Opto-electronic counters for postlarvae and juveniles of Machrobrachium rosenbergii (de Man); design and tests of Type A counter

Postlarvae or juveniles of Macrobrachium rosenbergii produced in Mauritius are sold at unit price to prawn farmers and are counted individually by hand. This process is time consuming, labour intensive, subject to error and may stress the juveniles; safe rapid electronic counting would be highly advantageous.The design of the hydraulic component of a successful counter prototype, Type A, is described. The counter has 10 parallel channels through which juveniles are carried in a water current. The detection of juveniles passing through these channels can be done photo-electrically. The counter accepts juveniles up to 15 mm in length (eye orbit to tip telson) and has the significant advantage that it can be constructed using only simple hand tools.Survival and growth rate tests, and microscopic examination for damage to appendages of juveniles were done on machine counted and hand counted juveniles. There was no reduction of growth or survival caused by passage through the counter by comparison with the hand counting method, and it is concluded that the counter is not detrimental to juvenile prawns.About 70% of all juveniles tested showed highly variable amounts of damage. The great majority of this damage appears to originate in the hatchery tanks, prior to the experimental treatments. Only in the case of antennules could the use of the counter be shown to increase damage significantly, i.e. from 25% to 38%, but this has no detectable effect on subsequent survival or growth.This appears to be the first counter made specifically for counting macroscopic crustacean juveniles, and via suitable scaling, adaptation to counting other species of crustaceans, fish larvae, etc., should be possible.     

Opto-electronic counters for juveniles of macrobrachium rosenbergii (de Man): Supporting equipment and use of counter B under commercial hatchery conditions

To count postlarvae or juveniles of M. rosenbergii with the high speed opto-electronic Counter B, a clean water supply, a system for delivery of juveniles into the counting device, the collection of counted juveniles and partition of these into containers for delivery to ponds, are required. Juveniles are collected from rearing tanks without contamination by particles of detritus, which would otherwise be counted. The infrastructure, counter performance, juvenile handling procedures and survival tests done on machine-counted juveniles are described. Counting speed (juveniles per hour), counting errors and labour needs (men) are: 200 000, <1% and two for machine counting; 10 000, >2% and six for hand counting; a supervisor is required in both cases. Survival of machine-counted juveniles is the same as that of hand counted ones, indicating that the electronic counter and methods of using it have no adverse effects on juvenile prawns of this species. It is believed that this counter is unique, and that it has wide applicability to counting various aquatic organisms.     

A method to eliminate self-fertilization in a simultaneous hermaphrodite scallop. 1. Effects on growth and survival of larvae and juveniles

Argopecten purpuratus, as several other scallops of commercial importance is a functional hermaphrodite. The usual methods for breeding cause an unavoidable degree of self-fertilization with the subsequent deleterious effects on survival and growth rates. Some researchers have unsuccessfully attempted to eliminate self-fertilization in these bivalves. Applying our knowledge of the cellular mechanisms of spawning in this species we have developed a method by which self-fertilization was eliminated. By this in vitro fertilization method, oocytes were obtained by scraping the female portion of the gonad, meiosis resumption was induced by serotonin, and then the oocytes were fertilized with sperm from different individuals. The progeny obtained by this method were compared with others obtained by cross-fertilization and self-fertilization. In all these procedures, the oocytes were fertilized at a ratio of 60 sperm per oocyte. The fertilization rates and D larvae survival were determined at 2 and 48 h, respectively. After the D larval stage, the hatchery culture of larvae continued as usual with sampling for survival and growth rates every three days. When larvae looked ready to settle and metamorphose, netlon collectors were introduced into the tanks. After 12 days the collectors were placed in 1 mm mesh collection bags and transferred to the sea. Juveniles were brought back to the hatchery every 40-70 days, to be counted, measured, and placed in mesh pearl nets. The assay of the three procedures for fertilization showed similar fertilization rates but low survival for D larvae obtained from the in vitro method. However, this procedure resulted in the highest size and survival rates of larvae. Survival of juveniles coming from this fertilization procedure was also the highest. The growth of juveniles obtained by in vitro fertilization was also higher than that of those obtained by self-fertilization. We conclude that this in vitro fertilization procedure provides a methodology that results in higher survival and growth rates of juveniles than obtained by procedures involving any degree of inbreeding. This in vitro procedure should be applicable to other simultaneous hermaphroditic scallops.     

The culture of the diatom Chaetoceros gracilis and its use as a food for penaeid protozoeal larvae

The use of the solitary diatom Chaetoceros gracilis as the exclusive food for Penaeus stylirostris and P. vannamei protozoeal larvae produced high survival rates in 40 m³ tanks at a commercial shrimp hatchery. The diatom was maintained in the laboratory at 25–26°C and grew vigorously in the hatchery tanks at 28–30°C.Simple and inexpensive procedures for maintaining stock cultures of C. gracilis and for growth of cultures in the laboratory and hatchery were developed. A problem encountered in the hatchery tanks was the occasional development of a high-density bloom followed by its collapse. This resulted in mass larval mortality. Techniques to maintain the diatoms within desirable concentrations, and to prevent larval mortality if diatom concentrations fell short of or exceeded these limits, are described.     

Preliminary study on broodstock rearing,induced breeding and grow‐out culture of the sea cucumber Holothuria scabra in Sri Lanka

Experiments were conducted to identify suitable methods for broodstock rearing, induced breeding and grow‐out culture of Holothuria scabra in Sri Lanka. Two hundred and seventy‐two brooders (500–600 g) collected from off Mannar were individually packed in oxygen‐filled polythene bags with and without sea water and transported to a sea cucumber hatchery at Kalpitiya. Lagoon pens, sand‐filled fibreglass tanks and bare tanks were used in triplicates to maintain brooders. Spawning was initiated using air dry, water jet and thermal‐stimulation methods. Hatchery produced juveniles with an average weight of 11 ± 5 g were reared (2 individuals m^?2) in lagoon pens, mud ponds and fibreglass tanks in triplicates. The significantly high evisceration rate was observed when brooders were transported without sea water (t‐test, P < 0.05). Brooders maintained in bare tanks showed a significant weight reduction than the brooders in sand‐filled tanks and lagoon pens (anova , P < 0.05, d.f. = 2). Thermal stimulation (ambient temperature ± 3–5°C) was found to be the most successful method of spawning initiation of H. scabra. The mean (±SD) percentage males and females participated for spawning per trial was 9.2 (±10) and 4.6 (±5.6) respectively. On an average, 1.16 millions of eggs (±1.03 SD, n = 5) were obtained per spawning trial. H. scabra juveniles reared in tanks showed significantly lower growth rate than the juveniles in pens and ponds (anova , P < 0.05). Lagoon pens and sand‐filled tanks are suitable to maintain brooders and lagoon pens can be successfully used for mass rearing of juveniles.     

Differentiated predation risk on hatchery-reared juvenile masu salmon by white-spotted charr with different body sizes

Predation after release is one of the major concerns of hatchery fish propagation. However, size-specific interaction between predator and prey on the survival of hatchery-released salmonid fish is largely unknown. To understand the size-selective predation risk, 24-h predation experiments were conducted on masu salmon Oncorhynchus masou in tanks. Four ranges of fork length (FL) were examined for masu salmon as a prey, in combination with three ranges of FL for white-spotted charr Salvelinus leucomaenis as a predator. The results show that not only predator and prey sizes, but also interaction between prey size and predator size, strongly affected the survival rate of masu salmon. Predation on masu salmon with the FL exceeding 40% of the FL of white-spotted charr was rare in the experiment. A logistic regression suggests that 37% relative FL of masu salmon to white-spotted charr results in the 50% survival of masu salmon. Our results suggest that adjusting relative size of hatchery fish to the size of local fish predators at the time of hatchery release will have a significant impact on the survival of hatchery fish in the wild. From this perspective, site-specific, adaptive management might be important to improve the effectiveness of hatchery fish propagation.     

Evaluation of the growth and survival rate of the Caribbean Sea cucumber,Isostichopus badionotus (Selenka, 1867), early juveniles produced in captivity

Isostichopus badionotus is a new commercially important sea cucumber in the Caribbean that has rapidly shown signs of overexploitation. There is no information on the growth and survival rates for wild or hatchery‐produced early juveniles. Hatchery production and determination of juvenile growth rates are essential for future aquaculture efforts for this species. In this study, daily growth rate (GR), specific growth rate (SGR), weight gain (WG), survival rate (SR), and weight–length relation (WLR) were evaluated using small (0.01 g) and large (0.25 g) hatchery‐produced early juveniles under laboratory conditions. Growth was rapid compared to other cultured sea cucumber species; maximal values of SGR and WG were obtained for small juveniles (4.58%/day and 2,191%, respectively), while maximal GR and SR were obtained for large juveniles (0.29 mm/day and 99.17%, respectively). A wide range of sizes was evident at the end of each trial. The WLR of small and large juveniles fit with a negative allometric growth trend. This study highlights the potential production of I. badionotus as early juvenile stages withstand hatchery handling well and exhibit a high SR, reaching commonly released mean sizes (>0.7 g and 25 mm) after almost 3 months (120 days postfertilization).     

Bivalve hatchery technology: The current situation for the Pacific oyster Crassostrea gigas and the scallop Pecten maximus in France

The French oyster farming industry relies almost exclusively on juveniles collected from the natural environment; the supply of spat produced by hatcheries is low, about 10 % of the industry's requirements. Development through selective breeding of oyster stocks which are better suited for aquaculture purposes, is likely to reverse this tendency since only hatcheries will be able to supply such animals. Scallop farming, which at present is poorly developed in France, relies exclusively on hatchery produced spat. Although hatchery technology is constantly being improved, significant production problems remain which must be solved before hatcheries become a major supplier of juveniles for the industry. This paper describes the present state of hatchery technology in France based on experimental results obtained with the great scallop Pecten maximus and the Pacific oyster Crassostrea gigas, over the past ten years. Compared to the great scallop, the Pacific oyster has higher D larva yields (60 % for C. gigas vs. 30 % for P. maximus) and a faster larval growth rate (10 μm·d⁻¹ for C. gigas vs. 5 μm·d⁻¹ for P. maximus). However, Pacific oysters have a greater heterogeneity during larval development and higher mortality rates (40 % for C. gigas vs. 25 % for P. maximus) which produce lower overall yields of pediveliger larvae ready-to-set for Pacific oysters (15 %) compared to the great scallop (30 %). Development of continuous larval and post-larval culture methods along with development of continuous phytoplankton technology offers one of the most promising methods to improve molluscan hatchery techniques.     

Intensification of redclaw crayfish Cherax quadricarinatus culture: I. Hatchery and nursery system

Intensification of an indoor hatchery and nursery system for the Australian redclaw crayfish Cherax quadricarinatus (von Martens) (Decapoda: Parastacidae) was obtained by increasing the surface area available for the crayfish juveniles and by synchronizing the age of the hatchlings held in each tank. The former improvement was facilitated by distributing an artificial seaweed-like material throughout almost the entire volume of small (275 l) hatching tanks. As the number of egg-bearing females was increased from 3 to 8 per hatching tank, the number of juveniles per liter also increased to as many as 6.5 juveniles/l, without reaching an apparent upper limit. The hatchlings were kept in the tanks for 75 days from the day females were found to be gravid and then harvested and graded according to size. The average juvenile weight at harvest was 0.34±0.04 g. The weight distribution of the juvenile males was not significantly different from that of the juvenile females on the day of harvest, and in both the distribution was positively skewed.     

Early-weaning diets for gilthead sea bream (Sparus aurata L.) and their potential use in Hellenic marine fish hatcheries

The purpose of this experiment has been to evaluate the suitability of commercially available early-weaning microdiets (MDs) for the production of sea bream early juveniles and in comparison with late-weaning protocols already in use by Hellenic marine fish hatcheries. Four sea bream experimental groups were allocated in rearing tanks of a commercial Hellenic marine fish hatchery. Each group represented a different protocol (A, B, CA, and CB) based on the combination of two different early-weaning MDs (A and B) and a late-weaning diet (C). In addition, the late-weaning protocols have received Artemia instar II only and not Artemia instar I. In protocol A, Artemia instar I first feeding and the early-weaning diet A were administered at 17 days post-hatch (17 dph). In protocol B, Artemia instar I first feeding started at 15 dph and the early-weaning diet B was administered at 18 dph. In the C protocols, Artemia instar II first feeding started at 20 dph and the early-weaning diets (A or B) were administered at 25 dph. All protocols have received the late-weaning diet C only after the 50th dph. The experiment lasted for 65 days. By the end of the experiment, early-weaning protocol A and late-weaning protocol CB had similar wet weights, but still lower to the wet weights recorded for the late-weaning protocol CA. These results cannot be explained solely by the nutritional profile of each weaning diet. The larval fatty acid profile of each protocol and at various time intervals reveals the importance that the succession of Artemia, rotifers, and MDs has for each protocol and not just the nutritional profile of the weaning diets, per se.     

Swim bladder inflation and survival of Mugil cephalus to 50 days

Excessive inflation of the swim bladder causes immobility and mortality of 12-day old larvae of Mugil cephalus and other marine fish. Increased aeration or mechanical agitation of the water in rearing tanks maintained the larvae below the water surface, and the swim bladder formed and functioned normally. Survival was dramatically increased. Two experimental production tanks, stocked initially with 15 000 and 20 000 larvae, at a density of 5.25 and 7.0/l, respectively, yielded 33.5% and 16.9% juveniles on day 50. Only an outbreak of ciliated parasites prevented both tanks from having survival figures of over 30%.     

Antipredator responses to overhead fright stimuli in hatchery-reared and wild European sea bass (Dicentrarchus labrax L.) juveniles

In this study, shoals of hatchery‐reared and wild sea bass juveniles (Dicentrarchus labrax L.) were tested for differences in their antipredator responses towards a visual (shadow) and a mechanical (dummy bill) overhead stimulus. Two behavioural variables – distance from the bottom and freezing duration – were measured during post‐stimulus phases of each test and compared between wild and hatchery‐reared shoals, composed of 10 juveniles each. The results showed that in both hatchery‐reared and wild juveniles, stimulus exposure elicited a significant decrease in the mean shoal distance from the bottom. Similarly, individuals from both groups engaged a freezing reaction, but the mean freezing duration was significantly higher in wild‐ than in hatchery‐reared juveniles. Results are discussed in the light of their relevance for the enhancement of restocking programmes.     

Laboratory trials on the effects of different diets on growth and survival of the common whelk, Buccinum undatum L. 1758, as a candidate species for aquaculture

Newly hatched juvenile Buccinum undatum can be reared under laboratory conditions. Good was growth is achieved when juveniles were fed on combined diets (blue mussel, cod, and fish pellets). Juveniles reached shell heights of 33.0?±?4.2?mm, 26.9?±?3.8?mm, 23.2?±?2.2?mm, and 20.1?±?1.6?mm, after 14 months of fedding on a combined diet, blue mussel, cod, and fish pellets, respectively under ambient sea temperature and salinity. After 14 months juveniles fed blue mussel had the highest survival rates (67%) followed by those fed a combination of all other experimental diets (61%), cod waste (53%) and fish-feed pellets (46%). High mortalities were recorded in most treatments during the summer months between June and September. This species appears to have an aquaculture potential, as juveniles readily feed on artificial diets at an early age, show high survival rates and could potentially reach market size in 2 years or less. The major constraint in realising this potential at present, is the relatively low value of the species; if market values increased as a result of serious depletion of natural populations, hatchery production of juveniles for intensive aquaculture or restocking could become economically viable.     

The effect of shade on water quality parameters and survival and growth of juvenile fluted giant clams, Tridacna squamosa, cultured in a land-based growth trial

Juvenile fluted giant clams, Tridacna squamosa (mean shell length (SL) 15.7 ± 1.9 mm, mean total wet weight (TWW) 0.3 ± 0.1 g), were cultured for 4 months in land-based nursery tanks under four shade regimes (0, 10, 55 and 80 % shade). After 4 months, there were no significant differences in survival which ranged from 98.8 ± 0.1 % in juveniles cultured under 55 % shade to 100.0 ± 0.0 % in the other treatments. The SL of unshaded juveniles was only significantly greater than that of juveniles shaded 55 and 80 %. However, the TWW of juveniles cultured with no shade was significantly greater than that of the juveniles from all other treatments. The mean growth rate of juveniles varied from 5.0 mm month^?1 for juveniles shaded 80 % to 6.9 mm month^?1 for juveniles not shaded. There was no significant difference in mean growth rate of unshaded juveniles and juveniles shaded 10 %, but only the growth rate of juveniles cultured unshaded was significantly greater than that of juveniles shaded 55 and 80 %. The organic tissue content of juveniles cultured either unshaded or shaded 55 % was significantly higher than that of juveniles shaded 80 %. In contrast, the condition index (CI) of juveniles shaded 80 % was significantly higher than the CI of juveniles cultured either unshaded or shaded 55 %. However, there was no significant difference between the CI of juveniles shaded 80 and 10 %. Growth rate of juveniles was significantly correlated with temperature, dissolved oxygen content and flow rate of seawater. While low degrees of shade increase the growth rate of juvenile T. squamosa, they also negatively affect tissue condition. A shade level of 10 % appears to be optimal as it maximises growth and maintains a high CI.     

Nursery rearing of the Asian catfish, Clarias macrocephalus (Günther), at different stocking densities in cages suspended in tanks and ponds

Growth and survival of hatchery‐bred Asian catfish, Clarias macrocephalus (Günther), fry reared at different stocking densities in net cages suspended in tanks and ponds were measured. The stocking densities used were 285, 571 and 1143 fry m^?3 in tanks and 114, 228 and 457 fry m^?3 in ponds. Fish were fed a formulated diet throughout the 28‐day rearing period. Generally, fish reared in cages in ponds grew faster, with a specific growth rate (SGR) range of 10.3–14.6% day^?1, than those in cages suspended in tanks (SGR range 9–11.3% day^?1). This could be attributed to the presence of natural zooplankton (copepods and cladocerans) in the pond throughout the culture period, which served as additional food sources for catfish juveniles. In both scenarios, the fish reared at lower densities had significantly higher SGR than fish reared at higher densities. In the pond, the SGR of fish held at 228 and 457 m^?3 were similar to each other but were significantly lower than those of fish held at 114 m^?3. The zooplankton in ponds consisted mostly of copepods and cladocerans, in contrast to tanks, in which rotifers were more predominant. Per cent survival ranged from 85% to 89% in tanks and from 78% to 87% in ponds and did not differ significantly among stocking densities and between rearing systems. In conclusion, catfish nursery in cages suspended in tanks and ponds is density dependent. Catfish fry reared at 285 m^?3 in tanks and at 114 m^?3 in ponds had significantly faster growth rates than fish reared at higher densities. However, the desired fingerling size of 3–4 cm total length for stocking in grow‐out culture can still be attained at stocking densities of 457 m^?3 in nursery pond and 571 m^?3 in tanks.     

Combined effects of water exchange regimes and calcium carbonate additions on growth and survival of hatchery-reared juvenile spotted babylon (Babylonia areolata Link 1807) in recirculating grow-out system

To determine a suitable culture environment to maximize growth and survival, the hatchery‐reared juvenile spotted babylon, Babylonia areolata, were held in plastic rearing tanks at four calcium carbonate additions of 0, 100 and 300 g tonne^?1, and four water exchange regimes of 0‐, 15‐, 30‐ and 60‐day intervals in a recirculating grow‐out system for 120 days. The results clearly showed that growth was greatest between water exchange regimes of 15‐ and 30‐day intervals and all calcium carbonate additions, with water exchange regimes of 0‐ and 60‐day intervals resulting in poor growth. Final survival was highest between water exchange regimes of 15‐ and 30‐day intervals, and all calcium carbonate additions, with water exchange regimes of 0‐day intervals and all calcium carbonate additions resulting in high mortalities. This study showed that water exchange regimes had a stronger influence on the growth of juvenile B. areolata than calcium carbonate additions. It is recommended that B. areolata juveniles be maintained within the water exchange regimes range of 15–30‐day intervals and at calcium carbonate additions between 0 and 500 g tonne^?1, providing optimum conditions for production of this species in a recirculating grow‐out system.     

Feeding selectivity of the seahorse,Hippocampus kuda (Bleeker), juveniles under laboratory conditions

This study examined the feeding selectivity of Hippocampus kuda juveniles under captive conditions and evaluates different food organisms that could be used to improve hatchery‐rearing of this species. Newly born H. kuda were reared for 10 days in 60‐L capacity tanks and fed rotifers (Brachionus rotundiformis), zooplankton (mostly Pseudodiaptomus annandalei and Acartia tsuensis) alone or both food sources. The size and amount of food ingested increased as seahorses grew. Selective feeding of seahorses appeared to change as they develop, preferring copepod adults over nauplii and rotifers. A. tsuensis was highly selected by juveniles over P. annandalei. Specific growth rate in terms of body weight (SGR‐BW, 15% day^–1) was the highest and mortality rate (9% at day 10) the lowest in seahorses fed a mixed food sources. Slowest growth rate (0.3% day^–1) and highest mortality rate (60% at day 7) were observed in seahorses fed rotifers alone. These results indicate that copepods are suitable food for seahorse juveniles, but a mixture of food organisms in the rearing tank environment enhances survivorship and growth of H. kuda, thus potentially providing a source of cultured rather than wild specimens for characterizing the life history of this threatened species.     

Performance of Different Types of Commercial Feeds on the Growth of Juvenile Silver Pomfret, Pampus argenteus, under Tank Culture Conditions

The present study was carried out using juveniles of silver pomfret, Pampus argenteus, produced in the hatchery by stripping eggs from spawners collected from the wild during 2004 and 2005 culture period. The experiments carried out during 2004 investigated the efficiency of using salmon feed mixed with cyclopeeze, salmon feed mixed with shrimp meat, salmon feed alone, and shrimp meat alone in the diet using 1 m³ capacity round fiberglass tanks in three replicates for each treatment with flow‐through seawater. The results showed that inclusion of shrimp meat in the diet either with salmon feed or alone give significantly high (P < 0.01) weight gain of up to 1.10 ± 0.06 g/fish/d and significantly lower (P < 0.001) feed conversion ratio compared to that of feeds without shrimp meat. The experiments carried out during 2005 investigated the use of commercially available feeds with feed additives. The results showed that feeding with “Gemma” feed with 54.0% crude protein and 19.0% crude fat or salmon feed with 41.4% crude protein and 23.9% crude fat give significantly high (P < 0.01) growth rates compared to that of pompano feed with 43.0% crude protein and 6.0% crude fat. The results of these investigations although show high growth rate of the fish compared to that of previous observations; there is yet an urgent need to formulate a suitable feed for this fish species for commercial ventures.     

Experiments in rearing rabbitfish (Siganus rivulatus) in sea water

The initial experiments in rearing rabbitfish (Siganus rivulatus) in sea water indicate that this fish may be suitable for commercial cultivation. Juveniles are available in large numbers during summer and fall along the Mediterranean and Red Sea coast of Israel. Juveniles collected in Eilat (Red Sea) were kept in tanks and submerged cages and fed fish pellets. After 29 days in tanks, the first group of fish which averaged 0.91 g upon capture increased in weight to an average of 3.7 g. A subsequent school of juveniles kept in sea cages for 84 days increased in weight from an average of 7.3 g to 28.5 g.     

Cost comparisons of rearing larvae of freshwater shrimp,Macrobrachium acanthurus and M. rosenbergii,to juveniles

Costs were determined for rearing larval Macrobrachium rosenbergii (De Man) and M. acanthurus (Wiegmann). Rearing was conducted in two phases. First-phase rearing (first 10 days) was conducted in conical fiberglass tanks with recirculated, filtered, brackish water; second-phase rearing (final 30 days) was done in aerated 1 000-l concrete tanks. To minimize water quality problems and debris accumulation in second-phase rearing, larvae were transferred every 4 to 5 days to a new tank of clean water. Larval food consisted of newly hatched Artemia and either freshly ground fish or freeze-fried fish.Mean survival in five M. rosenbergii rearing trials was 43% in 40 days at a final density of 12 juveniles per liter. Mean survival of four M. acanthurus trials was 25% in 44 days at a final density of 3 juveniles per liter. Average total cost of labor, operation (electricity), food and water per thousand juveniles produced was $3.56 for M. rosenbergii and $13.42 for M. acanthurus.Extrapolation of results to larger (3 000-l) second-phase rearing tanks showed M. rosenbergii rearing cost could be reduced to $\text{\$1.87}\text{1 000}$.