Bruce M. Davis

Effects of Pulsed Turbidity and Vessel Traffic on Lake Herring Eggs and Larvae

Proposals to extend commercial shipping in the St. Marys River (connecting Lakes Superior and Huron) to include winter months have raised concerns regarding its effect on lake herring (Coregonus artedi). Because lake herring spawn in fall and their eggs overwinter in the river and hatch in spring, their hatching success could be impacted by early opening of the locks in spring. Our laboratory studies showed that under the range of turbidities expected in the river due to vessel traffic, lake herring eggs hatched and larvae fed adequately. Field incubation studies produced about 75% survival and 70% hatching success of lake herring eggs at two of three study sites. Collections in the river throughout the month following ice-out showed that sufficient plankton of appropriate size were available to ensure growth and survival of larval lake herring. We did not detect any negative impacts on the early life stages of lake herring as a result of sedimentation in the laboratory or field. However, detailing the spawning sites of lake herring and defining the normal survival-to-hatch in these areas are necessary before making accurate predictions of the effects of early season vessel traffic on lake herring hatching success.

Lake St. Clair Zooplankton: Evidence for Post-Dreissena Changes

ABSTRACT We surveyed the zooplankton of Lake St. Clair at 12 sites over ten dates from May to October 2000. Mean zooplankton density by site and date was 168.6 individuals/L, with Dreissena spp. veligers the most abundant taxon at 122.7 individuals/L. Rotifers, copepods, and cladocerans were far lower in mean abundance than in the early 1970s (rotifers, 20.9/L; copepods, 18.1/L; and cladocerans, 6.8/L). Species richness of zooplankton taxa in 2000 was 147, which was virtually unchanged from that of the first reported survey in 1894. Overall, the decline in abundance was greatest for rotifers (-90%) and about equal for cladocerans (-69%) and copepods (-66%). The decrease in abundance of Daphnia spp. was especially dramatic in Canadian waters. The decline in the southeastern region was significant for all three major groups of zooplankton, whereas in the northwestern region the decline was significant only for rotifers. From June to August 2000, Lake St. Clair open waters were numerically dominated by Dreissena spp. veligers, with a reduced abundance of rotifers and crustaceans compared to pre-Dreissena spp. surveys. Mean nutrient concentrations were not different from the 1970s, but Secchi depth (greater) and chlorophyll a concentration (lower) were. Disproportionate reduction in rotifer abundance is consistent with hypotheses implicating direct consumption by settled Dreissena spp. Reduction of crustaceans is likely due to more complex interactions including removal of nauplii as well as resource competition for phytoplankton.

Ichthyoplankton assemblages of coastal west-central Lake Erie and associated habitat characteristics

Early life stage survival often determines fish cohort strength and that survival is affected by habitat conditions. The structure and dynamics of ichthyoplankton assemblages can tell us much about biodiversity and fish population dynamics, but are poorly understood in nearshore areas of the Great Lakes, where most spawning and nursery habitats exist. Ichthyoplankton samples were collected with a neuston net in waters 2–13 m deep weekly or biweekly from mid-April through August, during 3 years (2000–2002) as part of a study of fish assemblages in west-central Lake Erie. A suite of abiotic variables was simultaneously measured to characterize habitat. Cluster and ordination analyses revealed several distinct ichthyoplankton assemblages that changed seasonally. A lake whitefish (Coregonus clupeaformis) dominated assemblage appeared first in April. In May, assemblages were dominated by several percid species. Summer assemblages were overwhelmingly dominated by emerald shiner (Notropis atherinoides), with large gizzard shad (Dorosoma cepedianum) and alewife (Alosa pseudoharengus) components. This seasonal trend in species assemblages was also associated with increasing temperature and water clarity. Water depth and drift processes may also play a role in structuring these assemblages. The most common and widely distributed assemblages were not associated with substratum type, which we characterized as either hard or soft. The timing of hatch and larval growth separated the major groups in time and may have adaptive significance for the members of each major assemblage. The quality and locations (with reference to lake circulation) of spawning and nursery grounds may determine larval success and affect year class strength.