Female Brook Trout Research Articles

Streamwide Evaluation of Survival and Reproduction of MYY and Wild Brook Trout Populations

AbstractBrook Trout Salvelinus fontinalis have been introduced across the western USA, where the species competes with and often replaces native salmonids. Nonnative Brook Trout are difficult to eradicate; thus, new removal strategies are needed. One novel methodology couples the partial suppression of wild Brook Trout with the replacement of MYY Brook Trout (males with two Y chromosomes). If MYY fish survive to reproduce with wild female Brook Trout, their progeny will be 100% male, which eventually shifts the sex ratio and theoretically extirpates the population. However, the effectiveness of this approach depends on survival and reproduction of MYY fish relative to the surviving wild conspecifics. From 2018 to 2020, we annually removed an estimated 45.7% of wild Brook Trout from three streams in New Mexico and stocked fingerling MYY Brook Trout (mean TL = 94 mm; range = 61–123 mm) targeting 50.0% of wild annual abundance estimates. Annual survival for MYY and wild Brook Trout was similar in Leandro Creek (MYY = 0.63 and wild = 0.63) and Rito de los Piños (MYY = 0.37 and wild = 0.46) but differed in Placer Creek (MYY = 0.28 and wild = 0.75). During spawning, we evaluated the reproductive potential of MYY Brook Trout by comparing the percentage of sexually mature male Brook Trout comprised of MYY fish to the percentage of hybrid (MYY × wild) F1 progeny. By the second spawning season (2019), MYY fish comprised 59.8, 50.4, and 34.5% of milt‐producing Brook Trout, which resulted in 55.1, 33.3, and 0% hybrid progeny in Leandro Creek, Rito de los Piños, and Placer Creek, respectively. We demonstrated that MYY fish exhibit similar vital rates compared with wild conspecifics in two of three streams; however, differences among streams highlights unforeseen variables that influence MYY survival and reproduction. The study offers promising results of the MYY approach for potentially eradicating unwanted Brook Trout populations.

A Nonlethal Genetic Assay for Discriminating Sex in Brook Trout

AbstractIdentifying sex in salmonids using nonlethal methods is valuable for controlled breeding in aquaculture and understanding behavior and dispersal for conservation. Although salmonids are male heterogametic, genetic sex identification has been problematic because a specific sex determination locus has been elusive. We implemented a blind analysis to identify sex in Brook Trout Salvelinus fontinalis using genetic primers (sdY) developed for Brown Trout Salmo trutta along with cytochrome c oxidase subunit I (COI_BT1) as an internal control. Fin clips were taken from 35 Brook Trout and analyzed via PCR and gel electrophoresis, and fish were simultaneously sacrificed and dissected to identify sex via gonads. The genetic and phenotypic analysis matched in 97% of the Brook Trout that were sampled, suggesting that this primer combination is successful in genetic sex determination. The ability to sex Brook Trout genetically will allow scientists to observe the difference in male and female Brook Trout behavior in the lab and natural stream ecosystems without using lethal methods.

Effect of double freezing fish semen on sperm motility and fertility

Refreezing freeze-thawed sperm has been found to improve the effectiveness of cryopreserved semen in mammals. However, such options have been never tested in fish. The objectives of this study were (i) to assess the effects of two freeze–thaw cycles on sperm motility parameters of different fish species (grayling, brown trout, Eurasian perch, Siberian sturgeon, rainbow trout, sex-reversed female (srf) rainbow trout, and sex-reversed female brook trout) and (ii) to test the fertilizing ability of rainbow trout sperm after two freeze–thaw cycles. Double freezing of semen resulted in the preservation of sperm motility in all samples, 59.2% post-thaw sperm motility was recorded for grayling, 49.4% for brown trout, 34.8% for Eurasian perch, 3.2% for srf rainbow trout and 7% for Siberian sturgeon semen. A significant correlation was found between the sperm motility of semen after single and double cryopreservation for most fish species. The fertilization rate at the eyed stage of rainbow trout fresh semen was approximately 64% and did not differ from the fertilization rate of semen after a single cycle of freezing. After two freeze-thaw cycles, significant decreases in the eyed stage (48.4%) and hatching stage (46.6%) were recorded. Our results clearly demonstrate the suitability of double cryopreservation of fish semen. The approach can be important for improved effectiveness of cryopreserved fish semen, which can potentially reduce costs and the space needed to maintain a bank of frozen semen. This method offers a new tool for the improved use of valuable samples of cryopreserved semen, which could increase the possibility of protecting endangered fish species and individuals that are extremely valuable for breeding aquaculture species.

The effects of a prolonged photoperiod and light source on growth, sexual maturation, fin condition, and vulnerability to fungal disease in brook troutSalvelinus fontinalis

The effects of an artificially prolonged photoperiod on growth, survival rate, colouration, and sexual maturation in brook trout Salvelinus fontinalis during pre- and post-spawning periods from 21 June to 06 November were investigated. Fish of mean initial weight ˜150 g were reared at ambient photoperiod as well as with an artificially prolonged photoperiod produced by either a light-emitting diode or a metal-halide light. The fish groups subjected to a prolonged period of artificial light grew significantly larger and had a higher survival rate (p < 0.05), regardless of sex, and showed lower occurrence of fungal disease compared to controls reared in the natural photoperiod. We found a significantly higher number of sexually mature fish in the control groups compared with experimental groups. The increased photoperiod effectively delayed gonad development and increased somatic growth in both male and female brook trout, and also increased resistance to fungal disease. The increased photoperiod produced an observable difference in fish colouration, with control groups exhibiting more intensive spawning colouration.

Comparative analysis of sperm freezability of sex-reversed female brook trout and sex-reversed female rainbow trout semen

The aim of our study was to compare quality of fresh and cryopreserved semen of sex-reversed female rainbow trout and sex-reversed female brook trout. The effect of different final sperm concentrations in straws with a constant extender concentration and final glucose concentration on the sperm motility parameters of cryopreserved semen was tested. Furthermore, we examined the effect of post-thaw storage time on sperm motility parameters at optimal sperm and glucose concentrations. The effects of semen cryopreservation from sex-reversed female rainbow trout and sex-reversed female brook trout on sperm fertilizing ability at the sperm-to-egg ratios 500,000:1 and 1,000,000:1 after 0 and 60 min of post-thaw storage of semen were investigated. We have demonstrated that final sperm concentrations in straws as well as final glucose concentrations in extended semen influenced post-thaw sperm motility parameters in both species. The high post-thaw sperm motility was recorded for sperm concentrations within the range of 1.0–4.0 × 109 spermatozoa ml−1 for both species. Furthermore, the glucose concentration appeared to be very important for cryopreservation success and its effect was species-specific. The final glucose concentrations of 0.15 M and 0.19 M, for sex-reversed female rainbow trout and sex-reversed female brook trout, respectively, produced the highest results for sperm motility after cryopreservation. The post-thaw sperm motility was unaffected by 60 and 360 min after thawing for sex-reversed female rainbow trout and sex-reversed female brook trout, respectively. The fertilization rates of cryopreserved semen of sex-reversed female rainbow trout were high and did not differ between the investigated sperm:egg ratios after 0 and 60 min of post-thaw storage. However, fertilization rates of cryopreserved semen of sex-reversed females brook trout were lower and decreased after 60 min of post-thaw storage of semen at sperm: egg ratio 1,000,000:1. In conclusion, our results demonstrated differences in freezability between the semen of sex-reversed female rainbow trout and sex-reversed female brook trout. The semen of both species can be cryopreserved at the final sperm concentration of 3.0 × 109 spermatozoa ml−1. However, the influence of final glucose concentration on cryopreservation success, as well as the duration of post-thaw storage time and fertilization rates, are species-specific. In our opinion, standardizing the procedure of semen cryopreservation presented in this study is a prerequisite for the development of repeatable procedures and the future implementation of cryopreserved semen from sex-reversed females into hatchery practice.

Sex reversal of brook trout (Salvelinus fontinalis) by 17α-methyltestosterone exposure: A serial experimental approach to determine optimal timing and delivery regimes

Commercial culture of Brook trout (Salvelinus fontinalis) in Tasmania was partly abandoned due to sexual maturation of male fish early on during the estuarine rearing phase. Maturation adversely affects body mass, flesh quality and immunocompetency effectively. Sex reversal techniques such as the in-feed addition of a synthetic androgen have proven difficult to adapt in brook trout. An appropriate timing, duration and delivery vehicle for administration of 17α-methyltestosterone (MT) to produce phenotypic males (neomales) from genotypically female brook trout required further investigation. In this study, groups of brook trout eggs (n=1000) maintained at 9.5±0.15−10±0.14°C, were immersed in MT (400μgL−1) for four hours on two alternate days (two immersions/group) staggered over a two week period surrounding the hatch of embryos (control groups excluded). The groups were then split and half received MT-supplemented feed for 60days and the other a standard diet. Following an 11 month on-growing period sex phenotypes were determined by gross & histological gonad morphology. The highest proportion of male phenotypes (75%) was found in fish immersed six and four days pre-hatch and subsequently fed a normal diet. Fish fed a MT supplemented diet and immersed in MT showed significantly higher proportions of sterile fish. These data indicate that a pre-hatch immersion-only regime (4–6days pre-hatch at 9.5°C) should be pursued as a target for optimization studies to further refine the effective concentration and duration of exposure to MT for the successful production of neo-male brook trout.

Effects of maternal stress and cortisol exposure at the egg stage on learning, boldness and neophobia in brook trout

The environment experienced by females can have long-lasting effects on offspring phenotype. The objective of this study was to determine if maternal stress-induced behaviour reprogramming in offspring is found in brook char and to test whether cortisol is the main mediator, by separating the potential effects of cortisol from that of other potential maternal factors. We exposed female brook trout (Salvelinus fontinalis) to different parallel treatments during the oogenesis period: undisturbed as controls (1) fed cortisol through food (2) or physically stressed by handling once a week (3). Additionally, we exposed half of the control eggs to a cortisol suspension before fertilisation (4). Cortisol consumption and handling did not elevate either maternal plasma or egg cortisol, although egg cortisol level was significantly increased when eggs were bathed in the suspension. We measured spatial learning and memory, boldness and neophobia in 6 month-old offspring and found no effects of treatments on learning, memory or behaviour. Our results suggest that the relationship between maternal stress, circulating and egg cortisol levels, other maternal factors, and behavioural reprogramming is context and species-specific.

Monthly variations in the profile of sex steroids and ovarian development of brook trout (Salvelinus fontinalis Mitchill) during the annual reproductive cycle in Tasmania

Control of maturation in all-female salmonid stock can be achieved by manipulating photoperiod. This initially requires an assessment of the annual reproductive cycle under ambient photoperiod. This study therefore describes the seasonal variations in ovarian development and plasma profiles of oestradiol-17β and testosterone during the second year of a population of female brook trout. It was found that fish recruited for maturation following the summer solstice in December. Shortening of photoperiod likely stimulated the brain–pituitary–gonadal axis for vigorous progress of ovarian development, as indicated by increasing levels of oestradiol-17β and gonadosomatic index values. The highest gonadosomatic index value (18.26 ± 1.1%) was observed in June. During July, eggs were released from mature females by manual stripping. Total fecundity was observed as 4266 ± 341 oocytes per mature fish. Profiles of sex steroids observed here were found to be finely entrained within the succession of seasonal photoperiod. This finding indicates that maturation suppression by photoperiod manipulation should be instigated before initiation of oocyte recruitment that commenced during December.

Monthly variations in the profile of sex steroids and gonadal development of brook trout (Salvelinus fontinalis Mitchill) during puberty in Tasmania

Puberty is an important reproductive event that has not been studied in brook trout in the Southern Hemisphere. The present study describes the histological development of gonads and variations in the levels of oestradiol-17β (E2), testosterone (T) and 11-ketotestosterone (11-KT) during the first year of male and female brook trout. The study started when the trout were at an age of 8 months (n = 10) and continued until they were 15 months. Of the males, 60% attained puberty at an age of 14 months. The peak of the gonadosomatic index in males was observed at 15 months of age (3.0 ± 0.9%). Female fish did not achieve maturation during their first year and ovarian development did not progress beyond the vesicular oocyte stage within the study period.

Rapid Visual Assessment to Determine Sex in Brook Trout

AbstractAlthough sex‐specific processes play a considerable role in the ecology of many fishes, nonlethal tools to determine sex in most species outside of the spawning season are lacking. We identified a suite of sexual characteristics in Brook Trout Salvelinus fontinalis by surveying the available literature, consulting biologists, and reviewing images of known‐sex individuals. Using pretraining and posttraining testing of fisheries professionals, we assessed the utility of color and morphology for sexing fish across a length gradient (110–251 mm TL) by presenting images of mature fish collected during the spawning season. Rapid visual assessment proved to be an effective approach for determining the sex of Brook Trout. Average accuracy significantly improved from 71.5% before training to 92% after training, and the proportion of fish scored as unknown was reduced (from 9.5% before to 2.0% after training). We found a small yet significant positive relationship between TL and the proportion of fish that were correctly sexed. With the exception of a single 113‐mm male, all individuals were correctly sexed by at least 79% of respondents after training. No significant differences were found among respondents based on education, experience, or confidence level. The effectiveness of rapid visual assessment also did not significantly differ between male and female Brook Trout. Rapid visual assessment is a viable technique for the determination of sex in Brook Trout.Received September 18, 2012; accepted March 6, 2013

Perturbation in protein expression of the sterile salmonid hybrids between female brook trout Salvelinus fontinalis and male masu salmon Oncorhynchus masou during early spermatogenesis

Most males and females of intergeneric hybrid (BM) between female brook trout (Bt) Salvelinus fontinalis and male masu salmon (Ms) Oncorhynchus masou had undeveloped gonads, with abnormal germ cell development shown by histological examination. To understand the cause of this hybrid sterility, expression profiles of testicular proteins in the BM and parental species were examined with 2-DE coupled with MALDI-TOF/TOF MS. Compared with the parental species, more than 60% of differentially expressed protein spots were down-regulated in BM. A total of 16 up-regulated and 48 down-regulated proteins were identified in BM. Up-regulated were transferrin and other somatic cell-predominant proteins, whereas down-regulated were some germ cell-specific proteins such as DEAD box RNA helicase Vasa. Other pronouncedly down-regulated proteins included tubulins and heat shock proteins that are supposed to have roles in spermatogenesis. The present findings suggest direct association of the observed perturbation in protein expression with the failure of spermatogenesis and the sterility in the examined salmonid hybrids.

Influence of Variable Interannual Summer Water Temperatures on Brook Trout Growth, Consumption, Reproduction, and Mortality in an Unstratified Adirondack Lake

AbstractStressful water temperatures negatively affect physiological processes in fishes, yet evidence for how elevated temperatures influence population‐level characteristics is rare. An 8‐year field study of brook trout Salvelinus fontinalis in an unstratified Adirondack lake revealed that an aggregate measure of chronically stressful summer water temperatures strongly influenced brook trout population‐level characteristics. We quantified chronic thermal stress using the cumulative degree‐days over which bottom temperatures exceeded a range of thresholds (18–22°C) and found that the strongest relationships were with cumulative degree‐days over 20°C (DD&gt;20). Across years with similar brook trout densities, warmer summer water temperatures resulted in decreased brook trout growth, but growth was not reduced in a year with high water temperatures and low density. Maximum stomach fullness was negatively related to water temperature. Reproductive activity was negatively correlated with stressful summer thermal conditions and was less dependent on female brook trout density. Periods of chronically stressful summer water temperatures resulted in the apparent mortality of age‐2 and older fish during a moderately stressful summer (156 DD&gt;20) and apparent mortality of age‐1 and older fish during the most stressful summer (210 DD&gt;20). In contrast, extensive mortality was not evident in any year‐classes when DD&gt;20 was less than 115. Anthropogenic impacts on the thermal conditions of aquatic ecosystems occur at small (hydrology) and large (climate change) spatial extents, and in situ studies at the margin of an organism's thermal range can provide important insights into changes that will occur if temperatures increase.

Successful production of monosex female brook trout Salvelinus fontinalis using gynogenetic sex reversed males by a combination of methyltestosterone immersion and oral treatments

Recent advances towards the production of all-female populations of brook trout Salvelinus fontinalis have shown that methyltestosterone treatments applied after first feeding were not efficient in inducing masculinisation in neomales. A process to create all-female populations of brook trout using methyltestosterone was determined from 4 experiments. Different protocols of masculinisation by oral and/or hormonal immersion treatments were tested and the sex ratio of progeny produced from the cross of sex-reversed gynogenetic males with normal females was checked. No sexual dimorphism for growth was recorded, at least until two years of age. Masculinisation trials using oral treatments failed to produce any reversal of sex. Only the application of four weekly immersions beginning one week before hatching, combined with an oral treatment, provided 100% masculinisation. Gynogenetic sex-reversed males crossed with normal females produced 100% females, demonstrating the efficiency of this protocol in reversing the sex of brook trout. This result also highlights an earlier and longer period of sex lability, compared with most other Salmonids, and the need to initiate immersion treatments a long time before the initiation of gonadal differentiation.

Maternal transfer of carbon and nitrogen to progeny of sea-run and resident brook trout (Salvelinus fontinalis)

Early-life history characteristics of sea-run and resident salmonid fishes that co-exist in sympatry are poorly understood. To assess differences in characteristics of offspring of these two strategies following their emergence to the stream, stable isotope ratios (δ13C and δ15N) were used to identify the progeny of resident and sea-run female brook trout ( Salvelinus fontinalis ) in a tributary of the Miramichi River, New Brunswick, Canada. Upon emergence, sea-run progeny captured in emergence traps were longer, heavier, and had higher carbon to nitrogen ratios (C/N, an indicator of lipid content) than resident progeny, but did not have a higher body condition. After some feeding and growth in the stream, sea-run progeny (still identifiable by their δ13C and δ15N) captured in dip-nets became longer, heavier, had higher C/N, and had higher condition than resident progeny. The proportion of fish caught in dip nets at sites where the two forms co-existed was dominated by offspring of sea-run females (sea-run = 56%, resident = 19%, unknown = 25%). These analyses suggest that offspring of sea-run, female trout benefit from maternally derived energy reserves gained at sea and thus have potential adaptive advantages over offspring of residents.

Reproductive status in female perch ( Perca fluviatilis) outside a sewage treatment plant processing leachate from a refuse dump

Reproductive failure and endocrine disruption in female perch ( Perca fluviatilis) and female brook trout ( Salvelinus fontinalis), naturally exposed to toxic leachate from a municipal refuse dump, has raised concerns about the reproductive status in female fish outside the sewage treatment plant (STP), which now treats the same leachate. Female perch from the receiving water, Lake Siljan, were therefore sampled in a gradient from the STP in Fornby and Österviken and compared with reference females from Isunda or Sollerön. A high resolution gas chromatography/high resolution mass spectrometry (HRGC/HRMS) technique enabled the analysis of circulating steroids in blood plasma: the sex steroids progesterone (P), 17α-hydroxyprogesterone (17α-OHP), androstenedione (A), testosterone (T), estrone (E1), and 17β-oestradiol (E2), the maturation-inducing hormone (MIH) 17α,20β-dihydroxy-4-pregnen-3-one (17α,20β-P), and the glucocorticoids 11-deoxycortisol (11-DC) and cortisol (C). Body and organ measurements, ratio of sexually mature (SM) females, and frequencies of body lesions, were also recorded. The percentages of SM females found in Fornby on Oct 23 (83.8%) and in Österviken on May 14 (54.0%) were lower compared with Isunda (100%). The lack of differences in gonadosomatic index (GSI) and liver-somatic index (LSI) indicated a normal vitellogenesis in SM females from Fornby and Österviken. No differences in circulating levels of steroids were observed between the sites, except at the time of spawning, when elevated P and 17α-OHP levels in females from Österviken indicated a steroidogenic shift from the synthesis of sex steroids towards the production of the MIH 17α,20β-P. The significant peak-levels of both 17α,20β-P and 11-DC in female perch from Österviken at the same time suggest that both are involved in controlling final oocyte maturation in perch. The elevated levels of androgens (A and T) in females caught on the same occasion in Fornby implied that these females were at an earlier stage of oocyte maturation, prior to the steroidogenic shift towards MIH-production. This study confirms the results of an earlier study revealing low numbers of SM female perch outside the STP in Fornby. The lack of associated effects on organ indexes and circulating levels of steroids does not indicate exposure to endocrine disrupting substances (EDSs) from the treated sewage. The consistently high numbers of SM females in the northern part of Siljan suggest, however, that natural factors are not responsible for the reproductive failures in the southern part of the lake.

A partial-reuse system for coldwater aquaculture

A model partial-reuse system is described that provides an alternative to salmonid production in serial-reuse raceway systems and has potential application in other fish-culture situations. The partial-reuse system contained three 10m3 circular ‘Cornell-type’ dual-drain culture tanks. The side-wall discharge from the culture tanks was treated across a microscreen drum filter, then the water was pumped to the head of the system where dissolved carbon dioxide (CO2) stripping and pure oxygen (O2) supplementation took place before the water returned to the culture tanks. Dilution with make-up water controlled accumulations of total ammonia nitrogen (TAN). An automatic pH control system that modulated the stripping column fan ‘on’ and ‘off’ was used to limit the fractions of CO2 and unionized ammonia nitrogen (NH3N). The partial-reuse system was evaluated during the culture of eight separate cohorts of advanced fingerlings, i.e., Arctic char, rainbow trout, and an all female brook trout × Arctic char hybrid. The fish performed well, even under intensive conditions, which were indicated by dissolved O2 consumption across the culture tank that went as high as 13mg/L and fish-culture densities that were often between 100 and 148kg/m3. Over all cohorts, feed conversion rates ranged from 1.0 to 1.3, specific growth rates (SGR) ranged from 1.32 to 2.45% body weight per day, and thermal growth coefficients ranged from 0.00132 to 0.00218. The partial-reuse system maintained safe water quality in all cases except for the first cohort—when the stripping column fan failed. The ‘Cornell-type’ dual-drain tank was found to rapidly (within only 1–2min) and gently concentrate and flush approximately 68–88% (79% overall average) of the TSS produced daily within only 12–18% of the tank’s total water flow. Mean TSS concentrations discharged through the three culture tanks’ bottom-center drains (average of 17.1mg/L) was 8.7 times greater than the TSS concentration discharged through the three culture tanks’ side-wall drains (average of 2.2mg/L). Overall, approximately 82% of the TSS produced in the partial-reuse system was captured in an off-line settling tank, which is better TSS removal than others have estimated for serial-reuse systems (approximately 25–50%). For the two cohorts of rainbow trout, the partial-reuse system sustained a production level of 35–45kg per year of fish for every 1L/min of make-up water, which is approximately six to seven times greater than the typical 6kg per year of trout produced for every 1L/min of water in Idaho serial-reuse raceway systems.

Altered steroid metabolism in several teleost species exposed to endocrine disrupting substances in refuse dump leachate

Endocrine disruption associated with reproductive failure has been reported previously in female perch ( Perca fluviatilis) and roach ( Rutilus rutilus) from Lake Molnbyggen in Sweden and in female brook trout ( Salvelinus fontinalis) from Vadbäcken, a stream emptying into Molnbyggen. Both Molnbyggen and Vadbäcken have been contaminated by toxic leachate from a municipal refuse dump. In this study, female perch were caught in Molnbyggen and the reference lake, Lake Djursjön, to further investigate the endocrine mechanism behind the significant numbers of sexually immature (SIM) female perch in Molnbyggen. Blood plasma analysis of progesterone (P), 17α-hydroxyprogesterone (17α-OHP), testosterone (T), and 17β-oestradiol (E2), as well as analysis of brain aromatase activity (P450arom), were carried out. The exceptional high numbers of SIM female perch in Molnbyggen was confirmed in February 1999. In July 1999, at an early stage of oogenesis, perch from Molnbyggen showed significantly decreased gonadosomatic index (GSI) and aromatase activity. The presence of aromatase inhibiting substances in lake sediments were therefore tested in vitro. The aromatase activity was dose-dependently inhibited by clotrimazole, reaching 50% inhibition at a concentration of 0.9 μM. Aromatase inhibiting substances were found both in Molnbyggen and reference sediment extracts, indicating that they were naturally occurring substances and not of anthrophogenic origin. The similar decrease in levels of circulating steroids (P, 17α-OHP, T, and E2), aromatase, and GSI therefore suggest that the low aromatase activity is due to down-regulation rather than inhibition. To further investigate the steroidogenesis prior to T, P, and 17α-OHP were analysed in perch caught in 1997 and 1998 in Lakes Molnbyggen, Kvarntjärn (downstream), Yxen (upstream), and Djursjön, in female roach caught in Molnbyggen and Djursjön in 1997, and in brook trout caught in Vadbäcken and the reference stream Björntjärnsbäcken in 1998. The absence of differences in P and 17α-OHP levels, combined with a significantly lower T level in female perch and roach from Molnbyggen in 1997, could be the result of either increased metabolism and excretion of T, or a disruption downstream of 17α-OHP formation. The unaffected P levels and significantly lower 17α-OHP levels, together with significantly decreased T and E2 levels, found in adult (>45 g) female brook trout from Vadbäcken, further indicated that an altered steroidogenesis downstream of P is one possible mechanism underlying the low T levels and thus the high number of SIM female fish, since too low T levels might be insufficient to activate the brain–pituitary–gonadal axis.

In vitro steroid production by triploid ovarian follicles

Triploid female fish are unlike normal diploids in that they show impaired oocyte growth and are unable to produce viable offspring. Compared to diploid siblings, triploid female brook trout ( Salvelinus fontinalis) have significantly lower levels of plasma vitellogenin, testosterone (T), and estradiol-17β (E 2), and show significantly lower gonadosomatic indices. The purpose of this study was to compare in vitro steroid production by diploid and triploid ovarian follicles in order to determine whether delayed oocyte growth in triploids is the result of a decreased capacity for E 2 synthesis by triploid follicles. Intact oocytes (i.e., including follicular tissue) from several age 3 + triploids were pooled and sorted according to small (<2 mm), medium (2.2–3.0 mm), and large (4.0–5.0 mm) diameter size, and compared to same-sized vitellogenic oocytes from 3 + diploids. Oocytes were incubated for 8 h at 15 °C in the presence or absence of gonadotropin stimulus (SG-G100) and the precursors E 2, T, and cholesterol. Estradiol-17β output following SG-G100 treatment was significantly higher than in controls for both diploid and triploid follicles. Production of E 2 was dependent on stage of oocyte and follicular development in triploids, with triploid and diploid follicles of similar diameter producing similar amounts of E 2 with SG-G100 stimulus. This suggests that ovarian steroidogenesis is similar in diploids and triploids.

Probable Pheromonal Attraction of Sexually Mature Brook Trout to Mature Male Conspecifics

Invasions of nonnative species are a leading cause of the loss of populations of cutthroat trout Oncorhynchus clarki in the western United States, but effective control of invasions of species such as brook trout Salvelinus fontinalis are expensive, labor intensive, and not species specific. Because salmonids respond to chemical cues, we hypothesized that pheromonal attraction could be exploited to capture brook trout. We conducted two tests as brook trout spawning began in 1999 and 2000 in two streams in Wyoming, using hoop nets seeded with sexually mature male brook trout, sexually mature female brook trout, or no fish. Catches of sexually mature fish in both tests were nonrandom (P < 0.001); traps seeded with sexually mature male brook trout resulted in a disproportionately high number of captures. Catches of fish that were not sexually mature were not significantly different from expected values (P > 0.075). In the second test, we estimated that 23% of adult fish were captured in hoop nets in 8 d. Catches were biased toward males by 10:1, and once-captured fish were rarely recaptured. We believe that with further refinements, this approach may be useful for sampling or manipulating brook trout and perhaps other salmonid populations.

Endocrine disruption in brook trout ( Salvelinus fontinalis) exposed to leachate from a public refuse dump

Lake Molnbyggen was previously found to harbour a large number of sexually immature female perch ( Perca fluviatilis) suffering from endocrine disruption. In an attempt to pin-point the source of the endocrine-disrupting substance(s) (EDSs), brook trout ( Salvelinus fontinalis) from Vadbäcken, a stream contaminated by leachate from a public refuse dump and which empties into Lake Molnbyggen, were investigated. In addition, female perch from Lakes Yxen and Kvarntjärn, located up-stream and down-stream of Lake Molnbyggen, were investigated. Only 16.7% of the adult female brook trout in Vadbäcken were sexually mature, associated with decreased gonadosomatic index, lower brain aromatase activity, and lower circulating levels of testosterone and 17 β-oestradiol, in comparison to female brook trout from the reference stream Björntjärnsbäcken. Male brook trout showed decreased gonadosomatic index, in addition to bile duct hyperplasia in the liver, which was also found in female brook trout livers from Vadbäcken. In Lake Molnbyggen, 57.6% of the female perch were found to be sexually immature with high frequencies of skin lesions, such as sores and fin erosion, significantly decreased gonadosomatic index, lower aromatase activity, and lower levels of testosterone and 17 β-oestradiol. No signs of reproductive disorders or endocrine disruption were seen in female perch from Lakes Yxen and Kvarntjärn compared to female perch from the reference lake, Lake Djursjön. Since brook trout of both sexes from Vadbäcken displayed the same kind of serious adverse impairment of gonad development and endocrine disruption as perch from Lake Molnbyggen, very strong evidence are provided that the refuse dump is the source for the responsible EDS(s), since both Vadbäcken and Lake Molnbyggen are known to be contaminated by leachate from that dump. The low levels of PAHs and PCBs in the surface sediments of Lake Molnbyggen suggest that these pollutants are not the responsible EDS(s).