INTRODUCTION

Killer whales, Orcinus orca (family Delphinidae), are easily identified by their distinctive appearance, exhibiting a robust body, a black-and-white color pattern, a tall and erect dorsal fin, large oval-shaped flippers, and a characteristic postocular white spot (^{Heyning and Dahlheim 1988}). As members of the superfamily Odontoceti, they are toothed whales and thus have carnivorous habits. A cosmopolitan species whose range extends from polar to tropical waters and from offshore to nearshore habitats, including enclosed seas, shallow bays, estuaries, and river mouths, they are the most widely distributed marine mammal. Most of their described ecology and behavior refers to the populations in the Pacific along the northwestern coast of North America, in waters off northern Norway, and in the higher latitudes of the Southern Ocean (^{Wade and Forney 2006}, ^{Jefferson et al. 2008}).

The distribution and occurrence of killer whales in most tropical, coastal, and offshore areas along the Caribbean Sea are sparse. For example, ^{Bolaños-Jiménez et al. (2014)} found that from 1866 to 2012 there were 176 records, which included 145 sightings (82.3%), 27 catches by whaling and fishing (15.3%), and 4 strandings (2.4%); the reports were most frequent in the Eastern Caribbean (n = 71), followed by the Southern Caribbean (n = 54), Greater Antilles (n = 38) and Western Caribbean (n = 13). Water depth at the location of the sightings and catches ranged from 18 to 8,396 m (^{Bolaños-Jiménez et al. 2014}).

The presence of O. orca in Caribbean waters off Colombia has been recorded from longline fishing cruises and direct sightings between 1986 and 2013 off the departments of La Guajira, Magdalena, and Antioquia (^{Alvarez-León 2002}, ^{Bolaños-Jiménez et al. 2014}), including unpublished observations registered in the Marine Environmental Information System (SiAM, Colombia) biodiversity platform (https://siam.invemar.org.co). The aim of this study was to explore and describe the potential causes of a single killer whale observation in Colombian inshore waters.

MATERIALS AND METHODS

A killer whale was sighted in the Gulf of Morrosquillo (9º22′-9º45′ N, 75º33′- 75º55′ W), off the Córdoba and Sucre departments. The gulf has an area of approximately 1,000 km², depths down to 55 m, and a continental shelf 70 km wide at maximum extension, the widest on the Caribbean coast of Colombia. The coastline is surrounded by mangroves, lagoons, and estuaries, but there are coral reef formations and seagrass meadows in the north and south sectors. At the southwestern extreme of the gulf lie the Bay of Cispatá and the delta of the Sinú River, which encompass approximately 4,622 ha of mangroves and an estuarine system of great influence in the gulf’s marine environment (^{Patiño and Flórez 1993}, ^{Díaz et al. 2003}, ^{CVS-INVEMAR 2010}, ^{MoAm 2013}).

The physiographic features in the gulf enabled the establishment of the main port for oil export in Colombia since the 1980s, with ongoing industrial activity up to date. The gulf is also the scenario for the development of an artificial reef program lead by Ecopetrol (formerly known as the Colombian Petroleum Company) to improve artisanal fishery activities, advance conservation, and promote ecotourism (^{Delgadillo-Garzón et al. 2010}).

Between 16 and 26 March 2015, a series of artificial reef monitoring surveys were carried out by researchers on a 24-m length boat with an outboard engine. Each day the boat navigated an average of 42.8 km, and a total of 342.8-km displacements were completed in the permitted areas for artificial reef development in the localities of San Antero, Coveñas, and Tolú (Fig. 1).

Figure 1 Location of the Orcinus orca sighting and other reference sites in the Gulf of Morrosquillo, Caribbean coast of Colombia. 

In order to explore the possible ecological relationships between the characteristics of the sighting reported here and other observations in the Caribbean, the supplementary material on O. orca records available in ^{Bolaños-Jiménez et al. (2014)} was analyzed with GIS tools to find the specific location, abundance, depth, and distance from the nearest point to land.

RESULTS

The opportunistic sighting of the O. orca individual took place in the Gulf of Morrosquillo on 22 March 2015 at 12:55 PM. The odontocete was located at 9º37′44.0″ N and 75º38′49.9″ W over a depth range between 20 and 24 m, at 6.39 km from the nearest coastal point, which was the mouth of a stream called Zaragocilla (Fig. 1). The location was also near the Bajo Pajarito natural reef (4.38 km), Berrugas village (8.06 km), and artificial reefs 31 and 32 (5.58 km). The sighting lasted a total of 17 min (12:55 PM to 13:12 PM) and consisted of a solitary individual (Fig. 2) identified as a killer whale by its distinctive coloration, black body with white patches behind the eyes and on the ventral area, and a very high dorsal fin. The body was approximately 7 m in length, with a dorsal fin that was slightly falcate and rounded at the tip. These features suggest the individual might have been a female or a subadult male, but the photographs taken lack the necessary details to confirm the sex. The individual also had some distinctive traits, like multiple scars on the dorsal area near the fin. The observed behavior was repeated diving, surfacing, and swimming as in an apparent search for food and even passing under our boat.

Figure 2 Different photographs of the killer whale, Orcinus orca, sighted in inshore waters of the Gulf of Morrosquillo, Caribbean coast of Colombia. 

DISCUSSION

The O. orca sighting reported in this study is unusual given the rarity of killer whales in the Gulf of Morrosquillo. For instance, ^{Bolaños-Jiménez et al. (2014)} reported 4 killer whale sightings in Colombia (Table 1), one in Panama, 2 in Costa Rica, 8 in Honduras, and zero in Nicaragua, but 34 in Venezuela. These authors documented detections mostly in the neritic and oceanic zones of the Eastern and Southern Caribbean all year round. This suggests killer whale occurrence is expected, however rare.

Killer whales have been observed in the Caribbean mainly during the northern spring season (1 March to 31 May, 35%) and the rainy tropical season (April to November, 62%), with the highest number in April (15%) and the lowest in September (3%), but no differences between seasons were found (^{Bolaños-Jiménez et al. 2014}). In Venezuelan waters O. orca was predominantly observed between December and May (83%) (^{Bolaños-Jiménez et al. 2009}). The timing of the record from the present study (22 March 2015) is consistent with the months in which killer whales are mostly observed in the Caribbean, but the opportunistic nature of this and other sightings in Colombia precludes any establishment of the origin or residence pattern of this species.

Regarding ecological requirements, some of the observed features in Caribbean killer whales indicate their diet is variable, including sea turtles, marine mammals, and probably fish, as they are generalist predators, a description that has been suggested for O. orca in tropical waters (^{Baird et al. 2006}, ^{Oviedo et al. 2009}, ^{Bolaños-Jiménez et al. 2014}). The whale was observed in the vicinity of the Bajo Pajarito natural reef, which has plenty of large schools of jacks (Carangidae) and grunts (Haemulidae) (^{MoAm 2013}). Additionally, in the enhancement zone of the artificial reefs near the sighting, there are herring (Clupeidae: Opisthonema oglinum, Harengula spp.) and anchovy (Engraulidae: Anchoa spp.) aggregations of more than 2,000 individuals per module, as well as large prey items belonging to the Scombridae family (^{Delgadillo-Garzón and García 2009}). These fishes could represent food items for O. orca, as was mentioned for the observations in Aruba (^{Bolaños-Jiménez et al. 2014}).

The Gulf of Morrosquillo and the Bay of Cispatá are of special interest given the presence of other cohabiting odontocete species, such as the Guyana dolphin (Sotalia guianensis), which is predominant, and the bottlenose dolphin (Tursiops truncatus), with fewer reported numbers. The presence and distribution of these odontocetes have been related to environmental and oceanographic conditions and to the abundance of food (fish), as they were described predominantly foraging and feeding (^{Ávila 1995}, ^{García and Trujillo 2004}, ^{Dussan 2013}). Furthermore, Morrosquillo and the archipelago of San Bernardo, located in the northern part of the gulf, have large extensions of coral reefs, seagrass meadows, and sandy beaches, some of which are reportedly home to sea turtles, especially the green turtle (Chelonia mydas) and hawksbill turtle (Eretmochelys imbricata), with identified nesting beaches and feeding locations (^{Díaz et al. 2001}, ²⁰⁰³; ^{Ceballos-Fonseca 2004}). Since Caribbean killer whales feed mostly on other marine mammals and sea turtles, their presence in the gulf could be related to the availability and diversity of food items in the area.

As for behavioral aspects, although O. orca is known for its high degree of social organization (^{Bigg et al. 1990}, ^{Ford et al. 2000}), “transient mammal-eating” offspring often disperse permanently (^{Baird and Dill 1995}, ^{Ford and Ellis 2006}). Accordingly, in the supplementary data of ^{Bolaños-Jiménez et al. (2014)}, the proportion of observations of solitary individuals was 38.6% and that of pods of 2 to 5 individuals was 43.1%. This indicates that the sighting of the solitary individual reported in the present study is a relative common pattern in the region and is also consistent with previous records for Colombian waters (Table 1).

The presence of solitary killer whales in the Caribbean may be related to the documented features of the different ecotypes in other regions. For instance, residents form large pods made up of up to 11 matrilines and have strong bonds, with no dispersal, while transients form smaller pods made up of a single matriline and a mean of 3.6 individuals, where subadult males and even females (less common) tend to disperse (^{Bigg et al. 1990}, ^{Baird and Whitehead 2000}). This contrasting behavior depends on the cost-efficiency for the survival of the group and is influenced by multiple factors, such as food availability, reproductive dispersal, social dynamics, intraspecific variability, behavioral plasticity, and individual experience. In this regard, smaller pods seen in the transient ecotype have maximum feeding efficiency when hunting prey that are usually distributed in smaller groups (^{Bigg et al 1990}, ^{Baird and Whitehead 2000}, ^{Müller and Bossley 2002}). Whenever the pod size surpasses the optimum maximizing energy, subadult males often disperse permanently and become loners (^{Baird and Dill 1995}, ^{Baird and Whitehead 2000}, ^{Ford and Ellis 2006}, ^{Dalla-Rosa and Secchi 2007}). This is the case for sightings in the Caribbean (^{Bolaños-Jiménez et al. 2014}) and other tropical areas, such as Perú (^{García-Godos 2004}), Brazil (^{Lodi and Farias-Junior 2011}), and the Galápagos Islands (^{Denkinger et al. 2020}). Since no ecotype has yet been determined for Caribbean killer whale populations, it is likely their structure and behavior resemble the transient type.

Despite the relative frequency of solitary killer whales in the Caribbean and other areas of the world, the sighting reported in the present study is unusual in terms of depth and distance from the coast. As documented by ^{Bolaños-Jiménez et al. (2014)} in their supplementary data, the records for single individuals report an average depth of 964.5 m (±1,315.4 SD), range between 2.5 to 8,150.0 m, mainly over the 50-m isobath (66.2%); the mean distance from the coast was 24.9 km (±45.4 SD), with values between 0.2 and 260.0 km, but chiefly over 6.0 km (58.8%). Moreover, most of the sightings within 6 km from the coast occurred in areas with a narrow continental shelf and an average depth of 124 m (±229.9 SD) (^{Bolaños-Jiménez et al. 2014}). In tropical environments O. orca tends to be more abundant in areas where upwellings and cold currents occur, possibly because of the higher productivity and the potential prey available for them (^{Bolaños-Jiménez et al. 2009}, ^{Lodi and Farias-Junior 2011}, ^{Denkinger et al. 2020}). In contrast, the extended continental shelf in the Gulf of Morrosquillo, with shallow bottoms close to the coast, allow for very warm water conditions throughout the year, with average sea surface temperature of 29.5 ºC, ranges between 27.5 and 32.0 ºC (^{Delgadillo-Garzón and García 2009}). However, the individual reported in the present study was observed swimming and diving in an apparent search for food. Despite the fact that no predation was observed, transient mammal-eating killer whales spend 90% to 96% of their time foraging (^{Similä 1997}). Thus, the individual reported here could possibly be a transient that happened to be exhibiting foraging activity and exploring the area, since there is no previous evidence of the species in the gulf.

There are external factors, such as anthropogenic noise, that might influence the presence and movement of killer whales in the region. During 2014 and 2015 a total of 65,175 km of 2D seismic offshore exploration was conducted by Anadarko Colombia Company and Ecopetrol along the Caribbean and Pacific waters off Colombia, including the Sinú offshore basin and the Fuerte Norte and Fuerte Sur blocks on the western margin of the Gulf of Morrosquillo. This led to the gas well perforation Kronos-1 in the Fuerte Sur block in August 2015 (^{ANH 2015a}, ^2015b). There are evident effects of this activity on cetacean populations and behavior, such as displacement from important habitats (e.g., feeding and reproductive) (^{Nowacek et al. 2007}), strong avoidance of the noise producing machine (usually seismic water guns), and changes in vocal behavior (i.e., cessation or increment of their vocalizations in response to masking noises) (^{Bowles et al. 1994}, ^{Foote et al. 2004}). Killer whales and harbor porpoises have been found to dramatically change their locations for years to avoid loud acoustic harassment, returning when it has stopped (^{Morton and Symonds 2002}, ^{Stone and Tasker 2006}). The recent seismic activity conducted during the time of the sighting may have influenced the presence of this solitary odontocete in the area, as a mechanism of noise avoidance.

According to the information provided by 15 active lifelong artisanal fishermen (ages between 45 and 80 years) from the municipality of Santiago de Tolú in the Gulf of Morrosquillo, no killer whales have been observed or reported in this and nearby areas, despite the ease of recognition of this distinctive species. Therefore, this study represents the first report of O. orca in the area, contributing to the existing information of its distribution along the Caribbean. This sighting and the presence of other cetacean species in the area suggest the Gulf of Morrosquillo may play an important role in the ecology of these mammals, and it is thus important to preserve its environmental characteristics. Additionally, it is pertinent to consistently invest in marine mammal prospection surveys to establish the possible routes and common habitats of these organisms in Colombian coastal and offshore waters.