Life-history traits of Plesionika martia (Decapoda: Caridea) from the eastern-central Mediterranean Sea

Life-history traits of Plesionika martia (Milne Edwards, 1883) were studied through data collected during six seasonal trawl surveys carried out in the Ionian Sea (eastern-central Mediterranean) between July 1997 and September 1998. P. martia was found at between 304 and 676 m depth, with the highest density in the 400-600 m range. Intraspecific, size-related depth segregation was shown. Recruitment occurred in summer at the shallowest depths. Juveniles moved to the deepest grounds as they grew. The largest female and male were 26 and 25 mm carapace length, respectively. The sex ratio was slightly in favour of females at depths >400 m. Although a seasonal spawning peak was shown, the reproduction appears to be rather prolonged throughout the year. Females with ripe gonads were found from spring to autumn. Ovigerous females with eggs in late maturity stage were found year round. Large females could spawn more than one time within their annual reproductive cycle. The size at first maturity (50% of the ovigerous females) was 15.5 mm CL. Average brood size of eggs with a well-developed embryo was 2,966ǃ,521. Iteroparity, low fecundity and large egg size patterns were observed. Brood size increased according to the carapace length. Two main annual groups were found in the field population of the Ionian Sea. Estimates of the Von Bertalanffy growth parameters are: L_X=30.5 mm, k=0.44 year^-1 in females; L_X=28.0 mm, k=0.50 year^-1 in males. A negative allometry was detected mostly in the ovigerous females. The life cycle of P. martia is discussed in the light of life-history adaptations shown in other deep-water shrimp species.     

A re-evaluation of the diel feeding hypothesis for marine herbivorous fishes

We re-evaluated the "diel feeding hypothesis" by measuring diel variation in starch, protein, and floridoside in three algal "types" collected from a fringing coral reef at Lizard Island, Great Barrier Reef, Australia. Samples of two species of rhodophyte algae, Gracilaria arcuata and Acanthophora spicifera, and the turf assemblage from the territories of the herbivorous pomacentrid Stegastes nigricans were collected at four time periods through the day: 0630-0730, 1000-1100, 1330-1430, and 1630-1730 hours. We also measured the ability of several species of marine fish (the herbivores Acanthurus nigricans, A. lineatus, A. olivaceus, and Parma alboscapularis and the detritivore Ctenochaetus striatus) to hydrolyse floridoside by estimating !-galactosidase activity in tissue from the anterior intestine. We detected no diel pattern in protein content of the algae but found a significant steady increase in starch content throughout the day. Floridoside content increased in the morning and decreased in the afternoon, a pattern that may be driven by midday photoinhibition of the algae. All the fishes tested could utilise floridoside. Our results support the diel feeding hypothesis. Although floridoside content decreased in the afternoon, our results suggest floridoside was used during the day by the algae to synthesise starch. Thus the algae increased in nutritional value until photoinhibition occurred at midday then subsequently maintained their nutritional value during the afternoon. This pattern of algal nutrients increasing to a midday peak and remaining relatively constant throughout the afternoon correlates well with the diel feeding pattern in many species of marine herbivorous fish.     

The presence of eggs in the nest and female choice in common freshwater gobies (Rhinogobius brunneus)

In many species of fishes with paternal care, females prefer to spawn with males who are already guarding eggs. We studied the effect of egg presence on female mate choice in common freshwater gobies, Rhinogobius brunneus sp. OR. In our tests, females did not prefer males with eggs, suggesting egg presence per se may not act as a cue to attract females. We also examined the effects of brood size on paternal care and offspring survival to look for possible benefits females could obtain when choosing males with eggs. Both fanning by egg-guarding males and egg survival increased with brood size. The presence of neighboring males did have a significantly negative effect on males' parental activity, which subsequently results in a lower level of egg survival than in solitary egg-guarding males. This result provides a partial explanation for the result of eggs not attracting females to mate.     

Reproductive cycle and maternal effects on offspring size and number in the neogastropod Buccinum undatum (L.)

This paper examines relationships between maternal phenotype and offspring phenotype in the commercially exploited marine neogastropod Buccinum undatum (L.). The study also aimed to estimate the effects of female size on subsequent recruitment, and to determine the reproductive cycle of B. undatum in Swedish waters. The evaluation of gonad development in field-sampled animals from August 1998 to February 2000 revealed a definite annual reproductive cycle with a single major egg-laying period in the autumn. In addition, adult whelks of varying sizes from two locations and in two consecutive years were kept in aquaria to study the influence of female size on fecundity. Twenty-seven females deposited eggs in the laboratory. Offspring quality, measured as egg and hatchling size, was independent of female size, although larger females produced more eggs and hatchlings than smaller females, and thus probably account for a disproportionately large share of potential recruitment; the relative fecundity (number of eggs or hatchlings per gram of body weight) was constant with female size. I found no evidence for a longer time interval between successive egg-layings in larger females, i.e. mature females of all sizes probably spawn equally often. The present results therefore indicate that spawning-stock biomass is probably a better measure of spawning-stock size than the number of mature individuals in the population. Effects of an individual female's characteristics on stock productivity have largely been overlooked by fishery managers but could help to explain the weakness often found in stock-recruitment relationships.     

Synchronized development of gonad and bioluminescent light organ in a highly sexually dimorphic leiognathid fish, Photoplagios rivulatus

In order to investigate the possible coupling between sexual maturation and the light organ system (LOS) development in leiognathid fish, we determined the seasonal changes in the gonad maturation and the light organ enlargement in Photoplagios rivulatus, one of the leiognathid species possessing highly sexually dimorphic LOS. The fish specimens collected from March 2001 to August 2002 were assessed for standard length (SL), body weight (BW), sex, gonad weight (GW) and the light organ weight (LW). Gonadosomatic index (GSI = 10²GW/BW) and percent weight of the light organ to body weight (PLW = 10² LW/BW) were used to demonstrate the extent of development of gonad and the LOS, respectively. Mean GSIs in both male and female increased in June through August. The mean PLW in males showed a similar trend to that of the GSI, while PLW in females showed no clear seasonal trends. A significant positive correlation was found between the GSI and the PLW in males but not in females. The onset of sexual maturity in males (55 mm SL) coincided with that of the light organ enlargement. These results strongly support the functional coupling between reproduction and bioluminescence in this species of leiognathid. From an evolutionary perspective, sexual dimorphism in the LOS is likely to have evolved through sexual selection for reproductive success in leiognathid fishes.     

Egg production in the arrow squid Nototodarus gouldi (Cephalopoda: Ommastrephidae), fast and furious or slow and steady?

The process of reproductive maturation and egg release was examined in the temperate shelf squid Nototodarus gouldi. The energy allocation between somatic and reproductive growth from juvenile to mature adult was investigated throughout the life span to determine the underlying energetic strategy adopted by individuals. The relative weight of the mantle, fin and digestive gland remained unchanged during ovarian development, with no significant correlations found between the mantle length (ML)-gonad residuals and the ML-mantle (r=0.01, P>0.05), ML-fin (r=0.07, P>0.05) and ML-digestive gland (r=0.07, P>0.05) residuals. This suggested that energy was not being diverted away from somatic growth during sexual development, and consequently neither muscle nor digestive gland was being utilised as an energy store. Since squid in all maturity stages contained some food in their stomachs (e.g. 66.7% of mature animals), it is likely that the cost of maturation in this species is largely being met by food intake. The energy investment in reproductive tissues was relatively low (mean gonado-somatic index for mature individuals was 9.29% - 0.40%), indicating that only small amounts of energy were being allocated to reproduction at anyone point in time, which is characteristic of a multiple-spawning strategy. Furthermore, oviduct weight was not correlated with body size (r=0.256, P>0.05), suggesting that eggs are not stored for a single release. In all except one individual, ovary weight was consistently heavier than oviduct weight, suggesting that the ovary is not being depleted of oocytes as mature ova move into the oviducts. Additionally, the ovaries of mature females contained a range of oocyte sizes with discrete peaks, indicating a continued production and development of oocyte cohorts. The presence of some individuals with stretched empty oviducts is further evidence that the reproductive strategy of N. gouldi is slow and steady, with eggs possibly being released in discrete batches over a period of time.     

Reproductive cycle of Zidona dufresnei (Caenogastropoda: Volutidae) from the southwestern Atlantic Ocean

Gonadal development of Zidona dufresnei (Donovan, 1823) (Caenogastropoda: Volutidae) was studied over a period of two consecutive years, through analysis of gonadal tissue. Individuals were sampled monthly at the Mar del Plata area, Argentina. Gonadosomatic index was estimated for males and oocyte size was used to estimate the stage of gonadal development in females. The reproductive season in this species in the sampled locality extended from October to March (austral spring-summer). During summer, a stage of advanced gonadal development and spawning predominated in the adult population. In autumn, gonads were generally under atresia; during wintertime (June-August), they underwent a period of recovery that lasted until the spring months, when gametes were released again. Synchronism between both sexes was evident. Marked periods of spawning were followed by resorption periods and then a growing phase; it was very clear that reproductive seasonality was linked to changes in bottom water temperature. These results suggest that Z. dufresnei gonads have a yearly cycle of gamete production, with two major activity peaks in September-October and January-February.     

Ultra-structural changes in the reproductive system of overwintering females of Acartia longiremis

As the only member of its genus, and along with only a few other marine copepod species, Acartia longiremis Lilljeborg overwinters as adult females in "active diapause" in the plankton. This study describes the changes in ultra-structure of the resting reproductive system from August to January, and evidence for fertilization in overwintering North Norwegian A. longiremis. Gonads in overwintering individuals remain in an undifferentiated state from September to January, i.e. without oocyte development or oocyte migration into the oviduct and its diverticulae. Lipid stores in close association with the reproductive system appear to be utilized for the development of primary oocytes after the overwintering period.     

Abundance, growth and life cycle of the mesopelagic amphipod Primno abyssalis (Hyperiidea: Phrosinidae) in the Oyashio region, western subarctic Pacific

Abundance and life-cycle features of the mesopelagic hyperiid amphipod Primno abyssalis (formerly P. macropa) in the Oyashio region, western subarctic Pacific, were investigated using samples collected between July 1996 and July 1998. P. abyssalis was collected throughout the entire survey period, with abundance peaks occurring in spring to autumn. While all maturity stages of males and females were observed throughout the study period, the peak reproduction season was in summer. Instar analysis based on the segment number of the pleopod rami indicated that hatched juveniles molted 10 times before becoming adult males and 13 times before becoming adult females. Judging from the dry and ash-free dry weights of each instar, males and females continued to feed throughout the final instar stage. Based on cohort analysis of seasonal samples and laboratory observations on molting frequencies, growth in body length of P. abyssalis was linear with time, and estimated generation lengths were 2.3-3.8 years for females and 1.4-1.9 years for males. Brood size of females ranged from 66 to 337 and increased with increasing female body length. Lifetime fecundity, calculated as the sum of six successive broods, was 1,004. Compared with P. abyssalis in the southern Sea of Japan, those in the Oyashio region have a larger number of adult instars (six versus five for females, three 3 vs one for males), a lower growth rate (0.014 mm day^-1 vs 0.021 mm day^-1), and mature earlier (instar 13 vs instar 15 for females; instar 10 vs instar 11 for males). These characteristics are considered to be advantageous life-history traits to counteract higher niche competition within the mesopelagic community and higher predation pressure by mesopelagic fishes in the Oyashio region than in the Sea of Japan.     

Ovarian allometry and the use of the gonosomatic index: a case study in the Mediterranean sardine,<Emphasis Type="Italic"> Sardina pilchardus</Emphasis>

The gonosomatic or gonadosomatic index [GSI=(ovary weight/fish weight)×100] has been widely used in fisheries science and experimental reproductive studies as a simple, low-cost measure of reproductive condition. However, its properties have not been fully evaluated, and several pitfalls, such as size-dependence and changes in ovarian allometry, may invalidate its use. In the present study, we examined ovarian allometry and the appropriateness of GSI for assessing ovarian activity in the Mediterranean sardine Sardina pilchardus. The analysis was based on a large sample of histologically scored females collected over an annual cycle in the eastern Mediterranean (Aegean and Ionian Seas). First, we examined GSI dependence on fish size by comparing ovary weight–on–fish weight relationships in different stages of oocyte development. The effects of recent spawning (incidence of postovulatory follicles) and intensity of follicular atresia were also addressed. In a subsequent step of the analysis, we applied generalized linear models (GLMs) to analyze the relationship between GSI and histological stage, taking into account the parallel effects of additional factors related to geographical region, month of capture, fish length and energetic reserves (fat stage, hepatosomatic index—HSI). Ovarian growth was isometric in all stages of oocyte development and states of atresia, but altered to positive allometric at the stage of oocyte hydration. Oocyte growth and intensity of atresia significantly affected GSI. Fish length, geographical region, month of capture, fat stage and HSI did not substantially affect GSI, further strengthening the appropriateness of the index for the Mediterranean sardine. Finally, we provide first evidence, based mainly on available (but yet limited) published information for other fish species, that: (1) the pattern of ovarian allometry may not be altered by fecundity variations and (2) the pattern of ovarian allometry may reflect the pattern of oocyte growth (i.e. isometric vs. allometric ovarian growth could reflect group-synchronous vs. -asynchronous oocyte development). Alternatively, the pattern of ovarian allometry could reflect the presence or absence of size dependency in the relative fecundity (eggs per gram of body weight) and/or egg size. The isometric ovarian growth in the Mediterranean sardine could be attributed to the lack of such size-dependent effects.Communicated by O. Kinne, Oldendorf/Luhe     

Spawning pattern and type of fecundity in relation to ovarian allometry in the round herring <Emphasis Type="Italic">Etrumeus teres</Emphasis>

Spawning pattern (assessed by seasonal changes in ovarian developmental stages) and type of fecundity (assessed by analysis of oocyte-size frequency distributions) of the round herring Etrumeus teres were studied in relation to ovarian growth and seasonal changes in the gonadosomatic (GSI), hepatosomatic (HSI) and liposomatic (LSI) index as well as the somatic condition of spawners (CS) in a spawning ground of southern Japan. Except for summer, mature and recently spawned ovaries occurred all year round. Oogonia and primary oocytes were present in all ovaries, and cortical alveoli stage (CA) oocytes occurred in all mature, hydrated and partially spent (PS) females (PS: females containing post-ovulatory follicles). Before hydration, a clutch of larger yolked oocytes, undergoing synchronous growth (range 0.7–1.1 mm), was present in mature ovaries which was completely separated from a more heterogeneous clutch of oogonia, primary and secondary oocytes (<0.150 mm) and oocytes in the CA stage (range 0.15–0.60 mm). As vitellogenesis progressed, the yolked clutch increased in size but the CA oocytes remained arrested. The latter entered into the secondary growth phase when hydration started in the advanced batch. Ovarian growth was isometric in all developmental stages, validating the use of GSI, which showed a consistent monthly evolution among years. Spawning stopped in summer (July and August) and peaked in winter and spring. HSI correlated positively with GSI on both a monthly mean basis (r = 0.76) and individual fish basis (liver weight explained 67–83% of the variability in ovary weight when females were grouped into 1-unit GSI intervals) suggesting a significant role of liver in vitellogenesis. LSI and CS also showed marked seasonal changes peaking from summer to middle autumn. Overall results suggest that E. teres is a multiple spawner with a group-synchronous ovarian development and indeterminate annual fecundity, with the three processes linked to an isometric growth of the ovary. We propose that such a reproductive pattern is an adaptation to produce batches of large pelagic eggs through a protracted spawning season.     

Gonad morphology and oocyte development in<Emphasis Type="Italic"> Pseudocalanus</Emphasis> spp. in relation to spawning activity

Gonad maturation processes were studied in Pseudocalanus spp. females from Georges Bank and Cape Cod Bay (northwest Atlantic) using a combination of morphological analysis and experiments. For light microscopy of the oocytes, females of different maturation stages were preserved immediately after capture. The maturation processes during the spawning cycle were described from observations of live females which were exposed to feeding and starvation at two temperatures, 8 and 15°C, for 12 days. The gonad morphology of these females was examined in 24 h intervals, and spawning events were recorded. Both light microscopy and whole animal observation revealed that oocytes during maturation change in shape and size, in the morphology of the nucleus, and in the appearance of the ooplasm. Due to these modifications of oocyte morphology and due to oocyte migration, the morphology of the gonads changed distinctly during a spawning cycle. Five oocyte development stages were identified by light microscopy and related to a macroscopic system of four gonad development stages, that can be applied to whole animals and allows the identification of females ready to spawn. The experiment showed that food and temperature had strong effects on gonad maturation processes. High proportions of mature females were found when food was available, whereas the proportion of immature females increased shortly after exposure to starvation. Compared to 15°C, gonad maturation at 8°C was prolonged and thus spawning frequency was lower. The final maturation processes at food saturated conditions were slower than the embryonic development, and no indication was found that mature oocytes are stored in the diverticula waiting to be released. The duration of the interspawning interval would thus be determined by the duration of final oocyte maturation which is dependent on both temperature and food supply.Communicated by O. Kinne, Oldendorf/Luhe     

A histological study of reproduction in the serpulids<Emphasis Type="Italic"> Pomatoceros triqueter</Emphasis> and<Emphasis Type="Italic"> Pomatoceros lamarckii</Emphasis> (Annelida: Polychaeta)

The reproductive cycles of the serpulid polychaetes Pomatoceros lamarckii (Quatrefages, 1865, as Vermilia) and P. triqueter (Linnaeus, 1767, as Serpula triquetra) were studied by histological examination during 1999 and 2000 at Bantry Bay, south-west Ireland. Gametogenesis, sex ratios and hermaphroditism were also investigated. The male/female ratio in P. lamarckii was approximately equal, but in P. triqueter it deviated significantly in favour of females. There was no significant variation in the sex ratios throughout the year in either species. Results confirm that the species are protandric hermaphrodites. In P. lamarckii, but not in P. triqueter, sexual dimorphism was exhibited in body size, the females being larger than the males of the same species. In both species, the proportion of females increased with increasing body size. Simultaneous hermaphroditism was recorded in P. lamarckii, but not in P. triqueter. There was no clearly defined annual gametogenic cycle in either species. Both species appear to have an extended reproductive season, with numerous small-scale peaks in reproductive maturity that can vary annually. Spawning was broadly synchronous between sexes. Visual observation alone (i.e. without histology) was insufficient to accurately assess reproductive condition in individuals not in, or close to, a ripe state.     

Spatial and temporal variation in growth rates and maturity in the Indo-Pacific squid <Emphasis Type="Italic">Sepioteuthis lessoniana</Emphasis> (Cephalopoda: Loliginidae)

Age, growth and maturity parameters are described for the Indo-Pacific squid Sepioteuthis lessoniana both temporally and spatially (equatorial, tropical and subtropical). Tropical squid that grew through periods of warming water temperatures grew 9% faster than squid that grew through periods of cool water temperatures. The tropical spring-hatched and equatorial squid had similar growth rates (3.24, 3.18 g/day) and these were significantly faster than the tropical summer/autumn hatched squid (2.89 g/day). The oldest squid aged was 224 days, but the majority of individuals were <200 days. Subtropical squid were larger, older and matured later than equatorial and tropical spring-hatched squid. The mean weight of subtropical squid aged between 100 and 150 days was >400 g, 85% greater than tropical and equatorial squid. Geographical differences revealed that subtropical mature winter females and males had mean ages >150 days, respectively 17% and 23% older than their tropical mature winter counterparts. Temporal differences in age at maturity were also evident with tropical winter females and males having a mean age of ~140 days, respectively 41% and 25% older than their summer counterparts. Cooler subtropical and winter tropical squid had the heaviest gonads (>15 g ovaries, >1.5 g testes) compared to summer tropical and equatorial squid. However, relative gonad investment (GSI) values of the cooler squid were significantly lower with cool subtropical and winter tropical females having GSI values <3, which was about half the value of the warmer water females. This study revealed considerable plasticity in the size-at-age of this species. The tropical population had growth parameters that fluctuated between an equatorial strategy (fast growth, small body size, and small gonads) and a subtropical strategy (large body size, slower growth, and large gonads) depending on season.     

Sexual selection and species recognition in the pygmy swordtail, Xiphophorus pygmaeus: conflicting preferences

Sexual selection and species recognition play important roles in mate choice; however, sexual selection preferences may overlap with traits found in heterospecifics, producing a conflict between sexual selection and species recognition. We examined female preferences in Xiphophorus pygmaeus for male traits that could provide both types of information to determine how females use multiple cues when preferences for these cues would conflict. We also examined X. pygmaeus behavior in the field to determine if females have the opportunity to choose mates. As no male-male competition was observed in the field, and females occasionally chased males from feeding areas, females apparently have the opportunity to exercise mate choice in their natural habitat. In the laboratory, female X. pygmaeus used body size as a sexual selection cue, preferring large heterospecifics (X. cortezi) to small conspecifics. Females also preferred barless X. cortezi over barred X. cortezi when males were size matched. Because X. pygmaeus males do not have bars, this preference suggests that X. pygmaeus females use vertical bars in species recognition, and that large body size and vertical bars are conflicting cues. However, X. pygmaeus females did not have a preference for males of either species when sexual selection and species recognition cues were presented concurrently. This result was surprising, because preferences for species recognition cues are often assumed to be stronger than sexual selection cues. We suggest that females may be using additional species-specific cues in mate choice to prevent hybridization.     

Effects of size and nuptial gifts on butterfly reproduction: can females compensate for a smaller size through male-derived nutrients?

In butterflies and other insects, fecundity generally increases with female adult weight. Hence, most butterflies are essentially "capital breeders", because nutrients acquired during the larval stage are stored and subsequently used for egg production during the adult stage. However, in some species, males transfer a large nutritious ejaculate to the female at mating. These females can partly be characterized as "income breeders", and female mass can potentially be decoupled from fecundity to some extent. In the gift-giving green-veined white butterfly Pieris napi, it has been shown that female fecundity and longevity increase with number of matings and also that females mature at smaller size under poor food conditions compared to males. So it has been suggested that females can compensate for their smaller size through nuptial feeding. Here we test this hypothesis in P. napi by assessing female fecundity and longevity in relation to female mass and polyandry. The results showed no support for the hypothesis. Smaller females were not capable of increasing their mating rate to compensate for a low weight at eclosion. Instead, larger females remated sooner. Also, smaller females suffered from both a reduced daily and total fecundity compared to larger females and this decrease in fecundity was independent of female mating status, i.e. females allowed to mate only once and multiply mated females suffered to the same extent from their smaller size.     

Reproduction and growth of the freshwater prawn, Palaemon paucidens (Decapoda: Palaemonidae) in a lake of Korea

This study analyzed the reproduction and growth of freshwater prawn, Palaemon paucidens De Haan 1844 from Suk-dang lake(Korea). The analysis of the sex ratio showed a higher proportion of males than that of females. The average size of the eggs was 6.12 (+/-0.55) mm3 in the non-eyed stage (stageA) and 7.20 (+/-0.86)mm3 in the eyed stage (stage B). The reproductive output (RO) calculated with the dried-body weight of an incubating female and the weight of the eggs in dried condition was 26.97% (n = 17) of the average females weight. Incubating prawn appeared in April, and the gonadosomatic indices(GSI) showed the highest value during three months from January to March. Based on the month when there was a high proportion of incubating females with a high GSI, the estimated spawning season of P. paucidens was April. The maturity of the female was evaluated by the development of the ovaries and the existence of eggs. The average body length when 50% of the females in the group reached maturity was 8.55 (+/-2.74) mm. The analysis of the length-frequency distribution showed that the life span of P. paucidens ranged from 12 to 13 months. Females mature faster than males.     

No reason to sneak: why males of all sizes can breed in the hole-nesting blenny, Aidablennius sphinx

Most blennioid fishes show a resource-based, promiscuous mating system with alternative 'sneaking' mating tactics. The hole-nesting species, Aidablennius sphinx, however, appears to be an exception; small males did not mimic females, most had their own nest and not once in 20 h of spawning observations did a second male enter a nest. In this field study, we ask if sneaker tactics are constrained and what factors favour independent nesting by small males. Larger males received more eggs sooner and attracted more females than smaller males. Larger males may be preferred because they released more sperm per ejaculation and fanned their eggs more often. In the long-term, however, larger males did not spawn more often than smaller males since: (1) the nests of large males filled up with eggs sooner than the nests of small males; (2) large males refused females more often than small males; and (3) a highly biased ratio of females to preferred large males meant many females had no option but to spawn with small males. Thus, small males nest independently, despite potential for female mimicry tactics (multiple females spawned simultaneously) and simultaneous parasitic spawning (males exited the nest during spawning)     

Seasonal cycle of gonadal development and serum levels of vitellogenin of the red porgy, Pagrus pagrus (Teleostei: Sparidae)

Histological examination of gonads of female and male red porgy, Pagrus pagrus, reared in captivity, was carried out in order to describe the main gonadal changes related to gametogenesis and the seasonal changes of environmental factors. The gonadosomatic index (GSI) and the concentration of serum vitellogenin (VTG) were also determined. The frequency distribution of gonad development stages and the GSI and vitellogenin concentration during the annual cycle indicated the separation of the female and male reproductive cycles into three main periods. The autumn period when gametogenesis begins (October-November), the period of exogenous vitellogenesis (December-March) or spermiation (December-March), and the spawning season (March-May). The spawning period coincided with an increase in temperature (15-19°C) and daylength. Serum levels of vitellogenin rose significantly in January, reached a peak at the beginning of the spawning period (March, 405.5 µg ml^-1) and remained high until the end of the spawning period (May).