Fathead Minnow Populations Research Articles

Predicting the Effects of Endocrine Disrupting Chemicals on Fish Populations

This study evaluates the applicability and sensitivity of fish population dynamics modeling in assessing the potential effects of individual chemicals on population sustainability and recovery. Fish reproductive health is an increasingly important issue for ecological risk assessment following international concern over endocrine disruption. Life-history data from natural brook trout and fathead minnow populations were combined with effects data from laboratory-based studies, mainly concerning species other than brook trout and fathead minnows, to assess the likely impact of nonylphenol (NP) and methoxychlor (MXC) on brook trout (Salvelinus fontinalis) and fathead minnow (Pimephales promelas) population size. A delay differential equation (DDE) model with a 1-day timestep was used to predict the population dynamics of the brook trout and fathead minnows. The model predicts that NP, could enhance populations by up to 17% at a concentration of 30 µg l−1 based on the results of reduction in survival and increased fecundity from life-cycle toxicity tests, however attempting to allow for growth reduction and its effect on fecundity results in a prediction of a 28% reduction in population numbers. For fathead minnows the DDE model predicts that the same concentration of NP could cause a population reduction of 21%. The differences in these predictions are related to these two species having different life history strategies, which are considered in the parameterization of the model. Post-application concentrations of MXC may peak around 300 µg l−1 and then decline rapidly with time. Predictions show that such applications could cause a reduction of up to 30% in brook trout populations if the application occurs at the peak of the spawning season on successive years but that the effect would be less than 1% if the spawning season is avoided. Effects on the fathead minnow population size are predicted to be smaller (<4%) even if application occurs during the spawning period. Risk based statistics generated by the population dynamics models, such as interval decline risk or quasiextinction risk and predicted time to recovery complement traditional effects parameters such as LC50 and LOEC and may ultimately prove to be more useful in risk assessment.

Natural Disturbances and Fish: Local and Regional Influences on Winterkill of Fathead Minnows in Boreal Lakes

Abstract We investigated the population dynamics of fathead minnow Pimephales promelas and the environmental factors of four small lakes in the boreal forest of Alberta, Canada, for 5 years to determine the influence of local and regional factors on the development of hypoxia and the occurrence of fish winterkill. Fathead minnow densities varied considerably among lakes and years, with dramatic (47–94%) year-to-year declines occurring when dissolved oxygen levels were extremely low in the intervening winter. Large declines (presumed winterkills) occurred after 25% of the “lake-winters,” affecting three of four study lakes and 2 of 5 years. A fifth population in the same region, monitored for 15 years, displayed both a comparable frequency and temporal synchrony of large density decreases, suggesting that winterkill is a pervasive natural disturbance in small lakes of the Boreal Plains. In contrast to patterns displayed by larger fish species, smaller individuals in the fathead minnow populations were more...

Size distribution of aquatic invertebrates in two prairie wetlands, with and without fish, with implications for community production

1. We compared the size distribution of aquatic invertebrates in two prairie wetlands, one supporting a population of fathead minnows and the other fishless. Both wetlands were sampled in three depth zones on three dates, allowing assessment of temporal and spatial variation.2. We determined biomass of aquatic invertebrates in 17 log2 size classes, and used these data to develop normalized size spectra. We also coupled size distributions with an allometric model to estimate relative production at the community level.3. The composition of the invertebrate communities differed greatly between sites, and invertebrate biomass was higher in nearly all size classes in the fishless wetland. Intercepts of normalized size spectra were significantly different between wetlands, but slopes generally were not, indicating differences in standing‐stock biomass but similar size structures between the two invertebrate communities. Higher standing‐stock biomass in the fishless wetland resulted in higher relative production per unit area, but similar size distributions resulted in similar mass‐specific production (P/B) between wetlands.4. Our results indicate that invertebrate communities in prairie wetlands may have relatively consistent size structures in spite of large differences in community composition and standing‐stock biomass. We hypothesize that the observed differences are because of predation by the minnow population and/or differences in the macrophyte communities between the two sites. However, the relative importance of macrophytes and fish predation in structuring invertebrate communities in prairie wetlands is poorly known.

Population dynamics of Ornithodiplostomum ptychocheilus metacercariae in fathead minnows (Pimephales promelas) from four northern-Alberta lakes.

Annual, seasonal, and interlake variation in prevalence and intensity of Ornithodiplostomum ptychocheilus (Faust) metacercariae was assessed in populations of fathead minnows (Pimephales promelas) collected from 4 lakes in north-central Alberta. Mean metacercariae intensity in young-of-the-year minnows varied extensively (5-123 metacercariae/host) among year, month, and lakes. In 2 of the lakes, prevalence always reached 100%, and mean intensity always peaked in September or October. The high spatial and annual variation in metacercarial recruitment was partly attributable to variation in host size. but variation in water depth, temperature, snail densities, and bird visitation likely also played a role. A laboratory experiment demonstrated that host and metacercariae survival was intensity-independent during a period of simulated winter. Thus, metacercariae recruited in the fall survive until the following spring.

Effects of Fathead Minnow Colonization and Removal on a Prairie Wetland Ecosystem

Fish communities in prairie wetlands are extremely dynamic. Due to complete winterkills and periodic colonization, individual basins alternate between supporting a fish population and being fishless. Here we assess the ecological consequences of colonization and subsequent extinction of a fathead minnow population in a prairie wetland. We used a BACI-type approach (before-after control impact) in which data from the colonized wetland were paired with data from a similar fishless site first when both wetlands were fishless (1996), then when the minnow population reached moderate densities in the colonized site (1998), and then again when the colonized site became fishless after treatment with rotenone (1999). Fish colonization resulted in significant increases in turbidity, total phosphorus, and chlorophyll a in the water column; it also caused significant decreases in the abundance of aquatic insects and large cladocerans. Elimination of the minnow population largely reversed the effects of minnow colonization. Our results indicate that characteristics of prairie wetlands can vary as they alternate between supporting fathead minnow populations and being fishless and that ecological characteristics may change rapidly in response to minnow colonization or elimination.

Effects of social structure on reproductive activity in male fathead minnows (Pimephales promelas)

The selection of alternative reproductive phenotypes is often thought to be the result of physiological state, with small individuals forced energetically to postpone the allocation of resources to reproduction. However, for male fathead minnows (Pimephales promelas), we show that seasonal reproductive activity is modulated by social status. In enclosure and pond experiments, small males advanced their reproductive condition, held nesting territories, and spawned earlier in the reproductive season only when large males were absent or removed from the population. Since differences in the timing of reproduction among small males were not sizeor condition-dependent, the common explanation for the selection of alternative reproductive phenotypes, based on state-dependence, is insufficient. In the absence of large, socially dominant individuals, small males produced comparable numbers of offspring as the treatment with large males, although the offspring of these uninhibited small males were smaller at the end of the growing season than the young of large males. Thus, interactions among conspecifics may account for much of the phenotypic diversity observed within and among natural fathead minnow populations, through their direct and indirect effects on growth, recruitment and survival.

Influences of fathead minnows and aquatic macrophytes on nutrient partitioning and ecosystem structure in two prairie wetlands

Ecosystem structure and sizes of nutrient pools were compared between two prairie wetlands with contrasting food-web configurations, one site supporting a population of fathead minnows and the other fishless. It was hypothesized that the fishless wetland would typify the clear-water state, the wetland with fish the turbid-water state, and that macrophytes, epiphyton, and invertebrates would be the largest nutrient pools in the fishless site, while phytoplankton and fish would be the largest pools in the wetland with fish. Turbidity, water-column sedimentation rates, and amounts of phosphorus and nitrogen in epiphyton, macrophytes, metaphyton, phytoplankton, aquatic invertebrates, fathead minnows, seston, interstitial water, and sediment were assessed in each wetland. Nutrients were apportioned quite differently in these two wetlands. Major phosphorus and nitrogen pools in the wetland with fish were phytoplankton, seston, fathead minnows, and macrophytes, whereas macrophytes, epiphyton, and aquatic invertebrates were the largest pools in the fishless site. These differences likely reflect direct and indirect influences of fathead minnows and submerged aquatic plants. Our results support the hypothesis that low macrophyte abundance and dense populations of fathead minnows favor turbid conditions in deep, semipermanent prairie wetlands.

Brain-encysting parasites affect visually-mediated behaviours of fathead minnows

Many populations of fathead minnows (Pimephales promelas Rafinesque) in northern Alberta, Canada, contain individuals with hundreds of trematode (Ornithodiplostomum ptychocheilus Faust) cysts on the surface of their brains. Most cysts are located on the optic tecta, a region known to play a role in integrating visual and motor stimuli, especially in schooling fish. We determined the effect of infection on visually-mediated behaviours of fathead minnows by evaluating host performance in an optomotor swimming task. Monitoring this task involved recording the time minnows spent following a spinning drum, onto which alternating black and white stripes had been painted. After controlling for host activity and host size, minnows containing an average of 18 (low-intensity) or 98 larvae (high-intensity) reduced their time spent following the spinning drum by 42% and 26%, respectively, compared to uninfected controls. Low-intensity minnows also took longer than controls to respond to a change in the direction of the spinning drum. Reduced optomotor performance has the potential to affect a host’s ability to detect and respond to prey, predators and conspecifics.

Dispersion and Distribution of Marked Fathead Minnows (Pimephales promelas) in Prairie Wetlands

ABSTRACT Use of mark-recapture techniques to estimate the size of fish populations relies on several assumptions, one being that marked and unmarked fish have equal probabilities of capture. This requires that marked fish, if released at their site of capture, mix randomly with unmarked fish in the population before recapture. We tested whether marked fathead minnows (Pimephales promelas) in four wetlands mixed randomly 1) when released at the capture site and 2) when transported and released in the center of the wetland. We captured and marked fish at two locations on opposite sides of each wetland. We recaptured fish three weeks later at these two original capture stations and at two additional sites spaced equal distances from the two capture stations. Results using fish released at their original capture location and those released in the center of the wetland indicated no difference between the observed and the expected number recaptured, with the expectation being equal numbers caught at each station. Our results indicate that researchers using mark-recapture methods to estimate fathead minnow population size may release marked fish at the site of capture and obtain accurate estimates.

Differential survival of fathead minnows, Pimephales promelas, as affected by copper exposure, prior population stress, and allozyme genotypes

AbstractTwo populations of fathead minnows (F1, offspring of survivors of an acute fluoranthene exposure; N1, naive hatchery fish born in outdoor ponds) were concurrently exposed to approximately 850 μg/L of copper for 132 h. During the exposure, 49% of the F1 and 85% of the N1 minnows died. A curled operculum deformity, detected in 54% of the F1 population, was significantly related to mortality. A failure time regression model, combining both the F1 and N1 populations together, was fit to examine the relationship between population type (F1 or N1), body condition (weight/length3), presence of an operculum deformity, and different allozymes on time to death (TTD). The model indicated that type of population, body condition, the presence of an operculum deformity, and three loci (GPI‐1*, IDHP‐1*, and MDH‐2*) were significantly related to TTD. The F1 minnows had a higher survival rate and longer average TTD compared to N1 minnows. In comparison to the N1 population, the F1 population possessed a higher frequency of genotypes associated with increased survivorship at the IDHP‐1* and MDH‐2* loci. Weight (and body condition) was negatively correlated with survivorship. Minnows with a severe operculum deformity, believed to be a result of parental exposure to fluoranthene, had a 100% mortality rate and exhibited a considerably reduced TTD compared to nondeformed minnows. Multilocus heterozygosity was not related to TTD for either population. This study indicates that genetic factors may exhibit stronger effects on survivorship than other factors (e.g., weight/body condition) commonly associated with fitness.

Adaptation to fluoranthene exposure in a laboratory population of fathead minnows

AbstractTolerance of toxicant exposure is common in polluted sites in nature. However, in most cases, the processes underlying tolerance acquisition are not well understood. In the case of exposure to polycyclic aromatic hydrocarbons (PAHs) in particular, reports are lacking on the capacity for tolerance acquisition. To evaluate the potential for adaptation to this class of contaminants we exposed a large laboratory population of fathead minnows to fluoranthene, commonly a major constituent of PAH contamination. The exposure concentration chosen did not cause mortality but was sufficient to diminish reproductive capacity in exposed breeding fish relative to a control population. This level of exposure also significantly diminished hatching success and larval survivorship in the exposed population. The estimated value of selection against susceptibility (the product of all assessments of effect) was 0.67. Tests for tolerance acquisition included comparative larval 96‐h LC50 determinations, and comparative juvenile median time to death (LT50) determinations. Enhanced tolerance was not apparent in the LC50 determinations, although an examination of the concentration‐response distribution suggested an adverse effect due to egg and larval exposure to fluoranthene prior to the test. In contrast, results of the comparative LT50 determination indicated that the exposed population was approximately 30% more tolerant, relative to the control population, when exposed to an acutely toxic concentration of fluoranthene. These results suggest that tolerance to PAH exposure can occur in nature and that this population‐level response needs to be examined relative to other recognized effects in PAH‐contaminated areas.

Population and sex differences in antipredator responses of breeding fathead minnows (Pimephales promelas) to chemical stimuli from garter snakes (Thamnophis radix andT. sirtalis)

We conducted a predator bite survey on a population of fathead minnows (Pimephales promelas) considered to be under substantial predation pressure by western plains garter snakes (Thamnophis radix). Scarring, due to failed predation attempts by garter snakes and crayfish (Orconectes virilis), was observed significantly more often in breeding males than in breeding females and nonbreeding minnows. Likely, territorial nest defense under the edges of rocks along the water's edge, a habitat occupied by crayfish and frequented by snakes, caused the breeding males to be differentially vulnerable to predation. Under controlled laboratory conditions, breeding males from this population exhibited an antipredator response to chemical stimuli from live snakes (T. sirtalis andT. radix) significantly more often than breeding female minnows from the same population and breeding minnows of both sexes from a population that was presumed to be under lower predation pressure from snakes.

Acclimation‐induced changes in toxicity and induction of metallothionein‐like proteins in the fathead minnow following sublethal exposure to zinc

AbstractExperiments were conducted to determine if a correlation exists between acclimation‐induced alterations in Zn toxicity and the concentration of metallothionein‐like proteins (MTP) in a cyprinid fish. Populations of subadult fathead minnows,Pimephales promelas, were maintained in dechlorinated tap water and exposed to 0, 0.60 and 1.80 mg Zn/L for 35 d. Groups of animals (120 each) were removed from each acclimation population after 0, 7, 14, 21 and 35 d to conduct 96‐h acute tests. Tolerance and resistance were monitored as changes in the 96‐h LC50 and LT50 values, respectively. Tolerance increased a significant 28% above control values after 14 d of exposure to 0.60 mg Zn/L, but decreased significantly to 63 and 74% of control values following exposure to 1.80 mg Zn/L for 7 and 14 d (p&lt; 0.05). Acute toxicity values returned to control levels in both treatment groups after 21 d of exposure and remained stable to 35 d. The concentration of MTP was measured in the viscera (i.e., liver, spleen, intestine and pancreas) indirectly as the total Zn‐binding capacity in MTP‐containing fractions. MTP levels increased after 7, 14 and 21 d and plateaued to 35 d. Metal concentration in the MTP fraction reached a maximum of 31.6 μg Zn/g viscera, and following 28 d of exposure to 1.80 mg Zn/L concentrations declined to control levels after 7 d in diluent water. Whole‐body residues of Zn reached steady state after 7 d of exposure to 0.60 mg Zn/L but continued to increase after 35 d of exposure to 1.80 mg Zn/L. Zn‐acclimated animals were cross‐resistant but not cross‐tolerant to acute Cd challenge. There was no correlation of alterations in toxicity with observed changes in MTP concentration. Growth effects were observed at both acclimation exposure levels.

Effects of chronic exposure to acidified water on chemoreception of feeding stimuli in fathead minnows (Pimephales promelas): Mechanisms and ecological implications

AbstractA computer‐automated behavioral assay was used to assess the effects of reduced ambient pH on the response of fathead minnows to chemical feeding stimuli. The testing system quantified eight behavioral parameters based on the activity level of individual fish before and after exposure to a single‐pulse dose of a feeding stimulus—a tissue culture medium (1 ml; Eagle minimum essential medium with Earl's salts and L‐glutamine) — injected into the water circulation. Twentyfive adult minnows were tested after exposure to one of five pH levels: 8.0 (control), 7.0, 6.5, 6.0 and 5.5. Complete elimination of the feeding response occurred at pH = 6 and lower; responses at higher pH levels were not statistically different from those in the controls. Differences in water hardness (&lt;10 mg/L CaCO3 versus &gt; 160 mg/L CaCO3) and duration of exposure (72 h versus 30 d) had no influence on the pattern or degree of the pH effects. Scanning electron microscopy revealed no pathological or gross morphological correlates of the behavioral changes. To test for possible recovery, minnows that did not respond at pH = 6 were placed in water of control pH (8.0) for 24 h and then tested again; a statistically significant response to the feeding stimulus was restored. These results indicate that acute and chronic sublethal levels of acidification caused a reversible impairment of chemoreception and that the impairment can occur at a relatively high pH. The toxic effect probably involves mechanical and chemical inhibition of receptor cells in the olfactory and gustatory epithelia rather than actual destruction of chemosensory tissue. In nature, reduction of pH to approximately 6.0 could impair feeding behavior and reduce food intake, fecundity and long‐term survival of fathead minnow populations. Our results correspond well with environmental studies that show that fathead minnows are eliminated from natural waters when pH levels reach 5.8 to 6.0.

Heavy metal tolerance and metallothionein induction in fathead minnows: Results from field and laboratory investigations

AbstractToxicity studies were performed on two natural populations of fathead minnows (Pimephales promelas). One group of organisms was taken from a metal‐contaminated flyash pond associated with a coal‐fired power plant and the other population was collected from Newtown hatchery ponds. As determined in acute toxicity tests, ash pond minnows were significantly more tolerant of cadmium and copper than were hatchery minnows. At an exposure concentration of 6.0 mg Cd/L in moderately hard water, the median period of survival (LT50) for flyash pond minnows was 50.0 h, compared with 6.8 h for hatchery minnows. The LT50 values for ash pond and Newtown animals exposed to 0.50 mg Cu/L in moderately hard water were 17.0 and 4.5 h, respectively. Both populations were about equally tolerant of zinc. After ash pond minnows were transferred to reconstituted water in the laboratory, tolerance to cadmium and copper decreased significantly after 7 d. Conversely, tolerance was increased in hatchery minnows following acclimation in the laboratory to sublethal concentrations of cadmium. For example, after exposing hatchery minnows for 35 d to 10 μg Cd/L, an acute toxicity test gave LC50 values of 2.88 and 1.71 mg Cd/L for acclimated and control organisms, respectively. This induced tolerance in cadmium‐acclimated hatchery minnows was lost when organisms were transferred to cadmium‐free water for 7 d. The results justify the conclusion that fathead minnows develop increased tolerance to cadmium and copper following prolonged sublethal exposure to these metals, and that this metal‐induced tolerance is not sustained once organisms are removed from toxicant stress. As determined in biochemical studies, gill metallothionein concentrations followed the fluctuations in tolerance exhibited during cadmium acclimation and deacclimation. Thus, tolerance induction was, in part, attributed to increased production of metallothionein, a protein which selectively binds and deactivates cadmium, copper and certain other metals.

HEAVY METAL TOLERANCE AND METALLOTHIONEIN INDUCTION IN FATHEAD MINNOWS: RESULTS FROM FIELD AND LABORATORY INVESTIGATIONS

Toxicity studies were performed on two natural populations of fathead minnows (Pimephales promelas). One group of organisms was taken from a metal-contaminated flyash pond associated with a coal-fired power plant and the other population was collected from Newtown hatchery ponds. As determined in acute toxicity tests, ash pond minnows were significantly more tolerant of cadmium and copper than were hatchery minnows. At an exposure concentration of 6.0 mg Cd/L in moderately hard water, the median period of survival (LT50) for flyash pond minnows was 50.0 h, compared with 6.8 h for hatchery minnows. The LT50 values for ash pond and Newtown animals exposed to 0.50 mg Cu/L in moderately hard water were 17.0 and 4.5 h, respectively. Both populations were about equally tolerant of zinc. After ash pond minnows were transferred to reconstituted water in the laboratory, tolerance to cadmium and copper decreased significantly after 7 d. Conversely, tolerance was increased in hatchery minnows following acclimation in the laboratory to sublethal concentrations of cadmium. For example, after exposing hatchery minnows for 35 d to 10 μg Cd/L, an acute toxicity test gave LC50 values of 2.88 and 1.71 mg Cd/L for acclimated and control organisms, respectively. This induced tolerance in cadmium-acclimated hatchery minnows was lost when organisms were transferred to cadmium-free water for 7 d. The results justify the conclusion that fathead minnows develop increased tolerance to cadmium and copper following prolonged sublethal exposure to these metals, and that this metal-induced tolerance is not sustained once organisms are removed from toxicant stress. As determined in biochemical studies, gill metallothionein concentrations followed the fluctuations in tolerance exhibited during cadmium acclimation and deacclimation. Thus, tolerance induction was, in part, attributed to increased production of metallothionein, a protein which selectively binds and deactivates cadmium, copper and certain other metals.

Role of Fish in Regulation of Plant and Animal Communities in Eutrophic Ponds

Alteration of fish communities resulted in marked changes in the Zooplankton, phytoplankton, and benthic plant communities in nutrient-rich ponds. In ponds containing dense populations of fathead minnows (Pimephales promelas) and brook sticklebacks (Culaea inconstans), intense fish predation resulted in sparse cladoceran zooplankton populations and turbid water dominated by dense blue-green algal blooms. In ponds containing no fish, or dense populations of largemouth bass (Micropterus salmoides), abundant Cladocera reduced phytoplankton densities through heavy grazing, and these ponds were characterized by clear water and dense growths of Elodea canadensis, Potamogeton spp., and Cladophora sp. Manipulation of fish populations to control unwanted algal blooms in shallow eutrophic lakes may yield dense growths of equally undesirable macrophytes and periphyton.

Acidification effects on macroinvertebrates and fathead minnows ( Pimephales promelas) in outdoor experimental channels

Three outdoor experimental channels at the U.S. EPA Monticello (Minnesota) Ecological Research Station were employed to assess in the field acidification effects on macroinvertebrate communities and populations of fathead minnows ( Pimephales promelas). One channel served as ambient (pH 8), the second was acidified with sulfuric acid to pH 6 and the third to pH 5 for 17 weeks. Acidification did not markedly increase toxic metal concentrations in the channels; however, there was some evidence of possible mercury mobilization. Benthic macroinvertebrate densities were lower in the acidified channels during most of the study and final diversity indices were 2.1, 1.7 and 1.2 in the ambient, pH 6 and pH 5 channels, respectively. Drift of amphipods and leeches was stimulated in the pH 5 channel early in the study. The number of insect taxa and percentages emerging from the ambient, pH 6 and pH 5 channels, respectively, were 30 and 56%, 28 and 31%, and 18 and 13%. Macroinvertebrate tolerance to the acidification was classified as follows: damselflies, isopods and leeches most tolerant; chironomids, some amphipods ( Crangonyx) and flatworms of intermediate tolerance; and other amphipods ( Hyalella azteca) and snails ( Physa gyrina) most sensitive. Fathead minnow spawning and embryo production were similar in the ambient and pH 6 channels with little spawning and no eyed-embryos found in the pH 5 channel. Newly-hatched larval fish did not survive to the juvenile stage in the pH 6 channel. Continued acidification would likely have further decreased the diversity of the macroinvertebrate community, increased relative abundance of more tolerant species, and caused extinction of the minnow population.

Population and Production Estimates of Fathead Minnows in a South Dakota Prairie Wetland

Abstract Population and production estimates were obtained for a native population of fathead minnows (Pimephales promelas) in a 16.2-ha prairie wetland in South Dakota from May to September 1976. The estimated population of adult minnows declined from 194 in May and June to 26 in July. Production of adult minnows in the study area was estimated to be 0.10 kg (0.008 kg/ha per year). The estimated populations of young-of-the-year minnows were 21,020 in July, 126,505 in August, and 105,297 in September. The production of young-of-the-year minnows in the study area was estimated to be 174 kg (17.5 kg/ha per year). Young-of-the-year minnows contributed more than 99% to the 175.09 kg total production (17.5 kg/ha per year). Production by fathead minnows during the present study was underestimated because of the following factors: low initial brood stock density (less than 12/ha), receding water levels during the study period, exclusion of sexual products, exclusion of production by fry not surviving to be censu...

A Computer Simulation of the Effects of Superimposed Mortality Due to Pollutants on Populations of Fathead Minnows (Pimephales promelas)

This computer simulation study tested the effects of 99 levels of proportional mortality on fathead minnow (Pimephales promelas) populations, assuming that the parent–progeny relationship is of the form proposed by Ricker. Limited data from two studies on three lakes were available to fit the model and obtain estimates of parameters for fathead minnow populations. Based on these estimates, 25 replications of a 50-generation cycle were simulated under two levels of environmental variability. Arbitrary extinction levels of 5, 100, and 500 females had little effect on the results. Increasing environmental variation lowered the percentage mortality at which population extinctions occurred. In general, the results are compatible with the recommendation of the U.S. Committee on Water Quality Criteria, that the maximum concentration of zinc to which fish could be continuously exposed should not exceed [Formula: see text] the 96-hr TLm (median tolerance limit) — a concentration that caused a 50% reduction in the mean number of eggs laid per female by fathead minnows in a laboratory study.