SOUTHEASTERN NATURALIST 2008 7(3):475–482 Size of Spawning Population, Residence Time, and Territory Shifts of Individuals in the Spawning Aggregation of a Riverine Catostomid Timothy B. Grabowski 1,2,* and J. Jeffrey Isely 3 Abstract - Little is known about the behavior of individual fish in a spawning aggregation, specifically how long an individual remains in an aggregation. We monitored Moxostoma robustum (Cope) (Robust Redhorse) in a Savannah River spawning aggregation during spring 2004 and 2005 to provide an estimate of the total number of adults and the number of males comprising the aggregation and to determine male residence time and movements within a spawning aggregation. Robust Redhorse were captured using prepostioned grid electrofishers, identified to sex, weighed, measured, and implanted with a passive integrated transponder. Spawning aggregation size was estimated using a multiple census mark-and-re- capture procedure. The spawning aggregation seemed to consist of approximately the same number of individuals (82–85) and males (50–56) during both years of this study. Individual males were present for a mean of 3.6 ± 0.24 days (± SE) during the 12-day spawning period. The mean distance between successive re- captures of individual males was 15.9 ± 1.29 m (± SE). We conclude that males establish spawning territories on a daily basis and are present within the spawning aggregation for at least 3–4 days. The relatively short duration of the aggregation may be the result of an extremely small population of adults. However, the behav- ior of individuals has the potential to influence population estimates made while fish are aggregated for spawning. Introduction Fishes have been hypothesized to form spawning aggregations for a number of reasons including spawning habitat limitations, fertilization enhancement, and swamping potential egg predators (Claydon 2005, Domeier and Colin 1997). Although this behavior is exhibited over a wide range of taxa, little is known about the length of time individuals remain a part of a spawning aggregation or how the positions or territo- ries of individuals change over that period of time. The majority of work on spawning aggregations has focused on coral reef and estuarine fishes. Serranids, such as Epinephelus guttatus (Linnaeus) (Red Hind; Colin et al. 1987, Whiteman et al. 2005) and E. striatus (Bloch) (Nassau Grouper; Smith 1972); lutjanids such as Lutjanus cyanopterus (Cuvier) (Cubera 1 Department of Biological Sciences, Clemson University, Clemson, SC 29634-0326. 2 Current address - Institute of Biology, University of Iceland, Sturlugata 7, Is-101 Reykjavik, Iceland . 3 US Geological Survey, South Carolina Cooperative Fish and Wildlife Research Unit, Clemson University, Clemson, SC 29634-0372. * Corre- sponding author - tbg@hi.is.