Science Centres: Fisheries

 

An orange roughy otolith

NIWA scientists have recently completed a study funded by MFish to validate the theory that two species of deepsea fish, orange roughy and black cardinalfish, live to extreme ages – something that has been assumed for over two decades, but not investigated in detail until now.

Conventional techniques for ageing fish include counting growth zones on otoliths ’ bony structures found inside the fish’s ear. There are various age validation methods used worldwide to prove that the zones counted are annual, but often these methods cannot be used for deepsea fish. A new technique was sought and an advanced radiometric method was found to be suitable.

Using this radiometric method, the activity of two radionuclides (lead and radium) present in the otolith was measured. The method showed that the ages of the two species of fish approached and even exceeded 100 years. Growth zone counts on otoliths were compared with ages determined by the radiometric technique, and importantly, the results showed that the conventional zone count is a valid means of age estimation for these species.

NIWA scientist Di Tracey, who led the study, says: ‘The findings are important because we can confidently use our zone counts in stock assessment. Also, by validating longevity in these deepsea fish we have shown there is a need to cautiously manage some of our slow growing, long-lived fish species.’