Cephalopods and mesoscale oceanography at the Antarctic Polar Front:satellite tracked predators locate pelagic trophic interactions

Abstract

Predator data and exploratory fishing in the Scotia Sea have revealed the presence of cephalopod stocks in the Antarctic Polar Frontal Zone (PFZ). This is a vast, remote region where large epipelagic cephalopods aggregate into highly mobde schools making them difficult to locate and sample. We used satellite tagged predators and shipboard acoustics for coarse and fine scale location of cephalopod concentrations, and sampled them with commercial and scientific nets to determine the relationship between cephalopod distribution and mesoscale oceanographic features at the PFZ. Satellite tags were attached to 9 grey-headed albatrosses Diomedea chrysostoma, breeding at Bird Island, South Georgia, to monitor foraging at sea in January-March 1994. A foraging area at the PFZ, north of South Georgia, was located, an acoustic survey undertaken and a fixed station established where acoustic targets were found. A net survey was carried out with a commercial pelagic trawl, a rectangular midwater trawl 25 mZ (RMT25), a horizontal multiple plankton sampler and a neuston net. Acoustic layers were targeted and the RMT25 sampled 200 m layers to 1000 m in daylight and darkness. Cephalopods were sinlultaneously recovered from food samples fed to D chrysostoma chicks at Bird Island. Two CTD transects, approximately normal to the major current flow, were undertaken across the PFZ and remote-sensed sea-surface temperature images from NOAA polar orbiting satellites were obtained aboard ship. The pelagic trawl sampled a cephalopod community that closely resembled that exploited by D. chrysostorna. The largest and most conspicuous species was the ommastrephid squid Martialia hyadesi which is the most important cephalopod prey species. Net-sampled M. hyadesi had been feeding on crustaceans and mesopelagic fish. The cephalopod community was sampled in a feature, interpreted as a warm core ring, in an area characterised by mesoscale features associated with the bathymetry of the northern end of the Northeast Georgia Rise and near a gap in the Falkland Ridge. The association of these mesoscale features with the bathyrnetry suggests that they may be predictable foraging locations for the cephalopods and their predators.

Review article: Thirty years of analysing and modelling avian habitat relationships using satellite imagery data: a review

“…Ginati et al 1995, Hatch et al 2000, Berthold et al 2001, Hashmi et al 2001, Watzke et al 2001, only three of the papers reviewed by us referred to these techniques (Jouventin et al 1994, Guinet et al 1995, Rodhouse et al 1996. This may be due to current inaccuracies in location determination, which actually lies at best around ¡30-150 m and would only allow broad-scale analyses.…”

“…Mostly, simple counts of sightings of bird species or their nests were conducted. This was carried out in systematic and non-systematic ways, and sometimes with the help of tools such as aircraft (Briggs et al 1984, Hodgson et al 1987, 1988, Haney 1989b, helicopter (Wallin et al 1992, Herr and Queen 1993, Morrison 1997, Jenkins et al 2003a, boat (Briggs et al 1984, Haney and McGillivary 1985a, Briggs et al 1987, Haney 1989b, Huettmann and Diamond 2001, Yen et al 2004, canoe (Groom and Grubb 2002), radio telemetry (Young et al 1987, Hunsaker et al 2002, Fearer and Stauffer 2003 or satellite telemetry (Guinet et al 1995, Jouventin et al 1994, Rodhouse et al 1996. A direct link between bird breeding sites and satellite imagery was applied by Guinet et al (1995) and Schwaller et al (1984Schwaller et al ( , 1986Schwaller et al ( , 1989.…”

“…With the help of NOAA AVHRR imagery often the actual or integrated normalized difference vegetation index (NDVI) was calculated to produce maps of primary production or to detect green biomass (Wallin et al 1992, Andries et al 1994, Hepinstall and Sader 1997, Verlinden and Masogo 1997, Osborne et al 2001, Saveraid et al 2001, in order to detect habitat change (Sader et al 1991, Debinski et al 2000, to map wetland distribution (Roshier et al 2001) or to explain the distribution of centres of bird endemism (Johnson et al 1998). The NOAA AVHRR Climate Data Set incorporating values of atmospheric temperatures and sea surface temperatures in the Atlantic and the Pacific, were used for instance by Haney (1989b), Jouventin et al (1994), Rodhouse et al (1996), Huettmann and Diamond (2001), Meyer et al (2002) and Yen et al (2004) to analyse seabird habitats or to model their distribution. A radar ERS image was applied by Thibault et al (1998) to investigate ice-cover on a river.…”

“…Except for nine studies (Priede 1983, Haney and McGillivary 1985a, Briggs et al 1987, Haney 1989b, Jouventin et al 1994, Guinet et al 1995, Rodhouse et al 1996, Huettmann and Diamond 2001, Yen et al 2004, the use of remote sensing imagery for bird habitat investigations deals with terrestrial habitats. Most studies do not provide an exact percentage of the different types of habitat that are investigated.…”

School characteristics of mesopelagic fish at South Georgia

“…Furthermore, our study focussed only on acoustic targets that formed discrete and regular schools, which reduces the likelihood of siphonophores biasing our results because they do not aggregate densely. Although squid, such as Martial hyadesi and Loligo gahi, have been reported around South Georgia, particularly during austral winter (June), these organisms have a distribution that is highly associated with the Antarctic Polar Front and are found predominantly in regions beyond the WCB (Anderson and Rodhouse, 2001;Gonzalez and Rodhouse, 1998;Rodhouse et al, 1996). Ground-truthed acoustic observations also showed that aggregations of squid in the region tended to have a relatively weak acoustic response and exhibit positive S v120-38 kHz backscatter (Goss et al, 2001;), so we advocate that squid were an unlikely source of bias in the study.…”