Variation in larval sea lamprey demographics among Great Lakes tributaries: A mixed-effects model analysis of historical survey data

Understanding variation in fish populations is valuable from both a management and an ecological perspective. Great Lakes sea lampreys are controlled primarily by treating tributaries with lampricides that target the larval stage. Great Lakes streams were divided into four categories based on their regularity of parasitic lamprey production inferred from the historic regularity of chemical treatments. This categorization was intended to direct future assessment efforts, but may also reflect differences in early demographics. We analyzed assessment data collected from 1959 to 2005 using mixed-effects models and variance components analyses to test for differences in recruitment and growth to age 1 among stream categories. Recruitment was twice as large in regularly treated streams as in irregularly treated streams, indicating that age-1 year-class strength is correlated with consistent chemical treatments. We found no differences in length at age 1 among stream categories; however, Lake Superior streams with irregular treatment histories exhibit more variation in length at age 1 than streams that are treated regularly. The majority of variation in length at age 1 was due to within-year variation, which was fairly consistent across stream types within each lake. Our results indicate that early life history differs among subsets of the Great Lakes sea lamprey population, and management practices should be modified to account for these differences. Mixed-effects models and variance components analyses are useful tools for analyzing large historical datasets for patterns of demographic variation within and among populations, whether the ultimate goal is pest control, harvesting, or conservation.