ARTÍCULO

Occurrence of fin and humpback whales in the Scotia Sea and the protected marine area of the South Orkney Islands, Antarctica

 

José L. Orgeira^{1, 2}, Juan C. González², Yohana G. Jiménez², and Lidia M. Benítez³

¹ Departamento Biología de Predadores Tope, Instituto Antártico Argentino. Balcarce 290, C1064AAF Ciudad Autónoma de Buenos Aires, Argentina. [Correspondence: <jlorgeira@dna.gov.ar>]
² Cátedra Ecología General, Facultad de Ciencias Naturales e Instituto Miguel Lillo, Universidad Nacional de Tucumán. Miguel Lillo 205, 4000 San Miguel de Tucumán, Tucumán, Argentina.
³ Cátedra de Matemática, Facultad de Ciencias Naturales e Instituto Miguel Lillo, Universidad Nacional de Tucumán. Miguel Lillo 205, 4000 San Miguel de Tucumán, Tucumán, Argentina.

Recibido 4 agosto 2016.
Aceptado 4 noviembre 2016.
Editor asociado: MF Grandi

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ABSTRACT.

The water masses around the South Orkney Islands (SOI) and their Protected Marine Area (PMA) in the Scotia Sea, Antarctica, are among the biologically richest areas of the Southern Ocean, supporting a large community of top predators. Information about the occurrence of whales in the southern Scotia Sea is scarce, partly due to the difficulty of conducting research in Antarctic waters. The aims of this paper are to report new observations of fin and humpback whales in the southern Scotia Sea and SOI PMA, and to describe environmental conditions associated with their occurrence. The study was conducted onboard R/V Puerto Deseado between 58ºS and 64ºS latitude, covering a total of 588 nm from 15 February to 15 March 2014. A principal component analysis was used to identify patterns of occurrence. Fin whales were associated with higher values of sea surface temperature (SST), salinity and depth. The largest aggregations of fin whales were located NW of the SOI. By contrast, humpback whales occurred in shallower areas with lower SST and salinity, and the largest aggregations were south of 60ºS, in or near the PMA. Our analyses are based on a single 25-day cruise but provide new information on the summer occurrence of fin and humpback whales in the southern Scotia Sea and SOI PMA, an area rarely visited by research vessels devoted to studies of top predators.

RESUMEN.

Ocurrencia de ballenas fin y jorobadas en el Mar de Scotia y Área Marina Protegida de las islas Orcadas del Sur, Antártida.

Las aguas que rodean a las islas Orcadas del Sur (IOS) y su Área Marina Protegida (AMP) están entre las regiones biológicamente más ricas del Océano Atlántico Sur, sustentando una gran comunidad de predadores tope. La información disponible de la ocurrencia de cetáceos en el Mar de Scotia es escasa debido, en parte, a la dificultad de llevar a cabo estudios de investigación a bordo de buques en Antártida. El objetivo de este estudio es reportar nuevas observaciones de ballenas fin o rorcual común y ballenas jorobadas o yubartas en aguas circundantes a las IOS y su AMP, y describir las condiciones ambientales asociadas a su ocurrencia. El estudio fue llevado a cabo a bordo del buque oceanográfico Puerto Deseado entre las latitudes 58º a 64ºS cubriendo un total de 588 millas náuticas desde el 15 de febrero al 15 de marzo de 2014. Se utilizó un análisis de componentes principales para identificar patrones de ocurrencia en función de las variables ambientales presentes al momento de la observación. La presencia de las ballenas fin ha resultado asociada a altos valores de salinidad, profundidad y temperatura superficial del mar (TSM) y sus mayores agregaciones se localizaron al NW de las IOS. En contraste, las ballenas jorobadas fueron registradas en áreas someras de baja salinidad y baja TSM, con las mayores agregaciones de individuos al sur de las IOS, dentro o cerca de la AMP. Nuestros análisis están basados en un único crucero de investigación de 25 días, pero aportan nueva información de la ocurrencia de ballenas fin y jorobadas al sur del Mar de Scotia y el AMP de las IOS, una zona raramente visitada por buques de investigación dedicados al estudio de los predadores tope.

Key words: Antarctica; Distribution; Fin whales; Humpback whales.

Palabras clave: Antártida; Ballenas fin; Ballenas jorobadas; Distribución.

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INTRODUCTION

The aim of studying the upper trophic levels species in the Austral Ocean is to understand the species interactions and how they are affected by the different environmental variables and the fisheries activities. The knowledge of the habitats used by the top predators gives us information about the general ecology of the species, and during the austral summer, it indicates the presence and distribution of their food resources (Joiris and Dochy, 2013). Among the top predators, baleen whales are one of the most interesting taxa because they show different patterns of spatial distribution (Sirovic et al., 2006). However, although whales are the largest and most conspicuous creatures of the Southern Ocean ecosystem, they are among the least studied (Ducklow et al., 2007). Our current understanding of the large-scale distribution of whales comes from the three circumpolar cetacean sighting surveys (CP I–III) carried out under the auspices of the International Whaling Commission’s (IWC) International Decade of Cetacean Research (IDCR) and Southern Ocean Whale Ecosystem Research (SOWER) programmes from 1978/1979 to 2003/2004 (Herr et al., 2016). In Antarctica many cetaceans studies have been conducted west of the Antarctic Peninsula and South Shetland Is. (Dalla Rosa et al., 2008; Nowacek et al., 2011; Burkhardt and Lanfredi, 2012; Friedlaender et al., 2013; Joiris and Dochy, 2013; Santora et al., 2014; Herr et al., 2016, among others). Comparatively the oceanic region of the Scotia Sea, SOI and its PMA remain less studied (Leaper et al., 2000; Sirovic et al., 2006; Rossi-Santos et al., 2007; Reyes Reyes and Iñíguez, 2013; Reyes Reyes et al., 2014; Orgeira et al., 2015). Fin (Balaenoptera physalus) and humpback (Megaptera novaeangliae) whales are among the most abundant baleen whales in Scotia Sea (Orgeira et al., 2015) and have been widely recorded in different Antarctic and sub-Antarctic areas (Joiris, 1991; De la Mare, 1997; Orgeira, 2004; Rossi-Santos et al., 2007; Nowacek et al., 2011; Joiris and Dochy, 2013). Identified as Subarea 48.2 by CCAMLR, the Scotia Sea is one of the areas of the Southern Ocean where the krill fishing effort is mostly concentrated (CCAMLR, 2013). The krill fishery intensively targeted west and south of the SOI in the same area where the highest concentrations of seabirds and cetaceans were observed during a five consecutive summer season monitorings (Orgeira et al., 2015). In an area where feeding grounds and fisheries overlap, new whale surveys can help us know their habitat preferences.

During the summer season 2014, we conducted a ship-based visual survey of cetacean species in the Scotia Sea and SOI PMA. We aim to report new observations of fin and humpback whales in the Scotia Sea and SOI PMA, Antarctica, and to describe the environmental conditions that were associated with their occurrence.

MATERIALS AND METHODS

The study was conducted onboard R/V Puerto Deseado from 15 February to 15 March 2014, between latitudes 58º00’S to 64º00’S and longitudes 40º00’W and 49º00’W including the PMA, between latitudes 61º30’S and 64º00’S and longitudes 41º00’W and 48º00’W (Fig. 1A). Fin and humpback whales were chosen as a case study because they were the most abundant cetaceans recorded. Data was collected from the ship’s bridge (15 meters above sea level) and from outdoor ailerons. Observations were continuously made during daylight (~05:00 to 20:00 h) by two observers working simultaneously while sailing. Species were detected using the naked eye and identification was made to the lowest possible taxonomic level with the help of 16 x 50 binoculars. When identification was not possible, the individual was recorded as “unidentified”. A ‘passing mode’ method was used during surveys which meant that the vessel continued travelling along the established transect line even after a group of marine mammals was seen (Dawson et al., 2008). Sampling effort was restricted to sea conditions ranging from Beaufort scale 0-4 to reduce effects of lower sighting probability during increased Beaufort levels. Survey tracks were determined by the navigation schedule of the vessel. As R/V Puerto Deseado was also scheduled for oceanographic stations, there was no specific route for whale survey. The occurrence of cetaceans is presented in two ways: number of whales and cetacean encounter rate (number of sighted cetaceans per nautical mile surveyed and estimated for each species; Reyes Reyes and Iñíguez, 2013) as a measure of density.

Fig. 1. Survey effort (A), 15 February to 15 March 2014, SST (B) and Salinity (C) data collected on board R/V Puerto Deseado. The rectangle indicates the SOI PMA.

Ship speed and geographical position (given by portable GPS) were recorded and updated every 20 minutes with the following environmental variables: ice-cover (evaluated from the bridge and expressed as % coverage within an approximate range of 500 m around the ship), wind speed and direction, Beaufort scale, depth (provided by ship’s instruments), salinity and sea surface temperature (SST). SST and salinity were taken in ºC at 3 m depth by a SBE 21 thermo-salinograph which performed a scan every 30 sec.

The relationships between environmental variables and the whales’ occurrence were explored using a PCA on four environmental variables recorded during each sighting—sea surface temperature (SST), ice-cover, salinity and sea depth from a correlation matrix. T tests were performed to evaluate whether the observed differences in the environmental variables for fin and humpback whales were significant.

RESULTS

During 25 days of active sighting effort, the total number of hours and distance covered of on-effort observations were 294 hours and 588 nm (1090 km), respectively.

Of this total, 38% of the observations (222 nm, 111.33 hours of observation) were within PMA. The study was suspended for 5 days due to bad weather conditions. A total of 394 whales (fin, humpback, southern right Eubalaena australis and unidentified whales) were recorded (Table 1), 168 fin and 83 humpback were recorded from 35 and 31 sightings respectively.

Table 1
Number of whales and cetacean encounter rate (whale/nautical mile) of the cetacean species recorded.

Simultaneously 135 fin whales (80%) were sighted in the north of 60ºS. All humpback whales but one were recorded south 60ºS. The largest aggregations of fin whales occurred NW of SOI (south of 58ºS) while humpbacks were seen in more southern areas, reaching 63ºS (Fig. 2A). Within the PMA, 24% and 58% of fin and humpback whales were recorded, respectively, throughout the study (Table 1). The cetacean encounter rate was 0.29 and 0.14 whale/nautical mile for fin and humpback whales respectively. Fin whales were more commonly seen in monospecific groups formed by 3-5 individuals (13 sightings), while humpback whales was more frequently sighted in monospecific pairs (20 sightings). Mixed groups (both fin and humpback) were observed in 4 sightings. No minke whales were observed.

Fig. 2. Number and spatial distribution of all whales species sighted: A) Humpback and fin whales, B) unidentified whales (UW) and southern right whales (SRW). The rectangle indicates the SOI PMA.

The study area comprises the Southern Boundary of the Antarctic Circumpolar Current (ACC) which presents different values of salinity and SST north and south of SOI (Fig. 1B, C). These values and other variables were taken into account to explain the distribution of the fin and humpback whales. The first two PCA components explained 73.5% of the variability in the data (Table 2). The first component (PC1) divided the sightings in relation to environmental variables where whales were recorded (Fig. 3). Variables that most contribute to PC1 were SST, depth, salinity. Therefore the greatest variability was explained by these variables. Fin whales were mostly distributed NW of SOI, associated to higher values of salinity, depth and SST.

Table 2
Relationships between environmental variables and the whales occurrence. Variables provided greater variations in the study are indicated in bold. N indicates the number of sightings in which there was at least one fin or humpback whale present. SST, sea surface temperature.

Fig. 3. Distribution of sightings on the first two components of the PCA analysis. SAL, salinity; SST, sea surface temperature; SD, sea depth; IC, ice-cover.

South of SOI and PMA were covered by several transects from 24 February to 6 March. Humpback whales occurred in this area at low salinity in shallow zones and lower SSTs. PC2 has included a single variable (ice-cover) as the most important which does not contribute significantly to this component. However, we observed humpback whales associated to ice-covered waters north of SOI (Fig. 2A). Most of the ice-cover was found north of SOI (Fig. 2A). The T test gave significant differences at 5% level for both species as the sea depth; ice-cover, salinity and SST average in which they were recorded had been already shown in the PCA (1.16 ºC for fin whales and 0.85 ºC for humpback; p = 0.023).

DISCUSSION

The Southern Ocean system is probably the largest marine ecosystem on the globe (Knox, 2006) and the Scotia Sea is one of the richest areas of the Southern Ocean (Hofmann et al., 1997; Tynan, 1998). It is known that the Scotia Sea supports a large community of top predators which feed on important plankton biomass, particularly krill Euphausia superba (CCAMLR, 2013). All our observations were obtained at the Southern Boundary of the ACC, a critical component of the global ocean circulation (Orsi et al., 1995). The association of primary production, krill, and whales with the Southern Boundary suggests that it provides predictable productive foraging for many species, and it’s crucial for to the function of the Southern Ocean ecosystem (Tynan, 1998).

Over the years, both cetacean species have been recorded in the Southern Boundary of the ACC (Van Franeker, 1992; Orgeira, 2004; Sirovic et al., 2006; Reyes Reyes and Iñiguez, 2013; Reyes Reyes et al., 2014). This suggests that these areas, probably feeding zones, remain roughly constant in time and space, which is supported by records of large seabird aggregations in the same locations over successive summer seasons (Orgeira and Alderete, 2013; Orgeira et al., 2015). Most humpback whales were recorded south of 60°S in the Southern Boundary of the ACC, in areas with 900 m average depth and most of them in ice-covered waters, which coincides with Reyes Reyes and Iñiguez (2013). As for the fin whales, most of them were recorded north of 60°S in the central Scotia Sea in ice-free waters (Kasamatsu et al., 1996; Sirovic et al., 2006; Reyes Reyes et al., 2014) and in areas with 2000 m average depth, although some records were obtained in shallow waters, less than 300 m (Burkhardt and Lanfredi, 2012). Our cetacean encounter rates were higher than the ones reported by Reyes Reyes and Iñiguez (2013) (fin whales 0.053 and humpback 0.073). This difference may be due to the cruise track (these authors did not cover PMA) and less time devoted and distance covered of on-effort observations, among others. Our records indicate that whales’ movements were usually monospecific, either in groups (fin) or in couples (humpback). Sightings of mixed groups (both species) were low, which is consistent with previous observations in the same area (Reyes Reyes and Iñiguez, 2013).

According to Herr et al. (2016) there is a horizontal niche separation between humpback and fin whales, with humpback whales preferring the coastal parts and fin whales residing in habitats around the shelf edge. This horizontal niche separation between both whale species appears to be due to differences in their trophic preferences. Santora et al. (2010) suggested that humpback whales showed a preference for small juvenile krill mainly residing in shallow waters in the Bransfield Strait/Mar de la Flota, as opposed to large, fast swimming mature krill, mainly found further offshore. Recent studies in the Drake Passage and west of Antarctic Peninsula suggest that fin whales feed primarily on krill Thysanoessa macrura, and humpback whales on Euphausia superba (Herr et al., 2016). All these behavior differences in both species were interpreted as a strategy to avoid interspecific competition (Murase et al., 2002) and help to explain spatial segregation observed in Fig. 2A, where most fin whales were observed in the north of IOS, in deeper waters, while humpback whales, in the south, in shallower waters.

On 3 March, heading south of 61°S, an increase in SST was recorded, from 0.4 °C to 1.4 °C. As the SST increased, groups and pairs of both cetacean species were observed, particularly fin whales (39 individuals). This area of increased SST and salinity probably corresponded to marine fronts or eddies. There is some evidence that fin whale foraging habitats are regions with complex bathymetry and hydrographic circulation patterns (Santora et al., 2014). Fin whales may target areas with complex water circulation because their speed and agility would enable them to exploit increased concentrations of forage species which may be entrained within eddies and fronts (Johnston et al., 2005). Marine fronts occur worldwide at several spatial and temporal scales (Acha et al., 2015) and play an important role in ecological processes in the oceans by increasing the primary productivity and providing suitable habitats for fish and squid, which attract seabirds and marine mammals (Acha et al., 2004). Fronts are therefore “hot spots” of intense biological and physical activity (Olson et al., 1994). Eddies are hydrodynamic processes that concentrate or dilute plankton and suspended material by physical processes of accumulation, retention, or dispersion (Aldeco et al., 2009). Its biological implications in the Scotia Sea and Weddell-Scotia Confluence were described in detail by Kahru et al. (2007). Top predators are so closely related to changes in the water masses properties caused by eddies or marine fronts that seabirds are considered indicators of the marine environment (Parsons et al., 2008) and whales may be able to locate physical oceanographic features and processes that enhance prey aggregation (Friedlaender et al., 2006). Twenty-two % and 23% of all fin and humpback whales, respectively, were sighted in this unique 25 nm transect south of SOI where the SST had an increase of 1 °C. Thus, the cetaceans aggregations registered on March 3 (especially fin whales) could be the result of any of these oceanographic processes that led to a concentration of food.

Minke whales, which generally associated to ice-cover waters (Bombosch et al., 2014; Sirovic et al., 2006), were not recorded in this study. This may suggest that the absence of minke whales may be due to the scarce presence of ice during the cruise. It seems that prey distribution may not be the only driver of habitat selection: the avoidance of interspecific competition appears to be another reason for minke whales to prefer ice-laden habitats (Kock et al., 2009). The percentage of unidentified whales in this study was high (32%). Many of them were registered either north of SOI, near fin whales or south, near humpbacks (Fig. 2B). If the minke whales usually avoid interspecific competition as suggested by Kock et al. (2009), their presence among the “unidentified” whales may be unlikely. During a long-term study of the at-sea distribution of seabirds and cetaceans in the Scotia Sea from summer 2010 to 2014 (Orgeira et al., 2015), minke whales were observed only in the summer of 2010, in contrast with previous studies that reported this species as the most abundant of all cetaceans (Reilly et al., 2004, Sirovic et al., 2006).

The most ambitious dedicated cetacean sighting program in the Southern Ocean was conducted by the International Whaling Commission (IWC). It is known as the International Decade of Cetacean Research (IDCR) and its follow-up program the Southern Ocean Whale and Ecosystem Research (SOWER)(1978/79– 2008/09), circumnavigating the Southern Ocean three times (Bombosch et al., 2014). Despite these efforts, some areas like SOI PMA remain poorly studied. The sightings presented in this paper were based on a single 25-day cruise but it provides new information on the occurrence of fin and humpback whales in the southern Scotia Sea and the SOI PMA, an area rarely visited by research vessels devoted to top predators monitoring (Engel and Martin, 2009). Only a ship-based monitoring program sustained over time will increase the knowledge on the current status of whales in this part of the Southern Ocean.

ACKNOWLEDGEMENTS

The authors are grateful to Mónica Torres and Natalia Dellabianca for their valuable help in cetaceans sightings, and to the crew of the R/V Puerto Deseado for assistance provided during the surveys. We also thank Javier Pardiñas who provided the thermosalinograph data, and the anonymous reviewers for their collaboration in improving the manuscript. This work was carried out with logistic and financial support from the Instituto Antártico Argentino.

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