Early out-of-season induced spawning of channel catfish Ictalurus punctatus (Rafinesque) conditioned in heated earthen ponds

This study documents early out-of-season induced spawning of channel catfish Ictalurus punctatus. During the early spring (February–April) of 1999, 2000 and 2001, ponds containing (1) male and female channel catfish (mixed-sex ponds) or (2) male channel and blue catfish I. furcatus only, or female channel catfish only (single-sex ponds) were heated to 24–30°C to encourage gonadal maturation and spawning. Unheated ponds were stocked with males and females and were monitored during the duration of heating. When natural spawning occurred in the heated ponds, the fish were captured by seining and unspawned females were injected with 100 μg kg⁻¹ of synthetic leutenizing hormone-releasing hormone. Injected females were either paired with males or held in communal all-female groups, and monitored for ovulation. Eggs were collected and fertilized with sperm of channel catfish or blue catfish. Females paired with males were induced to spawn 44 days (mixed-sex ponds) and 50 days (single-sex ponds) before natural spawning occurred in unheated ponds. Spawning latency (the time between injection and ovulation) and the percentage of neurulated embryos from eggs fertilized using channel catfish sperm was not different between spawning before the natural season (P=0.68) and during the natural season in fish from mixed-sex ponds (P=0.57). Females held in all-female groups produced eggs 34 days before the onset of spawning in unheated ponds. Spawning latency was not different between spawns before and during the natural season (P=0.16), and the percentages of neurulated embryos from eggs fertilized with channel catfish sperm (P=0.76) or blue catfish sperm (P=0.77) before or during the natural season were not different. This study demonstrates the feasibility of conditioning of channel catfish females for early out-of-season induced spawning in the laboratory.     

Voluntary Spawning of Captive Nassau Groupers Epinephelus striatus in a Concrete Raceway

Nassau groupers Epinephelus striatus held in a 37-m³ concrete raceway were conditioned to spawn 3 mo later than in their home waters. After being held in the raceway for 15 mo, four female and two male groupers spawned voluntarily over a 4-d period in March 1994, producing 10.3 million eggs. By December 1994, two females had been removed. During March 1995, one of the remaining two females spawned on three consecutive days (60 cm TL, 1,172,000 eggs) and the other only on the third day (47 cm TL, 488,000 eggs). In April 1995, three females (including one held in isolation for 6 mo, then replaced 6 d before the first spawn) produced more than 9.8 million eggs in 4 d. The isolated female spawned 8 d after being placed in a cage in the raceway and 4 d after release in the raceway.
Individual females spawned as many as nine times a day for 1–4 d. Spawning occurred from 3.5 d before until 8.5 d after the full moon. A 30-d interval elapsed between March and April 1995 spawning periods. Fertilization was in the range 83–100% and hatching 90–100%. Spawns occurred in a temperature range of 23.1–27.9 C; however, based on spawning frequency and volume and on egg development, 24–27 C seems most suitable. These results and other evidence support the view that Nassau groupers can be conditioned to spawn any month of the year, mainly by manipulating temperature.     

Effect of dietary protein level on the reproductive performance of Nile tilapia, Oreochromis niloticus (L.)

The effects of five dietary protein levels (25%, 30%, 35%, 40% and 45%) on the individual spawning frequency and the egg production of 135 tagged female Nile tilapia, Oreochromis niloticus (L.), were studied from 8 May to 19 November 1996 in outdoor concrete tanks. Virtually identical spawning patterns were found in all treatments, but there was a great deal of variation in the frequency of spawning and egg production. Overall, individual spawning frequency varied from one to 14 and individual egg production from 31 to 2828 eggs per spawning. The average number of spawnings and average number of eggs per spawning for fish receiving dietary protein levels of 25%, 30%, 35%, 40% and 45% were 8.0 ±1.6, 8.4 ± 2.2, 8.5 ± 2.7, 8.4 ± 2.5 and 9.4 ± 2.1 spawnings, and 1167 ± 305, 1082 ± 410, 1288 ± 324, 1145 ± 389 and 1328 ± 311 eggs per spawning, respectively. Fish receiving 45% dietary protein spawned more frequently than fish receiving 25% dietary protein. The total number of eggs produced per female was significantly higher for females fed 45% protein feed than females receiving 25% and 30% protein feeds. No definite trend was found in the number of eggs produced per spawning and the number of eggs produced per gram in females fed at different protein levels. Based on weekly checking, the time interval between spawnings varied from 7 to 77 days, and mean spawning intervals ranged from 15.8 to 17.1 days. Sixty per cent of females spawned after an interval 14 days, 15% after 21 days, 13.6% after 7 days, 7.2% after 28 days, 1.8% after 35 days and 1.0% after 42 days, and the time interval was 49-77 days for the rest of the females (1.5%). In all treatments, maximum spawning activity was recorded from May to August, and thereafter, it gradually decreased and no spawning females were found in November.     

Development and Evaluation of an Ultrasound Imaging Reproductive Index Based on the Ovarian Cycle of Channel Catfish,Ictalurus punctatus

Commercial production of hybrid catfish (female channel catfish Ictalurus punctatus × male blue catfish Ictalurus furcatus) is reliant on interdependent biological, environmental, and technical procedures. This study addresses one of these critical components – the evaluation of channel catfish ovarian development based on sonographic analysis. The objectives were to: (1) develop a channel catfish ovarian ultrasonography index, (2) test the effect of the spawning trials on fertilization estimates of fish assessed using the ultrasonography index, and (3) evaluate the expected (hypothesized) and observed outcome of the ultrasonography assessments. Seven ovarian morphology classifications were developed based on ultrasonography of 915 channel catfish (N = 915 images), and 210 females were selected for spawning. The predictions based on the classifications prior to hormone injection showed a significant effect (P < 0.002) on the observed outcomes (viable or nonviable eggs). The probability of correct classification was 0.86–0.89 for Categories 3 (developing), 4 (advanced), and 5 (mature), and 0.93–1.0 for Categories 1 (undeveloped), 2 (underdeveloped), 6 (spawned), and 7 (atretic). The ultrasonography index covered the full range of ovarian dynamics (i.e., recrudescence through spawning). It provided an unobtrusive, direct method of ovarian assessment to work toward improving the efficiency of broodstock selection.     

Reproduction of striped bass, Morone saxatilis (Walbaum), broodstock: monitoring maturation and hormonal induction of spawning

Abstract Levels of gonadal steroid hormones were quantified in an adult striped bass, Morone saxatilis (Walbaum), broodstock during their gametogenic cycle. Blood plasma concentrations of Estradiol (E₂) and testosterone in females, or 11-ketotestosterone(11-KT) and testosterone (T) in males, were used as indicators of maturation. In both sexes, hormone levels were low in summer but increased significantly by late October to intermediate levels which were then maintained until late January. They then increased again rapidly to maximum pre-spawning values attained in late February or March, and subsequently decreased during the spawning period (April and May) with an increased incidence of spent fish with low hormone levels. The changes in blood hormone concentrations coincided with annual changes in photoperiod and water temperature that may be useful landmarks for maturation in captive broodstock. Mature females were implanted with pellets containing a dose of approximately 20 μg/kg body weight of [D-Ala⁶-Pro⁹-Net]-LHRH (GnRHa) in a matrix of cholesterol (CH) and cellulose. In April, they had not yet begun final oocyte maturation (FOM) and were too immature for conventional induction of spawning by injection with human chorionic gonadotropin (hCG). In early April, females given two 95% CH (slow hormone-release) GnRHa pellets (95/95) or females given one 80% CH (fast hormone-release) GnRHa pellet and one 95% CH GnRHa pellet (80/95) spawned within 13 days treatment (n= 4) with good egg fertility (76 ± 7% of total) and hatch rates (62 ± 15% of fertile). Females given dual fast-release GnRHa pellets (80/80) or control (Sham) pellets did not spawn or show evidence of increased oocyte diameter or development. In late April, four of six females given the 80/95 GnRHa pellet combination spawned within 9 days. Three fish produced fertile eggs (54 ± 18%). one spawned overripe eggs, and the remaining two increased oocyte diameter and maturation. Three corresponding controls did not spawn, and two of these showed clear signs of atresia within 11 days. In early May, some females were undergoing early FOM and were mature enough to be spawned by hCG injection. Three were given a single 80% CH GnRHa pellet and spawned within 6 days of treatment to produce fertile eggs (44 ± 6%). Of two other females given dual 80% CH GnRHa pellets, one spawned infertile eggs and the other failed to spawn within 9 days. GnRHa implants show promise as a technique for inducing spawning of captive striped bass broodstock although the optimum hormone delivery systems, dosages and release rates should be verified for fish at specific maturational stages.     

Improving spawning synchrony in the Nile tilapia, Oreochromis niloticus (L.)

Abstract. The development stage and number of seed (eggs and fry) removed from individual incubating Oreochromis niloticus (L.) broodfish at 10-day interharvest intervals (IHI) was compared with the harvest of free-swimming fry from undisturbed fish held in a tank within the same system. Spawning synchrony and productivity were compared for (1) total exchange of females for ‘conditioned’ females after each harvest, (2) exchange of only spawned females, and (3) return of females back into the spawning tank. Improved synchrony and seed production were positively related to removal of eggs and fry from incubating females and could have important implications for broodstock management in commercial hatcheries. Allowing fish to remain undisturbed in clear water tanks may lead to inhibition of spawning activity related to the formation of hierarchies. Individual weight loss by females during incubation was correlated with stage of seed harvested and thus with timing of spawning during the interharvest interval.     

Effects of different feeding regimes on ovarian maturation and spawning of pond-reared giant tiger prawn in Thailand

Large pond-reared Penaeus monodon were used in three maturation and spawning trials lasting 60 days. One group of prawns was fed a fresh diet only; a second group was supplied a fresh diet combined with a formulated pelleted diet; a third group of prawns received the formulated diet only. Total maturation (N = 66 and 55) and spawning events (N = 63 and 48) were much greater for prawns receiving only the fresh diet, and the combined fresh with formulated diets, respectively. Prawns receiving only the formulated diet matured only 12 times, and spawned 11 times. Prawns fed a fresh diet produced significantly more eggs than those given a formulated diet alone. No significant differences were seen in eggs produced per spawning event, % fertilization, % hatch or % metamorphosis from egg to protozoea. Female prawns exhibited greater mortality than did males, while females on the fresh diet only had greater mortality than did other females, presumably due to greater handling and other stress factors. Female prawns on a combined diet had lower mortality than other females. The formulated diet used is suitable for supplementation, but not as a sole maturation food.     

Spawning of injured compared to uninjured female Atlantic salmon, Salmo salar L.

Abstract. Observations of spawning by uninjured and injured Atlantic salmon, Salmo salar L., indicated that uninjured fish usually spawned in the outlets, or inlets, to pools and on coarse bottom sediments, whereas injured females spawned in different places. Frequently, the spawning sites lay in very shallow water with a slow current and on a bed of sand or fine gravel. Subsequent excavation of the spawning redds revealed that about 28% of the eggs laid by uninjured females were recovered, compared with only about 8% of those laid by injured females. This difference was significant (P < 0·01).     

Captive Breeding of Endangered Ompok pabda with Ovatide

The Pabdah catfish, Ompok pabda, was successfully spawned with the synthetic hormone Ovatide at two different doses (0.4 and 0.6 ml/kg of body weight of female). The latency period was 9–10 h at 0.6 ml compared to 10–12 h at 0.4 ml kg^?1 of body weight. All the females spawned successfully. Average rate of fertilization (75.5 %) and hatching (60.5%) was higher with a dose of 0.6 ml kg^?1, while lower fertilization (65.1%) and hatching rate (45%) were observed at 0.3 ml kg^?1 of body weight. Fecundity ranged from 80–130 eggs gm^?1 body weight of female. Eggs hatched between 12–14 h after spawning at a water temperature 28°–32°C. The mean egg diameter was 0.95 ± 0.15 mm.     

Induction of Spawning in the Sea Cucumber Holothuria scabra (Echinodermata: Holothuroidea)

With the advent of sea cucumber aquaculture in the South Pacific region, a reliable method is needed to induce large numbers of animals to spawn in captivity. Broodstock of the sea cucumber Holothuria scabra , collected from Stradbroke Island, Moreton Bay (27°90'N, 153°24'E) Australia, during the reproductive season from October to January, were used in spawning trials. During the 1997–1998 summer between one to five weeks of captivity, 100% of animals were induced to spawn in four trials at dusk on or close to a new or full moon, using nine males and nine females contained in a Reln tank and 30 cm of filtered sea water, using a 3–5 C temperature shock. H. scabra was induced to spawn in small numbers during the 1996–1997 summer despite a marked degree of weight loss, and all animals spawned during 1997–1998 with minimal loss of weight. The difference in the number of spawned eggs between animals of similar size and mean numbers of spawned eggs in consecutive trials decreased the longer animals were held in captivity before spawning. The hatch rate of eggs was reduced significantly for broodstock held for more than one month. Hatch rate and numbers of spawned eggs are important indicators of egg viability of broodstock maintained in captivity for an extended period.     

Progress in Controlled Maturation and Spawning of Summer Flounder Paralichthys dentatus Broodstock

Abstract— Wild-caught, adult summer flounder Paralichthys dentatus (avg. wt. = 740 g; range = 264–1,540 g; N = 60), collected in northeastern US coastal waters during October 1994, were transported to Vero Beach, Florida in March 1995 and held in 2.6-m³ indoor tanks through November 1995 under two artificial photothermal regimes: (1) natural regime, simulating natural habitat conditions; and (2) accelerated thermal regime, with seasonal temperature changes advanced by one month. A third group of fish was held in outdoor tanks under ambient photothermal conditions. Under all photothermal conditions, onset of vitellogenesis was associated with declining daylength and temperature, beginning in the accelerated group, then progressing to the natural and the ambient groups. From 20 September to 28 November 1995, 23 vitellogenic stage females from the accelerated and natural regimes were implanted with a cholesterol-cellulose pellet containing LHRH-a (100 μg/kg body wt). Females with initial mean oocyte diameters ranging from 258–456 μm spawned voluntarily 2.5–5.5 d postimplantation, while no maturational response was obtained from females with mean diameters ranging from 165–231 μm. Two females were spawned twice during the study period by LHRH-a pellet implantation. Infrequent, natural spawning without hormone intervention was also obtained. Females released from 22.7–396.9 × 10³ eggs on the first day of spawning, with fertilization and hatching rates of 0–93.470 and 0–81.1%, respectively.     

The Use of Dairy Protocols for Sperm Cryopreservation of Blue Catfish Ictalurus furcatus

The commercial‐scale production of fish by use of artificial (induced) spawning would require reliable, large‐volume sources of sperm. Cryopreservation can be used to preserve and store sperm within commercial and research germplasm repositories, but is limited in its application to aquaculture. Straw volume and cooling chamber size restrict the quantity of sperm that can be frozen, and straws must be filled by hand. In contrast, the dairy industry has refined methods for freezing of bull sperm, including automation of straw filling and the use of large cooling chambers. These methods could be used for commercial‐scale cryopreservation of fish sperm, although application would require testing. To supply sperm in large volumes, bags originally developed for swine semen could be cooled using dairy protocols and used as a container for fish sperm. The current study documented the use of commercial‐scale dairy cryopreservation techniques for the production of hybrids of channel catfish Ictalurus punctatus (female) by blue catfish Ictalurus furcarus. Four cryoprotectants (methanol, dimethyl sulfoxide, dimethyl acetamide, and glycerol) were initially evaluated for use with blue catfish sperm. During May 2000 and March to April 2001, suspensions of blue catfish sperm were cryopreserved with 10% methanol in 0.5‐mL French straws and in commercial swine semen bags (Cochette^* bags, IMV International. Minneapolis, Minnesota, USA). Cryopreservation took place at a dairy breeding cooperative, using technology employed for bull semen. Sperm motility before freezing was 26 ± 18% during Year 1 (2000) and 62 ± 30% during 2001. Sperm were thawed at 40 C and used to fertilize the eggs of channel catfish (yielding hybrids). Motility after thawing for sperm frozen in 0.5‐mL straws was 11 ± 10% during 2000 and 50 ± 24% during 2001. Motility after thawing was 41 ± 17% for sperm frozen in swine semen bags in 5‐mL aliquots and 43 ± 10% for sperm frozen in 10‐mL aliquots. Neurulation of eggs fertilized with thawed sperm from straws was 83 ± 13% during 2000 and 54 ± 27% during 2001. Neurulation was 57 ± 24% using sperm frozen in swine semen bags in 5‐mL aliquots and 55 ± 10% using sperm frozen in 10‐mL aliquots. There was no correlation between sperm motility before freezing (in 0.5‐mL straws) and after thawing during 2000 (r= 0.52) or during 2001 (r= 0.49). In addition, there was no correlation between initial motility and neurulation of channel catfish eggs fertilized using thawed sperm during 2000 (r= 0.14) or during 2001 (r= 0.29). Sperm of blue catfish can thus be cryopreserved at a commercial scale using dairy protocols and can be made available for the production of hybrid catfish when viable eggs are available.     

Reproductive Performance of Pigmented and Albino Female Channel Catfish Induced to Spawn with HCG or Ovaprim

Reproductive performance of albino and pigmented channel catfish ( Ictalurus punctarus ) females was compared when spawning was induced with HCG (human chorionic gonadotropin) or Ovaprim ³ (domperidone and [d-Ala⁶, Pro⁹ NEt]-luteinizing hormone-releasing hormone). In 1985, only HCG was used to induce spawning. Albino females required more injections, had a longer latency period from time of first injection to spawning, and had lower spawning success (30%) than pigmented females (77%). No differences were observed between albino and pigmented females for time-of-day of spawning, number of eggs obtained by manual stripping, and number of eggs/kg body weight. In 1988, HCG, Ovaprim, and saline (control) were used to induce spawning. HCG induced spawning in 80% of albino and 71% of pigmented females. Ovaprim induced 67% of pigmented females to spawn, but only 2 of 5 albino females spawned. One of four pigmented females injected with saline spawned. Albino females yielded 40–50% fewer eggs/kg body weight than pigmented females. Other performance characteristics for albino and pigmented females were similar whether injected with HCG or Ovaprim. High temperatures encountered during the 1985 spawning season my have had a differential effect on albino and pigmented fish and may account for some differences in spawning success.     

Growth and maturation of Pacific saury Cololabis saira under laboratory conditions

This study details the growth and maturation processes of Pacific saury Cololabis saira from eggs to first spawning under laboratory conditions. They were reared at 20°C, and fed almost to satiation every day. There was no significant difference in the knob length (KnL) between males and females, and so data for both sexes were combined in the following Gompertz growth formula: KnL _t  = 277.1 exp[?exp(?0.015 (t ? 83.8))]. The first spawning was observed 243 days after hatching (DAH). At the beginning of spawning, when the mean KnL was about 250 mm, only several hundred eggs were spawned at most. The spawning continued, and the number of spawned eggs increased notably after 260 DAH. Correlation and stepwise multiple regression analysis of the gonad somatic index (GSI) of Pacific saury versus KnL, CF, and DAH revealed that only DAH (R = 0.88 and 0.72 for males and females, respectively) was significantly correlated with GSI of Pacific saury (P < 0.001). This result suggests that DAH is one of the most influential factors for maturation in this species.     

Effect of water temperature on individual reproductive activity of pollack (Pollachius pollachius)

The effects of water temperature on pollack (Pollachius pollachius) spawning features were individually assessed. During the spawning period, nine fish sets, each composed of one female and two males (mean body weight: 2.5±1.0 kg), were placed in small-volume tanks (2 m³). Fish sets were held at 8, 10, or 12 °C (n=3 for each temperature).Three females released eggs at 8 and 10 °C, while only two spawned at 12 °C. One hundred thirty-one individual spawns were collected from the eight spawning fish sets. The number of spawns collected per female was significantly lower at 12 °C (3.0±3.6), compared to 10 °C (17.3±10.1) and 8 °C (23.3±2.5). Egg number (eggs kg⁻¹ BW) was significantly lowered at 12 °C (26,068±35,989) compared to 10 °C (323,230±136,796) and 8 °C (599,612±249,545). The number of viable eggs (eggs kg⁻¹ BW) was significantly lower at 12 °C (4,175±7,167) compared to 8 °C (192,034±145,870).When incubated at a common temperature of 10 °C, hatching and malformation percentages were significantly enhanced for breeders maintained at 8 °C (respectively, 35.8±8.5–7.3±1.4%) compared to 10 °C (9.8±8.0–2.3±1.8%). Then, pooling data were recorded at the three temperatures; a significant decrease of egg diameter with time was observed.The individual reproductive activity of pollack experienced by breeders was deeply affected by temperature during the spawning period. A temperature of 12 °C is suggested to be close to the temperature reproduction upper limit in this species.     

Production of triploid eastern little tuna,Euthynnus affinis (Cantor, 1849)

Herein, we developed a triploidization method using spawned eggs collected immediately after spawning of eastern little tuna (ELT), Euthynnus affinis. ELT broodstock induced the spawning by hormonal treatment in the tank, with the resulting spawned eggs being used for the triploidy induction. Under optimal conditions, the mean ± SEM triploidization and hatching rates were 97.2 ± 2.8% and 84.5 ± 10.3%, respectively. Although triploid ELT showed growth performance equivalent to that of diploids, the triploids died at a higher rate than the diploids during 2–4 weeks post‐hatching when triploids and diploids were reared in the same tank. Therefore, we propose that it would be necessary in a practical operation to use triploid‐only ELT seedlings to avoid selective cannibalism by the diploids. The ELT triploids exhibited an all‐female phenotype. Because previous studies have reported that female triploids show a greater probability of sterility than male triploids, this characteristic could be a major advantage. Since this triploidization method, using spawned eggs, can be performed without handling the broodstock, it is possible to avoid the physical damage caused by the process of artificial insemination, making it possible to repeatedly produce triploid populations without valuable broodstock loss. Thus, we have developed an efficient method to produce ELT triploids, although further study is essential to evaluate sterility of the triploid ELT.     

Evaluation of Dual Injection of LHRHa and the Dopamine Receptor Antagonist Pimozide in Cage Spawning of Channel Catfish Ictalurus punctatus

Abstract.— Control of reproduction in cultured fishes has been improved by the use of gonadotropin releasing hormones (GnRH's) and, in many fishes, the use of dopamine (DA) receptor antagonists. However, little successful use of these treatments has been reported for channel catfish Ictalurus punctatus . This study was conducted to investigate the potential of treatment with a gonadotropin hormone releasing hormone (GnRH) and a dopamine (DA) receptor antagonist to improve cage spawning success in channel catfish. Two and 3-yr-old catfish were selected from ponds. All males were injected with des-Gly¹⁰, [D-Ala⁶]-Leuteinizing hormone releasing hormone ethylamide (LHRHa, 50 μg/kg body weight). Females were randomly assigned to the various injection treatments, control (vehicle injected, C), LHRHa only (L), DA receptor antagonist (pimozide) only (P), and LHRHa and pimozide (LP). After injections, one male and one female were paired and placed into 1 m wide × 1.5 m long × 1.2 m deep spawning cages with a metal spawning can. Spawning cans were checked from 1 to 10 d after injection and successful spawning events were recorded. Five trials were conducted between 13 May and 5 June 1998. The LP treatment yielded significantly more spawns than any other treatment (LP, N = 24, 83% > L, N = 25, 44% > C, N = 23, 43 % > P, N = 11, 36%). The greatest promotion of spawning with LP treatment was in the first two induction trials. In the subsequent three trials, differences between treatments were not significant, although LP tre.ated females tended to spawn more frequently. Percent hatch (60.5 ± 3.4%) did not differ between treatments. Treatment of females with LHRHa + pimozide is a potentially useful method for increasing spawning success, particularly early in the spawning season.     

The relationship between channel catfish female body weight and relative fecundity and fry production when induced to ovulate with carp pituitary extract and fertilized with blue catfish sperm

ABSTRACT

Little is known regarding the correlation between channel catfish female body weight and relative fecundity and fry production when the females are induced to ovulate with carp pituitary extract and fertilized with blue catfish sperm. A total of 145 female channel catfish were used in the early, middle, and late spawning season. Female body weight of channel catfish had negative correlations with relative fecundity (r = ?0.33 and ?0.25, P < .05; r = ?0.23, P > .05) and before the late spawning season with fry/kg (r = ?0.21, P > .05; r = ?0.30, P < .05). Eggs/kg female body weight and fry/kg female body weight followed a parallel pattern throughout the season, illustrating the logical relationship between relative fecundity and fry output. The size of females used is a consideration to maximize hybrid catfish embryo production during the early and middle spawning season but not late in the spawning season. However, these correlations are relatively small even though significant; thus a dramatic change in fry production would not be predicted. Since reproductive relationships can change during the spawning season, conclusions and recommendations could be flawed if data are not collected periodically during an entire spawning season.     

Multiple spawning of amphidromous type ayu <Emphasis Type="Italic">Plecoglossus altivelis</Emphasis> in a large river,Mogami River System

Female ayu Plecoglossus altivelis were sampled from the Mogami River System, Yamagata Prefecture to investigate the occurrence of multiple spawning that has previously been reported in small rivers. Spawning frequencies of the females were estimated by histological observations of the ovary mainly from the degree of degeneration of postovulatory follicles. In the middle and late part of the spawning period, most females showed characteristics of having spawned two or three times, indicating that multiple spawning is a common phenomenon also in a large river. Mean relative batch fecundity (RBF) and condition factor (without gonads) decreased with the number of spawning batches. Size dependency in the occurrence of the multiple spawning and the RBF values was not observed. These results are similar to those reported in previous studies of ayu from a small river near the Mogami River System, and indicate that the occurrence and properties of multiple spawning are almost the same regardless of the river scale. However, it is possible that the ratios of multiply spawned females may differ due to variations in survival rates during the spawning season.     

Artificial Spawning of Tilapia Eggs1

A stripspawning methodology was evaluated for tilapia (oreochromid) species. This technique achieved an average hatching success of 68.6 ± 3.6% (N= 31). Female fecundity and spawning frequency were dependant on both genetical and husbandry factors. Egg yields for Oreochromis niloticus, O. mossambicus, and O. niloticus±O. mossambicus hybrids averaged 4.54, 10.86 and 10.36 eggs/g female/spawn, respectively. Female broodstock that were adapted to an intensive spawning regime exhibited a significant increase in fecundity. Additionally, egg survival was not affected by hydration for up to 15 minutes prior to fertilization. Results suggest that the strip spawning of tilapia species may be an efficient method of providing viable gametes for hatchery purposes.