NOTE Secor: Otolith microchemistry analysis of Morone saxatilis anadromy 



799 



in microprobe analysis (Kalish 1991). Otolith sections were carbon- 

 coated in a high-vacuum evaporator. 



A sagitta from a juvenile striped bass sampled from the Patux- 

 ent River (Chesapeake Bay tributary) was similarly prepared and 

 polished so that the core and all increments were sectioned (Secor 

 et al. 1991). The juvenile's parentage, a 20kg female that was 

 assumed to be migratory based on its size (Kohlenstein 1981), was 

 known because the juvenile was a marked hatchery fish released 

 as a 9-day-old larva. 



X-ray intensities for Sr and Ca elements were quantified using 

 a JXA-840A JEOL wave-length dispersive electron microprobe 

 (Central Facility for Microanalysis, Univ. Maryland, College Park 

 MD 20742), with Calcite (CaCOg) and Strontianite (SrCOg) as 

 standards. Striped bass otoliths were resilient to high-beam power 

 densities compared with previous work on salmonid otoliths 

 (Kalish 1990) and showed no diffusion of elements over a 32-sec 

 counting period (Table 2). This permitted analysis of small 



sample points (5 x 5fim) at high accelerating 

 voltage (25 kV) and probe current (20 n A). 

 Background and peak counting times were 

 each 20 sec for Sr, and 5 sec for Ca. Back- 

 ground counting times were equally divided 

 below and above the peak position. The 

 detection limit for Sr was 580 ppm. Preci- 

 sion was calculated at <1% for Ca counts 

 and 8.2% for Sr counts (at Sr/Ca= 0.003) 

 (1.96o; Goldstein et al. 1981). The electron 

 beam caused a physical disruption (a pit) at 

 the section's surface which limited the prox- 

 imity of adjacent points that could be accur- 

 ately sampled. Initial analyses of Chesa- 

 peake sample otoliths at "step" distances of 

 8/.im resulted in no Sr X-ray counts. This 

 was probably due to physical disruptions 

 among adjacent points because surface 

 structure can cause artifacts in microprobe 

 analysis (Kalish 1991). Analysis was there- 

 fore conducted at 13 and 20j:.(m step sizes 

 where positive counts occurred (Table 3). 

 Transects (700-2600 ^.im in length) across 

 annuli in the otolith sections were selected. 

 The electron microprobe sampled 60-130 

 points along these transects. Each point re- 

 quired ~70sec of microprobe time. X-ray in- 

 tensities were calculated using the ZAP pro- 

 cedure (Reed 1975), normalized to stan- 

 dards, and converted to elemental (atomic 

 weight) ratios. 



Due to their close proximity, individual 

 points were not always visible in probed 

 otolith sections. To relate Sr/Ca ratios to the 



