AMLR 2006 Weekly Report No. 10
19 March 2006
1. The R/V Yuhzmorgeologiya is currently in
the Straits of Magellan, enroute to
2. We successfully completed 64 trawl stations
using a random depth stratified sampling design (5 strata between 80-500 m). Of
these hauls, 62 hauls were conducted off the
3. A total of 1918 Kg of finfish (7990 individuals) of 52 species were captured and processed. The finfish species with the greatest biomass was Gobionotothen gibberifrons, followed by Chionodraco rastrospinosus. The species with the greatest number of individuals was G. gibberifrons, followed by Trematomus newnesi, and Pleuragramma antarcticum, while the most ubiquitous species was C. rastrospinosus (encountered at 59 of the 64 stations).
4. The dominant element of the Antarctic fish
fauna both in terms of biomass and numbers is the suborder Notothenioidei
The benthic fish fauna on the shelf of the western tip of the
5. The other two families of the suborder, the Bathydraconidae (dragon fish) and the Artedidraconidae
(plunderfish), were poorly represented
in our samples. The only bathydraconids caught were Gymnodraco acuticeps, Parachaenichthys charcoti and
Gerlachea australis. Artedidraconids were represented by Artedidraco skottsbergi and members of the genus Pogonophryne. A
third and fourth faunal element with a very low biomass but a larger number of
individuals were pelagic myctophids (5 species of the
genera Gymnoscopelus, Electrona
6. Reproductive investigations of icefish - Gonads of Chaenodraco wilsoni were all in resting stage (stage 2). A small proportion (< 2%) of the fish had gonads in transition from stage 5 (spent) to stage 2 which indicates that spawning had been completed by early January. Another abundant icefish species, Cryodrao antarcticus, was mostly represented by fish less than 45 cm long which were either juvenile fish or males in an early stage of maturation, indicating that the maturation process takes longer than one year as has been described also for C. aceratus in the low-Antarctic (Everson et al., 1997) which occupies the same ecological niche. The few males in pre-spawning and spawning condition were all larger than 45 cm. The few adult females were longer than 53 cm. They were either in pre-spawning condition (stage 3) or were spent already (stage 5). This was consistent with previous results which pointed at February-March as the spawning season for C. antarcticus (Kock and Jones, 2002). Chionodraco rastrospinosus were caught in pre-spawning, spawning and post-spawning condition which suggested that the species was in the middle of the spawning season. Oocyte diameter of running ripe fish was 4.7-4.9 mm.
7. Reproductive investigations among the nototheniids - the few females of Trematomus pennellii had spent ovaries. Spawning had probably occurred 4 weeks earlier. T. hansoni and L. squamifrons were close to spawning. T. eulepidotus, T. newnesi, L. nudifrons, and N. coriiceps were likely to commence spawning in 3-6 weeks’ time. Gonads of L. larseni were in an early stage of maturation. They do not commence spawning before late June. Gonads of G. gibberifrons were in resting stage which confirms observations from other areas in the southern Scotia Arc that the species is spawning in austral winter (August-September).
8. Stomach content analyses (stomach content wet weight, filling degree, degree of digestion, species composition of the diet) was conducted for most icefish and nototheniid species captured during the course of the survey. Feeding intensity was high in C. wilsoni; with one or two exceptions, all individuals had fed on krill (Euphausia superba). In C. antarcticus, the proportion of empty stomachs was high. Sometimes, stomachs were regurgitated. Individuals < 30 cm fed primarily on krill while larger individuals took Pleuragramma antarcticum and benthic nototheniids and a small proportion of krill. Prespawning, spawning and post-spawning C. rastrospinosus hardly fed. Only juvenile and fish with gonads in resting stage preyed heavily, primarily on krill and to a lesser extent on benthic nototheniids.
9. Nototheniids are much more diverse in terms of their diet than are icefish. G. gibberifrons and Lepidonotothen nudifrons were primarily benthic feeders with polychaetes as one of the most abundant prey items in their stomachs. T. eulepidotus, L. squamifrons and T. newnesi preyed overwhelmingly on krill, salps, jelly fish and salps while L. larseni took primarily krill. N. coriiceps was almost omnivorous with a food spectrum ranging from benthic algae over krill and fish to amphipods and salps.
10. Observations on age - With very few exceptions, all C. wilsoni were 26-34 cm long, likely representing 1 or 2 age classes only. Otoliths of C. wilsoni and C. antarcticus were collected and will be processed and read by Italian collegues. Age class 1+ C. rastrospinosus (14-17 cm) were caught for the first time. Together with age class 0+ fish (5-8 cm) which are found regularly as by-catch in the krill fishery (Iwami et al., 1996) they will allow to better verify age determinations of the species from otoliths.
11. The benthic invertebrate megafaunal
bycatch of all 64 successful hauls this leg were analysed in terms of megafaunal
abundance and biomass. A grand total of
23.5 metric tons of benthic megafauna was collected
The largest recorded (2.4 tons) was at Station 81, north of
12. Euphausia superba densities averaged 44.3 g/m2 for 64 transects. Mean transect length is 4.3 nautical miles with a range in densities from 0.097 to 338.5 g/m2. Bottom typing data is still being processed.
13. The week started with warm air temperatures
around 3°C (maximum of 5.2°C) and a rising air pressure which reached 992 millibars. On Tuesday the air pressure dropped to 976 millibars, and this was associated with strong winds from
the South East reaching 37 knots and air temperature dropping to a minimum of
-2.0°C. For the rest of the week the winds were mainly from the
14. A total of 41 CTDs were successfully completed during the leg of the cruise. Measurements were made for temperature, salinity and oxygen using a SeaBird 911 instrument. No water samples were collected during the leg, so no calibrations were done on the instruments. An Expendable Bathy Thermograph (XBT) transect was completed on the north bound crossing, starting at around Latitude 61°S till after the Antarctic convergence was crossed around latitude 57°S, with 18 units being deployed every 90 minutes.
C. Jones sends.