Morpho-physiological predictors of ovulatory success in white sturgeon,Acipenser transmontanus Richardson

Body length, oocyte diameter, germinal vesicle position, in vitro oocyte maturation response to progesterone, and plasma concentrations of progesterone, cortocosteroids, and 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-diOHP) were examined in white sturgeon females prior to their spawning induction and correlated with their subsequent ovulatory response. The relationship between broodfish size and ovulatory success was insignificant. Responsive females had larger oocytes and elevated plasma concentrations of corticosteroids and 17α,20β-diOHP, but did not differ significantly from nonresponsive females in progesterone concentrations. Germinal vesicle position and in vitro oocyte maturation response exhibited the closest relationships with ovulation, and can be used as practical predictors of ovulation for hormonally induced spawning of white sturgeon.     

Photoperiodic manipulations of the reproductive cycle of sea bass (Dicentrarchus labrax) and their effects on gonadal development, and plasma 17ß-estradiol and vitellogenin levels

The annual profile of plasma vitellogenin (VTG) and 17ß-estradiol (E₂) levels, as well as gonadal development and spawning characteristics were investigated in captive female sea bass (Dicentrarchus labrax). Endocrine and gonadal changes were studied in fish reared under natural conditions or exposed to manipulated photothermal cycles. In natural conditions of photoperiod and temperature, sea bass spawned from February through March (East coast of Spain, 40°N 0°E). One or two months of constant long-days (15L/9D) in a constant short-day photoperiod regime (9L/15D) all-year-round, given early in the year (March and March–April), advanced spawning by 3 months. The same treatment applied later in the year (September–October) delayed spawning by 1 month, compared to controls.In all groups, changes in plasma VTG levels were correlated with E₂ levels, oocyte growth and spawning time. In control females, VTG was low (<100 ng ml^-1) during the summer, until its first surge in plasma 4 months before the beginning of spawning. The VTG (3.1 ± 0.3 mg ml^-1) and E₂ (4.1 ± 0.5 ng ml^-1) levels showed a single annual peak during late vitellogenesis, the time of the highest proportion of vitellogenic oocytes in the ovary. Constant high levels of VTG (1–1.4 mg ml^-1) and E₂ (1.6–1.9ng ml^-1) were maintained during the entire spawning time, together with the presence of vitellogenic oocytes, suggesting the existence of several waves of oocyte growth in the ovary and thus, several spawns per female. Endocrine profiles and oocyte development in fish exposed to constant photoperiods were similar to controls, but were shifted in time in relation to the displacement of the spawning time. In the fish showing advanced spawns, the duration of the gametogenic proces was compressed when compared to controls. The differences observed in the evolution of the reproductive-related factors in the advanced groups, which were exposed to a reduction in temperature to 15°C, suggest an influence of the temperature in the early stages of the reproductive cycle in sea bass.     

Molecular endocrinology of oocyte growth and maturation in fish

Pituitary gonadotropins (GTHs) are of primary importance in triggering oocyte growth and maturation. However, the actions of GTHs are not direct, but are mediated by the ovarian production of steroidal mediators of oocyte growth (estradiol-17β) and maturation (maturation-inducing hormone, MIH; 17α,20β-dihydroxy-4-pregnen-3-one, 17α,20β-DP in salmonid fishes; 17α,20β,21-trihydroxy-4-pregnen-3-one, 20β-S in sciaenid fishes). It is established that production of estradiol-17β and 17α,20β-DP by salmonid ovarian follicles occurs via the interaction of two cell layers, the thecal and granulosa cell layers (two-cell type model). A distinct shift in the salmonid steroidogenesis from estradiol-17β to 17α,20β-DP occurs in the ovarian follicle layer immediately prior to oocyte maturation. It is possible that this shift is a consequence of dramatic changes in the expression of the genes encoding various steroidogenic enzymes. As an initial step to address this question, we have isolated and characterized the cDNAs encoding a number of ovarian steroidogenic enzymes including the rainbow trout cholesterol side-chain cleavage cytochrome P-450, 3β-hydroxysteroid dehydrogenase (HSD), 17α-hydroxylase/17,20 lyase cytochrome P-450, aromatase cytochrome P-450 cDNAS as well as the pig 20β-HSD cDNA. Estradiol-17β stimulates the hepatic synthesis and secretion of a yolk precursor, vitellogenin. Vitellogenin is then transported to the ovary where it is selectively taken up into the oocyte by a receptor-mediated process involving specific cell-surface receptors. Estradiol-17β was also shown to induce the synthesis of egg membrane proteins in the liver. The maturation-inducing action of 17α,20β-DP and 20β-S is through the binding to the oocyte plasma membrane. This initial MIH-surface interaction is followed by the formation of the major mediator of MIH, maturation-promoting factor (MPF). We have purified MPF from mature oocytes of carp. Carp MPF consists of two components: the homolog of the cdc2⁺ gene product of fission yeast (p34^cdc2) and cyclin B. The cdc2 kinase protein is present in immature oocytes as well as in oocytes induced to mature by 17α,20β-DP treatment, while cyclin B proteins can be detected only in mature oocytes. Addition of bacterially expressed goldfish cyclin B to the extracts of immature goldfish oocytes induced MPF activation. These results suggest that the appearance of cyclin B protein is a crucial step for 17α,20β-DP-induced oocyte maturation in fish.     

Reproductive cycle of the Amazonian planktivorous catfish Hypophthalmus marginatus (Siluriformes,Pimelodidae)

We evaluated the annual process of oocyte and ovarian development of the planktivorous Amazonian catfish, the mapará (Hypophthalmus marginatus), aiming to support captive reproduction for domestication. A total of 149 females were captured from the Tocantins River at the Tucuruí Dam (3°49'55"S, 49°39'9"W) in 13 sampling events. For each individual, we evaluated ovary histology and plasma steroid concentrations. Two annual reproductive cycles, with similar characteristics, including a long period of rest, a short vitellogenic stage and a well‐defined spawning season from November to March, were described. A 17β‐estradiol (E₂) rise and a 17α,20β‐dihydroxy‐4‐pregnen‐3‐one (DHP) peak were associated with vitellogenesis and spawning season, respectively, in the first cycle but not in the second reproductive cycle. In conclusion, the studied species presents a reproductive cycle similar to that of other migratory total spawners; however, unusually for this group of fish, there were three (but not two) batches of vitellogenic oocytes in “in maturation” and “mature” stage ovaries, pointing to the possibility of more than one spawn during the spawning season.     

Influence of photoperiod and temperature manipulation on gonadal development and spawning in Caspian roach (Rutilus rutilus caspicus): Implications for artificial propagation

Caspian roach (Rutilus rutilus caspicus), a spring spawning teleost, were subjected to various photoperiod and temperature regimes to study the feasibility of shifting the timing of spawning for artificial propagation purposes. A total of 650 female reproductively mature R. rutilus caspicus were subjected to different photoperiod and temperature regimes including four light regimes (natural light (NL), 16 hr of light (L):8 hr of darkness (D), 9L:15D, 11L:13D), each affected by three temperature regimes (14, 20 and 24°C) for 70 days. Five fish per tank were randomly sampled on Feb. 10, Feb. 20, March 28, April 15 and April 30 (natural spawning time). Ovarian tissue sections were studied using light microscope and transmission electron microscope (TEM). The levels of 17‐β estradiol (E2) and 17α‐hydroxyprogesterone (OHP) were also measured in the serum samples. In late winter (March 28th), the gonadal maturation and spawning were accelerated in fish treated with the long day length (16L/8D) and warm temperature (20°C). While, the maturation of oocytes and spawning delayed in fish exposed to low temperature (14°C) and short day length (9L/15D and 11L/13D). Photoperiod seems to play a more important role in the ovarian development of the R. rutilus caspicus compared to temperature; since even among the fish treated with the lowest temperature (14°C), those exposed to a longer day length (16L/8D), matured and spawned earlier than the others. Considering that the earliest spawning occurred in R. rutilus caspicus treated with 16L/8D at 20°C and the latest spawning occurred in fish exposed to low temperature and short photoperiod, it can be concluded that temperature and photoperiod play an important role in accelerating oocyte maturation and spawning.     

Effects of Phase-Shifted Photoperiod Regimes on Oocyte Growth and Hormonal Profiles in Female Striped Bass Morone saxatilis

Abstract.— The use of 12–mo long, but phase-shifted advanced and delayed photoperiod cycles in the regulation of the reproductive cycle was investigated in captive-reared female striped bass Morone saxatilis during the 3-yr study in an attempt to control the timing of sexual maturation under simulated photoperiod conditions. Phase-shifted photoperiod cycles did not induce a full shift in oogenesis during the first year cycles, but did in the following years. Spawning time, indicated by maximum oocyte diameters, was advanced up to 4 mo in females maintained under the phase-shifted advanced photoperiod, and delayed up to 4 mo when they exposed to the phase-shifted delayed photoperiod, compared to the natural spawning time in Spring (March-May). Phase-shifted photoperiod regimes shifted the profiles of plasma testosterone (T) and estradiol (E₂), corresponding to the shift of oogenesis in the respective groups. Significant increases in T and E₂ levels occurred during the vitellogenic phase, and these levels peaked before the occurrence of maximum oocyte diameters. The studies demonstrate that phase-shifted photoperiod regimes can be used to control oogenesis, and have implications for ensuring the year-round supply of mature female striped bass, particularly in domesticated striped bass.     

Delayed gametogenesis and spawning of sea bass (Dicentrarchus labrax L.) kept under different photoperiod and temperature regimes

The present work investigates the importance of day length and temperature in the control of reproduction of sea bass, as well as the effectiveness of LHRHa and HCG in inducing spawning out of season in this species. A controlled regime was produced and seasonal cycles with high components of temperature and photoperiod were extended from the summer solstice for at least 6 months, followed by a short photoperiod regime for 3 months before a new increase in these components. Natural spawning in the control fish occurred more frequently in mid February, although it was also observed in January and early March. Temperature manipulation delayed the spawning one month with respect to the controls, although some of the animals entered into gonadal regression. Photoperiodic manipulation delayed maturation for three months with respect to controls but it was necessary to perform hormonal induction of spawning. Although LHRHa and HCG were both applied, only intraperitoneal injections of LHRHa were effective in inducing spawning of sea bass out of season when the temperatures were 17°C. Dephasing between the annual changes in photoperiod in relation to the coordination of the different events of the sexual cycle of sea bass is considered.     

The use of luteinizing hormone releasing hormone analogue for ovulation induction in black sea bass (Centropristis striata)

Interest in commercial production of black sea bass has increased in recent years, but reliable spawning methods remain problematic. The objective of this study was to evaluate the effects of oocyte size and luteinizing hormone releasing hormone analogue (LHRHa) dosage and delivery systems on ovulatory success for in vitro fertilization. Vitellogenic females with maximum oocyte diameters of 400–625 μm were implanted with a 95% cholesterol–5% cellulose pellet containing 50 μg of LHRHa. Fish with maximum oocyte diameters < 450 μm failed to ovulate. In contrast, 90% of fish with 500 μm oocytes spawned within 36 h and 40% of this group ovulated a second time. All of the females containing oocytes > 550 μm ovulated. In a second experiment, females with uniformly vitellogenic oocytes (> 500 μm) and implanted with 50 μg LHRHa ovulated substantial numbers of eggs (45,000–192,000 eggs/kg body weight (BW), but fertility was consistently low (0–15%). In a third experiment, 19 of 39 females receiving implants containing 6.3–23.6 μg LHRHa/kg BW during the spawning season ovulated, but fecundity (17,000–339,000 eggs/kg) and fertilization (0–98%) were highly variable. When fish were grouped by developmental index, calculated as the number of oocytes with diameters > 400 μm/total number of oocytes measured, there were no statistical differences among groups with respect to the number of spawns, fecundity or fertilization success. In a fourth experiment, 11 of 13 females with a clutch of fully vitellogenic oocytes that were injected with 20 or 100 μg/kg BW LHRHa ovulated between 1 and 2 times on consecutive days. Five of seven given an implant containing 12.5 μg LHRHa ovulated one or more times. Fish implanted with shams or injected with vehicle alone did not ovulate in any of the experiments. No differences were found in the number of spawns, fecundity or fertilization success from fish receiving different doses of injected or implanted LHRHa. Incubation of pooled eggs produced 155,000 larvae (60% hatch) and 95,000 one-gram juveniles. These results demonstrate that injected or implanted LHRHa is effective for inducing ovulation in black sea bass with maximum oocyte diameters > 500 μm.     

Intra‐annual changes in reproductive indices of male and female Himalayan snow trout,Schizothorax richardsonii (Gray, 1832)

Periodic changes in reproductive hormone levels, gonadal histology and gonadosomatic index (GSI) of snow trout, Schizothorax richardsonii, were examined to ascertain annual cycle of gonadal development and reproductive status in their natural habitat. In females, there were coherent changes in plasma 17β‐oestradiol and vitellogenin along with GSI, oocyte maturation and vitellogenic progression, collectively indicating two distinct maturation peaks during the months of September and February. Coinciding with this, in males, plasma 11‐keto testosterone was also noticeably higher during September and February, with highest GSI values in September. However, plasma 17α, 20β‐dihydroxyprogesterone levels in males were found to be persistently high from September to February. This observation suggests the potential presence of matured oozing males over a longer period, unlike in females. Overall, the close association between reproductive hormone levels, GSI and gonadal maturation stages in males and females (particularly, the presence of postovulatory follicle complexes) with apparent natural synchronization clearly indicates that S. richardsonii breeds twice in a year, possibly during late September to early November and late February to early April in the coldwater riverine habitats of the Indian Himalayan region.     

The modulating effect of vitellogenin on the synthesis of 17β-estradiol by rainbow trout (Oncorhynchus mykiss) ovary

In vivo induction of vitellogenin (VTG) in response to the administration of 17-estradiol (E₂) was achieved and the protein was isolated by gel filtration column chromatography of plasma samples. Adult female trout were injected with the vitellogenic fraction every ten days from July to November and levels were measured by RIA from September to December. The results showed a significant decrease (p<0.01) in plasma E₂ levels in injected females compared with the controls. In December, after finishing the treatment, the plasma E₂ concentration increased, in injected females to reach a level similar to that of control females at vitellogenesis. The in vitro study showed that in early vitellogenic oocyte (from September) the presence of the vitellogenic fraction in the incubation medium causes a significant decrease (p<0.01) in the synthesis of E₂ by the oocytes. These data suggest that the concentration of the VTG into the oocyte can alter VTG production by the liver, moderating the production of E₂ by the ovary.     

Seasonal changes in plasma gonadal steroid concentrations and gonadal morphology of male and female tench (Tinca tinca, L.)

In order to gain a better understanding of the reproductive cycles of male and female tench (Tinca tinca), gonadosomatic index, gonad histology and plasma concentrations of estradiol‐17β (E₂), testosterone, an drostenedione, 11‐ketotestosterone (11‐KT), 17,20β, 21‐trihydroxy‐4‐pregnen‐3‐one (17,20β,21‐P), 17,20β‐dihydroxy‐4‐pregnen‐3‐one (17,20β‐P) and 17,20α‐dihydroxy‐4‐pregnen‐3‐one (17,20α‐P) were measured at the four seasons of the year, plus a further sampling coincident with the peak of spawning in early July. As expected, in both males and females, the plasma concentrations of androgens (excluding 11‐KT in females – undetectable) and C₂₁ steroids were significantly more elevated in the spring and summer (when most gonadal development took place) than in the autumn and winter. The only unexpected finding was that 17,20β‐P and 17,20β,21‐P, the steroids that are normally associated with oocyte final maturation in females and spermiation in males, were found in substantial amounts in both pre‐vitellogenic, pre‐spermatogenic and post‐spawning fish. This suggests that these steroids may have other as yet unidentified roles in this species.     

Changes in the expression of pituitary gonadotropin subunits during reproductive cycle of multiple spawning female chub mackerel Scomber japonicus

The endocrine regulation of reproduction in a multiple spawning fish with an asynchronous-type ovary remains largely unknown. The objectives of this study were to monitor changes in the mRNA expression of three gonadotropin (GtH) subunits (GPα, FSHβ, and LHβ) during the reproductive cycle of the female chub mackerel Scomber japonicus. Cloning and subsequent sequence analysis revealed that the cDNAs of chub mackerel GPα, FSHβ, and LHβ were 658, 535, and 599 nucleotides in length and encoded 117, 115, and 147 amino acids, respectively. We applied a quantitative real-time PCR assay to quantify the mRNA expression levels of these GtH subunits. During the seasonal reproductive cycle, FSHβ mRNA levels remained high during the vitellogenic stages, while GPα and LHβ mRNA levels peaked at the end of vitellogenesis. The expression of all three GtH subunits decreased during the post-spawning period. These results suggest that follicle-stimulating hormone (FSH) is involved in vitellogenesis, while luteinizing hormone (LH) functions during final oocyte maturation (FOM). Both GPα and FSHβ mRNA levels remained high during the FOM stages of the spawning cycle and increased further just after spawning. Thus, FSH synthesis may be strongly activated just after spawning to accelerate vitellogenesis in preparation for the next spawning. Alternatively, LHβ mRNA levels declined during hydration and then increased after ovulation. This study demonstrates that chub mackerel are a good model for investigating GtH functions in multiple spawning fish.     

Additive effects of advanced temperature and photoperiod regimes and LHRHa injection on ovulation in Atlantic salmon (Salmo salar)

In order to evaluate manipulation of spawning time as a potential means to extend 0+ Atlantic salmon (Salmo salar) smolt production in Tasmania, Australia, female salmon were exposed to a natural/simulated natural (42°S) photoperiod or an advanced (L:D 9:15) photoperiod from the austral summer solstice (20 December) under natural or advanced (~ 6 °C below natural temperature) temperature conditions. In late summer (26 February) injections of a commercial LHRHa preparation or vehicle (propylene glycol) commenced. Regular ovulation checks were conducted and ova were fertilised using milt from LHRHa-injected males held under matching photo-thermal conditions. Plasma levels of 17β-estradiol (E₂) and testosterone (T) were monitored and reproductive success (cumulative % ovulation, % fertilisation and % survival to the eyed-egg stage) was recorded. Ovulations commenced first (09 March) in LHRHa-treated fish that experienced advanced photoperiod and thermal regimes whereas sham-treated fish exposed to natural photoperiod and temperature conditions where the last to ovulate (22 May-08 June). Treatment-related sequential changes in the timing of ovulations were reflected by sequential advances in the timing of peaks in plasma levels of E₂ and T. The fertilisation of ova from LHRHa-treated fish that experienced advanced photoperiod and thermal regimes was significantly reduced (~ 52%) relative to all other treatments (> 80%) but there were no significant treatment-related differences in the survivals of eggs to the eyed stage (~ 50-90%). Consequently, a maximum advance in the timing of median ovulation of 71 days and commercially acceptable eyed-egg yields were generated, demonstrating that combinations of photoperiod, thermal and hormone treatments may be employed to significantly extend spawning and thereafter increase the availability of 0+ smolts for grow-out.     

Diurnal rhythm of serum steroid hormone levels in the Japanese whiting,Sillago japonica,a daily-spawning teleost

Ovarian developmental stages and serum steroid hormone levels were examined at six different times of day (0100, 0600, 1000, 1300, 1600, 2000 h) in a marine teleost, the Japanese whiting Sillago japonica, which has an asynchronous-type ovary containing oocytes at various stages of development and spawns every day during a period ranging up to three months. The largest oocytes in the ovaries at the active vitellogenic or post-vitellogenic stages were found between 0100 and 1300 h. Oocyte maturation indicated by germinal vesicle breakdown (GVBD) occurred at 1600 h, and ovulated oocytes were observed in the ovaries collected at 2000 h. These processes were accompanied by a significant daily change in serum steroid hormone levels. The serum level of estradiol-17β showed a peak in fish with mature oocytes sampled at 1600 h. In these fish, the second-largest oocytes in the ovaries were at the initial stage of vigorous vitellogenesis, the secondary yolk stage. Therefore the highest level of serum estradiol-17β was considered to be due to the second-largest oocytes. Testosterone levels remained low and constant throughout the experimental period. The serum levels of 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-diOHprog) peaked at 1600 h at which time all fish had mature oocytes. These results indicate that the Japanese whiting possesses a diurnal rhythm of oocyte development including vitellogenesis, oocyte maturation and ovulation, and further suggest that daily cycles in oocyte growth and maturation which simultaneously take place in an ovary are regulated by diurnal secretions of estradiol-17β and the maturation-inducing steroid, 17α,20β-diOHprog.     

Changes in gonadal hormones during oocyte development in the striped bass,Morone saxatilis

Wild striped bass,Morone saxatilis, were collected from coastal waters and spawning areas to describe the endocrine correlates of oocyte development in non-captive, migratory fish. The fish were classified according to their most advanced oocytes. Serum levels of estradiol (E₂), testosterone (T) and 17-20-dihydroxyprogesterone (DHP) were measured by radioimmunoassay (RIA). Females in the primary growth phase and early secondary growth phase (pre-vitellogenic) had low levels of plasma steroids, ovarian lipid content and gonadosomatic indices (GSIs). Significant increases in E₂, T, ovarian lipid content and GSIs occurred during the vitellogenic phase. Maximum levels of all reproductive parameters were found in prespawning fish sampled in the Hudson River. Mean levels of E₂, T, ovarian lipids and GSIs for these fish were 2.0±0.5 ng/ml, 3.0±0.3 ng/ml, 24±1 mg/g, and 5.6±0.3% (mean±SEM), respectively. In fish induced to spawn with human chorionic gonadotropin (HCG), DHP levels (1.9±0.4 ng/ml) were significantly elevated. Similar levels were found in two fish captured during the spawning season, suggesting that DHP may serve as the maturation-inducing steroid in this species.     

Effect of in vitro xenoestrogens on steroidogenesis in mature female fish, Chasmichthys dolichognathus

In this study, fully vitellogenic oocytes of the longchin goby (Chasmichthys dolichognathus) were exposed to in vitro xenoestrogens such as diethylstilbestrol (DES), bisphenol A (BPA) and nonylphenol (NP) at concentrations of 100 ng/ml in the presence of [³H]17α-hydroxyprogesterone as precursor. The major metabolites produced in vitro are progestogens [17α-hydroxy,20α-dihydroprogesterone (17α20αOHP) and 17α-hydroxy,20β-dihydroprogesterone (17α20βOHP)], androgens [androstenedione (A4) and testosterone (T)] and estrogens [estrone (E1) and estradiol-17β (E2)]. Comparative activities of these chemicals in oocyte steroid biosynthesis showed as follows: NP treatment resulted in clear stimulation of estrogen production, while the opposite occurred in DES treated incubation. Treatment with DES resulted in a higher synthesis of 17α20αOHP. BPA treatment increased the androgen, while estrogen synthesis was slightly inhibited. These results suggest that NP exhibited clearly estrogenic activity on the three chemicals.     

Correlation between oocyte development and plasma concentrations of steroids and vitellogenin in greenback flounder Rhombosolea tapirina

The development of oocytes in association with changes in plasma concentrations of vitellogenin (Vtg), 17β-estradiol (E₂) and testosterone (T) was investigated in maturing female greenback flounder Rhombosolea tapirina over the first part of a reproductive season. During vitellogenesis, increasing oocyte size was accompanied by the sustained elevation of plasma Vtg and E₂. There was a marked increase in T towards the end of vitellogenesis.     

Effects of light and temperature conditions on the expression of GnRH and GtH genes and levels of plasma steroids in <Emphasis Type="Italic">Odontesthes bonariensis</Emphasis> females

In this study we examined the endocrine mediation between environmental factors (temperature and photoperiod) and the brain–pituitary–gonadal axis in females of pejerrey Odontesthes bonariensis. Changes in the expression of brain gonadotropin-releasing hormones (GnRHs) and gonadotropin (GtH) subunit [follicle stimulating-β (FSH-β), luteinizing hormone-β (LH-β), glycoprotein hormone-α (GPH-α)] genes, plasma gonadal steroids [estradiol (E₂) and testosterone (T)], gonadal histology, and gonadosomatic index (GSI) in adult females exposed to combinations of short-day (8 h) or long-day (16 h) photoperiods and low (12°C) or high (20°C) temperatures after winter conditions (8 h light, 12°C) were analyzed. Pejerrey females kept under the short photoperiod had low GSIs, and their ovaries contained only previtellogenic oocytes regardless of the experimental temperature. In contrast, females exposed to the long photoperiod had high GSIs and ovaries with vitellogenic oocytes at both temperatures. These fish also showed a significantly higher expression of sGnRH, pjGnRH, cGnRH-II (the three different GnRH variants found to date in the pejerrey brain), FSH-β, LH-β and GPH-α genes and plasma E₂ levels than those at the shorter photoperiod. No significant changes were observed in plasma T levels. Based on these results, we concluded that the increase in day length but not that of temperature triggers the maturation of pejerrey females after the winter period of gonadal rest and that this occurs by an integrated stimulation of the various components of the brain–pituitary–gonad axis.