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Diet of the South American Sea Lion Otaria Flavescens During the Summer Season at Río Negro, Patagonia, Argentina

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Diet of the South American Sea Lion Otaria Flavescens During the Summer Season at Río Negro, Patagonia, Argentina This authors' personal copy may not be publicly or systematically copied or distributed, or posted on the Open Web, except with written permission of the copyright holder(s). It may be distributed to interested individuals on request. Vol. 20: 235–243, 2014 AQUATIC BIOLOGY Published online May 7 doi: 10.3354/ab00557 Aquat Biol Diet of the South American sea lion Otaria flavescens during the summer season at Río Negro, Patagonia, Argentina Raimundo Lucas Bustos1,*, Gustavo Adolfo Daneri1, Alejandra Vanina Volpedo2, Ana Harrington1, Esperanza Amalia Varela1 1Museo Argentino de Ciencias Naturales ‘Bernardino Rivadavia’, Div. Mastozoología, Av. Angel Gallardo 470 (C1405DJR), Ciudad de Buenos Aires, Argentina 2Centro de estudios Transdisciplinarios del Agua-CONICET, Facultad de Ciencias Veterinarias, Universidad de Buenos Aires, Av. Chorroarín 280 (1428), Ciudad de Buenos Aires, Argentina ABSTRACT: The South American sea lion Otaria flavescens population from northern Patagonia was under intensive commercial harvesting pressure between 1930 and 1950 and is currently increasing at a rate of nearly 6% yr−1. Food availability in the oceans is one of the most important factors influencing the survival and dynamics of marine mammal populations. The objective of the present study was to determine the summer diet of O. flavescens by faecal analysis, in order to assess its trophic behavior, determine whether there are interannual differences in diet composi- tion, and evaluate the potential interaction between fisheries operating in the study area and the species. Present results show that O. flavescens preys mainly on fish species, with Raneya brasiliensis being the most frequent (frequency of occurrence, %FO = 65.7) and abundant (%N = 47.9) fish prey. Cephalopods were the second most important prey item in terms of %FO (42.6%), with the highest index of relative importance corresponding to the octopod Octopus tehuelches. Since these prey taxa are not targeted by the commercial fishing fleet that operates in the study area, the overlap in the use of resources by O. flavescens and fisheries may not be substantial. Given that the biomass of the prey species mentioned above is not directly affected by fishery activities during summer, a greater amount of food may be available for sea lions, with this factor possibly contributing to the population recovery in the area. KEY WORDS: Otaria flavescens · Summer diet · Scats · Patagonia · Argentina Resale or republication not permitted without written consent of the publisher INTRODUCTION ties, while the latter involves the direct competition of fishermen and pinnipeds for a particular prey (e.g. Food availability is one of the most important fac- Wickens 1995, Crespo et al. 1997). A potential cause tors influencing the survival and reproduction of of the decline in pinniped populations is the deple- marine mammal populations (Trillmich & Dellinger tion of their most important prey species, which re - 1991, Roux 1997). Many studies have focused on the sults from ecological interactions (Trites & Larkin interactions between pinnipeds and fisheries, which 1992, Merrick et al. 2009). are classified as either direct (operational) or indirect The South American sea lion Otaria flavescens is (ecological). The former involves abandonment of a distributed from southern Brazil to southern Argen - foraging area because of disturbance by a fishery or tina (southwestern Atlantic Ocean) and along the the disruption of foraging patterns by fishing activi- Chilean and Peruvian coasts (southeastern Pacific *Corresponding author: [email protected] © Inter-Research 2014 · www.int-res.com 236 Aquat Biol 20: 235–243, 2014 Ocean) (King 1983, Bastida et al. 2007). The popula- Negro province by means of scat analysis; (2) deter- tion inhabiting the Argentine coast dramatically de- mine whether interannual differences exist in diet copy clined between the 1930s and 1950s due to commer- composition, in order to better understand the tro - cial hunting, but since 1990 it has been increasing at pho dynamics of the species; and (3) investigate the a rate of nearly 6% yr−1 (Dans et al. 2004, Grandi et potential interaction between O. flavescens and fish- al. 2010). eries, which may be delaying the recovery of the sea Author The fisheries that operate in the San Matías Gulf, lion population in Argentina. Río Negro province, target demersal and pelagic- demersal species, with the Argentine hake Merluc- cius hubbsi being the main species caught through- MATERIALS AND METHODS out the year (Irusta et al. 2003). A trend exists to - wards major diversification in landings and Fresh faecal samples were collected from the Ota - in creased fishing activity (Romero et al. 2011). The ria flavescens rookery at Punta Bermeja (41° 09’ S, fact that the stock of Argentine hake in the region 63° 05’ W), which is located within a Natural Pro- was reported to have collapsed in the 1990s due to tected Area managed by the Secretaría de Medio over fishing (Aubone 2000), combined with the ex - Ambiente of Río Negro province (Fig. 1). Samples treme reduction of the sea lion population, led to a were collected in the summers of 2007, 2008 and reorganization of the ecosystem (Koen Alonso & Yod - 2009 (n = 44, 48 and 45, respectively), between the zis 2005). This is now characterized by larger popula- last week of February and first week of March. Dur- tions of the Magellanic penguin Spheniscus magel - ing these collection periods, the mean number of sea lan icus, Argen tine short-fin squid Illex argen ti nus lions ashore was 2650, with 70% of the colony being and Argen tine anchovy Engraulis anchoita (Koen represented by juveniles and females, 27% by new- Alonso & Yodzis 2005). Furthermore, predation on borns and 3% by adult and subadult males. hake de creased, while predation on alternative prey, A total of 137 samples of scat were collected and such as Squalus acanthias, increased (Koen Alonso et stored in plastic bags with 70% ethanol until further al. 2000). All foraging trips of the South American sea lions from Patagonia are on the continental shelf of up to 400 km in width, with males moving further from the coast than females (Campagna et al. 2001). In the austral summer, adult females give birth to 1 pup, copu late about 1 wk later and then alternate foraging trips with offspring provisioning (Campagna & Le Boeuf 1988). South American sea lions seem to be generalist and opportunistic predators, with fish being the main prey group, followed by cephalopods and crustaceans (Thompson et al. 1998, Koen Alonso et al. 2000, Bustos et al. 2012). However, studies on the diet of Otaria flavescens populations distributed along the Argentine coast are scarce, and most of them reported only preliminary results (Koen Alonso et al. 2000, Suárez et al. 2005, Romero et al. 2011, Bustos et al. 2012). These studies were based on sta- ble isotope techniques and the analysis of stomach contents and scats. The largest sea lion rookery in Patagonia is at Punta Bermeja, in the San Matías Gulf. The size of this per- manent colony is maximum in winter and minimum in summer, and >80% of the sea lions from the colony are non-breeding individuals (Dans et al. 2004). Based on the considerations mentioned above, the Fig. 1. Geographical location of the South American sea lion aims of the present study were to: (1) evaluate the Otaria flavescens rookery at Punta Bermeja in Río Negro diet of Otaria flavescens on the coasts of the Río province, Argentina Bustos et al.: Summer diet of the South American sea lion in Patagonia, Argentina 237 analysis in the laboratory. Hard prey remains were Xavier & Cherel 2009), while those relating the lower extracted using sieves of different mesh sizes (range: hood length (LHL, mm) to mantle length (mm) and W copy 2.5 to 0.5 mm) and rinsing water, and they were clas- were used for octopods (Pineda et al. 1996). sified into 3 categories: fish, cephalopods and crusta - In addition, the index of relative importance (IRI) ceans. Fish items were recognized from bones, oto- was estimated with an equation modified from liths, scales and eye lenses in the scat; cephalopod Pinkas et al. (1971) to include percentage wet weight Author items from beaks, eye lenses and pens; and crusta- instead of volume (Reid 1995). This index was calcu- cean items from exoskeletal remains. When possible, lated for prey species with a %FO >5%, and it was prey items were identified to the lowest taxonomic expressed as a percentage (%IRI) to facilitate its level. Fish and cephalopod species were identified interpretation (Cortés 1997). by comparing otoliths and beaks with available cata- A cumulative prey curve was constructed to deter- logs (Volpedo & Echeverría 2000, Xavier & Cherel mine whether the number of scats analyzed gave an 2009) and reference material. accurate description of summer diet composition. The frequency of occurrence (%FO) and numerical The curve was constructed using randomization pro- abundance (%N) were calculated for both fish and cedures (EstimateS v. 8) based on the occurrence of cephalopod items. Only scats containing identifiable primary prey. Sample-based curves were plotted by prey remains were used to calculate %FO. The mini- calculating the mean number of prey items identi- mum number of individuals in those remains was fied, followed by repeated re-sampling of scats with- determined based on the most commonly found fish out replacement and 1000 randomizations of scat otolith (left or right) and the number of cephalopod order. Sampling adequacy was evaluated by visual lower beaks. inspection of curve stabilization. The size and weight of the main prey species con- A nested ANOVA was used to detect interannual sumed by Otaria flavescens were estimated using differences in the size of prey items consumed by sea regression equations that relate fish otolith length to lions, with scats (random factor) nested in years.
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