CCAMLR

In 2008 WG-FSA recommended that the Secretariat undertake to identify the tagging event details for all tags recovered (WG-FSA 2008 paragraph 3.55). The Secretariat has been able to verify the tagging details of a greater proportion of tags than reported last year which has resulted in an improved proportion of the tags that can now be linked to tagging events in exploratory fisheries (Table 1). The recapture linkage rate for rajids has been lower than for toothfish in Subareas 88.1 and 88.2. The linking rate for tags recovered in established fisheries is also variable due to the partial inclusion in the CCAMLR database of some information on tag deployments and recaptures from tagging schemes that are operated by individual Members. WG-FSA has requested Members who have previously deployed tags to provide inventories of these tags to the Secretariat (WG-FSA 08 paragraph 3.56). This request, including a tag inventory template, has been sent to all Members. The Secretariat urges all Members with this type of information to provide these details as soon as possible. A copy of the tag inventory template can be obtained from the Secretariat.

A submersible multifrequency acoustic TS-probe was used for measuring the target strength of Antarctic krill in situ at short range. The experimental methods, procedure for data retrieval and analysis with some results for one selected station are presented. A Simrad EK60 split beam system operating at 38, 120 and 200 kHz is installed in the probe, connected to three pressure resistant transducers with 7o half power beam opening angles. The orientation of the probe was monitored with sensors for pressure, compass, pitch and roll. A stereo camera system was also mounted directly on the transducer platform with the purpose of measuring the orientation of the organisms. All system communication between the ship and the probe was through an optical Ethernet link. Firstly, krill presence was documented through recordings with hull-mounted ship echosounders and verified through net samples. The probe was subsequently lowered to suitable depth from the vessel in fixed position and data were acquired at short pulse duration, high ping rate and corresponding short maximum detection range, often limited to 25 – 50 meters below the transducers. The data analysis could then be performed by selecting single target tracks from the echograms. From synchronized detections at the three frequencies within single target tracks, individually based TS frequency response could be determined with fair accuracy. The results from the target strength measurements can further be compared with theoretical model predictions and further utilized in the biomass evaluation.

Target strength (TS) determinations can be classified into several types: 1) theoretical, 2) ex situ measures on dead or living fish under experimental conditions, 3) in situ measures on free swimming fish in their natural habitat. A canvassing of the literature suggested that the type of study may influence the result. For example, some authors suggested that freezing the fish reduce the over-all backscatter of 30% (Sun et al. 1985), others that TS measurements on living fish ex situ would be several dB higher than in situ (Nakken and Olsen, 1977). On other hand the many factors likely to influence TS measurements in situ (swimming movements and aspect, day-night behaviour, physiological state, stomach fullness, sea conditions, trawl type and efficiency) may differ from time to time and place to place (Ona, 1990), making comparisons difficult. For those reasons theoretical models have been thought a necessary support to in situ and ex situ results. Antarctic Silverfish is an important component of Middle Trophic Level of the Ross Sea and several studies have been published on its biology and ecology (De Witt, 1970; De Witt and Hopkins, 1977; Hubold and Hagen, 1997; Vacchi et al., 2004). However, there are no studies concerning TS of P. antarcticum and thus no acoustic assessment of its distribution and abundance in the Ross Sea. In this paper we present results of three different studies on Antarctic Silverfish TS. In situ experiments were conducted on juveniles (length<90mm) on the SE of the Continental Slope (three hauls between Lat. 71°-73° S and Long. 170°-175° E) and of Continental Shelf (one haul around Lat 75° and Long. 167°) during the Italian expedition of January 2004 to the Ross Sea. The mean backscattering cross section of Antarctic Silverfish was estimated comparing the fish density determined acoustically (at 38, 120 and 200 kHz) along the towing track of a pelagic trawl and that determined by the catch in number of fish. The catch was constituted of P. antarcticum for more than 95% in number and weight. Ex situ experiments were conducted on preserved (frozen and defrosted) adults of Antarctic Silverfish, ranging in length from 90 to 210 mm, by means of an underwater framework with a floating platform, moored in Ancona Bay, in May 2007 and in February 2009. The same split beam acoustic system (EK500) as in situ experiments was used, with 38, 120 and 200 kHz transducers bolted to the floating platform over the centre of the frame. The fish were divided in five length classes of eight (2007)/four (2009) individuals: 90-115 mm; 115-130 mm; 130-150 mm; 150-170 mm; 170-210 mm. Single fish were tied by their upper lip and tail to monofilament line stretched between two opposite fibreglass hollow bars of the frame, at a depth of 5-7.4 m from the transducers. The TS was estimated both for only one fish and for all the fish together of each class. We then used a general linear modelling approach to predict TS of Silver fish and its dependence on morphological characteristics of its vertebrae and body.

In order to describe an aquatic ecosystem scientists require a variety of information. Some of this information can be obtained using underwater acoustics. As examples acoustics is often used to provide information about biomass densities, biological species compositions, size distributions, behavior in space and time, and links to other environmental information such as habitats. Examples of underwater acoustic instrumentation used by scientists to obtain such information are scientific multi frequency split beam echo sounders, multi beam echo sounders, omni directional sonars, 3D matrix sonars, and various types of trawl gear instrumentation. Data are often obtained from fixed installations on board dedicated research vessels, but are sometimes also obtained from various kinds of moored installations, acoustic landers, remote buoys, autonomous underwater vehicles (AUV’s), and commercial fishery vessels. In this paper a variety of underwater acoustic technologies are presented to serve as input for the discussion of what kind of underwater acoustic instrumentation is needed for both research vessels, potentially commercial fishing vessels, and other types of installations in order to obtain improved information about the marine ecosystem in the Antarctic.

Fishery for krill is a major economical activity in the Antarctic Ocean. The increased Norwegian fishery inherently gives Norway a responsibility to contribute to the management of the marine resources in the southern ocean. During the International Polar Year 2008, RV “G.O. Sars” spent 3 months in the Antarctic Ocean to do investigations on euphausiids and other key components of the ecosystem. Major acoustic activities were to acoustically identify krill and estimate specimen size, investigate behaviour, measure target strength in situ, and of course verify the acoustic measurements biologically. Krill were identified from the relative frequency response of a 6-frequency hull-mounted echo-sounder system, and specimen size was estimated acoustically by means of several acoustic scattering models implemented in an optimized framework in the post-processing system LSSS.