Evidence of Introgressive Hybridization in the Genus Stizostedion: Interspecific Transfer of Mitochondrial DNA between Sauger and Walleye

The mitochondrial genotypes of walleye (Stizostedion vitreum) and sauger (Stizostedion canadense) are readily differentiated, permitting an analysis of the incidence of hybridization between the taxa. We provide evidence that introgression of sauger into walleye has occurred: two female walleye collected from Lake Simcoe contained a mitochondrial genome identical to that of sauger. Not only were these two fish morphologically indistinguishable from typical walleye, they were homozygous for the diagnostic alleles characteristic of walleye at the Mdh-3 and Pgm-1 allozyme loci. Sauger have not been recorded in Lake Simcoe, but a few individuals may have been inadvertently introduced during stocking programs and introgressed with walleye due to a lack of conspecific mates.

Feeding Ecology of the Sauger (Stizostedion canadense) in a Large River

In stomachs of 550 sauger (Stizostedion canadense) collected by gillnetting in the Ohio River during June 1980 through August 1981, Dorosoma cepedianum were the most abundant prey (42% of all identified prey), followed by Notropis atherinoides (28%), Aplodinotus grunniens (16%), and Ictalurus punctatus (12%). Annual food consumption of sauger averaged 1.1% of the body weight per day. Growth and daily ration followed similar patterns, being high from September to January (maximum in October) and low from March to August. Rapid growth and high food consumption throughout fall and early winter were related to an extended period of high temperatures and increased availability of Dorosoma cepedianum. The annual period of growth and food consumption was longer and peaked later in the year in the river than those previously observed in other types of aquatic systems.

Measurements of Growth and Consumption of Sauger (Stizostedion canadense): Implication for Fish Energetics Studies

The energy budget of sauger (Stizostedion canadense), in Watts Bar Reservoir, Tennessee, was studied over a 17-mo period. Field determinations of consumption and laboratory determinations of metabolism were incorporated into the balanced energy equation of Winberg to predict growth in energy value of sauger in age-classes I+ and II+. The equation included energetic costs of specific dynamic action and the metabolic efficiency penalty of metabolizing body energy stores. Observed and predicted growth during the warm months, March through October, agreed within 2.5–9.6% for the I+ and II+ age-classes, respectively. Observed and predicted growth over the winter for the only age-class modeled, the I+, did not agree. High food consumption during winter did not result in the amount of growth expected, suggesting that the physiological assumptions of the balanced energy equation are invalid for describing the winter energetics of sauger. We emphasize the potential error in inferring consumption from growth and metabolism.Key words: sauger, Stizostedion, bioenergetics, feeding, growth, modeling

Changes in Percid Populations and Species Interactions in Lake Erie

Changes in the percid community of Lake Erie from 1915 to 1975 are delineated mainly from recently published material. Sauger (Stizostedion canadense), blue pike (S. vitreum glaucum), walleye (S. vitreum vitreum), and yellow perch (Perca ftavescens) are the percids which have been affected by exploitation, eutrophication and the presence and proliferation of colonizing fish species. Changes in habitat, fishing techniques and gear have also affected percid and nonpercid interactions.Yellow perch, presently the most abundant percid, have experienced irregularity in recruitment, lack of strong year-classes, and increased exploitation. The concern over the status of yellow perch and walleye has led to some important management initiatives. Improved regulation of exploitation and present efforts by United States and Canadian government agencies to improve Great Lakes water quality, are expected to improve the success for long-term maintenance of percids in Lake Erie. Key words: Percidae, populations, Lake Erie, habitat change, exploitation, management, Stizostedion vitreum vitreum, S. canadense, S. vitreum glaucum, Perca flavescens

Population Dynamics of Yellow Perch (Perca flavescens), Sauger (Stizostedion canadense), and Walleye (S. vitreum vitreum) in Four Main Stem Missouri River Reservoirs

Percid populations have generally increased in abundance relative to other groups of fishes in the 15–25 yr four large reservoirs have been operational on the Missouri River in South Dakota. Saugers (Stizostedion canadense) were initially the most abundant percid, but their numbers gradually declined, probably because of the reduction in river spawning habitat and the increase in water clarity. Yellow perch (Perca flavescens) increased to high population levels as the reservoirs filled but later decreased in abundance. Walleye (S. vitreum vitreum) populations were the slowest to develop in the reservoirs, probably because of initial lack of suitable spawning habitat; they later became well established.The yellow perch was one of the most important forage species in the reservoirs, and walleyes and saugers the primary predator and sport fishes. Sportfishing quality 15–25 yr after impoundment was good, but harvest remained low because of the large size of the reservoirs, inadequacy of facilities for anglers, and the relatively sparse human population. Reproductive success was the primary factor regulating the abundance of percids. Year-class strength was primarily determined by climatic rather than biological factors. Above-average precipitation presumably enhanced reproductive success, both directly by increasing the quantity and quality of spawning substrate through higher stream flows and water levels, and indirectly by providing a larger forage supply, thereby apparently reducing predation on young percids, as well as cannibalism. Kew words: Percidae, reservoirs, Missouri River, Stizostedion, Perca, population dynamics, ecology, precipitation, water level

Retinal Structure and Function in the Walleye (Stizostedion vitreum vitreum) and Sauger (S. canadense)

In the sauger, Stizostedion canadense, the tapetum lucidum of the retina was found to be more highly developed than in the walleye, S. vitreum vitreum. The tapetum is uniformly distributed throughout the sauger's eye, whereas it reaches a comparable development only in the ventral region of the walleye's retina. Other structural differences also suggested that the sauger's retina is more sensitive to light than the walleye's. In both species, photomechanical changes in response to light- and dark-adaptation were restricted to the retinal epithelial pigment (melanin) and the rods. Electroretinographic recordings appeared to be correlated with the differences in tapetal development and habits of the two species. Key words: Percidae, Stizostedion vitreum vitreum, S. canadense, light intensity, retinal structure, histology, electrophysiology, behavior

Thermal Requirements of the Early Life History Stages of Walleye, Stizostedion vitreum vitreum, and Sauger, Stizostedion canadense

Walleye (Stizostedion vitreum vitreum) and sauger (Stizostedion canadense) eggs were exposed to temperatures ranging from 6 to 21 C to determine temperature effects on fertilization, incubation, and fry survival. Optimum fertilization temperatures were 6–12 C for walleye and 9 C for sauger. Optimum incubation temperatures were 9–15 C for both walleye and sauger. A sharp drop or rise in temperature had no great effect on walleye fry and juvenile survival. Optimum temperature for juvenile walleye and sauger growth was 22 C. When acclimated at 8–26 C, 96-h upper lethal temperatures for walleye juveniles were 27.0–31.6 C, with the lower acclimation temperatures producing the lower upper lethal limits and intermediate acclimation producing intermediate upper lethal limits in order as acclimation temperatures increased. When acclimated at 10–26 C, the 96-h upper lethal temperatures for sauger juveniles was 26.6–30.4 C, in order as with the walleye.

Gastric Digestion, Food Consumption, Feeding Periodicity, and Food Conversion Efficiency in Walleye (Stizostedion vitreum vitreum)

Gastric digestion of walleye, Stizostedion vitreum vitreum, and sauger, Stizostedion canadense, was measured by pumping stomachs at various intervals after experimental meals had been force-fed or voluntarily consumed. No significant differences in digestion rates were found between species or among walleye taken from three lakes. Repeated use of fish in digestion experiments failed to influence results. Gastric digestion rate increased with fish size, but was depressed by increased meal size, food particle size, and by force-feeding experimental meals.Food consumption rate and feeding chronology of the strong 1966 year-class of Lake of the Woods walleye were determined by applying results of laboratory digestion studies to stomach samples. An estimating technique that takes into account the effects of factors shown to influence digestion in the laboratory was developed and used to make calculations for the field population. Consumption averaged 1% of body weight during June, 2% during July, and 3% during August and September. Average temperatures were similar during June and September. Greater consumption during September resulted from higher food availability. Feeding was greatest during night and early morning hours in all months except June when it was continuous. Conversion efficiency of the 1966 year-class was described from estimates of growth and consumption rates for June through September and approximated 20% during each month.