Effect of cold shock and hormone on growth and male morphotypes of freshwater prawn,Macrobrachium rosenbergii (de Man)

A study was carried out to determine the effects of cold shock and 17α‐methyl testosterone hormone on growth in male morphotypes of giant freshwater prawn Macrobrachium rosenbergii. Juvenile males were exposed at 18 ± 0.25°C for 24 h with normal feed and cultured at 27 ± 0.7°C for 70 days as well as administrated feed with hormone separately at 400 mg kg^?1 feed for the first 30 days of culture period (70 days). The growth (Mean ± SD) of male juveniles at harvest was closely homogenous in the cold shock (CLS) treated group; while it was heterogeneous in hormone‐treated (MH) and control (CO) groups. Although total body weight of blue claw (BC) males was not significantly different in the controls and the treatments, the 2nd pereiopods (claw) weight and length of the BC was significantly lower in CLS (0.52 ± 0.05 g, 10.02 ± 0.65 cm) than that in the CO (0.71 ± 0.16 g, 12.53 ± 0.57 cm) and MH (0.75 ± 0.08 g, 12.85 ± 1.18 cm) treatments. Survival of prawns was similar in treatments. The morphotypes of male prawn were significantly different with 59% of BC males with small‐sized claws in CLS and, 45% and 30% of BC males with large‐sized claws in MH and CO respectively. Cold shock adversely affected male morphotypes of the prawn so that numbers of BC males with small‐sized claw was the highest in CLS tank. However, the CLS treatment resulted in more uniform marketable prawns and suggested a high production of BC males in an all‐male culture system.     

Should only females of giant freshwater prawn Macrobrachium rosenbergii be selected in genetic improvement programmes?

Sexual dimorphism is widely observed in almost all farmed aquatic animal species but giant freshwater pawn (GFP) is unique, with males characterized by three main morphotypes (blue claw, orange claw and small males) and females by different reproduction status (ovary, berried egg and already‐spawned females). There has been reported evidence that the effect of male morphotype may have masked genetic variation in growth‐related traits, as a result the heritability for male body weight was lower than that estimated in female. A pending question has arisen whether selection should be made in female only. To answer this question, we used an 8‐year data set from a long‐term selection programme (2008–2015) for high growth in this species comprising 106,756 individuals that were offspring of 515 sires and 810 dams. The body weight data of female and male GFP or of each morphotype was treated as a separate trait and a multi‐trait approach was used to estimate genetic correlations for homologous traits between sexes and between morphotypes. Our analysis showed that there were little differences in the heritability estimates between female and male. In female, mature ovary individual displayed higher heritability than berried egg and already‐spawned females. For male, the heritability for blue claw, orange claw and small males were 0.11, 0.06 and 0.00 respectively. Between‐sex genetic correlation was moderate (0.55 ± 0.11) for body weight, suggesting that the trait expressions in female and male may be genetically different. In female, the genetic correlations for body weight among three female types were close to one (0.91–0.94). In contrast, the genetic correlations for body weight between male morphotypes especially between blue claw or orange claw and small males were low (0.15–0.25). Furthermore, we estimated genetic gain as the difference in least square means (LSM) or estimated breeding values (EBV) between the selection line and control group. The genetic gain in body weight was smaller in females than in males. It is concluded that there is no need to run separate breeding programme for female and male GFP. A combined selection using both female and male data can achieve selection response for body weight as demonstrated in the present study.     

Effect of Intensification on Grow Out of the Amazon River Prawn, Macrobrachium amazonicum

The effects of intensification on growth, survival, productivity, population structure, and distribution of harvested biomass in individual size classes of Macrobrachium amazonicum in semi‐intensive culture were evaluated. Postlarvae (0.01 g) were stocked in 12 ponds at densities of 10, 20, 40, and 80/m² (three replicates per treatment) and raised for 5.5 mo. Average individual weight significantly decreased and productivity significantly increased as stocking density increased (P < 0.001), while survival was not affected (P > 0.05). Prawn mean weight at harvest ranged from 3.6 (80/m²) to 7.0 g (10/m²). Average survival ranged from 65.5% (40/m²) to 72.8% (20/m²), while productivity ranged from 508 (10/m²) to 2051 kg/ha (80/m²). Harvested biomass showed a clear bimodal distribution in individual size classes indicating the occurrence of heterogeneous growth, which may affect management and market strategies. Harvested biomass of prawns weighing more than 7 g (the best market size) increases for stocking densities up to 40/m² and stabilizes between 40 and 80/m². Growth reduction was associated with a decreasing frequency and average weight of green claw 1 and green claw 2 male morphotypes and adult females as density increased. Thus, the distribution of male morphotypes and sexually mature females are affected by density‐dependent factors. Results suggest that prawn density plays an important role on M. amazonicum grow‐out phase, as has been demonstrated for other species of the genus Macrobrachium. M. amazonicum tolerates grow‐out intensification and may be raised in both semi‐intensive and intensive systems stocked at very high densities yielding high productivity.     

The case for all‐female freshwater prawn,Macrobrachium rosenbergii (De Man), culture

The prevailing notion in the prawn (Macrobrachium rosenbergii) culture industry is that the large male prawns that dominate the product value in research and commercial populations should, alone, be the target of commercial mono‐sex (all‐male) culture. However, studies have shown that the male prawn's response to increasing density (intensification) is controlled by a strong hierarchical dominance‐based social structure resulting in a large, disproportionate, increase in low value small animals in the ‘lower’ modal class of the male prawn size distribution. In contrast, prawn females and other aquatics, such as marine shrimp, display a moderate and uniform response to intensification in all size classes. Indeed, the densities in which prawn male superiority has been demonstrated are well below those used in intensive marine shrimp culture. This article: (1) discusses the background to the issue, (2) discusses the notion that because female prawns appear to be ‘marine shrimp‐like’ – with a normal size distribution indicating a lack of a strong social dominance – this will render them superior to all‐males under intensive growing conditions, (3) presents a ‘rate‐of‐response‐to‐density’ model projecting a female‐superior response to high previously untested densities, (4) summarizes results of a pond field test conducted at Auburn University (Auburn, AL, USA) to test the model's results whereby hand‐sexed prawns in treatment‐replicated in‐pond cages at densities of 10, 25, 40 and 55/m² showed that all‐female prawns give higher production and product value than males under intensive conditions, (5) discusses the use of biotechnological methods to sex‐reverse brood‐stock to produce all‐females and (6) discusses the notion that the potential of genetic selection to increase prawn production is only possible using females.     

Production Characteristics of All‐male and Mixed‐sex Giant River Prawns,Macrobrachium rosenbergii,Grown in Earthen Ponds in Kentucky and Mississippi USA

Production and population characteristics of monosex male (all‐male) giant river prawns, Macrobrachium rosenbergii, were compared with a normal (mixed‐sex) population in separate studies in Mississippi and Kentucky (USA) under low and high density stocking conditions, respectively. In Study 1 (Mississippi), juvenile prawns were stocked into eight 0.05–0.06 ha ponds at 24,700/ha. The mean stocking weight of all‐male was 0.34 g and mixed‐sex was 0.39 g. Prawns were fed 23% crude protein “range cubes” and harvested after 120 d for the all‐male prawns and 112 d for mixed‐sex prawns. In Study 2 (Kentucky), juvenile prawns from each group were stocked into six 0.04 ha ponds at 60,000 juveniles per hectare. The mean stocking weight for all‐male was 0.38 g and for mixed‐sex juveniles was 0.34 g. Prawns were fed a commercial sinking pellet (33% protein) once daily at a standardized rate and harvested after 105 d. In both locations survival of mixed‐sex prawns and all‐male prawns was not significantly different and the final average weight of all‐male prawns was significantly greater than the average weight of mixed‐sex prawns. For the study in Kentucky, total production was not significantly different between treatments, whereas in Mississippi total production in the all‐male ponds was significantly higher than in the mixed‐sex ponds. For both studies, the production size index of all‐male prawns was significantly greater than that of mixed‐sex prawns. In terms of population structure, in all‐male ponds there was a significant increase in orange claw (OC) males compared with the mixed‐sex ponds both as a percent of sex and a percent of total population. The increase in OC numbers in all‐male populations may be due to a lack of females to stimulate the transition of males to the final, sexually mature, blue claw stage. As target weights increase from 20, 30, and 40 g, the all‐male populations were increasingly superior in terms of production (kg/ha) of those target sizes. The economic benefit of all‐male over mixed‐sex populations will be principally based on an examination of tradeoffs that primarily consider the cost difference of juveniles relative to the price differences for different final harvest weights.     

Individual food intake measurement of freshwater crayfish (Pacifastacus leniusculus Dana) juveniles

The inert marker method for individual food intake measurement of signal crayfish (Pacifastacus leniusculus Dana) juveniles (carapace length (CL) ranged from 16.5 to 25.6 mm and wet weight (WW) from 0.88 to 4.47 g) was evaluated in a series of experiments. The measurement technique was tested with two types of diet: a gelatin‐based semi‐moist diet and an agar‐based dry diet. Diets were labelled with X‐ray dense ballotini glass beads. After feeding, juveniles were X‐rayed, and the amount of ingested glass beads in the digestive track was counted from the X‐ray images. The method proved reliable only with the gelatin diet. In the ingested gelatin food, 92.80±8.55% of the glass beads originally in the food were found in comparison with 40.99±24.72% for the agar diet. Subsequently, the growth efficiency of the gelatin diet was compared with typically used raw fish diet (roach, Rutilus rutilus L.), and possible harmful effects of X‐raying on the growth and survival of the juveniles were tested on 2‐year‐old crayfish (CL 22.90±1.66 mm and WW 3.15±0.71 g). Roach‐fed juveniles grew significantly faster than the gelatin‐fed counterparts. However, the growth performance and survival of the juveniles on gelatin diet was considered good enough to allow long‐term growth experiments with individual feed intake monitoring, e.g. for studies on social behaviour of the crayfish. The results of this study confirmed that the X‐radiography technique together with a semi‐moist gelatin‐based diet makes it possible to measure individual food consumption of freshwater crayfish juveniles reared communally.     

The progeny of sex-inverted female Atlantic salmon (Salmo salar L.)

A group of Atlantic salmon juveniles received dietary 17α-methyl-testosterone (3 mg/kg of food) for 60 days after first-feeding. Many of these fish became sexually mature as males at 1.5 years of age, while they remained in freshwater. Supposed sex-inverted females among these were identified by their failure to strip manually. Milt from the excised gonads of five of these fish was used separately to fertilise mixed ova from normal adult females. The progeny of each male, when examined 8 weeks after first-feeding, all had typically female gonads (n = 50).     

Effect of glycine betaine, a feed attractant affecting growth and feed conversion of juvenile freshwater prawn Macrobrachium rosenbergii

A feeding experiment was conducted to determine the effect of the feeding attractant, glycine betaine (betaine hydrochloride) on the growth and feed conversion of juveniles of Macrobrachium rosenbergii (deMan) (mean initial weight 1.12 g). Three types of diets were prepared with the incorporation of glycine betaine at 5, 10 and 15 g kg^?1 levels along with an unsupplemented control. After 60 days, weight gain, feed intake and food conversion ratio (FCR) were higher in prawn fed the three glycine betaine‐added diets compared with the control feed. Among the glycine betaine‐added diets, prawn fed glycine betaine at 5 g kg^?1 level showed highest weight gain (2.73 g) by registering 61.5% increase in growth over control and also higher feed intake (5.79 g) and good FCR (2.12). There were highly significant differences (P < 0.01) in weight gain, feed intake, per day growth and FCR among treatments. There was no significant difference (P > 0.05) in survival among treatments.     

Phenotypic and genetic parameters for body traits in the giant freshwater prawn (Macrobrachium rosenbergii) in India

Phenotypic and genetic parameters were estimated for three body traits (harvest weight, carapace length and standard length) and for adult male morphotypes of the giant freshwater prawn Macrobrachium rosenbergii in a fully pedigreed synthetic population in India. The data set included 9,173 progeny produced over four generations from 162 sires and 234 dams. Variance components and genetic parameters were estimated fitting an animal model using the residual maximum‐likelihood methodology. The heritabilities for harvest body weight (HW), carapace length (CL) and standard length (SL) were moderate (0.22 ± 0.056, 0.22 ± 0.055 and 0.25 ± 0.059 respectively). The common environmental effects for HW, CL and SL were 0.10 ± 0.020, 0.08 ± 0.018 and 0.10 ± 0.021 respectively. As M. rosenbergii is sexually dimorphic, we estimated heritabilities within each sex. Heritability of HW in females (0.27 ± 0.068) was greater than that in males (0.15 ± 0.057). CL and SL followed the same pattern. The occurrence of male morphotypes is a unique characteristic of adult populations of M. rosenbergii. Populations from culture ponds exhibit a wide range of sizes. To examine whether there was a heritable component in male morphotype frequencies, we treated male morphotypes as traits. The additive genetic variance (and hence the heritability) was zero for male morphotype, indicating that selective breeding to increase the proportion of desirable male morphotypes would not be effective. The genetic correlations among body traits were all positive, high and approaching unity. The results are discussed in relation to selection plans for the giant freshwater prawn.     

Size rank and growth potential in redclaw crayfish (Cherax quadricarinatus): are stunted juveniles suitable for grow‐out?

Considerable size variation emerges at an early stage among redclaw (Cherax quadricarinatus) juveniles. In order to explore whether juveniles from different size fractions of the same population differ in their growth potential, they were reared individually, in the absence of social effects. In the first experiment, post‐release siblings were reared for 5 weeks and then categorized into three size fractions: small, med‐sized and large. Subsequently, juveniles of each size fraction (n=24, 18, and 18, respectively) were reared in individual cells for 46 days and their moult increments and moult intervals were compared. In addition, the growth indices of the med‐ and large‐sized juveniles were compared with those of juveniles from other broods that were similar in age and size but belonged to a different size fraction, in order to separate the effect of absolute size from that of size rank. In a second experiment, juveniles from the small‐, med‐ and large‐size fractions (n=21) of a commercial nursery‐reared population were individually reared for 14 weeks and their growth indices were compared. Survival of individuals from the small‐size fraction was low, particularly in the first experiment, and increased with fraction size. However, small‐sized juveniles did not show reduced growth potential and even grew at a faster rate than large‐sized juveniles. The results indicate that social interactions plays an important role in the development of size variation, and that small juveniles have the potential to grow rapidly when reared in individual compartments. Overall, the size rank was a poor predictor of growth potential.     

The androgenic gland and monosex culture of freshwater prawn Macrobrachium rosenbergii (De Man): a biotechnological perspective

Males of the freshwater prawn Macrobrachium rosenbergii (De Man) grow faster and reach a larger size at harvest than females of the species. It is thus obvious that culture of monosex all‐male populations would be economically advantageous. Sexual differentiation in crustaceans is regulated by the androgenic gland (AG), which plays a pivotal role in the regulation of male differentiation and in the inhibition of female differentiation. In M. rosenbergii, AG removal from immature males resulted in sex reversal, with complete female differentiation. Similarly, AG implantations into immature females lead to the development of the male reproductive system. Sex‐reversed M. rosenbergii animals were capable of mating with normal specimens to produce offspring. Early attempts in Israel and more recently, attempts in other countries to establish all‐male populations through manual segregation showed that for the production of monosex prawn populations to be economically feasible, intervention via the AG is probably required. However, a suitable biotechnology is still to be developed, and an androgenic hormone has yet to be identified in decapods. Three lines of aquacultural and biotechnological research and development are proposed for the future: (1) Establishment of monosex cultures through manual segregation, together with the application of selective harvesting and claw ablation, as well as examination of different monosex culture strategies under a variety of economic conditions. (2) Microsurgical intervention in the AG, leading to the development of functional neo‐females, which would subsequently be mated with normal males to produce all‐male progeny. (3) Elucidation of AG bioactive products to enable biochemical or molecular manipulation of sex differentiation.     

Gonadal structure analysis of Macrobrachium amazonicum (Heller, 1862) from a wild population: a new insight into the morphotype characterization

The aim of this work is to correlate the histological gonad analysis of adult male Macrobrachium amazonicum of a wild population with morphotypes identified in populations reared in earthen ponds. A total of 500 adult male specimens were collected in the Mosqueiro Island, Municipality of Belém, State of Pará, Brazil. The animals were classified on the basis of body and cheliped colour, second right cheliped spination, total length, cheliped length and weight. The gonads of each morphotype were fixed, processed by histological techniques and stained with haematoxylin and eosin. All four morphotypes, translucent claw (TC), cinnamon claw (CC), green claw 1 (GC1) and green claw 2 (GC2), were identified in the wild population, indicating the same population structure described for cultured animals. The M. amazonicum testes are composed of multiple lobes. The TC gonads showed a population of germ cells in the eccentric portion of the seminiferous tubule and few spermatozoa in the central region. Cinnamon claw males exhibited a large cell population with intense meiotic activity and sparse spermatozoa. Regardless of the different external morphologies of GC1 and GC2, no histological differences were found in their gonads; both had a large number of gametes and similar spermatogenic cell population. Despite the external morphological evidences of the four morphotypes, the histological criteria indicate the existence of only three morphotypes: TC, CC and green claw (GC).     

Current status and prospects of farming the giant river prawn Macrobrachium rosenbergii (De Man) in Bangladesh

Giant river prawn (Macrobrachium rosenbergii) farming plays an important role in the economy of Bangladesh. Presently, it is cultured in around 50 000 ha area with total annual production of 23 240 t. Traditional extensive prawn farming has been expanding over the last three decades through the introduction and adoption of improved culture systems, such as culture of prawn‐carps, prawn‐shrimp‐fish and prawn‐fish‐rice as concurrent and rotational systems. Efforts for the development of improved techniques on broodstock management, seed production and rearing and grow‐out of prawn have been made over the last decade. The outcomes are low‐cost feed for broodstock, production of post‐larvae in net cages (hapa), all‐male prawn culture, periphyton based prawn‐tilapia culture, C/N based prawn culture, organic prawn farming, prawn‐mola culture and prawn‐carp‐mola polyculture. Despite the development of culture technologies, a number of challenges for sustainable development of prawn farming need to be overcome to realize the potentials of this promising sector. Good aquaculture practises at all levels and application of measures for quality control and food safety would ensure sustainable development of prawn farming in Bangladesh.     

Liver oil of pharaoh cuttlefish Sepia pharaonis Ehrenberg, 1831 as a lipid source in the feed of giant freshwater prawn, Macrobrachium rosenbergii (De Man 1879)

The efficacy of pharaoh cuttlefish ( Sepia pharaonis Ehrenberg, 1831) liver lipid in the feed for juveniles of giant freshwater prawn Macrobrachium rosenbergii (De Man 1879) was tested by feeding five experimental diets prepared using clam meat, groundnut oil cake, wheat bran, tapioca powder, vitamin–mineral mixture and cellulose powder as the major ingredients. The test diets T₁, T₂, T₃, T₄ and T₅, containing five levels (1, 2, 3, 4 and 5%, respectively) of lipid, extracted from the liver of pharaoh cuttlefish, were fed to 30-day-old prawn juveniles for 60 days, in triplicates and compared against a control. Analysis of variance of the growth parameters showed that the juveniles fed with T₂, containing 2% cuttlefish liver lipid with a total lipid content of 9.85% and lipid–carbohydrate ratio of 1 : 3.8, showed significantly better growth ( P  < 0.05) with the highest weight gain of 118%, greatest food conversion and protein utilization efficiency, and the highest body protein content, although the survival rates remained unaffected among treatments. Growth of the juveniles, however, declined at lipid levels >2%. Addition of cuttlefish liver lipid led to an increase of ω-3 and ω-6 fatty acids in T₂, the proportion of ω-6 being higher than the former. The crude lipid content of the body of test animals also increased with the increase in dietary lipid from 1.51% in T₁ to 3.27% in T₅. The present study indicates that cuttlefish liver lipid can be successfully used as a substitute for conventional lipid sources in rearing juveniles of giant freshwater prawn, an eco-friendly practice in recycling cuttlefish waste – a discarded fishery by-product.     

POPULATION PROFILE DEVELOPMENT AND MORPHOTYPIC DIFFERENTIATION IN THE GIANT FRESHWATER PRAWN Macrobrachium rosenbergii (de Man)

Post-larval populations of Macrobrachium rosenbergii are characterized by a homogenous size distribution curve which changes, with time, to a positively skewed curve, with some individuals demonstrating a higher growth rate than the rest of the population. No morphotypic variation is observed at the post-larval and juvenile stages, and the ratio of males to females is 1:1, equally distributed along the whole size distribution curve. Both the mean weight and the development of the skewed distribution curve are inversely affected by increasing density. In a mature population, three male morphotypes can be clearly differentiated by claw color, relative size within the population and the ratio of claw length to body length. The Small Male (SM) morphotype is characterized by short clear claws, while the two larger morphotypes are characterized by Orange Claws (OC) and Blue Claws (BC). Their relative occurrence in the population is SM 20–25%, OC 15–25%, and BC 5–10%. No apparent morphotypic variation exists in females and their size distribution is generally homogenous. Increased population density does not change the relative position, nor the relative proportions of the three male morphotypes and the females, however, the degree of skewness of the curve is inversely related to the density.     

脊尾白虾全人工繁育技术初步研究

脊尾白虾是我国沿海地区池塘养殖重要品种,但目前尚未实现全人工繁育,严重制约了产业进一步发展。选取野生脊尾白虾作为亲虾,经过越冬培育、促熟交尾、幼体孵化培育、仔虾选育等手段,进行了脊尾白虾全人工繁育技术研究。试验结果显示,在盐度31.3、水温18.1~26.2℃、pH 8.1条件下,室内越冬脊尾白虾亲虾可成功培养至性腺成熟,并交尾抱卵,平均抱卵率达70%,平均孵化率为61.02%。幼体最佳培育密度20尾/L,仔虾适宜养殖密度0.13~0.53尾/L,培育至2cm时存活率可达80%,继续培育3个月后发育至性成熟。一年可繁殖2~3代。在此基础上建立了脊尾白虾近交家系,现已培育至第6代,各家系繁殖力、生长速度和存活率均未出现近交衰退现象。本研究初步解决了脊尾白虾室内全人工繁育技术,并为试验动物培育奠定了良好的技术基础。     

Growth,Processing Measurements,Tail Meat Yield,and Tail Meat Proximate Composition of Male and Female Australian Red Claw Crayfish,Cherax quadricarinatus,Stocked into Earthen Ponds

Abstract

Small (0.7 g) juvenile red claw, Cherax quadricarinatus, were stocked in earthen ponds (0.04 ha) in Kentucky and grown for 86 days so that measures of growth, survival, processing characteristics, and proximate composition of tail muscle in male and female Australian red claw crayfish could be obtained. A commercial marine shrimp was fed to all red in two separate feedings, each consisting of one-half of the total daily ration between 0800-0830 and between 1530-1600 hours for the duration of the culture period. Total weight and number of red claw from each pond were recorded at the harvest. Forty red claw from each sex were randomly sampled, chill-killed by lowering the body temperature in an ice-bath, and frozen (-20°C) for processing measurements and proximate composition. Red claw were removed from the freezer, thawed, individually weighed to the nearest 0.01 g, and then were hand-processed so that weights of chelae, tail, and tail muscle were obtained to the nearest 0.01 g. After 87 days, red claw had a final average individual weight of 59.6 g, weight gain of 8,413%, survival of 64.0%, yield of 903 kg/ha, and feed conversion ratio (FCR) of 4.63. When analyzed by sex, males had significantly (P < 0.05) higher final individual weight (71.05 g) compared to females (53.65 g); higher (P < 0.05) chelae weight (13.1 g for males compared to 6.84 g for females); higher tail weight (25.53 g for males compared to 20.38 g for females); higher tail muscle weight (18.99 g for males compared to 14.89 g for females); and higher cephalothorax weight (32.41 g for males compared to 26.44 g for females; Table 2). There were no significant differences (P > 0.05) in the percentage moisture, protein, lipid, fiber, and ash in the tail muscle of male and female red claw when analyzed either on a wet-weight basis and averaged 81.0%, percentage protein averaged 16.46%, percentage lipid averaged 0.16%, fiber averaged 0.1%, and percentage ash averaged 1.42%. Results from the present study indicate that red claw can be grown as a commercial aquaculture species in locations with limited growing seasons, and that male red claw grow larger, have larger chelae, and higher tail muscle weights compared to females so that it might be economically advantageous to stock all-male populations of red claw in ponds to achieve maximum production. Further research on growth and processing yields of red claw should be conducted to assist the industry.     

Growth performance of European whitefish [Coregonus lavaretus (L.)] under a constant light and temperature regime

Growth, feed intake and maturation were monitored in a fast‐growing benthivore morph of European whitefish [Coregonus lavaretus (L.)] held under constant light (LD 24:0) and temperature conditions (10°C) for 415 days. Their growth was best described by polynomial regression model and no indication of seasonal patterns (e.g. winter depression) in growth rates was found. There was no significant difference in food conversion ratio (FCR) between periods during the experiment and the FCR was favourable (0.84). Furthermore, there were no differences in growth between males and females. None of the fish became sexually mature by the time the experiment was terminated, although they had reached a mean size of about 444 g. The European whitefish seems to be a promising aquaculture species as the growth rate was relatively high with a remarkable homogenous growth performance between individual fish within each experimental unit, a relatively favourable FCR, a low degree of agonistic behaviour and negligible growth depression due to internal biological rhythms or gonad maturation. These results were obtained at relatively low temperatures that should be beneficial for sustainable cost‐effective farming conditions in cold climate condition in northern Scandinavia.     

Effects and Interactions of Stocking Density and Added Substrate on Production and Population Structure of Freshwater Prawns Macrobrachiurn rosenbergii

The effects and interactions of added artificial substrate with different stocking densities of prawns were evaluated. Juvenile prawns (0.2 ± 0.1 g) were stocked into 12 0.04-ha ponds at 60,000ha and 120,000/ha with and without substrate. Added substrate consisted of horizontal plastic mesh and strips of "oyster netting" sutlicient to increase available surface area 80%. There were no statistically significant ( P > 0.05) interactions between stocking density and presence of added substrate, allowing these main effects to be compared separately. Increasing stocking density produced a significant increase ( P < 0.01) in total production but a significant decrease ( P < 0.01) in average prawn size and production ( P < 0.05) of market size prawns (>20 g, -27%; > 30 g, -56%). Added substrate also produced a significant increase ( P < 0.05) in total production (18%). However, average prawn size was not decreased and production of marketable shrimp was increased (>20 g, 25%; >30 g, 19%). Feed conversion ratios were significantly decreased (P < 0.01) by the presence of substrate. Increased stocking density significantly increased ( P < 0.05) the percentage of males which were small males (SM) and decreased ( P < 0.01) the percentage of orange claw males (OC), but had no impact ( P > 0.05) on numbers of reproductive (RF) and virgin (VF) female morphotypes. Increased stocking density also produced a significant decrease ( P < 0.05) in average size of OC, RF, and VF morphotypes while the addition of substrate had no statistically significant impact (P > 0.05) on the number or size of different morphotypes. The mathematical relationship between available surface area and average prawn size should be determined to produce recommended inclusion rates for added substrate based on desired levels of total production, stocking rates, and optimum market sizes.     

Sex differentiation of summer flounder (Paralichthys dentates L.) raised at practical hatchery temperatures

Female summer flounder grow considerably faster than males, and rearing temperature has been shown to influence sex differentiation in related species. The present studies were conducted to examine sex differentiation of post‐metamorphic juveniles reared at temperatures conducive for commercial production. In experiments 1 and 2, offspring produced from crosses with meiogynogenetic, sex‐reversed males and normal females were raised at 14, 16 or 18°C for ≥300 days or 12°C for 30, 60 or 120 days and then transferred to a male‐determining temperature (21°C). In all cases, the fish developed predominantly as phenotypic males (≥74%). In a third experiment, offspring produced from crosses with normal males and females were reared at 15, 17 and 19°C for 111, 227 and 278 days and then transferred to 21°C. Most fish (≥92.1%) developed as phenotypic males irrespective of rearing temperature or length of exposure. Several hypotheses are proposed for these findings and further research is necessary to understand the sex‐determining mechanisms in this species before the economic advantages associated with sexually dimorphic growth can be realized.