Executive Summary

The goals of this study were to record the rapid increase in the distribution and abundance of burrowing mayflies (Ephemeridae: Hexagenia spp.) in the soft bottom sediments of western Lake Erie, to determine the timing and relative magnitude of adult swarms onshore, and to determine the extent to which these mayflies were incorporated into the diets of forage fishes in the same area. Further, the results were to be used to refine the potential of Hexagenia to serve as a mesotrophic indicator organism.

The burrowing nymphs increased in abundance and distribution throughout the basin in 1995 and 1996 except in a relatively small central area where it is possible that oxygen depletion prevents recolonization. Peak emergence of the adults occurred in the third week in June both years, with very large numbers on a single night each year. The number of adults appeared to be about ten times greater in 1996 than in 1995. The increase in the size of the onshore swarms corresponded with increases in the distribution and abundance of nymphs in the sediments. Trout perch and silver chubs incorporated an increasing proportion of the mayflies in their diets as the density and distribution of the nymphs increased, whereas spottail shiners and emerald shiners, which fed primarily on zooplankton, did not.

This report includes a recommendation for the refinement of the Mayfly rating for the Key Indicator Species metric of the Lake Erie Quality Index published in 1998 by the Ohio Lake Erie Office. It is suggested that the rating of "Excellent" currently assigned to densities of more than 450 nymphs per square meter be reduced to the range of 200 to 249 nymphs per square meter, and that lower ratings be assigned to densities that deviate above or below that range. Average density in the western basin should be calculated as a moving average based on several consecutive years rather than a simple average for one year.

Introduction

Major changes have been a hallmark of Lake Erie since Europeans began to exploit its resources as early as the eighteenth century. Two obvious changes have been shifts in the abundances of sport and commercial fish species, largely in response to overharvest and changes in habitat quality, and a long term degradation of water quality that threatened public health as well as the aquatic ecosystem. Less noticeable among the changes in Lake Erie in the past 200 years has been a major alteration in the "keystone" aquatic invertebrates, those that exert a major influence on the invertebrate community structure and dynamics. Some of the changes in the invertebrate community have affected the kinds and amounts of food available to important fish species.

Two species of burrowing mayflies, Hexagenia limbata and H. rigida, were keystone species in western Lake Erie and parts of the central and eastern basins. An adult and burrowing nymph of Hexagenia are shown in Figure 1. For centuries until the mid-1950s, they were abundant in the western basin (Reynoldson and Hamilton 1993) and comprised an important food resource for many of the forage fish species upon which commercially and recreationally valuable fish, such as walleye (Stizostedion vitreum vitreum), fed. Some important fish species, such as yellow perch (Perca flavescens), fed directly on the bottom-dwelling nymphs of the large mayflies. Among numerous other fish that fed on Hexagenia in Lake Erie were white perch (Morone americana), freshwater drum (Aplodinotus grunniens), and channel catfish (Ictalurus punctatus) (Boesel 1937, Daiber 1952, Price 1963). Hexagenia were most vulnerable to fish predation in June and July, when the nymphs swam to the surface to molt into winged subadults, and they were frequently the primary food in fish diets during that period. In the 1940s and early 1950s, newspapers and other media reported nuisance swarms comprised of millions of the winged, nonbiting subadults and adults (Teale 1960, Burns 1985).

Between 1930 and the early 1950s, densities of Hexagenia nymphs in the sediments of the western basin averaged around 300 to 500 per square meter, and at some locations in the island area, densities occasionally exceeded 1000 per square meter (Britt 1955, Wright 1955). This large population of mayflies died suddenly, however, in September 1953 as a direct result of the depletion of dissolved oxygen in the bottom waters of the lake (Britt 1955). Other factors, such as increased application of insecticides in the Great Lakes basin, have been suggested as additional mechanisms in the demise of the burrowing mayfly population of Lake Erie, although there exists little evidence to support those contentions (Burns 1985). Oxygen depletion was an increasing problem in much of the central basin (Burns 1985), but the shallowness of the western basin seems to have ensured that a sufficient supply of dissolved oxygen was maintained, even near the bottom sediments, prior to the 1950s.

Under the Great Lakes Water Quality Agreement of 1972, signed by Canada and the U.S., an extensive binational effort was launched to reduce and eliminate sources of pollution to Lake Erie. Water quality in the lake gradually responded to those measures (Makarewicz and Bertram 1991); however, the burrowing mayflies apparently did not begin to recolonize the sediments until the early 1990s, at which time rapid recolonization took place (Krieger et al. 1996). Evidence of the recolonization included the widespread appearance onshore of small swarms of winged subadults and adults.

Adult female

Burrowing nymph

Figure 1. Adult and nymph of the burrowing mayfly Hexagenia. Both individuals are approximately 1.25 inches (3 cm) long, excluding the terminal filaments.

The changes that permitted the re-establishment of Hexagenia in the lake sediments are not known (Krieger et al. 1996). Pollution abatement measures undertaken during the two decades prior to the 1990s resulted in an improvement in some lake quality indices, such as secchi depth (Makarewicz and Bertram 1991). Minimum summer concentrations of dissolved oxygen at the sediment-water interface may have increased above the threshold needed to sustain the nymphs, although appropriate measurements to confirm this possibility were not taken. Dramatic changes wrought by the invasion of the zebra mussel (Dreissena polymorpha), and later the quagga mussel (D. bugensis), may have played a role in the ability of Hexagenia to recolonize the sediments. Those changes included a further increase in water clarity and the consequent re-establishment of regions of submersed aquatic plants. Studies regarding the possibility of a substantial decline in toxic contaminants in the sediments, within which Hexagenia nymphs construct their burrows, have not been conducted.

The resurgence of the burrowing mayfly populations of Lake Erie is important because of (1) their impact on the benthic-pelagic food web, which extends to key commercial and recreational fishes and ultimately to humans; (2) their high visibility to the public while swarming; and (3) their potential economic impact on public utilities, such as causing electrical shorts at power generating plants (The Ann Arbor News, 25 June 1996), and on public services, such as creating slick streets and fouled beaches (The Blade, 1 July 1996). Hexagenia may be bioconcentrating toxic contaminants remaining in lake sediments and transferring them to fishes and ultimately humans. Indeed, adult Hexagenia have been shown to possess varying body burdens of organic contaminants that vary depending on where in the western basin the individuals grew in the sediments (Corkum et al. 1997).

Hypotheses and Objectives

This project was proposed to test the following hypotheses:

1. That the Hexagenia population in western Lake Erie would continue to expand in area and density.
2. That Hexagenia would again become a major food item for forage fishes as well as important commercial and sport fishes.
3. That recolonization of the lake bottom by the mayflies would be limited in some areas because of modification of otherwise suitable habitat by Dreissena or by continued high levels of sediment contamination.
4. That recolonization by nymphs would be enhanced by additional food resources provided by Dreissena.

The third and fourth hypotheses were not tested because the project was funded at a lower level than requested and these hypotheses were considered by the Lake Erie Office to be basic research rather than applied research. The last two hypotheses were recently tested via funding through Ohio Sea Grant project R/ER-36 in a joint grant to Dr. David J. Berg at Miami University and Ken Krieger at Heidelberg College. Therefore, this LEPF project was limited to the study of population changes of Hexagenia (Hypothesis 1) and the use of Hexagenia as a food resource by forage fishes (Hypothesis 2).

In order to test the hypotheses, we accomplished the following objectives:

1. To sample the bottom sediments of western Lake Erie at 16 historical stations in the open lake and from those samples to measure the abundance (density) of mayfly nymphs, and similarly, to collect sediment samples at two historical stations in two Areas of Concern (AOCs): Maumee Bay in the western basin, and Cleveland Harbor in the central basin.
2. To record the period of emergence and the relative abundance of adult Hexagenia during summer in the island region of the western basin.
3. To relate the range and densities of Hexagenia to their relative abundance in the stomachs of fishes from the western basin.
4. To integrate the results with other information to refine the management potential for the use of Hexagenia as a mesotrophic indicator organism in the Great Lakes, especially Lake Erie.

Methods

Objective 1: Nymph Distribution and Abundance. The locations of the stations that were sampled for mayflies in 1995 and 1996 are listed in Table 1 and are shown in Figure 2. Some of the stations have been sampled by the Biological Resources Division, USGS in Ann Arbor since early in the twentieth century, several stations were sampled annually in the 1950s and 1960s by Dr. Wilson Britt of The Ohio State University (Britt et al. 1973), and some stations were first sampled in 1993 (Krieger et al. 1996) or in this study. The objective in selecting the stations was to provide a broad, representative survey of the western basin and, where possible, to permit comparison of our results with historical data.

Minor changes in the stations sampled were made between 1995 and 1996; they are detailed in the paper by Krieger et al. (1996). That paper also describes details of the sampling methods. Our study design was slightly modified from that proposed: We reduced the number of replicate samples collected at each station from five to four in order to increase the total number of stations that were sampled, thereby providing better spatial coverage of the basin. As proposed, we measured dissolved oxygen concentrations and temperature near the surface and bottom at each station.

The samples were processed in the field and laboratory as described by Krieger et al. (1996). From the samples, the number of Hexagenia nymphs per square meter was estimated for each station. We determined size:dry weight regression relationships for the nymphs at each station, where sufficient numbers of nymphs permitted. The slopes of the regression lines will serve at a later time as the basis for comparing the relative health of the nymphs.

Objective 2: Emergence Period and Relative Abundance of Adults. Subimagoes and imagoes (winged subadults and adults) of Hexagenia were collected nightly for 15 minutes beginning at 11:00 p.m. from the middle of June through late August of 1995 and 1996. They were hand-picked from a designated area beneath a mercury light atop a hill at the Lime Kiln ferry landing on the southwestern end of South Bass Island, Lake Erie. The mayflies were kept in a large jar until the next afternoon to allow final emergence to the imago stage. They were then preserved in 70% ethanol and were counted according to species and sex. The data were used to determine the general period of the emergence, dates of peak abundance, and composition by species and sex. These data were compared with similar results obtained in 1993 and 1994 from Gibraltar Island (in Put-in-Bay) using basically the same methods.

Objective 3: Fish Foraging on Hexagenia. We obtained frozen fish that had been collected in May, June, and July 1995 and 1996 on trawling runs by the Ohio Department of Natural Resources, Fisheries Research Unit, Sandusky, Ohio. We observed the stomach contents of trout perch (Percopsis omiscomaycus), silver chubs (Hybopsis storeriana), and spottail shiners (Notropis hudsonius). The species were chosen in consultation with staff of the Fisheries Research Unit. We avoided studying yellow perch (Perca flavescens) and white perch (Morone saxatilis) because they were already under study by other investigators. For each month, we included at least 25 individuals of each species from the eastern part of the western basin and 25 individuals from the western part of the basin as defined by the Fisheries Research Unit. Occasionally there were fewer than 25 individuals of a given species in the collections from one or both parts of the basin. The number of specimens of each species that were analyzed is shown in Table 2. We chose the months of May, June, and July because May represents the period when the nymphs are near their maximum size, June is the period when emergence begins as nymphs swim to the water surface, and mid- to late July is the period of waning numbers of mayflies in the nightly emergences. Thus, we expected that May, June and July are the months when the nymphs are most vulnerable to fish predation. The data that were generated permitted us to determine to what extent the selected species of fish incorporated Hexagenia into their diets as the mayflies increased in abundance from year to year.

Objective 4: Value of Hexagenia as a Mesotrophic Indicator Organism. Some assessment of Hexagenia as an indicator was accomplished through telephone conversations with the Lake Erie Office staff, and Hexagenia was incorporated as one of the benchmarks of lake quality in the State of Lake Erie Report (OLEC 1998). In this report, some suggested refinements of the criteria for that benchmark are presented.

Results and Discussion

1. Nymph Distribution and Abundance. Details of our studies of the range expansion of Hexagenia and increases in its density (number per unit area) within that range in western Lake Erie have been presented in two peer-reviewed papers and one submitted manuscript (refer to Krieger et al. 1996, Madenjian et al. 1998, Schloesser et al. in preparation, and abstracts in Appendix). The overall observation was that Hexagenia expanded its range and density in western Lake Erie from 1991 through 1997, and declined markedly in density but not range in 1998. The average number of Hexagenia nymphs in the soft bottom sediments increased from around 10 per square meter in 1993, to 34 in 1995, 104 in 1996, and 451 in 1997, then declined to 157 in 1998. Their distribution increased from 64% of sampled stations in 1995 to 77% in 1996, 91% in 1997, and 95% in 1998. Densities remained relatively low in an area east of Middle Sister Island to west and northwest of Pelee Island, and none were ever found at a station (5P) east of West Sister Island (Figure 2). The data for 1997 and 1998 were collected under a separate project (LEPF-97-30) and are included in this report to facilitate the discussion which follows regarding the "Key Indicator Species Metric" of the Lake Erie Quality Index (Ohio Lake Erie Commission 1998).

2. Emergence Period and Relative Abundance of Adults. The results of the nightly collections in 1995 and 1996 near the Lime Kiln Dock on South Bass Island are shown in Figure 3. These data unfortunately are of limited value because (a) the collection period began three days later in 1995 than in 1996 and ended ten days earlier; and (b) no sampling area was recorded for nine days of the 1996 collection period, so no quantitative information could be determined for those days, including the night when a very large number (>7,000) of mayflies was collected.

The 1995 collections began on the night of maximum emergence (J. Hageman, Stone Laboratory, personal communication, 20 Oct 95). Peak emergence appeared to occur in the third week in June both years, with very large numbers on one night each year followed by a few other nights with particularly heavy emergences, seen best in the 1996 graph. To compare the data quantitatively, the one night of maximum emergence was removed from the data set for each year because it would mask the results of all the remaining nights and because the number collected in 1996 is unknown. The number of Hexagenia collected was about ten times higher in 1996 than in 1995, with the average collection containing 23 + 6 (S.E.) Hexagenia in 1995 and 195 + 60 in 1996. The median collection contained 6 mayflies in 1995 and 80 mayflies in 1996. It must be remembered that collections began earlier and ended later in 1996 (6/16-7/31) than in 1995 (6/19-7/21). The total length of the emergence period each year appears to be indefinite, as a few adults appear nightly into late summer and occasionally in the fall. For example, one notable emergence was seen at the Research Building of Stone Laboratory on the night of 12 Oct 95 (J. Hageman, Stone Laboratory, pers. comm., 20 Oct 95).

Figure 2. Locations of stations in western Lake Erie and density of nymphs at each station, 1995 through 1998.

Figure 3. Numbers of winged Hexagenia collected nightly near Lime Kiln Dock, South Bass Island.

Large swarms in June and July 1995 caught the attention of the public and the news media along the shore of the western basin. In late June 1996, swarms of millions of mayflies were attracted to lights near the lakeshore in cities from Toledo (Point Place) to Port Clinton, Ohio, and wire services and newspapers carried stories throughout Ohio and the region about their impact (e.g., The Blade, 26 June 1996; The Ann Arbor News, 25 June 1996; The Plain Dealer, 24 June 1996). The increase in the size of the onshore swarms corresponded with increases in the distribution and abundance of nymphs in the lake sediments (Krieger et al. 1996).

3. Fish Foraging on Hexagenia. Results of the fish foraging studies have been reported at several conferences (see abstracts in Appendix). In brief, we found that trout perch and silver chubs fed on Hexagenia in 1995 and 1996, whereas spottail shiners fed mostly on zooplankton rather than bottom invertebrates. As we had hypothesized, both trout perch and silver chubs included an increasing proportion of Hexagenia in their diets as the density and distribution of the mayflies increased. Spottail shiners only rarely contained mayflies in their stomachs. Work by others (French 1994; M. Kershner and J. Mion, Ohio State Univ., pers. commun., 1995) has shown that freshwater drum, yellow perch, and white perch also fed on Hexagenia.

4. Value of Hexagenia as a Mesotrophic Indicator Organism. The Lake Erie Quality Index (LEQI) published by the Ohio Lake Erie Commission in 1998 (OLEC 1998) consists of ten "indicators" which are rated with descriptive scores of "Excellent", "Good", "Fair", or "Poor". Each descriptive score is determined on the basis of weighted numerical scores assigned to one or more (up to five) "metrics" (28 total). The "Biological Indicator" is scored on the basis of two metrics: "Key Indicator Species" and "Index of Biotic Integrity". The Key Indicator Species metric is based on population or reproduction trends of three species: bald eagles, walleyes, and Hexagenia mayflies.

The LEQI has initially established the following scores for Mayflies (OLEC 1998, pp. 44-46), derived from the density of nymphs per square meter of soft sediment on the lake bottom:

Descriptive Score	Mayfly Nymphs per Square Meter	Numerical Score
Excellent	More than 450	4
Good	400-450	3
Fair	350-399	2
Poor	Fewer than 350	1

A refinement of this scoring method is suggested here based on the mayfly densities observed since the 1997 survey and on new experimental evidence. The suggested scoring method is as follows:

Descriptive Score	Mayfly Nymphs per Square Meter (in May)	Numerical Score
Fair	More than 400	2
Good	200-400	3
Excellent	200-249	4
Good	100-199	3
Fair	30-99	2
Poor	Fewer than 30	1

There are several reasons for this recommendation. First, more and more mayflies is not necessarily a positive development, either for people on shore in the summer or for the ecology of Lake Erie. The State of Lake Erie report (OLEC 1998, p. 46) states, "The best available data, from 1930 to the early 1950s, indicate average densities of up to 500 nymphs per square meter. The Lake Erie Commission has set a goal of re-establishing mayfly nymphs in the western basin of Lake Erie to this average density of 500 nymphs per square meter" (OLEC's italics). It should be noted that the data collected from 1929 to the 1950s (Britt 1955, Wright 1955, Britt et al. 1973) were collected during a period of massive nutrient enrichment of the lake, and the 1929-1930 study of Wright (1955) was established because large-scale impacts of nutrients and toxic organics in the western basin were already suspected. As long as the sediments are not too toxic and dissolved oxygen levels remain above minimal concentrations throughout their nymph stage, Hexagenia mayflies respond to increasing food supplies (nutrient enrichment) by growing more rapidly, and the lake sediments support increasing numbers of nymphs per square meter.

As shown by Reynoldson and Hamilton (1993) from historical evidence supplied by sediment cores, Hexagenia appears to have undergone a large population increase during the period from the beginning of European settlement, deforestation, and drainage of the Lake Erie basin up until the time of its sudden disappearance from the western basin in the 1950s. Therefore, it appears that average densities of 300 to 500 nymphs per square meter, and individual station densities up to 9,000 per square meter, observed in the first half of this century were artificially high. The consensus (not unanimous) among Lake Erie benthic biologists is that it was suffocation by severely low dissolved oxygen levels at sporadic periods during summers that first caused the demise of the Hexagenia population and then prevented its recovery for decades. The oxygen depletion was brought about by over-enrichment (cultural eutrophication) of the lake which resulted in very high sediment oxygen demand. Thus, in the absence of severe oxygen depletion, Hexagenia responds favorably to increasing nutrient enrichment (accompanied by increasing oxygen demand) through increased growth rates and higher densities of nymphs. It is probable that a maximum density would be reached above which further increases in density would be limited by effects of crowding. When the oxygen demand reaches a critical point, periods of hypoxia (very low oxygen concentrations) suffocate the Hexagenia and the population suddenly disappears. This is precisely what Britt (1955) recorded in the late summer of 1953. The presence of toxic concentrations of anthropogenic chemicals in the sediments could present additional limitations to the survival and repopulation of Hexagenia in the western basin, but several independent experiments over the years (Burns 1985; J.J.H. Ciborowski, Univ. of Windsor, pers. comm. May 1999) have repeatedly shown that the eggs can hatch and the nymphs can grow and mature in western basin sediments where it continues to be absent, as long as sufficient oxygen is provided.

On the basis of the above insights, it is suggested that the Mayfly measure be modified to incorporate a single range of Hexagenia densities designated as "Excellent", but with two separate ranges of density designating "Good", "Fair", and "Poor". That is, too few mayflies is not good for the Lake Erie fishery, but too many mayflies is an indication of over-enrichment and potential dissolved oxygen problems. "Excellent" would include 200 to 249 nymphs per square meter. In order for the western basin to support Hexagenia within any particular range, its food supply has to be provided at the appropriate level, and that level will most likely be determined by management decisions related to the desired catch of walleye, yellow perch, and other sport and commercial fishes. Therefore, the range that actually should be designated as "Excellent" must be flexible. It seems likely that managers will want to maintain the productivity of western Lake Erie well above the pristine productivity that existed prior to the impacts of large-scale industrialization, urbanization, and agriculture. However, it appears that a carrying capacity based on average densities of the early 1950s is much too high to permit maintenance of the lake quality that we now experience.

Finally, the number of nymphs per square meter of lake bottom should be derived from a moving average of the results of several consecutive years, perhaps four or five, rather than on a simple one-year average. The reason for this is that, just as perch and walleye experience highly variable success in different year classes, so also Hexagenia has shown much variability in either hatching success or the survival of nymphs in each year class (Figure 2). Although the mechanisms responsible for this variation are not known, it is unlikely that the difference from year to year (such as between 1996 and 1997, and 1997 and 1998) is a reflection of sudden changes in lake quality. Application of a moving average to the annual densities is more likely to show the real trend in lake quality.

Benefits and Information Dissemination

The recolonization of western Lake Erie by burrowing mayflies has become a classical example in population biology. It has generated much interest by biologists, managers and the general public. Interest in the mayflies is particularly great in June and July, as the mayflies emerge from the lake sediments, molt into subadults at the water surface, and swarm on land. As a result, there have been numerous newspaper, television, and radio stories about the increase in abundance of Hexagenia, the impact of that increase on the ecology of Lake Erie, the indication from that increase that the lake is now healthier than it has been for many years, and the brief nuisance the swarms create once again on an annual basis. The project director has given many talks at public meetings, university seminar series, and professional meetings. Those are listed in Tables 3 and 4. To date, two peer-reviewed publications have resulted, with a third pending; those are listed in Table 5. A list of the news media with which the project director has had contact over the course of this project is presented in Table 6.

Acknowledgments

The nightly censuses of winged Hexagenia were conducted by Lisa Kutschbach-Brohl, who was assisted by Matthew Thomas and Deborah Vallance. Lisa supervised students from Stone Laboratory in the identification of species, sexing, and counting. John Hageman assigned Stone Laboratory students to work on the project and provided oversight; those students included Deepika Bhardwaj, Theresa Colson, Mike Pennington, Todd Semanco, Derek Smith.

Numerous Heidelberg College students assisted in both field collections and laboratory analyses of nymphs as well as the dissection of fish stomachs and analysis of their contents. They included Kevin Boggs, Shad Bowman, Laura Shields, Liberty Trissell, Brian Villalon, and Stacie Wildman. Nancy Miller provided administrative assistance throughout this project.

The sampling of nymphs was made possible through the highly capable assistance of Mike Burr, director of the USGS Lake Erie Biological Station in Sandusky, Ohio, and his assistant, Brian Ickes. Their professionalism and dedication during long and arduous days aboard the R/V Pike are greatly appreciated.

Literature Cited

The Ann Arbor News. 25 June 1996. Mayflies short-circuit power plant. p. 1. The Associated Press.

The Blade, Toledo, OH. 26 June 1996. Point Place bugged by the big comeback. Story by Marie-Anne Hogarth.

The Blade, Toledo, OH. 1 July 1996. Blizzard of mayflies coats roads.

Boesel, M.W. 1937. The food of nine species of fish from the western end of Lake Erie. Trans. Am. Fish. Soc. 67:215-233.

Britt, N.W. 1955. Stratification in western lake Erie in summer of 1953: effects on the Hexagenia (Ephemeroptera) population. Ecology 36:239-244.

Britt, N.W., J.T. Addis, and R. Engel. 1973. Limnological studies of the island area of western Lake Erie. Bull. Ohio Biol. Survey (n.s.) 4(3).

Burns, N.M. 1985. Erie, the lake that survived. Rowman & Allanheld Publ., Totawa, NJ. 230 pp.

Corkum, L.D., J.J.H. Ciborowski, and R. Lazar. 1997. The distribution and contaminant burdens of adults of the burrowing mayfly, Hexagenia, in Lake Erie. J. Great Lakes Res. 23:383-390.

Daiber, F.C. 1952. The food and feeding relationships of the freshwater drum, Aplodinotus grunniens Rafinesque in western Lake Erie. Ohio J. Sci. 52:35-46.

French, J.R.P. III. 1994. Burrowing mayflies (Hexagenia spp.) consumed by freshwater drum (Aplodinotus grunniens) in western Lake Erie, 1990-1992. (Abstract). 37^th Annual Conf., Internat. Assoc. Great Lakes Res., Windsor, ON, June 5-9, 1994.

Krieger, K.A., D.W. Schloesser, B.A. Manny, C.E. Trisler, S.E. Heady, J.J.H. Ciborowski, and K.M. Muth. 1996. Recovery of burrowing mayflies (Ephemeroptera: Ephemeridae: Hexagenia) in western Lake Erie. J. Great Lakes Res. 22:254-263.

Madenjian, C.P., D.W. Schloesser, and K.A. Krieger. 1998. Population models of burrowing mayfly recolonization in western Lake Erie. Ecol. Applications 8:1206-1212.

Makarewicz, J.C., and P. Bertram. 1991. Evidence for the restoration of the Lake Erie ecosystem. BioScience 41:216-223.

OLEC (Ohio Lake Erie Commission). 1998. State of Ohio 1998 state of the lake report: Lake Erie Quality Index. Ohio Lake Erie Office, Toledo, OH. 88 pp.

The Plain Dealer , Cleveland, OH. 24 June 1996. Bad news: mayflies back; good news: lake cleaner. The Associated Press.

Price, J.W. 1963. A study of the food habits of some Lake Erie fishes. Bull. Ohio Biol. Survey 11(1).

Reynoldson, T.B., and A.L. Hamilton. 1993. Historic changes in populations of burrowing mayflies (Hexagenia limbata) from Lake Erie based on sediment tusk profiles. J. Great Lakes Res. 19:250-257.

Schloesser, D.W., K.A. Krieger, J.J.H. Ciborowski, and L.D. Corkum. In preparation. Densities of burrowing mayflies (Ephemeroptera: Ephemeridae: Hexagenia spp.) in western Lake Erie: historical (1929) to extirpation (1950s) to recovery (1990s).

Teale, E.W. 1960. Journey into summer. Ch. 5. Mayfly island. Dodd, Mead & Co., NY. 366 pp.

Wright, S. 1955. Limnological survey of western Lake Erie. U.S. Fish and Wildlife Service Spec. Rep., Fisheries No. 139. Ann Arbor, MI.

Table 1. Locations of sediment stations sampled in this project
STATION	N LATITUDE	W LONGITUDE
5B	41°41.50'	82°46.00'
6B	41°52.00'	82°49.00'
3D	41°56.33'	83°12.17'
8D	41°57.33'	83°07.17'
15D	42°02.00'	83°09.17'
1K	41°45.00'	82°45.00'
2K	41°46.00'	82°52.00'
6K	41°40.00'	82°40.00'
7K	41°34.00'	82°40.00'
2L	41°47.83'	83°13.83'
6L	41°50.83'	83°07.00'
7L	41°49.00'	83°00.00'
10L	41°53.67'	82°59.17'
1M	41°42.83'	83°25.50'
7M	41°44.00'	83°17.83'
8M	41°47.33'	83°21.33'
1P	41°32.92'	82°55.00'
3P	41°39.00'	83°09.00'
3P ALT	41°39.79'	83°09.09'
4P	41°45.00'	83°06.25'
5P	41°44.00'	82°58.25'
6P	41°38.42'	82°56.67'
7P	41°41.25'	83°02.42'
4R	41°52.83'	83°17.83'
1T	41°41.81'	83°28.13'
2T	41°44.55'	83°26.86'
3T	41°44.21'	83°27.77'

Table 2. Number of fish analyzed from the western basin of Lake Erie, collected in May, June, and July 1995 and 1996
		Number of Specimens Analyzed
Species		Western End	Eastern End
trout perch	1995	68	65
	1996	75	40
silver chub	1995	50	39
	1996	66	8
spottail shiner	1995	23	29
	1996	25	18
emerald shiner	1995	0	0
	1996	42	50
Total
349	249

Table 3. Presentations related to this project (no abstracts available)

• Heidelberg College Biology Seminar Series, Tiffin, Ohio. 23 March 1995. "Return of the Canadian Soldiers (Burrowing Mayflies) to Lake Erie".
• Fourth Annual Ohio Lake Erie Conference, Wickliffe, Ohio. 13 September 1995. "The Resurgence of Lake Erie Mayflies".
• Fifth Annual Ohio Lake Erie Conference, Toledo, Ohio. 25 September 1996. "The Benthic Community and Mayflies".
• Miami University, Department of Zoology Ecology Seminar Series, Oxford, Ohio. 7 November 1996. "Changes in the Environmental Quality of Lake Erie as Indicated by Benthic Invertebrate Communities: Implications for Lake Management".
• University of Toledo, Department of Biological Sciences, Toledo, Ohio. 31 January 1997. "Recent Changes in the Lake Erie Ecosystem as Indicated by Benthic Invertebrate Communities".
• Bowling Green State University, Department of Geology Colloquium, Tuesday Lecture Series, Bowling Green, Ohio. 15 April 1997. "Recent Changes in the Lake Erie Ecosystem as Indicated by Benthic Invertebrate Communities".

Table 4. Presentations of project results at professional meetings (abstracts available)

• Krieger, K.A., L.A. Trissell, and K.G. Boggs. June 1997. Burrowing mayflies (Ephemeridea: Hexagenia spp.) in the diets of forage fishes in western Lake Erie. Annual Meeting, International Association for Great Lakes Research, University of Buffalo, New York.
• Schloesser, D.W., K.A. Krieger, C.P. Madenjian, M. Bur, and J. Ciborowski. Increasing densities of burrowing mayflies (Hexagenia spp.) in western Lake Erie. Annual Meeting, International Association for Great Lakes Research, University of Buffalo, New York.
• Schloesser, D.W., and K.A. Krieger. May 1997. Rapid range expansion of burrowing mayflies (Ephemeridae: Hexagenia spp.) in western Lake Erie. Annual Meeting, International Association for Great Lakes Research, Erindale College, University of Toronto.
• Ciborowski, J., L. Corkum, K. Krieger, and D.W. Schloesser. October 1997. Burrowing mayfly (Hexagenia) populations in Lake Erie. 24th Annual Aquatic Toxicity Workshop, Niagara Falls, Ontario.
• Ciborowski, J. J. H., L. D. Corkum, J. Gerlofsma, D. W. Schloesser, and K. A. Krieger. May 1998. Burrowing mayfly (Hexagenia) populations in Lake Erie. International Association for Great Lakes Research, McMaster Univ., Hamilton, Ontario.
• Schloesser, D.W., J.J.H. Ciborowski, K.A. Krieger, and T. Nalepa. December 1998. Abundance of burrowing mayfly nymphs in western Lake Erie: exponential increases 1991-1997 and a decline in 1998. 60th Midwest Fish & Wildlife Conference, Cincinnati, Ohio.
• Shields, L.A. February 1999. The burrowing mayfly influences the diets of forage fish in Lake Erie. Fifth Annual Student Research Conference, Heidelberg College, Tiffin, Ohio.
• Krieger, K.A., L.A. Shields, S.L. Wildman, J.B. Saxton, and K.G. Boggs. April 1999. Changes in the diets of four forage fishes in western Lake Erie in relation to changes in the benthic invertebrate community. Annual Meeting, Ohio Academy of Science, Cleveland, Ohio.
• Ciborowski, J.J.H., L.D. Corkum, A. Grgicak, M.E. Chase, D.W. Schloesser, and K.A. Krieger. May 1999. Estimated flux of nutrients and contaminants associated with Hexagenia mayflies in western Lake Erie. Annual Meeting, International Association for Great Lakes Research, Cleveland, Ohio.
• Krieger, K.A., S.L. Wildman, L.A. Shields, J.B. Saxton, and K.G. Boggs. May 1999. Forage fishes in Lake Erie respond to changes in benthic food resources. Annual Meeting, International Association of Great Lakes Research, Cleveland, Ohio.

Table 5. Peer-reviewed papers published or in preparation that directly resulted from this project

• Krieger, K.A., D.W. Schloesser, B.A. Manny, C.E. Trisler, S.E. Heady, J.J.H. Ciborowski, and K.M. Muth. 1996. Recovery of burrowing mayflies (Ephemeroptera: Ephemeridae: Hexagenia) in western Lake Erie. Journal of Great Lakes Research 22(2):254-263.
• Madenjian, C.P., D.W. Schloesser, and K.A. Krieger. 1998. Population models of burrowing mayfly recolonization in western Lake Erie. Ecological Applications 8(4):1206-1212.
• Schloesser, D. W., K. A. Krieger, J. J. H. Ciborowski, and L.D. Corkum. In preparation. Burrowing mayflies (Hexagenia spp.) in western Lake Erie: increasing densities in the 1990s. J. Great Lakes Res.

Table 6. News media that have contacted the project director regarding this project
Correspondent	Affiliation	Date of Initial Contact
Ann Hogarth, Kim Bates	Toledo Blade	6/96
Tom Henry, George Tamber		6/98
Jeff Barnhill, Lou Hebert	WTOL-TV (Toledo)	5/16/97
Mark Boleky	News-Herald (Willoughby)	6/8/99
Scott Carpenter	Fremont News-Messenger	5/22/97
	& Port Clinton News-Herald
Joe Case	Audio News Service (Columbus)	6/14/99
Molly Cavanaugh	Plain Dealer (Cleveland)	6/25/97
Darcy Egan		6/18/96
Michael Culp	Advertiser Tribune (Tiffin)	6/15/98
	& Port Clinton News-Herald
Bob Gross	Times Herald (Port Huron, MI)	2/26/97
Eileen Guy	Sandusky Register	6/96
Ellen Tietjen, Kris Weiss		6/97
Debbie Katterheinrich	Ohio Lake Erie Office	6/97
Maggie Kelch	Lakefront News	6/13/97
Karin Messner	free-lance (Port Clinton)	5/20/97
Frank Savukinas	Port Clinton News-Herald	9/16/98
Ron Truman	free-lance (Belleville, Ontario)	11/3/98
Ron Vidica	Lorain Journal	6/4/98
Art Weber	Milbury Press (Toledo)	7/18/97