Application of enzyme-linked immunosorbent assay (ELISA) for the study of reproduction in the Manila clam Ruditapes philippinarum (Mollusca: Bivalvia): II. Impacts of Perkinsus olseni on clam reproduction

We investigated the effects of infection by the protozoan Perkinsus olseni on the reproduction of female Manila clams, Ruditapes philippinarum, from a population in Gomso Bay, Korea. The reproductive effort of the clams was assessed by ELISA using a clam egg-specific antibody and was expressed as a weight-based gonadosomatic index (GSI). The number of Perkinsus infecting each clam was estimated from the gills using Ray's fluid thioglycollate medium (RFTM) along with a NaOH digestion assay. We found that reproductive effort was negatively correlated with the intensity of the Perkinsus infection: more heavily infected clams produced fewer eggs during the spawning period from May to August. Frequency of spawning was also negatively correlated with the level of Perkinsus infection; heavily infected clams (HIC) exhibited a single spawning pulse in late July, whereas lightly infected clams (LIC) showed three spawning peaks in mid-May, late July, and late August. Egg production of HIC was only 30–75% of LIC during spawning. The level of total protein in LIC was also higher year round than in HIC. In conclusion, our investigation demonstrates that a high level of Perkinsus infection affects spawning frequency and reduces egg production, which may have long-term impacts on clam recruitment and population growth.     

Growth of the hard clam Meretrix lusoria in Taiwan

The growth of hard clam Meretrix lusoria in Taiwan was observed as it was grown at six different stocking rates (55, 109, 172, 244, 344 and 455 clams/m²), from November 1979 to September 1980. It was found that the stocking rate had more effect on the increase of the total weight of the clam than on the shell length. Under the environmental condition of the study site, the optimum stocking rate was 244 clams/m². When the net production of the clam was lower than 1103 g/m², the growth of the individual clam was not affected by the stocking rate. But when the net production was higher than 1589 g/m², the growth of the individual clam seemed to be retarded. The average pH of the sea water in the clam culture area was between 7.9 and 8.4. The average dissolved oxygen content was between 4.2 and 11.2 mg/l. And the average water salinity was between 30‰ and 35‰. All these three factors did not seem to have direct relationship with the growth of the clams. Sea water temperature, however, showed great effects on the growth of the clams. It is shown that the growth of the hard clam was slow at a water temperature of 15–18°C, increased at 20–22°C, and accelerated at 25–32°C.     

Reproductive performance and offspring quality of Chinese mitten crab Eriocheir sinensis (H. Milne-Edwards) females fed an optimized formulated diet and the razor clam Sinonovacula constricta

After feeding female Eriocheir sinensis on an optimized formulated diet or fresh razor clam Sinonovacula constricta for 7 months, their reproductive performance and offspring quality were compared. To evaluate diet nutrient contents, the proximate, fatty acid and amino acid compositions of the formulated diet and the razor clam were analysed. The nutritional value of the diets was determined by assessing survival, gonadosomatic index (GSI) and hepatosomatic index (HSI) of female crabs from both diet treatments, together with the percentage of females that spawned, total egg production per female and fecundity (number of eggs g^?1 female wet weight). Furthermore, the quality of eggs and newly hatched larvae from the two dietary treatments were determined using the following parameters: egg diameter, wet weight and dry weight, hatchability, proximate and fatty acid profile of eggs, larval carapace length, resistant to starvation and osmotic shock, larval survival and development to the zoea II stage. Higher protein, phospholipids (PL) and amino acids (AA) contents were found in the razor clam while the formulated diet contains higher levels of ash, total lipid (TL) and 18:1n‐9, 18:2n‐6 and 22:6n‐3 fatty acids. Although female crabs fed the two different diets showed similar reproductive performances, newly hatched zoea I larvae produced by the crabs fed the formulated diet had significantly longer mean carapace length and shorter development time to the zoea II stage under identical culture condition (P<0.05). Moreover, dietary fatty acid appeared to have more significant effects on the fatty acid composition of the hepatopancreas than it did on mature ovaries or eggs. This suggests that the fatty acid profile of mature ovaries is indicative of the specific fatty acid required for ovarian development in E. sinensis. In conclusion, our results show that the optimized formulated diet developed in this laboratory can totally replace the razor clam, a broodstock food widely used in E. sinensis hatcheries in China. This encouraging result should facilitate more reliable hatchery production of this important aquaculture species.     

A technique for delaying embryogenesis of vendace (Coregonus albula L.) eggs in order to synchronize mass hatching with optimal conditions for lake stocking

To improve the efficiency of stocking lakes with vendace (Coregonus albula) larvae, a technique for delaying egg hatching was developed. This synchronizes mass hatching with the development of suitable thermal conditions and food availability in the lake.Water temperatures were measured in Polish hatcheries and a mathematical model of the dependence of C. albula embryogenesis on incubation temperature was utilized to predict the possibilities of delaying egg hatching. Vendace hatching can be delayed by cooling the circulation water (to 1 or 2°C) during March and April. At the beginning of May, when lake conditions are optimal for stocking, the incubation temperature should be raised (at a rate of 1.5°C h^?1) to that of the water supply. This ensures mass hatching of strong, normally developed larvae within 2–3 days. This technique facilitates both the delay of vendace hatching at minimal cost and the production of larvae on demand for lake stocking.     

The effects of bioturbation by the Venus clam Cyclina sinensis on the fluxes of nutrients across the sediment–water interface in aquaculture ponds

A mesocosm experiment was conducted to study the effects of bioturbation by the marine bivalve Cyclina sinensis in an aquatic system and to examine how the bioactivities of this clam alter the rate at which nutrients are exchanged across the sediment–water interface. A dark incubation experiment was performed to determine the fluxes of inorganic nutrients (NH₄ ⁺, NO₃ ^? + NO₂ ^? and PO₄ ^3?), benthic chlorophyll a (Chl a), organic matter and sediment oxygen consumption (SOC) on days 1, 10, 20 and 30. C. sinensis destroyed the initial sediment surface and partially enhanced oxygen penetration into the sediment. The clam also increased the water content and volume of the oxic sediment. The analysis showed that increases in the clam density and time period produced higher fluxes of all the nutrients (NH₄ ⁺, NO₃ ^? + NO₂ ^? and PO₄ ^3?) in the bioturbated chambers compared with the control (non-bioturbated chambers). The concentration of Chl a in the sediment increased significantly at the end of the experiment (day 30) (p < 0.05). The SOC rate increased significantly with the increase in density and with the time period from day 10 to day 30 (p < 0.05). As a result, the fluxes of nutrients increased with increased SOC, producing oxygen-induced sediment mineralization. Our findings suggest that the activities of the clams could positively influence SOC and cause an increase in the fluxes of inorganic nutrients. These effects may substantially improve the primary productivity and water quality of earthen pond ecosystems.     

Seasonal dynamics and yearly change in the abundance of metacercariae of <Emphasis Type="Italic">Parabucephalopsis parasiluri</Emphasis> (Trematoda: Bucephalidae) in the second intermediate host in the Uji–Yodo River,central Japan

An introduced pathogenic fish trematode, Parabucephalopsis parasiluri Wang, 1985 (Digenea: Bucephalidae), was studied to determine the seasonal fluctuation in its metacercariae abundance. The timing of the recruitment period of the metacercariae and the relationship between yearly fluctuation in metacercarial abundance and some environmental factors were also examined to develop an effective plan for disease control. Monthly changes in metacercarial abundance in the caudal fin of pale chub (Zacco platypus) and of the prevalence of the first intermediate host (golden mussel Limnoperna fortunei) suggest that P. parasiluri generally has an annual life cycle. The metacercarial abundance in chub increased when the water temperature fell below 7°C in winter, but the rapidity of the increase varied between 2007 and 2008. The fluctuations in the abundance of metacercariae in winter from 2001 to 2008 showed that mean flow rate and mean water level in January were negatively related to the infection level each year, and positively related to the mean water temperature. Based on these results, we propose a control plan for parabucephalopsiosis involving discharge control; that is, more than 50 m³/s (corresponding to 150 m³/s in the Yodo River) of water should be flushed from upstream dams for a period of at least one week starting on the day when the water temperature falls below 7°C.     

Abnormal mortalities of the carpet shell clam Ruditapes decussatus (Linnaeus 1756) in natural bed populations: a practical approach

In situ controlled systems based on PVC cages surrounded by a net mesh and buried in sand were designed to determine the causes of important mortalities in producing locations of carpet shell clam, Ruditapes decussatus, in two important production areas of Galicia (NW Spain). The pathogenic status of the clam populations (focusing mainly on Perkinsus sp. and clam haplosporidian) as well as changes in water temperature and salinity were recorded. Sampling was conducted during a short period of time when high mortalities were present in the two locations and in a 1‐year study when no important mortalities were registered. We found that the environmental conditions and not the presence of pathogens seemed to be related to mortalities in the study. Our results suggest that mortality rates, especially in natural beds, can emerge from a complex synergy of different factors. Moreover, we propose in situ controlled systems based on PVC cages as a useful method to control infaunal bivalve mortalities in natural beds.     

Experimental challenges of juvenile and adult Manila clams with the protozoan Perkinsus olseni at different temperatures

Infection with Perkinsus species, primarily P. olseni, is thought to be a major cause of the decline of Manila clam populations in Japan since the 1980s. However, the pathogenicity of the infection has not been sufficiently evaluated to estimate the impact of infection on wild Manila clam populations. We experimentally challenged juvenile (3- to 6-mm shell length) and adult (18- to 22-mm shell length) Manila clams with P. olseni at 18, 23, 28, and 30 °C. Mortality was significantly higher in challenged groups than in control groups. The difference in mean mortality between the challenged and control groups (all life stages and temperatures) was only significant above a threshold of infection intensity ~10⁶ cells/g soft wet tissue (SWT). As temperature increased, the onset of mortality occurred more rapidly. The increase in mortality occurred earlier in juveniles than adults at 28 °C and lower. Our results suggest that the pathogenicity of P. olseni is higher in juveniles than in adults and at higher water temperatures. Given the infection intensities (ca. 10⁶ cells/g SWT) previously reported in wild Manila clams, the parasite likely has considerable impact on wild Manila clam populations, particularly juveniles during periods of high temperature.     

Pattern of physicochemical changes in the water during hatching of Indian carp eggs in a Chinese hatchery system

Physicochemical changes in the water were monitored at different hours of embryonic and larval development of Indian carp (Cirrhinus mrigala) eggs in a Chinese hatchery system with 75–85% hatching success. At a water temperature of 29.3°C, mass hatching of the embryos started at about 11–12 h after fertilization and was completed within 4 h. The amount of dissolved oxygen and chemical oxygen demand ranged between 4.8 and 9.63 mg l^?1 and 16.5 and 185.0 mg l^?1, respectively. A marked rise in the concentration of bicarbonate and total hardness of the water after egg transfer into hatching tanks was perhaps due to liberation of Ca²⁺ and Mg²⁺ from the egg mass. A sharp decline in phosphate content during hatching was associated with the building of bones, while the ammonia level was greatly increased due to a high rate of nitrogenous excretion during hatching.     

Diphasic sedimentation technique complemented by a statistical approach as a useful tool to the quantification and the determination of dynamics of eggs produced by adult monogenean parasitizing groupers,Epinephelus marginatus (Lowe, 1834), in captivity

Monogenean parasites create problems in confined fish, such as at aquaculture and aquariums facilities. Strategic timing of antihelminthic treatments should be based on the dynamics of monogenean egg production which requires egg quantification, but there is no standardized analytical method. The WHO diphasic sedimentation method for helminth quantification of eggs was applied in samples with tetrahedral and fusiform monogenean eggs from six specimens of confined groupers (Epinephelus marginatus) (31 m³ sea water tank; water temperature 21.1–14.4°C; photoperiod 9:53–15 h light; November 2011 to June 2012). Helminth eggs remain in the pellet, while most organic matter was removed forming a cap. In contrast, monogenean eggs are distributed within both the pellet and cap. The diphasic sedimentation method is suitable to quantify tetrahedral and total egg number by counting those eggs in the pellet and incorporing water temperature data to obtain a statistically significant model expression (R² = 0.943; R² = 0.809), while this approach only weakly predicted fusiform egg numbers (R² = 0.297). Fusiform egg numbers declined along November–December, and increased in February–March, until June. Tetrahedral eggs decreased in November–December, but did not increase until March. The trend of egg populations and best time for antihelminthic treatment application can be set by WHO technique with the statistical approach proposed.     

维生素E、C和HUFA交互作用对中华绒螯蟹生殖性能的影响

通过投喂添加或不添加VE、VC和HUFA的四组实验饲料,经198d饲养实验,以雌蟹的性腺系数、产卵力、孵化率、各组织超氧化物歧化酶(SOD)活性和过氧化产物丙二醛(MDA)含量等为指标,研究这两类营养素交互作用对其生殖性能的影响。结果表明,VE、VC和HUFA对雌蟹生殖性能有显著的影响(P<0.05):在产卵力方面,1组为3790个卵细胞·g-1体重,极显著高于2组(为2317个卵细胞·g-1体重)和3组(为2129个卵细胞·g-1体重)(P<0.01),显著高于4组(为3050个卵细胞·g-1体重)和5组(对照组)(为3010个卵细胞·g-1体重)(P<0.05);而孵化率方面,1组(86.17%)极显著高于2组(36.06%)、3组(29.18%)和4组(25.73%)(P<0.01),也显著高于5组(71.12%)(P<0.05)。1组和2组雌蟹性腺中SOD活性分别为40.09Nu·mL-1和39.87Nu·mL-1,相应地,MDA含量分别为6.87nmol·mL-1和3.33nmol·mL-1;而3组和4组雌蟹,尽管其性腺中SOD活性较高,分别为79.21Nu·mL-1和85.2Nu·mL-1,但由于饲料中抗氧化性VE、VC的缺乏,导致脂质过氧化发生,其MDA含量显著增加(P<0.05),分别为12.65nmol·mL-1和25.18nmol·mL-1。4组雌蟹的产卵力较大,但因VE、VC的缺乏发生脂质过氧化,导致所产的卵质低下,从而影响了卵子的孵化率。结果表明,VE、VC和HUFA都是河蟹保持良好的生殖性能的必需营     

Spawning of captive Senegal sole (Solea senegalensis) under a naturally fluctuating temperature regime

Senegal sole aquaculture is at present limited due to poor reproduction of captive breeders in many facilities. Temperature seems to play an important role in controlling reproduction of Solea senegalensis, and differences in temperature regimes followed by various hatcheries are likely to be responsible for lack of success in some of them. This work describes the reproduction of captive soles, held in facilities that used water at ambient temperature, from a marshy environment where this species naturally breeds. Acclimated sole breeders were kept for two consecutive years. The main spawning period occurred from February to May, with a secondary spawning in autumn. Total yearly fecundity ranged from 1.15×10⁶ to 1.65×10⁶ eggs kg⁻¹ body weight. Of the total egg batches produced, only 5.4% corresponded to autumn spawns. The male population was found to produce sperm all year round, with a maximum proportion of 100% occurring in spring, and a minimum proportion of around 50% in summer. Females showed the more developed ovary stages from October to May, with partial regression in the summer months. During the main spawning period, eggs were produced between 46% and 69% of days.Spawning took place at temperatures from 13 to 23 °C, although higher fecundities (P<0.05) occurred between 15 and 21 °C. Within the range between 17 and 20 °C, the mean number of spawned eggs was 29,600±21,600 eggs day⁻¹ kg⁻¹. Most of the eggs (65–73%) were produced after temperature increased up to 2.5 °C within 3 days prior to spawning. Mean egg fertilization was 63.1±17% (year 2002) and 44.9±18% (year 2003), and hatching rates varied from 69.7±24% (2002) to 56.5 ±25% (2003). Weak correlations were found between either fertilization or hatching and fecundity, whereas a positive regression (P<0.05) indicated that higher hatching rates were achieved when fertilization increased. A weak, but significantly (P<0.05) positive correlation was found between egg fertilization and the spawning temperature. Present results indicate temperature is an important control factor for reproduction of S. senegalensis, and suggest it can be used to properly manage controlled captive reproduction of this species.     

Effect of dietary supplementation of phospholipids and highly unsaturated fatty acids on reproductive performance and offspring quality of Chinese mitten crab, Eriocheir sinensis (H. Milne-Edwards), female broodstock

This study was conducted to determine the optimum phospholipids: highly unsaturated fatty acids (PL/HUFA) ratio in maturation diets for female Chinese mitten crab, Eriocheir sinensis broodstock. Five isolipidtic and isonitrogenous diets were formulated to contain different amounts of pork lard, fish oil and soy lecithin with resulting PL/HUFA levels (%dry weight) of 1.4/0.3 (Diet 1), 1.6/2.5 (Diet 2), 2.3/2.0 (Diet 3), 3.8/1.5 (Diet 4) and 5.1/0.8 (Diet 5). Diet 1 lacked supplemental fish oil and soy lecithin and served as the control. Each of the five formulated diets was fed to a total of 150 female crabs for a period of 7 months. All diet treatments were triplicated with 50 crabs stocked in a pond as a replicate. After 6 months, male crabs were introduced into the ponds where females were kept, mating, spawning and egg hatching (in seawater) occurred in the following month. The nutritional value of various formulated diets was assessed based on survival, gonadosomatic index (GSI), egg production per female, fecundity (eggs/g female weight) of the females fed those diets and egg and larval (newly hatched zoea I larvae) quality.Females fed Diet 1 had the lowest GSI, egg production per female and fecundity while no significant difference were found for survival, hepatosomatic index (HSI) and the percentage of female spawned among the diet treatments (P > 0.05). Although there were no significant differences in egg diameter, egg wet or dry weights among all treatments (P > 0.05), the highest and the second highest proportions of essential fatty acids (EFA), i.e. 20:5n-3 (EPA), 22:6n-3 (DHA) and HUFA were found in the eggs produced by the females fed Diet 3 and Diet 2, respectively. Statistical analysis showed that EPA, DHA and HUFA in eggs produced by females fed Diet 3 were significantly higher than those from the other treatments (P < 0.05). Meanwhile, zoea I larvae from crabs fed Diet 3 had significantly larger carapace length than those from the other four treatments (P < 0.05). The zoea I larvae from crabs fed Diet 2 and Diet 3 also showed generally better tolerance to starvation and osmotic shock.In conclusion, our results indicated that diet included levels of PL/HUFA around 1.6/2.5 or 2.3/2.0 (% dry weight) fed female E. sinensis broodstock supported increased fecundity and elevated level of HUFA in egg, which in turn resulted in improved overall quality of newly hatched larvae.     

Water Management to Control Clam Shrimp,Cyzicus morsie in Walleye,Stizostedion vitreum,Production Ponds

Clam shrimp, Cyzicus morsie , can occur in hatchery ponds in such dense numbers that they interfere with production of fish. Hatchery ponds are frequently left dry during the fall and winter and are filed in spring or early summer, simulating the vernal pools where clam shrimp naturally occur. Ponds left dry over winter and ponds that were full over winter, but were drained and dried for a period of time immediately prior to stocking, had the highest numbers of clam shrimp (P = 0.001). Few or no clam shrimp were collected in ponds that were full over winter and were not dried in the spring. High turbidities were observed in ponds with high numbers of clam shrimp; however, clam shrimp were not the only cause of turbidity. The largest number of walleye were produced in ponds which contained few or no clam shrimp. Clam shrimp were controlled by preventing hatchery pond substrate from drying during winter and early spring; however, the absence of drying may cause other problems that interfere with fish production.     

Tropical production of tilapia (Sarotherodon aurea) and tomatoes (Lycopersicon esculentum) in a small-scale recirculating water system

An integrated fish and hydroponic tomato production system was evaluated for use in the US Virgin Islands. The system was constructed from readily available materials, and designed to minimize capital costs, energy and water use, and the technological skill needed for operation. Tilapia (Sarotherodon aurea) and tomatoes (Lycopersicon esculentum) were cultured outdoors for 181 days in a closed system containing 7.34 m³ of water. Water quality suitable for fish production was maintained by aeration, biological filtration, sedimentation, hydroponic vegetable production and the addition of make-up water. Fish metabolites, wasted feed, and small quantities of inorganic fertilizers served as nutrient sources for tomato production. Fish survival (97.5%) and growth (2.54 g per day) were excellent with 63.6 kg of fish averaging 521 g each recovered at harvest. The total yield of ripe tomato fruit was 87.0 kg, of which 87.4% was marketable. The yield and quality of fruit produced hydroponically exceeded that produced under field trial conditions. Capital costs for the complete system were estimated to be US$612 (1979). Electrical energy and water use were 9.12 kWh day^?1 and 2.6% of the total volume per day, respectively. Cost and return projections indicate the system could be operated on a profitable basis in the US Virgin Islands.     

THE ARTIFICIAL PROPAGATION AND CULTURE OF BEAR SHRIMP,Penaeus semisulcatus de HAAN,IN HONG KONG

Penaeus semisulcatus (bear shrimp) was used for biological and culture studies. A peak of reproductive periodicity was shown in the months of May, June and July. Increase in body weight and in gonad weight coincided with the increase in sea water temperature. This suggested that sexual maturation might be a direct response to increased sea water temperature. Fecundity of mature female shrimp was estimated as 415,000 to 479,000 ova. In most cases, half-spent spawnings led to the production of poor eggs, characterized by irregular cytoplasmic formation and final autolysis. The embryonic and larval development of P. semisulcatus proceeded satisfactorily in a slightly alkaline medium (pH range from 7.5–8.5), and in water of salinity ranging from 28 to 35 ppt for egg and nauplius stages, and thereafter 25–35 ppt for zoea and mysis stages. The shrimps exhibited faster increase in body length than body weight in early growth. Later, after the shrimps had reached a body length of about 7.0 cm, the growth rate of body weight increased more markedly than body length. The rate of daily increase in weight was 1.01% in 7.0 cm shrimp. The feed efficiency of the formulated shrimp pellets was found to be 31.4%. During the nutritional study of P. semisulcatus, it was found that the combination of high dietary protein (about 40%) and low dietary lipid produced best growth and survival of bear shrimp. High increase of biomass of the shrimp fed with clam meat and high quality fish meal demonstrated the favorable response of shrimp to these diets. The use of cages for culturing bear shrimp was found to be practicable, but not efficient.     

Culture possibilities of the razor clam Ensis arcuatus (Pharidae: Bivalvia)

This study investigated razor clam Ensis arcuatus culture from larval rearing to commercial size to assess it's aquaculture potential. Broodstock was spawned several times in hatchery facilities to demonstrate that larval availability is not a constraint for hatchery production. Larval culture lasted 20 days, showing an average survival of 14.35%. The razor clam seed reached a size of 28.5 mm at 4 months. Seed culture is feasible; however, it is constrained by the fact that razor clams need to be buried. Higher survival was observed when culturing razor clam seed with a substrate (82–83%), irrespective of the substrate grain size, while containers held without a substrate showed a significantly lower survival (56.5%). We showed that it is possible to rear E. arcuatus juveniles in bottles, with survival as high as 94–95% being achieved. The length increase of the hatchery‐produced E. arcuatus over 600 days was described by the equation Ln L=4.5+(?141.17/t), where L is length in mm and t is time in days. On‐growing showed adequate survival rates and it took 2–3 years to reach commercial size.     

Comparison of low temperature adaptation ability in three native and two hybrid strains of the rotifer <Emphasis Type="Italic">Brachionus plicatilis</Emphasis> species complex

The low temperature adaptation ability of five selected strains of the Brachionus plicatilis species complex, i.e., three native strains [ Japanese (NH1L), Australian, German] and two hybrid strains [♀NH1L and ♂Australian (N × A) and ♀NH1L and ♂German (N × G),was investigated in terms of life history traits, reproductive characteristics, and mobility under different thermal conditions (12 and 25 °C). The life history traits of these five strains included a longer lifespan, reproduction period and generation times at 12 °C than at 25 °C, combined with reduced lifetime egg and offspring production. At 12 °C, the intrinsic rate of natural increase was higher in NH1L and N × A strains. Reproductive characteristics determined at 12 °C by batch culture showed active population growth for NH1L and N × G strains, while no resting egg production was observed in all of the strains tested. The ratio of swimming rotifers at 12 °C was monitored every hour for 6 h (short term) and every day for 10 days (long term). In the short-term study there was a 81% ratio of swimming rotifers of the NH1L strain, while other strains exhibited low swimming ratios (<60%). In the long-term study NH1L and two hybrid strains showed a >75% swimming ratio from the initial day of the study. These results suggest that outcrossing of rotifer strains is useful to obtain live food resources for the larviculture of cold water fish.     

Total egg harvest by the calanoid copepod Acartia tonsa (Dana) in intensive culture – effects of high stocking densities on daily egg harvest and egg quality

Understanding the factors limiting copepod productivity in dense cultures is a prerequisite for the partial or entire replacement of Artemia and rotifers as live feed for finfish larvae. In dense cultures, high encounter rates between individuals may increase stress, cannibalism incidents and potentially trigger resting egg production. We conducted an experiment to evaluate the potential egg production and egg quality of Acartia tonsa stocked at densities ranging from 10 to >5000 ind. L^?1. Egg Production (EP), Delayed Hatching Eggs production (DHE), hatching success (HS), egg mortality and water quality were used as end points. In the present system, A. tonsa was raised at >5000 ind. L^?1 without affecting the mortality, confirming that attaining this high density in culture is possible. However, egg harvest reached an optimum of 12 000 egg L^?1 day^?1 at ~2500 ind. L^?1 indicating that increasing stocking density above this level is not of practical interest. Calculations showed that the loss in egg harvest at stocking densities <2500 ind. L^?1 is of 1.3% for every additional 100 adult copepods L^?1. The increasing adult density did not affected the proportion of DHE produced (~10% of harvest) but decreased significantly the HS, though not to a point that would be problematic in a commercial production. Understanding the biology of copepods when stocked at high density is important to improve copepod culture systems and increase egg harvest yields. Technical solutions such as the continuous separation of eggs from adults in the water column, recirculation and the continuous provision of food are seen as potential solutions.     

Characterization of the pelagic ecosystem in surface waters of the northern Gulf of St, Lawrence in early summer: the larval redfish-Calanus-microplankton interaction

In June 1989, the water column along a transect in the north-central Gulf of St Lawrence was thermally stratified (10–14^oC at the surface; 0-l^oC at 30 m). In the surface layer, nitrate and chlorophyll concentrations were very low; the mean concentration of chlorophyll a > 5 μm in the subsurface maximum was 0.26 μg1^-1. Autotrophic and (presumably) heterotrophic flagellates and dinoflagellates were the most abundant micro-plankton. In this system, redfish (Sebastes spp.) larvae and the planktonic copepod Calanus finmarchicus overwhelmingly dominated the ichthyoplankton and zoo-plankton, respectively. Redfish larvae, Calanus females and Calanus eggs were most abundant in the surface layer (0–25 m) day and night. Daily specific egg production rates of Calanus, calculated from shipboard incubations of females, approached the predicted maximal level for this species at the ambient temperature of the surface layer, indicating no or little food limitation. The redfish larvae were feeding almost exclusively on the Calanus eggs and were found in greatest abundance along the transect where Calanus egg production rates (eggs m^-2 day^-1), calculated from the product of the specific egg production rate and female concentration, were highest. The mono-specificity of the larval redfish diet and the codom-