Natural Spawning Period Research Articles

Effects of Photoperiod, Water temperature, and Exogenous Hormones on Spawning and Plasma gonadal Steroid in Starry Flounder, Platichthys stellatus

The sexual maturation and spawning of teleosts are regulated by the external environment and the endocrine system. When the environmental conditions are artificially adjusted at a fish farm, the maturity and spawning of fish can be controlled. In this study, sexual maturation and spawning of the starry flounder, Platichthys stellatus, were artificially induced by adjusting the water temperature and photoperiod at a fish farm to accelerate the species’ natural spawning period. One experimental group acted as a control and was exposed to a natural photoperiod and natural water temperature (NPNT). In contrast, another experimental group was exposed to an adjusted environment consisting of a regulated photoperiod and temperature (RPRT). Daylight time was reduced by 10 minutes every 3 days from 13 hours to a duration of 8 hours. The water temperature was first reduced by 1oC every day, starting at 22oC and ending at 8oC, and then raised to 10oC until the spawning period. Both experimental groups were treated with gonadotropin-releasing hormone analog (GnRHa) pellets to induce ovulation. The results show that when the water temperature and photoperiod were artificially controlled, ovulation could be induced 97 days earlier than the natural spawning. Plasma testosterone levels of RPRT and NPNT tended to increase and then decrease 1–2 months before spawning, and plasma levels of 17α,20β-dihydroxy-4-pregnen-3-one increased 1–2 months before spawning. The concentration of estradiol-17β (E2) in plasma was not associated with spawning.

Sperm motility and lipid composition in internally fertilizing ocellate river stingray Potamotrygon motoro

All extant groups of Elasmobranches have internal fertilization and the structure of the male reproductive organs is very specific: sperm passes from the internal organs via the cloaca, but the male copulating organ (clasper) is distant from the cloaca. This suggests that sperm can contact the surrounding medium before fertilization. Because of this involvement with the environment, external signaling in sperm motility activation could occur in these species even though their fertilization mode is internal. In this case, spermatozoa of Elasmobranches should hypothetically possess a specific structure and membrane lipid composition which supports physiological functions of the sperm associated with environmental tonicity changes occurring at fertilization. Additionally, sperm motility properties in these taxa are poorly understood. The current study examined sperm lipid composition and motility under different environmental conditions for the ocellate river stingray, Potamotrygon motoro, an endemic South America freshwater species. Sperm samples were collected from six mature males during the natural spawning period. Sperm motility was examined in seminal fluid and fresh water by light video microscopy. Helical flagellar motion was observed in seminal fluid and resulted in spermatozoon progression; however, when diluted in fresh water, spermatozoa were immotile and had compromised structure. Lipid class and fatty acid (FA) composition of spermatozoa was analyzed by thin layer and gas chromatography. Spermatozoa FAs consisted of 33 ± 1% saturated FAs, 28 ± 1% monounsaturated FAs (MUFAs), and 41 ± 1% polyunsaturated FAs (PUFAs), and a high content of n-6 FAs (32 ± 2%) was measured. These results allowed us to conclude that sperm transfer from P. motoro male into female should occur without coming into contact with the hypotonic environment so as to preserve potent motility. In addition, this unusual reproductive strategy is associated with specific spermatozoa structure and lipid composition. Low level of docosahexaenoic acid and relatively low PUFA/MUFA ratio probably account for the relatively low fluidity of freshwater stingray membrane and can be the main reason for its low tolerance to hypotonicity.

Sperm motility in ocellate river stingrays: evidence for post‐testicular sperm maturation and capacitation in Chondrichthyes

AbstractSperm maturation, as a process through which spermatozoa acquire the ability for motility activation and fertilization, is well‐known for many groups of animals. This process takes place in a taxon‐specific manner in different parts of male reproductive tract. In contrast to most fish with external fertilization, a large number of animals have internal fertilization where transit through the epididymis of the male reproductive tract is necessary for the completion of sperm maturation. Although Chondrichthyes are fishes, they exhibit internal fertilization and possess specific male genital anatomy that resembles mammals. The existence of sperm maturation in the epididymis of cartilaginous fishes has been proposed but has yet to be confirmed. In this study, we used mature ocellate river stingrays Potamotrygon motoro, as a representative of Chondrichthyes, collected during the natural spawning period and evaluated sperm motility parameters during transit through the male reproductive tract, and upon contact with uterine fluid. Our data demonstrate that spermatozoa acquire motility after transit through the epididymis and are stored in a motile state within a specific organ – the seminal vesicles. Moreover, these motile spermatozoa are able to increase their velocity under the influence of uterine fluid in the female reproductive tract.

Seasonal-and dose-dependent effects of recombinant gonadotropins on sperm production and quality in the flatfish Solea senegalensis

Consecutive treatments with recombinant follicle-stimulating and luteinizing hormones (rFsh and rLh, respectively) stimulate spermatogenesis and potentiate sperm production in pubescent specimens of the oligospermic Senegalese sole (Solea senegalensis). However, sperm production in response to the hormones is highly variable, and the steroidogenic potential of the testis may be diminished due to sustained hormone supply. Here, we compared the effectiveness of low (9 μg/kg) and high (18 μg/kg) doses of rFsh and rLh to improve sperm production in adult sole during late winter-early spring (onset of the natural spawning period), and in autumn under a controlled temperature of 12 °C (period of testicular recrudescence). Treatment with rFsh over six weeks during spring, followed by a single rLh injection, did not enhance sperm production, possibly because of an advanced stage of sexual maturation of the males, as reflected by high Lh plasma levels (~17 ng/ml) before rFsh treatment. In contrast, in autumn, when the Lh circulating levels were much lower (~3 ng/ml), the low doses of rFsh and rLh generated a four-times increase in sperm production, whereas the high doses of the hormones were ineffective. However, treatment with rLh, regardless of the effect of rFsh, improved the motility of spermatozoa during both spring and autumn. These data confirm that consecutive rFsh and rLh treatments increase sperm production and quality in adult sole males, although they seem to be highly sensitive to the rFsh dose. The efficiency of recombinant gonadotropins also appears to be season-dependent despite the asynchronous nature of the sole testis.

The quality of great scallop (Pecten maximus) sperm after thawing

Most publications devoted to the cryopreservation of mollusc sperm have focused on the definition of technical protocols, avoiding the description of sperm quality after thawing. The present study investigated the effects of cryopreservation on sperm quality in the great scallop. Wild scallop were fished during the natural spawning period and conditioned in the hatchery before use. Sperm samples were obtained after intragonadal injection of serotonin and cryopreserved using a previously published protocol.Sperm quality was assessed using a panel of four parameters: sperm motility characteristics, using a computer assisted sperm analysis plugin with Image J, intracellular ATP content using an ATP-Lite kit, sperm integrity, using flow cytometry and sperm morphology, using transmission electron microscopy. For each parameter, fresh (control) and thawed spermatozoa were compared.A significant decrease of both the percentage of motile spermatozoa (reduction: 75%) and sperm swimming speed (86%) were observed for thawed sperm compared with fresh sperm. The percentage of living spermatozoa, as assessed using flow cytometry, was significantly lower for thawed sperm (72.4±2.5%) compared with fresh sperm (86.4±1.1). However, no significant difference of intracellular sperm ATP content was observed between fresh and thawed sperm. Post thawing, while some spermatozoa showed little or no morphological differences compared with fresh sperm, others had undergone drastic changes, including swelling of the plasma membrane, structural alterations of the chromatin and damage to mitochondria.In conclusion, the descriptive parameters studied in the present work showed that the quality of thawed great scallop sperm was lower than that of fresh cells but was still sufficient for use in aquaculture programs and sperm cryobanking for this species.

Endocrine-disruptor molecular responses, occurrence of intersex and gonado-histopathological changes in tilapia species from a tropical freshwater dam (Awba Dam) in Ibadan, Nigeria

In the present study, the occurrence of endocrine disruptive responses in Tilapia species from Awba Dam has been investigated, and compared to a reference site (Modete Dam). The Awba Dam is a recipient of effluents from University of Ibadan (Nigeria) and several other anthropogenic sources. A total of 132 Tilapia species (Sarotherodon malenotheron (n=57 and 32, males and females, respectively) and Tilapia guineensis (n=23 and 20, males and females, respectively)) were collected from June to September 2014. At the reference site, samples of adult male and female S. melanotheron (48 males and 47 females) and T. guineensis (84 males and 27 females) were collected. Gonads were morphologically and histologically examined and gonadosomatic index (GSI) was calculated. Hepatic mRNA transcriptions of vitellogenin (Vtg) and zona radiata protein (Zrp) genes were analyzed using validated RT-qPCR. Significant increase in Vtg and Zrp transcripts were observed in male tilapias from Awba Dam, compared to males from the reference site. In addition, male tilapias from Awba Dam produced significantly higher Vtg and Zrp mRNA, compared to females in June and July. However, at the natural peak spawning period in August and September, females produced, significantly higher Vtg and Zrp mRNA, compared to males. Fish gonads revealed varying incidence of intersex with a striking presence of two (2) pairs of testes and a pair of ovary in S. melanotheron from Awba Dam. The entire fish population examined at Awba Dam showed a high prevalence of intersex (34.8%), involving phenotypic males and females of both species. Analysis of sediment contaminant levels revealed that As, Cd, Pb, Hg and Ni (heavy metals), monobutyltin cation, 4-iso-nonyphenol and PCB congeners (138, 153 and 180) were significantly higher in Awba Dam, compared to the reference site. Principal component analysis (PCA) showed that fish variables were positively correlated with sediment contaminant burden at Awba Dam, indicating that the observed endocrine disruptive responses are associated with contaminant concentrations. Overall, the occurrence of intersex and elevated expressions of Vtg and Zrp in male fish, suggest that the measured contaminants were eliciting severe endocrine disruptive effects in Awba Dam biota, which is an important source of domestic water supply and fisheries for the University of Ibadan community.

Effects of Broodstock Age and Sperm Collection Time over a Natural Spawning Period on Sperm Cryopreservation in Farmed Greenlip Abalone, Haliotis laevigata

ABSTRACTThis study investigated the effects of broodstock age (2‐ and 3‐yr‐old), and sperm collection time at the beginning, middle, and end of a natural spawning period, on the ability of sperm to tolerate cryopreservation in farmed greenlip abalone, Haliotis laevigata. The quality of sperm was assessed by motility, fertilization rate, plasma membrane integrity, mitochondrial membrane potential (MMP), and acrosome integrity (AI). The sperm collected at the middle of a natural spawning period had significantly higher quality than those collected at the beginning and the end of the spawning period in terms of plasma membrane integrity, MMP, and AI of fresh and post‐thaw sperm, post‐thaw sperm motility, and fertilization rate. No significant difference was found between 2‐ and 3‐yr‐old animals in most quality parameters evaluated. The results suggested that sperm collected during the middle of the spawning season should be used for cryopreservation. The efficiency of genetic improvement programs can be further enhanced by using sperm collected from 2‐yr‐old abalone which has a similar ability to tolerate cryopreservation as 3‐yr‐old counterparts.

072 Further improvements in the sea urchin embryo cryopreservation protocol (Paracentrotus lividus)

Previous studies indicated the existence of some amount of variability in the embryonic development success of cryopreserved sea-urchin ( Paracentrotus lividus ) embryos. The present work studied the covariation of cryopreservation results fluctuations and reproductive parameters (condition index, CI and oocytes quality) during the reproductive season, in order to verify the hypothesis of the effect of sea-urchin energy allocated to reproduction on cryopreservation results on variability. Some methodological issues were also addressed, such as: the use of MiliQ water or Artificial Sea Water (ASW) for preparation of cryoprotectant solutions (CPAs); the determination of the optimum density of embryos, and the suitability of two different containers for cryopreservation (2 mL cryovials and 0.25 mL straws) and the possible differences in the cryopreservation outcome. The natural spawning period can be located when the condition Index decreases, at the beginning of summer. There is a positive correlation between the results obtained for the CI calculation among males and females (r = 0.63; p =0.02; n = 13). There was also a positive correlation between female CI and the oocytes weight but, in both cases, no correlation was found between the cryopreservation outcome and either the CI of the female sea urchins or their oocyte weight. Embryos cryopreserved with CPAs prepared with ASW yielded 68 ± 23% (average ± standard deviation) growth after 96 h incubation, but when using CPAs prepared in MiliQ water, embryonic development was impaired and most blastulas were partially destroyed or lysed. Cryopreservation outcome achieved after 96 h incubation was not affected by the embryo density (10000; 5000; 2500; 1250; 500 larvae/vial), and the average larval growth was ca. 50 ± 16% of the control larvae. No significant differences were found when using 0.25 mL straws or 2 mL cryovials, the average growth was 62 ± 12% of the control larvae. In conclusion, the variability in cryopreservation results is not due to seasonality in sea-urchins reproductive condition and might be attributed to experimental variability. Once the reproductive season starts, we only need to check for healthy blastulas previous to cryopreservation, in order to achieve at least 50% larval growth. Finally, although both straws and cryovials may be used indistinctly, it is advisable to use ASW for preparing CPA. Source of funding: E. Paredes was sponsored by the Spanish government through a PhD fellowship (FPU). Conflict of interest: None declared. eparedes@uvigo.es

Reproductive parameters of common carp (Cyprinus carpio L) spawners during natural season and out-of-season spawning

Common carp spawning characteristics were compared between season and out-of-season periods. There were no differences in the percentage of ovulation, spermatozoa motility and embryo survival to the eyed-egg-stage between the spawning periods. Fish spawned during the season produced oocytes of higher total weight than those spawned out-of-season. On the day exogenous feeding began, larvae obtained in out-of-season spawning were shorter than those obtained during the spawning season. However, the out-of-season larvae started feeding two days earlier than in-season larvae. As a result, seven days after fertilization, larvae obtained during out-of-season had the same length as larvae obtained during season. It appears to be possible to obtain common carp larvae half a year before the natural spawning period without decreasing the quality of gametes and larvae.

Aquaculture of the Atlantic Red Porgy

AbstractAquaculture of red porgy Pagrus pagrus (Sparidae) in North America was assessed by the investigation of broodstock conditioning and egg production, larval growth and survivorship, and juvenile grow out. Red porgy broodstock were collected off the coast of North Carolina and held in an outdoor recirculating seawater tank under ambient photoperiod and offshore bottom temperatures. Red porgy broodstock (n = 20) produced up to 300,000 viable eggs/d during their natural spawning period between January and March 2005. Larval survival to 10 d posthatch (dph) was 75.0 ± 2.2% (mean ± SE). Survival declined markedly after 13 dph and was 2.4% by 35 dph, when 1,200 postmetamorphic‐stage juveniles remained. Larvae reached 11.2 ± 1.12 mm and 29.3 ± 0.55 mg at 35 dph, and juveniles reached 55 mm total length (TL) at 90 dph. Juvenile grow‐out trials in recirculating tanks resulted in red porgy reaching 195 ± 0.32 mm TL and 158 ± 0.14 g at 313 dph and a weight‐specific growth rate of 6.8%. The results suggest a lower larval growth rate for western Atlantic red porgy compared with Mediterranean red porgy culture; however, juvenile growth rates were significantly higher than previously reported. Given the high market demand for reef fish species, the red porgy appears to be a good candidate for marine fish culture in North America.

Preparation and Administration of Gonadotropin-Releasing Hormone Agonist (GnRHa) Implants for the Artificial Control of Reproductive Maturation in Captive-Reared Atlantic Bluefin Tuna (Thunnus thynnus thynnus)

Mature migrating Atlantic bluefin tuna (Thunnus thynnus thynnus) were captured in the Mediterranean Sea with a purse seine and reared in floating cages for 2 to 3 years. During the natural spawning period (June–July) of two consecutive years, fish were randomly implanted underwater with a controlled-release delivery system (implant) loaded with gonadotropin-releasing hormone agonist (GnRHa), in order to induce final oocyte maturation (FOM), ovulation/spermiation, and spawning. At the time of sampling, males were significantly larger than females (ANOVA, P < 0.001), having a mean (± SE) fork length and body weight of 190 ± 3 cm and 122 ± 5 kg, compared to 176 ± 3 cm and 94 ± 4 kg of females, respectively. All fish were reproductively mature, with their age ranging between 5 and 12 years and males being a year older, on average. After GnRHa implantation, fish were monitored for spawning and the release of eggs, and were sacrificed at different times after hormone treatment in order to examine the progressive effect of the treatment on gonad maturation. The in vitro GnRHa release from the produced implants was maximal during the first 2 d, with a mean (± SE) release of 525 ± 166 μ g GnRHa implant−1 day−1. The plasma GnRHa profile in vivo reflected the release in vitro, and statistically significant elevations in plasma GnRHa levels were measured until 7 d after treatment (ANOVA, P < 0.01). The underwater implantation procedure was improved between 2004 and 2005, requiring an average (± SD) of 3.1 ± 1.4 min for each fish, and was 64 and 84% successful in 2004 and 2005, respectively. There were no differences between the histological appearance of the testes of GnRHa-treated and control males, and almost all of them contained intra-testicular spermatozoa. However, the proportion of spermiating control males (n = 17) was only 12% compared to 26% for the GnRHa-implanted males (n = 19). Also, there were no differences between controls and GnRHa-implanted fish in sperm concentration, initial spermatozoa motility, or duration of forward motility, which ranged between 29.02–48.54 × 1010 spermatozoa ml−1, 58–63% and 8–9 min, respectively. Final oocyte maturation (FOM) and post-ovulatory follicles (POFs) occurred in 63% and 88%, respectively, of the GnRHa implanted females (n = 16), compared to 0% and 21%, respectively, of the control females (n = 14). In addition, two GnRHa-implanted females in 2005 were found to be ovulated at the time of sacrifice, and their eggs were fertilized in vitro with sperm from spermiating males, which resulted in viable embryos and larvae. Finally, although spawning was not observed, fertilized eggs were collected from the cages. Larvae produced from these eggs were identified as Atlantic bluefin tuna, demonstrating that the present GnRHa implantation method can be used to induce FOM, ovulation/spermiation, and spawning in captive-reared Atlantic bluefin tuna.

Continuous light delays sexual maturation and increases growth of Atlantic cod (Gadus morhua L.) in sea cages

Abstract Prevention of early sexual maturation is essential in Atlantic cod (Gadus morhua) farming because maturity results in reduced growth, affects flesh quality, and may lead to increased mortality. In farmed cod, almost 100% of the fish mature at two years of age and often at a size of 1.5–2 kg. Two pilot experiments were conducted with cod in sea cages at a commercial fish farm in western Norway (60°N) to test the effect of additional continuous light (LL) on the timing of sexual maturation and somatic growth compared with controls under natural light (NL). In the NL groups, 100% maturation was indicated during the natural spawning period from February to April at the age of two years. By contrast, LL treatment from 27 June (15-month-old cod) or 2 September (18-month-old cod) onwards delayed gonad development by three to five months, reduced reproductive investment, and enhanced winter growth compared with the controls. Fish held at NL decreased in body weight during the spawning season (February–April), whereas LL-exposed fish appeared to continue to grow during their spawning season (May–August). LL-treated cod reached mean body weights of 2.90–3.13 kg within 28 months of hatching, whereas the controls reached 2.20–2.42 kg during the same period.

Efficacy of releasing captive reared broodstock into an imperilled wild Atlantic salmon population as a recovery strategy

The strategy of releasing captive reared adult Atlantic salmon Salmo salar into the Magaguadavic River, New Brunswick, Canada, to spawn, was not an effective tool for rebuilding a seriously depressed wild population. The fish were first generation progeny from wild parents, and had spent their entire lives in captivity in either sea or fresh water. No differences in movement or behaviour patterns were observed between freshwater and seawater reared groups. Fish released in the lower river early (35 to 80 days prior to the natural spawning period) moved into a lake low in the system, and most stayed there near the commercial hatchery where they had been reared from egg to smolt. During the spawning season, none moved to the upper river reaches where most spawning habitat exists. Most broodstock released in the upper river reaches near the time of spawning stayed there during the spawning period. The following year few to no Atlantic salmon fry were found, and most appeared not to be offspring of released adults.

Induction of Out-of-Season Spawning of Pikeperch, Sander Lucioperca (L.)

Pikeperch were induced to spawn 3 months prior to the natural spawning period through photothermal and hormonal stimulation. Females (five specimens in each group) were stimulated with injection of human chorionic gonadotropin (hCG) once (200 IU kg−1), twice (200 IU kg−1, second dose after 48 h–400 IU kg−1) or three times (200 IU kg−1, after 24 h–200 IU kg−1 and after another 24 h–200 IU kg−1). The control group was injected once with 0.9% NaCl. The males were stimulated with a single hormone dose of 200 IU kg−1. Eggs were obtained from all the hormonally treated fish. None of the control group females, which were only stimulated photothermally, ovulated any eggs. The time of ovulation was 66–71 h following the first injection, and the eggs viability until the eyed stage (from 71.5 to 77.5%) did not depend on the number of hormone doses (P > 0.05). The out-of-season spawning method described in this paper could be used to provide pikeperch larvae for intensive culture systems (recirculating water systems) before natural spawning season and to produce larger-sized pikeperch fingerlings for stocking.

Movements of non-maturing cultured Atlantic salmon (Salmo salar) in a Canadian river

High numbers of non-maturing, cultured Atlantic salmon that escaped after one winter in sea-cages ascended into fresh water in the Magaguadavic River, New Brunswick. Acoustic telemetry was used to investigate the movements and behaviour of 14 of these fish in the river over the natural spawning period. Four of the fish remained in deep, slow areas within 1 km of the dam, five moved less than 10 km upstream, and five moved no further than 17.5 km. They then remained at these sites for the rest of the study. Thus, all fish failed to reach the known spawning areas located further upstream. It is presumed that the limited movement of the cultured fish was related to their failure to sexually mature. Why these non-maturing salmon entered fresh water remains unanswered, but their residence in the river could have ecological implications.

Effects of aromatase inhibitors on reproduction in male three-spined sticklebacks, Gasterosteus aculeatus, exposed to long and short photoperiods

Three-spined stickleback Gasterosteus aculeatus, males were implanted with Silastic capsules filled with different aromatase inhibitors; 1,4,6-androstatriene-3,17-dione or the non-steroidal CGS16949 A, 4-(5,6,7,8-tetrahydrimidazol [1,5-a]pyridin-5-yl) benzonitrile monohydrochloride or empty capsules. The fish were then exposed to long or short photoperiod. Under the long photoperiod most fish in all treatments displayed a hypertrophied kidney (a secondary sexual character in sticklebacks) and completed, quiescent spermatogenesis, similar as in the natural spawning period. Under the short photoperiod the controls had unstimulated kidneys and an active spermatogenesis, whereas the males implanted with both aromatase inhibitors had stimulated kidneys, though not to the extent as in the long photoperiod, and completed, quiescent spermatogenesis. These findings suggest that aromatization is of importance for the inhibitory effects of short photoperiod on reproduction in the stickleback.

Monthly variation in sperm motility in common carp assessed using computer‐assisted sperm analysis (CASA)

Sperm motility variables from the milt of the common carp Cyprinus carpio were assessed using a computer‐assisted sperm analysis (CASA) system across several months (March‐August 1992) known to encompass the natural spawning period. Two‐year‐old pond‐raised males obtained each month were primed with an exogeneous hormone injection of carp pituitary extract and stripped of milt 18–24h later. The milt was diluted, activated and videotaped using a high‐speed (200 Hz) videocamera and recorder. Videotaped samples were subsequently analysed using the CellTrak/S CASA system (Motion Analysis Corp.) for percent motility, curvilinear and straight‐line velocity. In addition to assessing changes in motility parameters across several months, a comparison was made between two predilution/ activation media combinations (homologous seminal plasma/NaCl+HEPES v. 2‐h incubation in 200 mM KCl+Tris/Tris). The percentage of motile cells assessed immediately after activation was significantly greater in the summer months (May, June, July) when compared to a spring sampling point (March); when assessed 1 min after activation, cells prediluted in seminal plasma maintained a higher percentage of motility than those prediluted in KC1. Curvilinear and straight‐line velocities exhibited a slight seasonal trend; variations in response to the predilution treatments were observed with these measurements. Sperm count gradually increased through April and May (9–63 × 109 to 2–38 × lO10ml– 1 milt), declined in June and July (to 1–83 × lO10ml– 1 milt), and was followed by a steep increase in August (2–74 × 1010 ml– 1 milt). Mean seminal plasma osmolality remained relatively constant (250–265 mOsmol kg – 1) throughout the sampling period.

Influence of photoperiod, frequency of stripping and presence of females on sperm output in turbot, Scophthalmus maximus (L.)

Abstract. The effect of four environmental conditions was investigated upon sperm output in turbot, Scophthalmus maximus (L.), submitted to three different rhythms of stripping. Males kept under a natural light cycle and under a 6-month contracted light programme released a similar sperm output in terms of total volume of semen produced per fish during the experimental period (4·9 ± 0·9ml), mean sperm concentration (29·4 ± 2·8 × 109 spermatozoa/ml) and total sperm number (163·2 ± 40·5 × 109 spermatozoa). Attempts to stimulate spermiation for a second time just after the end of the natural reproduction period resulted in the release of low sperm output (total volume of semen: 1·6 ± 0·4 ml; mean sperm motility: 2 min 36s ± 0 min 47s; mean sperm concentration: 47·6 ± 10·2 × 109 spermatozoa/ml; total sperm number: 84·5 ± 25·3 × 109 spermatozoa). Stripping frequency had no effect on total volume of semen, mean sperm motility and total sperm number. Monthly collection did not modify sperm samples in relation to stripping rank. However, decreasing volume, motility and sperm concentration were observed when males were stripped fortnightly and weekly. During the natural spawning period, the presence of females in the tank enhanced mean sperm motility (from 3 min 27s + 0 min 52s to 6 min 38s ± 1 min 38s).

Phenotypic analysis in the Chilean oyster, Ostrea chilensis Philippi 1845: relationship between juvenile and adult growth rates

. The Chilean oyster, Ostrea chilensis Philippi, shows high phenotypic variability for growth rate within populations. In order to determine if the serial correlation of growth rate in oysters is positive, the correlation between shell height and live weight at different stages of the life cycle in a cohort of the Chilean oyster was estimated. Four size groups of individually tagged oysters selected on the basis of their shell heights were examined. The relationships for juvenile and adult oysters were found to be positively correlated throughout the study period. The correlation coefficients ranged from r = 0.28 to r = 0.83. For shell height the coefficient of determination for market size oysters was higher in 2-year-old (r2=0.88) than in 1-year-old oysters (r2=0.09). Shell height and live weight were more highly correlated at later stages of the life cycle. The results indicate that 2-year-old oysters have a size that is indicative of performance to market size and hence is an appropriate age to select the parents to be used in breeding programmes to improve the growth rate in O. chilensis. A high mortality was detected after the natural spawning period. A physiological stress hypothesis is proposed to explain the high summer mortality in O. chilensis in the Quempillen estuary, southern Chile.

Oocyte Development and Induced Spawning of Smallmouth Bass (Micropterus dolomieui) in Response to Photoperiod and Temperature Manipulation: Preliminary Results

AbstractSmallmouth bass (Micropierus dolomieui) in Quebec spawn between the end of May and mid‐June. Wild broodstock (4 to 8 years old) and laboratory‐reared ones (0+ years old) reared in a water recycling system were submitted to different temperature and photoperiod regimes to extend their natural spawning period. Oocyte maturation followed in vivo showed that oocytes developed to stage III in both cases but final maturation and ovulation was limited to the laboratory‐reared bass and this, a month before the normal spawning period, at 20.5 cm TL. Exogenous factors are examined, specifically duration of long photoperiod and vital space related to non‐spawning of wild broodstock.