Exploratory analysis of acoustic data from 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 using a Simrad ES60 38 kHz echosounder on the New Zealand commercial longliner FV Janas during the 2002–03 exploratory fishery. Data were recorded continuously from 28 December to 2 February 2003, then during line setting only during 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. Each ‘line’ recording was between 20 and 50 min long, corresponding to 2–4 nmi.
Data quality was generally good. Of the 84 line recordings, 68 were considered suitable for acoustic analysis. The other 16 files were rejected because data quality was too poor (11 files), the file was corrupted (1 file), or the longline was lost so there were no corresponding catch data (4 files). Poor data quality was associated with strong winds (greater than Beaufort 5) and/or high seas (swell heights greater than 2 m): conditions that led to bubble interference on the hull-mounted transducer. Other issues with data quality were interference from another echosounder before 11 January 2003, and the occurrence of a double bottom echo caused by too high a ping rate from 23–30 January.
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 an acoustic system deployed at depth.
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…[please contact the Secretariat for the full version of the abstract]