Temporal and Spatial Distribution of Pelagic Age-0 ...

Transactions of the American Fisheries Society 124 :84-93, 1995 © Copyright by the American Fisheries Society 1995

Temporal and Spatial Distribution of Pelagic Age-0 Fish in Lake Mendota, Wisconsin JOHN R. POST Division of Ecology, Department of Biological Sciences, University of Calgary Calgary, Alberta T2N 1N4, Canada

LARS

G.

RUDSTAM1

AND DENISE M. SCHAEL 2

Center for Limnology, University of Wisconsin, Madison, Wisconsin 53706, USA Abstract. -Age-0 yellow perch Perca flavescens, sunfish Lepomis spp ., black crappies Pomoxis nigromaculatus, and freshwater drums Aplodinotus grunniens had pelagic life history phases in Lake Mendota in 1988, 1989, and 1990, as determined by purse-seine sampling . The duration of the pelagic phase varied from 3 to 7 weeks and differed between species and in some cases between years within species . Age-0 yellow perch were numerically dominant through most of June and as summer progressed were replaced sequentially by black crappies, freshwater drums, and Lepomis spp . By August, the pelagic community of age-0 fish was numerically dominated by Lepomis spp. This temporal succession of taxa was consistent among years, although the absolute and relative abundance of taxa differed between years . Most species were distributed across the surface of the lake early in their pelagic phase, became relatively more abundant near shore later in their pelagic phase, and then disappeared from the pelagic zone . Other published studies have demonstrated that age-0 fish migrated into the littoral zone of Lake Mendota after their pelagic residence . Variability in density of age-0 yellow perch among replicate sites within depth strata was high, with coefficients of variation ranging from 48 to 154% . The mean-to-variance relationship indicated that the age-0 yellow perch were aggregated . There were significant differences in the density of age-0 yellow perch among years but not among depth strata within years . The purse seine was our preferred gear because it has been shown to catch a wider range of sizes of age-0 fish than most other sampling gear. Therefore, we believe that the temporal and spatial patterns we describe during the pelagic life history phase of age-0 fish in Lake Mendota is more accurate than those described in other studies because our data include a better representation of the larger size-classes of age-0 fish . Larvae of many lacustrine fishes that spawn in the littoral zone move into open water for weeks or months before returning to the littoral zone, where they reside as juveniles and adults (Faber 1967 ; Amundrud et al . 1974 ; Floyd et al . 1984 ; Mathews 1984 ; Gregory and Powles 1985 ; Treasurer 1988 ; also see review on Perca spp . by Post and McQueen 1988). The timing and duration of these movements are poorly known and may be influenced by water temperature, photoperiod, body size, food density, or predator abundance . Because these potential influences on the timing and duration of these movements can vary with water body, weather, and species, the temporal and spatial distribution of pelagic age-0 fish should be dynamic in diverse fish communities .

1 Present address: Cornell Biology Field Station, Department of Natural Resources, 900 Shackelton Point Road, Bridgeport, New York 13030, USA . 2 Present address : Aquatic Ecology Laboratory, Ohio State University, 1314 Kinnear Road, Columbus, Ohio 43212-1194, USA .

The gears used in most studies (towed nets, pushed nets, high-speed samplers) underestimate abundance of the larger age-0 fish, or are incapable of capturing them, because the ability of age-0 fish to avoid the samplers increases exponentially with body length (Tranter and Smith 1968 ; Noble 1970 ; Murphy and Clutter 1972 ; Snyder 1983). Therefore, a reduction over time in the pelagic catch of age-0 fish in conventional gears may indicate (1) a change in the spatial distribution, (2) reduced abundance due to mortality, or (3) growth-related improvement in the ability of age-0 fish to avoid the gear. Because of the pronounced size-selective nature of most gears used to capture age-0 fish in the pelagic habitat, the published ontogenetic patterns in temporal and spatial distribution may be representative only of the smallest and youngest age-0 fishes that are unable to avoid the gears . In this study we used a purse seine to assess the temporal succession and spatial distribution of a diverse ichthyoplankton . Although the critical experiments have not been done to fully quantify the size selectivity of purse seines, several studies

have shown that this gear captures substantially larger fishes than conventional towed, pushed, or high-speed gears (Coles 1981 ; Evans and Johannes 1988 ; McQueen et al . 1989 ; Post and Evans 1989 ; Post and McQueen 1994, in press) . Collectively, these studies indicate that purse-seine catch data for pelagic age-0 fish are less biased by size-dependent gear avoidance than catch data from conventional gears. The objectives of this study were to (1) describe the temporal and spatial distribution of pelagic age-0 fish within years, (2) determine interannual variability in temporal and spatial distribution, (3) examine interannual variability in year-class strength of fish at the pelagic stage of their life history, and (4) estimate sampling variability and determine the level of aggregation . Methods Lake Mendota (43°4'N, 89°24'W) is a 3,980-ha, 25-m-deep eutrophic lake in southern Wisconsin. It presently has a diverse community of at least 40 native and introduced fishes (Lyons 1989 ; Magnuson and Lathrop 1992). The fish populations of the lake have been studied extensively (Magnuson and Lathrop 1992), but there has been no system-

atic examination of age-0 fish during their pelagic phase. We sampled pelagic age-0 fish in Lake Mendota in 1988, 1989, and 1990 with a 1-mm-square-mesh purse seine, 30 .5 m long and 9 m deep, that was similar to those used by Evans and Johannes (1988) and Post and Evans (1989) . All sampling was conducted between 1000 and 1600 hours. The net was set over the gunwale of a small boat while the boat was maneuvered along the perimeter of a circle about 10 m in diameter. When the maneuver was completed, the purse line was pulled from both ends of the net to enclose the sample volume . When deployed in this manner, the net enclosed 360 m3 of water (surface area 74 m2) in the top 6 m of the water column . Setting and closing the net took 2-3 min . The net was then pulled slowly into the boat to concentrate the fish in a 20-L pocket that was emptied into a pail in the boat . In order to describe temporal and spatial patterns of pelagic age-0 fish distribution, we sampled from late May to early August on 6 dates in 1988, 9 in 1989, and 10 in 1990 . On most sample dates, one purse-seine sample was collected at each of nine stations along transect 1, which extended from University Bay to Governors Island (Figure 1) .

Two stations were located over each of the 5-, 10-, 15- and 20-m depth contours, and a single station was located within the 25-m depth contour. Strong prevailing winds from the west prevented us from sampling the more exposed stations at the Governors Island end of the transect on July 26, 1988, August 8, 1988, and July 2, 1990 . To estimate the variability in density of age-0 fish between locations within depth strata, we did additional sampling on June 12-16, 1989, and June 19-22, 1990 . We sampled at seven stations on each of the 5-, 10-, and 15-m depth contours and at nine stations in water 20-25 m deep . These additional stations were located on transects around the lake (Figure 1) . Age-0 fish were preserved in 70% ethanol and stored for analysis . In the laboratory, all larvae were identified to genus and species when possible by means of the keys of Auer (1982) . These fish were then measured (total length, TL) to the nearest millimeter. Water temperatures were measured weekly, from May to August, with a YSI temperature meter at 1-m depth intervals at the 25-m station on transect 1 (Figure 1) . The mean epilimnetic temperature was calculated as the arithmetic mean of the measurements for the top 6 m . Mean daily wind velocity data were collected at the meteorological station at the Dane County Airport in Madison, Wisconsin, which is adjacent to Lake Mendota. The hypothesis that age-0 fish were distributed evenly among stations was tested for each species on each sample date with the Kolmogorov-Smirnov goodness-of-fit test . Mean, standard deviation, and coefficient of variation (CV = 100.SD/mean) of catches were calculated from log-transformed catches (log10[catch + 1]) and then back-transformed for presentation as geometric means. A two-factor analysis of variance (ANOVA ; a = 0.05) was used to test the hypothesis that the geometric mean catches were the same among years and among depth strata, and that there was no interaction of year and depth stratum. Results and Discussion The Pelagic Community

Of the 40 species of fish recorded in Lake Mendota, only species in 8 families and 15 genera were found in the pelagic region of the lake (Table 1) . Of these, only yellow perch, sunfish (primarily bluegill Lepomis macrochirus, but also pumpkinseed L. gibbosis), black crappie, and freshwater drum were sufficiently common (>5% of the

catch) to evaluate their spatial and temporal distribution. These species were all abundant in the open water at some time during our sampling . Represented in the group of common pelagic taxa in Table 1 are species with a wide variety of life history strategies, including spring and summer spawning, littoral and pelagic spawning, broadcast spawning, and nest building . Although a pelagic early life history phase occurs in species with a diversity of life history strategies, a pelagic phase is clearly not universal for fish in Lake Mendota because many species found as adults in the lake were not found in the pelagic zone as age-0 fish . A pelagic phase in the early life history has been previously observed for some of the species that occur in Lake Mendota and was best described by Post and McQueen (1988) for age-0 yellow perch. Pelagic age-0 Lepomis spp. (Werner 1967 ; Amundrud et al . 1974), black crappies (Amundrud et al . 1974), and freshwater drums (Mathews 1984) have also been reported, but the spatial and temporal extent of the pelagic phases of these species remained uncertain because of the size-selective bias of the gears used in most of these studies. The mean density of age-0 fish in the pelagic zone of Lake Mendota was 0.088-0 .369 fish/m 3

(32-133 individuals per net lift) over the 3 years of the study (Table 2) . Yellow perch and black crappie were most variable between years and Lepomis spp. and freshwater drum were less variable (Table 2) . The relative abundance of the four taxa differed between years (chi-square, df = 6, P < 0 .05) . The most abundant species was different in each of the

3 years of the study, and the four species were not simultaneously most abundant in a particular year. This lack of synchrony in abundance between years suggests that no single environmental factor, such as prey abundance, predator abundance, temperature, or storm events, controlled abundance across species . Recent studies have shown that interannual variability in the year-class strength of age-0 yellow perch in Lake Mendota is not correlated with the abundance of their major prey, copepods (Schael et al . 1991 ; Lathrop and Carpenter 1992 ; Rudstam et al . 1993) .

Catch of the four main taxa per unit effort (Table 2) was not significantly correlated with either the mean or standard deviation of wind speed (P > 0.05 ; Figure 2) . The power to detect significant correlations was low because there were only 3 years of data, but the catch per unit effort in six of the eight comparisons was negatively related to mean wind speed (yellow perch r = -0 .34; freshwater drum r = -0 .52 ; black crappie r = -0 .39 ; Lepomis spp. r = 0.66) and the standard deviation of wind speed (yellow perch r = -0 .80; freshwater drum r = 0.97; black crappie r = -0 .76; Lepomis spp. r = -0 .92) . Other studies on the effect of wind on year-class strength or survival of age-0 fish are inconclusive ; some authors reported no correlation (Serns 1982 ; Kallemeyn 1987), whereas others reported negative correlations (Clady 1976 ; Peterman and Bradford 1987) . Definitive tests of the wind hypothesis will require studies at finer spatial and temporal scales than those that have been used to date . Temporal Succession

The temporal succession of pelagic age-0 fish was consistent among the 3 years of the study (Figure 3) . Yellow perch were dominant through most of June and were replaced sequentially by

black crappies, freshwater drums, and Lepomis spp. as summer progressed . By August the community was dominated by Lepomis spp . The yearto-year consistency in the seasonal chronology of species dominance is probably due to differences in temperature requirements for spawning, incubation, and growth, which vary among species (Becker 1983) . Other studies have found similar seasonal chronologies in the same or related species (Faber 1967 ; Amundrud et al . 1974 ; Floyd et al . 1984 ; Gregory and Powles 1985) . Spatial Distribution

We first caught age-0 yellow perch in relatively high densities in the open water of Lake Mendota in late May or early June in all 3 years (Table 3) . Age-0 yellow perch were not evenly distributed across transect 1 on any date for which the total catch was greater than five individuals (P < 0.05) . They occurred at all stations across the lake but were generally less abundant at the deepest station than at stations closer to shore. They occupied the open water habitat for 2-6 weeks annually . Length data of age-0 yellow perch suggest that yellow perch spawned synchronously and that their progeny grew at similar rates throughout the lake while in the pelagic habitat (Table 4) . Growth rates seem

to have been lower in 1990, perhaps because of lower temperatures in that year (Figure 4) . Three lines of evidence suggest that the age-0 yellow perch had migrated to the littoral zone when we no longer captured them in pelagic habitats. First, age-0 yellow perch were probably not capable of avoiding the purse seine. Thus, the absence of age-0 yellow perch from our samples collected after June indicates that they were no longer in the upper 6 m of the epilimnion (the effective depth of the purse seine) . The evidence supporting this contention is that similar purse seines captured as many as 1,500 pelagic age-0 yellow perch 6090 mm TL per purse-seine set in Lake St . George, Ontario (Johannes et al . 1989 ; McQueen et al . 1989 ; Post and McQueen 1994, in press), yet a purse seine captured none in Lake Mendota when the fish would have been about 30-50 mm TL . The relative abundance of age-0 yellow perch is typ-

ically lower in Lake Mendota than in Lake St . George, but the maximum catch of 17-mm yellow perch in Lake Mendota was 4,313 in a single purseseine set (Tables 3, 4) . Second, analysis of highfrequency (200 kHz) sonar data collected during the daytime in Lake Mendota identified small targets the size of age-0 fish in the upper 6 m of the epilimnion but at no time were small targets seen below 6 m (L . G. Rudstam, unpublished data). This indicates that age-0 yellow perch were absent from epilimnetic water deeper than the effective sampling depth of the purse seine. Third, after age-0 yellow perch disappeared from the pelagic zone they appeared in the catches of fyke nets and gill nets in the littoral zone (Johnson et al . 1991) . Age-0 black crappies were also caught at all sites across the pelagic transect (Table 3) . The duration of their pelagic residence varied from 2 to 6 weeks among years. They were not evenly distributed across the lake (Table 3) but seemed to be concentrated at the deeper stations early in their pelagic residence and at the shallowest station in University Bay towards the end of their pelagic residence. Length data suggest that the spawning period for black crappie was more protracted in 1988 than in 1989 and 1990 (Table 4) . Small individuals were captured in the pelagic zone for about 1 month in 1988 . When age-0 black crappies disappeared from the pelagic zone, they appeared in the catches of fyke nets and gill nets in the littoral zone (Johnson et al . 1991) . Age-0 freshwater drums were resident in the pelagic zone for 3-5 weeks in all 3 years studied (Table 3) . They were not evenly distributed across the lake (Table 3) and were generally least abundant in the center of the lake . During July in 1988 and 1990, near the end of their pelagic residence, they were most abundant in the University Bay end of the transect, which is more sheltered from

the prevailing winds than the Governor's Island end. However, in 1989 they were more abundant at the Governor's Island end of the transect . These differences in distribution between years are not related to wind velocity because July wind velocities were similar in the 3 years (3 .47, 3 .44 and 3 .53 m/s for 1988, 1989, and 1990). Spawning was probably of short duration because mean, maximum, and minimum total lengths increased through time (Table 4) . Age-0 Lepomis spp. were captured for 4-6 weeks in 1988 and 1989 but only for 2-3 weeks in 1990 (Table 3) . Age-0 Lepomis spp. were not evenly distributed across the lake (Table 3) . Their densities were always substantially higher at the shallower sites, particularly in University Bay. Length data suggest that spawning was protracted in 1988 and 1989 and that it began later and was of shorter duration in 1990 . Water temperatures in late May and early June were 3-4°C lower in 1990 than in 1988 and 1989 and may have delayed spawning in 1990 (Figure 4) . Sampling Variability and Aggregation

To determine the variability between replicate purse-seine sets, we focused on age-0 yellow perch, which made up 99 .7 and 88 .4% of the catch during the additional sampling in June of 1989 and 1990 . In 1989, age-0 yellow perch densities were about 10 times higher and the CVs of replicate samples lower (64-108%) than in 1990 (107156% ; Table 5) . There was no consistent pattern between CVs and depth strata . Variability between replicates at the deeper strata was higher in 1989 and lower in 1990 than at the shallower strata . Similar CVs were found for replicate purse-seine catches of coregonine larvae in Lake Opeongo, Ontario (Evans and Johannes 1988). The variability between replicate purse-seine

samples in Lake Mendota is typical of that found for larval and juvenile fishes in general by Cyr et al . (1992) . The mean versus variance relationship for our data indicates that age-0 yellow perch were spatially aggregated (Table 5) and had the same slope (t = -0 .0734, df = 707, P > 0.05) but a larger intercept (t = 2.3432, df = 708, P < 0.05) than the general relationship developed by Cyr et al . (1992) for eggs and for larvae, older age-0, and age-1 fishes . The higher intercept for age-0 yellow perch in Lake Mendota suggests that they were more highly aggregated than young fish in general. This may simply be an age effect because the general relationship shown by Cyr et al . (1992) included older fish that were typically less aggregated. Conclusions

Many common gears progressively undersample age-0 fish as the fish grow (Tranter and Smith 1968 ; Noble 1970 ; Murphy and Clutter 1972 ; Snyder 1983). In contrast, purse seines, although probably not without size bias, capture substantially larger age-0 fish than conventional sampling gears (Coles 1981 ; Evans and Johannes 1988 ; McQueen et al . 1989 ; Post and Evans 1989 ; Post and McQueen 1994, in press) . For example, in Lake St . George, Ontario, a purse seine was successfully used to capture 60-90-mm TL pelagic age-0 yellow perch and adult (100-180-mm TL) golden shiners Notemigonis crysoleucas (Johannes et al . 1989 ; McQueen et al . 1989 ; Post and Evans 1989). The largest age-0 yellow perch caught consistently with a 50-cm-diameter bridled tow net and a highspeed Miller sampler were 20 and 30 mm TL, respectively, even though we knew larger fish were present from purse-seine sampling and visual observation (J . R . Post, University of Calgary, and M . R. S . Johannes, Pacific Biological Station, unpublished data). Therefore, the temporal and spatial patterns and levels of aggregation that we describe for age-0 fish in Lake Mendota from purse-seine catches are increasingly more reliable as the age-0 fish grow through larval and juvenile stages than are those from studies that used conventional sampling gears . The seasonal succession of age-0 yellow perch, black crappie, freshwater drum, and Lepomis spp . in the pelagic zone of Lake Mendota was consistent among years, but relative abundance differed among years. Early in their development, age-0 fish were found at all pelagic stations across the lake, but as they grew larger, their relative abundance increased at the shallower pelagic sites. Al-

though there were differences between species and years, the age-0 fish typically resided in the pelagic zone for 3-7 weeks and then migrated to the littoral zone . The variability in catches of age-0 yellow perch among replicate purse-seine samples is typical of that found for a wide range of age-0 fishes, which indicates that they are highly aggregated, not randomly distributed . The occurrence of a pelagic phase in the early life history, which involves high densities of spatially aggregated fish, may significantly affect the pelagic trophic structure and community composition of freshwater lakes. Acknowledgments

We thank Chris Luecke, Terry Schenk, Jo Temte, Steve Anglea, Deborah Shosteck, and Lance Gardner for help in purse seining and Yvonne Allen for help in all aspects of this study. We also thank Jim Kitchell for the opportunity to be involved in the "Lake Mendota Project." Comments by Dennis DeVries, Mike Allen, Kevin Pope, Allen Keast, R. G. Werner, Perce Powles, and Don Stewart on an earlier draft are appreciated . This study was supported by the Federal Aid in Sport Fish Restoration Act under project F-95-P and the Wisconsin Department of Natural Resources, with additional support from a Natural Sciences and Engineering Research Council of Canada Postdoctoral Fellowship and a Guyer Postdoctoral Fellowship from the Department of Zoology, University of Wisconsin (JRP), and a postdoctoral fellowship from the Swedish Council for Forestry and Agricultural Research (LGR). References Amundrud, J. R., D. J. Faber, and A. Keast . 1974 . Seasonal succession of perciform larvae in Lake Opinicon, Ontario. Journal of the Fisheries Research Board of Canada 31 :1661-1665 . Auer, N. A., editor. 1982 . Identification of larval fishes of the Great Lakes Basin with emphasis on the Lake Michigan drainage . Great Lakes Fishery Commis sion Special Publication 82-3, Ann Arbor, Michigan. Becker, G . C . 1983 . Fishes of Wisconsin. University of Wisconsin Press, Madison. Clady, M. D. 1976 . Influence of temperature and wind on the survival of early stages of yellow perch, Perca flavescens. Journal of the Fisheries Research Board of Canada 33 :1887-1893 . Coles, T E 1981 . The distribution of perch, Perca fluviatilis, throughout their first year of life in Llyn Tegid, North Wales. Journal of Fish Biology 18 :1530 . Cyr, H., J . A. Downing, S. Lalonde, S . B . Baines, and

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