Journal of Experimental Marine Biology and Ecology 545 (2021) 151629 0022-0981/© 2021 Published by Elsevier B.V. Prey composition and nutritional strategies in two sympatric pinnipeds Pablo Denuncio a, b, * , Joaquín C.M. Gana a , Gisela V. Giardino a , Diego H. Rodríguez a , Gabriel E. Machovsky-Capuska c, d a Instituto de Investigaciones Marinas y Costeras (IIMyC), Facultad de Ciencias Exactas y Naturales (FCEyN), Universidad Nacional de Mar del Plata (UNMdP), Consejo Nacional de Investigaciones Científcas y Tecnol´ ogicas (CONICET), Funes 3350. CC1260. 7600 Mar del Plata. Argentina b Asociaci´ on de Naturalistas Geselinos, Villa Gesell, Argentina c The Charles Perkins Centre, The University of Sydney, Sydney, NSW 2006, Australia d Cetacean Ecology Research Group, Massey University, Albany, AKL 0745, New Zealand A R T I C L E INFO Keywords: Marine predators Otaria favescens Arctocephalus australis Multidimensional nutritional niche framework Nutritional ecology ABSTRACT Niche segregation has been recognized as a valuable mechanism for sympatric species to reduce interspecifc competition and facilitate coexistence. The differential use of habitats is one of the behavioural mechanisms that may shape coexistence among marine predators. In this study, we provide a dietary and nutritional assessment of two pinnipeds, the South American sea lion (SASL) and the South American fur seal (SAFS) and explore their sympatric coexistence within the Warm Temperate Southwestern Atlantic biogeographic province (WTSA province). Pelagic prey species within the WTSA province showed signifcantly higher proportional composition of lipids than demersal counterparts, evidencing a nutritional variability in a vertical dimension accessible to marine predators. By modelling the dietary niches of these pinnipeds through a nutritional lens, we showed high overlapping prey composition niche breadths suggesting that both species consumed prey with similar nutri- tional composition; however, distinct realized nutritional niches showed that diets are likely shaped by differ- ences in foraging behaviours. The SAFS combined pelagic and demersal prey, whereas SASL mostly preyed upon demersal species. This paper provides crucial information on how nutritional variability in the water column likely drives the feeding strategies of both pinnipeds in the WTSA province. Given that this variation can in- fuence the stability of the contrasting population trends shown by these two pinnipeds, nutritional dynamics must be taken into consideration when defning conservation strategies. 1. Introduction Naturalists, evolutionary biologists, and ecologists have long been fascinated on understanding how species within the same community exploit their resources (Schoener, 1974). By linking individuals with their habitats, foods and time, the niche concept reconceptualizes ecological interactions providing a precise framework for the descrip- tion of resource partitioning (Pianka, 1974). Thus, niche segregation has been recognized as a valuable mechanism for sympatric species to reduce interspecifc competition and facilitate coexistence, particularly in carnivores (Costa-Pereira et al., 2019; Hearn et al., 2018; Rosenzweig, 1966). The displacement of morphological characters has been suggested to enable the sympatric coexistence of terrestrial (Grant, 1975) and marine predators (Drago et al., 2017). The literature provides extensive evidence on the behavioural mechanisms that may shape coexistence among marine predators, with emphasis in intraspecifc competition (Drago et al., 2015), the differential use of habitats (Jones et al., 2015), prey consumption (Loizaga de Castro et al., 2017), and temporal-spatial segregation (Fossette et al., 2017; Thiebot et al., 2012). The Warm Temperate Southwestern Atlantic biogeographic province (hereafter WTSA province, sensu Spalding et al., 2007) is infuenced by the encounter of the Brazil Current (southern-fowing, oligotrophic, warm and high salinity tropical waters) with the Malvinas/Falkland Current (northward-fowing, nutrient-rich, cold and low salinity sub- Antarctic waters) and the Río de la Plata (the second fuvial basin of South America) that outfows a high volume of freshwater into coastal marine systems (Fig. 1A). The convergence of these different water- masses creates a highly dynamic and productive environment that dominates the adjacent areas of the northern coast of Argentina, * Corresponding author at: Instituto de Investigaciones Marinas y Costeras, Departamento de Ciencias Marinas, Facultad de Ciencias Exactas y Naturales, Uni- versidad Nacional de Mar del Plata, CONICET, Funes 3350, Mar del Plata B7602AYL, Argentina. E-mail address: pdenunci@mdp.edu.ar (P. Denuncio). Contents lists available at ScienceDirect Journal of Experimental Marine Biology and Ecology journal homepage: www.elsevier.com/locate/jembe https://doi.org/10.1016/j.jembe.2021.151629 Received 11 May 2021; Received in revised form 17 August 2021; Accepted 9 September 2021