Decreased genetic diversity and a reduced effective population size in cultured silver-lipped pearl oysters (Pinctada maxima)

Before selection programmes for aquaculture species can commence, a fundamental knowledge of how the culture process affects genetic diversity is requisite. “South Sea” pearl production, through the culture of the silver-lipped pearl oyster, Pinctada maxima, stands to gain substantially from genetic selection. Aquaculture techniques for P. maxima have progressed vastly, to a point where the majority of current operations are based on hatchery-propagated stock; however, there has been no investigation on how the culture process is affecting the genetic diversity of this species. To address this deficiency, we investigated the genetic diversity and effective population sizes (N_e) in three wild and five hatchery-produced P. maxima populations. Using six microsatellite DNA markers, cultured populations exhibited low N_e (3.5–9.2) and reduced allelic richness (29–44%) compared with their wild progenitors. Observed heterozygosities were not affected greatly, however, an increased mean pairwise genetic relatedness was observed in all cultured populations. The practice of mass spawning was identified as a major factor in the reduction of diversity, although diversity was not necessarily maintained when a more controlled spawning approach was utilised. Pedigree reconstructions revealed genetic diversity and N_e is affected by large skews in full-sib family representations (up to 40% from a single family, in a cohort using 28 broodstock), and could be due to differential survival rates amongst communally reared families. In order to maximise future selective breeding efforts, further research should be directed towards identifying critical stages of the culture process in which genetic diversity is lost.     

Family by environment interactions in shell size of 43-day old silver-lip pearl oyster (Pinctada maxima), five families reared under different nursery conditions

To understand the influence the environment and associated genotype by environment interactions will exert on future silver-lip pearl oyster (Pinctada maxima) selective breeding programs, this study assessed the relative performance in four shell growth traits of spat from five full-sib families, when spat were communally reared at different salinities (29, 34 and 40 ppt), food availability (high, medium and low), food quality (high, medium and low), and in a hatchery vs. ocean environment for 43 days.Rearing environment was found to influence growth expression, with significant differences evident when oysters were grown at different salinities in the ocean instead of hatchery, or when fed algae of differing nutritional quality. As indicated by MANOVA, family comparative growth performances were also altered when the environment changed, with significant environment by family interactions apparent in the food quality, food availability and hatchery vs. ocean rearing treatments. Changes in salinity, however, did not affect relevant family performances.These results indicate that growth and relative family performance in P. maxima may change dependent on local environmental conditions and that genotype by environment effects may need to be considered in breeding programs for this species.     

How to determine the appropriate mortality in experimental larval rearing?

Survival in larval rearing experiments is difficult to estimate due to accidental losses and periodic sampling. The number of sampled fish can be a large proportion of the stocked ones, making it difficult to calculate the overall survival rate and mortality coefficient as this is based on the initial number. Here, a new method of calculating survival is proposed using the mortality coefficient. When the initial stocking density and sampled and final numbers are known, and assuming that mortality coefficient is constant, the final number of fishes can be represented by the formula N _t = e^−mt (N ₀ − ΣN _Sne ^mdn), where t is rearing period (days), N ₀ indicates initial number, N _t indicates the survival number at t days of rearing, m is the natural mortality coefficient, N _Sn is the sampled number in the nth sampling, and dn is the rearing period until removal of the nth sample. The provisional mortality coefficient is calculated from initial and final stocking numbers. Then values for the natural mortality coefficient are substituted into the formula with successive approximation. The coefficient, which most closely approximates the actual survival, is determined as the best fit natural mortality coefficient. Examples of larval experiments are provided to demonstrate the method and show that survival is often underestimated using traditional methods.     

Growth and survival of hatchery-bred giant clams (<Emphasis Type="Italic">Tridacna gigas</Emphasis>) in an ocean nursery in Sagay Marine Reserve,Philippines

To restore the diminishing population of the giant clam Tridacna gigas in Sagay Marine Reserve (SMR), Negros Occidental, central Philippines, two size classes [8- and 10-cm shell length (SL)] of hatchery-bred T. gigas were reared in an adjacent ocean nursery for restocking to Carbin Reef later upon reaching grow-out size of ≥20 cm SL. Growth rates did not significantly differ for both sizes and were on average 0.67 cm month⁻¹. However, survival after 382 days of rearing T. gigas was significantly higher in the 10-cm SL clams than the 8-cm SL clams (96 and 83%, respectively). For future restocking projects, the use of 8-cm SL clams is recommended because the lower survival of this size class is compensated by its cheaper price. While rearing the clams to attain grow-out size, the population of wild clams (Family Tridacnidae) in Carbin Reef was assessed using ten 50 × 2-m belt transects. Four species of tridacnid clams have been recorded: Hippopus hippopus, Tridacna crocea, T. maxima, and T. squamosa. T. crocea comprised 12.5–93.9% of all the clams observed in all ten transects. There was a significant difference in clam density between species (ANOVA, F = 6.94, P < 0.001), with T. crocea having the highest density. Living T. gigas were absent, but presence of dead shells was indicative of its presence in the reef in the past. It can be expected that the release of hatchery-bred T. gigas juveniles in Carbin Reef could provide future breeders that will repopulate this reef and the adjacent reef communities.     

Estimating effective population size of large marine populations,is it feasible?

Sustainable exploitation of marine populations is a challenging task relying on information about their current and past abundance. Fisheries‐related data can be scarce and unreliable making them unsuitable for quantitative modelling. One fishery independent method that has attracted attention in this context consists in estimating the effective population size (N_e), a concept founded in population genetics. We reviewed recent empirical studies on N_e and carried out a simulation study to evaluate the feasibility of estimating N_e in large fish populations with the currently available methods. The detailed review of 26 studies found that published empirical N_e values were very similar despite differences in species and total population sizes (N). Genetic simulations for an age‐structured fish population were carried out for a range of population and samples sizes, and N_e was estimated using the Linkage Disequilibrium method. The results showed that already for medium‐sized populations (1 million individuals) and common sample sizes (50 individuals), negative estimates were likely to occur which for real applications is commonly interpreted as indicating very large (infinite) N_e. Moreover, on average, N_e estimates were negatively biased. The simulations further indicated that around 1% of the total number of individuals might have to be sampled to ensure sufficiently precise estimates of N_e. For large marine populations, this implies rather large samples (several thousands to millions of individuals). If however such large samples were to be collected, many more population parameters than only N_e could be estimated.     

Effect of parental stock size on F1 genetic structure in the bay scallop, Argopecten irradians (Lamarck, 1819)

Three F₁ families of the bay scallop, Argopecten irradians, were produced from one, two and 10 individuals. The genetic changes in these populations, which suffered recent and different levels of bottleneck, were analysed using amplified fragment length polymorphism (AFLP) techniques. In the parental stock, a total of 330 bands were detected using seven AFLP primer pairs, and 70% of the loci were polymorphic. All F₁ groups had a significantly lower proportion of polymorphic loci when compared with the initial stock, and loss of the rare loci and reduction in heterozygosity both occurred. The progeny of the larger population (i.e., N=10) exhibited a lesser amount of genetic differentiation compared with the progeny from N=2, which showed lesser differentiation than progeny from N=1. The effective population sizes (N_e) in N=1, 2 and 10 were estimated as 1.50, 1.61 and 2.49. Based on regression analysis, we recommend that at least 340 individuals be used in hatchery populations to maintain genetic variation.     

Suitability of European green frogs for intensive culture: Comparison between different phenotypes of the esculenta hybridogenetic complex

European countries are probably the greatest consumers of frogs' legs, although this is occurring in a context of amphibian decline. Imports of frogs' legs have been increasing over the last few decades in relation to the development of deep-frozen products. This demand has stimulated farming with neotropical species for international trade. Today, in spite of some trials on European species, no production is effective. From the early 1990s, small-scale experimental rearing trials have been undertaken in Brittany (France) with a local green frog belonging to the esculenta complex (Rana ridibunda) which accepts granulated feed. This complex comprises two Mendelian species R. lessonae (LL) and R. ridibunda (RR) as well as their hybridogenetic hybrid Rana esculenta, which can be either diploid (RL) or triploid (RLL, RRL).The purpose of this study is to analyse the ability of each taxon to be reared. During the two months following metamorphosis, froglets were trained to eat pellets. The surviving individuals were reared for one year under regulated conditions to estimate the survival rate, growth and production of each frog type.The results show low survival rates (14.8 to 26.2%) for LL and RLL, higher rates (39–46.4%) for F₁ and F₂ from wild RR and RLL, with the highest rates (63.0 to 77.4%) for RL, RRL and all RR, using data from several years of rearing. After three years under rearing conditions, the adult survival rate is highest for rearing strain RR-Rivan 92* (53.7%). The growth rate varies greatly according to phenotype, and only frogs with an RR phenotype reach marketable size. In the wild, LL exhibits some difficulty in reaching this limit. As a result, production increases from 1.5–8.7 kg/m² for individuals with an L hemiclone to 22.9–35.7 kg/m² for each cohort of phenotype RR from at least two generations under rearing conditions.In relation to hybridogenesis processes, hybrids with diploid or triploid genomes do not seem to offer any advantage for production purposes. However, R. lessonae individuals from intensive rearing can produce fertile eggs and tadpoles, which could enable production for restocking to preserve wild diversity or build up new populations in suitable habitats.     

Studies of the Thermal and Haline Influences on Growth and Survival of Litopenaeus vannamei and Litopenaeus setiferus

A series of experiments were conducted to examine the effects of salinity (1–48 g/L) on the biological performance, as evaluated by growth and survival, of the Pacific white shrimp, Litopenaeus vannamei, and the Atlantic white shrimp, Litopenaeus setiferus, reared at temperatures of 20, 24 or 28 C. Poor growth and survival of L. vannamei was observed after 21–28 d of culture at low salinity (2 and 4 g/L) at 20 C. Raising salinity to 8 and up to 32 g/L significantly increased survival at this temperature, indicating that avoiding low temperatures is critical for survival of this species when reared at low salinity. A major improvement in the growth rate of L. vannamei was observed at 24 C, but it still was sub‐optimal compared to growth observed at 28 C. Irrespective of salinity, high survival rates were observed at both 24 and 28 C, but variable growth rates were recorded. Contrary to L. vannamei, the Atlantic white shrimp, L. setiferus, which was reared for 28 d at 24 C only, had better growth performance at 8 g/L compared to 2, 16 and 32 g/L. Under equal experimental conditions, L. setiferus had considerably lower weight gain and survival than L. vannamei.     

Effects of size and nitrite exposure on respiration,oxygen partitioning,and growth of obligate air-breathing fish Channa striata

The effects of fish size and nitrite level on metabolic rate and growth were investigated in the obligate air-breathing snakehead Channa striata, which is an important aquaculture species in Vietnam. Channa striata displayed respiratory size dependence, whereby the standard metabolic rate (SMR) and routine metabolic rate (RMR) decreased progressively in an exponential manner as fish size increased from 50 to 200 g. A mildly elevated nitrite level of 5% of the LC₅₀ 96 h (12 mg NO₂^?/L or safe concentration) induced significant increases in Channa striata SMR and RMR, which were almost double that of the control at the same size. At mild elevation, nitrite caused no significant effect on fish growth and survival during 3 months of rearing. However, both growth and survival rates of fish reared at severely elevated nitrite levels were significantly lower than those of the control; in particular, survival rates were under 50%. While changes in size reduced SMR and RMR, the percentage of air oxygen partitioning remained unchanged. Channa striata upregulation of SMR and RMR and air-breathing regulation were not significantly proven in this study. In summary, maintaining water environments at levels lower than 12 mg NO₂^?/L with ample oxygenation will not affect the growth and survival rate of snakeheads.

     

Is it possible to successfully rear meagre (Argyrosomus regius Asso 1801) larvae without using rotifers?

In hatcheries, meagre Argyrosomus regius larvae still depend on an adequate supply of rotifers and Artemia, as no artificial diet can totally fulfil their nutritional requirements. However, production of live feed is highly expensive and demands intensive labour and specific facilities. This study investigated the effect of a dietary regime without the use of rotifers, to simplify the meagre larval rearing protocol. Two feeding treatments (T₁ & T₂) are compared to investigate their effects on survival and growth of meagre larvae. In T₁, larvae were fed rotifers from 2 to 5 days post hatch (dph), and Artemia from 4 to 15 dph. In T_2, larvae were kept under dark conditions and fed Artemia from 6 to 15 dph. Standard larval length (SL) was significantly higher in T₁ (p < .01) until 8 dph in comparison with larvae reared initially without rotifers. No significant difference in SL was found among treatments (p = .187) at 15 dph. Significant difference was found among treatments in survival rate at 15 dph (p < .003). The survival rate observed at 15 dph in T₂ (30 ± 4.2%) represents an important finding, although the highest survival rate was observed in T₁ (45.0 ± 3.4%). This study showed that it is possible to conduct larval rearing of meagre without using rotifers. Nevertheless, further research efforts are still needed to improve these results in comparison with the common larval rearing protocol.     

Assessment of temperature or salinity effects on larval development by catecholamine‐induced metamorphosis of hatchery‐reared flat oyster,Ostrea angasi (Sowerby 1871) larvae

A need to improve larval rearing techniques led to the development of protocols for catecholamine‐induced settlement of flat oyster, Ostrea angasi, larvae. To further refine these techniques and optimize settlement percentages, the influence of salinity or temperature on development of O. angasi larvae was assessed using epinephrine‐induced metamorphosis. Larvae were reared between salinities of 15–35 and temperatures between 14.5 and 31°C. The greatest percentage survival, growth, development occurred when larvae were reared between 26 and 29°C and between salinities of 30 and 35. Larvae reared outside this salinity and temperature range exhibited reduced growth, survival and/or delayed development. Short‐term (1 h) reduction in larval rearing temperature from 26°C to 23.5°C significantly increased larval metamorphosis without affecting larval survival. Short‐term (1 h) increase in larval rearing temperature from 26°C to 29 and 31°C decreased larval survival and metamorphosis. To ensure repeatability in outcomes, tests showed that larvae sourced from different estuaries did not vary significantly in their metamorphic response to short‐term temperature manipulation and epinephrine‐induced metamorphosis.     

Family × Environment interactions in the growth and survival of Arctic charr (Salvelinus alpinus) grown in brackish and fresh water

We investigated whether Fraser strain Arctic charr Salvelinus alpinus L. reared in fresh water (FW) differed in survival and growth from full siblings transferred to brackish water (BW) and whether family performance was consistent throughout 1 year of commercial growth. Fish were grown communally (n = 3600) in either FW or BW from May 2008 to May 2009. Mean family survival in BW was moderately correlated with mean family survival in FW (r = 0.342; P = 0.032). Fish which survived until the conclusion of the experiment were significantly heavier in body weight at the baseline assessment than their full‐sibs which died. Family by treatment interactions were detected for body weight, condition factor and specific growth across all measurement dates (May 2008, October 2008 and May 2009) and growth intervals. Despite the varied response of families to treatment, mean family body weight in FW was correlated phenotypically with BW body weight in October 2008 (r = 0.633, P < 0.0001) and marginally correlated in May 2009 (r = 0.289, P = 0.061). These data suggest that body weight in FW and body weight in BW should be analysed as separate, but correlated traits in Arctic charr breeding programs.     

The effects of holding space on juvenile red king crab,Paralithodes camtschaticus (Tilesius, 1815), growth and survival

Rearing crustaceans communally for aquaculture, stock enhancement or research often results in high rates of cannibalism and low yields. One potential strategy to reduce loss from cannibalism is to rear crustaceans in individual cells. As small holding cell size can result in decreased growth or increased mortality, it is essential to identify the optimal holding cell size, both for mass culturing efforts and for experimental design purposes. In this study, we reared juvenile red king crab, Paralithodes camtschaticus (3.67–8.30 mm carapace length) in 20, 40 and 77 mm diameter holding cells, and monitored growth and survival over a 274‐day experiment. A trend of lower growth per molt in the smallest holding cells resulted in crab 17% smaller than those in the large holding cells at the end of the experiment. In addition, mortality rates were an order of magnitude higher in the small holding cells compared with the large or medium cells. For individual rearing of this size of juvenile red king crab, the medium‐sized cells (40 mm diameter) are the optimal size as there was no increase in mortality and only marginally lower growth rates compared to the large‐sized cells.     

Effect of tank proportions on survival of seven-band grouper Epinephelus septemfasciatus (Thunberg) and devil stinger Inimicus japonicus (Cuvier) larvae

We examined the effects of rearing‐tank proportions on early survival, surface death and growth of the seven‐band grouper Epinephelus septemfasciatus (Thunberg) and the devil stinger Inimicus japonicus (Cuvier). Fertilized eggs were introduced into three differently shaped 100 L rearing tanks. The three tanks had different water surface areas, and included a shallow tank (S; 71 × 26 cm in diameter and depth, respectively), an intermediate tank (I; 57 × 39 cm) and a deep tank (D; 44 × 70 cm). Both species showed their highest survival rate and the lowest numbers of surface death in the D tank (P<0.05). There were no significant differences between fish reared in the three tank shapes in notochord length, total length, growth rate and dry weight. Rearing‐tank shape affected larval movement in the water column, with the duration of larval movement under the water surface being the shortest in the D tank. These results suggest that using a rearing tank of a suitable shape could significantly reduce the surface death of marine fish larvae.     

Growout of hatchery-reared juvenile spotted babylon (<Emphasis Type="Italic">Babylonia areolate</Emphasis> link 1807) to marketable size at four stocking densities in flow-through and recirculating seawater systems

Hatchery reared juvenile spotted babylon, Babylonia areolata, were raised in canvas rearing tanks at four stocking densities (100, 200, 300 and 400 ind. m^–2) in flow-through and recirculating seawater systems until they reached the marketable sizes of 100–140 snails/kg. At all stocking densities, final increments in mean length and weight of snails held in flow-through systems were higher than those in recirculating systems, there was a significant effect (p < 0.05) of culture system on final length and weight. Mean (±SE) survival of snails held at 100 ind. m^–2 in the flow-through system was 100.0 ± 0.1% but was not significantly higher than survival in any other treatment (p > 0.05). Mean survival of snails held in the recirculating system was not significantly lower than at any stocking density in the flow-through system (p > 0.05).     

Growth, Survival and Reproductive Performance of Domesticated Australian Stocks of the Giant Tiger Prawn, Penaeus monodon, Reared in Tanks and Raceways

The growth, survival and reproductive performance of domesticated Australian stocks of the giant tiger shrimp Penaeus monodon were evaluated in trials conducted in 1997 and 2003. The 1997 trials assessed the performance of first generation progeny of wild broodstock from the northeast coast of Australia and fourth generation progeny of pond‐reared broodstock, which also originated from northeast coast wild stocks. In these trials, growth and survival of the shrimp were assessed when reared for 17 mo in tanks. Reproductive performance of the shrimp was assessed at 14.5 mo and 17 mo. The 2003 trials assessed the performance of first generation progeny of wild broodstock from the Gulf of Carpentaria (north coast of Australia). In these trials, growth and survival of shrimp were assessed when reared for 14 mo in tanks and raceways. Reproductive performance of the shrimp was assessed at 11 mo, 12 mo, and 15 mo. Growth and reproductive performance of the stocks varied between trials, families, ages and rearing systems. The most pronounced differences in growth and reproductive performance were between the 1997 and 2003 trials. At 11 mo of age, the average wet weight of the shrimp in the 2003 trials (females 117.1 ± 5.8 g; males 87.9 ± 7.6 g) was 200% greater than the average wet weight of shrimp in the 1997 trials (females 55.2 ± 6.8 g; males 41.2 ± 3.4 g). The reproductive performance of the shrimp was also higher in the 2003 trials in terms of the percentage of spawnings hatching (52.0% in 1997; 77.1% in 2003) and mean hatch rate (21.5% in 1997; 31.6% in 2003). Differences in the growth and reproductive performance of the tank‐reared stocks between years were indicative of significant improvements in the rearing environment, diet and husbandry techniques. Variation in the reproductive performance between families was consistent across rearing environments and at different ages and suggests the potential to improve reproductive performance through genetic selection. Notably, this study identified hatch rates of nauplii from the spawned eggs as a key area for future improvement of domesticated stocks reared in tanks and raceways. Future efforts to improve the growth and reproductive performance of domesticated P. monodon could benefit from integrating incremental improvements to husbandry with genetic selection.     

Scale matters: performance of European sea bass,Dicentrarchus labrax,L. (1758), reared in cages of different volumes

The effects of rearing volume on on‐growing European sea bass performance and stress parameters were investigated for the first time in a pilot aquaculture farm. Fish were held under the same initial stocking densities in triplicate net‐pen cages of different sizes (1.4, 45 and 252 m³) for a period of 8 months. Results showed significant differences among the experimental groups in most parameters tested, with better performance in the two larger rearing volumes. In particular, growth rate showed a linear association with rearing volume, being 0.68 g day^?1 for the large cage group, 0.56 g day^?1 and 0.32 g day^?1 for the medium and the small groups respectively. The feed conversion ratio and per cent survival (%) were also better in the large cage group. Fish reared in the small and medium‐sized cages showed higher plasma cortisol concentrations than those reared in large cages, which showed low basal cortisol concentrations. Additionally, after an acute chasing stress challenge, fish in the large and small groups, but not the medium group, showed increased cortisol concentrations. Differences also occurred in the ratio of the expression of cortisol receptors, namely the mineralocorticoid (mr) and glucocorticoid receptors (gr). In specific, the ratio of mr to grmRNA expression in the liver was higher in fish reared in the small cages. These findings verify that experimental scale significantly affects experimental results and is a critical factor for the interpretation of results.     

Increasing the stocking density in Paracentrotus lividus larviculture: Effects on survival and metamorphosis rates

The development of sustainable methods for sea urchin juvenile production is currently constrained by high mortality rates during larval growth and the high costs of larval rearing systems management. With the aim of developing a method for the production of juveniles of the purple sea urchin Paracentrotus lividus in a medium‐scale recirculating system, the present study focused on the effects of high stocking densities on larval growth. Plutei larvae were reared at three different densities (up to 7 ind/ml) in a semi‐static culture system. Larval survival and metamorphosis success were evaluated in order to identify the most effective density range. The highest metamorphosis rates (80%–95%) were obtained at 4 and 7 larvae/ml. These results are comparable with (and in some cases higher than) those reported for the same species at much lower larval densities. In conclusion, the rearing conditions tested here show for the first time that a significantly higher (4 ind/ml) stocking density than those of traditional P. lividus rearing methods (based on large volumes and low densities) can be adopted, thus supporting the feasibility of an increase in the final output of competent larvae with no increase in rearing volumes.     

Improving culture techniques for village-based farming of giant clams (Tridacnidae)

Eight experiments aimed at improving methods for the village-based farming of giant clams were conducted in the Solomon Islands. The experiments focused on either improving the fitness of seed clams delivered to village farmers, assessing whether differential growth rates of seed clams in nursery tanks persisted during grow-out at farms, or testing the effects of alterations to the design of grow-out cages on the growth and survival of clams. We found that Tridacna squamosa (Lamarck) ‘seed’ transferred from land-based nursery tanks to a floating ocean nursery (FON) for ≈ 3 months at the end of the nursery phase were significantly larger than seed reared only in land-based nursery tanks. Similarly, T. maxima (Röding) placed in a FON for 2–5 months generally grew at a significantly greater rate than tank-reared ‘seed’. However, the use of FONs did not improve survival. There were no consistent differences in the growth and survival of fast- and slow-growing seed of T. derasa (Röding) at village sites when slow-growing seed were retained in the nursery until reaching a larger size. The survival of T. maxima was enhanced significantly by placing an insert of smaller mesh (a ‘settlement ring’) in grow-out cages for the first 2 months after delivery of seed to farmers. The settlement ring retained clams in cages until they found a suitable place to attach their byssal threads. Attempts to remove the sediment which impedes the attachment of T. maxima to the base of grow-out cages by perforating the substrate did not improve survival: the perforated substrate resulted in poor attachment of clams and harboured predators (Cymatium spp.). The survival of T. crocea (Lamarck) was not improved by ‘softening’ the concrete base of grow-out cages to simulate dead coral rock and to encourage the clams to burrow in the substrate. The survival of T. crocea in grow-out cages was enhanced significantly by enclosing the cages in fine mesh after the delivery of the seed clams to prevent predation and disturbance by juvenile wrasse, Thalassoma spp. The experiments indicate that the critical stage for village farming of giant clams is during the initial weeks following distribution of seed. Further research is needed to improve the survival of T. crocea and T. maxima during this phase.