The diet of the South American sea lion (Otaria flavescens) at Río Negro, Patagonia, Argentina, during the winter-spring period

Abstract

ABSTRACT. The South American sea lion, Otaria flavescens (Shaw, 1800) population is steadily expanding along the Patagonian coast of Argentina in the last decades. However, little is known about the feeding ecology of the species in the area. The aim of this study was to analyze the food habits of O. flavescens from 91 scats collected at Río Negro province, during the winter and spring of 2005. Fish occurred in 96% of scats containing prey remains, followed by cephalopods (26%). Raneya brasiliensis (Kaup, 1856) was the most frequent and abundant species occurring in 58.6% of samples and constituting almost 50% of fish predated. Second in importance were Porichthys porosissimus (Cuvier, 1829) and Cynoscion guatucupa (Cuvier, 1830) in terms of occurrence (%FO 20.7) and numbers (29.6%) respectively. The squid Loligo gahi (d'Orbigny, 1835) was the most frequent cephalopod prey (42.1%), whereas Octopus tehuelchus (d'Orbigny, 1834) was the most abundant (77%). The higher amount and diversity of prey found in the spring in comparison with the winter season might be related to a higher feeding activity of seals or to a seasonal increase in food availability in the area. KEYWORDS.Trophic ecology, Pinnipedia, fishes, cephalopods.RESUMEN. Dieta del lobo marino de un pelo sudamericano (Otaria flavescens) en Río Negro, Patagonia, Argentina, durante el invierno y primavera. La población del lobo marino de un pelo sudamericano Otaria flavescens (Shaw, 1800) ha experimentado un crecimiento continuo en las ultimas décadas en las costas de la Patagonia Argentina. Sin embargo, poco se conoce sobre la ecología trófica de la especie en el área. El objetivo de este estudio fue analizar los hábitos alimentarios de O. flavescens a partir de 91 fecas colectadas en la provincia de Río Negro, durante el invierno y la primavera del 2005. Los peces estuvieron presentes en el 96% de las fecas que contenían remanentes presas, seguidos por los cefalópodos (26%). Raneya brasiliensis (Kaup, 1856) fue la especie mas frecuente y abundante ocurriendo en el 58.6% de las muestras y constituyendo casi el 50% de los peces predados. Le siguieron en importancia Porichthys porosissimus (Cuvier, 1829) y Cynoscion guatucupa (Cuvier, 1830) en términos de frecuencia de ocurrencia (%FO 20,7) y abundancia numérica (29,6%) respectivamente. El calamar Loligo gahi (d'Orbigny, 1835) fue el cefalópodo presa mas frecuente (42,1%), mientras que Octopus tehuelchus (d'Orbigny, 1834) fue el mas abundante (77%). La mayor cantidad y diversidad de presas observada en primavera en comparación con el periodo invernal podría estar relacionada a una mayor actividad de forrajeo de los lobos o a un incremento en la disponibilidad de presas en el área.

Diet of the South American sea lion Otaria flavescens during the summer season at Río Negro, Patagonia, Argentina

“…Nevertheless, the use of the same resource at different times or spatial locations and of different age classes does not necessarily imply that the interaction between sea lions and fisheries is less strong (Szteren et al 2004). Still, considering that the main prey item (R. brasiliensis) of O. flavescens is not targeted by commercial fisheries and that it only represents a small percentage of the discards, the overlap in the use of resources by the former and the latter may not be substantial, at least during the summer season.On the other hand, the present results show that Octopus tehuelchus was the most important cephalopod prey of South American sea lions in summer, which coincides with previous results reported by Bustos et al (2012) for the spring of 2005. This octopod species is an important coastal resource for artisanal fishing in the San Matías Gulf, with this activity taking place during the spring and summer seasons (Storero et al 2010).…”

“…2). This is in line with previous dietary studies that also reported fish to be the dominant prey items consumed by this otariid (Thompson et al 1998, Koen Alonso et al 2000, Suárez et al 2005, Romero et al 2011, Bustos et al 2012.In the present study, Raneya brasiliensis was clearly the dominant prey species throughout the sampling period, since it was the most frequent and abundant fish item and contributed 75% to the IRI (Table 1) On the other hand, some authors have suggested that hake Merluccius hubbsi make an important contribution to the diet of the South American sea lions from Patagonia, after analysis of the stomach contents of individuals found dead on the beach or recovered from incidental catches (Koen Alonso et al , Romero et al 2011). However, in other studies based on scat analysis, including the present one, hake was shown to be absent or poorly represented in the diet of Otaria flavescens (Table 2) (Suárez et al 2005, Bustos et al 2012).…”

“…However, a low number of exoskeletal fragments and appendages were recovered from fecal samples. The fact that the most important prey taxa consumed by South American sea lions (fish and octopods) are, at least partially, carcinophagous may indicate that crustaceans are secondary prey items (prey of prey), as previously proposed by Bustos et al (2012).In summary, the present study suggests that Otaria flavescens feed mainly on demersal species and secondarily on benthic species associated with the continental shelf and the shelf break. Based on satellite 241 Aquat Biol 20: [235][236][237][238][239][240][241][242][243] 2014 telemetry, it is assumed that, during summer, South American sea lions focus their foraging activity on the temperate waters of the Patagonian continental shelf (Campagna et al 2001).…”

“…Scat analysis is a powerful tool for studying mammalian diets and has been widely used for several pinniped species (Reid & Arnould 1996, Daneri et al 1999, Daneri et al 2005, Páez-Rosas & AuriolesGamboa 2010, Bustos et al 2012. One of the advantages of this method is that, in most cases, feces are easily collected in large numbers, with minor shortterm disturbance to the seal colonies (Kucey & Trites 2006).…”

“…A correction factor was applied to compensate for the erosion (10% for Group 1 and 20% for Group 2). The otoliths included in the third category were not taken into account for the estimation of fish size and biomass (Reid & Arnould 1996, Daneri et al 2008, Bustos et al 2012.Previously reported allometric equations relating the lower rostral length (LRL, mm) to dorsal mantle length (DML, mm) and wet mass (W, g) were used for squids (Pineda et al 1996, Koen Alonso et al 2000, Xavier & Cherel 2009, while those relating the lower hood length (LHL, mm) to mantle length (mm) and W were used for octopods (Pineda et al 1996).In addition, the index of relative importance (IRI) was estimated with an equation modified from Pinkas et al (1971) to include percentage wet weight instead of volume (Reid 1995). This index was calculated for prey species with a %FO > 5%, and it was expressed as a percentage (%IRI) to facilitate its interpretation (Cortés 1997).…”