Random movement pattern of the sea urchin Strongylocentrotus droebachiensis Clément P. Dumont a, ⁎ , John H. Himmelman a , Shawn M.C. Robinson b a Département de Biologie and Québec Océan, Université Laval, Québec, Canada GIK7P4 b Applied Aquaculture Section, St. Andrews Biological Station, Department of Fisheries and Oceans, 531 Brandy Cove Road, St. Andrews, New Brunswick, Canada E5B 2L9 Received 20 January 2006; received in revised form 16 February 2006; accepted 21 August 2006 Abstract We describe the fine-scale movement of the sea urchin Strongylocentrotus droebachiensis based on analyses of video recordings of undisturbed individuals in the two habitats which mainly differed in food availability, urchin barrens and grazing front. Urchin activity decreased as urchin density increased. Individuals alternated between moving and being stationary and their behaviour did not appear to be affected by either current velocity (within the range from 0 to 15 cm s - 1 ) and temperature (2.3 to 6.0 °C). Movement of individuals at each location was compared to that predicted by a random walk model. Mean move length (linear distance between two stationary periods), turning angle and net squared displacement were calculated for each individual. The distribution of turning angles was uniform at each location and there was no evidence of a relationship between urchin density and either move length or urchin velocity. The random model predicted a higher dispersal rate at locations with low urchin densities, such as barrens habitats. However, the movement was sometimes greater or less than predicted by the model, suggesting the influence of local environmental factors. The deviation of individual paths from the model revealed that urchins can be stationary or adopt a local (displacement less than random), random or directional movement. The net daily distance displaced on the barrens, predicted by a random walk model, was similar to the observed movement recorded in our previous study of tagged urchins at one site, but less than that observed at a second site. We postulate that the random dispersal of urchins allows individuals on barrens to reach the kelp zone where food is more abundant although the time required to reach the kelp zone may be considerable (months to years). Urchins decrease their rate of dispersal once they reach the kelp zone so that they likely remain close to this abundant food sources for long periods. © 2006 Elsevier B.V. All rights reserved. Keywords: Grazing front; Movement pattern; Random walk; Strongylocentrotus droebachiensis; Urchin barrens; Video recording 1. Introduction Quantifying movement is fundamental to elucidating temporal and spatial changes in populations. Most animals move in search of food and shelter and to avoid predation and environmental stress (Swingland and Greenwood, 1982; Bell, 1991). Studying movement patterns at a fine-scale may provide information on foraging strategies and on the ability of individuals to exploit different habitats. Individual-based models are an attractive tool for characterizing small-scale movement patterns and for studying changes in behavioural decisions (movement rules) over time (Lima and Zollner, Journal of Experimental Marine Biology and Ecology 340 (2007) 80 – 89 www.elsevier.com/locate/jembe ⁎ Corresponding author. Present address: CEAZA, Departemento de Biologia Marina, Universidad Catolica del Norte, Larrondo 1281, Casilla 117, Coquimbo, Chile. E-mail address: cdumont@ucn.cl (C.P. Dumont). 0022-0981/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jembe.2006.08.013