NOTE Fenton and Short: Radiometric analysis of Macruronus novaezelandiae otolith cores 



395 



0.603, r 2 = 0.910 for the Kenchington and Au- 

 gustine [1987]data set). 



The 226 Ra concentrations in the blue grenadier 

 otolith (0.022 ±0.004 dpm-gr 11 cores are low compared 

 with other fish species. For example, orange roughy, 

 Hoplostethus atlanticus, whole otoliths had a mean 

 226 Ra concentration 2-3 times higher (Fenton et al., 

 1991), and the otolith cores of three species of tropi- 

 cal snapper had 226 Ra values 6-10 times higher 

 (Milton et al., 1995). The main consequence of the 

 relatively low radium in blue grenadier cores is that 

 the error associated with their age estimates are 

 higher ±4—8 years than those for tropical snapper 

 otolith cores ±1-2 years (Milton et al., 1995). 



Furthermore, the 226 Ra concentrations in the blue 

 grenadier otolith cores were lower than the concen- 



trations we reported earlier in whole otoliths from 

 east coast 1+ fish (0.050 ±0.005 to 0.075 ±0.008 

 dpm-g" 11 but higher than the values for 4+ fish 

 (0.016±0.010 to 0.018±0.009 dpm.g" 1 ) (Fenton et al., 

 1990). The 226 Ra concentrations found in Fenton et 

 al. (1990) for whole blue grenadier otoliths ranged 

 from 0.084 ±0.009 dpmg 1 for 0+ fish caught in the 

 Derwent estuary to 0.005 ±0.005 dpm-g" 1 for 21+ fish 

 caught off the west coast of Tasmania and exhibited 

 a clear exponential reduction in 226 Ra with fish 

 length/age. The 226 Ra found in cores in the present 

 study are consistent with our earlier data for whole 

 otoliths and also confirm that the core represents 

 approximately a 2+ fish. 



Reexamination of the radiometric data in our ear- 

 lier study shows that data for a 1+ fish sampled from 

 the east coast of Maria Island can also be used to 

 estimate age. The sample had a 226 Ra value of 0.050 

 ±0.005 dprn-g" 1 and 210 Pb of 0.003 ±0.003 dpm-g" 1 

 which corresponds to a linear otolith mass growth 

 radiometric age (see Fenton and Short, 1992, for the 

 linear mass growth radiometric equation) of 0.68 

 ±0.68 years. This is consistent with the age of 1+ 

 determined by Kenchington and Augustine ( 1987 ). The 

 remaining data in our earlier study cannot be used to 

 determine age because of the exponential reduction in 

 226 Ra through life and the lack of a priori knowledge of 

 the relationship between R and increasing otolith mass. 



The age of Australian blue grenadier derived by 

 radiometric age of otolith cores gives age estimates 

 in approximate agreement with those derived by 

 Kenchington and Augustine ( 1987) and New Zealand 

 studies (reviewed Paul, 1992). Further radiometric 

 ageing of otolith cores may be useful in examining 

 apparent differences in the growth curves for New 

 Zealand and Australian blue grenadier fisheries. For 

 such a comparison, it would be worthwhile to include 

 analysis with the radioisotope pair 228 Th/ 228 Ra be- 

 cause this should increase the precision of the age 

 estimates particularly for fish under 5 years (Smith 

 et al., 1991; Campana et al., 1993). In conclusion, 

 the results of this study offer an independent assess- 

 ment of age in blue grenadier and in doing so dem- 

 onstrate the applicability of radiometric ageing of 

 otolith cores for medium-aged temperate fish. 



Acknowledgments 



Thanks are extended to the scientists of CSIRO Di- 

 vision of Fisheries Research and the Tasmanian De- 

 partment of Sea Fisheries who provided all otolith 

 material and associated biological data. Thanks are also 

 extended to Russell Bradford for technical assistance. 

 Robert Chisari, ANSTO, is thanked for assistance with 



