240 J. Great Lakes Res. 29 (Supplement 1):240–252 Internat. Assoc. Great Lakes Res., 2003 Estimating Lake-wide Abundance of Spawning-phase Sea Lampreys (Petromyzon marinus) in the Great Lakes: Extrapolating from Sampled Streams Using Regression Models Katherine M. Mullett 1,* , John W. Heinrich 1 , Jean V. Adams 2 , Robert J. Young 3 , Mary P. Henson 1 , Rodney B. McDonald 3 , and Michael F. Fodale 1 1 U.S. Fish and Wildlife Service Marquette Biological Station 1924 Industrial Parkway Marquette, Michigan 49855-1699 2 U.S. Geological Survey Great Lakes Science Center 1451 Green Road Ann Arbor, Michigan 48105 3 Department of Fisheries and Oceans Canada Sea Lamprey Control Centre 1 Canal Drive Sault Ste. Marie, Ontario P6A 6W4 ABSTRACT. Lake-wide abundance of spawning-phase sea lampreys (Petromyzon marinus) can be used as one means to evaluate sea lamprey control efforts in the Great Lakes. Lake-wide abundance in each Great Lake was the sum of estimates for all streams thought to contribute substantial numbers of sea lampreys. A subset of these streams was sampled with traps and mark-recapture studies were con- ducted. When sea lampreys were captured in traps, but no mark-recapture study was conducted, abun- dance was estimated from a relation between trap catch and mark-recapture estimates observed in other years. In non-sampled streams, a regression model that used stream drainage area, geographic region, larval sea lamprey, production potential, the number of years since the last lampricide treatment, and spawning year was used to predict abundance of spawning-phase sea lampreys. The combination of esti- mates from sampled and non-sampled streams provided a 20-year time series of spawning-phase sea lam- prey abundance estimates in the Great Lakes. INDEX WORDS: Lake-wide abundance, sea lamprey, non-sampled streams, regression model. INTRODUCTION The abundance of parasitic-phase sea lampreys in the Laurentian Great Lakes is one indicator of whether the sea lamprey management program is achieving its objectives. Although the parasitic stage is the one that inflicts damage to fish, difficul- ties in sampling this stage led to the assessment of spawning-phase sea lampreys as a surrogate. Since 1980, traps have been the primary tool to monitor changes in spawning-phase sea lamprey populations * Corresponding author. E-mail: katherine_mullett@fws.gov (Schuldt and Heinrich 1982, Heinrich et al. 2003). Spawning-phase sea lampreys were monitored in a subset of streams each year to assess lake-wide abundance. To use this information required devel- opment of a method to extrapolate information col- lected from sampled streams to an entire lake. Stream discharge was considered to be an impor- tant factor influencing sea lamprey spawning runs in streams (Applegate 1950, Wigley 1959, Morman et al. 1980). Correlative tests carried out on 13 physical and chemical factors demonstrated a strong positive correlation (r = 0.77) between trap