Comparative life history of female mosquito fish,Gambusia affinis,in tidal freshwater and oligohaline habitats

Synopsis Female Gambusia affinis were collected from low salinity/near-neutral pH and tidal freshwater/low pH localities to document life history variation between these two habitats. Mosquito fish were 2.7 times more abundant in low salinity/near-neutral pH (2.37 ± 0.72 ppt; pH = 5.73 ± 0.11) than in tidal freshwater/low pH (0.27 ± 0.19 ppt; pH = 4.46 ± 0.18) habitats in Old Fort Bayou, a black-water tidal river of the Biloxi Bay estuary, Mississippi. Fish were common in both areas from June 1985 through February 1986. There were no significant differences between habitats in the length-weight relationships during the reproductive season with the exception of February. However, there were significant differences in life histories, although the areas were only 4 km apart. Ovarian recrudescence was observed in fish from both areas in February, but ovaries of low salinity fish were more developed than ovaries of freshwater fish. All fish from both areas had regressed ovaries by late September. The monthly slopes of the brood size vs. standard length regressions were significantly different between areas throughout the reproductive season with the exception of June. Although freshwater fish produced heavier tail-free embryos, fish from low salinity had significantly higher GSI values and brood size throughout the reproductive season. These observations suggest that Gambusia affonis may have a greater reproductive effort in low salinity/near-neutral pH environments than in freshwater/low pH habitats.     

Reproductive biology of the prawn Melicertus kerathurus (Decapoda: Penaeidae) in Thermaikos Gulf (N. Aegean Sea)

The reproductive biology including insemination frequency, ovarian maturation, gonadosomatic index (GSI), size at first maturity, and fecundity of Melicertus kerathurus were investigated using monthly samples from Thermaikos Gulf. Insemination was recorded by the presence of a spermatophore in the thelycum, and ovarian development was based on macroscopic ovarian staging (ST1–ST5). Inseminated females were found throughout the year with high percentages recorded from April to July. Although all ovarian stages were represented in mated females, insemination increased with size and ovarian maturation. High percentages of vitellogenic or mature ovarian stages were observed from May to July, while immature and developing ovaries were predominant mainly in winter. Spawned ovaries occurred from May to October. Carapace length at first maturity based on the presence of a spermatophore (CL_50sp) was estimated at 39.20 mm, while that based on the presence of vitellogenic and mature ovaries (CL_50ov) at 40.70 mm. The seasonal peak in the proportion of mature females (ST4) varied with size. Inseminated females at ST4 and GSI peaked in June–July. GSI varied in relation to insemination status and ovarian stage. In large females (>50 mm CL), the decline in mature ovaries and GSI increment with size indicates a relative reduction in the reproductive output. The number of oocytes ranged from 62,742 to 602,947 (mean ± SD: 268,000 ± 113,000). As the prawns are targeted during the spawning season, mainly by the artisanal fishery, and female size at first maturity is selected by artisanal net size, managerial measures toward artisanal fishery should be implemented.     

Age,growth and reproductive traits of invasive goby Taenioides cirratus in the Chaohu Lake,China

The eel goby Taenioides cirratus (Blyth, 1,860) is a small fish inhabits muddy bottoms of brackish-water in the Indo-West Pacific. It has invaded many inland freshwater lakes in China, such as the Chaohu Lake, Gaoyou Lake and Nansi Lake, and its population increased rapidly in these freshwater lakes in recent years. The age, growth and reproductive traits of T. cirratus invading the Chaohu Lake were studied. A total of 482 specimens (210 females, 204 males and 68 juveniles) with total length (TL) ranging from 9.4 to 20.6 cm were collected using the benthic fyke nets at monthly intervals from March 2018 to February 2019. The sagittal otolith was used for age determination. Monthly variation of marginal increment ratio indicated that the annual forming of opaque band on sagittal otolith was completed during March and April. For both sexes, only four (from 0+ to 3+ years) age groups were observed and 1+ and 2+ years age individuals dominated the population. Back calculated length at age showed males grew faster than females. Both sexes reached maturity at 1+ year age and the TL at first maturity (TL₅₀) was 12.6 cm for females and 11.9 cm for males. Monthly variation of gonado-somatic index indicated that the spawning occurred from May to August. The fecundity ranged from 967 ova to 5,114 ova, with a mean of 3,205 ova. Our study provides a comprehensive data on the key life history traits of T. cirratus for the first time.     

Effects of salinity on survival,daily food intake and growth of juvenile round goby Neogobius melanostomus (Pallas, 1814) from a brackish water system

The growth response of juvenile round gobies was tested in various salinities to assess the potential performance of the species in widely differing salinities in the Baltic Sea and adjacent waters. An experimental approach analysed the survival, daily food intake and growth of juvenile Neogobius melanostomus from the brackish water region of the Kiel Canal, Germany. Round gobies (TL = 61.5 ±5.3 mm; n = 40) were fed ad libitum (diet = mysid shrimp; energy content = 4.7 cal per mg dry weight) at salinity levels of 0.1, 7.5, 15 and 30 over 12 weeks. Two replicate tanks (125 × 30 × 30 cm; 100 L) were maintained for each salinity level, each holding five specimens at a mean temperature of 19.90°C (±0.64°C; n = 136). Survival at all levels was 100%, except for 0.1 with 90%. Mean daily food intake (% of body weight) increased from salinity level 0.1 to 15 (from 17.76% to 19.95%); specimens at level 30 showed a more diversified and lower mean food intake (18.44%). Mean cumulative body weight gain appeared greatest at a level of 7.5 (224.52%), closely followed by 0.1 (206.57%) and 15 (191.92%). In contrast, fish growth was significantly lower at a salinity level of 30 (100.57%). Results of the experiment indicate that salinity tolerance estimates of the round goby made by other authors are too low regarding specimens that stem from brackish habitats. Growth patterns of some specimens, even in the salinity 30 treatment, show that a further spread of this species into regions with higher salinities is quite possible.     

Length‐weight relationships,condition factor,gonadosomatic index‐based size at first sexual maturity,spawning season and fecundity of Aspidoparia morar (Cyprinidae) in the Jamuna River (Brahmaputra River distributary), northern Bangladesh

The present study describes the length‐weight relationships (LWRs), length‐length relationships (LLRs), Fulton's condition factor (K_F), size at first sexual maturity, spawning season, sex ratio and fecundity of the Morari Aspidoparia morar (Hamilton, 1822) (Cyprinidae). Sampling was done using traditional fishing gear jhaki jal (cast net) from July 2010 to June 2011. Total length (TL), fork length (FL) and standard length (SL) were measured with digital slide calipers. Individual body weight (BW) and gonad weight (GW) were determined to an accuracy of 0.01 g for all specimens. The gonadosomatic index (GSI) was calculated and size at first maturity for males and females estimated using GSI and TL as indicators. Female ≥ size at first maturity was used to determine fecundity. A total of 1200 specimens (males = 552, females = 648) ranging from 4.06–12.84 cm TL and 0.53–16.75 g BW were analyzed. The overall coefficient b for the LWR indicated positive allometric growth (>3.00) in males and isometric growth in females (~ 3.00). ancova (analysis of covariance) revealed significant differences between males and females (P < 0.001). All LLRs were highly correlated (r² > 0.973, P < 0.001). Sizes at first sexual maturity for males and females were 6.0 and 7.0 cm TL, respectively. K_F changed little throughout the year and GSI peaked in November to April, indicating the spawning season (GSImax = 15.0 in females, 2.0 in males). Mature females were dominant during the entire spawning season except in April. Mean total fecundity was 6700 ± 3500, ranging from 1860 to 19680. In addition, relative fecundity ranged from 190 to 1200 (mean 560 ± 235) in the Jamuna River. To ensure sustainable management of this species, the protection of mature individuals during the peak spawning season is highly recommended.     

Age,growth, reproduction and status of resource development of Ptychidio jordani,a critically endangered freshwater fish in the Hongshui River,China

The age, growth, reproduction and resource development status of Ptychidio jordani, as a critically endangered freshwater fish in the Hongshui River, China, was studied in this work. A total of 525 specimens were collected monthly using the cages and gillnets from October 2021 to September 2022 in the Hongshui River. The scale was used for age determination, and the maximum age for both female and male was estimated to be 5 years and 3 years, respectively. Female and male P. jordani showed different growth patterns, which were expressed as L_t = 261.3 (1-e^{−0.4885(t−0.1476)}) and L_t = 251.2 (1-e^{−0.4758(t+0.9643)}), respectively. The overall sex ratio was 1:0.47 (female:male). Female attained sex maturity at 2.34 years (192 mm body length). Month variation of the gonad somatic index indicated that the spawning period occurred from April to October. The absolute fecundity was estimated at 9046 ± 3434 eggs per individual, and the relative fecundity was 38.08 ± 15.77 eggs per gram. The exploitation rate of female and male was 0.233 and 0.495, which indicated that P. jordani was not overfishing. This study provided data on the key life-history traits of P. jordani, which has not been known previously and is essential for conservation strategy and policy development.     

Cyclic variations of gonad development of snow trout,Schizopyge niger from the river Jhelum of Kashmir Himalaya

The aim of the present study was to know the cyclic changes in gonadal maturation and to investigate the various developmental stages of oocytes and testicular germ cells of snow trout, Schizopyge niger. A total of 316 fish specimens were procured from three sites of river Jhelum during November 2016 to October 2017 and the histological analysis of their gonad was carried out. The results of the present study indicates that the male population was found significantly (p < 0.05) higher compared to female population and were reported as (1.6:1; M:F). The highest mean GSI value was recorded in male (12.24 ± 0.89) during the month of March and for female (13.74 ± 0.92) also in the month of March, which indicated that the gonadal development reached its peak during this month. The highest mean oocyte diameter was 1.04 ± 0.20 mm observed in March, showing that the oocyte reached its maturity during this month. Histological study of ovary revealed the evidence of vitellogenic yolk granule stage and ripe mature stage from December to April. While in case of male, four stages of spermatogenesis were distinguished and spermatozoa were highly abundant in the months of March and April. Mean ± SE of absolute fecundity was 3,224 ± 421 eggs and also, the relative fecundity was 69 egg/g body weight. Based on the results of GSI and the analysis of gonad histology, it was observed that the peak breeding season of S. niger occurred in the month of March. It was also noted that spawning period of this fish begins in the month of February and lasts till April with a peak breeding season in the month of March. The present study will provide a baseline information about the appropriate peak breeding of the fish, which will be useful for artificial breeding as well for prevention of fishing from natural habitat.     

Life cycle of a small endemic goby Economidichthys pygmaeus (Family: Gobiidae) in Lake Pamvotis,northwestern Greece

The life history tactics of the small endemic freshwater goby Economidichhtys pygmaeus were examined in a Mediterranean shallow lake located in the northwestern part of Greece (Lake Pamvotis). The purpose of the study was to define the population structure, growth, length at sexual maturity, sex ratio, annual cycle of gonad development, spawning characteristics and fecundity. Fish samples were collected using a hand net from October 2006 to September 2007. The fish grew rapidly in summer after hatching, matured after their first winter, and bred from February to June. The length frequency distribution indicated the appearance of two cohorts per year. Juveniles occurred in samples from April to September. Maturity length (ML₅₀) of males and females were estimated to be 23.54 and 23.28 mm TL, respectively. Sexual dimorphism related to the male caudal fin was observed and males grew to greater lengths than females. The overall ratio of males to females was 1:1.35. Spawning was from February to June. Ripe females appeared to be multiple spawners, with oocytes at different stages of development. Size of oocytes changed during maturation and increased in diameter from about 0.125 to more than 0.9 mm. Absolute fecundity was positively related to TL and TW and varied between 59 and 309 eggs. After reproduction, the number of parental individuals decreased.     

Reproductive biology of the endemic cyprinid fish Hypselobarbus thomassi (Day, 1874) from Kallada River in the Western Ghats,India

The study provides new information on the reproductive biology of Hypselobarbus thomassi (Day, 1874) an endemic cyprinid fish in the Western Ghats–a biodiversity hotspot of India. A total of 384 specimens (males = 244 and females = 140) were collected monthly from April 2009 to March 2011 from Kallada River of Southern part of Western Ghats, India. Monthly sex ratio indicated that males predominated in the fishery, and an overall sex ratio was significantly different from the expected value of 1:1 (male:female = 1:0.57, x² = 28.17, p < .01). Based on the monthly analysis of gonadosomatic index (GSI) and the proportion of mature individuals, the spawning season of H. thomassi occurred between May to October with a peak in June to August. The length at first maturity (L₅₀) recorded was 290 mm TL in males and 330 mm TL in females. The absolute fecundity ranged between 305 (234 mm TL) to 1,089 (414 mm TL) and relative fecundity oscillated between 77.3–220.69 oocytes/g of fish. The fecundity established a linear relationship with total length, body weight and ovary weight. The findings of this study would be very effective to impose sustainable conservation plan for this threatened species in Kallada River and other Western Ghats river systems.     

Effects of salinity on the growth and morphology of the invasive,euryhaline hydroid Cordylophora (Phylum Cnidaria,Class Hydrozoa)

The invasive, euryhaline hydroid Cordylophora sp. is a colonial cnidarian present in both freshwater and brackish water habitats. Individuals contend with osmotic stress at the tissue and cellular level. It has been suggested that this hydroid's ability to expand its range of distribution by invading new habitats is due in large part to an ability to acclimate to new salinities. The purpose of this study was to assess colony growth and morphological changes at various salinities in freshwater and brackish genotypes of Cordylophora sp. Single genotypes from a known freshwater clade (0.5 psu; Des Plaines River) and a known brackish clade (16 psu; Napa River) were cultured and gradually transitioned to 12 different salinities ranging 0.5–22 psu, and we characterized the growth rates and hydranth morphological features at each salinity. Colony growth was optimal at 0.5 psu for the freshwater genotype and 10 psu for the brackish genotype. Changes in hydranth morphology in the freshwater genotype were primarily observed at higher salinities, while morphological changes in the brackish genotype primarily occurred at lower salinities. Our results for the brackish genotype generally concur with previous work, but this study is the first to document the response of a freshwater genotype of Cordylophora sp. to various salinities. Differences in growth between these two genotypes strongly support the previously proposed existence of multiple cryptic species. Furthermore, because this hydroid is quite prevalent in freshwater and brackish systems as a fouling organism, understanding the effects of various salinities on the successful establishment of Cordylophora sp. is an important contribution to the understanding of the ecophysiology and management of this invasive hydroid.     

A multilevel trait-based approach to the ecological performance of Microcystis aeruginosa complex from headwaters to the ocean

The Microcystis aeruginosa complex (MAC) clusters cosmopolitan and conspicuous harmful bloom-forming cyanobacteria able to produce cyanotoxins. It is hypothesized that low temperatures and brackish salinities are the main barriers to MAC proliferation. Here, patterns at multiple levels of organization irrespective of taxonomic identity (i.e. a trait-based approach) were analyzed. MAC responses from the intracellular (e.g. respiratory activity) to the ecosystem level (e.g. blooms) were evaluated in wide environmental gradients. Experimental results on buoyancy and respiratory activity in response to increased salinity (0–35) and a literature review of maximum growth rates under different temperatures and salinities were combined with field sampling from headwaters (800 km upstream) to the marine end of the Rio de la Plata estuary (Uruguay-South America). Salinity and temperature were the major variables affecting MAC responses. Experimentally, freshwater MAC cells remained active for 24 h in brackish waters (salinity = 15) while colonies increased their flotation velocity. At the population level, maximum growth rate decreased with salinity and presented a unimodal exponential response with temperature, showing an optimum at 27.5 °C and a rapid decrease thereafter. At the community and ecosystem levels, MAC occurred from fresh to marine waters (salinity 30) with a sustained relative increase of large mucilaginous colonies biovolume with respect to individual cells. Similarly, total biomass and, specific and morphological richness decreased with salinity while blooms were only detected in freshwater both at high (33 °C) and low (11 °C) temperatures. In brackish waters, large mucilaginous colonies presented advantages under osmotic restrictive conditions. These traits values have also been associated with higher toxicity potential. This suggest salinity or low temperatures would not represent effective barriers for the survival and transport of potentially toxic MAC under likely near future scenarios of increasing human impacts (i.e. eutrophication, dam construction and climate change).     

Determination of batch fecundity in Uranoscopus scaber Linnaeus, 1758 from the Aegean Sea, Turkey

This work focuses on reproductive biology aspects of Uranoscopus scaber from the coast of the Aegean Sea. In total, 175 U. scaber specimens were collected in Izmir Bay using trawl nets from January to December 2005. The reproduction period according to gonadosomatic index (GSI) values extents from March to September, with a peak in May (6.32 ± 1.2). Oocyte diameters taken from 59 mature gonads ranged from 62 to 1860 μm. Maximum batch fecundity (BF) was 58091 (mean: 11556 ± 1445) in large oocytes in May, and 8408 (mean: 2821 ± 945) in hydrated oocytes in September. The BF increased with total length and body weight, excluding the ovaries. Total length (TL)–BF relationships and fish weight–BF relationships were BF = 0.0034TL^4.6248 (r = 0.47) and BF = 59.166W?3101 (r = 0.49), respectively, according to the oocyte size frequency method. There was a linear relationship between BF and fish size.     

Effects of salinity on the survival and aggression of the invasive Rio Grande cichlid (Herichthys cyanoguttatus)

The spread of non-native Rio Grande cichlids (Herichthys cyanoguttatus) in southeast Louisiana includes brackish habitats. We studied the effects of three different salinity levels on the biology of juvenile H. cyanoguttatus for 13 months to determine the potential of this species to spread through local estuaries. The highest salinity tolerated was 30.0 psu, and these fish did not survive acclimation to the 32 psu treatment. Fish in brackish conditions grew slower than fish in freshwater conditions, indicating a potential long-term detriment to juvenile fish living in brackish conditions. Aggression levels were notably higher for fish in brackish (15 psu) vs. freshwater conditions. This persisted through the entire experiment even after acclimation back to freshwater. This study indicates that higher salinity habitats in Louisiana can be tolerated by this species. It also raises a question about the effect of higher salinities on aggressive behavior.     

Reproductive biology of the freshwater goby Knipowitschia croatica Mrakovčić, Kerovec,Mišetić & Schneider 1996 (Actinopterygii,Gobiidae)

The reproductive biology and gonadogenesis cycle of the Vrgorac goby, Knipowitschia croatica is described [Conservation of Endangered Freshwater Fish in Europe, Birkhauser Verlag, Basel]. The species displays sexual dimorphism during the spawning period. Sexual maturity is achieved at an early age, with 50% of males and females sexually mature at total lengths of 40–45 mm. Fecundity of gravid females ranged from 188 to 593 eggs, with an egg diameter of 0.22–1.11 mm. Though the extended spawning period lasts from March to November, the highest intensity is observed from April to September. A comparison is made of the reproductive biology of this species with other sand goby species of the genera Knipowitschia and Pomatoschistus.     

The salinity preference of members of the genus Macroplea (Coleoptera, Chrysomelidae, Donaciinae), fully aquatic leaf beetles that occur in brackish water

Reed beetles (Donaciinae) of the genus Macroplea Samouelle, 1819 live permanently submerged. Literature indicates that Macroplea mutica occurs in brackish water, whereas Macroplea appendiculata is restricted to freshwater. The salinity preference of these two species was tested in a linear and a circular device that offered a continuous salinity gradient. The distribution of animals in the devices was monitored over at least 3 h in each of the 21 experiments. Both species preferred freshwater (salinity 0) over brackish water (salinity 10). In particular, this holds true for specimens collected in brackish water. Likewise, immediate reactions could be observed when during such experiments the direction of the gradient was reversed. While M. mutica can be regarded as a truly marine insect, this marine environment does not strictly reflect its fundamental niche with respect to salinity preference. This is in line with accumulating evidence that M. mutica can be found in freshwater habitats (and M. appendiculata in brackish water). This indicates that the species’ distribution might be influenced by other factors like host plant preference or dispersal mechanisms. It is discussed if—in spite of similar fundamental niches—differences in salinity tolerance (and hence performance in brackish water) may have contributed to speciation in the genus Macroplea.     

The reproductive cycle,size at maturity and fecundity of garfish (<Emphasis Type="Italic">Belone belone</Emphasis>, L. 1761) in the eastern Adriatic Sea

A recent study concerning the reproductive biology of the garfish (Belone belone, L. 1761) has been carried out in the eastern part of the Adriatic Sea along the Croatian coastline. Specimens of the fish (N = 3,393) were collected over a 6-year period (2003–2008). Their length varied between 20.8 and 75.4 cm (mean ± SD = 38.3 ± 7.94). Female garfish were dominant in larger length groups being most apparent during the resting phase of sexual cycle and in the peak of the spawning period, occurring in April and May. The sexual ratio of all specimens was m/f = 0.98. Males were prevalent in March—at the beginning of the highest spawning activities. Fifty percentage of the garfish population sexually matured at 28.5 cm of total length. Males and females reached their sexual maturity at 28.0 and 31.5 cm of total length, respectively. Spawning began in January peaking during March to May. According to their maturity stages, gonad weight and the gonadosomatic index, males began to spawn one month earlier (April) than females (May). The mean batch fecundity of garfish was 1,242.46 ± 843.64 of matured oocytes per ovary. Matured oocyte diameters ranged from 1.223 to 4.283 mm with the mean value of 2.269 ± 0.332 mm.     

Survival and behavioral characteristics of amphidromous goby larvae of Sicyopterus japonicus (Tanaka, 1909) during their downstream migration

To understand the ecology and environmental tolerances of newly hatched larvae of the amphidromous fish Sicyopterus japonicus during their downstream migration, the salinity tolerance of eggs, 0-15 day old larvae, and adults, and the temperature tolerance, specific gravity and phototaxis of hatched larvae were examined. Tolerances of adults were measured as survival after a 24 h challenge in freshwater (FW), brackish water (1/3 SW) and seawater (SW). The survival rate of adult S. japonicus was 100% in FW and 1/3 SW, while none survived in SW. Hatching success of eggs (30 eggs each) was significantly higher in FW (mean: 73%) and 1/3 SW (73%) than in SW (19%). Tolerance of newly hatched larvae to salinity and temperature was investigated in different combinations of salinities (FW, 1/3 SW and SW) and temperatures (18, 23 and 28 °C). Larval survival was significantly different in each salinity and temperature. Survival rate was significantly higher in 1/3 SW than in FW and higher in SW than in FW at 23 °C and 28 °C. At the latter part of the experiment, there was no survival in FW and at 28 °C. Survival was higher in lower temperatures, but larval development did not occur in FW. Specific gravity of newly hatched larvae was 1.036 at 28 °C and 1.034 at 23 °C. When exposed to a light source on one side of an aquarium, larval distribution was not affected. Our results indicated larval S. japonicus are more adapted to brackish water and seawater than freshwater, while the adults and eggs are more adapted to freshwater and brackish water than seawater. This is consistent with their amphidromous life history with growth and spawning occurring in freshwater and the larval stage utilizing marine habitats.     

Estuarine fish communities respond to climate variability over both river and ocean basins

Estuaries are dynamic environments at the land–sea interface that are strongly affected by interannual climate variability. Ocean–atmosphere processes propagate into estuaries from the sea, and atmospheric processes over land propagate into estuaries from watersheds. We examined the effects of these two separate climate‐driven processes on pelagic and demersal fish community structure along the salinity gradient in the San Francisco Estuary, California, USA. A 33‐year data set (1980–2012) on pelagic and demersal fishes spanning the freshwater to marine regions of the estuary suggested the existence of five estuarine salinity fish guilds: limnetic (salinity = 0–1), oligohaline (salinity = 1–12), mesohaline (salinity = 6–19), polyhaline (salinity = 19–28), and euhaline (salinity = 29–32). Climatic effects propagating from the adjacent Pacific Ocean, indexed by the North Pacific Gyre Oscillation (NPGO), affected demersal and pelagic fish community structure in the euhaline and polyhaline guilds. Climatic effects propagating over land, indexed as freshwater outflow from the watershed (OUT), affected demersal and pelagic fish community structure in the oligohaline, mesohaline, polyhaline, and euhaline guilds. The effects of OUT propagated further down the estuary salinity gradient than the effects of NPGO that propagated up the estuary salinity gradient, exemplifying the role of variable freshwater outflow as an important driver of biotic communities in river‐dominated estuaries. These results illustrate how unique sources of climate variability interact to drive biotic communities and, therefore, that climate change is likely to be an important driver in shaping the future trajectory of biotic communities in estuaries and other transitional habitats.     

Growth pattern and survival in populations of Poecilia vivipara (Teleostei; Poeciliidae) inhabiting an environmental gradient: a common garden study

We performed a common garden experiment to assess the existence of genetic differences on growth and body size between two populations of Poecilia vivipara inhabiting extremes of an environmental gradient caused by water salinity in lagoons of Northern Rio de Janeiro State, Brazil: the Campelo lagoon (freshwater) and Açu lagoon (brackish/saltwater). The two populations show extreme differences in average phenotypes for body size, shape and life history (freshwater populations with smaller body size, lower fecundity and larger reproductive allotment). Pregnant females were brought to the lab and the offspring from both groups were kept in a common recirculating system with freshwater. Standard length and survival were measured weekly over a period of 200 days and growth models were fitted and selected with information criteria. The offspring originally from the brackish water lagoon presented larger asymptotic length, higher maximum growth rate but lower survival than the offspring originally from the freshwater lagoon. Potential confounding variables such as density differences due to mortality and maternal effects (offspring size) were included as covariates in comparisons of growth rates between groups. The results are consistent with phenotypic differences among populations having some genetic basis, and with the existence of a trade-off between growth and maintenance due to the high growth/low survival observed in the group that changed from salt to freshwater. Comparisons of captive and natural populations suggest that the influence of environmental factors, such as salinity, food availability, fish density and predation should also be considered relevant to explain phenotypic variation in this system.