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Scientists cast doubts on fish ages - 11/03/1998

A group of international scientists visiting New Zealand this week have cast doubt on accepted views about the lifespan of some commercial fish species.

At a two-day workshop in Wellington the scientists unveiled evidence which suggests that the lifespan of some New Zealand commercial species is significantly less than currently accepted.

Establishing the age and longevity of fish, particularly deepwater species, has been a controversial topic for decades. The lifespan and biomass (stock size) of fish species are the main considerations when assessing the sustainability of commercially fished species.

This leads to decisions on the allocations of commercial, Maori customary and recreational fishing quotas.
The workshop was organised by the Institute of Geological & Nuclear Sciences Limited which has a long history in age dating biological samples such as fish bones, and geological events such as pre-historic earthquakes and volcanic eruptions.

Institute fisheries researcher Bob Gauldie said the impetus for holding the workshop was the Institute's concern about sustainability of commercial fish stocks and a desire to strengthen research in deep ocean fish species.

Dr Gauldie said conventional models used to age fish did not look as robust as they used to. Estimating the age of fish centres on the anlaysis of bands found in a bone-like structure in the fish's ear called an otolith -- lierally ear-stone.

Some bands in the otolith are thought to be laid down annually, but other much narrower bands are thought to be laid down daily. However, Dr Gauldie said the total number of daily bands was often far too low to be consistent with the number of annual bands.

'' Only one of the two interpretations can be correct,'' Dr Gauldie said.

For the past 10 years fisheries scientists, particularly in New Zealand, have relied heavily on radiometric methods of age estimation to support the annual interpretation. Evidence presented at the workshop showed that radiometric methods did not support annual band interpretations.
Dr gauldie said models presented at the workshop strongly supported the daily band interpretation.

''This means that orange roughy, for example, are now thought to have a maximum age of less than 20 years -- not 120 to 200 years as previously believed.

'' The implication of this is that fish stocks regenerate faster than previously believed. It also suggests that there are biological processes occurring in the ocean that we don't yet understand.''

Institute scientists were developing new hardware and software to convert their new modelling approach into a user-friendly technology.
''Our aim is to provide more accurate growth rate and age information to guide decisions on the management of New Zealand's fish resources.''
Dr Gauldie said a bonus of the workshop was the opportunity to discuss new thinking in deep-water research with some of the world's most experienced deep-water fisheries scientists.

There was wide acceptance that in New Zealand much had yet to be learned about regeneration rates in deep-water species. Dr Gauldie said the fishing industry supported the Institute's involvement in the scientific debate and was helping to fund the Institute's research in this area.