Improvement of larval rearing technique for mass seed production of snow crab <Emphasis Type="Italic">Chionoecetes opilio</Emphasis>

Larval rearing experiments were conducted to examine the potential for mass seed production of the snow crab Chionoecetes opilio using a total of eight 20-kL tanks. Tanks were equipped with agitators, which move the water using a rectangular blade to prevent the zoeas sinking to the bottom of the tank. Larval rearing water was treated with sodium nifurstyrenate once a week to reduce the chance of larval infection by pathogenic bacteria. Zo eas were fed with rotifers and Artemia nauplii. A total of 122 830 megalops and 16 660 first-stage crabs were produced. Thus, the potential for mass seed production of snow crab was determined. Survival rates up to the megalopal stage were high in tanks with a feeding regime that fed rotifers to larvae through an entire zoeal stage. This study also describes the fatty acid composition of snow crab larvae. It revealed that the first zoeas had a high DHA content and DHA/EPA ratio, but these values significantly decreased in the second stage zoeas and megalops. Improving the DHA content and/or DHA/EPA ratio of larvae should be important in studies on mass seed production technology of the snow crab.     

Survival,development and growth of larvae of the coconut crab,Birgus latro,cultured under different photoperiod conditions

Populations of the coconut crab, Birgus latro, have been severely depleted on most inhabited islands throughout the Indo‐Pacific regions because of overharvesting and environmental degradation. To assist in the development of artificial propagation technologies for supportive breeding programmes in conserving this species, this study was designed to elucidate the effect of photoperiod on the survival, developmental period of zoeae and body size of megalopae through laboratory experiments. We tested six light (L) and dark (D) photoperiod regimes times of 0–24 h, thus 0L:24D, 6L:18D, 12L:12D, 18L:6D and 24L:0D. Photoperiod conditions significantly affected larval survival. Survival was lower in the 0L:24D condition than in other photoperiod conditions, reflecting higher mortality during the zoeal stage in the 0L:24D condition. On the other hand, survival rates were high during the periods when larvae metamorphosed into megalopae, regardless of photoperiod conditions. In the 24L:0D condition, larval survival rate was slightly lower than in other conditions containing a light phase. Zoeal duration and megalopal body size were not different in light durations ranging from 0 to 18 h, but they were longer and larger, respectively, in the 24L:0D condition. Thus, continuous dark or continuous light conditions impaired the survival and development of larvae, and such photoperiod regimes should be avoided for culturing larvae of the coconut crab.     

Effects of photoperiod and night‐time aeration rate on swim bladder inflation and survival in Pacific bluefin tuna,Thunnus orientalis (Temminck & Schlegel), larvae

Success of swim bladder inflation (SBI) is crucial for early survival of Pacific bluefin tuna (PBF) larvae, because it reduces larval sinking death by enhancing buoyancy. In Experiment 1, we examined the effect of photoperiod on SBI and survival in PBF larvae by comparing photoperiods of 9L: 15D (9L), 14L: 10D (14L: natural photoperiod), 19L: 5D (19L) and 24L: 0D (24L) during 2–10 days post hatch (dph). In Experiment 2, the combined effects of photoperiod (24L and 14L) and nighttime aeration rate (enhanced night‐time aeration: ENA of 1300 mL min^?1 as a countermeasure for sinking death and 130 mL min^?1) on the survival and SBI were also examined during 2–10 dph. Moreover, in Experiment 3 the effect of photoperiod on vertical distribution of larvae in night‐time was examined on 3–5 dph. Photoperiod of 24L in Experiment 1 significantly inhibited SBI compared with 14L and 19L; nevertheless, it significantly improved survival compared with other photoperiods with a dark period. On the other hand, the shortened light period (9L) showed significantly reduced SBI and also survival. In Experiment 2, the countermeasure for sinking death of ENA under 24L did not further improve the survival; rather it tended to reduce the survival. In Experiment 3, larvae distributed less in the bottom layer in 24L than in 14L, suggesting the reducing effect of 24L on sinking death. The results indicate that 24L without ENA is suitable for survival which is the most serious problem in PBF larviculture.     

Effects of n-3 highly unsaturated fatty acid content in Artemia on survival and development of laboratory-reared horsehair crab Erimacrus isenbeckii larvae

The effect of varying levels of dietary n-3 highly unsaturated fatty acid (HUFA) content in Artemia sp. on survival, the number of days required to reach each larval stage, and the growth and morphogenesis of larvae of the horsehair crab Erimacrus isenbeckii were examined. Artemia enriched with materials of six types [condensed Nannochloropsis sp., ethyl oleate, two enrichment levels of eicosapentaenoic acid (EPA), and those of docosahexaenoic acid], and unenriched Artemia were fed to larvae. Newly hatched larvae were reared in groups of 50 individuals in two 2-l beakers until they reached the first crab stage. No significant difference was found in survival rate, growth, or morphogenesis of larvae, but fewer days tended to be required to reach the first crab stage in the group enriched with high EPA compared to the other groups. Results show that, although the amount of n-3 HUFA in unenriched Artemia might meet the demands of horsehair crab larvae for their survival, growth, and morphogenesis, Artemia should be enriched with high EPA to improve the rate of larval development.     

Photoperiod affects the survival rate but not the development of larval swimming crab Portunus trituberculatus

Photoperiod is a crucial environmental factor affecting the survival and development of crustacean larvae. The full-spectrum light-emitting diodes (LEDs) were used as light sources to investigate the effect of five photoperiods, i.e., 0 L:24D (constant darkness), 6 L:18D (6 h photophase), 12 L:12D (12 h photophase), 18 L:6D (18 h photophase), and 24 L:0D (constant light) on the survival and development of the swimming crab Portunus trituberculatus larvae (from zoea I to juvenile crab). Natural photoperiod (ambient) was used as a control group. Each treatment had four replicates (n?=?4, 100 larvae per replicate). The experiment lasted 20 days when all the larvae died or metamorphosed into juvenile crabs. The results revealed that the P. trituberculatus could not metamorphose into juvenile crab under constant darkness, 6 h photophase, and constant light, and all the larvae died in zoeal III, zoeal IV, and zoeal IV stage, respectively. The highest survival and the shortest developmental duration of larvae were found in the 12 h photophase group (1.75%, 16.80 d) but were not statistically different from those reared in the 18 h photophase group (0.75%, 17.00 d) and natural photoperiod (0.25%, 18.90 d). The above results indicated that full-spectrum LEDs could be a considerable alternation for natural light, and the optimal photoperiod for P. trituberculatus larvae is 12–18 h photophase.

     

Effect of larval and prey density, prey dose and light conditions on first feeding common dentex (Dentex dentex L.) larvae

Initial larval stocking density, prey density, daily prey ration and light conditions (light intensity and photoperiod) were tested for common dentex larval rearing under experimental conditions. Experiments continued until the first peak of larval mortality. The best results in larval survival were obtained with an initial stocking density of between 10 and 40 larvae L^?1, fed with at least 10 rotifers mL^?1, maintaining ratios of 500–1000 rotifers larva^?1, with one or two adjustments of prey density per day. The use of more than 2000 rotifers larva^?1 or three daily adjustments of live prey density had negative effects on larval survival. The best light conditions for common dentex larval rearing were found using a photoperiod of 24 h L:0 h D and an intensity of at least 3.4 μmol m^?2 s^?1.     

Influence of water management,photoperiod and aeration on growth,survival, and early spat settlement of the hatchery-reared green mussel,Perna viridis

In an attempt to induce early spat settlement and improve mussel seed production, this study aims to determine the influence of water management, photoperiod, and aeration, on the growth, survival and settlement of green mussel (Perna viridis). Water in the pediveliger rearing tanks was changed every day, every 3 days and every 5 days for the water-management experiment. Pediveligers were exposed in 24L:0D h (light: dark), 12L:12D h and 0L:24D h conditions for the photoperiod experiment. Three aeration intensities were also tested—mild (10 L h−1), moderate (20 L h−1), and strong (30 L h−1). This study demonstrated that changing water every 3 days was effective in maintaining the rearing water quality and improving the growth and survival of P. viridis larvae. Highest growth and survival rates were observed in P. viridis spats grown in 0L:24D h photoperiod. There was no significant difference in the settlement rate of larvae exposed to different photoperiods. Mild aeration has shown to improve the growth of P. viridis larvae, but higher survival and settlement rates were attained in the strongly-aerated conditions. Therefore, when the larvae start to settle, it is recommended to expose them to darkness, change the water every 3 days and provide a strong aeration to be able to attain high survival and settlement rates, and bigger spats.     

Comparison of Two Environmental Regimes for Culture of Australian Snapper, Pagrus auratus, Larvae in Commercial-scale Tanks

The performance of Australian snapper, Pagrus auratus, larvae from 4 to 33 days posthatch (dph) under two environmental rearing regimes was evaluated in 2000‐L commercial‐scale larval rearing tanks (N = 3 tanks/treatment). The treatments were the following: (1) a varying regime of salinity (20–35 ppt), temperature (24 C), and photoperiod (12 light [L] : 12 dark [D] to swim bladder inflation and then 18L : 06D) and (2) a constant regime of salinity (35 ppt), temperature (21 C), and photoperiod (14L : 10D). The final total length (TL) and wet and dry weights (mean ± SEM) of larvae grown in the varying regime were greater (15.6 ± 0.5 mm; 42.4 ± 3.4 mg wet weight; and 7.3 ± 0.6 mg dry weight) than those of larvae grown in the constant regime (11.1 ± 0.2 mm; 12.9 ± 0.8 mg wet weight; and 2.1 ± 0.2 mg dry weight). By 33 dph, larvae in the varying regime were fully weaned from live feeds to a formulated pellet diet and were suitable for transfer from the hatchery to a nursery facility. In contrast, larvae in the constant regime were not weaned onto a pellet diet and still required live feeds. Neither survival (Treatment 1, 14.2 ± 3.0% and Treatment 2, 13.3 ± 1.9%) nor swim bladder inflation (Treatment 1, 70.0 ± 17.3% and Treatment 2, 70.0 ± 11.5%, by 13 dph) was affected by rearing regime. The incidence of urinary calculi at 7 dph was greatest initially in the varying regime; however, by 19 dph, when larvae were 8.0 ± 0.28 mm TL, very few larvae in this treatment had urinary calculi. In contrast, many larvae in the constant regime had developed urinary calculi and this continued until the end of the experiment. The incidence of urinary calculi was not associated with larval mortality. Extrapolation of the snapper larval growth curves for the constant larval rearing regime predicts that a further 15–18 d, or approximately 1.5 times longer, will be required until these larvae attain the same size and development of larvae reared in the varying regime.     

Effects of light intensity, tank colour and photoperiod on swimbladder inflation success in larval striped bass, Morone saxatilis (Walbaum)

Striped bass, Morone saxatilis (Walbaum), larvae, although physoclistous as adults, initially need to inflate their swimbladders by passing ingested air through a pneumatic duct to the swimbladder. Failure to inflate the swimbladder results in swimbladder atrophy, dysfunctional buoyancy control, deformities, and poor larval survival and growth. The present investigation studied the effects of light intensity, tank colour and photoperiod on the initial swimbladder inflation success of striped bass. In the first experiment, newly hatched larvae were reared for 2 weeks in black or white 350-L circular tanks exposed to incandescent lighting at 163 or 10 lux. Larvae were fed Artemia from 6 days post-hatching (dph). The standard length, and oil globule and yolk areas were measured. Swimbladder condition and evidence of feeding were recorded. To study the effects of photoperiod on inflation, larvae were reared in black tanks exposed to either 8 h:16 h or 16 h:8 h light:dark photoperiods at 139 lux. Striped bass inflated their swimbladders between 6 and 9 dph at 17 ^oC. Inflation success was significantly higher in black tanks (65%) compared with white tanks (42%), but light intensity did not significantly affect swimbladder inflation. Larval growth and food consumption were better in black tanks, primarily because this treatment resulted in more larvae with inflated swimbladders, although larvae with uninflated swimbladders in black tanks were longer at 12 dph than larvae with inflated swimbladders in white tanks. Larvae that successfully inflated their swimbladders fed earlier and exhibited better growth than larvae with noninflated swimbladders. Larvae reared at higher light intensities used up yolk reserves faster. Exposing striped bass larvae to 8 h:16 h light:dark photoperiods resulted in higher inflation rates (55%) compared to larvae reared at 16 h:8 h light:dark photoperiods (30%). Therefore, black tanks and short photoperiods are preferable for early rearing of striped bass larvae to maximize swimbladder inflation success.     

Optimal temperature and photoperiod for the spawning of blue crab,Callinectes sapidus,in captivity

Like all poikilotherms, the growth and reproduction of blue crab, Callinectes sapidus depends on temperature and season. Warmer water temperatures in the Chesapeake Bay allow for ovarian development and spawning, while colder water temperatures slow their metabolism and reproduction. The current study aimed to identify optimal environmental conditions for inducing reproduction in animals held in long‐term captivity for year round production in aquaculture through environmental manipulations. Temperature and photoperiod were the main environmental factors tested for 25 weeks: 11°C and 21°C, with the following photoperiods: 0L:24D, 8L:16D, 16L:8D and 24L:0D. At 21°C, the females increased spawning frequency, which was arrested at 11°C. Shorter light exposure at 21°C increased spawning frequency, while constant light inhibited and did not produce spawning. Constant dark (0L:24D) at 21°C produced the most (86%) spawns, but yielded poor larval quality. At 21°C with all photoperiod conditions except constant light, the first spawning took 94.8 ± 32.4 days to occur (n = 17). With females producing multiple spawns, the intervals between the first and second spawns and the second and third spawns were 37.7 ± 8.7 days (n = 6) and 31.0 ± 7.1 days (n = 2) respectively. Analysis of our data using response surface methodology (RSM) predicts the following conditions: at 15–19°C and 0–10 hr darkness for maximal survival and at 19–22°C and 0–8 hr darkness for spawning. The number of larvae produced was positively correlated with size (weight) of the female C. sapidus, suggesting the importance of female size in reproduction.     

Factors affecting the feeding response of larval southern bluefin tuna,Thunnus maccoyii (Castelnau, 1872)

Southern bluefin tuna, Thunnus maccoyii, are cultured in Australia following collection of wild juveniles. Hatchery culture from egg is in the experimental stage. High early mortality has hindered the production of quality juveniles in the hatchery. This study investigated the visual capacity of T. maccoyii during early larval ontogeny in order to describe the best larval rearing conditions to produce high‐quality seed stock. Functional visual ability, determined through behavioural experimentation, identified the effect of light intensity, prey density, turbidity, tank colour and turbulence on the feeding response. Larvae were visually challenged to feed under a range of conditions in short‐duration (4 h) feeding experiments. Feeding performance was measured as the proportion of larvae feeding and the intensity of feeding. First‐feeding performance was positively affected by increasing prey density and lower turbidities and unaffected by light intensity, tank colour, turbulence, prey size and larval density. The key findings from feeding experiments on 6 and 9 dph larvae was that as T. maccoyii aged, lower light intensities and higher prey densities significantly increased feeding performance. In addition, the study has identified that high light intensity and high air‐driven turbulence induced significant mortality. The proficient first‐feeding response indicated that early mortality common in culture is unlikely to be associated with a failure to initiate feeding. Our findings show the use of low light intensity has the potential to significantly improve survival and feeding response during the first two critical weeks of culture, when the major bottleneck in hatchery production is currently experienced.     

Influence of light intensity on feeding, growth, and early survival of leopard coral grouper (Plectropomus leopardus) larvae under mass-scale rearing conditions

This study investigated the effect of different light intensities on feeding, growth and survival of early stage leopard coral grouper Plectropomus leopardus larvae. Four different light intensities (0, 500, 1000 and 3000 lx) were used and larvae were kept under constant light conditions from 0 day after hatching (DAH) to 5 DAH. The larvae were fed a small S-type of Thai strain rotifers at a density of 20 individuals/mL from 2 DAH. The number of rotifers in larval digestive organ and total length of larvae were examined at 3 h intervals between 04:00 and 22:00 h on 3 DAH, and thereafter at 6 h intervals until the end of the experiment (5 DAH). Four experimental trials of the larval rearing were repeated using by 60 kL mass-scale rearing tanks. The results indicate that coral grouper larvae are visual feeders and their food intake increases with increasing light intensity. Food intake of larvae reared at 3000 lx was significantly higher than those reared at 0–1000 lx on 3 DAH despite being the first-feeding day (P < 0.01). On 4 DAH, total length of larvae reared at 3000 lx was significantly larger than those reared at the lower light intensities (0, 500 and 1000 lx), and thereafter light intensity significantly influenced larval feeding and growth until the end of the experiment. Survival on 5 DAH did not show a significant difference between light intensities, but survival rate at 3000 lx and 1000 lx had a tendency to be higher than those reared at the lower light intensities (0 and 500 lx). In contrast, larvae reared at 0 lx exhibited stagnant and/or negative growth. These results indicate that light intensity is significantly the factor affecting larval feeding, growth, and survival in coral grouper larvae under the rearing conditions.     

The effect of photoperiod on growth, survival, colour and activity of juvenile southern rock lobster, Jasus edwardsii

Juvenile (1–10 g) southern rock lobsters, Jasus edwardsii, were subjected to five photoperiods [0L(light):24D(dark); 6L:18D; 12L:12D; 18L:6D; 24L:0D] during a 112‐day trial, and growth, survival, colour, food consumption and activity were examined. Lobsters grown under the 6L:18D and 24L:0D photoperiods had significantly lower (P < 0.05) final mean weight and specific growth rate than any other treatments. Photoperiod had no effect on survival or colour of lobsters. Food consumption differed significantly during the first week of the trial; after 5 weeks, food consumption was similar in all treatments. Major peaks in activity occurred during dark periods for lobsters exposed to photoperiods that had a light:dark regime. Activity of lobsters exposed to continuous light or dark regimes remained constant and at low levels during the 24‐h period. Food consumption or activity does not appear to determine the rate of growth of lobsters in the different photoregimes. One over‐riding outcome of this and other studies on photoperiod is that the growth and survival response of juvenile J. edwardsii was not significantly better than those of the standard 12L:12D cycle.     

Reproduction,growth and stress response in adult red sea bream,Pagrus major (Temminck & Schlegel) exposed to different photoperiods at spawning season

Adult red sea bream, Pagrus major (body weight, 1.0–2.0 kg) was exposed to three photoperiods [12 h light:12 h dark (12L:12D), 16 h light:8 h dark (16L:8D) and 24 h light:0 h dark (24L:0D)] from 2 months before spawning till the end of the spawning season to investigate growth, spawning and stress response. During the spawning season, tanks were checked every morning for spawned eggs. The growth performance in fish under 24L:0D was stimulated with significantly higher feed intake than those under other photoperiods (P<0.05). The number of eggs and gonadal histology confirmed that three and five females out of six in each of duplicate tanks of the 16L:8D treatment spawned. In contrast, only two out of six females in one tank of the 24L:0D treatment spawned, and no spawns were observed in the 12L:12D treatment. At the end of the spawning period, both 17β‐estradiol and testosterone levels were significantly higher in fish exposed to 16L:8D followed by 12L:12D and 24L:0D photoperiods (P<0.05). Photoperiod manipulation did not cause significant stress response in fish (P>0.05). The results suggest that stimulating the growth performance of red sea bream at reproductive stage with a 24L:0D photoperiod is possible if the fish are subjected to this photoperiod long before the onset of the spawning season.     

Morphological development and allometric growth of yellowtail kingfish Seriola lalandi V. larvae under culture conditions

Morphological development and allometric growth patterns of Seriola lalandi larvae were assessed to characterize normal growth patterns under culture conditions. Early ontogenetic stages of yellowtail kingfish exhibited an exponential growth in terms of standard length as a function of age. Five development stages were characterized from hatching to the juvenile stage: larval stage I (0–2 days post hatch, dph) with endogenous feeding, characterized by a small yolk sac, unpigmented eyes, primordial finfold surrounding the body and a closed mouth; larval stage II (2–15 dph) characterized by mouth opening, complete pigmentation of eyes and the beginning of the exogenous feeding; subsequently, in the larval stage III (15–25 dph) the posterior tip of notochord of the larvae bended upward and the first rays appeared in fins, concomitant with a change in swimming behaviour; thereafter, larval stage IV (post‐flexion stage; 25–30 dph) began when larvae resembled in morphology to a juvenile organism; however, caudal and dorsal fins were not completely development. Lastly, the juvenile stage was reached 30 dph characterized by a morphology and fin structures similar to those of the adults. Growth and development of structures and organs associated with vital functions such as feeding, sensorial and breathing systems seemed to be more critical previous to 23 dph, which was reflected with a positive allometric growth of head and eyes during this period. The results from this study can be used as a tool‐guide to assess normal development in larval research with S. lalandi to improve existing rearing protocols in hatchery production.     

Combined effects of photoperiod and salinity on growth,survival, and osmoregulatory ability of larval southern flounder Paralichthys lethostigma

The southern flounder, Paralichthys lethostigma, is an important commercial and recreational marine flatfish that inhabits estuaries and shelf waters in the south Atlantic, from North Carolina through the Gulf coasts, with the exception of south Florida. Because juvenile and adult fish are highly euryhaline, it is a prime candidate for aquaculture. Methods for captive spawning of southern flounder are well developed; however, information on optimal culture requirements of the early larval stages is required for reliable mass production of juveniles.To determine the optimal photoperiod and salinity conditions for culture from hatching to day 15 post-hatching (d15ph), embryos were stocked into black 15-l tanks (75 l⁻¹) under four photoperiods (24L:0D, 18L:6D, 12L:12D, and 6L:18D) and two salinities (25 and 34 ppt) in a 4×2 factorial design. Temperature was 18 °C, light intensity was 150 lx, and aeration was 50 ml min⁻¹. Significant (P<0.05) effects of photoperiod and salinity on growth (notochord length, wet and dry weights) were obtained. Growth increased with increasing photoperiod and salinity and was significantly greater at 24L and 18L than at 12L or 6L, and at 34 than at 25 ppt. On d11ph and d15ph, significant interactive effects between photoperiod and salinity on growth (wet and dry weights) were also evident. Growth of larvae reared at 25 ppt increased with increasing photoperiod to a maximum at 24L, while growth of larvae at 34 ppt reached a plateau at 18L. While there were no significant photoperiod effects on these parameters, larval survival, body water percentage, and larval osmolality on d15ph were significantly higher at 34 than at 25 ppt (41% vs. 16% survival; 322 vs. 288 mosM kg⁻¹; and 84% vs. 76% water, respectively), suggesting stress and nonadaptation to 25 ppt, a salinity more nearly isoosmotic than full-strength seawater. Since larvae from both salinity treatments were neutrally or positively buoyant at 34 ppt, but negatively buoyant at 25 ppt, larvae reared at 25 ppt probably allocated energy to maintain vertical positioning, compromising growth and survival.The results demonstrate that growth and survival of early-stage southern flounder larvae are maximized under long photoperiods of 18–24L and in full-strength seawater. Longer photoperiods probably extend the time larvae have for feeding, while full-strength seawater salinity optimizes buoyancy and vertical positioning, conserving energy. The results show that early larval stage southern flounder larvae are not entirely euryhaline, which involves not only the ability to osmoregulate, but to conserve energy under reduced buoyancy. This is consistent with suboptimal vs. maximal growth of larvae reared at 25 and 34 ppt, respectively, under 18L (i.e., photoperiod×salinity interaction). This is also consistent with other reports that tolerance to lower salinities in these euryhaline flatfish increases post-metamorphosis when transition from a pelagic to benthic existence alleviates the need to counteract reduced buoyancy.     

A tank design for first feeding of Atlantic halibut, Hippoglossus hippoglossus L., larvae

An intensive method of juvenile Atlantic halibut, Hippoglossus hippoglossus (L.), production has been under development over the last decade because of the problems associated with the extensive method. The lack of initiation of feeding behaviour has been the main obstacle for successful indoor rearing under artificial light and feeding conditions. In the present paper, an intensive method for the first feeding of halibut larvae is described and verified by practical feeding trials. The method involves circular 1.5-m³ indoor tanks with a peripheral ring-shaped cover, the use of continuous light, UV-A radiation during the first 24 h, central up-welling of water made by aeration and the use of microalgae in the rearing water. The single most important factor in such systems is to maintain a current pattern which allows the larvae to orientate and position themselves to face the water current for easy capture of prey.     

First results of embryonic development,spawning and larval rearing of the Mediterranean spider crab Maja squinado (Herbst ) under laboratory conditions,a candidate species for a restocking program

The Mediterranean spider crab, Maja squinado, is depleted due to overfishing. The crab has virtually disappeared from areas where it was abundant, such as the Balearic Islands and the Catalan coast. Maja squinado, is economically and ecologically very valuable, and it is essential to obtain information on its biology and rearing conditions to attempt to repopulate the damaged stocks of the species in the Mediterranean basin. Herein, we describe the first successful rearing of M. squinado under laboratory conditions. Our results show that M. squinado is an excellent candidate for restocking using cultured juveniles. Two consecutive broods with a 1–4 day interbrood period were observed in the laboratory in wild‐caught females, the maximum observed duration of embryonic development of the egg mass being 32 days at 18.4 ± 0.9°C, and went through four different stages. The complete larval and first juvenile development was studied in laboratory cultures fed enriched Artemia metanauplius. At 19.6 ± 0.6°C, development from hatching to first crab moult took 17 days, and it comprised two zoeae stages and one megalopa stage. The survival rate at the different stages was monitored, and 7.13 ± 2.3% was achieved at the first crab instar.     

Effect of water surface condition on survival,growth and swim bladder inflation of yellowfin tuna,Thunnus albacares (Temminck and Schlegel), larvae

Early‐stage mortality due to surface water tension‐related death and due to sinking to the tank bottom was investigated for yellowfin tuna, Thunnus albacares (YFT), larvae. Different aeration rates and rearing water surface conditions were examined to evaluate the effect on larval survival, swim bladder inflation and growth. The percentage survival of yolk sac larvae was significantly higher when the rearing water surface was covered with fish oil at aeration rates of 0 and 50 mL min^?1. The highest mortality occurred at the highest aeration rate of 250 mL min^?1 regardless of surface water condition. A second experiment was conducted twice under different water surface conditions: the water surface was covered by fish oil (FO), skimmed of fish oil (SS), and was not treated (NC). The percentage survival was not significantly different between treatments after 7 days of feeding. In contrast to the survival, the proportion of larvae with inflated swim bladders was significantly higher for the NC and SS groups than that of the FO group. Results of these experiments indicate that the addition of oil to the rearing water surface without its removal interferes with the initial swim bladder inflation in YFT larvae. These results also indicate that YFT larvae need to obtain (gulp) air at the water surface for initial swim bladder inflation, and success of initial swim bladder inflation may be crucial for their survival.     

Improved techniques for rearing mud crab Scylla paramamosain (Estampador 1949) larvae

A series of rearing trials in small 1 L cones and large tanks of 30–100 L were carried out to develop optimal rearing techniques for mud crab (Scylla paramamosain) larvae. Using water exchange (discontinuous partial water renewal or continuous treatment through biofiltration) and micro‐algae (Chlorella or Chaetoceros) supplementation (daily supplementation at 0.1–0.2 million cells mL⁻¹ or maintenance at 1–2 millions cells mL⁻¹), six different types of rearing systems were tried. The combination of a green‐water batch system for early stages and a recirculating system with micro‐algae supplementation for later stages resulted in the best overall performance of the crab larvae. No clear effects of crab stocking density (50–200 larvae L⁻¹) and rotifer (30–60 rotifers mL⁻¹) and Artemia density (10–20 L⁻¹) were observed. A stocking density of 100–150 zoea 1 (Z1) L⁻¹, combined with rotifer of 30–45 mL⁻¹ for early stages and Artemia feeding at 10–15 nauplii mL⁻¹ for Z3–Z5 seemed to produce the best performance of S. paramamosain larvae. Optimal rations for crab larvae should, however, be adjusted depending on the species, larval stage, larval status, prey size, rearing system and techniques. A practical feeding schedule could be to increase live food density from 30 to 45 rotifers mL⁻¹ from Z1 to Z2 and increase the number of Artemia nauplii mL⁻¹ from 10 to 15 from Z3 to Z5. Bacterial disease remains one of the key factors underlying the high mortality in the zoea stages. Further research to develop safe prophylactic treatments is therefore warranted. Combined with proper live food enrichment techniques, application of these findings has sustained a survival rate from Z1 to crab 1–2 stages in large rearing tanks of 10–15% (maximum 30%).