Thresher et al.: Otolith analysis of Nemadactylus macropterus 



819 



would differ geographically. With regard to the num- 

 ber and location of nursery areas, we further hypoth- 

 esized that if there is only one nursery area, then 

 the composition of that part of the otolith deposited 

 during residence in the nursery ground would be 

 similar for all adults, irrespective of where they were 

 caught, and would match that of juveniles caught in 

 southeastern Tasmania. 



Methods 



Collection details for juvenile and adult N. macro- 

 pterus are provided in Figure 1 and Table 1. Recently 

 settled (0+) juveniles were collected by hand-lining 

 and trawling at six sites off Tasmania and southern 

 Victoria. Adult specimens were also obtained at six 

 sites, from commercial and scientific trawls on the 

 southeastern Australian continental shelf. To mini- 

 mize possible effects of interannual variation in 

 otolith composition, we minimized the number of year 

 classes in the sample by using only adults in the size 

 range of 30-35 cm fork length. Otolith macrostruc- 

 ture and published length-at-age keys for the spe- 

 cies (Smith, 1982) suggest that the specimens were a 

 mixture of the 1980 to 1984 year classes and that year- 

 class distributions overlapped broadly among sites. The 

 juveniles were from the 1987 and 1988 year classes. 



All specimens were frozen at -20°C shortly after 

 collection and remained frozen (up to 30 days) until 

 the otoliths were removed. After extraction, each 

 otolith was cleaned of adhering tissue with fine for- 

 ceps and a soft-bristled brush in millepore-filtered 

 distilled water. They were then dried in an oven at 40- 

 45°C for at least 6 hours, after which they were stored 

 in polyurethane capsules in a desiccating cabinet. 



Procedures for embedding, sectioning, and prepar- 

 ing otoliths for probe microanalysis are detailed in 

 Gunn et al. (1992). Only sagittae were used in this 

 study, because of their larger size. Prior to embed- 

 ding, a scaled diagram of the distal surface of each 

 otolith was made in order to guide subsequent sec- 

 tioning. The otolith was then fixed upright on its 

 ventral edge to the base of an embedding mold with 

 a drop of Araldite. The mold was then filled with a 

 harder-setting resin. After hardening, the otolith was 

 sectioned with a diamond-edged saw blade (350 urn 

 thick) on a rotary saw. Grinding to the plane of the 

 primordium was done by hand with 2400-grade sili- 

 con carbide wet/dry paper. Final polishing was done 

 by using progressively finer grades of diamond paste 

 (6-3 pm) and aluminum oxide powder (Linde B) on a 

 lapping machine. After polishing, the section was 

 ultrasonically cleaned and stored in a moisture-free 

 environment. Prior to probe microanalysis, the sec- 

 tion was heated on a hot-plate at 80°C for 10 min- 



Adults 



W. Tas. 



NSW 



E. Tas 



39 



45 



143° 



149° 



Figure 1 



Source locations of samples of adult and juvenile Nemadactylus macropterus examined by elec- 

 tron probe microanalysis. Sample details are provided in Table 1. Adult sites: GAB=Great Aus- 

 tralian Bight, W. Vict.=western Victoria; E. Vict.=eastern Victoria; W. Tas.=western Tasmania; 

 E. Tas.=eastern Tasmania; and NSW=New South Wales. Nutgrove and Derwent are two in- 

 shore sites sampled for juveniles, both located in the Storm Bay (SE Tasmanian) estuary. Phillip 

 Island is an inshore site off Victoria. 



