CCAMLR

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.

The spatial distribution characteristics of ice-fish in the near-bottom layer are analyzed using the acoustic data obtained in the South Georgia area during the Russian bottom trawl surveys 2002. Acoustic and trawl density samples obtained in the near-bottom layer covered by trawl survey are compared. It is shown the possibility to estimate the bottom trawl efficiency. The high heterogeneity of horizontal and vertical fish distribution in the near-bottom layer of the survey area is shown. Influence of this fish distribution heterogeneity on trawl surveys efficiency and reliability is analyzed. In addition, the author discuses the results of bottom trawl surveys carried out in the South Georgia area during season’s 2000 and 2002. It is shown, that observed inter-seasons and inter-vessels differences between fish biomass estimates from bottom surveys may be to a considerable degree stipulated by heterogeneity of fish distribution in the near-bottom layer relative to the fishing gear operation zone (Russian trawl HEK-4M and UK trawl FP120) and the trawl station location rather than by the variability of the stock state.

An anatomically detailed acoustic scattering model has been used to estimate the backscattered target strength (TS) of six mackerel icefish (Champsocephalus gunnari) of total length 26.8–35.0 cm, at 38 kHz. The model used computed tomography (CT) scans of fish to determine the sound speed and density throughout the fish and simulated the complex interaction of the incident sound wave with the acoustic impedance contrasts throughout the fish. A preliminary length to tilt-averaged TS relationship is derived from the model results: 148)(log4.7010-=LTSwhere L is the fish total length in cm and is applicable to fish of length 26–35 cm. Existing in situ TS estimates are for smaller fish (16–26 cm) and hence are not directly comparable, but extrapolation of the in situ and model results reveal a large different in TS (6 dB at 26 cm) between the two datasets.