A Spatially-Explicit Size-Structured Model for Identifying Areas of High Larval Sea Lamprey Density in a Large River

The St. Marys River is the largest contributor to the population of sea lampreys (Petromyzon marinus) in Lake Huron and upper Lake Michigan and presents a unique challenge to sea lamprey control due to its size.  A portion of the sea lamprey control program in the river focuses on controlling the larval life stage through the application of granular Bayluscide.  The ultimate goal of these treatments is to reduce sea lamprey mortality on lake trout and other fish species. The St. Marys River has been divided into 71 plots for the purposes of lamprey assessment and control.  Decisions about which plots to treat are currently based on only the most recent year’s data, although 18 years of plot-level data are available.  Sampling and treatments are conducted on a plot by plot basis, but resources are not available to adequately sample or treat all plots each year.  We developed a spatially-explicit size-structured modeling approach that estimated abundance in each plot over time in two size categories (Age 1: <47 mm, Age 2-6: > 47mm).  The model incorporated the full 18-year data set to predict areas of high larval lamprey abundance in the river.  Recruitment in each plot was based on adult spawner abundance and a Ricker stock-recruitment model.  Larval mortality was a function of natural mortality and treatment mortality.  Natural mortality was set at 58% based on previous studies and treatment mortality was estimated.  The probability of a larvae metamorphosing to the transformer life stage was assumed to be zero for ages 1-3, and 0.46, 0.57, and 1.0, for ages 4-6. The parameters of the Ricker stock-recruitment relationship and the proportion of recruits that settled in each plot were estimated within the model.  Larval abundance and transformer production decreased significantly through Bayluscide treatments and treatment mortality was estimated to be about 40%.  Ten of the 71 plots were received over 40% of the annual recruitment.  This work will help to refine sampling techniques and provide more accurate assessments of larval sea lamprey abundance leading to more effective and efficient control.