Survey finds 10 million orange roughy off eastern Tasmania
CSIRO have released preliminary results from the July 2013 SETFIA led stock survey of the Eastern Orange Roughy Zone. Two features known as St Patricks and St Helens in the Eastern Zone where roughy are known to aggregate to spawn were surveyed. St Helens Hill is typical of a roughy hill being a conical seamount rising from a depth of 1,100m to 600m. This is the 4th acoustic survey in the Eastern Zone since 2006.
CSIRO use an acoustic optical system or AOS designed by CSIRO Hobart. An AOS is a device that emits multiple frequency signals to calculate the amount of fish present. It is towed 200-300m above schools of aggregating roughy and can survey the size and densities of roughy schools and distinguish between roughy and other gas bladdered fish. It is particularly effective in assessing aggregations of deep-sea fish and has been used in Australasia to assess blue grenadier (hoki) and roughy.
By catching fish shortly before it spawns and studying reproductive organs called gonads scientists can tell which fish will spawn that year. They have found that not all roughy come to a hill each year to spawn. Previous eastern surveys have found that only 52-70% of roughy spawn each year meaning that there could be 1.4 to 1.9 more fish in the zone than surveyed.
The survey endured very bad weather last year. Unusually CSIRO found roughy on both eastern roughy hills and it was more widely distributed across the hills than in previous years with a declining trend over time at St Helens.
Previous surveys have found between 18,000 and 27,000 thousand tonnes of fish in the east. In this unusual 2013 year only 15,000 tonnes or about 10 million fish were found scattered widely across both hills.
To give readers some context about these numbers 40,000 horses weigh about 20,000 tonnes. Interestingly there are about 30,000 registered race horses in Australia. It is fair to say that roughy inside the eastern zone weigh more than all Australian race horses combined! Neither roughy nor race horses are going extinct any time soon.
New Zealand’s roughy stocks are estimated to be 168,000 tonnes from which an annual commercial catch of 7,000 tonnes is allocated. In the absence of a commercial fishery, the size of Australia’s stocks are less certain with somewhere in the vicinity of 21,000 – 48,000 tonnes in the east alone (allowing for fish not spawning) and an unknown quantity of fish in Australia’s four other roughy zones. A ban on targeted fishing and closures set down in the orange roughy conservation program means that the Australian roughy catch is only 200 tonnes in most years.