Abstract.-A thorough understanding of the early life history of lake sturgeon Acipenser fulvescens is critical for rehabilitation of this species. Recruitment of lake sturgeon is known to be variable, but the extent of that variation and mortality rates experienced by early life stages are unclear. The objective of this study was to quantify early life stage mortality and explore the variability in year-class strength by estimating total egg deposition and abundance of larval and age-0 juvenile lake sturgeon from the 2006 and 2007 year-classes in the Peshtigo River, Wisconsin. Egg mats, drift nets, and visual surveys were used to collect lake sturgeon eggs, larvae, and age-0 juveniles, respectively. Total egg deposition, larval abundance, and age-0 juvenile abundance were higher in 2007 than in 2006. The magnitude of difference ranged from 2 times for eggs to 11 times for age-0 juveniles. The rate of mortality from the larval stage to the age-0 juvenile stage was higher in 2006 (98.26%) than in 2007 (90.46%); overall mortality from the egg stage to the age-0 juvenile stage was also higher in 2006 (99.98%) than 2007 (99.93%). These results suggest that mortality rates for these life stages of lake sturgeon are high, and large variation in early life stage abundance may be common. Management strategies to reduce these mortality rates may increase recruitment and aid population recovery.

Mortality of early life stages can limit recruitment of fishes, and understanding the impacts of various sources of mortality has long been a goal of fisheries management. The impacts of predation on lake sturgeon Acipenser fulvescens are not well understood. The objective of this study was to identify and quantify sources of predation that affect lake sturgeon eggs, larvae, and age‐0 juveniles in the Peshtigo River, Wisconsin, during 2006 and 2007. Egg bags were used to assess the rate of lake sturgeon egg consumption by crayfishes Orconectes spp. Potential piscine predators on eggs, larvae, or age‐0 juveniles were captured using fyke nets, gill nets, hoop nets, and electrofishing for analysis of stomach contents. Crayfish consumed lake sturgeon eggs at an average rate of 9.4 eggs/d, and the population of crayfish within the lake sturgeon spawning habitat consumed an estimated 300,000 eggs during the incubation period. Numerous fish species were observed consuming lake sturgeon eggs, and piscine predators likely consumed most eggs that settled on the surface of the substrate. Within 862 predator stomachs, only a single lake sturgeon larva was observed, and there was no evidence of predation on age‐0 juveniles. These results suggest that predation could limit recruitment at the egg stage, but it does not appear to be limiting to the larval and age‐0 juvenile life stages in the Peshtigo River.

Natural rates of straying are difficult to quantify over large spatial scales using direct observations, particularly for long‐lived fish species characterized by delayed sexual maturity and long interspawning intervals. Using multilocus microsatellite genotypes and likelihood‐based statistical methods, we quantified rates of immigration and emigration for six genetically differentiated (mean FST = 0.041) lake sturgeon Acipenser fulvescens populations in Lake Michigan based on adults (n = 437) captured in tributaries during the spawning season. Estimated rates of straying were high (mean = 0.105), asymmetrical, and highly variable across populations. We found no significant association between the total length (a surrogate measure of age) of individuals that strayed and those that did not. Linear distance between streams was more predictive of straying rates and FST than least‐cost distances estimated based on lakescape features (bathymetry and lake current patterns). Historical rates of gene flow estimated using coalescent analysis indicated a fully parameterized model with variable evolutionarily effective population sizes (θ range, 0.684–0.989), and variable and nonsymmetrical migration rates best explained the genetic data. Comparatively high estimates of relative historical gene flow from several numerically depressed populations suggest that these populations were once larger contributors to basinwide gene flow than indicated by estimates of contemporary straying rates. High rates of interpopulation straying contrast with high FST, suggesting that straying rates are poor indicators of successful reproduction following dispersal.

Lack of information about the rates and sources of population‐specific mortality and habitat use during nonspawning periods has impeded the restoration of lake sturgeon Acipenser fulvescens. Using eight microsatellite loci and mixed‐stock analyses, we estimated the proportional contributions of spawning populations from throughout the Lake Michigan basin (n = 5) to the fall sport fishery in the lower Menominee River, Wisconsin. We compared estimates of harvest composition with estimates from collections made in adjacent open‐water habitats in Green Bay. The analyses revealed that 81% (90% confidence interval [CI] = 72.9–89.3%; N = 104) of harvested individuals originated from the Menominee River; all of the bycaught fish originated in adjacent streams. The harvest composition estimates differed significantly from those of open waters immediately offshore (26.7% Menominee River; 90% CI = 9.7–44.8%; N = 36) and across Green Bay (27% Menominee River; 90% CI = 19.5–34.7%; N = 214), indicating that the harvest was not a random sample from across the basin. The harvest composition estimates were not consistent with the estimates of individuals in prespawning condition (females = 50%; males = 83%), suggesting that not all of the harvested fish were staging for spring spawning. The contributions of nontargeted and numerically depressed populations to the fishery are of management concern given efforts to rehabilitate populations. Spatially restricted harvests during nonbreeding periods may not protect numerically depressed populations originating in nearby streams.

Coaster brook trout are a migratory form of brook trout Salvelinus fontinalis that spend part of their lives in the Great Lakes. Over the last century the abundance of coaster brook trout in Lake Superior has declined dramatically, and only remnant stocks remain. Recently, the rehabilitation of coaster brook trout in Lake Superior has become a goal of fish management agencies. The specific goal agreed upon by all of the agencies involved is to maintain widely distributed, self‐sustaining populations in as many of the historical habitats as practical. We discuss realistic expectations for rehabilitation and emphasize the need for management agencies, academia, and angling organizations to work cooperatively. We first present a brief history of coaster brook trout in Lake Superior, then discuss habitat requirements and protection, the regulations required for rehabilitation, stocking, species interactions, and the role that human dimensions play in rehabilitation. The management issues that must be addressed are implementation of a basinwide survey to identify remnant stocks and critical habitat, restrictive harvest regulations, watershed rehabilitation, critical biological review, and the formulation of expectations before experimental stocking programs are initiated, along with coordinated, basinwide information sharing and cooperative management among agencies similar to that undertaken during the rehabilitation of lake trout Salvelinus namaycush in Lake Superior. Future research needs include basic coaster biology and life history, habitat use in streams and the lake, interaction with other species in the Lake Superior fish community, and interaction between stream‐resident and coaster brook trout. Successful rehabilitation will require a shift from a harvest fishery to one with minimal or no harvest of coaster brook trout in the Lake Superior basin. Coaster brook trout rehabilitation will take time and will proceed at different rates at different locations, depending on the presence of remnant stocks, quality of habitat, angling pressure, and political will.