First record of orca predation on franciscana dolphins (Pontoporia blainvillei) in Argentina

Antonella D. Padula1*, Joaquín C.M. Gana1, Gisela V. Giardino1 , M. Carolina De Leon1, Andrea Elissamburu1, Diego H. Rodriguez1 and Pablo Denuncio1,2 1Instituto de Investigaciones Marinas y Costeras (IIMyC), Facultad de Ciencias Exactas y Naturales (FCEyN), Universidad Nacional de Mar del Plata (UNMdP), Consejo Nacional de Investigaciones Científicas y Tecnológicas (CONICET), Funes 3350, CC1260, 7600 Mar del Plata, Argentina 2Asociación de Naturalistas Geselinos, Villa Gesell, Argentina


Ecología y Conservación de Mamíferos Marinos, unpub. data).
In the last few years, there were two orca mass strandings in northern Argentina, along the Buenos Aires coast. The first event occurred on 25 August 2018, where six females live stranded between Mar de Ajó (36°43′13″ S, 56°41′16″ W) and Mar del Plata (38°00' S, 57°33' W) (Fig. 1); three of them died, and the other three were released. Unfortunately, it was not possible to take samples of these individuals.
The second event took place on 16 September 2019, when seven orcas stranded alive in La Caleta (37°46'44" S, 57°27'50" W; Fig. 1). Following the morphometric relationships adopted by Best et al. (2010), there were four mature females (> 6 m total length), two calves (< 6 m) of unidentified sex, and a mature male (6.5 m of total length -TL, and a percentage height of dorsal fin to TL > 14.8%). Six of the seven individuals were released, whereas the male died on the beach ( Fig. 2A) where a necropsy was performed.
The franciscana dolphin Pontoporia blainvillei is a small cetacean endemic to the Southwest Atlantic Ocean, from Itaúnas, Brazil (18°35' S, 64°48' W) to Peninsula Valdés, Argentina (42°35' S, 64°48' W) (Siciliano, 1994;Crespo et al., 1998;Bastida et al., 2007;Danilewicz et al., 2009). It is considered the most threatened small cetacean in the Southwest Atlantic Ocean, classified as 'Vulnerable' by the International Union for the Conservation of Nature (Zerbini et al., 2017) and the Society for Mammals in Argentina (Denuncio et al., 2019). The main threat to the species is incidental mortality in gillnet fisheries which has been observed since at least the mid-1960s (Van Erp, 1969. The latest mortality estimation ranged between 360-539 dolphins/year, representing > 2% of the species abundance (between 15,000 and19,000, Crespo et al., 2010;2020) in Buenos Aires, Argentina (Negri et al., 2012). Herein, we report the first evidence of an orca predation on the franciscana dolphin in Argentina, from stomach content analysis.
Highly digested prey items were identified in the stomach contents of an adult male orca, including beaks of four longfinned squids Loligo sanpaulensis (Class: Cephalopoda), a partially digested cetacean rib, and 14 odontocete teeth ( Fig. 2B and 2C). The size and the shape of the teeth found in the stomach The orca, or killer whale (Orcinus orca), is a cosmopolitan cetacean species that is known as a predator of a wide range of prey, including bony fishes, sharks, sea birds, sea turtles, sea otters, pinnipeds, dolphins, and whales (Jefferson et al., 1991;Visser, 2005;Ford, 2017;Wright et al., 2021).
In northern Argentina, the trophic ecology of orcas is poorly known. This may be related to the fact that historically orcas seemed to be sporadic visitors near the coast with only a few sightings per year, and that stranding events are very infrequent in this region. However, recent records of sightings suggest that there is an increasing trend in their occurrence in the area (Biología, revealed that they belonged to a franciscana dolphin. Kasuya and Brownell (1979) described decades ago that franciscana dolphin teeth range between 7 and 10 mm (length of teeth found: 8.41 mm ± 1.27), with a crown antero-posteriorly compressed, and a root of L or J-shape, and lingual-buccally flattened, characteristic particularly notorious in older specimens. Two measurements were taken at the external surface of each tooth, following Ramos et al. (2000): (1) tooth length (L-mm), measured from the apical extremity of the crown to the end of the root; and (2) cingulum width (CW-mm), measured in the maximum width in the intersection of the crown with the root (Fig. 2D). These measurements were compared to a reference teeth collection of P. blainvillei of known age (Table 1; Denuncio et al., 2013). Significant differences were found for cingulum width between the year class 1 and 2 of the reference collection and the ones in the stomach (K-W: CW: χ 2 = 40.764, d.f. = 3, p ≤ 0.01), but there were no significant differences between the year class 0 and the ones found in the present study (p = 0.69) (Fig. 2D). These findings indicate that the teeth found seem to belong to a young specimen less than one year of age (up to 108 cm TL; Denuncio et al., 2013).
Orca predation on franciscana dolphins has been previously reported in Brazilian waters. The first record was found in the stomach content of a stranded orca in Rio Grande do Sul (38°48'15" S, 50°32'45" W; Ott and Danilewicz, 1998; Franciscana Management Area III (FMA III; Secchi et al., 2003)). The second record was an observational study in Paraná (25°20' S, 48°05' W; Santos and Netto 2005; FMA II) ( Table 2). In Argentina, orcas have only been documented preying on two species of dolphins before (L. obscurus and D. delphis; Coscarella et al., 2015). To our knowledge, and despite a previous observational record of this behavior in Mar del Plata (Buenos Aires) during the late 1970s to early 1980s (Bastida et al., 2007), this is the first report of a confirmed franciscana dolphin consumed by an orca in Argentina. Only broadnose sevengill shark was known to be a franciscana predator in the area (Lucifora et al., 2005; Table 2).
Very scarce data on natural mortality is available for franciscana, and therefore orca predation could be an underrepresented source of natural mortality in coastal areas of northern Argentina. However, franciscana dolphins may constitute a profitable prey for orcas, as the franciscana has a predicted occurrence with limited home ranges (Bordino, 2002;Wells et al., 2013), confined to shallow waters (up to 30 m; Danilewicz et al., 2009). Furthermore, from the viewpoint of the predator, franciscana's average blubber layer represents 25-31% of body weight (Caon et al., 2007;Denuncio, 2012), and could therefore satisfy the energetic demands of an   Table 1. Measurements (in mm) of the total length and cingulum width of franciscana (Pontoporia blainvillei) teeth (n = 14) found in the stomach content of the orca (Orcinus orca) and those from a reference collection (ID) with known ages (0, 1, and 2 years old, respectively).  (Kriete,1995;Noren, 2011). Conversely, these characteristics, along with the existence of breeding areas reported near Bahía Samborombón (Denuncio et al., 2013) and their small size (< 150 cm TL and 25-30 kg of body weight, Denuncio et al., 2018), make franciscanas potentially vulnerable for active foraging of orcas. This note presents three valuable contributions to the trophic ecology of marine mammals in the waters of Argentina: (i) along with previous records in Brazil, it confirms orca predation of the franciscana dolphin in almost the entire distribution range of franciscana's, increasing to at least three the number of natural predators of this endangered small dolphin (Table 2); (ii) it provides valuable information on the previously poorly known diet of orca, this apex predator, from the analysis of stomach contents of a stranded animal; and (iii) it highlights the importance of conducting necropsies to provide insights to the ecology of understudied orca populations on the coast of Buenos Aires, Argentina. 1980-1983 Uruguay (34°52'00" S, 56°10'00" W) Stomach content Praderi, 1985Praderi, 1998Praderi, -2001 Anegada Bay (39°50' S, 62° W) Stomach content Lucifora et al., 2005 Tiger shark (Galeocerdo cuvieri)