Describing historic Cisco population structure in Lake Michigan with scale samples from 1930-1974

Amanda Ackiss (USGS, aackiss@usgs.gov), Cory Brant (USGS)

The landscape of coregonine diversity in Lake Michigan has been vastly altered by more than a century of anthropogenic impacts. In addition to the extinction or extirpation of all but one deepwater cisco, shallow-water Cisco (Coregonus artedi) populations in Lake Michigan have seen dramatic declines in abundance (Smith 1964; Eshenroder et al. 2016). Two main embayments in Lake Michigan, Green Bay and Grand Traverse Bay, served as critical habitat for Cisco in Lake Michigan (Koelz 1929; Smith 1956; Kao et al. 2020). Green Bay was once the main source of Cisco productivity in Lake Michigan (Koelz 1929), and in Michigan waters alone yielded a peak of 5.4 million kg of Cisco in 1908 (Hile et al. 1953). By the 1950s, however, habitat degradation, heavy fishing pressure, and the introduction of invasive species led to the total collapse of Green Bay Cisco populations (Great Lakes Fishery Commission, 2022; Rook et al. 2024). The Grand Traverse Bay Cisco population underwent similar declines but shows evidence of recent resurgence in the past two decades (Claramunt et al. 2019). Today, a number of threats that led to the decline of Cisco in Lake Michigan have been mitigated or reduced (reviewed in Claramunt et al. 2019, Kao et al. 2020) and this has led to growing interest in reestablishing Lake Michigan Cisco as a major forage base for piscivores, including Lake Trout (Salvelinus namaycush). Of particular interest are restoration strategies that would support diverse prey fish assemblages to promote a diverse piscivore assemblage and ecosystem resiliency in the face of changing environments (Bunnell et al. 2023). A better understanding of the diversity and location of discrete, historic Cisco spawning stocks would provide valuable information to managers leading efforts to restore critical prey fish habitat and/or Cisco diversity in Lake Michigan. In early 2020, PI Ackiss genotyped scales from historic Lake Michigan Cisco collections with a genotyping-in-thousands by sequencing (GTseq; Campbell et al. 2015) panel for a project focused on historic species diversity. The majority of historic Lake Michigan Cisco samples that were genotyped for that project were from non-spawning periods. Koelz (1929) defined peak spawning from mid-November through December. To take advantage of less limited sampling into December, Kao et al. (2020) assumed that Lake Michigan Cisco were on the spawning grounds from Oct 16th – Nov 19th. Of nine scale collections already genotyped in 2020, only one bundle from the East Central Main Basin was collected during this period (Oct. 22, 1955), with another bundle from Green Bay collected just prior (Oct. 1, 1930). Population analysis indicates the presence of a large amount of genetic diversity not represented by contemporary spawning populations. One caveat, however, is that because the previously analyzed genotyped data are largely comprised of mixed stocks, the nature of this diversity remains vague. Here, we propose to sample and genotype spawning or near-spawning period scale bundles so that differentiation can be mapped to geographic location and used to inform the delineation of historic population structure and reproductive spawning habitat in Lake Michigan Cisco, which would be a critical step in the restoration planning stage should Lake Michigan managers move forward with Cisco reintroduction in areas beyond their current distribution.