Abstract

Rodhouse, P. G., and Boyle, P. R. 2010. Large aggregations of pelagic squid near the ocean surface at the Antarctic Polar Front, and their capture by grey-headed albatrosses. – ICES Journal of Marine Science, 67: 1432–1435.

Satellite-tracked squid predators and fish-finding acoustics were used to locate squid concentrations at the Antarctic Polar Front, then to sample them with a midwater trawl. Near-surface hauls were dominated by the squid Martialia hyadesi similar in size to those fed to grey-headed albatross chicks. The characteristics of the squid and their proximity to the surface suggest that the birds locate squid concentrations by olfaction and catch them by plunging.

Introduction

How pelagic birds such as albatrosses, which are well adapted for soaring and gliding, are able to detect and capture deep-living, jet-propelled squid has long been a mystery and the subject of much theory and speculation. Although some albatross species can plunge to depths of about 10 m (Croxall and Prince, 1994), there has been little evidence that squid approach that close to the surface in large enough numbers to provide a reliable food source. It has been suggested that albatrosses feed primarily on species of squid with photophores, which can be detected visually (Imber, 1992), that they feed only on moribund post-spawning squid floating at the surface (Lipiński and Jackson, 1989) and, at least for wandering albatrosses (Diomedea exulans), that they consume squid regurgitated at the surface by sperm whales (Clarke et al., 1981).

Extensive research on the ecological relationships between albatrosses breeding at Bird Island, South Georgia, and their pelagic prey has revealed a considerable amount of information about predation on cephalopods (Collins and Rodhouse, 2006, and references therein). Here we describe new observations which, together with other assembled evidence, lead us to suggest how grey-headed albatrosses (Thalassarche chrysostoma, formerly Diomedia chrysostoma) might locate and capture their cephalopod prey, especially the ommastrephid Martialia hysadesi, at the Antarctic Polar Front (APF). Thalassarche chrysostoma is classified as vulnerable on the IUCN Red List 2004 and listed in Appendix II of the Convention on the Conservation of Migratory Species.

Material and methods

Three platform terminal transmitter (PTT) deployments on chick-rearing grey-headed albatrosses at Bird Island, South Georgia (Figure 1), were made in January 1996 to correspond with a biological research cruise on RRS “James Clark Ross”. On 25 January, three PTTs were deployed on parent birds rearing chicks. In late January, one PTT stopped transmitting, but the other two operated successfully throughout the cephalopod sampling phase of the cruise. Positions from the PTTs continued to be collected until after the ship had left the cephalopod sampling area, providing temporal coverage of 26 d.

Figure 1.

Polar projection of the Southern Ocean showing the location of South Georgia (arrow).

Figure 1.

Polar projection of the Southern Ocean showing the location of South Georgia (arrow).

A standard commerical pelagic midwater trawl (PMT), carrying a netsonde for gauging depth, was modified for deployment. A reinforced rubber paravane was fitted to the headline of the net, and extra weight (chain) was added to the footrope to increase the height of the net opening. Following these modifications, it was estimated that the net would work with a mouth (height) opening of 25–30 m. In comparison, a commercial net such as that used in the Illex argentinus high-seas fishery off the Patagonian Shelf would operate a net opening of about 100 m height and 150 m width.

Results and discussion

Position fixes received before the cephalopod sampling phase revealed that two birds were foraging in a region of the Polar Frontal Zone (PFZ) bounded by 48°15′–51°15′S and 37°15′–42°35′W. The foraging tracks of these birds were overlaid on false colour images of sea surface temperature (Figure 2a) and bathymetry (Figure 2b).

Figure 2.

(a) False colour sea surface temperature image of the Antarctic Polar Frontal Zone at the time of the sampling programme [Physical Oceanography DAAC; AVHRR Oceans Pathfinder Global Equal-angle Best SST (NOAA/NASA); NASA JPL Physical Oceanography DAAC, Pasadena, CA, February 1996). (b) False colour bathymetric image of the same area (IOC, IHO, and BODC, 2003; Centenary Edition of the GEBCO Digital Atlas, published on CD-ROM on behalf of the Intergovernmental Oceanographic Commission and the International Hydrographic Organization as part of the General Bathymetric Chart of the Oceans; British Oceanographic Data Centre, Liverpool). South Georgia is located in the southeast corner of the images, and Bird Island lies off the western tip. Overlaid are the tracks of three satellite-tagged grey-headed albatrosses, a box showing tracks of pelagic midwater trawl hauls, and the mean position of the Antarctic Polar Front (from Moore et al., 1997; Trathan et al., 1997). In (b), the arrows show the Maurice Ewing Bank to the north and the North Scotia Arc to the south.

Figure 2.

(a) False colour sea surface temperature image of the Antarctic Polar Frontal Zone at the time of the sampling programme [Physical Oceanography DAAC; AVHRR Oceans Pathfinder Global Equal-angle Best SST (NOAA/NASA); NASA JPL Physical Oceanography DAAC, Pasadena, CA, February 1996). (b) False colour bathymetric image of the same area (IOC, IHO, and BODC, 2003; Centenary Edition of the GEBCO Digital Atlas, published on CD-ROM on behalf of the Intergovernmental Oceanographic Commission and the International Hydrographic Organization as part of the General Bathymetric Chart of the Oceans; British Oceanographic Data Centre, Liverpool). South Georgia is located in the southeast corner of the images, and Bird Island lies off the western tip. Overlaid are the tracks of three satellite-tagged grey-headed albatrosses, a box showing tracks of pelagic midwater trawl hauls, and the mean position of the Antarctic Polar Front (from Moore et al., 1997; Trathan et al., 1997). In (b), the arrows show the Maurice Ewing Bank to the north and the North Scotia Arc to the south.

The foraging tracks of the birds were used to identify a suitable study area, and seven PMT hauls were carried out. The characteristics and squid catch data for each haul are listed in Table 1. All but two hauls caught squid, and the samples were dominated by Martialia hyadesi.

Table 1.

Data from seven net hauls made with the pelagic midwater trawl at the Antarctic Polar Front.

Haul Event Date Time UTC Depth (m) Start and end position Squid species caught (number/ML range, mm) 
415 10 Feb 15:23–16:15 150 50°44.30′S 40°1.23′W to 50°46.78′S 40°15.95′W None 
429 11 Feb 11:10–13:21 250 50°12.85′S 40°47.18′W to 50°9.54′S 40°30.33′W None 
435 11–12 Feb 23:00–11:12 25 50°16.98′S 40°37.27′W to 49°22.11′S 39°50.09′W Martialia hyadesia (∼2 300/see Table 2
461 13 Feb 01:51–03:20 20 50°9.58′S 40°43.51′W to 50°23.4′S 40°52.58′W Martialia hyadesi (30/140–220), Slosarczykovia circumantarctica (2/98 and 100), Galiteuthis glacialis (1/230), Kondakovia longimana (1/385) 
   03:20–05:45 100   
5b 468 13 Feb 17:52–19:37 130 49°32.03′S 39°56.08′W to 49°45.07′S 40°12.04′W Martialia hyadesi (28/195–264), Slosarczykovia circumantarctica (1/173), Galiteuthis glacialis (1/ 179), Kondakovia longimana (1/173), Moroteuthis knipovitchi (1/152) 
   19:37–20:06 80   
   20:06–22:00 20   
6c 484 14 Feb 08:40–10:09 150 49°36.35′S 39°49.83′W to 49°42.13′S 39°54.92′W Martialia hyadesi (7/170–233) 
7d 504 16 Feb 00:30–01:16 60 49°41.60′S 39°31.85′W to 49°29.68′S 40°02.89′W Martialia hyadesi (36/183–252), Gonatus antarcticus (3/154–225), Slosarczykovia circumantarctica (1/115), Galiteuthis glacialis (2/240, 265) 
   01:25–03:21 20   
   03:21–03:45 70   
   03:45–05:30 100   
Haul Event Date Time UTC Depth (m) Start and end position Squid species caught (number/ML range, mm) 
415 10 Feb 15:23–16:15 150 50°44.30′S 40°1.23′W to 50°46.78′S 40°15.95′W None 
429 11 Feb 11:10–13:21 250 50°12.85′S 40°47.18′W to 50°9.54′S 40°30.33′W None 
435 11–12 Feb 23:00–11:12 25 50°16.98′S 40°37.27′W to 49°22.11′S 39°50.09′W Martialia hyadesia (∼2 300/see Table 2
461 13 Feb 01:51–03:20 20 50°9.58′S 40°43.51′W to 50°23.4′S 40°52.58′W Martialia hyadesi (30/140–220), Slosarczykovia circumantarctica (2/98 and 100), Galiteuthis glacialis (1/230), Kondakovia longimana (1/385) 
   03:20–05:45 100   
5b 468 13 Feb 17:52–19:37 130 49°32.03′S 39°56.08′W to 49°45.07′S 40°12.04′W Martialia hyadesi (28/195–264), Slosarczykovia circumantarctica (1/173), Galiteuthis glacialis (1/ 179), Kondakovia longimana (1/173), Moroteuthis knipovitchi (1/152) 
   19:37–20:06 80   
   20:06–22:00 20   
6c 484 14 Feb 08:40–10:09 150 49°36.35′S 39°49.83′W to 49°42.13′S 39°54.92′W Martialia hyadesi (7/170–233) 
7d 504 16 Feb 00:30–01:16 60 49°41.60′S 39°31.85′W to 49°29.68′S 40°02.89′W Martialia hyadesi (36/183–252), Gonatus antarcticus (3/154–225), Slosarczykovia circumantarctica (1/115), Galiteuthis glacialis (2/240, 265) 
   01:25–03:21 20   
   03:21–03:45 70   
   03:45–05:30 100   

aMany of the squid were in fresh condition (mantles pulsating, chromathophores active), indicating that they had entered the trawl during the latter part of the haul.

bDuring daylight, a persistent acoustic layer at about 15 m, overlain by dense acoustic marks, was observed. This haul was designed to target the marks by fishing the PMT along the top of the layer then track them up through the water column in the late afternoon, as they dispersed towards the surface.

cThis haul was intended to sample the acoustic marks near the top of the acoustic layer at around 150 m in daylight.

dHaul planned to cross approximately normal to the latter part of Event 435, where the largest haul of M. hyadesi was presumed to have been taken. This was a haul in darkness that started to the east, where some recent grey-headed albatross positions had been recorded, and fished towards the west over a putative cold filament from the APF. On hauling, the headline of the net was found to have parted, resulting in a long tear in the net.

The basic morphometric parameters of the Martialia hyadesi sample are listed in Table 2. The squid were mostly at an early stage of sexual maturation (according to the scale of Lipiński, 1979; females II–III and male II). A few immature specimens were also present. The mantle length (ML) of these M. hyadesi was similar to those being fed to chicks at Bird Island in studies by Rodhouse et al. (1990, 1996) who estimated ML from beak size using the allometric equation given in Rodhouse and Yeatman (1990).

Table 2.

Basic morphometric parameters for Martialia hyadesi sampled with the pelagic midwater trawl during five hauls that caught squid.

Event Mantle length (mm ± s.d.) Maturity stage male/female 
435 229.6 ± 21.8 II/II–III 
461 173.1 ± 22.8 II/II 
468 225.8 ± 17.2 II/II–III 
484 212.0 ± 21.3 II/II–III 
504 223.7 ± 19.3 II/II–III 
Event Mantle length (mm ± s.d.) Maturity stage male/female 
435 229.6 ± 21.8 II/II–III 
461 173.1 ± 22.8 II/II 
468 225.8 ± 17.2 II/II–III 
484 212.0 ± 21.3 II/II–III 
504 223.7 ± 19.3 II/II–III 

The haul that caught most squid was the second haul in the series (Event 435), and was made close to the surface at night. The catch in this, and indeed in all other successful hauls, was dominated by Martialia hyadesi, the dominant cephalopod in the diet of grey-headed albatrosses foraging at the APF (Rodhouse et al., 1990, 1996). It has also been caught in the area by commercial squid-jiggers during exploratory fishing trials (Rodhouse, 1991; González and Rodhouse, 1998). Although commercial jiggers attract squid to the surface using lights, the results of the net hauls reveal that the squid are found naturally near the surface in large numbers at night.

We suggest that our observations throw new light on how grey-headed and probably other albatrosses locate and catch squid at the APF. Although the squid approach the surface at night, we discount the possibility that the albatrosses locate the squid visually by the light produced by photophores, because only one species (Galiteuthis glacialis) of those caught (Martialia hyadesi, Slosarczykovia circumantarctica, Galiteuthis glacialis, Kondakovia longimana, Moroteuthis knipovitchi, and Gonatus antarcticus) possesses photophores and those are only on the eyes. The main prey of the birds, M. hyadesi, has no photophores at all. It is also clear that the birds are not preying on moribund post-spawning squid, because all the M. hyadesi in the net samples were at maturity stage III or less. We speculate that the birds might detect the squid by olfaction. Fishers off the Shetland Islands can identify the presence of concentrations of another ommastrephid squid (Todarodes sagittatus) by the presence of an oily slick at the sea surface, which is probably released when the squid are feeding on oil-rich prey (Joy, 1990). Albatrosses possess a highly sensitive olfactory system, and Nevitt et al. (2004) demonstrated that birds are attracted to slicks of organic oil scented with extracts of prey species. Given the oily nature of squid prey, particularly that of M. hyadesi, which feeds on myctophid fish and euphausiids (Rodhouse et al., 1992; González et al., 1997), it is likely that chemical cues in the oil released by the feeding squid floats to the surface in sufficient concentration to be detectable by the seabirds. We know that grey-headed (and black-browed, Thalassarche melanophrys) albatrosses can plunge to a depth of 6 m in pursuit of prey (Croxall and Prince, 1994) and can therefore capture squid once they have been detected near the surface.

Our observations at the APF lead us to reject the possibility that grey-headed albatrosses detect ommastrephid squid at night using visual cues, or that they feed on moribund or dead post-spawners, or that they scavenge regurgitated squid from sperm whales, as suggested by Clarke et al. (1981) for wandering albatrosses.

Acknowledgements

We thank Nathan Cunningham for retrieving the data held by the BAS Polar Data Centre and for plotting the images in Figure 2, and Phil Trathan at BAS for critical comments on the manuscript. We also thank Ángel Guerra and Earl Dawe for their helpful suggestions on how to improve the manuscript and the officers and crew of RRS “James Clark Ross” for their commitment and patience when deploying an unfamiliar, very large trawl net. The second author (PRB) sadly passed away in April 2009 having bravely endured a long illness. His career in cephalopod research was an inspiration to his fellow marine scientists, and he is already sorely missed when squid research is discussed.

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