Major hotspots detected along the Scotia Ridge in autumn for southern right whales Eubalaena australis, Antarctic fur seals Arctocephalus gazella and Antarctic prions Pachyptila desolata

Claude R. Joiris, Grant R. W. Humphries, Diederik D’Hert, René-Marie Lafontaine,2, Henri Robert,2 & Roseline C. Beudels-Jamar,2

1 Laboratory for Polar Ecology (PolE), 1367 Ramillies, Belgium;

2 Conservation Biology Unit, Royal Belgian Institute for Natural Sciences, 1000 Brussels, Belgium;

3 Department of Ecology and Evolution. Stony Brook University, Stony Brook, New York, USA 11794

1 Introduction

The distribution of upper trophic level predators in the polar oceans reflects prey abundance and availability (e.g.,zooplankton, krill, nekton, and small fish), in turn responding to lower trophic level processes. These top predators therefore constitute the best indicators to localize areas of high biological production[1-6].

In the Antarctic, biological studies mainly concern the Weddell, Ross and Bellingshausen seas[7-9]. Appreciable work has been conducted in the Scotia Sea as well, but not during autumn except in the Scotia-Weddell Confluence(about 50 papers resulting from AMERIEZ: Antarctic Marine Ecosystem Research in the Ice Edge Zone). Following the description of an important autumn aggregation around the South Shetland Islands[6], this paper reports on autumn observations of top predators in the Scotia Ridge in northern Scotia Sea.

Water masses and fronts, pack ice and ice edge are well understood to be the main hydrological factors influencing the distribution of upper trophic level species (e.g., seabirds and marine mammals) in the ocean[10-18]. Bathymetry is often considered a factor in affecting species distribution and abundance due to its relationship with upwelling fronts and eddies[19-21]. Recent studies summarize the situation in the southern “ocean” in general[22]or for Antarctic Minke whaleBalaenoptera bonaerensisin particular[20], or an aggregation of fin whalesB. physalus, southern fulmarsFulmarus glacoidesand grey-headed albatrossesThalassarche chrysostomaaround the South Shetland Islands[6].

Our study area covers the southwest sector of the Atlantic Ocean at the boundary of the Antarctic Circumpolar Current(ACC) and the Polar Front (PF), as reflected by signatures of water temperature (SST) and salinity. We progressed longitudinally between Patagonia and the Scotia Ridge,including the vicinity of South Georgia and South Sandwich Islands, and the South Sandwich Trench. During both the outward leg, originating from Punta Arenas, Chile, and inward leg, terminating at the Falkland Islands, we conducted surveys of marine birds and mammals during daylight hours attempting to relate the results to hydrological variables.

2 Material and methods

During the PS81 (ANT-XXIX/4) expedition of icebreaker RVPolarsternfrom Punta Arenas, Chile to the North Scotia Sea Ridge, South Georgia, and the South Sandwich Trench and back to the Falkland Islands from 22 March till 26 April 2013, three independent datasets were collected in order to establish the at-sea distribution of seabirds and marine mammals. The main dataset was collected from the port side of the bridge (hereafter “l(fā)eft”,n= 365), another from the starboard side (“right”,n= 282); in both cases, one observer either side tallied marine birds and mammals. A third data set was derived from helicopter ぼights in a few locations (lasting two hours each, height 300 feet, speed 80 knots; two to three observers;n= 8). The transect counts, from 18 m above sea level, were partitioned into 30 min segments, without width limitation on a continuous basis, light and visibility permitting (see description and discussion in[4-5]).

Sea surface temperature (SST) and salinity were continuously recorded using a thermo-salinometer at keel depth (-10 m). Ice cover was evaluated from the bridge and expressed as per-cent coverage within an approximated range of 500 m around the ship.

Using the generalized boosted regression modelling technique[23], we tested the power of using seabird abundance as a predictor of the presence or absence of marine mammals:fur seals and whales. Due to low number of whale sightings,we combined whale sightings and converted abundance measurements to binary presences and absences. We performed three model runs: (1) oceanographic parameters(SST, distance to shore and bathymetry) and seabirds as predictors; (2) only oceanographic parameters as predictors,and (3) only seabird abundance as predictors. Crossvalidation assessment of model accuracy was performed on random subsets of observations using the area under the receiver operating characteristic curve (AUC).

Basic results were included in the Biodiversity dataset:http://www.gbif.org/dataset/0e8b0e10-1680-4d71-ae93-f61bd7933b1d.

3 Results and discussion

During the 365 counts of the main dataset from the “l(fā)eft” side of the bridge, 129 cetaceans were encountered, belonging to seven species of which three were baleen whalesMysticeti:41 humpback whalesMegaptera novaeangliae, six fin whalesBalaenoptera physalusand six southern right whalesEubalaena australis(which totalled 22?sightings, including one individual seen twice.when adding tallies from helicopter ぼights; unidentified whales might belong to this species). Among pinnipeds, the vast majority were Antarctic fur sealsArctocephalus gazella(1635 out of 1650 individuals).Among seabirds, we counted 18900 individuals belonging to 36 identified species, not including the strictly coastal Patagonian ones. The most numerous species was Antarctic prionPachyptila desolatawith 9800 individuals (Table 1).

3.1 Count reliability

The result was a ratio of 1.3 for the five most abundant seabird species (min. 0.74, max. 1.73), 1.1 for the cetaceans(four species, 0.65 and 2.37) and 1.3 for the pinnipeds (two species, 1.84 and 4.81). We conclude that left and right datasets provided confidence in the reproducibility of our counting method.

3.2 Species occurrence patterns

The main water masses and fronts were recognised on the basis of SST (Figure 1) since salinity differences were very limited[24-27]. From North-West to South-East they were: Sub-Antarctic Surface Water (SASW), Antarctic Front (AF), Polar Front Water (PFW), Polar Front (PF) and Antarctic Surface Water (AASW) (Table 2). Many of the observed species distributions were limited by these hydrological features:chinstrap penguinPygoscelisantarctica, southern fulmarFulmarus glacoides, black-bellied storm-petrelFregetta tropicaand southern right whale were restricted to AASW;Antarctic prion and Antarctic fur seal to PFW; hourglass dolphinLagenorhynchus crucigerand humpback whale to PFW and AASW; black-browed albatrossThalassarche melanophrysand white-chinned petrelProcellaria aequinoctialisto SASW and PFW; and South American fur sealArctocephalus australisto SASW (Table 3, Figure 2).Areas of aggregation along the Scotia Ridge / Scotia Trench were comprised of Antarctic prions and Antarctic fur seals in six successive counts, southern right whales in four counts and humpback whales in five counts (Table 4).

Table 1 Seabirds and marine mammals encountered along the North Scotia Ridge and South Sandwich Trench; total numbers recorded; n= number of half-an-hour transect counts on board Polarstern, and number of helicopter ぼights; mean per count: seabirds for total＞ 10, per hour respectively

Continued

Figure 1 Near-surface water temperature (SST, °C) registered on board RV Polarstern along the North Scotia Ridge and the South Sandwich Trench, March – April 2013; noon position; main water masses and fronts: Sub-Antarctic Surface Water (SASW, SST ＞ 5°C,red); Antarctic Front (AF); Polar Front Water (PFW, 1.7°C ＜ SST ＜ 4.8°C, yellow); Polar Front (PF); and Antarctic Surface Water (AASW,SST ＜ 1.7°C, green).

Table 2 Main water masses and fronts detected along the North Scotia Ridge and South Sandwich Trench, and their SST and salinity values

Table 3 Seabirds and marine mammals encountered along the North Scotia Ridge and South Sandwich Trench, grouped per water mass; main seabird species (total ＞ 500); n = number of counts from the bridge (left); N: total number; mean: per count

Figure 2 Distribution maps for the main species encountered on board RV Polarstern along the North Scotia Ridge and the South Sandwich Trench, March – April 2013 based on the “l(fā)eft” bridge dataset; number per transect count; species symbols and classes of abundance indicated on the map; large cetaceans: fin Balaenoptera physalus, southern right Eubanaena australis and humpback Megaptera novaeangliae whales (a); fur seal, mainly Antarctic Arctocephalus gazella (b); Antarctic prion Pachyptila desolata, red circles,and unidentified prion Pachyptila sp., yellow crosses (c); chinstrap penguin Pygoscelis antarctica (d).

Table 4 Hotspot concentration of selected species close to South Georgia; number per count from the bridge (left)

Southern right whales were encountered from 29 March till 9 April, with most individuals being observed in five consecutive days (29 March to 2 April),i.e., between 54.5°S and 58.5°S, 25°W and 35°W, East and South-East of South Georgia corresponding to Antarctic Water (Figure 1,Tables 4 and 5). Most individuals of the three species were actively feeding. The right whales were detected during the south-eastern journey, but not the return journey along the same route. Their position was clearly linked to the strong frontal system (SACCF) with an abrupt change in water temperature close to 3°C (Table 2, Figure 1). They were often seen at the edge of medium-sized icebergs and approached the ship when stationary. Attraction to ships by southern right whales has been noted in the past (e.g., in December 1925 in South Georgia[28]). One individual seen during this expedition deserves special attention (Figure 3). This whale was photoidentified in October 1972 at Peninsula Valdès, Argentina as an adult and therefore must currently be at least 45 years old(see more detail in Reference [29]). A large concentration of humpback whales was also observed from a single helicopter flight (26 individuals at 59°S). Additional concentrations of Antarctic fur seals and Antarctic prions were detected in the same area. For these species this could correspond to the proximity to their main breeding colonies, and may reflect post-breeding aggregations (Table 4). Large numbers of chinstrap penguins were also observed on icebergs (more than 4000 on ten icebergs). Some chinstrap penguins were also seen swimming close to southern right whales, seemingly exploiting the same prey.

Figure 3 Southern right whale Eubalaena australis, 6 April 2013,58°S?27°W, close to an iceberg. Photo was taken by H. Robert.The same individual was photographed on 18 October 1972 at Peninsula Valdès (more detail in Reference [29]).

Table 5 Synopsis of the records of southern right whales Eubaleana australis along the N Scotia Ridge and the S Sandwich Trench,March-April 2013, from the different platforms; n = number of transect counts, of ぼights respectively

3.3 Modelling: importance of hydrological variables in characterizing hotspots

When we modelled fur seal and whale occurrence along transects, bathymetry, distance to shore, and abundance of Antarctic prions were the most important predictor variables.When using only oceanographic data, depth and SST were the top two variables explaining the presence of whales,while distance to shore and bathymetry were the top two variables for fur seal. When we limited the predictors to only seabird abundance, Antarctic prion was the top predictor variable for both whales and fur seal, while southern fulmar and king penguin were the second most important variables (for whales and fur seal respectively). Models that included both oceanographic data and seabird abundance as predictors showed the top two predictors for whales to be bathymetry and abundance of Antarctic prion, and for fur seal to be distance to shore and abundance of blue petrel(Table 6). AUC values were lowest for the models using only seabird abundance (0.75 and 0.74 for whales and fur seal respectively). Models using only oceanographic data had AUC values of 0.85 and 0.84 for whales and fur seal respectively. The most accurate models were those that combined both seabird abundance data and oceanographic data (0.89 and 0.85 for whales and fur seal respectively;Table 6). Our models found that high concentrations of whales, fur seals and Antarctic (＞150 birds) coincided when compared to presences and absences of both mammal groups. However, whales were commonly located in deeper waters (mean 4424 m), while fur seal were mostly present in shallower waters (mean 1268 m; Table 7).

Table 6 Top two predictor variables contributing to presences of whales and fur seal from generalized boosted regression models for models run using only oceanographic data, only seabird data, and oceanographic data combined with seabird data. Area under the receiver’s operating characteristic curve (AUC) values are reported to assess model accuracy

Table 7 Means and standard deviations (in brackets) of the most important predictors for whales and fur seal (abundance of Antarctic prions and bathymetry) at locations where they were observed (Presences), and not observed (Absences)

3.4 Discussion

Southern right whale observations can be considered significant with a total of 22 individuals (all platforms pooled), as the global population is estimated to be only 7500 individuals. The ocean around South Georgia represents their main summering ground, while breeding populations were estimated for Peninsula Valdès as 2600 (in 1997) and in South Africa as 3600 (in 2008)[30-32]. During the whaling era (1785-1913), the distribution of southern right whales was defined as 20°S and 57°S, mainly between 30°S and 50°S[33], with the area north-west of South Georgia recognised as important[34].The majority of southern right wales, fur seals and prions as well as fin and humpback whales were concentrated in a very limited region (i.e., a “hotspot” clearly delimited in space and time). Such aggregations can be considered reflecting high local concentration of their prey[1-6,35]. The position of the fronts and water masses can strongly vary on a small time scale, and thus inぼuence seabird abundance[36]. This system also happens to inぼuence large-scale krill transport possibly from the west of the Antarctic Peninsula to South Georgia[37].

Most whales and penguins were encountered around free-drifting icebergs. Their presence could be linked to higher local prey abundance (mainly krillEuphausia sp.),due to higher primary production under and close to freedrifting icebergs[38], the basic mechanism being that meltwater can cause a vertical upwelling transport for nutrients and therefore increase primary production, zooplankton and seabirds (synopsis in Reference [39]). Previous work has highlighted a similar association between crabeater sealsLobodon carcinophagusand icebergs[40]. Icebergs can also provide a certain protection against predators such as leopard sealsHydrurga leptonyxand/ or killer whalesOrcinus orca.

Seasonal factors might be of importance for large cetacean aggregations, namely their seasonal migration towards the tropics in autumn, as well as for seabirds[41].They could also be correlated to the movements of adult krill towards the shelf slope in autumn[42]. Two large groups of humpbacks (～100 individuals) were observed on 22 January 2008 around 62°S, 0°E (Polarsternexpedition PS71,HR). Another major autumn feeding ground for fin whales,southern fulmars and grey-headed albatrossesThalassarche chrysotomawas detected around the South Shetland Islands[6]. A comparable high aggregation was observed in the Western Antarctic Peninsula in May 2009, with more than 300 humpback whales encountered in 65 km of transect,corresponding to an exceptionally large krill swarm[43].

Comparisons with other polar marine ecosystems based on data collected by the same team, same platform and same counting method allow us to express a kind of rough measurement for biodiversity based on number of species and individuals. In the Weddell Sea during the first leg of the EPOS 1??First European Polarstern Study.expedition, 31 seabird species were observed with a mean of 150 individuals per count[7]. In the South Shetland Islands area, 40 species were recorded for a mean of 340 individuals per count, and maximal value of 485 individuals[6].In the South Georgia area, 37 species were represented with a mean of 50 per count (Table 1) and a maximal value of 1320 per count in seven counts close to the Antarctic Front (5000 in one count, Table 4). We consider that these data reぼect a kind of low biodiversity index for the Antarctic marine areas characterized by small numbers of species and large number of individuals for some of them. Qualitative differences were detected as well, Antarctic prion being the most numerous species in this study, southern fulmar in the South Shetland,Antarctic petrelThalassoica antarcticain the south-eastern Atlantic, and Adélie penguinPygoscelis adeliaeand Cape petrelDaption capensein the Weddell Sea.

This study further investigated how seabirds could be used to predict the occurrence of whales and fur seal. Models using just seabirds proved reasonable with AUC values that are greater than 0.74. Of importance here is that a model combining seabirds and oceanographic data provided much better results than either oceanographic data or seabird data alone. This provides evidence towards the use of seabird data to help project the distributions of marine mammals (andvice versa). Ecosystem-style modelling efforts, where all species are taken into account simultaneously, would likely improve the management situation in the Southern Ocean.

3.5 Conservation

The South Georgia and South Sandwich Island Marine Protected Area, established in 2012 and covering more than one million km2, includes a large “no take zone” of 12 nmiles around each island. The objectives of the protected area already include the conservation of the environment, the restoration of native biodiversity, the sustainable management of fisheries to ensure minimal impact on non-target species and it underlines the need to encourage high quality scientific research[44]. Together with the seasonal closure of krill fishery, it was an essential step toward reducing potential competition between marine mammals, seabirds and krill, in an area previously known as a very important krill fishery[45]. Observations outlined in this paper stress again the international conservation importance of the South Sandwich Islands and South Georgia area for krill and top predators-seabirds and marine mammals especially southern right whale. An appropriate conservation strategy should ensure that krill fishery does not cause local reduction in krill abundance in key periods,i.e., at least during the breeding and post-breeding seasons of top predators. This area therefore deserves more protection and management measures, with spatial and temporal extension of the protected zone. These measures should include restrictions in krill fisheries and of“scientific” whaling.

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