Postspawning Mortality of Rainbow Smelt in Western Lake Superior

1. Great Lakes Res. 7(1):37-41 Internat. Assoc. Great Lakes Res. 1981

POSTSPAWNING MORTALITY OF RAINBOW SMELT IN WESTERN LAKE SUPERIOR Wayne F. Schaefer l Richard A. Heckmann William A. Swenson 2 Department of Zoology, Brigham Young University Provo, ur 84602

ABSTRACT. Extensive die-ofJs of rainbow smelt, Osmerus mordax (Mitchell), have occurred in the western end of Lake Superior during each of the past several springs. Sex, age composition, and incidence offungus infection in smeltfrom the 1977 die-ofJwere compared with the same characteristics ofsmelt ofthe extant spawningpopulation to determine the extent andpossible causes ofmortality. The die-ofJin 1977 began about 3 weeks after the end ofthe spawning run and continuedfor approximately 2 weeks. The occurrence of dead rainbow smelt was greatest near the Superior-Duluth harbor and decreased as distance from the harbor increased. Age structure ofsmelt that died was similar to that of the spawning population. Some factor or factors related to spawning may have caused the mortality. The most probable cause ofthe die-off was temperature stress on spawning smelt in the spawning areas which increased the susceptibility of smelt to the fungus Saprolegnia sp. and may have promoted osmoregulatory imbalance. Male smelt were more vulnerable to the cause of the die-ofJthanfemale smelt; young fish, especially females, were more resistant than older fish.

INTRODUCTION The rainbow smelt, Osmerus mordax, is the most abundant forage species in Lake Superior and supports important commercial and sport fisheries (Baldwin and Saalfeld 1962). During the past several years smelt have exhibited conspicuous spring dieoffs in the extreme western end of the lake. Massive smelt die-offs occurred in Lake Huron and Lake Michigan in 1941 which were attributed to an undetermined communicable disease that affected only smelt (Van Oosten 1947). Unlike the Lakes Huron and Michigan die-offs, that in western Lake Superior occurred each year after spawning and apparently was more localized. Two previous studies of smelt in western Lake Superior included information on biological characteristics of spawning populations (Hale 1960, Bailey 1964). However, neither these studies nor those for other waters (Baldwin 1950; Lievense 1954; McKenzie 1958, 1964; Nepszy and Dechtiar 1972)

described the population characteristics of postspawning die-offs. This study was undertaken to determine population characteristics and the extent and possible causes of the die-offs in western Lake Superior. METHODS During the springs of 1976 and 1977, 1,958 mature rainbow smelt, 1,916 rainbow smelt that survived spawning, and 1,014 post-spawning dead rainbow smelt were examined. Rainbow smelt were collected during spawning runs at five stations.'A 7.6 m bag seine was used to capture smelt in the Amnicon River, Brule River, and near-shore areas off Wisconsin Point (Figure 1). Collections from the Knife and Beaver rivers were made with hand nets. Three spawning samples were obtained at the Wisconsin Point and one spawning sample was obtained at each of the other four locations. Total length and sex of each spawning fish were recorded. Surviving rainbow smelt were collected from western Lake Superior off Wisconsin Point and Minnesota Point during the die-off period (17 May16 June) by bottom trawl. Ten representative samples were collected during 1976 and 1977 at depths from 4 to 15 m using a semiballoon trawl with

1 Present

address: Department of Biology, University of Wisconsin Center - Washington County, West Bend, WI 53095 2Department of Biology University of Wisconsin - Superior Superior, WI 54880

37

SCHAEFER, HECKMANN, and SWENSON

38

f

t

Scole in Miles

1~0 10

0

10

20

Scale in kilometers

...f c

~

FIG. 1. Study sitefor the western Lake Superior research.

a 9.5 m headrope, 18 mm bar mesh, and 6.4 mm cod liner. The length, sex, and incidence of fungus infection were recorded for each specimen. Comparisons were made between the degree of fungal infection on non-spawning and spawning size fish. Duration and intensity of the die-off were determined by counting dead fish along the beach. These counts were made at approximately 5-day intervals at 12 stations located east of SuperiorDuluth along the north and south shorelines to the Cascade River in Minnesota and Cornucopia Beach in Wisconsin (Table 1). Three counts of dead fish were made at each location each sampling day. Counts were made in areas 30-m long by 6-m wide and included all rainbow smelt that had washed onto the beach. Because of the difficulty in distinguishing recently dead fish from those previously counted, the total number of dead smelt on the beaches was determined and the number of dead fish added to anyone beach between sample days was estimated by subtracting the previous count. At two locations (Wisconsin Point and the Amnicon River), the die-off was intensive enough to provide large representative samples. Total length and sex of dead fish were recorded. Subsequently the age structures of the spawning and dead fish were determined by stratified age samples (Ricker 1975) using Schaefer's (1979) age-length samples for males, females, and sexes combined. Fresh mycelia were collected from dead fish and cultured for the purpose of fungus identification. Other fungal specimens were preserved in 10%

TABLE 1. Summary of beach counts of dead smelt 'made during the postspawning die-off of 1977.

Location Wisconsin Point Lester River Knife River Two Harbors Beaver River Temperance River Cascade River Amnicon River Brule River Iron River Herbster Cornucopia

Km from SuperiorDuluth harbor

o 16 44 48 83 128 159 13

32 41 60 71

Approximate spawning peak

Approximate die-off peak

4-30-77 5-4-77 5-4-77 5-5-77 5-6-77 5-6-77 5-6-77 5-4-77 5-4-77 5-4-77 4-30-77 4-30-77

6-4-77 6-8-77 6-6-77 5-28-77 No die-off No die-off No die-off 5-24-77 6-4-77 6-4-77 5-24-77 5-24-77

Number of dead smelt per 30 m of beach during die-off peak

3239 405 244 24

o o o

475 29 80

10

1

POSTSPAWNING SMELT MORTALITIES formalin and later prepared for viewing with a scanning electron microscope. RESULTS The die-off of smelt in western Lake Superior reached maximum intensity 3-5 weeks after the spawning peak at each location and lasted about 2 weeks as indicated by fresh, recently demised fish floating on the water and deposited by wave action on the beaches. No dead smelt were found on the beaches prior to or during spawning. Along both the north and south shores, the density of dead smelt on beaches decreased with distance from the SuperiorDuluth harbor (Table 1). In general, evidence ofthe die-off was restricted to within 50 km of the Superior-Duluth harbor in 1977. The highest average concentration of dead smelt (18/ m 2) occurred on Wisconsin Point near the harbor entry. Sex ratios varied greatly between the spawning population and dead fish located on the beach. The average sex ratio in the spawning population was 0.74 males per female, but the dead fish samples included 1.62 males per female. The observed numbers of males in all four die-off samples analyzed were significantly greater than expected under the hypothesis that half the dead fish were males and half females (p < .001, X2 =24.28, df =3). The composite age structure of rainbow smelt in the die-off (Table 2) did not significantly differ from that of the general spawning population (x 2 =0.44, df = 4), indicating that all age groups of spawning fish may have been equally subjected to the factors that caused the die-off. Comparison of age structures between the seven spawning samples showed variation (s =25%), and further illustrates the complications involved in obtaining representative population samples of Osmerus mordax. However, the use of multiple spawning samples obtained at different times from several locations

39

tended to minimize the effect of sample bias in the final estimate of spawning population age structure. All dead rainbow smelt were infected with fungus. The most common locations of fungus on the fish were the base of the pectoral and pelvic fins and the area around the vent. Fungus was also observed on the dorsal surface of the head and on the caudal peduncle. Fungus was also observed on live smelt, and the percent of live infected fish decreased as the die-off progressed (Table 3). No fish less than the minimum spawning length of 110 mm was infected with fungus. Over 50% of the live smelt of spawning size, collected from the lake early in the die-off period of 1976, were infected with fungus. The fungus, cultured at 5° C in the laboratory, showed hyphae containing small spherical zoospores within terminal sporangia but none ofthe sexual structures required for identification (Figure 2). The fungus was then sent to Dr. Roland Seymour, Ohio State University, who isolated and observed it for 2 months and noted no sexual structures. The fungus was determined to be a nonsexual form of Saprolegnia, probably S. parasitica. DISCUSSION The time of the rainbow smelt die-offs in western Lake Superior, similarity in age structures of the spawning and die-off populations (Table 2), and the lack of fungal infection in pre-spawning smelt (Table 3) suggest that the phenomenon is related to spawning. The decrease in intensity of the die-off with distance from the Superior-Duluth harbor (Table 1) suggests that a factor characteristic ofthe Superior-Duluth harbor may have caused or intensified the mortality. The prevailing westerly winds and counter-clockwise current flow in the western basin of Lake Superior would not tend to concentrate fish near the Superior-Duluth harbor. Variation in temperature is considerable between

TABLE 2. Percentage age composition ofspawning and die-offpopulations ofrainbow smelt in western LakeSuperior.

Sex

Group

Sample size

Male Male Female Female Composite" Composite"

Spawning Die-off Spawning Die-off Spawning Die-off

780 525 1,053 325 1,954 1,007

"Includes mature males, females, and spent fish of undetermined sex.

Age 2

3

4

5

18.5 19.6 15.0 10.9 18.2 19.0

54.4 53.6 47.9 46.6 50.5 50.7

24.7 24.6 31.8 36.1 27.5 26.7

2.4 2.2 5.3 6.6 3.8 3.6

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SCHAEFER, HECKMANN, and SWENSON TABLE 3. Variation in the occurrence of fungus infection with time and between spawning and prespawning size groups of rainbow smelt captured by trawl during the postspawning period.

Date 5-18-76 5-20-76 5-27-76 6-8-76 6-10-76 6-16-76 5-17-77 5-19-77 5-26-77 6-2-77

FIG. 2. Electron micrograph of Saprolegnia sp on Osmerus mordax. 200X.

waters in the harbor and the lake. In 1977, the surface water temperature in the harbor during the spawning period was approximately 15°C, about 6° C warmer than nearshore waters of the lake proper. The preferred temperature for rainbow smelt in Lake Erie is 7°C (Hart and Ferguson 1966). During spring spawning, rainbow smelt move into the harbor from the deep water of the lake and must undergo substantial temperature stress. Stanley and Colby (1971); Reaves, Houston, and Madden (1968); Hickman et al. (1964); Houston (1962); Woodhead and Woodhead (1959); and Meyer, Westall, and Platner (1955) suggested" a possible relationship between temperature stress and osmoregulatory imbalance in clupeids, cyprinids, and salmonids. Stanley and Colby (1971) noted that some alewives, Alosa pseudoharengus, subjected to thermal stress, developed fungal infection and died. They observed no deaths in noninfected fish in any of their experiments. Rainbow smelt may be vulnerable to osmoregulatory imbalance because of their history as a marine species. General physiological stress of spawning and osmoregulatory imbalance that may result appear to have made smelt highly susceptible to Saprolegnia parasitica in western Lake Superior.

Fish> 110 mm Number Sample with size fungus 51 76 104 99 32 27 67 194 234 233

32 36 8 4 0 2 6 12 6 4

Fish < 110 mm Number Sample with size fungus 48 66 228 125 70 71 75 117 3 41

0 0 0 0 0 0 0 0 0 0

This fungus appeared to be the actual cause of death for many of the moribund fish that were observed. The sex ratio of smelt was estimated to be 0.42 males per female (Schaefer 1979), but 1.62 males per female were found to have died. Schaefer (1979) found that annual mortality of male rainbow smelt in western Lake Superior is higher than that of females. The higher annual mortality of males appears to be related to their higher vulnerability to the cause of the postspawning die-off. The greatest contrast in susceptibility occurred during the early ages, in which females appeared to be more resistant to the cause of the die-off than males (Table 2). Male rainbow smelt entered the spawning area on the harbor side of the Wisconsin Point before females in 1977, and other studies showed that early arrival of males on the spawning grounds is typical (Langlois 1935; Warfel, Frost, and Jones 1943; Baldwin 1950; Bailey 1964; MacCallum and Regier 1970). The higher susceptibility of males to the cause of the die-off may relate to their earlier arrival on the spawning grounds, resulting in prolonged exposure to the conditions which result in die-off. In contrast to the characteristic of male predominance among the smelt that died in Lake Superior, Brown (1968) noted that male and female alewives were equally represented in the 1967 alewife die-off in Lake Michigan and found that dying fish had a low incidence of Saprolegnia infection. Brown (1972) attributed the 1967 alewife die-off to poor condition of the fish caused by overcrowding, and concluded that spawning attrition was not a major factor.

POSTSPAWNING SMELT MORTALITIES ACKNOWLEDGMENTS

We gratefully acknowledge the NSF students participating in the 1977 science training program at UW-Superior for their assistance in collecting many of these data. We also thank Dr. R. Seymour for his help in identifying the fungus and Drs. S. Rushforth and V. Crandall for their critical reviews of the manuscript. Contribution Number 43 from the Center for Lake Superior Environmental Studies, University of Wisconsin-Superior.

REFERENCES Bailey, M.M. 1964. Age, growth, maturity and sex composition of the American smelt, Osmerus mordax (Mitchill), of western Lake Superior. Trans. Amer. Fish. Soc. 93:382-395. Baldwin, N.S. 1950. The American smelt, Osmerus mordax (Mitchill), of South Bay, Manitoulin Island, Lake Huron. Trans. Amer. Fish. Soc. 78:176-180. _ _ _ _ , and Saalfeld, R.W. 1962 (plus supplement, 1970). Commercial fish production in the Great Lakes 1867-1970 (supplement 1961-68). Great Lakes Fishery Commission. Technical Report 3. Brown, E.H. 1968. Population characteristics and physical condition of alewives, Alosa pseudoharengus, in a massive die-off in Lake Michigan, 1967. Great Lakes Fishery Commission. Technical Report 13. _ _ _ _ . 1972. Population biology of alewives, Alosapseudoharengus, in Lake Michigan, 1949-70. J. Fish. Res. Board Can. 29:477-500. Hale, J. 1960. Some aspects of the life history of the smelt (Osmerus mordax) in western Lake Superior. Minnesota Fish and Game Investigations 2:25-41. Hart, J.L., and Ferguson, R.G. 1966. The American smelt. Trade News 18:22-23. Hickman, c.P., Jr., Me Nabb, R.A., Nelson, J.S., Van Breemen, E.D., and Comfort, D. 1964. Effect of cold acclimation on electrolyte distribution in rainbow trout (Salmo gairdneri). Can. J. Zoology 42:577-597. Houston, A.H. 1962. Some observations on water balance in the goldfish, Carassius auratus, during cold death. Can. J. Zoology 40:1169-1174. Langlois, T.H. 1935. Notes on the spawning habits ofthe Atlantic smelt. Copeia 1935: 141-142. Lievense, S.J. 1954. Spawning of American smelt

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Osmerus mordax in Crystal Lake, Benzie County, Michigan. Copeia 1954:232-233. MacCallum, W.R., and Regier, H.A. 1970. Distribution of smelt, Osmerus mordax, and the smelt fishery in Lake Erie in the early 1960's. J. Fish. Res. Board Can. 27:1823-1846. McKenzie, R. A. 1958. Age and growth of smelt, Osmerus mordax (Mitchill), of the Miramichi River, New Brunswick. J. Fish. Res. Board Can. 15: 1313-1327. _ _ _ _ . 1964. Smelt life history and fishery in the Miramichi River, New Brunswick. Fisheries Research Board of Canada Bulletin 144. Meyer, D.K., Westall, B.A., and Platner, W.S. 1955. Water and electrolyte balance of goldfish under conditions of anoxia, cold and inanition. Amer. J. Physio. 184-533-556. Nepszy, S.J., and Dechtiar, A.a. 1972. Occurrence of Glugea hertwigi in Lake Erie rainbow smelt (Osmerus mordax) and associated mortality of adult smelt. J. Fish. Res. Board Can. 29: 1639-1641. Reaves, R.S., Houston, A.H., and Madden, J.A. 1968. Environmental temperature and the body fluid system of the fresh-water teleost - II. Ionic regulation in rainbow trout, Salmo gairdneri, following abrupt thermal shock. Comparative Biochemistry and Physiology 24:849-860. Ricker, W. 1975. Computation and interpretation of biological statistics of fish populations. Fisheries Research Board of Canada Bulletin 191. Schaefer, W.F. 1979. Population dynamics of rainbow smelt in western Lake Superior. Doctoral dissertation. Brigham Young University, Provo, Utah, USA. Stanley, J.G., and Colby, P.J. 1971. Effects of temperature on electrolyte balance and osmoregulation in the alewife (Alosa pseudoharengus) in fresh and sea water. Trans. Amer. Fish. Soc. 100:624638. Van Oosten, J. 1947. Mortality of smelt, Osmerus mordax (Mitchill), in Lakes Huron and Michigan during the fall and winter of 1942-43. Trans. Amer. Fish. Soc. 74:310-337. Warfel, H.E., Frost, T.P., and Jones, W.H. 1943. The smelt, Osmerus mordax, in Great Bay, New Hampshire. Trans. Amer. Fish. Soc. 72:257-262. Woodhead, P.M.J., and Woodhead, A.D. 1959. The effects of low temperature on the physiology and distribution of the cod, Gadus morhua L., in the Barents Sea. Proc. Zoo. Soc. London 133: 181-199.