Fecundity of Walleyes in Western Lake Erie, 1966 and 1990-91

J. Great Lakes Res. 19(4):715-719

Internat. Assoc. Great Lakes Res., 1993

Fecundity of Walleyes in Western Lake Erie, 1966 and 1990-91 Kenneth M. Muth and Brian S. Ickes National Biological Survey, Great Lakes Center Sandusky Biological Station 6100 Columbus Avenue Sandusky, Ohio 44870

ABSTRACT. Ovaries were collected from walleyes (Stizostedion vitreum vitreum) in western Lake Erie just prior to spawning in 1990 and 1991 to determine current fecundity. Results were compared with fecundity determined in 1966 prior to stock rehabilitation when walleye abundance was lower and fish size at age was greater. Fecundity estimates determined from 121 fish aged 3-10 ranged from 53,000 to 426,000 eggs per female. Increases in egg production correlated with increases in length and weight, and weight accounting for most of the variability. In 1990-91 the mean egg production of the dominant age groups of spawners (ages 4 to 8) was approximately 25% lower than fishes of similar age in 1966. The mean egg diameter in 1990-91 (1.63 mm) was not related to the size or age of the fish and was not significantly smaller than the egg diameter in 1966 (1.72 mm). INDEX WORDS: Fecundity, walleye, reproduction, Lake Erie.

Fluctuations in walleye recruitment are often attributed to climatic and environmental conditions mediating success of egg incubation and survival of fry (Koenst and Smith 1976, Hokanson 1977, Smith 1977, Goodyear and Christensen 1984, Kallemeyn 1987). Density-dependent interactions between fish species that influence survival, growth, fecundity, and abundance of the spawning stocks also influence recruitment (Forney 1976; Colby and Nepszy 1981; Koslow 1984, 1992; Baccante and Reid 1988). The combined effects of climatic, environmental, and density-dependent factors in conjunction with variable annual exploitation rates produce uncertainty about specific causes of the loss of recruitment. Changes in fecundity must be a contributing factor under any circumstances. We quantified the fecundity of walleyes in western Lake Erie during 1990-91 and determined whether it differs from that in 1966 (Wolfert 1969). Changes in fecundity may provide insight into causes of recent and future fluctuations in recruitment.

INTRODUCTION

The walleye (Stizostedion vitreum vitreum) is a major game and commercial species in Lake Erie. Fluctuations of the catch affect socioeconomic values of the fishing industry. Successive years of declining catches may indicate the need for changes in management to compensate for such losses and restore higher catches. However, potential causes for declining stock abundance must be determined to serve as a basis for developing new management strategies that lead to higher sustainable harvest levels. The reproductive success of walleyes in western Lake Erie, as measured by annual young-of-year abundance indexes (Sandusky Biological Station, unpublished data), progressively declined from 1987 through 1989 for unknown reasons. The 198789 year classes subsequently provided very poor recruitment to the fishery. As stock abundance declined, game harvests annually decreased approximately 30% from 1989 through 1991 and resulted in a cumulative reduction in catch of nearly 68% from the 1988 catch (Ohio Department of Natural Resources 1989, 1990, 1991, 1992). The commercial harvest of walleyes also decreased about 17% from the 1988 harvest during this time (Ontario Ministry of Natural Resources 1989, 1990, 1991, 1992).

METHODS

Female walleyes were collected from commercial fishery trap nets just prior to spawning each spring during 1990-91 and returned to the labora715

716

Muth and Ickes

tory. Fish that were shedding eggs during handling were excluded from the samples. In the laboratory, each fish was assigned a number, weighed to the nearest gram, measured total length to nearest millimeter, and aged from a scale. Both ovaries from each fish were placed in a cloth bag marked with the sample number and preserved for later processing in a solution of approximately 10% formalin. Impressions of dried scales were made on cellulose acetate slides and read under a microprojector at a magnification of 42x to determine ages. Scales were independently aged by several individuals and, when they disagreed, the ages were assigned by consensus. Ovaries were removed from the preservative, blotted dry, and weighed to the nearest 0.01 g on a triple beam balance. A cross-section subsample from the median section of each ovary was independently weighed to the nearest 0.01 g, and the total number of eggs in it were counted with the aid of a binocular microscope. In addition, we measured the diameters of 15 randomly selected eggs in each subsample to the nearest 0.01 mm with the aid of a calibrated ocular in the microscope. The number of eggs in each ovary was calculated; the ratio of number of eggs in the subsample to subsample weight was assumed to be identical to the ratio of the total numbers of egg in the ovary to total ovary weight. The combined number of eggs from both ovaries was the estimated fecundity of the fish. The effects of variable concentrations of preservative and duration of preservation on egg size were tested to assure current egg measurements were comparable with previous measurements. Ovaries in 1966 were preserved in 10% formalin (Wolfert 1969), whereas the formalin concentration to preserve ovaries in 1990-91 was not consistently measured but was approximately 10-15%. We collected fresh eggs from one large old female and one small younger female and measured 20 eggs taken at random from each fish. Subsamples of eggs from each fish were preserved in 10%, 15%, and 20% (by volume) concentrations of formalin solutions and sequential measurements of egg diameters were recorded during a lO-week period. Significant differences of mean length and mean weight at age, mean eggs/gram female weight, and mean eggs/age female between 1990 and 1991 were determined with t-tests. No significant differences between years occurred for any of these characteristics and the data were combined for comparison with 1966 data. Regression lines were determined for the weight vs. age, fecundity vs. weight, and fe-

cundity vs. age relations in spawning females from both the 1966 and 1990-91 populations. Regression lines from the 1966 population and the 1990-91 population were compared with the analysis of covariance (ANCOVA) outlined in Neter and Wasserman (1974). T-tests were used to determine significant differences in mean egg diameters among preserved samples in 1992 and between 1966 and 1990-91 egg samples.

RESULTS Length and weight characteristics of the 121 walleyes sampled in 1990 and the 78 fish sampled in 1966 are presented in Table 1. No walleyes older than age 7 were present in the samples collected in 1990 but a few in age groups 8 through 10 were obtained in 1991. Fecundity was highly correlated with length, weight, and age in all years; the highest correlations were for 1966 (Table 2). Correlations between fecundity and age were slightly lower than the correlations between fecundity and length or weight but age can be an important characteristic for describing fecundity changes over time as age composition of the spawning stock changes. Mean fecundity increases progressively with age during all sampling years (Table 3) and the means from the pooled 1990-91 data were lower than mean fecundity values for similar aged fish in 1966. Average fecundity in 1990-91 from all age groups was about 25% lower than in 1966 (Fig. 1). Weight is the best indicator of fecundity during

TABLE 1. Age, mean total length (mm), and mean weight (g) of walleyes in western Lake Erie used to determine fecundity in 1966 and 1990-91. 1966*

Year

Mean Mean Length Weight

Age Group

N

3 4 5 6 7 8 9 10 11

14 37 5 2 3 3 9 3 2

465 531 593 629 674 709 711 763 765

1,126 1,799 2,625 2,958 4,120 4,198 4,798 6,523 6,160

Sample Means

574

2,582

*Data from Wolfert (1969).

1990-91 N

3 51 36 9 15 4 2 1

Mean Mean Length Weight 417 491 529 571 617 645 683 691

776 1,323 1,800 2,248 2,882 3,625 3,878 4,680

532

1,859

Walleye Fecundity Changes TABLE 2. Regression equations for walleye fecundity (F) versus length (L), weight (W), and age (A) in 1966 and 1990-91. Number (N) of sampled fish. Year

N

Equation

r

1966 1990-91

78 121

LENGTH F = -646931 + 1514.434L F = -440567 + 1125.603L

0.9358 0.8998

1966 1990-91

78 121

WEIGHT F = -22558 + 94.671 W F = -2607 + 86.542W

0.9489 0.9140

1966 1990-91

78 121

AGE F = -108419 + 63611A F = -81193 + 47347A

0.9357 0.8348

TABLE 3. Mean number of eggs (in thousands) (± SE) of walleyes in Lake Erie in 1966 and 1990-91. Number (N) offish sampled in each age group. Year Age

1966* N 14 37 5 2 3 3 9 3 2

3 4 5 6 7 8 9 10 11

Mean 90.7 139.7 213.5 260.6 374.3 363.6 501.7 503.2 519.7

1990-91 SE (7.0) (6.8) (11.0) (86.7) (64.6) (33.1 ) (25.5) (64.6) (75.6)

N 3 51 36 9 15 4 2 1

Mean 64.2 112.6 148.8 186.8 256.0 319.0 318.7 425.6

SE (9.4) (3.8) (7.9) (21.3) (13.8) (38.4) (27.4)

9

11

*Data from Wolfert (1969).

600 -1966

500

- - - - 1990-91

en

Cl Cl VOj

400

"

_c 0"

" ~

300

... 0

V<=

.at:;

E

:> Z

200 100 0 2

4

5

6

7

8

10

12

Age

FIG. 1. Fecundity of different age groups of walleye in Lake Erie in 1966 and 1990-91. Vertical bars indicate one standard error above and below the mean.

717

the 1966 and 1990-91 sampling periods (Table 2). An F-test for equality of regression lines for the 1966 vs. 1990-91 age-weight regressions found the lines were significantly different (P0.05) so data were pooled for comparison with egg size recorded in 1966. However, there was some concern that preservation technique may alter egg size sufficiently to invalidate comparability between the data sets. Ovaries in 1966 were preserved in 10% formalin (Wolfert 1969) whereas formalin concentration used to preserve ovaries in 1990-91 was not consistently measured but was approximately 10-15% so we conducted a series of tests to determine if varying formalin concentrations differentially altered preserved egg sizes. Average egg diameters for unpreserved eggs collected from one older large female and one younger smaller fish collected in the spring of 1992 were both 1.59 mm. Subsamples of these eggs were preserved in 10, 15, and 20% concentrations of formalin (by volume) and periodically measured to determine any changes in egg diameter measurements. Mean egg diameters decreases by 3.5, 4.4, and 5.0% after 5 days of preservation in 10, 15, and 20% formalin, respectively, but subsequent decreases in egg size were minimal. Mean egg diameters after 75 days of preservation in 10, 15, and 20% formalin concentrations were only 4.1,5.0, and 5.7% smaller than the original diameter of unpreserved eggs so we concluded that effects due to preservation technique on the 1990-91 eggs would be comparable to those for 1966 eggs. Mean egg diameter from the 1990-91 pooled data was 1.63 mm but this was not significantly smaller (F = 1.52, P>0.05) than the 1.72 mm egg diameter reported in 1966.

718

Muth and Ickes DISCUSSION

The fecundity of walleyes varies greatly among stocks in different ecosystems (Colby et ai. 1979, Colby and Nepszy 1981) and in a given ecosystem over time (Serns 1982, Baccante and Reid 1988). Variability in fecundity may be caused by a variety of factors but is often associated with changes in growth as stocks are exploited or as environmental conditions alter the fish community structure (Colby and Nepszy 1981). Ware (1980) hypothesized that piscivores have the ability to divert digested energy from the production of sex products to maintain somatic growth at times when growth rates are limited by lack of sufficient food. This may have occurred with walleye in Lake Erie. The growth of walleyes decreased during the past decade in association with stock rehabilitation (Muth and Wolfert 1986, Hatch et at. 1987) and th~ simultaneous decline of some prey fish species utilized by walleye (Muth 1985). Delayed onset of maturity also occurred as stock characteristics changed (Muth and Wolfert 1986), but fecundity was not examined at that time. This study clearly shows that walleye fecundity decreased from that which existed in 1966 prior to stock rehabilitation (Wolfert 1969) and the current time. This knowledge may be useful in understanding future fluctuations in the reproductive success of walleyes in Lake Erie. The fecundity of walleyes as a factor regulating reproductive success and recruitment is poorly understood and often debated. Koslow (1992) indicated that characterization of stock-recruitment relations of species with high fecundities such as walleye may not be possible. Conversely, Goodyear and Christensen (1984) suggested recruitment for a population of striped bass (Morone saxatiiis) in the Hudson River, a highly fecund species, was directly proportional to fecundity but also influenced by random environmental variables. Cowan et ai. (in press) indicated striped bass recruitment is dependent on many factors including size of spawning female, thus the larger more fecund females should produce better recruitment. Whatever the case, if walleye fecundity has any impact on recruitment, the current lower fecundity of Lake Erie stocks may present several possible scenarios that managers may want to consider when developing future management strategies. Egg survival and hatchability are optimal when climatic and environmental conditions are ideal during egg incubation, and the production of a

strong year class for future recruitment can occur regardless of fecundity or the size of the brood stock (as occurred in Lake Erie in 1977; Hatch et ai. 1987). Ideal spawning conditions however occur infrequently, and optimal egg survival and hatchability are rare. When spawning conditions are not ideal, total egg deposition may be an important factor influencing year class production and recruitment. Spawning stocks with low fecundity would produce lower total egg deposition. The impact of reduced fecundity on total egg deposition is compounded when brood stock abundance is low, the brood stock consists mainly of younger fish that produce fewer eggs, or both factors occur simultaneously. The combination of reduced fecundity and a brood stock dominated by younger fish existed during the 1987-89 period when successive years of poor spawning conditions produced a progressive decline in year class production and subsequent recruitment of walleyes in Lake Erie. By 1990 and 1991, increases in young-of-year abundance indexes (Sandusky Biological Station, unpublished data) suggested that spawning success was improving. This improved spawning success may be related to increased egg production from the strong 1986 year class as these fish grew older and became a more dominant component of the brood stock. ACKNOWLEDGMENTS

We thank P. Leidorf, commercial fisherman, for allowing us to collect the walleye samples from his trapnets. D. Wolfert collected and helped process the fish for this study. The production of computer graphics and assistance with the computer analysis of data provided by M. Bur and C. Madenjian are gratefully acknowledged. Help with counting and measuring egg samples was provided by summer students and technicians and their assistance is greatly appreciated. This article is contribution 840 of the National Biological Survey, Great Lakes Center, 1451 Green Road, Ann Arbor, MI 48105. REFERENCES Baccante, D.A., and Reid, D.M. 1988. Fecundity changes in the exploited walleye populations. North Am. J. Fish. Manage. 8:199-209. Colby, P.J., and Nepszy, S. J. 1981. Variation among stocks of walleye (Stizostedion vitreum vitreum): Management implications. Can. J. Fish. Aquat. Sci. 38:1814-1831. _ _ _, McNichol, R. E., and Ryder, R. A. 1979. Synopsis of the biological data on the walleye (Stizoste-

Walleye Fecundity Changes dion vitreum vitreum) (Mitchell 1818). FAO (Food and Agricultural Organization of the United Nations) Fish. Synop. 119. Cowan, J. H., Jr., Rose, K. A., Rutherford, E. S., and Houde, E. D. In press. Individual-based model of young-of-the-year striped bass population dynamics. II. Factors affecting recruitment in the Potomac River, Maryland. Trans. Am. Fish. Soc. 122:XXX-XXX. Forney, J.L. 1976. Year-class formation in the walleye (Stizostedion vitreum vitreum) population of Oneida Lake, New York, 1966-73. J. Fish. Res. Board Can. 33 :783-792. Goodyear, C.P., and Christensen, S. W. 1984. On the ability to detect the influence of spawning stock on recruitment. North Am. J. Fish. Manage. 4:186-193. Hatch, R.W., Nepszy, S. J., Muth, K. M., and Baker, C. T. 1987. Dynamics of the recovery of the western Lake Erie walleye (Stizostedion vitreum vitreum) stock. Can. J. Fish. Aquat. Sci. 44: 15-22. Hokanson, K.E.F. 1977. Temperature requirements of some percids and adaptations to the seasonal temperature cycle. J. Fish. Res. Board Can. 34:1524-1550. Kallemeyn, L.W. 1987. Correlations of regulated lake levels and climatic factors with abundance of youngof-the-year walleye and yellow perch in Four Lakes in Voyageurs National Park. North Am. J. Fish. Manage. 7:513-521. Koenst, W.M., and Smith, L. L. Jr. 1976. Thermal requirements of the early life history stages of walleye Stizostedion vitreum vitreum and sauger Stizostedion canadense. J. Fish. Res. Board Can. 33:1130-1138. Koslow, J.A. 1984. Recruitment patterns in northwest Atlantic fish stocks. Can. 1. Fish. Aquat. Sci. 41: 1722-1729. ____. 1992. Fecundity and the stock-recruitment relationship. Can. J. Fish. Aquat. Sci. 49:210-217. Muth, K.M. 1985. Changes in prey fish abundance in western Lake Erie during periods of high and low predator abundance. In Presented Papers from the Council of Lake Committees Plenary Session on Great Lakes Predator-Prey Issues, March 20, 1985, ed. R.L. Eshenroder, pp. 27-38. Great Lakes Fishery Commission Technical Report 85-3. _ _ _" and Wolfert, D. R. 1986. Changes in growth and maturity of walleyes associated with stock rehabilitation in western Lake Erie, 1964-1983. North Am. J. Fish. Manage. 6:168-175. Neter, J., and Wasserman, W. 1974. Applied linear statistics models. Richard D. Irwin, Inc. Homewood, Illinois. ODNR (Ohio Department of Natural Resources). 1989. Status and Trend Highlights Ohio's Lake Erie Fish

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and Fisheries, March 1989. Report for the Lake Erie Committee Meeting of the Great Lakes Fishery Commission, Grand Island, New York, March 22-23, 1989. _ _ _. 1990. Status and Trend Highlights Ohio's Lake Erie Fish and Fisheries, March 1990. Report for the Lake Erie Committee Meeting of the Great Lakes Fishery Commission, Ann Arbor, Michigan, March 22-23, 1990. _ _ _. 1991. Status and Trend Highlights Ohio's Lake Erie Fish and Fisheries, March 1991. Report for the Lake Erie Committee Meeting of the Great Lakes Fishery Commission, Niagara Falls, New York, March 25-26, 1991. _ _ _. 1992. Status and Trend Highlights Ohio's Lake Erie Fish and Fisheries, March 1992. Report for the Lake Erie Committee Meeting of the Great Lakes Fishery Commission, Erie, Pennsylvania, March 2324, 1992. OMNR (Ontario Ministry of Natural Resources). 1989. Lake Erie Fisheries Report 1988. Report for the Lake Erie Committee Meeting of the Great Lakes Fishery Commission, Grand Island, New York, March 22-23, 1989. _ _ _. 1990. Lake Erie Fisheries Report 1989. Report for the Lake Erie Committee Meeting of the Great Lakes Fishery Commission, Ann Arbor, Michigan, March 22-23, 1990. _ _ _. 1991. Lake Erie Fisheries Report 1990. Report for the Lake Erie Committee Meeting of the Great Lakes Fishery Commission, Niagara Falls, New York, March 25-26, 1991. _ _ _. 1992. Lake Erie Fisheries Report 1991. Report for the Lake Erie Committee Meeting of the Great Lakes Fishery Commission, Erie, Pennsylvania, March 23-24, 1992. Serns, S.L. 1982. Walleye fecundity, potential egg deposition, and survival from egg to fall young-of-year in Escanaba Lake, Wisconsin, 1979-1981. North Am. J. Fish. Manage. 4:388-394. Smith, L. L., Jr. 1977. Walleye (Stizostedion vitreum vitreum) and yellow perch (Perea flavescens) populations and fisheries of the Red Lakes, Minnesota, 1930-75. J. Fish. Res. Board Can. 34:1774-1783. Ware, D. M. 1980. Bioenergetics of stock and recruitment. Can. J. Fish. Aquat. Sci. 37:1012-1024. Wolfert, D.R. 1969. Maturity and fecundity of walleyes from the eastern and western basins of Lake Erie. J. Fish. Res. Board Can. 26: 1877-1888. Submitted: 25 June 1993 Accepted: 5 October 1993