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.     

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.     

Routine oxygen consumption rates of california halibut (Paralichthys californicus) juveniles under farm-like conditions

Fish oxygen requirement is a fundamental variable of aquaculture system design and management, as it is the basis for determining water flow rates for sustaining stock. A study on oxygen consumption of California halibut (Paralichthys californicus) between 3.2 and 165.6 g was conducted in small raceways (2.41 m long, 0.28 m wide, and 0.22 m high; operational water depth between 0.05 and 0.10 m with a quiescent zone 19 cm long in the effluent section) working as open respirometers in a recirculating system under farm-like conditions. The fish were fed commercial dry pelleted feeds at a ratio of ～0.70–3.00% of body weight (BW) and stocked at densities between 94% and 316% percent coverage area (PCA). Oxygen consumption rates were determined by mass balance calculations. The mean and maximum oxygen consumption rates (g O₂/kg fish/day) for juvenile California halibut under the conditions tested can be expressed by M_day = 15.077W^?0.2452 and M_day = 17.266W^?0.2033, respectively, where W is fish weight in grams. The determination of oxygen consumption by California halibut in farm-like conditions provides valuable information on the oxygen requirement of these fish in an aquacultural setting. This information can be used for designing and sizing a rearing facility for the intensive culture of California halibut.     

Routine oxygen consumption rates of california halibut (Paralichthys californicus) juveniles under farm-like conditions

Fish oxygen requirement is a fundamental variable of aquaculture system design and management, as it is the basis for determining water flow rates for sustaining stock. A study on oxygen consumption of California halibut (Paralichthys californicus) between 3.2 and 165.6 g was conducted in small raceways (2.41 m long, 0.28 m wide, and 0.22 m high; operational water depth between 0.05 and 0.10 m with a quiescent zone 19 cm long in the effluent section) working as open respirometers in a recirculating system under farm-like conditions. The fish were fed commercial dry pelleted feeds at a ratio of ∼0.70–3.00% of body weight (BW) and stocked at densities between 94% and 316% percent coverage area (PCA). Oxygen consumption rates were determined by mass balance calculations. The mean and maximum oxygen consumption rates (g O₂/kg fish/day) for juvenile California halibut under the conditions tested can be expressed by M_day = 15.077W^−0.2452 and M_day = 17.266W^−0.2033, respectively, where W is fish weight in grams. The determination of oxygen consumption by California halibut in farm-like conditions provides valuable information on the oxygen requirement of these fish in an aquacultural setting. This information can be used for designing and sizing a rearing facility for the intensive culture of California halibut.     

Evaluation of levamisole as a feed additive for growth and health management of hybrid striped bass (Morone chrysops × Morone saxatilis)

Three separate experiments were conducted with juvenile hybrid striped bass to determine the effects of levamisole as a feed supplement on growth performance, innate immunity and specific antibody responses, as well as resistance to Streptococcus iniae and Aeromonas hydrophila infection. A basal diet was formulated to contain 40% protein, 10% lipid and an estimated digestible energy level of 3.5 kcal/g. Four levels (100, 250, 500 and 1000 mg/kg) of levamisole were added to the basal diet in place of cellulose. After the 3-week feeding period in feeding trial 1, enhanced weight gain was generally observed in fish fed the diets supplemented with a low level (< 500 mg/kg) of levamisole. Dietary supplementation of levamisole at 100 mg/kg significantly (P < 0.05) enhanced growth and feed efficiency, compared to fish fed the basal diet. Fish fed diet supplemented with 1000 mg levamisole/kg diet showed chronic toxicity signs of inferior growth, reduced feed intake and feed efficiency. Although the extracellular superoxide anion production of head kidney cells, hematocrit, serum lysozyme and peroxidase were not significantly affected by dietary treatments, intracellular superoxide anion production of head kidney macrophages of fish fed 250 mg levamisole/kg diet was significantly (P < 0.05) higher than that of fish fed the other four diets. The hypothesized benefits of dietary supplementation of levamisole in reducing mortality from infection of pathogenic bacteria including S. iniae and A. hydrophila were not proven after two separate 3-week feeding trials and disease challenges in the present study. Agglutinating antibody titers of individual fish were highly varied and not significantly influenced by dietary levamisole at the tested levels.     

Loads applied to aquaculture nets by the biting behaviour of Atlantic cod (Gadus morhua)

Atlantic cod (Gadus morhua) bite at netting materials in sea-cages, which is known to weaken and destroy net materials, contribute to the formation of net holes and lead to escapes. We determined the lateral pulling forces applied to nets through biting by cod in a laboratory set-up. A sensor for automatic logging of the lateral pulling force was used to obtain data for over 1000 undisturbed, objectively collected biting events. Lateral pull force varied between cod of different size (180 and 610 g) and different net types, ranging from 0.9 to 9.4 N. Further, pull forces were on average 1.5–2.5 times stronger during lengthy biting events compared to single, brief bites. Maximum lateral pulling forces exerted by 610 g cod were similar to their body weight in air, while 180 g fish were able to pull with a force equivalent to twice their own weight in air. All recorded lateral pull strengths significantly exceeded the known force needed to break single nylon fibres of the netting material. Our results advance understanding of mechanisms and forces involved when cod interact with sea-cage nets and provide important baseline data for the design and production of more bite-resistant netting.     

Optimization of the lighting system for a Hydraulically Integrated Serial Turbidostat Algal Reactor (HISTAR): Economic implications

An estimated 28% of the production cost in HISTAR systems that are artificially illuminated is attributed to the lighting cost. This cost estimated is based on an operational configuration comprised of eight CFSTRs, a system dilution rate (D_s) of 0.640 d^?1, and 400 W metal halide lamps positioned at an elevation of 38.1 cm over the culture. Deterministic model simulations of the volumetric productivity (P_v), photosynthetic efficiency (E_o) and lighting cost (LC) under various management strategies, operational parameters and reactor design configurations were performed and compared to the simulation results obtained for the original configuration. The simulations showed that LC may be reduced by 35.5% by switching from a metal halide (MH) to high-pressure sodium (HPS) light source at an optimum system dilution rate D_s = 0.641 d^?1. LC may be reduced by an additional 17.8% through decreasing the lamp elevation to 25.4 cm. Increasing the wattage of the light source from 400 to 1000 W in the last six reactors would reduce the LC by 13% from the original cost. Overall, using HPS lamps at 25.4 cm height, with six 1000 W and two 400 W lamps at a D_s = 0.641 d^?1 will result in a 54% overall LC reduction compared to the original configuration of HISTAR. This represents a 13% reduction in the overall microalgal production cost for HISTAR.     

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.     

The effect of nearly closed RAS on the feed intake and growth of Nile tilapia (Oreochromis niloticus), African catfish (Clarias gariepinus) and European eel (Anguilla anguilla)

One of the challenges that Recirculating Aquaculture Systems (RAS) are still facing is the risk that in RAS fish grow less than in flow-through systems due to the accumulation of substances originating from feed, fish or bacteria associated with the water re-use. The present study investigated whether RAS with high and low accumulation levels of these substances affect feed intake and growth of Nile tilapia Oreochromis niloticus, African catfish Clarias gariepinus, and European eel Anguilla Anguilla. One-hundred and twenty individuals of each species were used (start body weights: Nile tilapia 264.8 ± 8.3 g; African catfish 253.2 ± 2.1 g and European eel 66.6 ± 1.3 g). For a period of 39 days, growth and feed intake were compared between high and low accumulation RAS. HIGH accumulation RAS was designed for maximal accumulation of substances in the water by operating the system at nearly-closed conditions (30 L/kg feed/d), using mature biofilters and high feed loads; and (2) LOW accumulation RAS was designed to be a proxy for flow-through systems by operating at high water exchange rates (1500 L/kg feed/d), new biofilters and low feed load. HIGH accumulation RAS induced a reduction in feed intake (42%) and growth (83%) of Nile tilapia, as compared to systems that are a proxy for flow-through conditions. This effect was not observed in European eel and African catfish. The cause of this reduced feed intake and growth rate of Nile tilapia is still unclear and should be addressed in further studies.     

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.     

Effect of different total suspended solids concentrations on the growth performance of Litopenaeus vannamei in a BFT system

Decreased Litopenaeus vannamei performance resulting from excess total suspended solids (TSS) has been highlighted in previous studies. Therefore, the aim of this study was to evaluate the effect of different TSS concentrations on the L. vannamei growth performance in a BFT system for 42 days. Five TSS concentrations were used—250, 500, 1000, 2000, and 4000 mg L⁻¹—in three replicates identified as T250, T500, T1000, T2000, and T4000, respectively, in 200 L-tanks each. Dissolved oxygen concentration (DO) was maintained above 5 mg L⁻¹. Shrimp with an initial average weight of 4.57 ± 1.07 g were stocked at a density of 277 shrimp m⁻². The physical and chemical parameters were monitored. Water quality parameters and animal performance were subjected to analysis of variance (ANOVA − one way). The physical and chemical parameters were within the recommended range for L. vannamei. Weekly weight gain, feed conversion rate, survival, and productivity showed no significant differences (p > 0.05). The high TSS concentrations did not seem to affect the performance of this species when DO concentrations were maintained above 5 mg L⁻¹.     

Reproductive biology and per-recruit analyses of striped trumpeter (Latris lineata) from Tasmania,Australia: Implications for management

This paper profiles the reproductive biology of and investigates the resilience of Tasmanian striped trumpeter under different levels of fishing pressure and size at entry to the fishery using per-recruit analysis. The spawning season around Tasmania occurs in the austral spring, with peak spawning activity in September and October. Size at 50% maturity was estimated at 543 mm fork length (FL) for females (estimated age = 6.8 years) and 529 mm FL for males (estimated age = 6.2 years). Striped trumpeter is a multiple spawner with batch fecundity estimates ranging from 205,054 for a 2 kg fish (540 mm FL) to 2,351,029 for a 9.5 kg fish (800 mm FL). At the current minimum legal size limit of 450 mm total length (equivalent to approximately 425 mm FL), yield-per-recruit was estimated to be close to maximum, and spawning biomass-per-recruit (SPR) ranged from 35 to 52% of virgin stock, depending on the mortality estimates used. Although these SPR are at a level considered sustainable, this methodology does not incorporate temporal variability, specifically recruitment variability. Therefore, conservative management measures are recommended until a dynamic model is applied to the striped trumpeter population.     

The lack of genetic differentiation of pacu (Piaractus mesopotamicus) populations in the Upper-Paraguay Basin revealed by the mitochondrial DNA D-loop region: Implications for fishery management

Pacu (Piaractus mesopotamicus) is an important fish species for professional and sport fisheries in the Alto-Paraguay Basin, located in the central western Brazil. In this study the D-loop control region of mtDNA sequence data was used to investigate genetic variability within and among populations of pacu. A total of 99 individuals were collected in the Paraguay River and in four other main tributaries. From these, 27 different haplotypes were observed. The overall nucleotide diversity and haplotype diversity were high in all sampling sites with 0.013 ± 0.001 and 0.840 ± 0.035, respectively. Hierarchical AMOVA analysis showed that pacu populations form a single panmictic population with low Φ_ST = ?0.0012 (p = 0.486) and high gene flow among rivers. Despite high genetic variability, the total capture officially landed in the Upper-Paraguay River Basin has been decreasing steadily during the last years with indications of overexploitation. Taking into account the heterogeneity of habitats, continuous anthropogenic disturbances and lack of genetic structure, it appears that fish from these sampling sites function as a metapopulation, a framework that may be applied to long term fishery management of pacu in the Panatanal.     

Production performance of Labeo calbasu (Hamilton) in polyculture with three Indian major carps Catla catla (Hamilton), Labeo rohita (Hamilton) and Cirrhinus mrigala (Hamilton) with provision of fertilizers,feed and periphytic substrate as varied inputs

Inclusion of kalbasu, Labeo calbasu as a candidate species in the Indian major carps based polyculture system was evaluated through a six-month grow-out trial in earthen ponds of 0.08 ha each. Species performance was assessed through provision of varied inputs viz., fertilizers (T-1), fertilizers + supplementary feed (T-2) and fertilizers + supplementary feed + periphytic substrate (T-3) as the three treatments, which were evaluated in replicates. Catla (35%), rohu (35%), mrigal (15%) and kalbasu (15%) were stocked at combined density of 7500 fingerlings/ha. While ponds were fertilized with cowdung, urea and single super phosphate, mixture of groundnut oilcake and rice bran at 1:1 (w/w) was provided as supplementary feed. The periphytic substrate, comprised stripe bamboo mat, was provided at 10% of the pond surface area. Provision of each additional input caused significantly higher increase in overall mean survival, growth, SGR and net biomass yield of carps. Among the carp species, while only rohu and kalbasu showed significantly higher weight gain (234.4 g and 170.3 g, respectively) in T-3, no such increase was noticed either in catla or mrigal. The net production in T-3 (1516.1 ± 24.3 kg ha^− 1 6 months^− 1) was 13.0 and 73.2% higher than those of T-2 (1341.7 ± 15.5 kg ha^− 1 6 months^− 1) and T-1 (875.2 ± 15.6 kg ha^− 1 6 months^− 1), respectively. The study revealed the relative advantage of using periphytic substrates in carp polyculture systems with kalbasu as a component species.     

Population dynamics of Melicertus kerathurus (Decapoda: Penaeidae) in Thermaikos Gulf (N. Aegean Sea)

The east Atlantic-Mediterranean penaeid Melicertus kerathurus constitutes a valuable resource for Mediterranean fisheries. In recent years, its world production is almost totally based on the yields from Greece and Tunisia. A two-year study of prawn stock in Thermaikos Gulf (NW Aegean Sea) provided information on age, growth and mortality of the species. M. kerathurus exhibits a marked sexual growth dimorphism, with females showing an overall wider size range and greater size than males. Mean monthly size, CL_max were also in favor of females. Monthly age classes varied from one to four for females and from one to three for males, and corresponded to three generations (0⁺, 1⁺, 2⁺). New generation (0⁺) cohorts recruited into fishery in waves, from late summer to February. According to the von Bertalanffy growth curve, females grow larger than males, but there is a prominent seasonal growth oscillation in both sexes (females: CL_∞ = 62.48, K = 1.15, C = 0.87, WP = 0.16, Rn = 0.170; males: CL_∞ = 47.78, K = 1.28, C = 0.97, WP = 0.12, Rn = 0.205). Growth performance indices (φ′, φ) and longevity were in favor of females. K values for both sexes in Thermaikos Gulf are the highest reported for the species, suggesting a higher growth rate and a lower longevity. Estimated values for total, natural and fishery mortality were greater in males. In both sexes, an exploitation rate E > 0.5 was found, implying an intense fishing impact on M. kerathurus stock in Thermaikos Gulf. Intense exploitation in other parts of the Mediterranean and eastern Atlantic, along with the replacement of the species by Eritrean penaeid prawns in the eastern Mediterranean, has resulted in a severe reduction in catches, in most countries. Thus, existing stocks of M. kerathurus should be considered as potentially endangered, and species-specific initiatives towards sustainable management of this resource should be implemented.     

Growth,survival and feed efficiency for post-metamorphosed Atlantic cod (Gadus morhua) reared at different temperatures

Two growth trials were completed on post-metamorphosed Atlantic cod (Gadus morhua) for a period of four weeks to determine the optimal temperature for best growth and feed efficiency. The same experiment was repeated twice under similar conditions to determine the effect of four temperature regimes (10, 12, 14 and 16 °C) on randomly selected juvenile cod with an initial weight ranging from 0.34 to 0.51 g. Post-metamorphosed cod grown at 14 and 16 °C were significantly larger at the end of the experiments than the fish grown at 10 or 12 °C, with specific growth rates following a similar trend (p < 0.05). Fish held at 16 °C utilized feed less efficiently than those held at the lower temperatures in experiment II and the highest feed efficiency was observed in fish held at 10 °C (p < 0.05). The results of the feed efficiency and the maximum growth per degree (dG / dT_max) in experiment II suggest that the feed efficiency was likely maximized at a lower temperature than those used in this study. By using data collected from sub-samples of fish in experiment II, maximum growth (G_max) was estimated at 14.5 °C using a growth temperature model. Cannibalism was a problem in the first experiment but was greatly reduced in experiment II by feeding a larger size feed pellet (2 mm).     

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.     

Practical applications of low-pressure hydrocyclone (LPH) for feed waste and fecal solid removal in a recirculating aquaculture system

In this work, the practical application of a low-pressure hydrocyclone was examined for feed waste and fecal solid removal for common carp (27 ± 3.1 g, average ± SD) and Nile tilapia (33 ± 3.4 g, average ± SD) in a recirculating aquaculture system. The dimensions of the low-pressure hydrocyclone included an inflow diameter of 30 mm, a cylinder length of 575 mm, an overflow diameter of 60 mm, an underflow diameter of 50 mm, a cylinder diameter of 335 mm and a cone angle of 68°. The different operating conditions tested were inflow rates of 400, 600, 800 and 1000 ml s⁻¹, and underflow rates of 25%, 25%, 20% and 10% of the inflow rates, respectively. Feed waste totals of 4.1 to 4.8% and 3.6 to 4.0% of the feed intake were produced by the common carp and Nile tilapia, respectively. The maximum separation efficiency (Et) for the feed waste from the common carp was 71% at an inflow rate of 600 ml s⁻¹ with an underflow rate of 25% of the inflow rate. The maximum separation efficiency for the feed waste from the tilapia was 59% at an inflow rate of 400 ml s⁻¹ with an underflow rate of 25% of the inflow rate. The fecal solid production estimated from the digestibility was 37.9% and 35.7% of the feed intake for the common carp and Nile tilapia, respectively. The maximum separation efficiency for the feces from the common carp was 60% for an inflow rate of 600 ml s⁻¹ and an underflow rate of 25% of the inflow rate. The maximum separation efficiency for the tilapia feces was 63% at an inflow of 400 ml s⁻¹ with an underflow rate of 25% of the inflow rate. The low-pressure hydrocyclone can be adopted for prefiltration and/or post-filtration for the removal of various sized solids. Furthermore, the solids separated from the underflow can be easily removed for further processing.     

Effect of different biofloc levels on microbial activity,water quality and performance of Litopenaeus vannamei in a tank system operated with no water exchange

In zero-exchange superintensive culture systems, flocculated particles (bioflocs) accumulate in the water column. Consequently, some control over the concentration of these particles must be performed. The objective of this study is to evaluate the effects of three concentrations of bioflocs on microbial activity, selected water quality indicators and performance of Litopenaeus vannamei in a tank system operated with no water exchange. A 44-day study was conducted with juvenile (6.8 g) shrimp stocked in twelve 850 L tanks at a stocking density of 459 shrimp m⁻³. Biofloc levels were expressed as three presets of total suspended solids (TSS) concentrations, as follows: 200 mg L⁻¹ (T200), 400–600 mg L⁻¹ (T400–600), and 800–1000 mg L⁻¹ (T800–1000). TSS levels were controlled by attaching a 40 L settling tank to each culture tank. Reduction of TSS to concentrations close to 200 mg L⁻¹ decreased the time of bacterial cell residence and significantly reduced the nitrification rates in the water (P < 0.05). The tanks in the T200 treatment had a greater variability of ammonia and nitrite (P < 0.05), which led to the need to increase the C:N ratio of the organic substrate to control ammonia through its assimilation into heterotrophic bacterial biomass. But the higher production of heterotrophic bacteria in T200 (P < 0.05) increased the dissolved oxygen demand. Nitrification rates were higher (P < 0.05) in tanks with TSS concentrations above 400 mg L⁻¹, and ammonia and nitrite were significantly lower than in the T200 tanks. We suggest that ammonia and nitrite in the T400–600 and T800–1000 tanks were controlled primarily by nitrifying bacteria, which provided higher stability of these parameters and of dissolved oxygen. Regarding shrimp performance, the reduction of TSS to levels close to 200 mg L⁻¹ was associated with better nutritional quality of bioflocs. Nevertheless, differences in biofloc levels and nutritional quality were not sufficient to affect the weight gain by shrimp. The rate of shrimp survival and the final shrimp biomass were lower (P < 0.05) when the TSS concentrations were higher than 800 mg L⁻¹. Analysis of the shrimps’ gills showed a higher degree of occlusion in the T800–1000 treatment (P < 0.05), which suggests that the shrimp have an intolerance to environments with a solids concentration above 800 mg L⁻¹. Our results show that intermediate levels of bioflocs (TSS between 400 and 600 mg L⁻¹) appear to be more suitable to superintensive culture of L. vannamei since they create factors propitious for maintaining the system’s productivity and stability     

Effect of increased water recirculation rate on algal supply and post-larval performance of scallop (Pecten maximus) reared in a partial open and continuous feeding system

In a commercial scallop hatchery spat production depends on a culture system which ensures high survival and good growth. Reuse of water with algae may increase the food exploitation and hence reduce the costs. Post-larvae of great scallop (Pecten maximus) were studied in a commercial hatchery using a partial open and continuous feeding tank system. Three different water recirculation rates (67, 83 and 92%) were tried out in two experiments with post-larvae originating from three spawning groups of ages between 43 and 57 days post-spawn, 316–886 μm shell-height and 1.1–9.6 μg ash-free dry weight. The post-larvae were held in sieves in tanks of 2500 l where a downwelling flow was maintained by airlifts. New water with a mix of monocultured algae was continuously added to the tanks at algal concentrations of 10 and 15 cells μl⁻¹ in experiment 1 (groups 1 and 2) and 2 (group 3), respectively. The algal supply to each sieve was reduced along with increased recirculation rate, but was kept between 6 and 13 cells μl⁻¹. Generally no significant differences in survival, growth or chemical content were found between the three recirculation rates, while few differences were found between and within groups. Large variation in survival was found between and within groups (1–81%). Highest survival was found in experiment 1, and where post-larvae from two settlements were used, the first settlement survived better than the second. The daily growth ranged from 15 to 62 μm shell-height and from 0.3 to 2.6 μg ash-free dry weight. The scallop post-larvae could well be reared at all three recirculation rates studied as an increase from 67 to 92% did not seem to affect the post-larval performance seriously. The algal supply, however, had to be compensated by an increasing number of cells (>10 cells μl⁻¹) when increasing the recirculation rate.