Crustacean zooplankton available for larval walleyes in a Lake Michigan embayment

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Abstract

Adequate densities of zooplankton prey are critical for growth and survival of larvae of many fish species. Little information exists on the density of zooplankton in Great Lakes inshore areas during early spring, when larvae of important fishes rely on zooplankton. Reduced age-0 walleye recruitment and the absence of data on zooplankton availability for larval walleyes in northern Green Bay, Lake Michigan, led us to assess zooplankton densities during this critical spring period. We conducted biweekly vertical plankton tows in 2014–2016 near reefs and river plumes used by spawning walleyes for periods when larval walleyes were expected to be relying on zooplankton prey. Densities of zooplankton were well below literature values identified for good growth and survival of larval walleyes, averaging 1.5 individuals L−1 for all taxa and 0.12 individuals L−1 for large-bodied taxa across all sites and sampling dates. Various factors could contribute to the low density of zooplankton observed. We found low but significantly higher densities of cyclopoid copepods, nauplii, Bosmina, and total zooplankton at river mouth sites compared to open water sites. These results suggest that food availability for larval walleye in our study area was severely limiting which is consistent with the paucity of strong year classes observed since 2000. We suspect northern Green Bay has limited potential for producing strong year classes of walleyes under such conditions. Fishery managers working in unproductive waters should consider assessing the zooplankton community during critical periods to identify potential bottlenecks to reproductive success and larval fish survival.

Introduction

Adequate densities of zooplankton prey are critical for growth and survival of larvae of many fish species and ultimately influence adult abundance (Houde, 1987, Miller et al., 1988). In the Great Lakes region, studies of walleye Sander vitreus in Lake Erie (Roseman et al., 2005) and yellow perch Perca flavescens in Lake Michigan (Dettmers et al., 2003, Redman et al., 2011) linked strong year classes to lake conditions that provided larvae with dense zooplankton populations and relatively warm water. Hoyle et al. (2011) reported that an 89% decline in zooplankton prey for larval lake whitefish Coregonus clupeaformis between the 1991–1996 and 2003–2005 periods appeared to be causing decreased growth and survival of larval fish. Forage availability during early development of larval walleye has been identified as a key factor influencing initial year class strength and subsequent abundance of walleyes at older ages (Barton, 2011, Fielder, 1992, Spykerman, 1974). Despite its importance, little information (e.g., Hoyle et al., 2011) exists on densities of zooplankton in inshore areas during early spring, when larvae of walleye and other important Great Lakes fishes rely on zooplankton resources.

Walleye are one of the most popular sport fishes in North America (Schramm and Gerard, 2004) and a favorite target of anglers in Lake Michigan’s Green Bay (Dembkowski et al., 2018, Zorn and Schneeberger, 2011). Walleye stocks here are largely self-sustaining (Dembkowski et al., 2018, Zorn, 2015) but have declined in northern Green Bay (Big and Little Bay de Noc) during the last two decades based upon survey data and statistical catch-at-age model estimates (Zorn, unpublished data). The lack of strong recruitment in recent years suggests that walleye recruitment may be limited by the availability of zooplankton as prey for larval walleyes. However, no studies documenting zooplankton availability at this time of year occur for inshore areas of northern or southern Green Bay, and data for quantifying the foraging value of inshore Great Lakes waters for larval fishes are lacking (Beletsky et al., 2007).

Previous research has sought to quantify the density of forage needed for survival and growth of larval walleyes, but results have varied. Several studies have found that optimal growth and survival rates of walleye fry occur when the densities of large-bodied zooplankton, primarily copepods and cladocerans, exceed 100 individuals L−1 (Hoxmeier et al., 2004, Li and Mathias, 1982, Peterson et al., 2006). In contrast, Engel et al. (2000) observed successful walleye recruitment in Escanaba Lake, Wisconsin, during a year when spring Daphnia densities were less than 10 individuals L−1. Plankton assessments in Green Bay have focused on southern Green Bay and offshore habitats during summer (e.g., De Stasio et al., 2018, Gannon et al., 1982), and no data exist for inshore habitats of northern Green Bay in spring. Given the strong south-north productivity gradient (Auer et al., 1986) and sharply contrasting trends in walleye recruitment and harvest between northern and southern Green Bay (Environment and Climate Change Canada and the U.S. Environmental Protection Agency Under review), it seems likely that spatio-temporal patterns in zooplankton resources for southern Green Bay may not apply to northern Green Bay.

Self-sustaining populations of walleyes in the Great Lakes often consist of a combination of river-spawning and reef-spawning fish (e.g., Roseman et al., 2010). Water from tributaries is often warmer and more nutrient rich than the Great Lakes proper, so plumes associated with spawning rivers can provide localized “hotspots” for walleye recruitment through a combination of increased forage availability and refuge from larval fish predation (Reichert et al., 2010, Roseman et al., 2005). Northern Green Bay does host river- and reef-spawning populations of walleyes, but the extent to which plumes from its relatively small tributaries (having largely-forested watersheds) locally influence zooplankton density is unknown.

The lack of information about the current potential for northern Green Bay habitats to support natural reproduction of walleyes provided the impetus for this study. Our overall objective was to characterize spatial and temporal patterns of zooplankton assemblage structure and abundance at or near reef and river habitats used by spawning walleyes in northern Green Bay during the critical period for growth and survival of newly hatched walleye fry. Our specific objectives were to determine if densities of zooplankton available to larval walleyes in spring were: 1) low, using 10 individuals L−1 as a benchmark; and 2) greater in river plume habitats than in open water habitats. We relate our findings to existing studies on zooplankton requirements of larval walleyes and trends in walleye populations and fisheries in these waters.

Section snippets

Study area

Little and Big Bays de Noc are located in the northern portion of Green Bay in northwestern Lake Michigan, and provide contrasting environments for walleye populations. Little Bay de Noc (LBDN) is smaller at 16,100 ha compared to 37,711 ha for Big Bay de Noc (BBDN) (Schneeberger, 2000). An abrupt contour break along much of LBDN’s length produces distinct shallow (<3 m) and deeper (12–30 m) habitats. Except for its southeastern shoreline, BBDN is generally shallow (over half of its area is <9 m

Results

The overall density of zooplankton in the Bay de Noc system in spring was well below 10 individuals L−1 and notably low for large-bodied taxa. Across all sites and sampling dates, the mean total crustacean zooplankton density was 1.5 (SE = 0.15) individuals L−1, and the median density was 0.8 individuals L−1. Similarly, the mean density of large-bodied taxa (Daphnia and calanoid copepods) was 0.12 (SE = 0.01) individuals L−1, the median density was 0.05 individuals L−1, and the highest observed

Current zooplankton densities and potential causes

Although there was some spatial and temporal variability in crustacean zooplankton density, values observed near the walleye spawning reefs and river mouth habitats we studied were consistently low, particularly for large-bodied taxa. To put our results in perspective, Bremigan et al. (2003) reported total zooplankton densities during late April-June in southern Green Bay of approximately 75 to over 300 individuals L−1, of which approximately 10% were large-bodied taxa. In contrast, our total

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank the Michigan Department of Natural Resources, Michigan Department of Environmental Quality, and the Department of Fisheries and Wildlife at Michigan State University for allowing us the time to pursue this project. Assistance in field sampling and data entry was provided by K. Sanford. Comments by J. Makarewicz and several anonymous reviewers helped to improve this manuscript. This study was supported with funds from the Federal Aid in Sport Fish Restoration Act (Study 747, Project

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    Present address – Michigan Department of Environmental Quality, 525 West Allegan, Lansing, MI 48913, USA.

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