Descriptive analysis of acoustic data collected during the 2003 exploratory fishery for toothfish in the Ross Sea

This report presents results from a pilot study to determine the feasibility of conducting acoustic surveys for toothfish and rattails in the Ross Sea. Acoustic data were collected during the 2002–03 exploratory fishery. Data were recorded continuously from 28 December to 2 February 2003, then during line setting only from 5–22 February 2003. Analyses were carried out to assess data quality, describe different mark types, and quantify acoustic backscatter by echo integration and echo counting. These analyses focused on the subset of acoustic data collected when setting longlines so acoustic recordings could be compared with longline catches.
Data quality was generally good. Of the 84 line recordings, 68 were considered suitable for acoustic analysis. Poor data quality was associated with strong winds and/or high seas: conditions that led to bubble interference on the hull-mounted transducer. Other issues with data quality were interference from another echosounder, and the occurrence of a double bottom echo caused by too high a ping rate.
All line recordings were in water over 1000 m deep. Because of the spreading of the acoustic beam, the acoustic deadzone at these depths is relatively large, especially if the bottom is rough or sloped. Simulations indicated that at 1500 m depth, the acoustic deadzone would be over 50 m high for a sea-bed with a slope of 20º. The problem of the acoustic deadzone was worsened by the occurrence of side-lobe echoes, produced as longlines were set on steep slopes parallel to the depth contours. Measurements indicated that side-lobe could create a deadzone of 50–100 m on apparently flat ground. Because both toothfish and rattails are considered to be demersal species, the inability of the acoustics to ‘see’ close to the bottom is a major limitation that could only be avoided with the use of a towed acoustic system.
Two types of pelagic layers were present in most acoustic recordings: a dense shallow layer between 30 and 200 m; and a more diffuse deep scattering layer between 300 and 800 m. Pelagic schools were also present in some recordings and these tended to occur at 150–400 m depth, between the layer marks. The most common demersal mark was single targets, which were present in 84% of line recordings. Most single targets occurred in a surface-referenced band between 800 and 1100 m depth, and were up to 500 m off the bottom. There was a significant positive correlation between the number of single targets counted from the echogram and the catch of rattails in the accompanying longline set. Bottom-referenced layers were present in 18% of line recordings and were also associated with higher catches of rattails. Demersal schools were present in 16% of recordings and were associated with higher catches of toothfish. Despite these associations, no acoustic marks could be reliably identified as being rattails or toothfish. It seems unlikely that the schools were toothfish or the single targets were rattails, as these were often more than 300 m off the bottom.
At this point, it is not practical to estimate toothfish or rattail abundance in the Ross Sea using hull-mounted acoustic systems. The acoustic deadzone was large, meaning it was impossible to detect demersal species close to the bottom. Echo integration was unreliable because there was a very low signal-to-noise ratio deeper than 1000 m. Echo counting showed more promise, but only relatively strong targets well separated from the bottom could be enumerated. As toothfish do not have a swimbladder, their acoustic target strength may be too weak to allow them to be counted.