Supporting evaluation components of the Lake Huron Technical Committee’s Cisco reintroduction study for FY24

Jose Bonilla-Gomez (USFWS, jose_bonillagomez@fws.gov), Amanda Ackiss (USGS), Andrew Honsey (USGS), Todd Hayden (USGS), Dave Fielder (MIDNR), Darryl Hondorp (USGS), Jason Smith (Sault Ste Marie Tribe of Chippewa Indians), Timothy O'Brien (USGS)

In 2024, the U.S. Fish and Wildlife Service conducted larval coregonine surveys to document the distribution, composition, and density of the larval coregonid community in Saginaw Bay. Sampling of the ichthyoplankton community included pelagic neuston net tows and beach seining. Pelagic surveys were conducted weekly from March 13 to May 6, 2024, at 40 randomly selected, spatially balanced, depth-stratified sites across the bay (Fig. 1). During each sampling event, larvae were collected within 1 m of the surface using a 500-µm, 2 × 1 m neuston net. Larval coregonines were detected in all depth strata greater than 3 m throughout the bay during the six-week sampling period. Observed densities from individual neuston tows ranged from 0 to 0.12 individuals/m³, with total lengths ranging from 4.67 to 20.77 mm. When pooled across weeks and depth bins, larval densities were consistently low, suggesting 2024 was a poor year for coregonid production (Fig. 2). Genetic methods were used to identify larval samples, Filtered GT-seq genotypes were compared to genotypes from reference spawning collections of populations in the Les Cheneaux Islands (hatchery stock source population, n=22) and reference of other Lake Huron different populations. Notably, monitoring efforts detected evidence of natural reproduction for the first time: among more than 249 larvae collected from 240 pelagic sites and six beaches, two individuals (>84% assignment probability) were identified as originating from the Les Cheneaux Islands Cisco strain (Fig. 3). In August 2024, the Michigan Department of Natural Resources conducted a Lake Huron acoustic survey (hydroacoustic transects and midwater trawling) to assess post-stocking survival of Cisco, but no individuals were captured. Post-stocking evaluations conducted from 2019 through 2024 have resulted in the capture of 67 adult Cisco (Table 1; mean length = 348.2 ± 51.7 mm; mean weight = 481.3 ± 181.2 g), including 65 of hatchery origin and two of wild origin. Mean catch per unit effort (CPUE) varied by survey type and year (Fig. 4). During juvenile surveys, CPUE remained low across all years, ranging from 0 fish/150 m in 2023 to 0.00394 ± 0.00175 in 2022. In contrast, spawning surveys showed higher CPUE values, peaking in 2022 (0.00665 ± 0.00329), followed by 2021 (0.00197 ± 0.00140) and 2023 (0.00150 ± 0.00080). When juvenile and spawning survey data were combined, the highest overall CPUE occurred in 2022 (0.00596 ± 0.00248), with notably lower values in 2021 and 2023. Among recaptured hatchery fish, 86.2% originated from spring releases and 13.8% from fall releases, while wild fish accounted for 3.0% of total captures (Table 2). Oxytetracycline (OTC) marks and genetic analyses confirmed species identity and release cohorts (Table 3, Fig. 5). These genetic results show that hatchery-reared Cisco stocked in Saginaw Bay return during the spawning season, and we also detected the presence of wild individuals (Fig. 6). To further evaluate spawning activity, a USGS-led complementary study assessed the spatial extent and timing of coregonine spawning in Saginaw Bay. Metal-ring egg traps (Weidel et al. 2023) were deployed at 24 locations and retrieved weekly or biweekly from November 6 to December 18, 2024 (Fig. 7). Coregonine eggs (N = 421) were collected at 20 of 24 stations (83%), with peak deposition occurring between November 20 and December 11. These results demonstrate that egg traps are an effective method for indexing spawning activity of Cisco. We collected 4 non-whitefish coregonine eggs between December 2 - 12, and the genetics results showed 3 genotyped as Bruce Peninsula stock Cisco with the GTseq panel and 1 did not genotype well (it had low amounts of DNA to work with comparatively after the first pass). Overall, the 2024 findings support ongoing restoration efforts and indicate that natural reproduction is possible. However, continued monitoring of larval production, juvenile survival, and adult spawning is essential to evaluate restoration success and guide management decisions. A recently published stochastic population model suggests that Cisco reintroduction in Saginaw Bay is feasible, with a 75-95% probability of success if stocking continues for at least 10 years (Fielder and McDonnell, 2024). Collectively, these studies document that stocked Cisco can survive to maturity and return to historical spawning areas, though it remains uncertain whether they can produce sufficient viable recruits to sustain the population.