Assessment of spat collector ropes in Galician mussel farming

The large amount of mussel seed required to support the present mussel farming production levels in Galicia (NW Spain) forces the development of new designs in artificial spat collectors for continual improvement of mussel seed gathering. In the present study, we have assessed both settlement and recruitment of Mytilus galloprovincialis on four different collector ropes in the Ría de Ares-Betanzos (Galicia). Besides the traditional collector ropes (lacing without loops and non-filamentous structure; NF-NL), three new rope designs with different lacing and structures were evaluated; ropes with a filamentous loop complement (F-L), ropes with a non-filamentous loop complement (NF-L) and filamentous ropes without loops (F-NL). Ropes with loops showed higher settlement densities (53,925 ± 4625 and 42,433 ± 5525 indiv./m for F-L and NF-L, respectively) than ropes without loops (26,475 ± 3875 and 13,033 ± 1136 indiv./m for F-NL and NF-NL, respectively). This may be explained by the increase in available surface area provided by the loops. Several studies recognized the importance of filamentous substrata for mussel spat settlement, which may help to explain greater settlement densities on filamentous structures between ropes with the same lacing. In recruitment evaluation, ropes with filamentous loops showed the highest densities expressed in indiv./m (5493 ± 587) as was the case of settlement. However, when density was expressed in kg/m, the ropes with non-filamentous loops had a higher yield (8.48 ± 0.22 kg/m), that could be a result of differences in adjusted shell length between ropes. Intra-specific competition and predation were identified as important factors affecting post-settlement mortality. The latter factors could also influence population length distribution. Ropes with rigid loops (NF-L) may supply refuges for spat from predators and therefore, enhance the recruitment of larger individuals, although other factors like size selective settlement could play a significant role in this result.     

Impact of environmental deviations on the larval and year-class abundances in the spring spawning herring (Clupea harengus membras L.) of the Gulf of Riga (Baltic Sea) in 1947–2004

Regular weekly sampling of larvae with a Hensen net in the stationary stations on the main spawning grounds of the brackishwater herring population in the Gulf of Riga was carried out in 1947–2004. Probably due to the rapidly increasing anthropogenic eutrophication and the rising winter temperature the yearly abundance of herring larvae has significantly increased with a shift in the average abundance in 1972/1973. Correlation of the average number of herring larvae with the year-class strength was significant for the intermediate (10–16 mm, P < 0.05) and large (≥17 mm, P < 0.001) larval size groups. The monthly average air temperature in the period from January to March was the main environmental variable regulating the abundance of the year classes (P < 0.001). The dependence between the spawning stock biomass (SSB) and year-class abundance differed by climate periods. In the period of cold winters environmental conditions dominated in determination of the year-class abundance whereas the role of the SSB was nonsignificant (P > 0.05). In the period of milder winters the importance of the SSB in determination of the abundance of year classes considerably increased and it significantly correlated with the abundance of year classes (P < 0.05). The winter–spring temperatures and other climatic variables presumably control the abundance of the forming year-class during the period of the highest larval mortality mainly via the production of their food.     

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.     

Effect of water recirculation on seawater quality and production of scallop (Pecten maximus) larvae

Scallop larval production systems in Norway have changed from the use of batch to continuous flow through systems (FTS) during the last decade. Energy use to heat water in both larval and spat nurseries is considerable. Two experiments (June 2010 and February 2011) using water recirculation technology (RAS) were performed in large scale systems (3500 L larval tanks) supplied with continuous addition of algal feed, and 20% renewal of seawater.In the RAS a gradual increase in CO₂, decrease in pH and dissolved oxygen was observed over time. This was most obvious during experiment two, when the total organic carbon content increased in both FTS and RAS. The total bacterial number was lower and more stable in FTS than in the RAS. The variations in seawater quality parameters were smaller during the first experiment compared to the second, when values of oxygen saturation were reduced to <70%, pH was 7.8 and NO₃⁻ reached 5 mg L⁻¹. Even though these changes would seem less beneficial for survival and growth of scallop larvae, results showed that the survival at the end of the larval stage was higher in the FTS, but the yield of competent larvae ready for settlement was not significant different (p > 0.05) due to large variations between tanks. The CV% was 28.9% in FTS, while it was 49.9% in RAS. In FTS the mean yield was 40.2%, while it was 26.5% of initial number of larvae in RAS. Large variations in survival and yield were found between the larval tanks as well as gradual reduction in pH and oxygen in RAS tanks. The results indicate that there is a large potential for 80% reduction in water use by utilizing recirculation technology.     

Effect of immersion time of cultch on spatfall of the scallop Argopecten purpuratus (Lamarck 1819) in the Marine Reserve at La Riconada, Antofagasta, Chile

A pilot program to evaluate the feasibility of collecting juvenile (seed) scallops using Japanese technology was carried out from October 2001 to April 2002 in the Marine Reserve at La Rinconada, Antofagasta, Chile. The continuous presence of larvae in waters in the reserve allowed for collection of 400–15,340 spat per collector and showed the high reproductive capability of local scallops and the relationship between numbers of larvae in the water and amount of spatfall. Highest rates of settlement were proportional to high numbers of umbone larvae in the water at >5000 larvae m⁻³ when collectors were first immersed. Absence of a relationship between larval numbers and postlarval settlement with trophic factors such as chlorophyll and particulate organic matter in the water suggests that productivity was not limited by these factors. Harvest of spat from collectors immersed for four and five months had recovery efficiencies of 5.5% and 59.4% compared with settlement in collectors after immersion for one month. Cohort analyses of recovered spat suggested that settlement occurred over the lengthy immersion period and was due to the continuous presence of larvae in the water. The loss of a high percentage of spat over the 4–5 immersion period compared with settlement in the first month can be attributed to intraspecific competition of spat density and growth as well as clogging of the collectors by sediment and marine fouling that impeded a flow of water through the nets.     

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.     

Effect of mussel bio-pumping on the drag on and flow around a mussel crop rope

Drag measurements are conducted to determine if inhalant and exhalant of fluid during mussel feeding has a detectable influence on the drag of a mussel-encrusted rope such as is commonly used in suspended aquaculture. The experiment is conducted using an artificial mussel crop rope constructed using the shells of Perna canaliculus, with 100 mussels (mean shell length 83.4 mm, S.D. 8.7 mm) attached over a length of 0.90 m. Fluid pumping from mussel feeding is simulated using inhalant and exhalant jets pumping at a rate of 7 L h^?1 per mussel. The mussel rope is towed at speeds between 0.05 and 0.4 m s^?1. No significant difference is found between drag with and without the mussels pumping indicating that assessments of the drag on or from mussel long-lines may safely neglect the effect of mussel feeding. We suggest using twice the mussel shell size to define mussel rope diameter which gives a drag coefficient of C_D ～1.0. A value of C_D ～1.3 is obtained if the projected area of the mussel rope is used. Particle tracking velocimetry (PTV) is also used on a similar but shorter crop rope (0.3 m length) in a recirculating flume which reveals that mussel pumping induces only small changes to mean velocity and turbulence distributions downstream of the rope. The wake of the crop rope is highly turbulent and dominated by shear instabilities formed in the free shear layer, similar to bluff body wakes. The sharp edges of the mussel shells provide many points for flow separation to occur. At typical ambient velocities, turbulent kinetic energy produced by the exhalant jets is small in comparison to that from flow around the crop rope.     

Molecular characterization and expression of the Prostaglandin reductase 1 gene and protein during ovarian development of the giant tiger shrimp Penaeus monodon

Prostaglandin reductase 1 (PTGR1; also called NADP⁺-dependent leukotriene B4 12-hydroxydehydrogenase, LTB4DH) is the key enzyme responsible for biological inactivation of prostaglandins and related eicosanoids. In this study, the full-length cDNA of PTGR1 in the giant tiger shrimp (Penaeus monodon) was characterized. PmPTGR1 was 2405 bp in length with an ORF of 1035 bp encoding a polypeptide of 344 amino acids. Interestingly, its 3′ UTR contained the nucleotide sequence (825 bp) that significantly matched positions 3–277 (with 4 amino acid variants) of the deduced P. monodon peritrophin2 protein. PmPTGR1 was more preferentially expressed in ovaries than testes of P. monodon broodstock. In intact broodstock, PmPTGR1 was up-regulated in early cortical rod (stage III) ovaries (P < 0.05) and comparably expressed afterwards (P > 0.05). In eyestalk-ablated broodstock, PmPTGR1 was temporally lower in early cortical rod compared to previtellogenic (I) and vitellogenic (II) ovaries (P < 0.05) and returned to the previous level in mature (IV) ovaries (P < 0.05). More importantly, the relative expression level of PmPTGR1 in each ovarian stage in eyestalk-ablated females was lower than that in intact P. monodon broodstock (P < 0.05). This strongly suggested that eyestalk ablation potentially affects the expression of PmPTGR1 allowing the stimulating effects of prostaglandins and related eicosanoids on vitellogenesis and ovarian maturation of P. monodon. The level of ovarian PmPTGR1 protein seemed to increase during ovarian development in intact broodstock but slightly reduced in mature ovaries in eyestalk-ablated broodstock. Results suggested the possible contribution of PmPTGR1 in ovarian development of P. monodon.     

Nursery of young Litopenaeus vannamei post-larvae reared in biofloc- and microalgae-based systems

A 13-day nursery trial was conducted to evaluate the performance of young Litopenaeus vannamei post-larvae (from PL₆ to PL₁₈) reared in both biofloc and microalgae-based systems at a stocking density of 67 PLs L⁻¹. The effects of different concentrations of total suspended solids (TSS) on PL performance were also evaluated. One experimental group was reared in a conventional microalgae-based system with daily water exchange and daily addition of microalgae (herein called microalgae treatment). The other two experimental groups were reared using biofloc technology (BFT) with daily dextrose addition and no water exchange, but in the “Biofloc-500” treatment, TSS were maintained at around 500 mg L⁻¹, while in the “Biofloc-700” treatment, TSS were maintained at around 700 mg L⁻¹. Water quality variables remained within the appropriate range for larval culture. In microalgae treatment, ammonia control was likely associated with its assimilation into microalgae biomass and daily water exchange. In biofloc tanks, however, the addition of dextrose stimulated the production of bacterial biomass from ammonia. This system required only 12.9% of the water used by the microalgae treatment since water was not exchanged during the culture. The nursery of young PLs resulted in similar (P > 0.05) performance in all treatments: survival >94%, PL length ∼ 11.5 mm, and PL dry weight ∼ 1.2 mg. In addition, the salinity stress test (>90.0%) was not significantly different among treatments. Our results indicate that BFT can be as effective as the microalgae-based system for the nursery of young L. vannamei post-larvae. We also found that post-larvae performance was similar (P > 0.05) between biofloc treatments, indicating that organisms can tolerate environments with large quantities of solids.     

The effects of food and the sum of effective temperature on the embryonic development of the seahorse,Hippocampus kuda Bleeker

The effect of different food (live Acetes spp., live Mysis spp., frozen Mysis spp., and mixed food of 50% live Acetes spp. and 50% frozen Mysis spp.) on gonad development of seahorses, Hippocampus kuda Bleeker, was evaluated in this experiment. The developmental durations of testes and ovary of seahorses were significantly different among the four experimental treatments. The live Acetes spp. treatment presented the shortest developmental durations to stage V, which were 87.6 ± 3.84 days and 89.2 ± 3.71 days, respectively, for ovary and testes. The frozen Mysis spp. treatment had the longest developmental durations (F_3,15 = 13.284, P < 0.05). The relationship between developmental duration of the ovary and gonad developmental stages could be formulated: Y₁ = 12.04x + 24.36 (r² = 0.9722, n = 16, P < 0.05). The gonadosomatic index (GSI) of parent seahorses among the four treatments differed significantly (F_3,15 = 18.364, P < 0.05). The standard GSI of seahorses fed live Acetes spp. was 15.64 ± 1.65%, which was the highest. Feeding live food had a significant effect on the fecundity and spawning of seahorses. The fecundity and spawning number of the live Acetes spp. treatment were 598 ± 45.49 and 552 ± 49.19 individuals, respectively, which were dramatically higher than those of frozen treatment (F_3,19 = 34.152, P < 0.05). Live food also had a similar effect on the fertilization and hatching rate during the embryonic development of seahorses (F_3,19 = 11.386, P < 0.05). Food treatment could also induce an indirect effect on survival rate of juveniles through gonadal and embryonic development of the parents (F_3,15 = 14.519, P < 0.05). In this experiment, the mortality within parent seahorses in the frozen Mysis spp. treatment was the highest (15.1 ± 6.55%), and the survival of juveniles was the highest in the live Acetes spp. treatment (90.4 ± 2.26% at 10 days). In conclusion, feeding live Acetes spp. significantly benefited the gonadal and embryonic development of H. kuda.The effect of temperature (22 °C, 24 °C, 26 °C, 28 °C, 30 °C and 32 °C) on the hatching time of H. kuda was also studied. We demonstrated that the higher the temperature, the shorter the hatching time, as well as the higher the hatching speed. The relationship between hatching time and temperature could be expressed: T = − 39.337 t + 677.75 (r² = 0.9755, n = 30, P < 0.05). In this finding, we provided the sum of effective temperature (SET) and threshold temperature of embryonic development of H. kuda (14066.9 °C h^− 1 and 13.7 °C, respectively). This new information on the effect of feeding type and culture temperature is beneficial for the commercial rearing and breeding industry of this species.     

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     

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.     

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.     

Effects of storage duration and storage media on initial and post-eyeing mortality of stored ova of rainbow trout Oncorhynchus mykiss

The effects of storage duration and storage media on the viability of unfertilized eggs of rainbow trout from fertilization to hatching stage was studied. The unfertilized eggs were stored in coelomic fluid and Cortland artificial media buffered with Tris–HCl (C₄H₁₁NO₃–HCl) and Hepes (C₈H₁₈N₂O₄S) at 2–3 °C. These eggs were fertilized after 0 (i.e. control eggs fertilized prior to storage), 2 and 9 days post-stripping using fresh, pooled sperm obtained from 4 to 5 males. According to the results of present study, after 2 days of storage, no significant (p > 0.05) change in survival to eyeing and hatching of eggs stored in coelomic fluid (85.5 ± 4.8%, 80.2 ± 6.3%) and Cortland medium buffered with Hepes (73.3 ± 4.1%,68.1 ± 4%) was observed in comparison to the control (82.6 ± 8%,78.7 ± 7.8%). However, eyeing and hatching survival rates of eggs stored for the same period in Cortland medium buffered with Tris–HCl (44.5 ± 7%,34.2 ± 8.2%) decreased significantly (p < 0.05) in comparison with the control. Also, eggs stored for 2 days in Cortland medium buffered with Tris–HCl (25 ± 6.7%) had significantly (p < 0.05) greater eyed egg mortality compared to the control (4.7 ± 0.6%), while no significant (p > 0.05) differences were observed between eyed egg mortality rates of eggs stored in coelomic fluid (6.7 ± 2.1%) and Cortland medium buffered with Hepes (7.3 ± 0.9%) compared to control. After 9 days of storage, the eggs that were stored in coelomic fluid showed no significant (p > 0.05) differences in eyeing (77.5 ± 7.2%) and hatching (67 ± 8.1%) rates in comparison with control, while both Cortland storage media exhibited significant (p < 0.05) loss in eyeing (44.4 ± 11.5%, 24.7 ± 13.8% for Hepes and Tris–HCl, respectively), and hatching (24.5 ± 5.9%, 19.2 ± 14.4% for Hepes and Tris–HCl, respectively) in comparison with control. Furthermore, eyed egg mortality increased significantly (p < 0.05) in coelomic fluid (14 ± 2.9%), media buffered with Hepes (43 ± 6.2%) and Tris–HCl (49.3 ± 26.9%) compared to control. Based on this study, unfertilized eggs of rainbow trout can be successfully stored in coelomic fluid for 9 days at (2–3 °C) without significant loss of fertility. However, storage within a similar period in artificial media showed lower fertilization rates and higher eyed egg mortality compared to coelomic fluid.     

Use of vegetated drainage ditches and low-grade weirs for aquaculture effluent mitigation: II. Suspended sediment

Total suspended solids are a priority pollutant under the Clean Water Act and a point of concern for aquaculture facilities. The use of ubiquitous vegetated ditches on the aquaculture landscape may serve as an environmentally and economically sustainable practice for reducing suspended sediment contributions to downstream environments. This study assessed effects of consecutive low-grade weirs on suspended solids retention and settling rates of aquaculture pond effluent in a single drainage ditch. Two control and nine treatment discharges were conducted in September and October 2012 at the Mississippi State University South Farm Aquaculture Facility. All discharges decreased total and volatile suspended solid loads. Total suspended solids were decreased 72–94%, with a significant removal rate of 0.02 ± 0.01 mg L⁻¹ min⁻¹ in both control (F = 6.12, P < 0.001) and treatment discharges (F = 16.02, P < 0.001). Volatile suspended solids comprised 2–80% of total suspended solids and had a significant removal rate of 0.02 ± 0.001 mg L⁻¹ min⁻¹ in both control (F = 10.46, P < 0.001) and treatment discharges (F = 6.28, P < 0.001). There was no significant difference in overall settling rates between control and treatment discharges; however, prior to weir 1, both total and volatile suspended solid concentrations increased in control discharges. Treatment discharges decreased both total and volatile suspended solids significantly (P < 0.001) prior to weir 1. Further analysis revealed flow rate to be a significant (P < 0.001) variable in total suspended solid removal while initial concentrations affected reduction rates of volatile suspended solids significantly (P < 0.001). These results suggest that the use of low-grade weirs could be a viable best management practice that easily integrates within the aquaculture landscape and creates hydraulic conditions conducive to sediment retention.     

Leaching properties of three different micropaticulate diets and preference of the diets in cod (Gadus morhua L.) larvae

Leakage of water soluble nutrients from larval microparticulated feeds is probably extensive and needs to be further investigated. Leaching rates of ¹⁴C-labelled serine, pepsin hydrolysed, protein enriched ¹⁴C-algae extract and intact protein enriched ¹⁴C-algae extract were measured from three microparticulated feeds for marine fish larvae (heat coagulated, protein bond feed; agglomerated feed; protein encapsulated feed). The effects of particle size (< 0.3 mm, 0.3–0.6 mm; 0.6–1.0 mm) and immersion time (1–60 min) in salt water were also tested. Leaching increased by decreasing molecular weight of leaching component (P < 10^− 5), by the feeds in order encapsulated, heat coagulated and agglomerated feeds (P < 10^− 5), by longer immersion time (P < 10^− 5), and by decreasing feed particle size (P < 10^− 5). Due to low protein content of the algae extract, the leaching rates of intact and hydrolysed algae extract did not represent absolute estimates for protein and hydrolysed protein leaching. A new estimate for leakage of hydrolysed protein was calculated based on molecular weight distribution of the hydrolysed algae extract analysed by cutoff centrifugation of the extract. Assuming that molecules < 300–600 or < 9–18 kD would leak, leakage of hydrolysed protein from the smallest feed particles after 5 min immersion would be 80–98%, 43–54% and 4–6% of the agglomerated, heat coagulated and protein encapsulated feeds, respectively. The feeds were also tested for preference in cod larvae of two different sizes (5.6 ± 2.5 mg and 15.8 ± 7.2 mg). The preference was highest for the heat coagulated feed in the first experiment (feed intake 0.32 ± 0.06 mg dry feed larvae^− 1) and the agglomerated in the second (2.04 ± 0.32 mg dry feed larvae^− 1), while the protein encapsulated feed was preferred at lower rates in both experiments (0.11–0.14 mg dry feed larvae^− 1).     

Evaluation of plankton surface pushnets and oblique tows for comparing the catch of diadromous larval fish

A bow-mounted surface pushnet and an obliquely towed plankton net were compared to evaluate gear efficiency and effectiveness in collecting larval fishes under daytime and nighttime conditions. The diadromous species targeted were striped bass Morone saxatilis, white perch, Morone americana, and river herring Alosa sp. We sampled the lower Roanoke River, North Carolina, from March through June of 2002 and 2003. Striped bass, white perch and river herring represented over 90% of the larvae collected during the study period. Mean larval densities (number/100 m³) were 63.4 for striped bass, 26.4 for river herring, and 17.7 for white perch. Striped bass larval densities were significantly higher in the surface pushnet for both years (P ≤ 0.05). In 2002, white perch mean larval density was significantly higher at night in the surface pushnet samples, but in 2003 there were no differences between day and night samples. River herring mean densities were significantly higher in the surface pushnets for both years, but showed no clear patterns between day and night samples. Larger larvae were consistently collected in the surface pushnets for all species. Overall, the surface pushnet was easier to operate. The pushnet was mounted on the bow of a small jon boat and required less specialized gear and fewer personnel than oblique sampling. The method also allows for sampling in shallow water or vegetated habitats. Because larvae were significantly larger in the surface samples, using surface pushnets may not allow for detection of the smaller-sized larvae therefore underestimating the abundance of smaller fish. Depending on the question being asked, we recommend that sampling programs should use both gear types to reduce any gear biases.     

The effect of different grading equipment on stress levels assessed by catecholamine measurements in Pacific oysters,Crassostrea gigas (Thunberg)

In this study the effects of exposure to two air temperatures (15 and 27.5 °C) and three graders commonly used in South Australia (Rotary, Flat Bed and Inside/Out) on Pacific oysters were investigated using post-exposure or post-grading catecholamine levels as a stress indicator. During the 48 h experimental period the circulating dopamine and noradrenaline levels of oysters exposed to 15 °C air temperature did not differ significantly from controls sampled directly from the 15 °C water tank. Conversely, in oysters exposed to 27.5 °C air temperature, dopamine and noradrenaline became significantly different from controls after 8 and 12 h air exposure, respectively, and then returned to the control levels at 24 h.Significant increases (P < 0.05) in circulating noradrenaline and dopamine levels were observed in oysters graded by the Flat Bed and the Inside/Out graders. The circulating noradrenaline concentrations in oysters subjected to the Flat Bed grader were significantly higher (P < 0.05) than those observed in the Rotary graded oysters and close to being significantly higher (P = 0.052) than those levels measured in the Inside/Out graded oysters. The circulating noradrenaline levels in oysters subjected to the Inside/Out grader were, in turn, significantly higher than those recorded in the Rotary graded oysters. These results suggest that in relation to the three graders used in South Australia, the Flat Bed induces the highest stress levels in oysters and the Rotary grader the lowest.     

Evaluation of methods for assessing the retention of seed mussels (Perna canaliculus) prior to seeding for grow-out

The process of catching and relaying wild seed green-lipped mussels, Perna canaliculus, onto grow-out farms in New Zealand is extremely inefficient with the majority of the seed mussels being lost after transfer, mostly due to secondary settlement behaviour of the seed. In an attempt to identify the causes of this behaviour the retention of seed mussels was experimentally determined after exposure to three sub-lethal stressors, desiccation, starvation, and temperature fluctuation. In addition, two types of mussel seed behaviour were tested for their potential for developing a commercially useful method for identifying seed mussels with high retention properties prior to seeding. The two behaviours were the ability of seed mussels to attach by byssus threads to the substrate in slow flowing water, and the ability of seed mussels to pedal walk vertically up through a substrate filled column. Of the three stressors tested only desiccation and starvation significantly reduced mussel seed retention compared to “unstressed” controls by 38% (P < 0.05) and 42% (P < 0.05) respectively. Of the two types of mussel seed behaviour used for attempting to sort seed mussels, only the ability of seed mussels to attach by byssus threads to the substrate in slow flowing water showed any sensitivity for identifying individuals with higher retention properties (P < 0.05). Seed mussels that failed to byssal attach had a 23% lower mean retention, after 10 d experimental grow-out compared to seed that successfully attached. Although significant, this difference was relatively small and is unlikely to provide the basis for the development of a useful commercial seed mussel sorting method. Desiccation and starvation are both stressors that seed mussels are likely to experience during their commercial harvesting, transportation and seeding process. Consequently, seed mussels need to be relayed quickly and under moist conditions to reduce the potential for starvation and desiccation stress. Identifying seed mussels in poor nutritional condition and feeding them prior to seeding out on farms could also help to prevent the intermittent large scale seed mussel losses currently experienced. The application of these findings will assist in making more efficient use of the wild seed mussel resource and significantly lower financial losses to growers in New Zealand.     

Effects of water depth on survival and growth of <Emphasis Type="Italic">Argopecten purpuratus</Emphasis> (Lamarck, 1819) spat in northern Chile

Collection and suspended culture of Argopecten purpuratus spat in Japanese-type collectors was undertaken in the Rinconada Marine Reserve (Antofagasta, Chile) to determine growth variations between surface and bottom waters. Scallop spat was collected at 16-m depth and grown at 1- and 16-m depths. An initial settlement of ca. 13,000 post-larvae per bottom collector (2 cohorts) was observed on 3 February 2001. Two new cohorts settled in bottom collectors on 3 March, while no spat settlement occurred in surface collectors. The four cohorts exhibited substantial and different mortality rates over the study period, depending on the cohort and on-growing depth. The first and second cohorts exhibited mortality rates of 80.3 and 53.1% in bottom collectors, respectively, versus 29 and 43.6% in surface structures. The third and fourth cohorts exhibited mortality rates of 21.4 and 59.7% in bottom collectors, respectively. Mean shell height of scallop spat after 88 days of culture was 6.46 mm for the first cohort and 4.37 mm for the second cohort in bottom collectors, versus 10.33 and 7.31 mm in surface collectors, respectively. Mean post-larval growth rate in bottom collectors (86 μm day⁻¹) was significantly lower than growth rate in surface collectors (146 μm day⁻¹). Factors explaining the depth-related mortality and growth rates are discussed to improve scallop culture in the Reserve. It was concluded from the results that culture improvement in Antofagasta Bay would require collecting spat in bottom waters and raising it near the surface during initial cultivation stages.