AMLR 2003 Weekly Report No. 11

23 March 2003

 

1. The R/V Yuhzmorgeologiya is currently northwest of King George Island conducting a trawl survey of demersal finfish, net and acoustic based characterization of invertebrate fauna, and habitat classification within the 500 m isobath of the South Shetland Islands, Antarctica.

 

2. We have completed 44 stations to date. On 20 March, we completed the Elephant Island component of the survey (N=32 Stations), and arrived the following morning to start our first deepwater stations off King George Island in the Lower South Shetland Islands. Most targeted stations on the Elephant Island Shelf were successfully completed, with the exception of some shallow stations on the shelf northwest of Elephant Island, where heavy concentrations of grounded icebergs prevented access to the targeted station coordinates. After searching for alternate suitable grounds west of Elephant Island and north of Aspland Island, most of these stations were relocated and successfully sampled according to the experimental design. Progress has been excellent due to favorable weather conditions, and at present we are ahead of schedule.

 

3. As of 23 March, a total of 4774.6 Kg of finfish (9,956 individuals) of 38 species have been captured and processed. Our greatest combined yields have occurred at stations on the western and northwestern shelf of Elephant Island and north of King George Island within the 100-200 m depth strata. The highest diversity of finfish species occurred north and east of King George Island within the 300-400 m depth strata. Other areas with high species diversity occurred north and east of Elephant Island. The species with the greatest yield in weight as of 23 March has been Gobionotoghen gibberifrons (1380 kg), followed by Notothenia coriiceps and two species of icefish: Champsocephalus gunnari and Chaenocephalus aceratus. The greatest catch in numbers has been C. gunnari followed by G. gibberifrons and the myctophid Gymnoscopelus nicholsi. The most frequently encountered species has been G. gibberifrons, followed by C. gunnari and C. aceratus.

 

4. The dominance of certain fish species within depth strata during the second week on the Elephant Island shelf was consistent with observations from the previous week. Around Elephant Island, C. gunnari and C. aceratus formed the most abundant species at shallow water stations (< 200 m). In deeper waters, C. gunnari declined in abundance and other species, such as C. rastrospinosus and G. gibberifrons, became more abundant and dominant. Catches of G. gibberifrons at all depth strata have thus far demonstrated an absence of juveniles. High-Antarctic species appeared in catches below 250 m; however, with the exception of C. rastrospinosus, always in small numbers. Two families of the suborder Notothenioidei which have primarily a high-Antarctic distribution, Bathydraconidae and Artedidraconidae, have been represented by three species: Gymnodraco acuticeps, Parachaenichtys charcoti, and Artedidraco skottsbergi. Species within these two families do not appear north of the southern Scotia Arc. A small number of rare high-Antarctic icefish species have been captured, such as Pagetopsis macropterus and Chionobathyscus dewitti. Important meso-pelagic fish species, Gymoscopelus nicholsi and Electrona antarctica, have appeared in catches below 300 m. Other meso-pelagic species taken in small numbers were E. carlsbergi, G. braueri and G. opistoptherus.

 

5. Catches of finfish off King George Island within specific strata have been similar in species composition to those of Elephant Island. C. gunnari appears to have declined in abundance relative to Elephant Islands. A dense concentration of large (>55 cm) sexually mature (stage 3) N. coriiceps was encountered at a station north of King George Island. The spatial distribution of this pre-spawning aggregation of N. coriiceps was also observed during the AMLR 1998 and 2001 South Shetland Islands surveys, suggesting that this area appears to be an important spawning ground for this species.

 

6. Results from investigations on the reproductive state of the abundant species indicated that the spawning season was slightly more advanced in the South Shetland Islands than at Elephant Island. Most C. gunnari had spawned already, and the proportion of spent C. rastrospinosus was higher than observed in all previous surveys.

 

7. Diet studies have been conducted on 2,158 individuals of most species encountered. C. gunnari have preyed on almost 100% krill. Feeding intensity in C. aceratus and Cryodraco antarcticus was low. They preyed on krill when small and fish as size increased. C. rastrospinposus had primarily empty stomachs when in prespawning and spawning condition. Spent individuals took krill and fish to a lesser extent. Gobionotothen gibberifrons fed on a variety of benthic invertebrates, but also took salps, jellyfish and tunicates, and to a small extent krill. Notothenia coriiceps took primarily krill and to a lesser extent fish and occasional benthic invertebrates.

 

8. We have continued collecting buoyancy data, ecomorphological variation, and material for molecular phylogenetic analysis for over 220 specimens, representing 23 notothenioid finfish species. The investigation of buoyancy variation among notothenioid species has benefited greatly from the collection of several rare high Antarctic species. Examples include Chionobathyscus dewittii, Artedidraco skottsbergi, Gymnodraco acuticeps, Neopagetopsis ionah, and Pagetopsis macropterus. We have conducted preliminary analyses of both the ecomorphological and buoyancy data and are finding interesting correlates with both diet and phylogenetic relationships.

 

9. We continued collecting blood and tissues samples from representative notothenoid species in the trawls. Of special interest are those nototheniids found predominantly in the high latitudes because understanding their freezing avoidance response to the warmer more northerly waters may correlate with their abundance in the trawl survey.

 

An excellent sampling of blood was made from 10 each large (25 cm) and from small (15 cm) (Pleuragramma antarcticum) that were caught in one trawl. Analysis of freezing points should show whether juveniles and adults utilize similar freezing avoidance strategies in waters distance from ice shelves where it has been reported that the juveniles are much more freezing resistant than the adults.

 

10. Krill abundance and dispersion is being acoustically mapped at each trawl location and a mean Nautical Area Scattering Coefficient (NASC) value is calculated for each station. NASC values may also be thought of as the number of krill (m2) per 1 nm2 of sea surface. For the stations surrounding Elephant Island, NASC values range between 3.5 and 2,491 with greatest concentrations of krill along the 500 m isobath along the north/northwest side of the island and also along the 300 m isobath on the shelf southeast of the island. Moderate concentrations are visible due west of the island between the 500 m and 1000 m isobath.

 

There are investigations underway to assess the feasibility of obtaining statistically valid, length frequency distributions of fresh krill from the stomachs of fishes that are primarily krill predators. Early comparisons between length frequency distributions from the IKMT trawls during Legs I and II and between Leg III stomachs are encouraging. If this method proves acceptable, it will then be possible to provide a biomass estimate for krill at each station.

 

11. The Questar Tangent seabed classification system continues to be inoperable due to software/hardware incompatibilities. We are awaiting the company to provide a patch that will allow their software to function as advertised with our computer systems. The raw acoustic waveform data is successfully being collected and prepared for processing while we are underway. Ground truthing of the acoustic seabed classification catalogue continues with nightly video/still camera transects and sediment grabs. To date, 1,028 high resolution (2100 1700 pixel) digital still images and > 8 hours of color video have been obtained from the nightly seabed/habitat transects.

 

12. While the primary goal of the imagery work is to groundtruth the acoustic seabed classifier, other studies benefit from this work. Seafloor still photos from 7 of the 12 photo-transects made so far have been subject to a preliminary analysis focusing on the most conspicuous elements of the benthic communities. An index of photos is being developed by which images among the 1000 or so already taken can be targeted for content such as large sponge masses, conspicuous echinoderm taxa (some of which can be identified to species), and fish. Initial and rough characterization of bottom type are also made for each photo in which taxa of interest are seen. This information will be used not only to qualitatively assess the behavior and distribution of target taxa, but to aid in later implementation of automated community- and bottom-typing classification algorithms.

 

13. The stations so far sampled have returned more than 10 metric tons of invertebrate bycatch from the bottom trawl, of which the overall greatest percentage by weight, 76%, has been sponges. Echinoderms (9%) and tunicates (6%) were the next two dominant taxa. Sponges dominated the biomass in 21 of the tows and echinoderms in 10. Tunicates, cnidarians and molluscs (octopus) dominated in a few tows each. In terms of diversity, stations at the northeast end of Elephant Island had the highest species counts (up to 76 species) while stations north and west of Elephant Island had the lowest with as few as 24 species. Sponge communities tended to have higher overall species counts than communities with high relative biomass of echinoderms, though often the echinoderm-dominated communities included single representatives of many species, and up to 35 species of seastars. Still and video images of the seafloor in areas similar to trawl paths confirm the selectivity of the trawl capture of invertebrate species. The Little BIT (Benthic Invertebrate Trawl) continues to sample these communities very differently. Higher proportions of smaller and infaunal taxa are evident in Little BIT hauls than in the bycatch from the fish trawls. The variation in representation by major taxonomic groups in the Little BIT seems to follow more closely the community structure in the seafloor photos. The most dramatic difference between bycatch and Little BIT sampling regimes is expressed in the number of ophiuroids, which frequently dominate by both number and weight in the latter. Similarly, tubicolous polychaetes are abundant in the Little BIT samples, but almost completely absent in the fish trawl bycatch.

 

 

C. Jones sends