Acoustic measurements of the population of orange roughy(Hoplostethus atlanticus)on the north Chatham Rise, New Zealand, in winter 1986

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Abstract
Orange roughy (Hoplostethus atlanticus) are found in depths from 700 to 1300 m around New Zealand and support an important fishery. An acoustic survey was carried out on the Chatham Rise to the east of New Zealand in mid winter 1986 on a spawning population of orange roughy. The great depth and poor weather conditions created a number of problems, particularly with respect to signal‐to‐noise ratio. These were tackled principally by use of a narrow‐beam transducer mounted in a very stable tow body, towed at a depth of 200 m. A scientific echo sounder operating at 38 kHz was used and echo data were recorded digitally on magnetic tape. A total of 1700 km2 was surveyed covering a bottom depth range from 750 to 1250m. The orange roughy were located near the bottom, 77% lying between 5 and 10 m. Highest density was found where the bottom depth exceeded 950 m. The overall areal density was 0.16 kg m‐2. One small area with very high fish densities extending up to 65 m was encountered during the survey. The areal density here was 3.41 kg m‐2. Target strength (TS) measurements were made on a sample of dead orange roughy giving a relationship with standard length (Ls) of: TS = 29.4 log(Ls) ‐22.6. The mean target strength for the population was ‐36.0 dB.

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