Stocking density positively influences the yield and farm profitability in cage aquaculture of sutchi catfish, Pangasius sutchi

The sutchi catfish (Pangasius sutchi Fowler 1937) was cultured at 10 different densities in cages suspended in a river‐fed channel during the summer of 2000. Catfish fingerlings (mean length 9.14–9.74 cm; mean weight 5.92–6.7 g) were stocked at densities of 60, 70, 80, 90, 100, 110, 120, 130, 140, and 150 fish m^?3, which were equivalent to 397, 427, 488, 551, 604, 656, 710, 871, 981, and 899 g m^?3 respectively. At the end of 150 days, the growth and yield parameters were studied and a simple economic analysis was performed to calculate profitability. Gross yields were 15.6 ± 0.27, 17.1 ± 0.31, 19.5 ± 0.30, 21.9 ± 0.29, 26.8 ± 0.22, 28.6 ± 0.40, 30.0 ± 0.37, 31.1 ± 0.45, 32.7 ± 0.31, and 34.5 ± 0.44 kg m^?3; net yields were 15.2 ± 0.22, 16.7 ± 0.28, 19.0 ± 0.29, 21.3 ± 0.21, 26.2 ± 0.19, 27.9 ± 0.33, 29.3 ± 0.33, 30.3±0.37, 31.8 ± 0.29 and 33.5 ± 0.36 kg m^?3, respectively, with stocking densities of 60, 70, 80, 90, 100, 110, 120, 130, 140, and 150 fish m^?3. Mean weights of fish at harvest were inversely related to stocking density. Both gross and net yields were significantly different and were directly influenced by stocking density, but the survival rates and feed conversion were unaffected. Higher stocking density resulted in higher yield per unit of production cost and lower cost per unit of yield, with a net revenue being higher with increasing stocking density. It was concluded that the density of 150 fish m^?3 produced the best results among the densities tested in this experiment.     

Effect of stocking density on growth and survival of Clarias batrachus (Linn.) larvae and fry during hatchery rearing

Experiments were conducted to determine optimum stocking density for Clarias batrachus larvae and fry during hatchery rearing. The increase in stocking density decreased the total weight, specific growth rate (SGR) and percent weight gain of Clarias larvae during a 13‐day experiment. Survival rate was highest at a stocking density of 1000 m^?2 and lowest at 5000 m^?2. Stocking density did not influence the total biomass production of larvae. Clarias batrachus fry performance was studied during a 28‐day hatchery rearing experiment whereby fry stocked at a density of 100 m^?2 attained the highest total body weight (P < 0.05). The survival rate greatly declined to 59–61% by a density increase to 300 m^?2 and above. Stocking density influenced growth and survival of C. batrachus larvae and fry during hatchery rearing. The best performance was obtained when larvae were stocked at 2000 m^?2; survival was highest with C. batrachus fry stocked at 200 m^?2.     

Stocking density‐dependent growth and survival of Asian sun catfish,Horabagrus brachysoma (Gunther 1861) larvae

The aim of the experiment was to evaluate the growth and survival of Horabagrus brachysoma larvae at different stocking densities (3, 7, 13, 20, 27 and 33 larvae L^?1) during their hatchery phase. Total length and wet weight of the larvae consistently decreased (P < 0.05) at the end of 14 and 28 days of rearing as the density increased. The specific growth rate was significantly (P < 0.05) highest at three larvae L^?1 compared to the other five densities. The percent weight gain and survival of larvae was also highest at lowest density. The observation corroborates that catfish larvae can be reared at low densities in stagnant water conditions. Considering the value of larval growth, survival and overall weight gain, the stocking density of seven larvae L^?1 has been identified as the maximum for larval rearing of H.  brachysoma under hatchery conditions.     

Effects of stocking density during live transportation on haematological parameters of Siberian sturgeon (Acipenser baerii,Brandt, 1869)

In this study, we investigated the effects of different stocking densities water quality and blood parameters during the transportation of the Siberian sturgeon (Acipenser baerii). Experiments were carried out in high-density polyethylene tank with three different stocking densities (50, 100 and 150 kg/m³) for 20 h. Ammonium nitrogen (NH₄-N) and nitrite nitrogen (NO₂-N) were measured in the water samples. The erythrocyte, leucocyte, haematocrit (HCT), haemoglobin (Hb), cortisol, and sodium ion (Na⁺) were measured in blood samples. Ammonium nitrogen and nitrite nitrogen concentrations in the water have increased in parallel with the stocking density depending on the time. In the highest stocking density, the maximum levels of NH₄-N and NO₂-N at the 16^th h reached 2.64 mg/L and 4.74 mg/L, respectively. Erythrocyte, leucocyte, HCT, and Hb values did not differ significantly between the experimental groups (p > .05). The results showed that fish could be transported safely for 20 h in 15℃ water temperature at 50 kg/m³ stocking density; however, stocking density of 100 kg/m³ and over could threaten fish welfare and health as from 16^th h.     

Season of grazing and stocking rate interactively affect fuel loads in Baikiaea plurijuga Harms woodland in northwestern Zimbabwe

Wildfire is a major disturbance in Baikiaea plurijuga Harms woodland savannas. We tested the hypothesis that the timing and intensity of herbivory influence fuel loads. We used three stocking rates namely light (three cows and four goats ha^?1), medium (six cows and eight goats ha^?1) and heavy (eleven cows and sixteen goats ha^?1) and three times of grazing namely early‐, middle‐ and late‐growing seasons. Season of grazing and stocking rate influenced herbaceous phytomass. Phytomass was generally the highest (53.5 g DM m^?2) in paddocks grazed during the early growing season and the lowest (27.8 g DM m^?2) in those grazed during the late growing season. Phytomass was also generally the highest (40.4 g DM m^?2) in lightly stocked paddocks and the lowest (32.7 g DM m^?2) in heavily stocked ones. Litter mass was the lowest (160.8 g DM m^?2) in paddocks grazed during the early season whereas there were no differences in ungrazed paddocks and those grazed during either mid‐ or late growing seasons (205.4 g DM m^?2). There was a negative relationship between litter mass and stocking rate. Baikiaea Benth. woodlands should be grazed during either the mid‐ or late‐growing season at stocking rates greater than 0.1 LU ha^?1 to reduce grass fuel loads.     

Growth,biochemical, fatty acid composition,and mRNA levels of hepatic enzymes in genetically improved farmed tilapia (GIFT,Oreochromis niloticus) (Linnaeus, 1758) at different stocking densities

Investigated were the effects of stocking density on juvenile growth performance, serum biochemistry, fatty acid composition, and mRNA levels of enzymes involved in lipid metabolism in genetically improved farmed tilapia (GIFT), Oreochromis niloticus. Juvenile fish (n = 900) were distributed into 15 tanks at five initial stocking densities (75, 150, 225, 300, and 375 g/m³ per 800‐L tank, represented as D1–D5, respectively). Temperature was maintained at 28 ± 0.3°C, and water flow rate into the tanks was about 8 L/min. The fish accepted floating pelleted feed (crude protein 28%, crude lipid 6%, and gross energy 149.2 KJ/g diet). The feed amount offered was 4%–8% of body weight. At the end of the 60‐day experiment, the final stocking densities were 1,454.18, 2,010.60, 2,913.13, 3,819.73, and 4,380.80 g/m³, respectively. Growth and body mass coefficient of variation were significantly affected (p < .05). No differences in feed conversion rate or survival were found (p > .05). Muscle lipid contents decreased with stocking density from D1 to D5. Serum total protein and cholesterol levels were not affected by the stocking density but the D5 group produced the highest serum cortisol and lactate levels and the lowest glucose levels compared to those of the other groups. Lower muscle saturated fatty acid and n?3 polyunsaturated fatty acid contents were observed in the D4 group than in the other groups, whereas the reverse was found in the n?6 polyunsaturated fatty acid content. Hepatic glucose 6‐phosphate dehydrogenase, fatty acid synthetase, hormone‐sensitive triglyceride lipase, and lipoprotein lipase mRNA levels were significantly upregulated in response to a high density. These data suggest that the GIFT adapted to high‐density conditions by altering the composition of muscle fatty acid and stimulating lipid metabolic enzymes. These mechanisms may help fish respond to high stocking density stress in intensive aquaculture systems.     

Effects of stocking density on survival,growth and size variation of juvenile Brachymystax lenok (Pallas, 1773)

In this experiment the effects of stocking density on growth and survival of juvenile Brachymystax lenok were studied. Experimental fishes were reared at different stocking densities (5, 25, 50 and 75 fish L^?1) for 40 days at ambient temperatures (11.2–16.4°C). During the experiment, dissolved oxygen (DO) was above 5.4 mg L^?1 and other major environmental factors were controlled at the same levels in each aquarium (DO 5.4–7.2 mg L^?1, pH 6.6–7.4, and flow‐through rates 0.3–0.4 L s^?1). Mortality data in each group was recorded daily and random samples of 30 juveniles were measured and weighed every 10 days. All statistical analyses were performed by anova . The survival rate was found to be significantly affected by stocking density, but not the condition factor (CF), specific growth rate (SGR) or coefficients of variation (CV).The results demonstrate that lenok juveniles can be reared at higher stocking densities, which can increase survival under intensive conditions.     

Stocking Density Affects Circadian Rhythms of Locomotor Activity in African Catfish,Clarias gariepinus

The effect of stocking density on the locomotor activity of African catfish C. gariepinus under different light regimes was investigated. C. gariepinus were stocked under different densities (1, 5, or 10 fish/tank), and their locomotor activity recorded under light-dark (LD), constant light (LL), constant darkness (DD), and LD-reversed (DL) regimens. Under the LD cycle, catfish showed a crepuscular activity pattern, irrespective of stocking density, with most of the daily activity concentrated around the light-onset and light-offset times. When fish were subjected to DD, all 4 tanks with medium (5 fish) and high (10 fish) stocking densities showed circadian rhythmicity, with an average period (τ) of 23.3?±?0.5 and 24.6?±?0.5?h, respectively. In contrast, only 2 low (1 fish) density tanks showed free-running rhythms. Under LL, activity levels decreased significantly in comparison with levels observed under LD and DD. Moreover, fish of 1, 2, and 3 out of the 4 tanks with low, medium, and high densities, respectively, showed free-running rhythms under these conditions. When the photocycle was reversed (DL), fish of 3, 2, and 4 out of the 4 tanks with low, medium, and high stocking densities, respectively, showed gradual resynchronization to the new phase, and transient cycles of activity were observed. These results suggest that stocking density of fish affected the display of circadian rhythmicity and the intensity of activity levels. Thus, fish kept in higher densities showed more robust rhythmicity and higher levels of daily activity, indicating that social interactions may have an influence on behavioral patterns in the African catfish. (Author correspondence: lmvera@um.es)     

GROWTH AND MORPHOLOGY OF YOUNG GAMETOPHYTES OF PORPHYRA ABBOTTAE (RHODOPHYTA): EFFECTS OF ENVIRONMENTAL FACTORS IN CULTURE1

Growth, blade shape and blade thickness of young gametophytes of Porphyra abbottae Krishnamurthy cultured from conchospores were determined at various combinations of temperature (8, 10, 12° C), photon flux density (17.5, 70, 140 μmol·m-^?2·S^?1), nutrient concentration (5, 25, 50, 100% f medium) and water motion (0, 50, 100, 150 rpm). Growth (as surface area) was light-saturated at 70 μmol· m^?2· S^?1, light-inhabited at 140 μmol·m^?2· S^?1, and nutrient-saturated an 25% f medium. Temperature had no significant effect on growth. Water motion and nutrients had an interactive effect on growth, with water motion having the greatest effect at the lowest nutrient concentrations. Water motion enhanced growth even at saturating nutrient concentrations. Blade length / width ratio was greater in low light (2.5) than in saturating light (1.9); with increasing water motion the ratio increased from 1.2 to 2.4. Blade thickness (53-88 μm) was greatest at the highest nutrient concentrations and at the lowest water motion levels. Temperature and light did not have a consistent effect on blade thickness.     

The effects of rearing density on growth,size heterogeneity and inter-individual variation of feed intake in monosex male Nile tilapia Oreochromis niloticus L.

The growth dispersion of farmed fish is a subject of increasing interest and one of the most important factors in stocking density. On a duration of 60 days, the effect of stocking density on the growth, coefficient of variation and inter-individual variation of feed intake (CV_FI) of juvenile Nile tilapia Oreochromis niloticus L. (14.9 ± 1.2 g) were studied in an experimental tank-based flow-through system. Groups of fish were stocked at four stocking densities: 200, 400, 600 and 800 fish/m³, corresponding to a density of ∼3, 6, 9 and 12 kg/m³ and referred to as D₁, D₂, D₃ and D₄, respectively. Each treatment was applied to triplicate groups in a completely randomized design. No treatment-related mortality was observed. The fish densities increased throughout the experiment from 3 to 23.5, 6 to 43.6, 9 to 56.6 and 12 to 69 kg/m³. Results show that mass gain and specific growth rate (SGR, %M/day) were negatively correlated with increased stocking density. Groups of the D₁ treatment reached a mean final body mass (FBM) of 119.3 g v. 88.9 g for the D₄ groups. Feed conversion ratios (FCRs) were 1.38, 1.54, 1.62 and 1.91 at D₁, D₂, D₃ and D₄ treatments, respectively. Growth heterogeneity, expressed by the inter-individual variations of fish mass (CV_M), was significantly affected by time (P < 0.001), stocking density (P < 0.001) and their interaction (P < 0.05). The difference in CV_M was particularly conspicuous towards the end of the experiment and was positively correlated with stocking density. Similarly, radiographic study shows that CV_FI was also found to be significantly greater for groups reared at high stocking densities (D₃ and D₄) than the other treatments (D₁ and D₂). These differences in both CV_M and CV_FI related to the stocking density need to be taken into account by husbandry practices to assure the production of more homogeneous fish size. A simple economic analysis indicates a parabolic relationship between profit and density with optimal final density at the peak of the curve. Given reasonable assumptions about production costs, the optimal final density (D_opt) is 73.7 kg/m³. A sensitivity analysis shows that changes in the fixed cost have no effects on the optimal final density. However, small change in variable costs, such as feed and juvenile costs, may have substantial effect on the optimal density.     

饲料脂肪水平对瓦氏黄颡鱼生长及脂蛋白脂酶基因表达的影响

研究采用脂肪水平分别为4.7%、7.9%、10.9%、15.4%、18.9%的五种等氮配合饲料饲喂瓦氏黄颡鱼早期幼鱼,进行了为期30d的生长实验,探讨了瓦氏黄颡鱼早期幼鱼的脂肪需求。并克隆了瓦氏黄颡鱼脂蛋白脂酶(LPL)cDNA序列片段,采用实时荧光定量PCR研究了饲料脂肪水平对肝脏LPL基因表达水平的影响。结果表明,饲料脂肪水平从4.7%增加到10.9%显著促进了瓦氏黄颡鱼早期幼鱼的生长(P<0.05)。饲料脂肪水平显著影响了实验鱼的鱼体体成分,随着饲料脂肪水平的升高,鱼体干物质和脂肪含量显著增加而蛋白含量显著下降(P<0.05)。高脂诱导了瓦氏黄颡鱼肝脏LPL基因表达,摄食15.4%、18.9%这两组较高脂肪水平的实验鱼肝脏LPLmRNA表达水平显著升高(P<0.05)。根据特定生长率通过折线回归分析得出瓦氏黄颡鱼早期幼鱼最适脂肪水平为11.2%。       

Ammonium uptake usingUlva (Chlorophyta) in intensive fishpond systems: mass culture and treatment of effluent

A vegetative clone ofUlva lactuca L. was selected for mass culture and nutrient uptake experiments with fish pond wastewater. Growth rates of over 55 g dry wt. d^?1 per 6001(1 m²) tank were obtained. Growth rate was linked to stocking density, tank flushing rates and aeration induced thallus movement. The plants could not survive on the macronutrients provided by a weekly pulse of wastewater. A continuous supply of fish pond wastewater was required to maintain good growth rates. An ‘uncoupling’ of growth rate and thallus nitrogen content was observed. The plants were able to store nitrogen from a pulsed ammonium supply and allot the nitrogen reserves to new tissue growth. Plants with slower growth rates or a continuous supply of ammonium had higher thallus nitrogen content.Ulva efficiently removed up to 85% of the ammonium from fish pond wastewater in darkness or light independently of temperature fluctuations.     

Effect of meal size on excess post-exercise oxygen consumption in fishes with different locomotive and digestive performance

Effects of feeding on pre-exercise VO₂ and excess post-exercise oxygen consumption (EPOC) after exhaustive exercise were investigated in sedentary southern catfish, active herbivorous grass carp, omnivorous crucian carp, and sluggish omnivorous darkbarbel catfish to test whether feeding had different effects on EPOC and to compare EPOC in fishes with different ecological habits. For fasting fish, the pre-exercise and peak post-exercise VO₂ were higher and recovery rates were faster in crucian carp and grass carp compared to those of darkbarbel catfish and southern catfish. EPOC magnitudes of grass carp and southern catfish were significantly larger than those of crucian carp and darkbarbel catfish. Feeding had no significant effect on peak post-exercise VO₂, recovery rate, and EPOC magnitude in grass carp. Both the pre-exercise and peak post-exercise VO₂ increased with meal size, while the EPOC magnitude and duration decreased significantly in the larger meal size groups of crucian carp and southern catfish. In darkbarbel catfish, both the pre-exercise and peak post-exercise VO₂ increased with meal size, but the VO₂ increment elicited by exercise was larger in feeding groups compared with the fasting group. These results suggest that (1) the characteristics of the post-exercise VO₂ profile, such as peak post-exercise VO₂ and recovery rate, were closely related to the activity of fishes, whereas the EPOC magnitude was not and (2) the effects of feeding on EPOC were more closely related to the postprandial increase in VO₂.     

Aggressive behavior as a mechanism of spatial differentiation of juvenile salmonid fishes (based on Black Sea brown trout Salmo trutta labrax (salmonidae))

Aggressive behavior of hatchery-reared juvenile Black Sea brown trout Salmo trutta labrax at the age of 5.5–6.0 months is investigated. Shortage of suitable territory leads to the separation of the fish into two spatial groups: demersal and pelagic. During the process of spatial differentiation of the fish, the individuals that have not selected permanent habitats (demersal or pelagic) are characterized by the highest aggression level. The duration of the formation of stable spatial differentiation of the fish depends on the stocking density. At a low stocking density (10–45 fish/m²), spatial differentiation is established by the beginning of the second day after the transfer of the fish to new conditions; at a high stocking density (182 fish/m²), it is established approximately by the seventh day. Following the establishment of the (secondary) spatial groups, aggressive acts are registered mainly between the individuals from the same spatial group. A role of aggressive behavior in intrapopulation differentiation of brown trout is discussed.     

The effect of exhaustive chasing training and detraining on swimming performance in juvenile darkbarbel catfish (<Emphasis Type="Italic">Peltebagrus vachelli</Emphasis>)

To investigate the effects of exhaustive chasing training and detraining on the swimming performance of juvenile darkbarbel catfish (Peltebagrus vachelli Richardson), we performed exhaustive chasing training daily for 14 days and subsequently detrained fish for 7 days. Chasing training resulted in significant increases in critical swimming speed (U _crit), post-chasing peak oxygen consumption rate (VO_{2 peak}), and heart and gill indexes compared with non-trained controls. Both resting oxygen consumption (VO_{2 rest}) and excess post-chasing VO₂ (EPOC) were unaffected by exhaustive chasing training. Fish that underwent chasing training had lower levels of whole-body lipid content and reduced food intake and growth compared with non-trained control fish; however, condition factor was not affected by chasing training. Seven days of detraining reversed the effects of exhaustive chasing training. Overall, these data suggested that short-term exhaustive chasing training improves aerobic swimming capacity in darkbarbel catfish, but the training effects are reversible over a short period of time.     

The effect of initial larval stocking density on growth and survival in common barbel Barbus barbus (L.)

Aim of the study was to determine the effect of initial larval density on growth and survival of the common barbel Barbus barbus (L.) under controlled conditions. The experiment examined fish density from 20 to 200 ind. L^?1 during intensive rearing at 25°C. Rearing lasted 21 days whereby the average total fish length, weight and survival were recorded. During the entire period fish were fed live nauplii Artemia sp. The data provided the basis for calculating the specific growth rate (SGR), Fulton’s condition index (K), and analysis of the relationship between biomass and the stocking density. The documentation was also used to determine the level of ontogeny. The study data obtained indicated that initial larval density has no effect on survival (over 98% in each treatment), level of ontogeny, or on the rearing indexes. Fish achieved the juvenile stage during the second week of rearing (<23 mm total length). At the end of the experiment fish reached 26.50–27.78 mm TL. SGR ranged from 22.75 to 23.53 (% day^?1). No statistical differences were found among all treatments (P > 0.05). The findings of this study suggest that stocking density in common barbel larvae is not a factor limiting growth or the ontogenetic development rate during initial rearing. This creates considerable potential for improvement in stocking material production under controlled conditions and should have a positive effect on the economic effectiveness of production.     

Growth and body composition of rohu, Labeo rohita (Hamilton), fed compound diet: winter feeding and rearing to marketable size

A 122‐day growth trial was conducted to observe the impact of polyhouse in winter on growth, conversion efficiencies and body composition of fingerling Labeo rohita (7.9 ± 1.1 cm; 4.60 ± 0.18 g). Fish were stocked (1.5 m^?2) in polyhouse and outdoor concrete tanks (250 m²) in duplicate and fed a compound diet [35% crude protein (CP)] to apparent satiation twice daily, at 09.00 and 17.00 hours. A natural photoperiod was maintained during the trials. At the end of the trials, polyhouse‐reared fish produced significantly (P < 0.01) higher values for weight increment, specific growth rate (SGR %), protein efficiency rate (PER), protein productive value (PPV) and better feed conversion ratio (FCR). Polyhouse‐reared fish showed higher (P < 0.01) CP and fat, and lower (P < 0.01) moisture and ash contents in the muscle. In the second part of the study, which also lasted 122 days, polyhouse‐reared young L. rohita (21.7 ± 1.3 cm; 127.8 ± 0.69 g) were raised to marketable size in outdoor earthen ponds (500 m²) at a stocking density of 0.37 m^?2 to examine the effects of feeding traditional feed mixture and compound diet (30% CP) on growth and body composition of fish. Labeo rohita fed the compound diet showed higher (P < 0.01) values for weight increment, SGR (%), PER and PPV, and better (P < 0.01) FCR than those fed traditional feed. Proximate composition of fish muscle showed higher (P < 0.05) CP and fat, and lower (P < 0.05) moisture and ash contents in fish fed the compound diet. The results suggest that polyhouse may be used successfully to culture L. rohita during winter, and that thereafter the fish may be reared in earthen ponds using the compound diet to obtain better production.     

Mass‐marking of farmed European eels (Anguilla anguilla (Linnaeus, 1758)) with alizarin red S

The Working Group on Eel of the International Council for the Exploration of the Sea (ICES) regularly reports that a significant amount of stocked eels in Europe was pre‐grown in aquaculture farms prior to stocking—so called “farmed eels.” The ICES advices chemical marking of stocked recruits to ensure their traceability throughout all life stages. To date, however, there was a lack of knowledge concerning the most suitable chemical substance and its application on farmed eels. The aim of this study was to fill this gap by presenting successful attempts of marking those eels with alizarin red S (ARS). An ARS concentration of 150 mg L^?1 buffered with 150 mg L^?1 Tris(hydroxymethyl)aminomethane applied as an immersion bath over 9 h was sufficient to mark a total of 3572 kg of farmed eels (6.5–8.0 g mean body weight). The marking success was 100% on otoliths and highest stocking density of up to 67.1 kg m^?3 (corresponding 54.0 kg m^?2) turned out to have no effect on mortality which was consistently below 1%.     

Movements of brown trout, Salmo trutta, and juvenile Atlantic salmon, Salmo salar, in a coastal stream in northern Norway

Movements of resident brown trout (age 2+ to 9+ years) and young Atlantic salmon (age 1+), stocked as fry, were studied in July, August and September in a coastal stream in northern Norway. Between 85 and 89% of the brown trout were recaptured in the study area (346m, 1326m²) within 45m of their release point throughout the investigation period. Most specimens had moved less than 150m. Trout movements were related to local variation in density. Trout occupying those sections of stream with the lowest fish densities (5.3–10.9 fish 100m^?2) had a significantly lower movement rate than fish from sections with densities between 13.7 and 31.5 fish 100m^?2. Trout that moved from their marking section were significantly larger than specimens that did not leave their original site. There was a significant correlation between permanence of station each month and the mean water level that month. The majority of the trout (47%) were caught at undercut stream banks or at sites in the proximity of these. A decrease in water level during the study period resulted in a high loss (36%) of such habitat, probably forcing some individuals to move. The recapture data indicate that the trout population consists of one small (c. 15–20%) mobile, and one large sedentary component. Young salmon displayed high station permanence in July and August (93% and 96%). However, in the autumn they exhibited a significant downstream movement, and only 73% were recaptured within their release section. This movement was significantly higher for larger specimens, and is thought to occur because of a pre-winter change in habitat, initiated by a decline in stream temperature. In contrast to trout, salmon in sections containing the lowest densities (22.0–25.0 fish 100m^?2) did not show significantly lower movement rates when compared with salmon at higher densities (32.2–46.3 and 51.8–60.6 fish 100m^?2). The spatial distribution of young salmon indicated the formation of territorial mosaics over the stream bed, which are thought to reduce intraspecific competition.     

Growth response of cultured larvae of silver therapon Leiopotherapon plumbeus (Kner, 1864) in outdoor tanks in relation to fertilizer type and fish density

This study evaluated the effects of fertilizer type and fish density on early growth and survival of silver therapon Leiopotherapon plumbeus (Kner, 1864) larvae reared in outdoor tanks. In the first experiment, larvae (1.92 ± 0.09 mm total length) were stocked into nine, 4 m³ tanks at an initial density of 0.5 larvae L^?1 and reared for 42 days at an ambient temperature of 28.8–30.7°C. Three treatments with three replicates each were compared: organic (chicken manure, OF) or inorganic fertilizers (ammonium phosphate, IF) applied once every 2 weeks, and the unfertilized (NF) tanks serving as the control group. Water quality, zooplankton densities, survival or growth of L. plumbeus larvae did not vary significantly in either fertilized or unfertilized tanks. Fertilization resulted in elevated nutrient concentrations, which did affect survival (2.10%–6.07%) of the fish larvae. In the second experiment, larvae were stocked at densities of 0.4 or 0.6 larvae L^?1 in tanks fertilized at 4–5 days interval with OF and IF for 30 days. Growth performance of L. plumbeus larvae was affected by fish density, with significantly larger (20.04 ± 2.65 mm in total length) and higher specific growth rate (SGR; 6.97 ± 0.48% day^?1) at 0.4 larvae L^?1 than at 0.6 L^?1. Fry production did not vary significantly between fish density treatment groups given the same fertilizer types, but survival rates were improved at 0.4 L^?1. Together, production of L. plumbeus larvae in outdoor tanks can be optimized at a lower stocking density, regardless of the type of fertilizer used.