Douglas G. Bone

Target strengths of Antarctic krill (Euphausia superba) at 38 and 120 kHz

Encaged aggregations of live krill in good to pristine condition have been ensonified at 38 and 120 kHz. Concurrent underwater television observations of behavior resemble those made by underwater divers in naturally occurring swarms, with comparably high densities of the order of 104 animals/m3 . Mean, single‐animal target strengths have been inferred from measurements of echo energy. For aggregations with mean lengths in the range [30,39] mm, the mean single‐krill target strengths are in the range [−88,−83] dB at 38 kHz and [−81,−74] dB at 120 kHz. Collateral measurements on some of the same encaged specimens determined a density contrast of 1.0357±0.0067 and sound‐speed contrast of 1.0279±0.0024, relative to seawater. These numbers have been used with the fluid‐sphere model as stated by Greenlaw [Limnol. Oceanogr. 24, 226–242 (1979)] . Computed backscattering cross sections have been averaged over the length distributions of each measured aggregation, resulting in target strength predictions in the ran...

Effectiveness of the RMT8 system for sampling krill (Euphausia superba) swarms

SummaryThe effectiveness of the Rectangular Midwater Trawl (RMT) for sampling krill swarms was studied by observing the behaviour of swarms in the vicinity of the net using echosounders. Large scale avoidance was observed by day but not at night. By day krill layers dispersed to such an extent that few krill were present less than 10 m below the net. Limited avoidance by night indicates that the krill are acting on visual clues of the nets presence.

Mitigation of seabird mortality on factory trawlers: trials of three devices to reduce warp cable strikes

Experimental trials were conducted onboard a stern trawler to identify the relative efficacy of three emerging mitigation measures (tori lines, warp scarer and Brady baffler) designed to reduce seabird mortality caused by warp cable strikes. The use of mitigation measures was clearly shown to substantially reduce seabird mortalities from collisions between seabirds and warp cables. Based on an established significant relationship between contact rate and seabird mortality, when using contact rate as an index of mortality there was a clear performance hierarchy of the three measures. Tori lines and the warp scarer were significantly more effective at reducing contacts than the Brady Baffler, whilst tori lines represent a smaller, but still significant, improvement on the warp scarer. While further testing would be required under local environmental and operational conditions, our findings are likely to have application for many trawl fisheries around the world.

The shelf break front to the east of the sub-Antarctic island of South Georgia

The sub-Antarctic island of South Georgia lies in polar waters to the south of the Polar Front and is influenced by the Weddell Scotia Confluence whose waters reach the island’s south east coast. In January 1996 a detailed hydrographic survey to the east of the island showed that on-shelf waters were both warmer and fresher than those off-shelf. These differences were not confined to surface waters. The inherent density differences resulted in a shelf break front, the position of which was defined as the point where the 26.96 kg m-3 isopycnal crossed 30 dbar as it rose towards the surface. The slope of the shelf break front between the on- and off-shelf waters was found to be approximately geostrophic. Although the position of the shelf break front was not linked to any particular bottom depth, its position was strongly influenced by the bottom topography in the region. Evidence is presented for upwelling in one specific region along the front. Differences between the on- and off-shelf waters are also observed in nutrient and chlorophyll a data: These data implied greater productivity on-shelf. Both physical and chemical data suggest that on- and off-shelf waters communicate at depth. This study also shows that shelf water can cross the shelf break front and remain in the vicinity of the island.

Physical oceanographic conditions to the northwest of the sub‐Antarctic Island of South Georgia

Ocean physics is a key aspect of the operation of island ecosystems, yet the hydrodynamics is sometimes poorly understood. A high-resolution oceanographic survey to the north of South Georgia identified two water masses. These are South Georgia Shelf Water (SGSW) and Antarctic Zone Water (AZW). At the surface, SGSW is both colder and fresher than AZW; at the near-surface temperature minimum these differences are reversed. There are two causes: SGSW is close to the island and has an additional contribution to the heat and salt balance from increased precipitation and island runoff, and second, AZW has advected from higher latitudes. Both of these mechanisms imply SGSW is retained around the island. Between these water masses is a variable width frontal region that has a mixture of both water masses. In general terms, the mean current of the AZW is to the west. In the SGSW the general flow also appears to be to the west, but at reduced magnitude; there is evidence of a stronger westerly current close to the shore. In both water masses there are easterly surface currents most likely driven by the prevailing wind. An approximation for the limit of SGSW is that it is constrained to water depths <500 m. A significant deviation is caused by an anticyclonic flow around a prominent bank that can be associated with significant upwelling. Finally, we comment on the ecological significance of the system observed.

A biological acoustic survey in the marginal ice-edge zone of the Bellingshausen Sea

An acoustic survey at 38 kHz was carried out from R.R.S. Discovery in the Bellingshausen Sea from 23 November to 7 December 1992 as part of the U.K. Joint Global Ocean Flux Study (JGOFS) Southern Ocean investigations [Turner D. and N. J. P. Owens (1995) Deep-Sea Research II, 42, 907–932]. A total of 285 targets were identified and described from the chart record of the echo sounder. Mean volume backscattering strength data were collected using an echo integration system. These data are used to describe the spatial and temporal variability of krill (Euphausia superba) distribution and biomass in the marginal ice-edge zone. Krill biomass density varied from a mean of 42 g m−2 during the first survey phase to 20 g m−2 on the second survey phase. The number of small swarms detected during the second phase was greater than during the first phase. On this first survey a large swarm (2.8 km in extent) had a potential biomass of 3.7 × 104 tonnes. Some calculations are presented to show the potential impact of krill on the flux of carbon in the area of the survey.

Target strengths of Antarctic krill

Encaged aggregations of Euphausia superba have been ensonified at 38 and 120 kHz and the echo energy measured. Derived estimates of target strength are considerably lower than previously measured values.