**ABSTRACT NOT FOR CITATION WITHOUT AUTHOR PERMISSION. The title, authors, and abstract for this completion report are provided below. For a copy of the full completion report, please contact the author via e-mail at bindert@msu.edu or tr.binder@gmail.com. Questions? Contact the GLFC via email at frp@glfc.org or via telephone at 734-662-3209.**

EVALUATION OF LAKE TROUT HABITAT SELECTION AT DRUMMOND ISLAND SPAWNING REEFS

Thomas Binder¹, Steven Farha², John Janssen³, Stephen Riley², Mike Hansen⁴, Charles Bronte⁵, Charles Krueger⁶

¹Department of Fisheries and Wildlife, Michigan State University, Hammond Bay Biological Station,11188 Ray Rd., Millersburg MI 49759

²Great Lakes Science Center, USGS, 1451 Green Rd., Ann Arbor, MI 48105

³U. WI-Milwaukee, 600 E. Greenfield, Milwaukee, WI 53204

⁴Hammond Bay Biological Station, USGS, 11188 Ray Rd., Millersburg MI 49759

⁵U.S. Fish and Wildlife Service, 2661 Scott Tower Dr., New Franken, WI 54229

⁶Center for Systems Integration and Sustainability, Department of Fisheries and Wildlife, Michigan State University, 115 Manly Miles Building, 1405 S. Harrison Rd., East Lansing, MI 48823

December 2018

ABSTRACT:

Restoration of self-sustaining lake trout populations in the Great Lakes has been slow, possibly due in part to a poor ability of hatchery-reared fish to select appropriate spawning habitat where embryos can successfully incubate and hatch. The process of spawning habitat selection by lake trout is poorly understood, and researchers have often been unable to determine why some habitats are used for spawning while other seemly similar habitats are not. Progress toward a complete understanding of spawning habitat selection has likely been impeded by three research shortcomings; 1) selection of spawning habitats occurs at very small spatial scales, but spawning habitat studies have examined habitats at coarse scales, 2) spawning habitat studies have often been limited to habitats that fit preconceived notions about what constitutes suitable spawning habitat, and 3) observations of spawning in 'unconventional' habitats have largely been dismissed as aberrant behavior. We used positional acoustic telemetry to study spawning habitat selection on two known spawning reefs in the Drummond Island Refuge, Lake Huron; our use of telemetry allowed us not only to examine habitat use at fine-scale, but also to focus our sampling effort based on the behavior of the fish, rather than pre-conceived notions of what habitats were most likely to be used. Our specific research objectives were: 1) to determine if physical characteristics of habitat selected by lake trout for egg deposition and incubation differ from those at sites on the same reefs not chosen for spawning, and 2) to determine if egg incubation success is higher on sites chosen by lake trout for egg deposition than on sites on the same reefs not used for spawning. Habitat characteristics (i.e., substrate size, interstitial depth, slope magnitude, substrate homogeneity) were measured at 58 sites (20 m x 20 m) on the two reefs, all of which had been identified as areas of frequent lake trout activity by the acoustic telemetry data. Egg deposition was confirmed at 25 sites, but the physical characteristics of sites with eggs varied among reefs and were not reliable predictors of spawning site selection. Slope magnitude was a significant predictor on one reef but not the other, which had little slope, and all other measured habitat variables were not significantly correlated with egg deposition. Embryo incubation success was assessed at a subset of sites using an incubation bioassay with hatchery-source fertilized eggs seeded in plexiglass incubators. Incubation success was higher in our study than in other studies elsewhere in the Great Lakes that have used this bioassay but did not differ among sites selected for spawning and those that were not, which suggests that some sites not selected for spawning were suitable for embryo incubation. We observed spawning on a third 'reef' in gravel-rubble substrates at the base of very large boulders, a behavior that has not been reported before. We surveyed for eggs under 40 out of an estimated 269 boulders (based on high resolution bathymetry) scattered over an area of 0.63 km². Deposited eggs were observed in the undercut space beneath overhanging edges of all 40 boulders, sometimes in substrates with seemingly insufficient interstitial space for protecting eggs. Successful incubation of embryos in substrates associated with boulders was confirmed using specialized traps and a modified ROV-based electroshocker. However, free embryo densities were much lower than at sites surveyed at the two adjacent more typical spawning reefs. This result may have been due to a high rate of post-hatch movement away from less-protected habitat under boulders. Numerous free embryos were discovered and caught on top of several boulders. Our observations of spawning in a wide range of substrates suggests that stony substrate characteristics are not the most important variables determining spawning habitat selection in lake trout. We propose that future studies should investigate the role of lake currents, including the interaction between flow and substrate and hyporheic upwellings, in determining spawning habitat selection in this species.