Radiometric analysis of 

 blue grenadier, 

 Macruronus novaezelandiae, 

 otolith cores 



Gwen E. Fenton 



Department of Zoology. University of Tasmania 

 GPO Box 252C Hobart. Tasmania 7001. Australia 



Stephen A. Short 



Environmental Radiochemistry Laboratory, ANSTO 

 Private Mail Bag I, Menai. NSW 2234. Australia 

 Present address: Kmgett Mitchell and Associates 

 PO Box 33-849, Takapuna. Auckland, New Zealand 



Ra. In addition, otolith cores have 

 been analyzed with ^Th/^Ra to 

 age flying fish, Hirundichthys 

 affinis (Smith et al., 1991; Camp- 

 ana et al., 1993), and silver hake, 

 Merluccis bilinearis (Smith et al., 

 1991 ). While the isotope pair 228 Th/ 

 228 Ra is useful for short-lived fish 

 up to about 5 years (Campana et 

 al., 1993), analysis of 210 Pb/ 226 Ra 

 is appropriate for medium-aged to 

 long-lived fish up to about 120 

 years. 



In the present study measure- 

 ments of 210 Pb/ 226 Ra disequilibria 

 have been conducted on cores of 

 adult female blue grenadier otoliths 

 in an attempt to provide indepen- 

 dent age data for this species. 



Blue grenadier, Macruronus novae- 

 zelandiae, is a major commercial 

 fish species in the upper continen- 

 tal slope waters of southeastern 

 Australia and New Zealand. An ac- 

 curate knowledge of age and growth 

 rate of this species is required for 

 management of the fishery. Several 

 ageing studies have provided esti- 

 mates of age for blue grenadier 

 (hoki) from New Zealand waters 

 (reviewed by Paul, 1992), with the 

 maximum age ranging from 12 to 

 15 years. Kenchington and Augus- 

 tine ( 1987 ) examined whole otoliths 

 and thin transverse sections from 

 blue grenadier caught in southeast- 

 ern Australian waters and reported 

 a maximum age of 25 years. How- 

 ever, Kenchington and Augustine 

 ( 1987) were unable to validate their 

 ages for fish >3 years of age. They 

 found that conventional techniques 

 of validating age estimates, such as 

 tagging, modal analysis, marginal 

 increment or edge-type analysis, 

 and back calculation of lengths at 

 age, could not be applied to blue 

 grenadier. 



The possibility that radiometric 

 analysis offish otoliths, a technique 

 pioneered by Bennett et al. (1982) 

 for ageing fish, might validate the 

 ages of blue grenadier led to a study 



by Fenton et al. (1990). Unfortu- 

 nately radiometric analysis of 210 Pb 

 and 226 Ra in whole blue grenadier 

 otoliths was unsuccessful for esti- 

 mating fish ages (Fenton et al., 

 1990). This was due to an exponen- 

 tial reduction in 226 Ra incorpora- 

 tion into the otoliths of blue grena- 

 dier during their life, a reduction 

 that was believed to result from an 

 ontogenetic change in habitat from 

 juvenile to adult fish. 



Recent advances with radiomet- 

 ric ageing offish indicate that ages 

 can be determined radiometrically 

 from otolith cores (Campana et al., 

 1990, 1993; Smith etal., 1991). The 

 coring method removes all the lay- 

 ers of otolith growth beyond the 

 earliest year(s) and thus avoids 

 problems of changes in 226 Ra or 

 210 Pb uptake patterns in later life. 

 Furthermore, an analysis of otolith 

 cores circumvents the need to 

 model the mass growth rate of 

 otoliths, which is necessary in de- 

 termining age from whole oto- 

 liths (Smith et al., 1991). 



Radiometric analysis of otolith 

 cores has been successfully used to 

 age redfish, Sebastes mentella 

 (Campana et al., 1990), and split- 

 nose rockfish, Sebastes diploproa 

 (Smith et al., 1991), with 2io Pb /226 



Materials and methods 



Field collection 



Blue grenadier, Macruronus novae- 

 zelandiae, were collected by bottom 

 trawling (500-850 m depth) during 

 cruises conducted northwest of 

 Sandy Cape off the west Coast of 

 Tasmania by the Tasmanian De- 

 partment Primary Industry, Divi- 

 sion of Sea Fisheries, and the 

 CSIRO Division of Fisheries be- 

 tween 1985 and 1986. Sagittal 

 otoliths were removed and stored 

 dry. This collection of otoliths was 

 used by Kenchington and August- 

 ine to estimate age by annulus 

 counts (1987) and was subse- 

 quently made available for the 

 present study. 



Otolith morphometries 



The linear dimensions of blue gren- 

 adier otoliths (maximum length, 

 width, and thickness with respect 

 to the primordia ) were measured to 

 the nearest 0.1 mm with calipers. 

 Otolith weight was also determined 

 for « = 172 blue grenadier otoliths 



Manuscript accepted 25 September 1994. 

 Fishery Bulletin 93:391-396 (1995). 



391 



