Accounting for responsive movement in line transect estimates of abundance

A method is developed to account for effects of animal movement in response to sighting platforms in line transect density estimates using data on animal orientation. Models of expected distributions of animal orientation show that presence of responsive movement is determined by the ratio of animal sightings with angles of orientation in the third quadrant relative to the first quadrant. The distance at which response began is estimated using logistic generalized additive models of the relationship between radial distance and orientation. Density corrected for responsive movement is estimated by applying the Buckland and Turnock two-team analysis method to data poststratified into regions “close” to and “far” from (beyond the distance that responsive movement began) the observation platform instead of the original stratification by observation team. For data collected in the North Atlantic, white-sided dolphins, harbor porpoises, and minke whales responded by avoiding the survey ship, and white-beaked dolphins were attracted to the ship. For these populations, our method to correct for responsive movement gave significantly higher estimates, from 1.4 to 2.7 times the uncorrected estimates. Résumé : On trouvera ici une méthode qui tient compte des déplacements des animaux en réaction à la présence de plates-formes d’observation dans les estimations de densité par transects linéaires basés sur des données sur l’orientation des animaux. Les modèles des distributions attendues des orientations des animaux indiquent que la présence de mouvements de réaction peut être déterminée par le rapport du nombre d’animaux aperçus avec un angle d’orientation dans le troisième quadrant par rapport au nombre dans le premier quadrant. La distance à partir de laquelle la réaction se fait sentir a pu être estimée à l’aide de modèles logistiques additifs généralisés de la relation entre la distance radiale et l’orientation. La densité, avec correction pour tenir compte du mouvement de réaction, est estimée par l’application de la méthode d’analyse à deux équipes de Buckland et Turnock; les données ont été stratifiées a posteriori en «proches» et «éloignées» (au delà de la distance où se manifeste le mouvement de réaction) de la plateforme d’observation, au lieu de la stratification originale faite par les observateurs. Dans les cas de données récoltées dans l’Atlantique Nord, les Dauphins à flancs blancs, les Marsouins communs et les petits Rorquals réagissent en évitant le bateau d’observation, alors que les Dauphins à nez blanc sont attirés par le bateau. Chez ces populations, notre méthode de correction des mouvements de réaction donne des estimations significativement plus élevées, de 1,4 à 2,7 fois les estimations non corrigées. [Traduit par la Rédaction] 787 Palka and Hammond Introduction Line transect estimators of animal density assume, among other things, that animals do not move. When animals move randomly, estimates of density are biased upward by a factor that depends on the speed of the animals relative to the speed of the survey platform and on the animal detection rate (Koopman 1980; Hiby 1982). The bias is small unless the ratio of animal speed to survey platform speed is around 0.5 or greater (Buckland et al. 1993). A potentially more serious violation occurs if animals move in response to the survey platform. If animals respond prior to detection by moving either towards (attraction) or away from (avoidance) the transect line, density will be overor under-estimated, respectively. In recent years, line transect sampling has become a primary tool for estimating abundance of cetacean populations (Hiby and Hammond 1989; Buckland et al. 1993). Aircraft typically survey at speeds that are sufficiently high for animal movement not to be considered. However, cetaceans have a well-developed sense of hearing and are capable of responding to ships at distances far greater than those at which they may be detected by an observer on a ship. For example, Au and Perryman (1982) found that dolphin schools of three Stenella species started avoiding a survey ship while still 7 –13 km away, and Barlow (1988) recorded a harbor porpoise (Phocoena phocoena) that drastically changed its swim direction to avoid a survey ship at a distance of 800 m. Thus, movement in response to a survey ship is potentially a source of substantial bias in line transect estimates of cetacean abundance. There are a number of ways to investigate effects of responsive movement. Previously, tracks of animals resighted one or more times were examined (e.g., Borchers and Haw Can. J. Fish. Aquat. Sci. 58: 777–787 (2001) © 2001 NRC Canada 777 DOI: 10.1139/cjfas-58-4-777 Received January 6, 2000. Accepted January 23, 2001. Published on the NRC Research Press Web site on March 19, 2001. J15515 D.L. Palka.1 Northeast Fisheries Science Center, National Marine Fisheries Service, 166 Water Street, Woods Hole, MA 02543, U.S.A. P.S. Hammond. Sea Mammal Research Unit, Gatty Marine Laboratory, University of St Andrews, St Andrews, Fife KY16 8LB, U.K. 1Corresponding author (e-mail: [email protected]). J:\cjfas\cjfas58\cjfas-04\F01-024.vp Friday, March 16, 2001 10:07:16 AM Color profile: Generic CMYK printer profile Composite Default screen