Minnow Populations Research Articles

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.

Pelagic C:N:P Stoichiometry in a Eutrophied Lake: Responses to a Whole-Lake Food-Web Manipulation

Changes in the ecological stoichiometry of C, N, and P in the pelagic zone are reported from a whole-lake manipulation of the food web of Lake 227, an experimentally eutrophied lake at the Experimental Lakes Area, Canada. Addition of northern pike eliminated populations of planktivorous minnows by the third year (1995) after pike introduction, and in the fourth year after pike addition (1996), a massive increase in the abundance of the large-bodied cladoceran Daphnia pulicaria occurred. Accompanying this increase in Daphnia abundance, zooplankton community N:P declined, seston concentration and C:P ratio decreased, and dissolved N and P pools increased. During peak abundance, zooplankton biomass comprised a significant proportion of total epilimnetic phosphorus (greater than 30%). During the period of increased Daphnia abundance, concentrations of dissolved inorganic nitrogen (TIN) increased more strongly than dissolved phosphorus (TDP), and thus TIN:TDP ratios were elevated. Sedimentation data indicated that increased grazing led to greatly reduced residence times of C, N, and especially P in the water column during 1996. Finally, previously dominant N-fixing cyanobacteria were absent during 1996. Our results show that strong effects of food-web structure can occur in eutrophic lakes and that stoichiometric mechanisms play a potentially important role in generating these effects.

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.

Stoichiometric Constraints on Food-Web Dynamics: A Whole-Lake Experiment on the Canadian Shield

A whole-lake manipulation of food-web structure (introduction of a top predator, northern pike, to a minnow-dominated lake) was performed in a Canadian Shield lake (L110) to examine the stoichiometric consequences of changes in planktonic community structure generated by altered food-web structure. Minnow abundance, zooplankton biomass and community composition, microconsumer abundance, and concentration and carbon–phosphorus (C:P) ratio of suspended particulate matter were monitored in L110 and unmanipulated L240 before (1992) and after (1993–95) pike introduction. Algal biomass in L110 determined from microscopic examination for postmanipulation and premanipulation periods was also compared with dynamics in a suite of unmanipulated reference lakes from long-term monitoring records. Pike were added in spring in 1993 and 1994 in sufficient quantity to raise pike biomass to levels of around 22 kg ha−1 by 1994. Minnow populations in L110 responded dramatically, decreasing to levels 30% (1993), 10% (1994), and less than 1% (1995) of premanipulation values. However, most components lower in the food web did not respond in a manner consistent with predictions of existing food-web theory, such as the idea of cascading trophic interactions (CTI). While Daphnia biomass increased in L110 in the first year following manipulation, consistent with CTI, this effect was temporary and Daphnia collapsed in 1995, the year of lowest minnow abundance. Total zooplankton biomass in both lakes declined during the study period and, contrary to CTI, this decline appeared somewhat stronger in L110 than in L240. Dominant microconsumers (heterotrophic microflagellates) did not differ among years in either lake and did not appear to respond to food-web manipulation. At the bottom of the food web, no changes in bacterial biomass occurred in either lake. However, total concentrations of particulate matter appeared to increase in L110 after manipulation (contrary to expectations based on the theory of CTI) while algal biomass did not change in the manipulated lake relative to reference systems. Finally, particulate C:P increased in both L110 and L240 during the study period. The lack of strong response of Daphnia, the lack of response of the microbial food web, decreases in zooplankton biomass and increases in particulate biomass following reduction of minnow populations after piscivore introduction are at odds with expectations from existing food-web theory, such as the idea of CTI as currently formulated. However, the extremely high C:P ratios in particulate matter at the base of the food webs in these lakes, the coincidence of zooplankton declines and increases in particulate C:P ratios, and the results of small-scale mesocosm food-quality experiments are consistent with a hypothesis of a stoichiometric constraint operating on food-web dynamics in this and similar ecosystems.

The ecology ofDiplostomum phoxiniinfections in two minnow (Phoxinus phoxinus) populations in Scotland

Data are presented on the infection dynamics of the brain-dwelling metacercariae of Diplostomum phoxini (Digenea: Trematoda) parasitizing European minnow (Phoxinus phoxinus) populations in a lowland river (River Endrick) and a highland loch (Loch Maragan) in central Scotland. Prevalence of the parasite approached 100% in all samples taken from both populations over a 12-month period, and within each population the intensity of infection increased with increasing host fork length. However, the two relationships differed, and size-matched minnows from the Endrick exhibited significantly higher intensities than those from Loch Maragan, suggesting that metacercarial acquisition occurred at different rates in the two populations. Data regarding seasonal trends in the acquisition of D. phoxini indicated that fish in both populations become infected with the parasites mainly during the spring and summer, with negligible infection occurring during winter months. Analysis of size-matched fish from individual samples revealed apparent differences in the distribution of D. phoxini within the two host populations. Whereas D. phoxini metacercariae appeared to be overdispersed amongst length-matched fish from Loch Maragan samples, the parasite was distributed normally amongst fish from the Endrick. Possible reasons for the observed differences in the infection characteristics of D. phoxini in the two minnow populations are discussed, with reference to the parasite's transmission dynamics and local environmental conditions.

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.

Genetic variability in the European minnow, Phoxinus phoxinus (L.)

An electrophoretic study of genetic variation at 11 loci was performedfor a population of European minnows, Phoxinus phoxinus (L.). Ten loci, EST-1*, EST-2*EST-3*,GPD-1*,GPD-2*,GPI-1*,GPI-2*,MPI*,6PGD* and PGM* were polymorphic. IDH*wasmonomorphic. The mean number of heterozygotic loci over all 176 fish was 3.05 ± 0.104(SE). Observed mean heterozygosity was 0.28±0.058(SE) and expected mean heterozygosity was 0.27±0.054(SE). EST-2*, EST-3* andPGM* were not in Hardy-Weinberg equilibrium. Length,condition, parasite numbers or male breeding characters, i.e. red colorationand tubercles, were not influenced by single enzyme loci.

Effects of metal contamination from mine tailings on allozyme distributions of populations of great plains fishes

This study was conducted to assess the effects of zinc and lead on genetic variability of minnow populations (Gambusia affinis, Pimephales notatus, and Fundulus notatus) sampled from two creeks, one receiving mine drainage (Willow Creek) and one reference (Brush Creek), in the Tri-State Mining district in Kansas. Zinc and lead concentrations were greater in water samples taken from sites in Willow Creek compared to sites sampled in Brush Creek. While total numbers of fish captured from Willow Creek were smaller than numbers taken from Brush Creek, there were no differences in these fish communities for estimates of species diversity, richness and evenness. Genetic differentiation within creeks was rare. However, there was significant differentiation between creeks for allozymes of the enzymes glucose phosphate isomerase (GPI) in F. notatus and P. notatus, malate dehydrogenase (MDH) in G. affinis, and phosphogluconate dehydrogenase (PGDH) in F. notatus. Proportions of heterozygous genotypes of P. notatus and G. affinis were higher in Willow Creek compared to Brush Creek. In vitro experiments were conducted to assess the effects of zinc and lead on sensitivity of enzyme staining during electrophoresis. Results indicated allozyme sensitivity to zinc for GPI and PGDH loci. Other studies suggest that zinc competitively competesmore » with magnesium as a cofactor for these enzymes. Allozymes did not exhibit differential sensitivity to lead. The significant differentiation between creek populations and the results of the in vitro tests indicate a selective effect of zinc on allozymic distributions of these fishes. These results demonstrate the potential use of genetic variability as a bioindicator of heavy metal contamination.« less

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.

Differences in response to liming in a lake-dwelling fish community

The Lake Fjorda water system in southern Norway consists of several lakes which exhibit a gradient in acidification. The system is inhabited by populations of brown trout, Arctic char, whitefish, perch, European minnows and Crucian carp. Populations of Arctic char, whitefish and brown trout were nearly wiped out in some of the locations, as a result of acidification, In 1985, Lake Fjorda was limed in order to improve water quality so the fish community would be recovered. Fish stock assessment by means of gill-net fishing in the epibenthic and pelagic zones was carried out before (1983) and three years after liming (1991–1993). Populations of Arctic char and whitefish have not recovered after eight years of liming. Brown trout are almost extinct and do not seem to be recovering. Perch were less affected by acidification, exhibiting good recruitment also before liming.

The Role of Experience in the Response of Fathead Minnows (Pimephales Promelas) To Skin Extract of Iowa Darters (Etheostoma Exile)

Detection of an alarm pheromone may allow conspecifics to lower their risk of predation if they avoid areas where alarm pheromones are detected. Similarly, individuals that learn to recognize and avoid alarm signals produced by heterospecifics of their prcy guild (i.e. those with which they share common predators) will likely receive similar benefits. In two separate field studies we tested whether fathead minnows (Pimephales promelas) avoid areas labelled with Iowa darter (Etheostoma exile) skin extract (which contains an alarm pheromone). One population of minnows tested was sympatric with darters while thc other was allopatric from darters. Swordtails (Xiphophorus helleri) are allopatric from fathead minnows in both of the populations tested. Swordtail skin lacks ostariophysan and darter alarm pheromones, and was used to control for a general response to damaged skin of an unfamiliar fish. In the darter-sympatric population we captured significantly fewer and smaller minnows in traps marked with darter skin extract (experimental traps) than traps marked with a control of swordtail skin extract. Given the correlation between age and size, and therefore experience, these data suggest that less experienced fish do not avoid darter skin extract to the same extent as do older more experienced fish. In the darter-allopatric population there was no difference in the number of minnows or the size of minnows captured in control versus experimental traps. Taken together, these results demonstrate that fathead minnows recognize and avoid areas where darter alarm pheromone is detected and that this is a learned response. Use of heterospecific alarm pheromones to assess predation risk may be a widespread phenomenon within communities of small fishes.

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.

The inheritance and development of minnow anti-predator behaviour

European minnow, Phoxinus phoxinus, populations that occur sympatrically with pike, Esox lucius, a piscivorous predator, form more cohesive schools and have better integrated evasion tactics than fish from pike-free waters. The present study, which used two laboratory-reared populations of minnows, showed that anti-predator behaviour is inherited, but that it is modified on the basis of early experience (of a simulated predator attack). The greatest adjustment in anti-predator behaviour occurred in the minnows naturally sympatric with pike, suggesting a genetic predisposition to respond to early experience that varies between populations.

Effects of contaminants on the frequencies of allozymes in populations of the central stoneroller

AbstractAllele and genotype frequencies for allozymes of phosphoglucomutase (PGM) differed significantly between populations of the central stoneroller (Campostoma anomalum, Rafinesque) collected from sites above and below impact from industrial activities. The frequencies of the PGM‒A allele and PGM‒AA and PGM‒AB genotypes were significantly less in populations of stoneroller minnows collected from impacted sites than in populations upstream of impact. Stonerollers with these sensitive PGM genotypes and stonerollers with genotypes of malate dehydrogenase MDH‒BB were more sensitive to the toxic effects of copper sulfate than other genotypes under laboratory conditions. These results suggest that individuals with certain allozymes may be more sensitive to the toxic effects of environmental contaminants than those with other genotypes. Selection or elimination of individuals with sensitive genotypes by contaminants may result in a narrowing of genetic diversity in populations and thus increase the susceptibility of these populations to further stress. Electrophoretic determination of genetic structure in fishes appears to be sensitive to changes in water quality and may be a useful tool for monitoring the health of aquatic populations.

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< 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.

Bioenergetics of growth of a cyprinid, Phoxinus phoxinm (L.): development and testing of a growth model

The development, verification and validation of a bioenergetics growth model for the European minnow, Phoxinus phoxinus, are described. Sensitivity analysis showed that the growth predicted by the model was sensitive to parameters used in the estimates of the energy and dry matter content of the fish and metabolic rate. The model gave good predictions of the form of the growth‐temperature relationship, but failed to predict the growth‐ration relationship well. Validation of the model against experimental data showed that the model underestimated growth at lower rations and overestimated growth at higher rations. Only at low ration levels did the model predict growth rates equivalent to those recorded for a natural population of minnows. Analyses of an empirical regression equation for growth rate, as well as the bioenergetics model, highlighted four areas for future research: (1) the energy content of fish in relation to environmental factors; (2) the feeding metabolism of fish in relation to environmental factors; (3) the regulatory mechanism of long‐term growth; (4) the effect of gonadal maturation on long‐term growth. The identification of these problems illustrates the advantage of a partially explanatory model, the bioenergetics model, over a descriptive empirical model.

Salsuginus bermudae sp.n. (Monogenea: Ancyrocephalidae) from Fundulus bermudae Gunther and Gambusia affinis (Baird and Girard) in Bermuda

Salsuginus bermudae sp.n. is described from gills of mangrove minnow, Fundulus bermudae Gunther, and mosquito fish, Gambusia affinis (Baird and Girard), from Bermuda. It differs from S. angularis by its shorter ventral hamuli (20–23 versus 22–26 μm); from S. fundulus, S. spirae, and S. heterocliti by its shorter dorsal and ventral hamuli (17–21 versus 22–26, 22–26, 20–24 μm, respectively, for dorsal hamuli and 20–23 versus 23–29, 23–27, 25–28 μm, respectively, for ventral hamuli); from S. bahamianus by its longer ventral hamulus superficial root (6–10 versus 5–6 μm); and from S. umbraensis by its shorter dorsal hamuli and dorsal bar (17–21 versus 23–24 and 19–26 versus 30–31 μm, respectively). It is indistinguishable morphometrically from S. seculus, yet differentiable from other species of Salsuginus by its dorsal and ventral hamuli and by its accessory piece. Mangrove minnows may be distributed in separate populations in the various Bermudian lakes. Differences in accessory piece morphology allowed discrimination of two morphotypes isolated in separate small mangrove minnow populations from two lakes (Mangrove and Lover's) which have different physicochemical characteristics. A possible isolating mechanism is discussed.

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 (<10 mg/L CaCO3 versus > 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.

Effects of Walleye Predation on the Population Dynamics of Small Littoral-Zone Fishes in a Northern Wisconsin Lake

Abstract We used a bioenergetics model to assess the effects of predation by juvenile walleyes Stizostedion vitreum vitreum on the populations of darters (Percidae) and minnows (Cyprinidae) in Sparkling Lake, Wisconsin. In 1982, when young-of-the-year yellow perch Perca flavescens were scarce, darters and minnows were important in walleye diets throughout the summer. Walleye predation accounted for approximately 100% of adult darter mortality and 75% of adult minnow mortality. Walleyes appeared to be important in determining abundance of young-of-the-year darters in October, but had little direct effect on abundance of young-of-the-year minnows. In 1983, when young-of-the-year yellow perch were abundant, darters and minnows were only important in walleye diets in June. Walleye predation accounted for 80% of adult darter mortality but only 35% of adult minnow mortality. The decrease in walleye consumption of adult darters and minnows in 1983 compared with 1982 is consistent with the hypothesis that abundan...