Science Centres: Fisheries

A cooperative study between NIWA and GNS scientists has successfully confirmed the high maximum ages of rubyfish and bluenose. The study, funded by the Ministry of Fisheries, was based on levels of radiocarbon (¹⁴C) derived from nuclear bomb tests in the South Pacific in the1960s in different parts of the otoliths of these fish compared with visible growth zones in the otoliths. The study showed that rubyfish reach surprisingly high ages (70 years, probably 90+), and that bluenose reach higher ages than previously suspected (i.e., probably 40+).

Determination of age and growth is a vital step in understanding the productivity and dynamics of fish populations. Bluenose and rubyfish otoliths have numerous narrow lines that are difficult to interpret using standard age-reading methods. To date, most studies which have used radiocarbon as an aid to interpreting otolith zones have used only the core of the otolith. A new feature of the NIWA/GNS study was that it took micro-samples at several points across each otolith cross-section. These samples were analysed to determine whether the sudden 1960s increase in environmental radiocarbon (¹⁴C) levels resulting from nuclear testing could be detected at some position across the section. These samples were compared with the growth zone count, and thus the back-calculated calendar year, at that position.

Large fish were selected which were likely to have been alive in the 1960s and earlier. The otolith core ¹⁴C level was determined by accelerator mass spectrometry. Based on fish capture dates, these core results indicated that the bluenose were at least 25 years old, and that the rubyfish were at least 45 years. Five fish of each species were chosen for further detailed analyses. Further samples were drilled from the otolith sections (a total of six for each bluenose, five for each rubyfish). Enlarged images of the zone-counted and drilled otolith sections were matched to allocate an age and calendar year to each drilled sample.

Stable isotope values were also determined from a larger number of cross-section samples, and these provided some interesting extra information. The X¹⁸O values showed changes in mean depth with age of each fish. Both species are near-surface as juveniles, moving deeper with age. Adult bluenose and rubyfish appear to live in 600–1000 m; this is deeper than most commercial fisheries catches suggest, but is supported by research trawling, and has stock assessment implications. The X¹³C values reflect fish metabolic rates, feeding levels, and oceanographic conditions. In combination, the stable isotopes record the environmental life history of each fish, and are of potential value in distinguishing geographic stocks and/or indicating migratory patterns.

The results support the idea that rubyfish lay down otolith growth zones at a rate of one per year, and hence zone counting can be used to determine the age of a fish. The bluenose data suggest that the zones were under-counted by up to a factor of two, possibly because of the difficulty in distinguishing true zones from minor checks. However, it is also possible that bluenose may not lay down an otolith growth zone each year.

Helen Neil, Larry Paul, & Peter Horn (NIWA), Rodger Sparks (GNS) [h.neil@niwa.co.nz]