Rehabilitation progress can’t be assessed without a measuring stick: Development of a recruitment index survey for lake sturgeon in Lake Superior

Abstract

Lake sturgeon Acipenser fulvescens remain a species of conservation concern in Lake Superior with a rehabilitation goal of maintaining spawning populations that are self-sustaining throughout their native range. The Lake Sturgeon Index Survey was developed to address rehabilitation research needs, determine rehabilitation progress, and monitor lake-wide and tributary-specific recruitment. Sampling with standardized gill nets occurred off the mouth of 19 known lake sturgeon spawning tributaries during 2011, 2016, and 2021. A target of 0.5 geometric mean catch per unit effort (CPUE) for cohort ages 4–8 was used as a quantitative recruitment indicator to determine progress towards Lake Superior’s Fish Community Objectives. In 2021, CPUE of the age 4–8 cohort met the target at the Bad, Ontonagon, Batchawana, Goulais, Michipicoten, and Pic/White rivers (7 of 19 locations), but was below target at the St. Louis, Montreal, North and South Entry Sturgeon, Pigeon, Kaministiquia, Black Sturgeon/Wolf, Nipigon, Gravel, Prairie, and Tahquamenon rivers. The Index Survey sampling design meets assessment needs by concentrating sampling effort near tributary mouths and by assessing recruitment over the age 4–8 cohort. Power analysis indicated that detecting a 50% change in CPUE was not likely at individual tributaries except Goulais River, but could detect recruitment changes when aggregated lake-wide. The Index Survey allows fishery managers to quantitatively measure recruitment for individual populations to inform evaluation or modification of rehabilitation efforts in a timely manner and help determine where rehabilitation efforts and funding should be focused.