), the levels of plasma cortisol, glucose, osmolality, Cl− and Na+ initially increased, exhibiting maximum elevations at 40 min and recovering within 2 h. At the end of a serial cohort-removal after 15 min, without chasing the fish (trial 3), the levels of plasma cortisol, glucose, Na+, Cl− and K+ did not significantly differ from the control levels. However, during recovery from netting and transfer of fish disturbed by this serial cohort-removal, the levels of plasma cortisol and glucose recovered after 5 h. It is indicated that serial cohort-removal, depending on its duration, may increase the alterations in stress responses, especially the secondary ones, and prolong the extent of cortisol and glucose elevations. It was concluded, that serial cohort-removal of gilthead sea bream should be mild and short-term and could be repeated the same day allowing, however, a sufficient period for cortisol and glucose recovery.

Growth and physiological changes in scaled carp and blue tilapia under behavioral stress in mono- and polyculture rearing using a recirculated water system

Growth performance, carcass composition, liver and blood parameters ofscaled carp (C), Cyprinus carpio, and blue tilapia (T),Oreochromis aureus, reared for eight weeks in twomonoculture (100%C and 100%T) and two polyculture (60%C–40%T and40%C-60%T) conditions were investigated. In polyculture 40%C–60%T bothspecies achieved the highest levels of specific growth rate and the lowestlevels of food conversion ratio and carcass lipid content. In addition, theyexhibited the highest values of plasma pO2 and pH and the lowestvalues of plasma pCO2, cholesterol and albumin, although thedifferences among treatments were not significant in the case of tilapia.Tilapia showed significantly lower plasma Cl− levels than underthe other conditions. Carp in monoculture and tilapia in polyculture60%C–40%T had the lowest levels of specific growth rate and significantlyhigher levels of liver lipids and plasma triglycerides than in the other groups.In addition, carp in monoculture exhibited a significantly higher haematocritthan in polyculture. No significant variations among treatments were observedconcerning plasma cortisol, glucose, osmolality, Na+, K+,HCO3− andHCO3−/H2CO3 in either species.The combination of scaled carp and blue tilapia, in which blue tilapia were themain species, proved to be the best for both species. It was suggested thatgrowth and physiological changes under mono- and polyculture rearing, in anintensive system, seem to be as a result of a different state of stress relatedto fish behavior.

Influence of acclimation temperature on the osmotic regulation and survival of rainbow trout (Salmo gairdneri) rapidly transferred from fresh water to sea water

Abstract Certain physiological responses of two groups of rainbow trout acclimated at 10 and 17°C to a sudden change in salinity were investigated in order to determine the most appropriate temperature for direct transfer in salt water. Plasma osmolarities of fish acclimated at 17°C increased at a much faster rate following transfer and approached a much higher value than that reached by the fish acclimated at 10°C. The same was apparent for body tissue dehydration. Haematocrit values, the number of red blood cells, as well as red cell volume showed no significant difference between the two groups of fish for most of the experimental period. The larger increases in plasma osmotic pressure and tissue dehydration observed at 17°C are discussed in connection with the physiological adjustments accompanying higher temperature acclimation.