The sei whale (Balaenoptera borealis) was intensively exploited throughout its range, with about 110.000 individuals hunted by pelagic fleets in Antarctic waters between 1960 and 1970. In addition, basic information on its distribution, migratory routes, and feeding grounds in the southeastern Pacific, has been poorly documented. In the case of Chile, recent information consists mainly of accidental records. This research presents the first sei whale photo-identification catalog for south-central Chile. From November 2019 to January 2020, 88 individuals were recorded from land-based and boat surveys at Caleta Chome. Of these, 12 individuals were photo-identified through scars or distinctive notches in the dorsal fins. The peak of sightings occurred during December 2019; two individuals were sighted on more than one occasion.

Balaenoptera borealis, Caleta Chome, Chile, Sei whale, South-eastern Pacific

The sei whale (Balaenoptera borealis) is an endangered mysticete (Cooke 2018) and the third largest whale after the blue whale (Balaenoptera musculus) and the fin whale (Balaenoptera physalus; Horwood 2018). This species presents a cosmopolitan distribution and pelagic with temperatures below 20 °C (Omura and Nemoto 1955; Gambell 1968, 1985). It migrates to the southern hemisphere during summer, from areas near the subtropical convergence where it reproduces, to areas near the Antarctic convergence (50°S–60°S) for feeding (Horwood 1987; Reeves et al. 1998; Rice 1998). Feeding zones are unpredictable, with a sudden influx into an area, followed by their disappearance and subsequent absence for years (Gambell 1985; Reeves et al. 2002; Jefferson et al. 2008). During the summer there are high concentrations of sei whales between 40° and 50°S with adult individuals reaching polar waters while juveniles or sub-adults stay north of the Antarctic convergence (Lockyer 1977; Acevedo et al. 2017). Six populations have been assumed for the southern hemisphere for management purposes; however, the scarce evidence has failed to identify separate populations within ocean basins (Kanda et al. 2006; Horwood 2018).

The International Whaling Commission estimated that by the 1940s the population declined from 191.000 to 37.000 individuals after the cessation of commercial catches in 1983 (Gambell 1985). Although the sei whale was not a target species for hunting until the early 1960s (Acevedo et al. 2017; Español-Jiménez et al. 2019) the decrease in the most profitable whales (B. musculus, B. physalus, Megaptera novaeangliae and Eubalaena australis) led to an increase in the hunting effort for this species. In South America, this species was heavily exploited throughout its range (Zerbini et al. 1997; Aguayo-Lobo et al. 1998a) where about 110.000 individuals were hunted by pelagic fleets in Antarctic waters between 1960 and 1970 (Horwood 2018). In Chile, it was the third most hunted whale species between 1929 and 1979 with at least 1,664 individuals captured (Aguayo 1974); however, due to the difficulty of differentiating from Bryde’s whale (Balaenoptera edeni) during this whaling period, its hunting numbers are probably overestimated since many Bryde’s whales would have been reported as sei whales (Valdivia et al. 1981; Gallardo et al. 1983; Aguayo-Lobo et al. 1998a). There were about 25 years (between 1974 and 1999) in which there was no research on sei whales, and the few studies that were conducted during this period did not consider the sei whale as an object of study (Gallardo and Pastene 1983; Gallardo et al. 1983; Guerra-Correa et al. 1987; Aguayo-Lobo et al. 1998b). Since the moratorium on whaling established in 1983, there has been a considerable reduction in sei whale research (Reeves et al. 2002); currently this species is one of the least known baleen whales in the world (Prieto et al. 2012; Acevedo et al. 2017; Horwood 2018).

Caleta Chome was founded by the Macaya Hnos. whaling industry on 1948 (Quiroz and Carreño 2019). By 1954, the sei whale was already within the productivity of the whaling plant in Caleta Chome and was the first documented record of sei whales for this region. The sei whale catch data for this area were for a long time under the name “S + B”, since they considered the bryde (B) and sei (S) whales together due to their similarities, therefore there are no clear records of the number of individuals of sei whales caught in this area (Pastene 1982). In Caleta Chome, between 1951 and 1983, active whaling was carried out by the Trinidad Whaler owned by the Macaya Family (Quiroz and Carreño 2019). Given that the sei whale is endangered, studies of its populations are crucial to support its conservation. This study presents the first sei whale photo-identification catalog in south-central Chile and information on sightings.

Between November 3 and January 18, we made 88 sei whale sightings. The number of sightings varied from November to January, registering a peak of sightings on December 11 and 20 (Fig. 2). We identified 17 groups of sei whale of 2 to 9 individuals (median = 4); most of these groups were registered during the month of December (10 groups). The SST range from 12.7 °C to 15.1 °C during the sightings, and the estimated depths of the sightings range from 16 to 137 m. On the other hand, sei whale sightings ware made at mean distance of 3.6 km from the coast. We highlight one event, where the distance was around 0.1 km. (Table 1).

Figure 2.

Frequency in the detection index of the Balaenoptera borealis sighted during November and December 2019 and January 2020. The detection rate was calculated based on the effective sightings per day for 10 days standardized by the sampling effort. The numbers above and within each bar indicate the number of total sightings and the sampling effort in each date range, respectively.

Twelve individuals were photo-identified through distinctive scars or notches on the dorsal fins (Fig. 3), 83% of the individuals have some notch and one of them has a broken fin (# 002) and another two had lacerations (# 007 and # 009). Most individuals (see photographic sequence from # 009 to # 012 in Fig. 3) were photographed in December 2019. Two individuals (# 003 and # 009) were sighted more than one day in the area, individual # 003 was sighted 7 days after the first sighting and individual # 009 on the seventh and tenth day after the first sighting. The presence of a mother with a calf was registered for 5 different days.

Figure 3.

Dorsal fin photographs of 12 identified sei whales (Balaenoptera borealis) at Caleta Chome, during the seasons from November 2019 to January 2020.

With the moratorium established by the International Whaling Commission (IWC) in 1983, the cetacean records associated with hunting decreased and the only records of the sei whale for central Chile were only three reports and one scientific publication (Pastene 1982; Gallardo and Pastene 1983; Gallardo et al. 1983; Aguayo-Lobo et al. 1998a). Although there are records of sightings of this species in previous years (F. Silva obs. pers.), a high abundance of individuals was not recorded in this area and neither was the presence of mothers with young.

The sei whale is described as predominantly found in deep waters, occupying mainly pelagic habitats at distances greater than 110 km from the coast (Best and Lockyer 2002; Prieto et al. 2012); however, many of our sightings occurred near the coast at distances no greater than 6 km (see Table 1) consistent with what was observed for populations of sei whales in the South Atlantic (Weir et al. 2020). This greater abundance of sei whales near the coast and the feeding activity during the day, agree with the results obtained by Español-Jiménez et al. (2019) on the coast of the Gulf of Penas and Tres Montes. Another data to highlight is the sightings of the same individual on more than one occasion; this suggests a stay of at least 7–10 days. In addition, the presence of mothers with young during the observation season may be giving signs of safety and good environmental conditions for rearing. More than 50% of the identified individuals had scars on their fins. Among the possible natural threats that could have damaged its dorsal fin, is predation by killer whales (Orcinus orca) which are considered its only significant natural predator (Jefferson et al. 1991; Springer et al. 2006).

The occurrence of sei whales in the coastal upwelling system of Chile was reported by Gallardo and Pastene (1983), who associated high primary productivity with the concentration of sightings, proposing that the coasts of the Biobío region can be feeding areas for some cetaceans during their migrations. The continental shelf off Biobío is an important upwelling zone and has been described as one of the most productive areas within the Humboldt Current System (Montecino et al. 1998; Montero et al. 2007; Thiel et al. 2007; Daneri et al. 2012; Iriarte et al. 2012). Bathymetric features such as underwater canyons generate foraging areas for a variety of cetaceans, contributing to primary productivity and biodiversity (Moors-Murphy 2014). Croll et al. (2005) mention that bathymetric ruptures and the coastal upwelling process are important factors in the density of euphausiids, contributing to the formation of feeding areas. The high occurrence of individuals in Caleta Chome could be related to a migratory route close to the Biobío Canyon located to the south of the study area.

The oceanographic conditions in the distribution of this species are variable in relation to the migratory routes and the permanence in feeding and / or reproduction sites (Omura and Nemoto 1955; Gregr and Trites 2001; Sasaki et al. 2013; Murase et al. 2014). On the coasts of Chile, the presence of sei whales has been reported in areas with sea surface temperatures of 14.5 °C in spring (Clarke et al. 1978). In the Magellan Strait Acevedo et al. (2017) reported a thermal range between 5.7 and 10.9 °C. Our sightings are also within this temperature range, agreeing with the values reported for this species in other parts of the world (Sasaki et al. 2013).

Acevedo et al. (2017) reported sei whales in the Magellan Strait from November to May, with a peak of sightings (83.6%) occurring during December and January. They suggest that although there are records that support the hypothesis that whales forage in southern Chile, none of them provide information on systematic annual occurrences in these same areas. But this, in turn, can be supported by the fact that sei whales are known for their unpredictable presence in an area followed by their subsequent disappearance, as well as having a greater variation in distribution in their feeding grounds than most species of baleen whales (Tønnessen and Johnsen 1982; Horwood 1987). All these records that support the presence of the sei whale on Chilean coasts are at the same time of the year (January–May) within relatively small latitudes and, although they are not in consecutive years, there is a certain periodicity that could support the hypothesis that the sei whale forages in Chilean waters but their foraging areas change over the years (Pastene and Shimada 1999; Aguayo-Lobo et al. 2006; Guzmán 2006; Acevedo et al 2017; Español-Jiménez et al. 2019) This may be due to an underestimation of the presence of this species in Chile, since there is no systematic monitoring in all the areas that it has been recorded. In this work, the occurrence of the sei whale off the Chilean coast shows the existence of a passage zone and possible feeding area within the waters near the coast. Ecological knowledge about sei whales along the Chilean coast is scarce, therefore we highlight the importance of increasing sighting efforts around the Hualpén Peninsula Nature Sanctuary during upwelling events to understand if the presence of sei whales follows some seasonal pattern or corresponds to sporadic events.

We thank the Turismo Chome Aventura company for the navigation services in Caleta Chome and the information provided through verbal communication of sei whale sightings and navigation routes commonly used by whales. A.C.C thanks the COPAS-coastal center, Universidad de Concepción, Chile and the Agencia Nacional de Investigación y Desarrollo de Chile (ANID).