Effects of stocking density on growth and skin color of juvenile darkbarbel catfish Pelteobagrus vachelli (Richardson)

A 3‐month experiment was conducted to investigate the effects of stocking density on growth performance and skin color of juvenile darkbarbel catfish. Experimental fish (5.0 ± 0.6 g; n = 160) were stocked in triplicate in 2 m² concrete tanks in a greenhouse at the initial density of 50, 150, 400 and 650 fish m^?2, respectively (0.42, 1.25, 3.33 and 5.42 kg m^?3). At the end of the experiment, 60 fish from each tank were sampled to record body weight and total length, and six fish from each tank were sampled to measure skin color by instrumental color analysis. Results showed that stocking density affected growth performance significantly (P < 0.05). Final mean weight (W), final mean total length (L) and specific growth rate (SGR) were significantly higher at 150 fish m^?2 treatment and lower in both the highest and lowest (650 and 50 fish m^?2) density treatments (P < 0.05). The condition factor (CF) was higher in 150 and 50 fish m^?2 and lowest at 650 fish m^?2 treatment; the coefficient of variation of weight (CV) tended to increase with the increase in stocking density, which was lowest in 50 fish m^?2 and highest in 650 fish m^?2. Stocking density also affected the skin color parameters, L* (lightness), a* (redness), b* (yellowness) and h* (hue), significantly (P < 0.05). The values of these parameters tended to decline with the increase in stocking density. Results of the present study suggest that growth performance and skin color responses to stock density exhibit different patterns for darkbarbel catfish juveniles: highest and lowest densities impaired growth rate, and higher density had a darkening effect on skin color.     

Welfare status of cultured seabass (Dicentrarchus labrax L.) and seabream (Sparus aurata L.) assessed by blood parameters and tissue characteristics

Seabass (SBS) and seabream (SBM) juveniles were reared with the goal of obtaining three different final densities (SINT = 40 kg m^?3; INT = 20 kg m^?3; SEM = 0.2 kg m^?3) to ascertain the effects thereof on the welfare of the fish. Significant blood metabolites and hepatic glycogen were determined every 3 months and at harvest. Trials lasted 18 months in seabass and 17 months in seabream. At the end of experiment the main biometric productive parameters and quality of body composition were also recorded. Regarding intensively reared seabass (SINT‐SBS, INT‐SBS), the plasma triglycerids, total cholesterol and transaminases (AST, ALT) were always significantly higher than in semi‐intensively maintained fish (SEM‐SBS). At the final sampling in the SINT‐SBS batch, the total protein and glucose were also markedly increased. Conversely, at harvest the liver glycogen content decreased in SINT‐SBS (34 ± 8 mg g^?1 liver) with respect to INT‐SBS (57 ± 12 mg g^?1 liver) and SEM‐SBS (63 ± 11 mg g^?1 liver). No differences among groups were observed for creatine kinase (CK) and lactate dehydrogenase (LDH). With regard to seabream, SINT‐SBM and INT‐SBM constantly showed plasma triglycerids and total cholesterol as being significantly higher when compared to SEM‐SBM. In the final two blood samplings, SINT‐SBM exhibited the most elevated values for LDH. At harvest, AST, ALT, total protein and glucose markedly increased in SINT‐SBM, whereas liver glycogen content was reduced (22.5 ± 9 mg g^?1 liver), more than in INT‐SBM (70 ± 16 mg g^?1 liver) and SEM‐SBM (75 ± 20 mg g^?1 liver). In both seabass and seabream, body composition was very similar in the different stocking densities, except for total cholesterol. Total n‐3 polyunsaturated fatty acid (PUFA) in seabream was significantly different from fish of the semi‐intensive groups; however, nutritional values and fatty acid profiles were equally good. The intermediate final density of seabass and seabream at 20 kg m^?3 seemed to give the best results in terms of their well being when compared to fish reared at 40 kg m^?3. The absence of differences in blood metabolites and hepatic glycogen levels between the intensive batch and the semi‐intensive groups until harvest was a reference to the positive status of the fish. A density of 20 kg m^?3 can be considered acceptable for farm strategy planning for raising healthy on‐growing seabass and seabream juveniles.     

Evaluation of the effects of different stocking densities on growth and stress responses of juvenile hybrid grouper ♀ Epinephelus fuscoguttatus × ♂ Epinephelus lanceolatus in recirculating aquaculture systems

This study was aimed at evaluating the physiological and metabolic responses of juvenile hybrid grouper ♀ Epinephelus fuscoguttatus × ♂ Epinephelus lanceolatus to stocking density. Hybrid grouper juveniles (mean ± SE = 25.43 ± 2.36 g live mass) were stocked for 22 weeks in a recirculating aquaculture system (RAS) under four different densities: low stocking density (LD; 1.03 kg m⁻³), medium stocking density (MD; 2.06 kg m⁻³), high stocking density (HD; 3.09 kg m⁻³) and extra-high stocking density (EHD; 4.11 kg m⁻³). Biometric variables were recorded and plasma, liver, intestine and stomach samples were taken for biochemical analysis at the end of the experimental period. Final stocking densities were 6.27, 16.04, 23.77 and 28.32 kg m⁻³, respectively, with significant differences in growth performance. Our results showed that the best growth rates and feed utilisation occurred in the MD group. Higher plasma cortisol and glucose levels and lower triglyceride levels reflected the stress responses in the EHD group. Moreover, the activity of aspartate and alanine transaminases was elevated in the HD and EHD groups due to enhanced gluconeogenesis. The activity of the digestive enzyme pepsin significantly increased in the MD group. We found that 2.06–3.09 kg m⁻³ is the most suitable starting density for culturing juvenile hybrid grouper in recirculating aquaculture systems.     

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.     

Growth performance of juvenile Russian sturgeon (Acipenser gueldenstaedtii Brandt & Ratzenburg, 1833) at two stocking densities in net cages

The growth performance of juvenile sturgeon (Acipenser gueldenstaedtii Brandt & Ratzenburg, 1833) was studied in experimental net cages (2 m × 2 m × 2 m) in freshwater Sapanca Lake. Fertilized eggs were imported from Krasnodar, Russia in January 2001. Two experimental stocking densities were employed (12 and 8 ind. m⁻³). Sturgeon were hand‐fed a pelleted commercial trout feed twice daily throughout the 203‐day experiment period. Fish were grown at ambient temperature (12.7–28.5°C) and natural photoperiod. Initial and final mean body weights were 279.5 g (±31.27) and 1112.8 g (±234.77) in the first group, and 271.1 g (±28.21) and 1140.5 g (±213.31) in the second group, respectively. Results were a fourfold biomass density increase in both trials from an initial 3.48 to 12.4 kg m⁻³ (high density) and 2.2 to 8.8 kg m⁻³ (low density). Mean specific growth rate (SGR) was determined as 2.69% day⁻¹ in both groups. Mean condition factor (CF) at the end of the experiment was 0.36 ± 0.05 in the high‐density group and 0.41 ± 0.05 in the low‐density group. Mean feed conversion ratio (FCR) was calculated to be 5.7 and 5.8, respectively. Survival was 92.7% in the first group (high density) and 96.1% in the second group (low density). The results demonstrated that sturgeon juveniles can be raised in cages and that growth performance (CF, SGR and FCR) and mortality of sturgeon were not significantly different at the two stocking densities (P > 0.05) tested.     

Responses to handling and confinement stressors in juvenile great sturgeon Huso huso

The effects of acute stressors on physiological responses of juvenile great sturgeon or beluga Huso huso L. were investigated in two experiments. In the first experiment, fish were handled by placing them in containers at either low density (LD, one fish l^?1) or high density (HD, four fish l^?1) for 60 s. Concentrations of plasma cortisol, glucose and lactate were determined from blood collected at 0, 1, 3, 6 and 12 h after application of the stressor. Plasma cortisol concentrations increased after the disturbance in H. huso from both handling treatments, but changes were not significant. Plasma glucose rose significantly by 22·9 and 31·6% in LD and HD handling treatments, respectively, after 3 h. Significant increases in plasma lactate occurred within 1 h in both treatment groups, but that of the HD group was much higher. In the second experiment, fish were held at two different densities, LD (2 kg m^?2 tank bottom surface area) and HD (7 kg m^?2), for 8 weeks and then subjected to an aerial emersion handling stressor in a net for 60 s; blood samples were taken before handling (resting, 0 h) and at 1, 3, 6 and 9 h after handling. Plasma cortisol increased significantly in fish from the HD treatment from 8·8 ± 0·3 to 19·2 ± 2·4 ng ml^?1 (mean ±s.e. ) by 1 h after stress, but post‐handling changes in the LD group were not significant. Significant increases in both plasma glucose and lactate were observed by 1 h in both treatment groups, with peak levels of plasma glucose evident at 3 h [69·4 ± 2·9 and 60·9 ± 1·7 mg dl^?1 (mean ±s.e .) in LD and HD groups, respectively]. Plasma glucose levels were significantly higher in the LD group than in the HD group at 3 and 6 h. Post‐handling haemoglobin content increased by 1 h and white blood cell numbers were reduced by 3 and 6 h in the HD treatment group compared with resting values, but changes in these blood features in the LD group were not significant. Acute handling did not affect haematocrit in either treatment. The results suggest that H. huso is relatively resistant to handling and confinement, and could tolerate normal hatchery practices associated with aquaculture. Because changes in cortisol concentrations were relatively low compared with those in most teleosts, glucose and lactate concentrations may be more useful as stress indicators in juvenile H. huso. This study also demonstrated that prior exposure to a chronic stressor, specifically high stocking density, could alter the physiological response to subsequent acute handling in H. huso.     

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.     

Mysid shrimp dynamics in relation to abiotic and biotic factors in the coastal lagoons of Maryland,Mid-West Atlantic,USA

The distribution and densities of two mysid species, Neomysis americana and Americamysis bahia in the coastal lagoons of Maryland were investigated using data collected monthly from March 2012 to December 2013. Mysid density was higher in the northern than southern bays in 2013 for Americamysis (P?=?0.043) and Neomysis (P?=?0.004). Neomysis americana density was inversely correlated with temperature (P?Neomysis densities (nos. m^?2?±?S.E) were negatively related to the catch-per-unit effort (CPUE?±?S.E) of fish predators (r?=??0.40, ?0.81; P?=?0.051, 0.002, respectively). Mysid density was higher in winter (March, 1.3?±?0.8 and spring, May, 1.4?±?1.3) 2012, when the CPUE of bay anchovy (Anchoa mitchilli) was relatively low (April: 5.8?±?2.3; and May: 11.9?±?4.5, 2012), than during summer (August, 0.003?±?0.002 nos. m^?2), when fish CPUE was relatively high (191.7?±?66.5). The observed seasonal patterns of mysid and fish densities, supported by the fact that mysids comprise 53–97% by weight of the diet of some of the abundant fish species in the bays, suggests a top-down control of mysids by fish predation. However, summer high temperatures in the bays exceeded 22°C, at which juveniles of N. americana have been reported to suffer high mortality, and perhaps contributed to the lower density of mysids in the summer.     

Survival,growth and physiological responses of juvenile Japanese flounder (Paralichthys olivaceus,Temminck & Schlegel, 1846) exposed to different dissolved oxygen concentrations and stocking densities

The aim of this study was to determine to what extent juvenile Japanese flounder can adapt to different stocking densities in captivity and to examine whether growth and some physiological parameters critical for welfare might be affected by different dissolved oxygen levels. Japanese flounder (Paralichthys olivaceus) juveniles (initial weight 1.27 ± 0.04 g/fish) were reared at five stocking densities (500, 1,000, 1,500, 2,000, 2,500 ind/m³) and two levels of dissolved oxygen (DO) concentration (5.5 ± 0.5 mg/L or 14 ± 2 mg/L) with duplicate tanks for each treatment in water‐recirculating systems for 40 days. Survival and activity of superoxide dismutase (SOD) were not affected either by stocking density or dissolved oxygen, whereas final body weight, specific growth rate (SGR) and feed conversion efficiency (FCE) of fish under low DO concentration decreased significantly with increasing stocking density. In contrast, growth of fish reared in high DO levels were unaffected by the stocking density. Furthermore, fish in this group had a higher feed intake and, consequently, grew faster (SGR) and achieved a higher final weight than fish reared at the low DO level. A significant reduction in hemoglobin (Hb) concentration and red blood cell (RBC) count of fish were recorded as DO concentration increased. Furthermore, the activity of protease decreased significantly with increasing stocking density and increased significantly with increasing DO concentration. The ventilation frequency results indicate that gill ventilation decreased significantly as DO levels increased. This study demonstrates that stocking density can affect the growth performance and physiological parameters critical for welfare of juvenile Japanese flounder. Also shown is that pure oxygen supplementation is an effective way to improve the growth of juvenile Japanese flounder when reared at a high stocking density.     

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.     

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.     

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 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.     

Estimating Esox lucius (Esocidae, Esociformes) density and population structure in a large European alluvial river: the Allier (France)

We developed a sampling protocol for pike (Esox lucius L., 1758) in large alluvial rivers in order to estimate adult population density and structure. This species is known as an indicator of lowland river biotic integrity in Europe evidencing the interest to determine population densities to a reliable degree of accuracy. A two occasion mark-recapture experiment based on a combination of net fishing and boat electro-fishing was specifically designed for monitoring of backwaters and channel sectors during the annual high flow level of an alluvial river, the Allier (France). Estimated densities calculated by using Chapman-Petersen and Loglinear models were weak, ranging from 0.68 (±0.16) fish.ha^?1 to 0.71 (±0.20) fish.ha^?1. However, when the densities are divided by the area of potential riverine habitats, pike abundance was calculated as being 15.3 (±3.7) fish.ha^?1 to 15.9 (±3.8) fish.ha^?1. Pike growth was high and 8 to 9 year-old females were captured. Age and sex distribution emerged as unbalanced, highlighting the precarious status of pike in the studied stretch of the river due to very limited recruitment. Pike were mainly captured in parapotamic side arms which underlines the importance of lateral habitats in semi regulated large alluvial rivers with wide riffle-pool-run sequences dominating geomorphological units.     

Density-dependent growth performance of Indian major carps in rainwater reservoirs

The study was carried out to evaluate growth performance of Indian major carps at different stocking densities in rainwater reservoirs. In this experiment, the absolute growth performance of Catla catla, Labeo rohita and Cirrhinus mrigala at stocking densities of 5000, 8000 and 11 000 fingerlings per hectare was 377.7, 215.4 and 241.9; 370.2, 186.7 and 219.1; and 306.7, 163.4 and 180.6 g, respectively. The recorded biomass yield at 5000, 8000 and 11 000 fingerlings per hectare was 1035, 1572 and 1573 kg ha^?1 per 180 days of culture. The effect of stocking density on production performance (performance index, PI) was highly significant (P < 0.05) at the higher stocking density of 11 000 ha^?1, while there was no significant variation between PI at stocking densities of 5000 and 8000 ha^?1. This indicates optimum production performance at 8000 ha^?1, where yield is significantly higher (P < 0.05) than yield at 5000 ha^?1 and almost equal to the yield at 11 000 ha^?1. An increase in stocking density from 8000 to 11 000 ha^?1, however, showed a sharp decline in average mean body weight of each species, even with supplemental feeding. With an increase in stocking density, the biomass yield increased to an optimum (1572 kg ha^?1), with no substantial increase thereafter. Reductions in growth, which occurred at high density, did not appear to be due to poor water quality as the water quality did not differ significantly among various treatments. Thus, the reduced survival and growth at high density appears to be a behavioural interaction or physiological response to density itself.     

Macrophytic, epipelic and epilithic primary production in a semiarid Mediterranean stream

Summary 1. Primary production by Chara vulgaris and by epipelic and epilithic algal assemblages was measured in a semiarid, Mediterranean stream (Chicamo stream, Murcia, Spain) during one annual cycle. 2. The rates of gross primary production (GPP) and community respiration (CR) were determined for each algal assemblage using oxygen change in chambers. The net daily metabolism (NDM) and the GPPd^?1 : CR₂₄ ratio were estimated by patch‐weighting the assemblage‐level metabolism values. 3. Gross primary production and CR showed significant differences between assemblages and dates. The highest rates were measured in summer and spring, while December was the only month when there were no significant differences in either parameters between assemblages. GPP was strongly correlated with respiration, but not with algal biomass. 4. Chara vulgaris showed the highest mean annual metabolic rates (GPP = 2.80 ± 0.83 gC m^?2 h^?1, CR = 0.76 ± 0.29 gC m^?2 h^?1), followed by the epilithic assemblage (GPP = 1.97 ± 0.73 gC m^?2 h^?1, CR = 0.41 ± 0.12 gC m^?2 h^?1) and epipelic algae (GPP = 1.36 ± 0.22 gC m^?2 h^?1, CR = 0.39 ± 0.06 gC m^?2 h^?1). 5. The epipelic assemblage dominated in terms of biomass (82%) and areal cover (88%), compared with the other primary producers. Epipelic algae contributed 84% of gross primary production and 86% of community respiration in the stream. 6. Mean monthly air temperature was the best single predictor of macrophyte respiration and of epipelic GPP and CR. However, ammonium concentration was the best single predictor of C. vulgaris GPP, and suspended solid concentration of epilithon GPP and CR. 7. Around 70% of the variation in both mean GPP and mean CR was explained by the mean monthly air temperature alone. A multiple regression model that included conductivity, PAR and nitrates in addition to mean monthly air temperature, explained 99.99% of the variation in mean CR. 8. Throughout the year, NDM was positive (mean value 7.03 gC m^?2 day^?1), while the GPP : CR₂₄ ratio was higher than 1, confirming the net autotrophy of the system.     

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.     

Effects of salinity,light intensity and sediment on growth,pigments, agar production and reproduction in Gracilaria tenuistipitata from Songkhla Lagoon in Thailand

The effects of salinity, light intensity and sediment on Gracilaria tenuistipitata C.F. Chang & B.M. Xia on growth, pigments, agar production, and net photosynthesis rate were examined in the laboratory under varying conditions of salinity (0, 25 and 33 psu), light intensity (150, 400, 700 and 1000 µmol photons m^?2 s^?1) and sediment (0, 0.67 and 2.28 mg L^?1). These conditions simulated field conditions, to gain some understanding of the best conditions for cultivation of G. tenuistipitata. The highest growth rate was at 25 psu, 700 µmol photons m^?2 s^?1 with no sediments, that provided a 6.7% increase in weight gain. The highest agar production (24.8 ± 3.0 %DW) was at 25 psu, 150–400 µmol photons m^?2 s^?1 and no sediment. The highest pigment contents were phycoerythrin (0.8 ± 0.5 mg g^?1FW) and phycocyanin (0.34 ± 0.05 mg g^?1 FW) produced in low light conditions, at 150 µmol photons m^?2 s^?1. The highest photosynthesis rate was 161.3 ± 32.7 mg O₂ g^?1 DW h^?1 in 25 psu, 400 µmol photons m^?2 s^?1 without sediment in the short period of cultivation, (3 days) and 60.3 ± 6.7 mg O₂ g^?1 DW h^?1 in 25 psu, 700 µmol photons m^?2 s^?1 without sediment in the long period of cultivation (20 days). The results indicated that salinity was the most crucial factor affecting G. tenuistipitata growth and production. This would help to promote the cultivation of Gracilaria cultivation back into the lagoon using these now determined baseline conditions. Extrapolation of the results from the laboratory study to field conditions indicated that it was possible to obtain two crops of Gracilaria a year in the lagoon, with good yields of agar, from mid‐January to the end of April (dry season), and from mid‐July to the end of September (first rainy season) when provided sediment was restricted.     

Experimental evidence that ecological effects of an invasive fish are reduced at high densities

Understanding the relationship between invasive species density and ecological impact is a pressing topic in ecology, with implications for environmental management and policy. Although it is widely assumed that invasive species impact will increase with density, theory suggests interspecific competition may diminish at high densities due to increased intraspecific interactions. To test this theory, we experimentally examined intra- and interspecific interactions between a globally invasive fish, round goby (Neogobius melanostomus), and three native species at different round goby densities in a tributary of the Laurentian Great Lakes. Eighteen 2.25 m² enclosures were stocked with native fish species at natural abundances, while round gobies were stocked at three different densities: 0 m^?2, 2.7 m^?2, and 10.7 m^?2. After 52 days, native fish growth rate was significantly reduced in the low density goby treatment, while growth in the high density goby treatment mirrored the goby-free treatment for two of three native species. Invertebrate density and gut content weight of native fishes did not differ among treatments. Conversely, gut content weight and growth of round gobies were lower in the high goby density treatment, suggesting interactions between round gobies and native fishes are mediated by interference competition amongst gobies. Our experiment provides evidence that invasive species effects may diminish at high densities, possibly due to increased intraspecific interactions. This is consistent with some ecological theory, and cautions against the assumption that invasive species at moderate densities have low impact.     

Bimodal oxygen uptake in a freshwater air-breathing fish,Notopterus chitala

Measurements of bimodal oxygen uptake have been made in a freshwater air-breathing fish,Notopterus chitala at 29.0±1(S.D.)°C. xhe mean oxygen uptake from continuously flowing water without any access to air, was found to be 3.58±0.37 (S.E.) ml O₂ · h^?1 and 56.84+4.29 (S.E.) ml O₂ · kg^?1 · h^?1 for a fish weighing 66.92 + 11.27 (S.E.) g body weight. In still water with access to air, the mean oxygen uptake through the gills were recorded to be 2.49 ± 0.31 (S.E.) ml O₂ · h^?1 and 38.78 ± 1.92 (S.E.) ml O₂ · kg^?1 · h^?1 and through the accessory respiratory organs (swim-bladder) 6.04±0.87 (S.E.) ml O₂ · h^?1 and 92.32±2.91 (S.E.) ml O₂ · kg^?1 · h^?1 for a fish averaging 66.92±11.27 (S.E.) g. Out of the total oxygen uptake (131.10 ml O₂ · kg^?1 · h^?1), about 70% was obtained through the aerial route and the remainder 30% through the gills.