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versão On-line ISSN 0718-1957
Rev. biol. mar. oceanogr. vol.46 no.3 Valparaíso dez. 2011
Revista de Biología Marina y Oceanografía
Vol. 46, Nº3: 463-469, diciembre de 2011
Records of Risso's dolphin, Grampus griseus, in coastal waters of southern Argentina
Registros del delfín gris, Grampus griseus, en aguas costeras del sur de Argentina
Luciana Riccialdelli1,2, Mónica A. Torres2, R. Natalie P. Goodall1,2, Natalia A. Dellabianca1,2, Lida E. Pimper2,3, Laura M. Reyes4, Alejandro Fernández-Ajó4 and Ricardo Bastida5
1Centro Austral de Investigaciones Científicas (CADIC), Bernardo Houssay 200, Ushuaia (9410), Tierra del Fuego, Argentina
2Museo Acatushún de Aves y Mamíferos Marinos Australes, Sarmiento 44, Ushuaia (9410), Tierra del Fuego, Argentina
3Departamento de Ecología, Genética y Evolución, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Intendente Güiraldes 2160, Ciudad Universitaria, Capital Federal (1427), Argentina
4Universidad Nacional de la Patagonia San Juan Bosco, Blvd. Brown 3050, Puerto Madryn (9120), Chubut, Argentina
5Universidad Nacional de Mar del Plata, Complejo Universitario, Peña y Funes, Mar del Plata (7600), Argentina
We review the records of published and unpublished sightings and strandings for Risso's dolphin (Grampus griseus) in subantarctic waters of the Southwestern South Atlantic Ocean. Based on 59 sighting (n = 521 individuals) and 33 stranding records (n = 88 individuals), we identified three main areas of Risso's dolphin concentration in Patagonian waters, which have been observed mostly during the austral summer. We were unable to find published or unpublished reports of Risso's dolphin in offshore or over deep-water areas in this region. Future studies covering other potential areas of the species' distribution are important to improve the scarce information known for this dolphin in this area.
Key words: Stranding, sightings, Southwestern South Atlantic Ocean
The subantarctic waters of the Southwestern South Atlantic Ocean are one of the most productive regions of the world (Campagna et al. 2006). These waters are found between two frontal systems, the Subtropical (~40ºS) and the Antarctic (~50-60ºS) Convergences, which provide unique conditions of high nutrient levels and primary production therefore constituting feeding areas for many species of marine mammals (Campagna et al. 2006, Bastida et al. 2007, Piola & Falabella 2009).
Risso's dolphin, Grampus griseus (Cuvier, 1812), is a small cetacean species which inhabits this area (Bastida et al. 2007), although in other parts of the world it is mostly found in temperate and tropical waters (Baird 2008). Due to its offshore habits, few sighting and stranding records have been documented along these coasts (Würsig & Würsig 1980, Goodall & Schiavini 19921, Reyes 2006, Goodall et al. 2008). While knowledge of other small cetacean and coastal species has increased in recent years (e.g., Commerson's dolphin, Cephalorynchus commersonii; dusky dolphin, Lagenorhynchus obscurus; and Franciscana dolphin, Pontoporia blainvillei; among others), very little is known about the distribution, movements, seasonal occurrence and feeding habits of Risso's dolphin in this region.
Stable isotope analyses suggest that in the Southwestern South Atlantic, Risso's dolphin is mainly an offshore predator (Riccialdelli et al. 2010), as it is in other parts of the world (Shane 1995a, b, Baird 2008, Bearzi et al. 2010). Nevertheless, its isotopic similarities to inshore species such as the Burmeister's porpoise, Phocoena spinipinnis (Riccialdelli et al. 2010), and several published coastal sighting records (Würsig & Würsig 1980, Reyes 2006), suggest that Risso's dolphin groups may move between inshore and offshore habitats, as noted near the Santa Catalina Islands (Shane 1995a, b) and in the NE Pacific Ocean (Leatherwood et al. 1980). According to bone collagen δ13C and δ15N values analyzed in Riccialdelli et al. (2010), the Risso's dolphin population in the Southwestern South Atlantic Ocean seems to prefer squid as a prey item, which has been recorded for other regions (Sekiguchi et al. 1992, Cockcroft et al. 1993, Blanco et al. 2006). The movements observed by Shane (1995a, b) near the Santa Catalina Islands in the NE Pacific were related to feeding activities, whereas the ones registered by Leatherwood et al. (1980) appeared to be related to surface temperatures. Therefore, distribution and seasonal movements of the species' main prey (squid) may influence its own distribution and movement patterns in Argentine waters.
The aim of this work is to update sighting and stranding information for Risso's dolphin of Patagonian Argentine waters to clarify the presence of this species in the Southwestern South Atlantic Ocean.
MATERIALS AND METHODS
The study region was divided into three areas: 1) Area I, north Patagonian waters (Península Valdés area, ~42ºS), 2) Area II, central Patagonian waters (Golfo San Jorge area, ~45ºS) and 3) Area III, southern Patagonian waters (southern Santa Cruz-Strait of Magellan-Tierra del Fuego area, ~50-54ºS) (Fig 1). We compiled and reviewed previously published records and interviewed colleagues for information on Risso's dolphin in Argentine waters as well as reviewing unpublished sighting and stranding events of the species in the study region (Table 1, Fig. 1). Most of the sightings were recorded opportunistically from the shore or small boats, and only sightings reported by Reyes (2006) were made by dedicated surveys at sea. Sighting reports were expressed as the total number of individuals recorded but also as a range of animals sighted. For the latter, we considered the midpoint of each group size estimated. When the midpoint was a non-integer, we used the lower integer.
Figure 1. Sighting and stranding records of Risso's dolphin (Grampus griseus) in the Southwestern Atlantic Ocean. Coastal areas visited by Risso's dolphin in Patagonian waters are: area I (Península Valdés area), area II (Golfo San Jorge area) and area III (southern Santa Cruz-Strait of Magellan-Tierra del Fuego). Strandings are shown as closed circles, sightings as open circles. Strandings at Bahía San Sebastián (and those from near this area), Tierra del Fuego, and sightings at Península Valdés, Chubut, are grouped in a square. The number of individuals involved in each record are shown
Figura 1. Registros de avistajes y varamientos de delfín gris (Grampus griseus) en el Océano Atlántico Sudoccidental. Áreas costeras visitadas por el delfín gris en aguas Patagónicas: área I (área Península Valdés), área II (área Golfo San Jorge) y área III (área sur de Santa Cruz-Estrecho de Magallanes-Tierra del Fuego). Los varamientos se muestran en círculos cerrados, los avistajes en círculos abiertos. Los varamientos en Bahía San Sebastián (y aquellos de áreas cercanas), Tierra del Fuego, y los avistajes en Península Valdés, Chubut están agrupados en un cuadrado. El número corresponde a los individuos involucrados en cada registro
A few stranding records were opportunistic, but most strandings were found during periodic beach surveys by the R. Natalie P. Goodall team from 1974/75 to the present, which primarily took place during austral spring-summer months (October to April). We considered each sighting and stranding event as a record.
RESULTS AND DISCUSSION
Fifty-nine sighting and 33 stranding records were available (Table 1, Fig. 1). We found 29 sighting and 16 stranding records which were previously published, and 30 new sighting records and 17 unpublished stranding records.
Table 1. Sighting and stranding records of Risso's dolphin (Grampus griseus) in the Southwestern Atlantic Ocean. RNP Goodall = R. Natalie P. Goodall Collection, Museo Acatushún de Aves y Mamíferos Marinos Australes, Tierra del Fuego, Argentina; IPPA = Instituto de la Patagonia, Punta Arenas, Chile; MRMB = Museo Regional 'Mayorino Borgatello' de la Congregación Salesiana, Punta Arenas, Chile
Tabla 1. Registros de avistajes y varamientos de delfín gris (Grampus griseus) en el Océano Atlántico Sudoccidental. RNP Goodall = Colección R. Natalie P. Goodall, Museo Acatushún de Aves y Mamíferos Marinos Australes, Tierra del Fuego, Argentina; IPPA = Instituto de la Patagonia, Punta Arenas, Chile; MRMB = Museo Regional 'Mayorino Borgatello' de la Congregación Salesiana, Punta Arenas, Chile
Sightings were made between the years 1962 and 2006. Of the total definite sightings (n = 57, excluding two possible sightings of Risso's dolphin), 67% corresponded to area I, 24% and 9% corresponded to areas II and III, respectively. The number of animals sighted ranged from one to an estimated 100 individuals, with a mean of 9.1 (± 16.2 SD), a mode of 1 and a median group size of 4. The mean group size of sighted animals is lower than average for the species, ~30 animals, which can increase to tens or even hundreds in response to abundant food resources (Baird 2008, Bearzi et al. 2010). Most sightings (85.2%) occurred between October and April (n = 46 records, 377 individuals sighted), with 25.9% occurring in November, 14.8% in January and 24.1% in February. Only 14.8% of the sightings were in winter months (n = 8 records, 25 individuals sighted) between May and September. Although Risso's dolphin occurs mainly in tropical and temperate waters, we found no sighting records for north of Península Valdés in Argentina, despite numerous marine mammal surveys in the area (Bastida & Lichtschein 1984, 19862, Würsig & Bastida 1986, Bastida et al. 19923). We believe this could be related to the lack of their main prey (squid) in these waters or it may indicate that their movements at northern latitudes are mainly in offshore waters.
Due to the lack of dedicated offshore studies we were unable to find any reports of sightings in offshore and deep waters and in the slope edge area of Argentina (Bastida & Lichtschein 1984, Lovrich 20104, Van Waerebeek et al. 2010), as is the case for other regions such as Chile, the Azores Archipelago, Gulf of Mexico and the Mediterranean Sea (Baumgartner 1997, Olavarría et al. 2001, Cañadas et al. 2002, Pereira 2008, Bearzi et al. 2010). The lack of records for this species around the Falkland (Malvinas) Islands might be due to the insufficient data available for most species of marine mammal in the area (Otley et al. 2008).
Stranding events were recorded between the years 1960 and 2003. Of the total strandings, 3% corresponded to area I, 9% and 88% corresponded to areas II and III, respectively. The coasts of Area III, where most stranding records (29 records including 84 individuals) of Risso's dolphins occurred, are characterized by high tides, reaching over 10m (e.g., Bahía San Sebastián, 53º18'S-68º16'W) and wide, shallow intertidal zones that provide ideal areas for stranding events (Goodall 1978). In northeastern Tierra del Fuego at least 75 animals were stranded from 1980 to 1999 (Goodall 1989, Goodall et al. 2008, Goodall unpublished data). Some of these events involved a high number of animals being stranded (15 individuals, Table 1). Mass stranding of this species is considered rare (Baird 2008), but there are a few records of mass stranding events in other regions, such as the west coast of South America (Olavarría et al. 2001, García-Godos & Cardich 2010).
The information about Risso's dolphin presented in this study advances our understanding of the species for this region. However, there are two gaps (areas with no sighting or stranding records) in its occurrence between the three coastal areas studied: one of ~300 km between Area I and II, and another of ~350 km between Areas II and III. These gaps may represent the actual absence of the species, or more likely, a limited survey effort carried out in those areas. Off Península Valdés, diving activities and whale watching surveys have taken place earlier than in any other areas (since 1957 and 1980, respectively). For these reasons, cetacean records have always been more numerous than for other areas. In Area II, few research programs have been carried out. In Area III, beach surveys began in 1974-75 for stranded or incidentally caught marine mammals, but sightings of live animals were on an opportunistic basis. The isolation of this area, due to long distances from nearby locations, few roads that are in poor condition, and extreme climatic conditions, have resulted in a lack of an organized observation program; nevertheless, there are a large number of stranding records.
The high number of sightings in austral summer months and the low number in winter along the Patagonian coast probably indicates a lack of research effort during winter or that during winter months, Risso's dolphins are in offshore areas or migrate to warmer ocean waters. Also, bad weather conditions and fewer observers may be largely responsible for the lack of sightings in austral fall and winter months. For Area I, sighting records in winter months could indicate the presence of late summer groups (records at the beginning of winter), early summer groups (records at the end of winter months) or the presence of resident groups living in northern Patagonian waters (sightings in mid-winter). The absence of records north of Peninsula Valdés suggests that this species could come in from offshore areas, probably at the southern part of the Province of Buenos Aires. The presence of this species so far south (~52º-55ºS), based primarily on stranding records from Area III, could be evidence of short term and seasonal movements of its principal prey. It is also possible that there are long-term fluctuations in the boundaries of its range in response to long-term environmental changes, as suggested by Leatherwood et al. (1980) for Risso's dolphins in the NE Pacific.
Future studies in other potential areas of the Risso's dolphin's distribution, such as the outer continental shelf and slope break, are important to increase the scarce information available on this species in this southern part of the world.
We gratefully thank G Harris, A Arias, GA Colombo, R Payne, ACM Schiavini, LG Benegas and A Golán for the important unpublished data shared with us. We also thank the collaboration of C Olavarría, M Iñiguez, A Lichter, J Gibbons, C de Haro, E Vermeulen, A Cammareri, J Acevedo, S Dans, Ch Torlaschi, M Píngaro, M Irurueta and JP Seco Pon in responding to our requests for data. C Olavarría provided helpful comments on a previous version of this paper. AC Jakle helped with the English version. This study was supported by Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) and the Committee for Research and Exploration (CRE) of the National Geographic Society to RNPG. LMR acknowledges the Rufford Small Grants Foundation and its continued support.
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Received 30 March 2011 and accepted 15 September 2011