Benthic cages versus floating cages in Octopus vulgaris: Biological performance and biochemical composition feeding on Boops boops discarded from fish farms

Some benthic cephalopods are considered potential candidates to diversify marine aquaculture, as they show fast growth and high market price. Most research on cephalopod culture is currently focusing on the development of specific enrichments and compound feeds, while little research has been conducted in order to test new rearing systems for cephalopods. The rigid characteristic of the floating cages commonly used for the ongrowing of Octopus vulgaris has restricted their use to calm water conditions (estuaries and harbors). Such sites are scarce and highly demanded, especially by the tourism industry; therefore the development of O. vulgaris grow out at these locations competes with touristic interests. The present study was set to compare the biological performance of O. vulgaris reared in a benthic cage (2 m²) as opposed to the traditional floating cage (2.5 m²), during two ongrowing trials. Initial rearing density was 10 kg m⁻³ and octopuses (892 ± 125 g) were fed on bogue Boops boops, discarded from fish farms, for 60–67 days. High growth (1.8–1.9% day⁻¹) and high survival (91–97%) were observed, regardless of the rearing system, and led to best biomass increment (178–212%) and food conversion rates (2.3–2.6) ever recorded for O. vulgaris under industrial rearing conditions. These results underline the adequacy of the benthic cage for the ongrowing of this species, and also the potential of aquaculture discarded fishes, particularly bogue, as a single diet for this species. High growth rates obtained and the high lipid content of bogue (44% dry weight) suggest efficient lipid utilization in this species. Proximate composition and fatty acid profile in octopus muscle was not affected by the rearing system. High dietary lipid content was not reflected in muscle proximate composition, which showed high protein (87% dw) and low lipid content (5% dw) by the end of the experimental period. Farmed octopus showed high levels of n-3 HUFA (42%), which should enhance its value for the consumers.     

Foam fractionation efficiency of a vacuum airlift—Application to particulate matter removal in recirculating systems

The accumulation of particulate organic matter (POM) in recirculating aquaculture systems (RAS) has become an important issue with the intensification of finfish production. The objective of this study was to assess the foam fractionation efficiency of a vacuum airlift in different conditions (POM concentrations, airflow rates, bubble sizes, water renewal rates and feed addition). In sea water, the vacuum airlift allowed removing 20% of the initial POM concentration per hour (foam fractionation efficiency), corresponding to a 20.7-fold concentration factor between the tank and the foam. In rearing conditions, efficiency increased with decreasing water renewal rate or increasing POM concentration. An increase in airflow rate from 10 to 80 L min⁻¹ in the vacuum airlift significantly decreased foam fractionation efficiency when feed was added to the water. The impact of feeding was only observed with high airflow rates where bubble coalescence occurred. Calculated POM production by fish ranged between 15.9 and 23.5 g h⁻¹ and was equivalent to estimations based on feed conversion ratio (FCR). This indicated that all the POM produced was extracted by the vacuum airlift.     

The effect of high ortho-phosphate water levels on growth,feed intake,nutrient utilization and health status of juvenile turbot (Psetta maxima) reared in intensive recirculating aquaculture systems (RAS)

In intensive recirculating aquaculture systems (RAS) ortho-phosphate (ortho-P) is one of the main accumulating substances, but effects of chronically elevated concentrations on fish health and production performance are still unknown. Therefore 120 juvenile turbot (Psetta maxima) were exposed to ortho-P concentrations of 3 mg/L (control – C), 26 mg/L (low – LP), 52 mg/L (medium – MP) and 82 mg/L (high – HP) for 56 days and fed until satiation with a commercial diet. Health status and feed conversion ratio (FCR) were not significantly affected by treatment (p > 0.05). Specific growth rates (SGR) and daily feed intake (DFI) of C were not considered significantly different from LP, MP and HP treatments, however LP showed significant higher DFI and SGR than HP (p < 0.05). Using non-linear regression between SGR and ortho-P concentrations, 27 mg/L ortho-P was found as the optimum for turbot growth. Although not reflected in blood plasma P levels (p > 0.05) a potential aqueous P uptake might result in metabolic benefits leading to the observed growth enhancement in the LP treatment.In a second experiment 114 juvenile turbot were exposed to ortho-P concentrations of 2 mg/L (C2) and 25 mg/L (LP2) for 63 days and fed until satiation with a low P diet (4.6 g digestible-P/kg diet). Overall production performance was low due to low voluntary feed intake. Whereas the FCR was unaffected by treatment (p > 0.05), significantly higher feed intake and biomass gain were observed for LP2 compared to C2 (p < 0.05). LP2 treatment showed a trend for higher protein retention efficiency and lower whole body lipid content (p < 0.1). The dry matter, ash, Phosphorus, Calcium and protein content in whole body did not significantly vary between treatments (p > 0.05).In conclusion the accumulation of ortho-P in RAS does not negatively affect health of turbot. Elevated ortho-P seems to have slight positive effects on production performance of juvenile turbot. Further research to quantify dietary P requirements for turbot in general, as well as for turbot raised under elevated ortho-P conditions in RAS is strongly required.     

The effect of light intensity on performance of larval pike-perch (Sander lucioperca)

In this work performance parameters of larval pike-perch (Sander lucioperca) reared under four different light intensities (100, 500, 1000 and 2500 lx) until 21 days post hatch (dph) were investigated. As performance parameters change in length and weight, swim bladder inflation, feed consumption, natural mortality, stress induced mortality and RNA-DNA ratio were measured. Aim was to investigate the influence of light intensity on pike-perch performance during the first three weeks of larval rearing. Significant differences were found in natural and stress induced mortality as well as in weight growth. No single light exposure level combined optimal performance of all tested performance parameters. Highest light intensity of 2500 lx showed good weight growth but an increase in stress induced mortality. Bright light of 500 and 1000 lx intensity was found to improve growth and stress mortality whereas dim light conditions of 100 lx showed significantly lower natural mortality. Thus data suggested that most favorable illumination during larval pike-perch rearing comprise a tradeoff between optimal natural mortality under dim light conditions (100 lx) or optimal larval growth and stress resistance under bright light conditions (500 and 1000 lx). It is shown that high light intensities during larval rearing can be beneficial for pike-perch rearing if offspring supply is not limited.     

Effect of alternate distribution of astaxanthin on rainbow trout (Oncorhynchus mykiss) muscle pigmentation

The aim of the present study was to determine the effects of the use of astaxanthin alternate feeding on rainbow trout pigmentation in term of astaxanthin serum concentration, muscle colour and astaxanthin muscle retention. Four hundred and fifty rainbow trout were fed the same basal diet supplemented with two different astaxanthin levels, 100, and 200 mg astaxanthin kg^? 1 of diet, hereafter designated as AX100 and AX200, respectively. An additional astaxanthin-free (AX0) diet was used. The experimental treatments were as follows: (1) REF = AX100 diet at each meal each day, served as reference; (2) SD1 = AX100 diet at each meal the first day followed by AX0 diet at each meal every second day; (3) SD2 = AX100 diet and AX0 diet in alternate meals each day; (4) R2 = AX200 diet and AX0 diet in alternate meals each day; (5) R4 = AX200 diet at the first meal the first day followed by AX0 diet at the second meal the first day and at each meal every second day. Fish were fed the experimental feeding schedule for 42 days.At the end of the experiment there were no significant differences among fish fed the different feeding schedules in term of final mean weight, specific growth rate and feed efficiency ratio. SD2 fish group displayed the highest (P < 0.05) astaxanthin serum concentration and the R4 fish group the lowest one. REF and R2 fish groups showed similar astaxanthin serum concentrations. Muscle chroma showed the most pronounced effect. It increased significantly for all fish groups during the experiment. At the end of the experiment REF and R2 fish groups displayed higher values than SD1 and R4 fish groups. Muscle astaxanthin concentrations increased significantly during the experiment whatever the astaxanthin feeding schedule. At the end of the experiment, the highest muscle astaxanthin concentration was recorded for R2 fish group while the lowest was noted for R4 fish group. Except for SD1 and R4 fish groups, muscle astaxanthin retention decreased significantly during the experiment. At the end of the experiment, muscle astaxanthin retention coefficients for SD2 fish group were significantly higher than those for REF fish group. The results reported here provide further evidence of the potential applicability of alternate astaxanthin feeding on rainbow trout pigmentation. Extending the optimisation of the SD2 treatment will therefore be subject for future studies. Its application could result in cost saving in the fish farming industry.     

Dietary phosphorus requirement of large yellow croaker,Pseudosciaena crocea R

A study was conducted to estimate the optimum requirement of dietary phosphorus for large yellow croaker in floating sea cages (1.0 × 1.0 × 1.5 m). Five practical diets were formulated to contain graded levels (0.30%, 0.55%, 0.69%, 0.91% and 1.16%) of available phosphorus from dietary ingredients and monocalcium phosphate (MCP). Each diet was randomly assigned to triplicate groups of 180 juvenile fish (initial body weight, 1.88 ± 0.02 g). Fish were fed twice daily (5:00 and 17:00) to satiation for 8 weeks. During the experimental period, the water temperature fluctuated from 26.5 to 32.5 °C, salinity from 32‰ to 36‰ and dissolved oxygen was more than 7 mg l^− 1. Specific growth rate (SGR) significantly increased with increasing available phosphorus from 0.30% to 0.69% of diet (P < 0.05), and then leveled off. The body composition analysis showed that the whole-body ash and lipid, as well as phosphorus content in the whole body, vertebrae and scales were significantly affected by dietary available phosphorus level (P < 0.05). Broken-line analysis based on SGR indicated the minimum available phosphorus requirement for the optimal growth of large yellow croaker was 0.70%. Based on the phosphorus content in either vertebrae or whole body, the requirements were 0.89% and 0.91%, respectively.     

Applications of computational fluid dynamics to modeling hydrodynamics in tilapia rearing tank of Recirculating Biofloc Technology system

Biofloc Technology (BFT) has been widely used in tilapia rearing. However, the Total Suspended Solids (TSS) at the bottom of rearing tank will increase to high concentration due to biofloc sedimentation and accumulation. Therefore, it will be have negative effects on the feeding enthusiasm, gill and even survival of cultured species, and especially in the initial stages of larvae culture. The Recirculating Biofloc Technology (RBFT) would be one of the potential solution for the above concerns. To achieve an accurate TSS regulation, the biofloc distribution should be as homogenous as possible. The aim of this work is to specify the optimal bubble size and Hydraulic Retention Time (HRT) to the uniformity of biofloc distribution in a RBFT system through Computational Fluid Dynamics (CFD) method. For this, a three-dimensional and three-phase unsteady transient model was developed to simulate the hydrodynamics in a gas-liquid-solid tilapia rearing tank based on the commercial software Workbenching 15.0. Firstly, to achieve a more reliable prediction, the influence of the grid types and the computation turbulence model (Standard, Renormalization-Group (RNG), Realizable) to the simulation result were discussed by contrasting the simulations and experiments results of solid holdup at dimensionless radial position. Secondly, appropriate mesh size (181395 elements) and mixture standard k-e model were implemented to study the effect of bubble size and HRT on solid distribution uniformity. Simulations were performed by using three different mean bubble sizes (diameter = 1, 2, 3 mm) and three different HRTs (0.56, 1.13, and 2.25 h) to study the sensitivity of the results to the uniformity of the biofloc distribution, respectively. The results showed that to get a better biofloc distribution, the bubble size should be range in 1 mm and 2 mm, and as the HRT decreasing, the distribution of biofloc distribution will be more homogeneous (0.45 ≤ HRT ≤ 0.56 h). This paper provides an essential data set for determining the bubble size and HRT in production, as well as evaluating the accuracy of various CFD models for capturing the complex flow field in a BFT rearing tank.     

Influence of feeding sequence,light and colour on the performance of a self-grading system designed for turbot (Scophthalmus maximus)

Grading of cultured flatfish is essential to maintain even sizes and to avoid potential feeding dominance or cannibalism. Current hand-grading and forced mechanical grading methods result in labour costs and additional stress to animals, reducing their growth performance. This study tested a self-grading system allowing turbot (Scophthalmus maximus) to self-grade, based on size, between tank sections within 8000 L tanks designated as “Graded (G)” and “Non-Graded (NG)”. The grading success (percentage of graded animals/total gradable individuals) with stimuli (light, feeding sequence and colour of the grading device) and without stimuli was analyzed over 9 days. Mean grading success without stimuli was 38.1 ± 11.5%. Grading success was not improved by changes in feeding sequence (30.6 ± 10.2%) or altered colour of the grading device (30.9 ± 13.8%). Marked increases in grading success were obtained by adding a light source to the G section (52.9 ± 12.4%), and by combining the presence of a light source and the feeding sequence (57.9 ± 14.4%). The combination of light sources and changes to the colour of the self-grading device resulted in the highest mean grading success (72.1 ± 18.2%) overall. Light and colour stimuli combined yielded the highest self-grading maximum (88.36 ± 5.3%), achieved on day 6 after experimental onset. A three-parameter asymptotic exponential equation was fitted to the data from each stimulus and parameters describing the curves of all conditions are reported. The present results indicate that the tested self-grading systems for turbot can be highly efficient if used with the appropriate stimuli. Self-grading systems operating at the measured levels of success offer a viable alternative to stressful grading events, avoiding significant commercial disadvantages as well as improving animal welfare and performance.     

A comparison of larval production of the northern scallop,Argopecten purpuratus,in closed and recirculating culture systems

Northern scallop Argopecten purpuratus aquaculture relies on an efficient all year-round larval supply. Larvae are generally produced in closed aquaculture systems (CAS) using the batch techniques with periodical water changes. For instance, survival rates are greatly variable and can range from 0 to 80% making production of scallop larvae uncertain. The main goal of this study was to determine the feasibility of rearing scallop larvae in a recirculating aquacultural system (RAS), and secondarily to compare scallop larval growth rate and time length to reach the settling stage when reared with a traditional Chilean CAS technique and in a novel RAS technique in an industrial-like approach.Several batches of larvae were cultured in CAS and RAS. Larvae were fed on Isochrysis galbana cultured in 35-L tubular photobioreactors. Growth rates were significantly different (F_11,2840 = 274.66; p < 0.001). All scallop larvae cultured in CAS showed lower growth rates ranging within 4.49 and 7.30 μm day^?1 and protracted period of culture until settlement (at least 10 more culture days) than those reared in RAS (growth rates between 9.56 and 13.15 μm day^?1). However, final survival (from D-larvae until settlement) of larvae reared in CAS showed higher values than those values recorded for larvae cultured on RAS. Higher growth rates observed in RAS could be attributed to a reduction in daily manipulation of the animals and/or more feed availability as well as higher temperatures and a steady state conditions in water quality. Even though, the reduction in time for rearing larvae until settlement in RAS was high, the comparison between systems is more significant in view of the reduction in make up seawater from 100% of system volume (CAS) to less than 10% of system volume (RAS). Therefore, RAS was independent from daily water quality variation from natural seawater by increasing water retention time, and with that improve water quality steady state conditions. Results of this research show that a more efficient use of water and heating systems than generally used in the Chilean hatchery industry is achievable. This is an important result since it could lead to significant reductions in the cost of operating a scallop hatchery, however further work is required to accurately compare the two systems (CAS and RAS). The main result from this research is that scallop larvae can be cultivated using recirculating aquaculture systems (RAS) as a method to increase production. The information reported in this paper will be useful for the improvement of scallop larvae culture techniques under controlled conditions.     

The effect of density on sea bass (Dicentrarchus labrax) performance in a tank-based recirculating system

Sea bass (Dicentrarchus labrax) (135 ± 4 g) were reared under tank-based recirculating aquaculture system for a 63-day period at four densities: 10, 40, 70, 100 kg m^?3. Fish performance, stress indicators (plasma cortisol, proteonemia plus other blood parameters—Na⁺, K⁺, glucose, pH, total CO₂^?) and water quality were monitored. At the end of the 63-day period, resistance to infection was also studied by a nodavirus challenge. A 25-day test was performed on fish from two extreme densities (10 and 100 kg m³) and one intermediate density (40 kg m³).With regards to the different density treatments, there was no significant difference between the daily feed intake (DFI) and the specific growth rate (SGR) up to a density of 70 kg m^?3. No significant difference was found between treatments concerning the feed conversion ratio (FCR) and the mortality rate. No density effect was observed on the fish stress level (plasma cortisol) or on sensitivity to the nodavirus challenge. Under these experimental rearing conditions, the density above 70 kg m^?3 has an impact on growth performance (DFI and SGR) indicators and also some blood parameters (CO₂) at the highest density tested (100 kg m^?3).This study suggests that a density up to 70 kg m^?3 has no influence on sea bass performance and welfare. At 100 kg m^?3, average specific growth rate was decreased by 14% without welfare deterioration according to the welfare indicators monitored.     

Inclusion of citric acid and/or amino acid-chelated trace elements in alternate plant protein source diets affects growth and excretion of nitrogen and phosphorus in red sea bream Pagrus major

Nutrient dense diets not only challenge the physiological capabilities of fish but also result in discharge of excess amounts into the environment, causing eutrophication of the receiving water. This study investigated the effect of dietary levels of citric acid (CA) and/or amino acid-chelated trace element (AA-CTE) on growth and nutrient retention in red sea bream and loading of phosphorus (P) and nitrogen (N). Six practical diets were formulated, the control being a fish meal-based diet (F0) without addition of P, CA and AA-CTE. Diet F1 was supplemented with 1% mono calcium phosphate (0.25% P). In diets C1 and C2 fishmeal was replaced with alternate plant protein ingredients and supplemented with 1% and 2% CA, respectively. Diets A1 and A2 were akin to diets C1 and C2, respectively but contained AA-CTE instead of the inorganic trace element. Fish weighing around 12 g were offered the respective diets to satiation during the 12-week trial. Growth and feed performance were lowest for fish fed F0 diet, but significant improvement (P < 0.05) was noticed among those fed diets supplemented with P, CA or AA-CTE. The F1 group had the best growth (final body weight 85.02 g) while A1 group had the best FCR (1). Absorption of P was significantly higher (P < 0.01) for fish that received CA and/or AA-CTE (C1, C2, A1 and A2) compared to group that did not receive them (F0 and F1). The P and nitrogen (N) retention values in fish fed diets with CA and/or AA-CTE were significantly (P < 0.01) greater compared to the rest and this facilitated a reduction in excretion rates. The diet supplemented with 1% CA to an alternate plant protein sources diet without inorganic P supplementation had better performance compared to the diets supplemented with 2% CA, 1% CA + AA-CTE, and 2% CA + AA-CTE. Thus, without addition of inorganic P, 1% CA supplementation to alternate plant protein sources diets significantly improved fish growth, FCR and nutrient retention, besides lowering N and P loading. An efficient level of CA is suggested to be 1% or less in alternative plant protein source diets (containing 35% fish meal). These findings would contribute to the pursuit of environment-friendly feed for red sea bream.     

Response to domestication and selection for growth in the European sea bass (Dicentrarchus labrax) in separate and mixed tanks

Selective breeding of European sea bass (Dicentrarchus labrax) receives a growing interest, as the estimated heritability of growth is medium to high. In this study, we compared the offspring of four groups of sea bass sires, mated with the same wild dams: wild (W), first generation of domestication (D), first generation of mass selection for length (M), first generation of PROSPER-like selection for length (P). The comparison was done both in replicated tanks (separate rearing) and in mixed tanks (mixed rearing) where sire origins were recovered by genotyping of eight microsatellite markers. Weight, length and growth rate were measured from day 238 post-fertilization (69 g mean weight) to day 611 post-fertilization (390 g mean weight). Both in mixed and separate tanks, both selected groups (P, M) were larger than unselected groups (W, D). No difference was seen at any time between W and D, nor between M and P. The selection response estimate on weight was larger in mixed tanks when compared to separate tanks (+ 42% in mixed tanks, + 23% in separate tanks at day 611), yielding realized heritability estimates of 0.60 and 0.34, respectively, and confirming the excellent potential of the species for growth improvement through selective breeding. Both selection response and the amplification effect between mixed and separate tanks decreased as rearing density increased. Our hypothesis is that selection response is magnified by competition in mixed tanks, while sub-optimal rearing conditions lower the observed selection response, more in separate tanks (where selected thus larger fish are at a higher density than unselected ones) than in mixed tanks (where all fish experience the same density effects).     

Effects of diet composition and ultrasound treatment on particle size distribution and carbon bioavailability in feces of rainbow trout

The effect of a high and low non-starch polysaccharide diet (HNSP and LNSP diet) and ultrasound treatment on particle size distribution and carbon bioavailability in fecal waste of rainbow trout (Oncorynchus mykiss) was studied. Feces were collected from four flow-through fish tanks, two tanks fed the HNSP diet and two the LNSP diet. The collected feces were sonicated (disintegrated) in duplicate with high-intensity (0.6 W/ml), low-frequency (f = 20 Hz) ultrasound at five different energy levels (0.6 W/ml for 0, 0.25, 1, 4, and 16 min). The particle size distribution of the treated feces samples was measured by wet sieving (1000, 500, 200, 100, 63, 36, 1.2 μm screen size) and total suspended solids (TSS) measurement. Carbon bioavailability in sonicated fecal waste samples was determined with oxygen uptake rate (OUR) tests. The results showed that: (1) feces from the HNSP diet contained significant more particulate material and bigger particles; (2) carbon bioavailability was almost three times higher in untreated LNSP feces when compared with HNSP feces; (3) almost 50% of HNSP feces could have been recovered on a microscreen of 36 μm after wet sieving, whereas it was only 10% for LNSP feces; (4) the production of small particles (1.2–36 μm), which could pass a drum filter screen and potentially accumulate in RAS, was approximately 50 g/kg feed, showing no significant differences between diets; (5) sonication increased fecal dry matter below 36 μm (p = 0.015), but it had no significant effect on the median particle size; (6) sonication increased carbon bioavailability with 7–10% for the HNSP feces (p = 0.037); (7) fecal particles withstood up to 16 min sonication at an intensity of 0.6 W/ml and a frequency of 20 Hz corresponding to specific energy input of 20,000 kJ/kg DM without major changes in particle size distribution. The results of this study indicate that the applied ultrasound treatment of fecal waste is not an effective method to increase short-term carbon bioavailability.     

Effects of two temperatures on the oxygen consumption rates of Seriolella violacea (palm fish) juveniles under rearing conditions

Oxygen consumption rates (mg O₂/kg fish/min; OC) of juvenile palm fish (average weight 420 g) were determined for temperatures of 14 and 18 °C. Three replicates of two tanks rearing fish at a density of 24 kg/m³ were used to measure OC at 34 ppt working as open respirometers in a recirculating system under farm-like conditions. The fish were fed commercial dry pelleted feeds at a ratio of 1% of total biomass. Oxygen consumption rates were determined by mass balance calculations. The OC increased from 1.6 to 2.4 g O₂/kg fish/day as temperature increased from 14 to 18 °C. The determination of oxygen consumption by palm fish in farm-like conditions provides valuable information on the oxygen requirement of these fish in an aquacultural setting. This bioengineering information can be used for designing and sizing a rearing facility for the intensive culture of palm fish.     

Design characteristics of pooled circular stepped cascade aeration system

Aeration experiments were conducted in a brick masonry tank of dimension 4 m × 4 m × 1.5 m to study the design characteristics of pooled circular stepped cascade (PCSC) aeration system. Based on dimensional analysis, non-dimensional numbers related to geometric, dynamic and process parameters were proposed. The non-dimensional geometric parameters – number of steps (N), ratio of total height of cascade (H) to the bottom radius of cascade (R_b), % coverage of circumference of each step by enclosure (P_e) and number of enclosures in each step (N_e) were optimized. Maintaining the optimized geometric parameters (N = 6, H/R_b = 0.25, P_e = 20% and N_e = 9), aeration experiments were further conducted at different discharges (Q) to develop simulation equations for prediction of aeration characteristics of PCSC aeration system at different dynamic conditions. Simulation equations for oxygen transfer and power consumption based on Froude (Fr) criterion were developed subject to 0.0014 ≤ Fr ≤ 0.0144. SAE of the developed prototype PCSC aerators based on estimated brake power ranged between 2.43 and 3.23 kg O₂/kWh.     

Effects of conventional and organic feed on the mineral composition of cultured European sea bass (Dicentrarchus labrax)

European sea bass (Dicentrarchus labrax) is a widely consumed marine fish in Mediterranean areas, and different farming techniques are applied for fish culturing to satisfy the growing demand for seafood. The aim of this study was to investigate the effects of conventional and organic feed on the quality of cultured European sea bass (Dicentrarchus labrax) collected during the growth period. The concentrations of ash, moisture, essential macro‐elements (Ca, K, Mg, Na and P), micro‐elements (Co, Cr, Cu, Fe, Mn, Ni and Zn) and toxic elements (As, Cd, and Pb) were determined in feeds and in fillets of cultured fish. The results were compared to those obtained from wild sea bass. Results showed that the differences between organic diet‐fed and conventional diet‐fed sea basses varied in relation to the specific element measured and the growth period. The former showed higher concentration of Fe, Mg and Cr, and lower Na content. The amount of P, Na, Fe, Cu and Cr in wild sea bass was significantly (P ≤ 0.05) higher than that found in farmed fish. The levels of toxic elements in cultured sea bass were always within the allowed limit for fishery products. Wild samples had significantly (P ≤ 0.05) higher content of arsenic and lead than farmed sea bass. Cultured fish represent a valuable dietary source of essential macro‐ and micro‐elements. Controlled rearing systems and feedings were related to a decrease in the presence of some toxic metals in cultured fish compared with wild fish.     

A mechanized harvest system for freshwater fishpond

Development of a prototype-mechanized fish harvesting system for carp from a rectangular shaped fishpond is described in this paper. The system consists of motorized trolleys on the longer dykes of fishpond, rail, boom pipe, netting arrangement and ON/OFF arrangements. The trolleys move on the rail tracks with constant speed of 3 m/min and takes about 15 min to complete the harvesting operation. The overall mean harvest efficiency of Indian major carps (IMC) with mechanized fish harvesting system has been estimated to be around 80% per operation. The whole harvesting operation is done mechanically except collection of harvested fishes at the end of operation, which presently requires two persons getting into the pond and lifting out the fishes from the net.     

Availability of essential amino acids,nutrient utilisation and growth in juvenile black tiger shrimp,Penaeus monodon,following fishmeal replacement by plant protein

Two trials with juvenile black tiger shrimp (Penaeus monodon) were undertaken to study the effects of replacing fishmeal by different levels of plant proteins on growth performances and nutrient utilisation of shrimp in semi-intensive conditions (Expt. 1) and on the availability of dietary nitrogen (N) and amino acids (Expt. 2). Five isoproteic diets (on crude protein basis) were formulated to contain 34, 24, 16, 8, or 0% fishmeal, with fishmeal being replaced by a mixture of plant protein (corn gluten meal, wheat gluten, and rapeseed meal). In Expt. 1, the shrimp (initial body weight, IBW 1.5 ± 0.1 g) were reared in earthen ponds for 144 days and fed one of the experimental diets. Apparent digestibility of nutrients and AA were assessed in Expt. 2, using 150 L tanks and shrimp of 12.8 ± 0.4 g IBW. After 144 days in grow-out ponds, shrimp fed the diet with 24% of fishmeal had similar growth as those fed the control diet containing 34% fishmeal (0% replacement). When 50% or more of the fishmeal were replaced, weight gain as well as N and energy gains significantly decreased. Digestibility of dry matter, protein and energy was also significantly lower in all fishmeal-replaced diets. In particular, leucine digestibility decreased by 26% at 100% replacement, which was significantly correlated to an increased incorporation of corn gluten meal. Our data confirm the need to improve our knowledge on AA availability and raw material quality in order to improve fishmeal replacement in P. monodon diets.     

Membrane performance and fouling behavior of membrane bioreactors installed in marine recirculating aquaculture systems

Accumulation of fine suspended solids and colloids in a recirculating aquaculture system (RAS) can be avoided by integrating a membrane filtration unit into the system, where the inclusion of a membrane bioreactor (MBR) may be an alternative. The main purpose of the study was to identify how the feeding regime affected membrane performance and fouling phenomena caused by dissolved and submicron colloidal particles in the system, and how the membrane impacted general water quality and particle characterization. To be able to evaluate membrane performance and fouling behavior, transmembrane pressure (TMP) was monitored and assessed in relation to changes in rearing conditions and different water quality parameters observed. From this study the positive influence on the chosen water quality parameters was apparent, where an improved water quality was observed when including a membrane filtration in RAS. Selected water quality parameters and TMP changed during the experimental period in response to the feeding regime, where algae paste, decaying rotifers and dry feed seemed to contribute the most to membrane fouling. Analysis of the concentration of submicron particles and particle size distribution (PSD) (particles < 1 μm) showed both a higher concentration and a more spread distribution in the rotifer/algae paste and dry feed period compared to the Artemia period, which might explain the observed increase in fouling. This study also showed that adapted procedures for concentrate removal are important to prevent hydrolysis of retained particles in the concentrate and leakage of nutrients and organic matter back to the system.     

Short-term hooking mortality of three marine fish species (Sparidae) caught by recreational angling in the south Portugal

Short-term hooking mortality was evaluated for three sparid species [Diplodus vulgaris (Geoffroy Saint-Hilaire), Sparus aurata L. and Spondyliosoma cantharus (L.)] in the Algarve, south Portugal. Fishes were caught from the shore during October 2009 at a fish farm reservoir (Ria Formosa), using three different hook sizes. The relationships between hooking mortality and seven independent variables were analyzed using logistic regression models. In all, 384 fishes representing the three target species were caught during the angling sessions. The most caught species was S. cantharus (n = 181; 100% undersized), followed by S. aurata (n = 137; 89% undersized) and D. vulgaris (n = 66; 97% undersized). Mortalities ranged between 0% for D. vulgaris and 12% for S. aurata (S. cantharus, 3%). For S. aurata, anatomical hooking location was the main predictor of mortality, with 63% of the fishes that died being deeply hooked. Our results support the current mandatory practices of releasing undersized fish for the studied species, given the low post-release mortality rates observed.