Leones Marinos

Molecular Ecology (2009) 18, 1088–1099

doi: 10.1111/j.1365-294X.2009.04093.x

Isolation by distance among California sea lion populations in Mexico: redefining management stocks
Blackwell Publishing Ltd

M. GONZÁLEZ-SUÁREZ,*‡ R. FLATZ,* D. AURIOLES-GAMBOA,† P. W. HEDRICK* and L. R. GERBER* *School of Life Sciences, Arizona State University, College & University Drive, Tempe, AZ 85287,USA, †CICIMAR-IPN, Avd IPN s/n Col. Playa Palo de Santa Rita, 23096 La Paz Baja California Sur, Mexico

Abstract
Understanding the spatial structure of a population is critical for effective assessment and management. However, direct observation of spatial dynamics is generally difficult, particularly for marine mammals. California sea lions (Zalophus californianus) are polygynous pinnipedsdistributed along the Pacific coast of North America. The species’ range has been subdivided into three management stocks based on differences in mitochondrial DNA, but to date no studies have considered nuclear genetic variation, and thus we lack a comprehensive understanding of gene flow patterns among sea lion colonies. In light of recent population declines in the Gulf of California, Mexico, it isimportant to understand spatial structure to determine if declining sea lion colonies are genetically isolated from others. To define population subdivision and identify sex biases in gene flow, we analysed a 355-bp sequence of the mitochondrial DNA control region and 10 polymorphic microsatellite loci from 355 tissue samples collected from six colonies distributed along Mexican waters. Using anovel approach to estimate sex biases in gene flow, we found that male sea lions disperse on average 6.75 times more frequently than females. Analyses of population subdivision strongly suggest a pattern of isolation by distance among colonies and challenge current stock definitions. Based on these results, we propose an alternative classification that identifies three Mexican management units: UpperGulf of California, Southern Baja Peninsula, and Upper Pacific Coast of Baja. This revised classification should be considered in future assessment and management of California sea lion populations in Mexican waters.
Keywords: Gulf of California, microsatellites, mitochondrial DNA, pinniped, population structure, sex-biased dispersal, Zalophus californianus Received 31 August 2008; revisionreceived 25 November 2008; accepted 8 December 2008

Introduction
Effective assessment and management of a population requires understanding spatial structure (i.e. how individuals are distributed and disperse over geographical space). Assumptions about spatial structure can significantly affect estimates of extinction risk (Gonzalez-Suarez et al. 2006; González-Suárez & Gerber 2008), andmanagement units are often defined based on the spatial structure of

Correspondence: M. González-Suárez, E-mail: manuela.gonzalez@snv.jussieu.fr ‡Present address: CNRS-UMR 7625, Fonctionnement et évolution des systèmes écologiques, Université Pierre et Marie Curie, Case 237, Bâtiment A, 7 Quai St Bernard, 75005 Paris, France.

populations (Dizon et al. 1992; Fraser & Bernatchez 2001). Unfortunately,in many cases spatial structure may be difficult to infer from observable dynamics. For example, many pinniped species (seals and sea lions) use seemingly continuous and extensive foraging habitats (oceans) and have the capacity to travel over large distances (Mate 1975; Campagna et al. 2001). However, pinnipeds also aggregate and mate in highly distinct terrestrial colonies (Riedman 1990) andoften exhibit high levels of philopatry (Pomeroy et al. 2000). Therefore, one could predict little spatial subdivision across the continuous foraging habitat or expect strong clustering around the distinct breeding colonies. In these cases, direct observation of dispersal events could help identify true spatial structure. However, direct observation of dispersal is generally difficult (e.g. Baguette...