Hatchery-reared Juveniles Research Articles

Optimizing release strategies for red king crab stock enhancement: Effects of release timing

Red king crab, Paralithodes camtschaticus, was commercially important around Kodiak, Alaska, USA, in the 1960s and 1970s; however, the stock crashed in the late 1970 s and has remained closed since 1983. The lack of recovery inspired consideration of stock enhancement through the release of hatchery-reared juveniles as a means to bolster the wild population. We examined the effects of release timing on in situ survival of hatchery-reared red king crab by releasing juveniles in June, August, and September 2015 in Trident Basin, Kodiak. We monitored densities inside and outside of release plots for six months using quadrat counts to determine loss and emigration rates. Relative predation risk was determined using tethering experiments performed after each release, and predator densities were quantified using quadrat counts and predator transect counts. Initial mortality over the first 24 h was approximately 53%, and subsequent mortality rates decreased with month-of-release, likely due to a combination of larger size-at-release and seasonal changes in predation. Although predator density was consistent over time, relative predation risk of tethered crabs decreased with season, suggesting later releases may be beneficial. However, the extended hatchery rearing period needed for later releases presents other challenges, including cannibalism, and the potential for developing maladaptive traits. Stock enhancement programs must balance these trade-offs to maximize overall success. Early releases of small juveniles immediately after settlement may be optimal if large-scale hatchery communal rearing results in significant juvenile production loss and/or hatchery conditioning is impractical.

Reduced lifetime fitness (growth, body condition and survivability) of hatchery-reared tiger pufferfish Takifugu rubripes compared to wild counterparts.

Tiger pufferfish Takifugu rubripes (order Tetraodontiformes, family Tetraodontidae) is a highly exploited species and stocks continue to decline, although hatchery-reared juveniles have been released since 1965 for stock enhancement. To determine why the stock has not recovered through hatchery-release practices, this study investigated and compared the population characteristics of wild and hatchery-origin fish. The length-mass relationship showed that hatchery-origin fish were skinnier, with males weighing less than 90% of the mass of wild males of the same length. The hepatosomatic index tended to be lower in hatchery-origin fish. Age was estimated using the otolith-based method, and the estimates were more accurate and precise than those obtained by the conventional vertebra-based method. At the age of 2.9 years, an age at which specimens were the most abundant in catches, hatchery-origin males weighed only 67% of wild males. The maximum observed age was 12 years for wild fish and 5 years for hatchery-origin fish. The instantaneous total mortality rates of hatchery-origin fish were more than twice as high as those of wild fish. In summary, the hatchery-origin fish had poor health status, poor growth and high mortality, and their fitness in natural environments was therefore hypothesized to be low throughout life.

Let them go free: Spatial behaviour following the experimental release of hatchery-reared juveniles Sciaena umbra and Dentex dentex

As marine stocks and biodiversity are rapidly declining, the release of hatchery-reared individuals into the wild is a species conservation measure aiming at replenishing stocks of freshwater, anadromous and marine fish species. The aim of this study is to assess for the first time the post-release exploration behaviours, survival and dispersal in the wild of released hatchery-reared juveniles of D. dentex and S. umbra. For this purpose, twenty fish from each species were tagged with acoustic transmitters and released in a marine no-take zone located in Corsica (France). Most individuals were considered alive until their last detection (95% of S. umbra and 74% of D. dentex detected individuals). Fish were detected within the receiver array on average up to 11.3 (± 20.7) and 10.3 (± 10.3) days after being released for juveniles of D. dentex and S.umbra, respectively. Dentex dentex juveniles dispersed mostly southward along the shore whereas S. umbra juveniles tended to disperse along the shore both northward and southward from the acoustic receiver array. Before their rapid dispersion (i.e. within a few days) outside the studied zone, juveniles of both species displayed increasing cumulative MCP areas and increased their vertical niche revealing their gradual coverage and exploration of deeper habitats in the area. The results from our study highlighted that hatchery-reared juveniles of both species did not settle inside the studied area but seemed to start to acclimate to the natural environment. This work is a first step towards the assessment of the viability of restocking for both species.

Sea Ranching Feasibility of the Hatchery-Reared Tropical Sea Cucumber Stichopus monotuberculatus in an Inshore Coral Reef Island Area in South China Sea (Sanya, China)

Sea ranching of tropical edible sea cucumbers is an effective way to relieve the overfishing stress on their natural resources and protect the coral reef ecosystem, yet only a few species have been applied in the sea ranching practice based on hatchery-reared juveniles around the world. In this study, an 8-month (April to December) sea ranching study for hatchery-reared edible sea cucumber Stichopus monotuberculatus juveniles was carried out at a tropical coral reef island area in Sanya, China. Several growth performance indexes and basal nutritional components were monitored. Results revealed that the sea cucumbers had a growth rate of 0.35~0.78 mm day-1 during the experimental period, reaching 15.9 cm long before winter. The weight gain reached 491.13% at the end, and most sea cucumbers were able to grow to the commercial size (over 150 g WW) in the first year of sea ranching. The overall specific growth rate (SGR) and survival rates were 0.73 and 27.5%. Most of the death occurred in the first month after release (25.0%–37.5%), and this is probably due to inadaptation to the sudden change of the environment from the hatchery to the wild, which is proved by the remarkable decrease in nutritional indexes (amino acids, total lipid, and crude protein). Stable isotope and lipid biomarkers revealed that the food source mainly comes from water deposits (with microbes), Sargassum sanyaense seaweed debris, phytoplankton, and coral mucus-derived organics. The study proved the feasibility of the sea ranching of the hatchery-reared S. monotuberculatus juveniles in the tropical coral reef island area. Also, it is highly recommended that appropriate acclimation operation before release should be carried out to improve the survival rate of this species.

Genetic Stock Identification and Adaptability of Hatchery-Reared Black Rockfish, Sebastes schlegelii, Released Into the North Yellow Sea waters

Successful stock enhancement refers to the maintenance of economic profit, whilst minimizing negative genetic and ecological effects. As hatchery environments differ from natural habitats, post-release identification of hatchery-reared juveniles, and estimation of their adaptability are essential steps in conducting responsible stock enhancement. The black rockfish is an important fishery species in Japan, South Korea and China. Overfishing has dramatically decreased natural resources and a stock enhancement program was first initiated in the Liaodong peninsula in 2019. We traced this program and used nine microsatellite markers to identify 279 individuals as being hatchery-reared, out of 710 captured fish, with a resource contribution rate of 39.30%. These results indicated that stock enhancement of this species is likely to maintain a positive economic performance. To understand the potential genetic impacts on the local wild population, the genetic patterns of S. schlegelii stocks (wild controls from 2017, wild controls from 2018, female broodstock from 2019, recaptured hatchery-reared fish from 2019 and recaptured non-hatchery-reared fish from 2019 stocks) from the same habitat were evaluated before and after artificial stocking. We found that the S. schlegelii population was able to maintain high genetic diversity and showed weak genetic differentiation, but potential genetic introgression derived from stock enhancement should be concerned. The hatchery-reared individuals showed good adaptability, as shown by feeding condition and growth status. There was no obvious morphology difference between hatchery-reared fish and their wild counterparts, but hatchery-reared fish seemed to be bigger in size (significantly higher in MDB and BW), had a larger head (significantly larger in PRL/HL and ID/HL) and smaller paired fins (significantly smaller PCFL/BL), when compared with wild individuals across all investigation months. Our results provided insights into management of black rockfish enhancement in the future.

Effect of predation‐risk and foraging opportunities on substrate choice and strength of brightness preference inSchizothorax wangchiachiiandPercocypris pingijuveniles

Fish substrate preference may be largely influenced by genetics. However, does predator-experience training (hatchery-reared fish exposed to their natural predators while in captivity) or the presence of conspecifics, prey or predator affect this preference? We determined the effect of predation-risk and foraging opportunities on substrate choice of Schizothorax wangchiachii and Percocypris pingi when in the presence of a conspecific, predator or prey. We found that both S. wangchiachii and P. pingi juveniles preferred the large gravel substrate, regardless of prey/predator presence or previous training. When in the presence of a predator, wild and predator-trained hatchery-reared S. wangchiachii juveniles chose the white substrate over the black substrate as it was the furthest away from the predator (Silurus meridionalis), whereas the predator-naïve individuals preferred the black substrate. This suggests that predation stress training could improve the ability to recognize and avoid predators in hatchery-reared S. wangchiachii juveniles. Wild and hatchery-reared P. pingi juveniles continued to choose the black substrate even though it was distant from the potential prey (S. wangchiachii). The time spent by wild P. pingi juveniles over black substrate was significantly lower than hatchery-reared juveniles. It is possible that the wild P. pingi juveniles' ability to recognize prey resulted in their more frequent movement towards the prey. The findings in the present study may lead to better predator/prey-recognition training of hatchery-reared S. wangchiachii and P. pingi juveniles, potentially increasing their survival rate when released to the wild.

No evident effect of domestication on the anti-predator behaviour of European abalone (Haliotis tuberculata): Implications for stock enhancement programs

Stock-enhancement programs for the European abalone (Haliotis tuberculata) might be an efficient way to restore collapsed natural populations or preserve fishing activities. Specifically, this practice consists of releasing hatchery-reared juveniles from wild or farmed broodstock into natural environments. However, the process of selection and adaptation to farming conditions may result in domestication, thus leading to genetic and behavioural changes in farmed animals. Therefore, evaluating the effects of the domestication process on juveniles is a critical first step toward the development of successful abalone stock-enhancement programs. To this end, the behaviour and survival of sixteen-month-old H. tuberculata juveniles from wild and from farmed broodstock (the product of three generations of shell-length based selection) were compared. We used three assays at different spatial and temporal scales: (1) short-term responses to simulated contact with predators, (2) the activity pattern in the presence of a natural predator (the velvet crab Necora puber) in aquariums during a 2-week trial, and (3) survival in large mesocosms in the presence of predators during a 3-week trial. No significant effects of the broodstock origin of the juveniles were detected on any of the studied parameters (P > 0.05). In addition, the estimated size effects were small compared to the overall mean for most of the variables, suggesting that the domestication process was too recent to induce important behavioural changes in juveniles from farmed broodstock.Introducing crabs into the mesocosms significantly decreased abalone survival rates (P < 0.05). In the activity pattern trial, most of the abalone mortality in aquariums with crabs (55%) occurred within the first three days, which was largely attributed to initial exploration of areas outside of shelters by the abalone. In the second week, abalone spent much less time moving when crabs were present (P < 0.05).These results suggest that juveniles from both origins would likely have the same survival rates if released in the wild. The commonly reported high abalone mortality immediately after seeding is likely due to a lack of hiding behaviour or exploration of the new environment during the first few days of exposure to predators in the wild. Therefore, current release systems may need to be adjusted to the ethological characteristics of abalone juveniles to avoid high mortalities due to an initial lack of predator avoidance.

Does Vaterite Otolith Deformation Affect Post-Release Survival and Predation Susceptibility of Hatchery-Reared Juvenile Atlantic Salmon?

Sagittal otoliths are calcareous structures in the inner ear of fishes involved in hearing and balance. They are usually composed of aragonite; however, aragonite can be replaced by vaterite, a deformity which is more common in hatchery-reared than in wild fish. Vaterite growth may impair hearing and balance and affect important fitness-related behaviours such as predator avoidance. Captive rearing techniques that prevent hearing loss may have the potential to improve fish welfare and the success of restocking programmes. The aim of this study was to test the effect of structural tank enrichment on vaterite development in the otoliths of hatchery-reared juvenile Atlantic salmon Salmo salar, and to assess the effects of vaterite on immediate predation mortality and long-term survival after release into the wild. Fry were reared in a structurally enriched or in a conventional rearing environment and given otolith marks using alizarin during the egg stage to distinguish between the treatment groups. Otoliths were scrutinised for the presence and coverage of vaterite at 6, 13, and 16 weeks after start feeding, and the growth traits were measured for enriched and control fry when housed in tanks. In a subsequent field experiment, juveniles were released in the Rasdalen river (western Norway), and otoliths of enriched reared and control reared fry were scrutinised from samples collected immediately prior to release, from predator (trout Salmo trutta) stomachs 48 h after release and from recaptures from the river 2–3 months after release. Vaterite otoliths occurred as early as 6 weeks after start feeding in hatchery-reared S. salar. Vaterite occurrence and coverage increased with fish length. Enriched rearing had no direct effect on vaterite formation, but enriched reared fry grew slower than control fry. After release into the wild, fewer salmon fry with vaterite otoliths had been eaten by predators, and a higher proportion of fry with vaterite otoliths than those lacking vaterite were recaptured in the river 2–3 months after release. Contrary to expectations, this suggests that vaterite does not increase predation mortality nor reduce survival rates in the wild during the early life stages.

Spawning, larval culture, settlement and juvenile production of the west Indian Sea egg, Tripneustes ventricosus (Lamarck, 1816), under hatchery conditions

Tripneustes ventricosus is a commercially important species of sea urchin native of the tropical Atlantic Ocean. With the purpose of providing a scientific basis for the development of its production by aquaculture, several experiments were performed under hatchery conditions, including spawning induction, larval culture, settlement and juvenile culture. The efficiency of different spawning stimuli was assessed, including the use of UV-irradiated seawater; thermal and electric shocks, and injections of potassium chloride (KCl) and acetylcholine (ACh). Growth and survival of larvae maintained at different culture conditions were compared, including culture densities (2, 6 and 12 zygotes mL−1; 0.5, 1 and 2 larvae mL−1), culture systems (9 L aquaria, 75 L and 500 L tanks), water renewal rates (total and partial), feed amount (15 and 30 cells μL−1 d−1), feeding regime (constant or increasing) and type of diet (Isochrysis galbana, and a mixture of I. galbana and Chaetoceros calcitrans). Different stimuli to induce the larval settlement were tested, including low temperature, KCl, l-glutamine and γ-aminobutyric acid (GABA), as well as different substrates (artificial and benthonic microalgae biofilms). Finally, the growth and survival of juveniles were assessed under indoor conditions. The highest percentage of spawning individuals and the shorter response times were obtained using ACh. The average fecundity was 1.5 × 106 oocytes female−1. This variable exhibited a positive correlation with the female body size. One of the most relevant results of the present study was the completion of the embryo and larval developmental stages of T. ventricosus. Higher levels of larval either survival or growth were found in the tanks with the highest volume (500 L), maintaining conditions of low culture densities and complete water renewal and a constant supply of microalgae during all the larval development stage (15 cells of I. galbana and C. calcitrans μL−1 day−1). The larval settlement was obtained using low temperatures in all the tested substrates, except the substrate with artificial biofilm containing Sargassum and Thalassia. After 9 months of culture in laboratory conditions, juveniles of 44-mm length were obtained, using a regime of complete water renewal, and feeding the larvae with artificial biofilm, benthic microalgae and macroalgae. Although the present study made possible to identify appropriate culture conditions for obtaining hatchery-reared juveniles of white sea urchin, the low survival rates observed during the larval culture and the settlement stage must be improved with further research on the feeding requirements and the optimal water quality conditions for this species.

High-temperature and starvation tolerances of juvenile Japanese Spanish mackerel Scomberomorus niphonius

Japanese Spanish mackerel Scomberomorus niphonius is a target species for stocking, but its biological characteristics at the juvenile stage are not well understood. Here, we investigated the high-temperature and starvation tolerances of hatchery-reared juveniles in captivity. We used juveniles of approximately 40 mm standard length, the size of juveniles released into the field. The upper incipient lethal temperature (50% lethal water temperature) was estimated to be 31.8 °C, higher than the maximum sea surface temperature in the field (approximately 29 °C). The critical thermal maximum was 34.8 °C, whereas it was 36.1 °C when juveniles were preliminarily acclimated to 31 °C for 24 h. Juveniles died from 3 to 11 days under nonfeeding conditions at 20 °C, but from 2 to 7 days at 27 °C. This information is expected to contribute to the development of the production of juvenile Japanese Spanish mackerel.

Early Juvenile Growth and Behavior of Laevistrombus turturella(Gastropoda: Strombidae) in a Laboratory Microcosm Experiment

The dog conch, Laevistrombus turturella, is an important marine resource of ecological and economic importance in the Southeast Asian region. Natural populations of L. turturella are under threat due to development and overexploitation, so efforts should be taken in culturing this species for commercial and conservation purposes. This study aimed to fill the knowledge gap in the life-history characteristics of L. turturella by investigating the growth and behavior of early juvenile L. turturella under laboratory conditions. Egg masses of L. turturella were collected from conch natural-spawning habitat. Upon reaching the late veliger stage (stage IV), L. turturella larvae was chemically induced, and newly metamorphosed juveniles were reared in small-scale laboratory microcosms. The microcosms were placed in an incubator at 28 °C and under 12:12 h light-dark condition, with very minimal aeration. Specific behavioral characteristics were observed at the onset and during the metamorphosis, and these characteristics resulted in highly camouflaged juveniles covered by surrounding sediment and debris. Burrowing behavior was first observed at day 56 post settlement, but they were only partially buried. Generally, juvenile conch showed fast and continuous growth, which were best modelled by fitting linear equations. Thus, a minimum size of 10 mm-shell-length juveniles is recommended for restocking. All these results indicated the technical viability of producing hatchery-reared juveniles for commercial purposes and for enhancing natural-stock populations.

Immersion can trigger detachment of juvenile mussels

The nursery stages of mussel aquaculture are highly inefficient with most juvenile mussels being lost within the first few months after seeding, although the precise timing of losses is largely unknown. One possibility is that many juveniles are lost at seeding because they are either impaired or killed by the process of transporting juveniles from collection sites to coastal nursery farms, or because they detach and move away from aquaculture substrata at, or soon after deployment, via secondary settlement behaviour. Three laboratory experiments were used to assess detachment of juvenile mussels from aquaculture substrates following periods of emersion for two batches of wild juveniles undergoing actual transport and one batch of hatchery-reared juveniles during simulated transport periods. The percentage of juveniles retained on their substrates (i.e., not detaching) after a two-minute immersion in seawater and the percentage of dead juveniles were quantified, as was the size of the mussels that were retained on, or detached from, their substrates. Detachment occurred in all experiments (16.9–53.8%) and was greater when the average size of the mussels was small (0.75 ± 0.01 mm). Across all the experiments, larger juvenile mussels were retained in greater numbers with mussels less than 1.75 mm in shell length having a greater propensity to detach from their substrates upon immersion. Very few mussels were dead at the end of the experiments and large proportions of mussels reattached in the aquaria, when given the opportunity. Together, the results of this study indicate that detachment of juvenile mussels is size-specific and driven by secondary settlement behaviour occurring upon or soon after (< 2 mins) immersion. New nursery techniques are required to grow small juvenile mussels to a size beyond which their migratory secondary settlement behaviour is reduced, so that they are less likely to detach upon or soon after immersion while seeding out to farm sites.

Aquaculture in Brazil: past, present and future

Aquaculture in Brazil probably started in the 17th century, during the Dutch occupation of the northeastern region. Currently, this activity can be divided into five main sectors, defined by tradition and type of cultured organism: freshwater fish, marine shrimp, mollusks, freshwater prawns and frogs. Production in 2019 was estimated at 800,000 tonnes, representing a gross revenue of US$ 1 billion. Freshwater fish is predominantly produced, followed by marine shrimp. The main farmed species are Nile tilapia (Oreochromis niloticus), tambaqui (Colossoma macropomum) and the Pacific white leg shrimp (Litopenaeus vannamei). Other species have great local socio-economic importance. The bulk of production comes from small farms: more than 80 % have less than 2 ha. Brazil has more than 200 thousand freshwater fish farms, about 3000 marine shrimp farms, and about 100 aquaculture research institutions. A large domestic market is available for edible fish and shellfish, ornamentals, baitfish and hatchery-reared juveniles for biomitigation purposes. The challenge is to develop truly sustainable production systems to support a perennial industry. New technologies, including digital devices and simple disruptive innovations, can increase productivity and support the shift to a circular economy, bioeconomics and sustainability supported by science-based innovations and knowledge.

Importance of experienced thermal history: Effect of acclimation temperatures on the high-temperature tolerance and growth performance of juvenile marbled flounder.

Experienced thermal history often affects the temperature tolerance of fish; however, the effect of thermal history on growth performance is unclear. To contribute to effective stocking (release of hatchery-reared juveniles in the field), we conducted four laboratory experiments using juvenile marbled flounder (Pseudopleuronectes yokohamae, around 30mm standard length and 0.3g body wet weight) acclimated at 12°C and 24°C for approximately 2 weeks to investigate the effects of acclimation temperature on high-temperature tolerance, food consumption, and growth performance. The acclimation to 24°C increased tolerance to high temperatures, as shown in a 24-h exposure experiment and in a temperature elevation experiment. The 50% lethal temperature (upper incipient lethal temperature) was estimated to be 25.9°C and 29.0°C for the 12°C and 24°C acclimation groups, respectively. In subsequent experiments, we tested the effects of high and low temperature acclimation on the food consumption and growth performance of two size groups of juveniles (28.7±2.0 and 34.5±2.9mm, mean±SD), that were reared at temperatures ranging from 14°C to 23°C. The optimal temperature for growth was 20°C and did not differ between the acclimation temperatures or between the size groups. However, food consumption and growth performance were suppressed by acute temperature changes. Specifically, feeding and growth were lower in the 24°C-acclimated group than in the 12°C-acclimated group when exposed to 14°C, which is close to the natural water temperature at release in the field. These results suggest that experienced thermal history does not affect the optimal temperature but can affect the growth performance of juveniles. To maximize the post-release growth of hatchery-reared juveniles, the influence of thermal history should be taken into consideration and acute thermal changes before release should be avoided.

Stock enhancement of Culter mongolicus: Assessment of growth, recapture and release size in the Yangtze lakes

The release of piscivorous fishes is considered crucial to alleviate eutrophication and fish miniaturization, as well as to develop sustainable fisheries management strategy for Yangtze lakes. In order to promote the stock enhancement programme of a new released piscivorous fish species Culter mongolicus, growth, recapture rates, reproduction and impact of size-at-release for hatchery-reared juveniles were evaluated in three Yangtze shallow lakes during a five-year period. C. mongolicus displayed important growth capability with a body weight reaching commercial size (> 500 g) during the second released year and about 1500 g in the third year. However, a sharp decline of growth appeared quickly after several years of releasing, indicating possible issue of density-dependent growth caused by over-stocking. The recapture rates for one-month-old juveniles were less than 5%, but increased to more than 20 % when one-year-old fingerlings were selected as release size. Gonad examination and catch analysis showed that released juveniles could not succeed in building the recruitment stock due to over-fishing. Thus hatchery-reared juveniles should be released annually. Management actions that prioritize the release of fewer but larger fingerlings will improve the recapture rate and subsequent economic benefit. A new stock enhancement strategy that changes the present release-catch cycle from two years to one was proposed accordingly.

‘The Bold are the Sociable’: Personality traits and laterality in an indigenous megafish, the Deccan Mahseer (Tor khudree)

The present study explored the relationships between the personality traits boldness, activity, exploration, and sociability, and lateralized utilisation of brain hemispheres in the hatchery-reared juveniles of Deccan Mahseer (Tor khudree), an endangered megafish inhabiting the rivers of central and southern India. Our results revealed a significant positive correlation between boldness, activity and exploratory traits in this species when tested in isolation. Boldness was also positively correlated with the time spent near a conspecific individual, but not with an alien invasive heterospecific tilapia (Oreochromis mossambicus). Although juvenile Deccan Mahseer exhibited significantly greater preference towards conspecific over heterospecific, no divergence in the utilisation of right or left eye was seen while observing these individuals, suggesting the lack of lateralised utilisation of brain hemispheres. Furthermore, laterality in visual perception did not show any significant correlation with any of the personality traits tested in this species. Positive correlations between boldness and sociability in hatchery-reared mahseer populations could result in divergent patterns of dispersion, while absence of laterality may reflect cognitive deficits induced by homogeneous rearing conditions. A greater understanding of such behavioural patterns could help improve rearing practices and inform release protocols for the purposes of conservation of this endangered species.

The effect of dietary protein, lipid, and carbohydrate levels on the performance, metabolic rate and nitrogen retention in juvenile European lobster (Homarus gammarus, L.)

Releasing hatchery-reared juveniles in the wild can mitigate the general decline in the natural stocks of European lobster, Homarus gammarus, L.. However, growth and survival rates in lobster culture are low, presumably due to suboptimal nutrition and feeding. With the aim of determining appropriate nutrient levels, we tested different formulated extruded feeds for the culture of juvenile European lobster. Baseline metabolism (standard metabolic rate, SMR), in combination with the metabolic cost of feeding (specific dynamic action, SDA), and nitrogen retention during digestion and assimilation was investigated for six experimental diets. Diets were formulated to contain two different levels of protein (400 and 500 g kg−1), with three lipid to carbohydrate (L:CHO) ratios (low, medium, and high). These experimental diets were tested over a 32-day period, against a conventional control diet (Antarctic krill, Euphausia superba). During this period, the growth performance of the juveniles was assessed as molting frequency, increments in carapace length and whole body wet weight. At the end of the growth performance trial, oxygen consumption (MO2) and nitrogen excretion rates of individual lobsters were determined prior to and following the ingestion of a single meal. Molting occurred more frequently in juveniles fed with krill and krill resulted in a significantly higher specific growth rate than experimental dry feeds except for the 500-low diet. However, lobsters fed any of the three 500 and the 400-low diets had carapace length increments, SMR, SDA, and nitrogen retention similar to those fed the krill diet. Results suggest that protein is an important macronutrient for juveniles of this species and must be included above 40%. Also, lobsters have a dietary requirement for carbohydrates ranging from 24% to 35% probably related to the need for glycogen in chitin synthesis. The lower the protein content, the higher the requirement in carbohydrates.

Targeted mutagenesis of the ryanodine receptor by Platinum TALENs causes slow swimming behaviour in Pacific bluefin tuna (Thunnus orientalis)

In bluefin tuna aquaculture, high mortalities of hatchery-reared juveniles occur in sea cages owing to wall collisions that are caused by high-speed swimming in panic due to changes in illuminance. Here, we report that targeted gene mutagenesis of the ryanodine receptor (RyR1b), which allows the sarcoplasmic reticulum to release Ca2+ in fast skeletal muscle, using highly active Platinum TALENs caused slow swimming behaviour in response to external stimuli in Pacific bluefin tuna (PBT) larvae. This characteristic would be a useful trait to prevent wall collisions in aquaculture production. A pair of Platinum TALENs targeting exons 2 and 43 of the PBT ryr1b gene induced deletions in each TALEN target site of the injected embryos with extremely high efficiency. In addition, ryr1b expression was significantly decreased in the mutated G0 larvae at 7 days after hatching (DAH). A touch-evoked escape behaviour assay revealed that the ryr1b-mutated PBT larvae swam away much less efficiently in response to mechanosensory stimulation at 7 DAH than did the wild-type larvae. Our results demonstrate that genome editing technologies are effective tools for determining the functional characterization of genes in a comparatively short period, and create avenues for facilitating genetic studies and breeding of bluefin tuna species.

Identification and adaptability of hatchery-origin Japanese flounder released into the Bohai Sea after the summer fishing moratorium

Because the artificial cultivation environment differs from the natural habitat of the Bohai Sea, it is essential to study the adaptability of hatchery-reared juveniles released into this area. Hatchery-origin Paralichthys olivaceus stocks were identified via morphological observation and genetic tags (i.e., mitochondrial DNA and microsatellites). Seven genomic microsatellite loci with a high PIC (polymorphic information content, 0.885–0.945) and a high exclusion probability (0.930–0.981) were selected and arranged into two multiplex PCRs. In total, 46.75% of the captures were assigned to our broodstocks. There were no significant differences in genetic diversity, such as Ar (allelic richness, P = .754) and He (expected heterozygosity, P = 0. 658), among the hatchery-origin stock (HS), their parents (P), the wild stock (WS) and the captured broodstocks (CB) in 2016. However, losses of Ar in the HS indicated that more attention should be given to the number of broodstocks in a reared population. The overall genetic divergence among these four groups was low (FST = 0.005), and the results of AMOVA also showed no genetic differences among them (P = .500), indicating that no pronounced genetic differentiation was found following the stock enhancement program in the Bohai Sea. There was no significant difference between HS and WS in terms of growth parameters (P > .05) and feeding ecology (P > .05). Furthermore, the stomach contents of the two groups were basically identical and consisted mainly of fish. Therefore, overall, the hatchery-origin juveniles had already adapted to the natural area after the summer fishing moratorium.

Experimental evidence on prevention of infection by the ectoparasitic protozoans Ichthyobodo salmonis and Trichodina truttae in juvenile chum salmon using ultraviolet disinfection of rearing water.

In northern Japan, juvenile chum salmon Oncorhynchus keta (Walbaum) are released from hatcheries to enhance the fishery resource. Infections with ectoparasitic protozoans, particularly the flagellate Ichthyobodo salmonis and the ciliate Trichodina truttae, occasionally cause severe mortality among hatchery-reared juveniles. This study examined the susceptibility of the two parasites to wide-ranging UV irradiation (experiment 1) and then investigated whether UV disinfection of the rearing water using a commercial device was useful for preventing infections among juveniles in a small-scale rearing system over a 28-day period (experiment 2). In experiment 1, parasite mortality reached 100% with UV irradiation doses of ≥9.60×105 μW s/cm2 for I.salmonis and ≥8.40×105 μW s/cm2 for T.truttae. In experiment 2, disinfection of the rearing water at a UV irradiation dose of 2.2×106 μW s/cm2 succeeded in complete prevention of both parasites in the juvenile salmon. These results elucidate the minimum dose of UV irradiation for inactivation of I.salmonis and T.truttae, and demonstrate the usefulness of water disinfection using a commercial UV irradiation device to prevent infections by these parasites in hatchery-reared juvenile chum salmon.