Population variation in spawning current speed selection in the blacktail shiner,Cyprinella venusta (Pisces: Cyprinidae)

Synopsis Many stream fishes lay demersal eggs. Some species simply scatter the eggs across the substrate, but many place the eggs in protected sites such as crevices (e.g., Cyprinidae:Cyprinella). Eggs laid in crevices may experience increased development times or lower embryo survival due to reduced water exchange rates compared to more open egg deposition sites. If so, the presence of an appreciable current near the crevice may be important for insuring water exchange. However, currents that are too strong may prevent complete insemination or prevent the eggs from adhering to the substratum before they are washed from the crevice. Thus, current speed preferences for spawning may be under strong stabilizing selection. Representatives of four populations of a crevice spawning stream fish (blacktail shiner,Cyprinella venusta), representing environments ranging from a swift stream to a lake, were studied in stream tanks to evaluate female spawning current use. The observed frequency of use of spawning currents was significantly different from random in all populations. Low current speeds were generally avoided, and a speed of ca. 30 cm sec^–1 was used most often. However, significant differences were observed among populations, and the variation correlated well with the apparent range of current speeds found in the four environments. Although other possible explanations cannot be dismissed, the results are consistent with the hypothesis that the populations have become adapted to their local current regimes.Authorship in order of seniority     

The interrelation of the size of fish eggs, the date of spawning and the production cycle

The variability in size of pelagic and demersal marine and freshwater fish eggs is examined. The difference between the smallest and largest volumes, based on published figures for the diameters, is large in many species. In marine species with planktonic eggs, the median percentage difference is just over 100%, and this is similar in species with demersal eggs and in freshwater fish.
The available evidence suggests that geographical differences in egg size are small, but in marine fish there is a well-known seasonal decline in egg size. In herring it has previously been shown that egg size in different spawning groups can be correlated with the timing of the production cycle. A similar correlation can be seen in the seasonal shift in time and locality of spawning, and egg size, of the plaice. Sufficient data on seasonal freshwater fish egg variations are not available, but the time of spawning does appear to be linked with the availability of food for the larvae in both lake and stream species.     

The effect of extreme northerly climatic conditions on the life history of the minnow, Phoxinus phoxinus (L.)

Over its exceptionally wide geographical range the minnow displays a remarkable variability in its life history. In southern England many fish mature after one year and few fish survive to their third birthday. Very few fish spawned for more than one season but a 65-mm female is estimated to spawn up to 3172 eggs over the extended spawning season. In the River Utsjoki in Finnish Lapland maturity was strongly size-dependent and delayed until the fish reached 5, 6 or even 7 years of age, with a maximum age of 13 years at a length of only 75 mm. Unlike some other small-sized species in unproductive environments, individual females continued to spawn successive clutches of eggs though over a much shorter spawning season. The maximum estimated egg production was 824 for a 65-mm fish. Temperature had a strong direct or indirect effect on growth, with significantly lower growth increments in cold summers. At a second Arctic site in eastern Lapland growth rates were higher and maturity earlier, yet clutch size was greatly reduced with a seasonal fecundity estimate of only 314. There were only small differences in size of eggs or larvae between the populations. Evidence from other species indicates that most of the observed changes over the range of the minnow are phenotypic responses to the contrasting environments. However, the different strategies displayed at the two Arctic sites could represent the results of selection for differences in pre- and post-reproductive survival.     

Divergence in reproductive ecology of the three-spined stickleback,Gasterosteus aculeatus

An anadromous population (trachurus form) and three freshwater populations (leiurus form) of the three-spined stickleback,Gasterosteus aculeatus, in central Japan were compared with one another in the reproductive traits: body length at maturity, egg size, clutch size, the swell of abdomen and the number of eggs per nest. The anadromous fish which become larger in body size at maturity spawn eggs smaller in size and greater in number than the freshwater fish. The abdomen swell of gravid females expressed by the proportion of abdomen width to body length was greater in the freshwater fish. The anadromous male fish collected a mean of 2,638 eggs with a range of 1,119 to 4,052 eggs from about 6–7 females. In the three freshwater (the Yamayoke, the Tsuya and the Jizo) populations, males must have mated with about 9–22, 7–18 and 4–7 females respectively. It seems that theleiurus form increases its reproductive success by its much more mating opportunities and the parental efforts of nesting males as well as by spawning large eggs. Furthermore, among the freshwater populations, the Jizo one inhabiting the upper stream was clearly larger in body size, in egg size and in clutch size than the Yamayoke and the Tsuya ones which inhabit stable waterbodies with springs. It is possible that the Jizo population adopted the strategy of spawning a few large eggs as an adaptation to its habitat. The causal and functional explanations in reproductive characteristics among the four populations are discussed in regard to differences in the environmental conditions.     

以标志物对长江上游漂流性鱼卵漂流方式的研究

2008年4月7日在长江上游重庆江津-珞璜31km的江段采用标志物对自然条件下大流量河流漂流性鱼卵的漂流方式进行模拟实验。结果显示在流量稳定的情况下, 在同一投放点投放的标志物的平均漂流速度具有显著差异, 标志物在河流中的漂流密度以高峰值和低峰值相间的方式出现, 高峰值随漂流时间的增加呈逐渐下降趋势。在水平方向上标志物的分布以主流水域的漂流密度高于缓流水域, 缓流水域未采集到标志物。采用传统产卵规模计算方法对标志物投放数量进行估算,各组的估算数量均小于实际投放数量, 估算数量的准确性随漂流时间的增加而降低, 总体估算数量为实际投放数量的16.6%。根据实验结果, 建议在长江上游鱼类产卵规模计算中引入时间系数以弥补随漂流时间的增加而引起的估算准确性降低。       

The Principles of Buoyancy in Marine Fish Eggs and Their Vertical Distributions across the World Oceans

Buoyancy acting on plankton, i.e. the difference in specific gravity between plankton and the ambient water, is a function of salinity and temperature. From specific gravity measurements of marine fish eggs salinity appears to be the only determinant of the buoyancy indicating that the thermal expansions of the fish egg and the ambient seawater are equal. We analyze the mechanisms behind thermal expansion in fish eggs in order to determine to what extent it can be justified to neglect the effects of temperature on buoyancy. Our results confirm the earlier assumptions that salinity is the basic determinant on buoyancy in marine fish eggs that, in turn, influence the vertical distributions and, consequently, the dispersal of fish eggs from the spawning areas. Fish populations have adapted accordingly by producing egg specific gravities that tune the egg buoyancy to create specific vertical distributions for each local population. A wide variety of buoyancy adaptations are found among fish populations. The ambient physical conditions at the spawning sites form a basic constraint for adaptation. In coastal regions where salinity increases with depth, and where the major fraction of the fish stocks spawns, pelagic and mesopelagic egg distributions dominate. However, in the larger part of worlds’ oceans salinity decreases with depth resulting in different egg distributions. Here, the principles of vertical distributions of fish eggs in the world oceans are presented in an overarching framework presenting the basic differences between regions, mainly coastal, where salinity increases with depth and the major part of the world oceans where salinity decreases with depth. We show that under these latter conditions, steady-state vertical distribution of mesopelagic fish eggs cannot exist as it does in most coastal regions. In fact, a critical spawning depth must exist where spawning below this depth threshold results in eggs sinking out of the water column and become lost for recruitment to the population. An example of adaptation to such conditions is Cape hake spawning above the critical layer in the Northern Benguela upwelling ecosystem. The eggs rise slowly in the onshore subsurface current below the Ekman layer, hence being advected inshore where the hatched larvae concentrate with optimal feeding conditions.     

Cannibalism and protective behavior of eggs in Arctic charr (Salvelinus alpinus)

From video recordings of spawning events, we quantified protective and cannibalistic behavior of Arctic charr occurring immediately after spawning. The number of fish cannibalizing on stray eggs was examined regarding (a) whether more than one male shed milt during the spawning event, that is, whether sperm competition occurred, (b) whether the sperm competition included few or many males, that is, the intensity of sperm competition, and (c) the density of fish at the spawning site. Response behavior toward egg cannibalism was also examined among females and dominant males in order to determine any parental investment toward protecting the eggs after spawning. Cannibalistic behavior was seen in almost 50% of the spawnings, and the multiple spawning events showed the highest numbers of fish cannibalizing on eggs. Both the number of males releasing milt and the number of fish approaching the spawning site were positively correlated with egg cannibalism. Sperm competition was, however, not a prerequisite for egg cannibalism. Although we also observed partial filial cannibalism, protective behavior of eggs was seen both among dominant males and females, suggesting that charr actually conduct parental care.     

Testing predictions of the critical period for survival concept using experiments with stocked Atlantic salmon

Two separate field experiments were performed in the U.S.A. and Norway with stocked Atlantic salmon Salmo salar . In the Norwegian experiment, the offspring of early‐spawning fish which had larger eggs and emerged a few days before offspring of later spawning fish had consistently higher survival rates. In the U.S.A. experiment, stream sections with higher proportions of favourable foraging locations during the critical period (the transition from dependence on maternally‐derived yolk reserves to independent feeding) had lower loss rates of fish stocked as unfed fry. These results provide support for the critical period concept (CPC) in Atlantic salmon, underscores the utility of a manipulative approach to achieve further advances in knowledge of Atlantic salmon ecology and provide additional guidance to management and restoration. A mechanistic, conceptual model for density dependence is presented to identify important knowledge gaps that remain to further evaluate the importance of the CPC for Atlantic salmon population regulation.     

Resident spawning of Japanese smelt,Hypomesus nipponensis,along gravel shorelines in Lake Nojiri,Central Japan

In Lake Nojiri, the Japanese smelt, Hypomesus nipponensis McAllister, 1963, population has been thought to be maintained by artificial spawning, and stocking efforts as natural spawning in in-flowing streams is unlikely due to the lack of inflowing stream habitat. In this study, novel resident Japanese smelt spawning was observed along the lakeshore in shallow areas with clean gravel and flow. Spawning occurred at night during early March to mid-April and eggs, confirmed on gravel substrate, progressed to the eyed state in 5–6 weeks. Although natural spawning was observed, the ratio of eggs that progressed to the eyed stage was minimal, possibly due to thick algae cover in areas with low flow velocity. Therefore, resident Japanese smelt spawning along the lake shoreline is physically possible and potentially contributes to the overall population in Lake Nojiri, but in-lake spawning alone presumably is not a biologically viable method for population sustainability due to low survival rates and egg density at present. Environmental improvements such as construction of fish ladders to suitable in-flowing spawning habitat, introduction of clean gravel to shorelines, and water level management adjusted to the smelt spawning run would contribute to higher recruitment by natural smelt reproduction, and consequently enhance the smelt production in Lake Nojiri.

     

Breeding biology of a non-diadromous galaxiid, Galaxias vulgaris Stokell, in a New Zealand river

In the Glentui River, Canterbury, New Zealand, Galaxias vulgaris Stokell spawned in late winter and early spring (from July to September), the onset of spawning being temperature-dependent. No females matured in their first year, but 95 and 68 % of age 0+ males were mature in consecutive years. With the exception of one female, all fish aged 1 + and older spawned. In mature fish, overies reached nearly 23%, and testes 11%, of somatic weight. Maturity in males depended on size, whereas in females, age appeared to be the major factor affecting maturity. Recruitment eggs for next season's spawning were distinct from primary oocytes soon after the ripe eggs were shed. Egg numbers varied between 284 and 1911 per fish, and weight was the best indicator of fecundity. Sex ratios of whole samples were not significantly different from 1 : 1, but ratios of potential spawners revealed an excess of males, due mainly to the presence of age 0 + fish. Age0+ males probably play an important role in adverse conditions and act as a form of insurance by significantly increasing the proportion of ripe males, thereby ensuring that all eggs are fertilized. The nest, egg mass and behaviour during the spawning period are described. More than one female may lay eggs in each nest and it is probable that each egg mass receives sperm from more than one male. It is suggested that the presence of fewer and larger eggs in G. vulgaris , compared with the diadromous galaxiids, is the result of a more precise fertilization process.     

Ovulation does not constrain egg parcel size in the simultaneous hermaphrodite Serranus subligarius

Among egg trading hermaphrodites, any factor which limits the number of eggs released by a female role hermaphrodite can potentially limit the mating success of the male role hermaphrodite fertilizing those eggs. This work examines the hypothesis that the timing of ovulation constrains the size of egg parcels and thereby limits male mating success in the egg parceling hermaphroditic fish Serranus subligarius. Two alternatives were evaluated: (1) Ovulation is a discrete event preceding spawning. It does not constrain the size of egg parcels and therefore does not limit mating success of male role partners. (2) Ovulation is an incremental process occurring throughout the spawning period. It limits the number of eggs available for release in each parcel and thereby limits mating success of the male role partner. Assessment of ovulation was conducted in a field stock of S. subligarius. Fish from size matched pairs were manually stripped at the onset of the spawning period or quarantined and sampled at the end of the spawning period. Fish sampled at either time point had the same number of eggs, suggesting that ovulation was a discrete event occurring at the onset of the spawning period. The diurnal cycle of ovulation was observed in naturally spawning hermaphrodites captured at intervals throughout the day. Ovulation began 2–4 h before spawning began. Some fish appeared to ovulate the entire day's clutch of eggs before spawning, while other fish released egg parcels before completing ovulation. I conclude that the pattern of ovulation is not uniform throughout the spawning stock. Because of the variability in timing of ovulation relative to parcel release, ovulation does not consistently limit the size of egg parcels and therefore is unlikely to be a physiological limit to male role mating success in S. subligarius hermaphrodites.     

Artificial Spawning Habitats Improve Egg Production of a Declining Diadromous Fish,Galaxias maculatus (Jenyns, 1842)

Anthropogenic impacts from urbanization, deforestation, and agriculture have degraded the riparian margins of waterways worldwide. In New Zealand, such impacts have caused changes in native vegetation, enhanced invasion by exotic grasses, and altered river bank morphology. One consequence has been a great reduction in obligate spawning habitat of a diadromous fish, Galaxias maculatus. Juvenile G. maculatus comprise a culturally important fishery that has been considerably reduced over recent decades. Rehabilitation of riparian vegetation needed for spawning is relatively straightforward, but slow. We hypothesized that artificial spawning habitats could accelerate restoration of fish egg production by creating an environment that would support at least the same density and survival of eggs as non‐impacted vegetation. We tested three artificial devices (straw bales, straw tubes, and moss tubes) in degraded and intact sites. Eggs were laid in all of these with numbers and survival usually exceeding that in riparian grasses. Where habitat was degraded, artificial spawning habitats yielded up to 10,000 eggs compared to none in nearby natural spawning habitat. The ground‐level environment of artificial habitat was similar to that of intact vegetation in buffering ambient temperature and humidity fluctuations. Crucial properties of the artificial habitats were (1) shelter to provide shade and hold moisture; (2) accessibility to allow adult fish to deposit and fertilize eggs; and (3) robustness to provide reliable surfaces and protection for the eggs during their development. We show that artificial spawning habitats are a viable short‐term alternative to rehabilitating spawning habitat while legacy effects abate and riparian vegetation recovers.     

NATURAL REPRODUCTION AND HYDROLOGIC REQUIREMENTS OF PELAGOPHIL FISH IN THE THREE GORGES RESERVOIR北大核心CSCD

To reveal the relationships between spawning activities of pelagophil fish and environmental factors in the Three Gorges Reservoir (TGR), fish eggs and larvae were sampled at Zhongxian section, from May to July in 2016 and 2017. 60 fish species including 30 pelagophil fishes were sampled. Hemiculter bleekeri and Squalidus argentatus were the dominant species in eggs, and Hemiculter and Pseudolaubuca were abundant in larvae. In 2016, 7.041 billion eggs and 103.763 billion larvae drifted through the sampling section, and in 2017, 3.454 billion eggs and 34.427 billion larvae drifted through the sampling section. The amount of eggs and larvae of the four major Chinese carps reached 2.909 billion in 2016 and 2.117 billionin in 2017. Spawning activities of pelagophils fish varies temporally in different river reaches, which mainly occurred after early-June in the river reach from Zhongxian to Fuling but begun in mid-May in the river reach upstream of Changshou. Fish eggs and larvae from the river reach upstream of Changshou provided a large resource for recruitment to the fishery in TGR. Classification and regression trees (CART) analysis explored that spawning activities of the four major Chinese carps were favord when water discharge was rasing. But, spawning activities of Hemiculter bleekeri and Squalidus argentatus had no relationship with water discharge. In order to protect the spawning grounds and meet environmental requirements for natural reproduction of pelagophils fish, we propose ecological regulation of the cascade hydro-power projects in the Jinsha River and maintenance of large free-flowing stretches upstream of TGR. © 2019, Institute of Hydrobiology, Chinese Academy of Sciences. All rights reserved.     

Comparison of brook trout reproductive success and recruitment in an acidic adirondack lake following whole lake liming and watershed liming

Limestone applications to the catchment of one tributary to Woods Lake were highly effective in reducing stream acidity and stabilizing seasonal fluctuations in pH. The resulting improvement in stream water quality also led to a dramatic shift in reproductive strategy of the Woods Lake brook trout population. Prior to catchment liming, brook trout in Woods Lake were restricted to spawning on poor quality near shore substrate with limited ground water seepage. Reproductive success was limited by high mortality of eggs and larvae and recruitment from in lake spawning was not successful. Spawning brook trout did not utilize the tributary for spawning prior to watershed liming. Mitigation of acidity in the tributary, by catchment liming, effectively extended the spawning habitat available to the Woods Lake brook trout population and one year following treatment brook trout spawned successfully in the tributary for the first time in 6 years of observation. Significant recruitment of young trout into the lake population occurred from 1991 through 1993, although the absolute number of fish captured was relatively small. In the fall of 1993, four year classes of naturally spawned brook trout were present in the lake. Although reproductive success was enhanced by improving tributary spawning habitat in the Woods Lake basin, self maintenance of the population may be limited by low recruitment rates of young trout, due to high levels of summer mortality resulting from predation. Mitigation of this constraint would require substantially higher levels of fry production than were observed in Woods Lake and/or enhanced refugia for young trout. The results of this experiment suggest that re-establishment of tributary spawning populations of brook trout may be possible, with future reductions in acidic deposition, in acidic Adirondack lakes with limited in-lake spawning habitat.     

Egg concealment in ascidians by females of the Japanese tubesnout, Aulichthys japonicus (Gasterosteiformes), and its subsequent copulation

The reproductive behaviors of the Japanese tubesnout, Aulichthys japonicus, the only fish among gasterosteiform fish that conceals eggs in ascidians, were observed in aquaria. Yellow nuptial-colored males attending at ascidian courted gravid females. The nuptial color could be an advertisement to females for depositing sites, and courtships seemed to facilitate female spawning. The females spawned eggs above the ascidian and immediately placed their eggs with their mouths into the peribranchial cavity. Males never ejaculated on eggs after spawning, but attempted to copulate with females during a period of several minutes, and one or a few males with the remarkable nuptial color frequently copulated using their urogenital papilla. An evolutionary scenario of unique egg concealment and subsequent copulation is proposed.     

中街山列岛海域鱼卵、仔稚鱼群落结构特征及其与环境因子的关系

为了解中街山列岛海域鱼卵仔稚鱼空间分布、种类组成及与环境因子的关系,于2010年5月—2012年2月在中街山列岛海域利用水平和垂直拖网开展了8个航次的鱼卵、仔稚鱼调查。运用非度量多维标度排序、聚类分析和典范对应分析(CCA)等方法对该海域鱼卵、仔稚鱼的空间分布进行了综合分析。结果表明:两年水平拖网共采集到鱼卵1783粒,仔稚鱼67尾;垂直拖网共采集到鱼卵576粒,仔稚鱼59尾。2010年平拖优势种为短吻红舌鳎(Cynoglossus joyneri),垂拖重要种为鲉形目未定种(Scorpaeniformes sp.)和扁舵鰹(Auxis thazard)。2011年平拖重要种为短吻红舌鳎、鰏属未定种(Leiognathuss sp.)、龙头鱼(Harpodon nehereus)和鳀(Engraulis japonicus);垂拖重要种为短吻红舌鳎、龙头鱼和中华小公鱼(Stolephorus chinensis)。经单因子相似性分析(ANOSIM),鱼卵仔稚鱼的优势种和重要种群落物种组成存在显著差异(P0.01)。研究结果表明,温度和盐度对鱼卵仔稚鱼的影响较大,夏季是中街山列岛海域鱼类产卵的重要时期。该海域以恋礁的小型底层鱼类为主,是褐菖鲉(Sebastiscus marmoratus)、龙头鱼等小型底层经济鱼类的重要产卵场和育幼场。     

三峡库区丰都江段鱼类早期资源现状

2014 年47 月鱼类主要繁殖季节于三峡库区(重庆丰都县)进行鱼类早期资源调查, 以了解三峡库区鱼类早期资源现状, 四大家鱼在库区是否仍有产卵场等问题。调查共采集到鱼类50 种, 分属5 目9 科。采集时, 卵汛出现在6 月后流速较高的时段, 其中贝氏的数量最多, 占86.24%。仔鱼出现的高峰期为6 月上旬至7 月中旬, 太湖新银鱼的数量最多, 占58.36%, 其次为子陵吻虎鱼, 占27.04%。在仔鱼中, 种类组成呈明显的月份差异, 4 月鲤、鲫占数量优势;5 月太湖新银鱼、子陵吻 虎鱼占数量优势;6 月后鲌亚科、鳅科、鳜等种类开始出现, 种类明显增多。通过估算, 调查期间流经丰都断面的鱼卵总量为4.37108 粒, 四大家鱼鱼卵为0.21107 粒;仔鱼总量为111.98108 尾, 四大家鱼仔鱼为0.21108 尾。通过产卵场的推算, 丰都断面采集的四大家鱼来自涪陵城区江段和三峡库区上游的重庆巴南区至朱沱镇江段的产卵场。通过对卵苗漂流密度的昼夜规律分析, 6 月之前夜间仔鱼密度显著高于白天, 而6 月后仔鱼昼夜漂流密度无显著差异。本研究表明, 从三峡库尾进入三峡水库的鱼类早期资源丰富, 大部分鱼类繁殖发生在6 月以后水温较高、入库流量大、库区流速较高的时段, 库尾部分江段在流速较高时期, 仍存在四大家鱼的产卵场。       

Intensive genetic assessment of the mating system and reproductive success in a semi-closed population of the mottled sculpin,Cottus bairdi

Most genetic surveys of parentage in nature sample only a small fraction of the breeding population. Here we apply microsatellite markers to deduce the genetic mating system and assess the reproductive success of females and males in an extensively collected, semi-closed stream population of the mottled sculpin fish, Cottus bairdi. In this species, males guard nest rocks where females deposit the eggs for fertilization. The potential exists for both males and females to mate with multiple partners and for males to provide parental care to genetically unrelated offspring. Four hundred and fifty-five adults and subadults, as well as 1,259 offspring from 23 nests, were genotyped at five polymorphic microsatellite loci. Multilocus maternal genotypes, deduced via genetic analyses of embryos, were reconstructed for more than 90% of the analysed nests, thus allowing both male and female reproductive success to be estimated accurately. There was no genetic evidence for cuckoldry, but one nest probably represents a takeover event. Successful males spawned with a mean of 2.8 partners, whereas each female apparently deposited her entire clutch of eggs in a single nest (mean fecundity = 66 eggs/female). On average, genetically deduced sires and dams were captured 1.6 and 9.3 metres from their respective nests, indicating little movement by breeders during the spawning season. Based on a 'genetic mark-recapture' estimate, the total number of potentially breeding adults (c. 570) was an order-of-magnitude larger than genetically based estimates of the effective number of breeders (c. 54). In addition, significantly fewer eggs per female were deposited in single than in multidam nests. Not only were perceived high-quality males spawning with multiple partners, but they were receiving more eggs from each female.     

Deceptive female oviposition behaviour elicits male ejaculation in the European bitterling

In European bitterling Rhodeus amarus , fish that lay their eggs in the gill chambers of living freshwater mussels, females perform conspicuous behaviours associated with spawning that increases the probability of males performing ejaculatory behaviour and participating in a spawning. A significant positive association was detected between behaviour in which a female performs a spawning action but without releasing eggs, here termed 'deceptive female oviposition', and ejaculatory behaviour by courting males.     

Reproductive biology of the endangered Oxleyan pygmy perch Nannoperca oxleyana Whitley

The reproductive biology of the Oxleyan pygmy perch Nannoperca oxleyana is described from simultaneous studies of wild populations in north-eastern New South Wales and mature fish held in aquaria. In the wild, 50% of males and females matured at total lengths of 24·0–25·9 and 28·0–29·9 mm, respectively. The species displays sexual dichromatism during the spawning season, with males developing more intense red and brown fin and body colouration, and black pelvic fins. Captive male broodfish displayed territoriality during the breeding season, closely guarding sites within artificial, plant-like substrata in which pairs of fish spawned adhesive eggs. Protracted serial spawning of wild and captive fish occurred from September to April and May at mean water temperatures ≥16·6° C and day length ≥10·7 h. Captive broodfish spawned on an average of 57% of days during the 256 day spawning period. Gonado-somatic indices averaged 0·7% for all ripe males and 4·1–4·2% for all ripe females collected. Mean total and batch fecundities of captive females were 1323 eggs per fish and 7·8 eggs per fish per day, respectively, and relative fecundity was 587 eggs g⁻¹ of body mass. Batch fecundity of wild females was estimated at 7·8 eggs per fish. The adaptive significance of this reproductive strategy in a harsh, variable environment is discussed.