800 



Fishery Bulletin 90(4). 1992 



opaque zones of annuli checks, 

 probed sections were viewed 

 under a compound microscope 

 and transect distances between 

 annuli measured with an ocular 

 scale. Because the distance be- 

 tween each microprobe mea- 

 surement was known, distances 

 between measurements can be 

 converted to distances between 

 annuli. Distances between annuli 

 (annular increments) became 

 narrow with increasing age (<50 

 i^m) (Fig. 1), and points did not 

 always sample directly on annuli. 

 Therefore, it was necessary to 

 assign an annulus to the closest 

 sampled point. Points between 

 annuli were assumed to sample 

 age in linear proportion. For in- 

 stance, if 10 points were sampled 

 between annuli 5 and 6, then 

 points would be assigned ages 

 5.0, 5.1, 5.2,. . .6.0. A replicate 

 scan was performed on two of 

 the otolith samples. In the 

 juvenile's otolith, not all daily in- 

 crements were visible along the 

 transect with scanning electron 

 microscopy or light microscopy. 

 Therefore, Sr/Ca ratios were 

 related to standard length using 

 an otolith/fish-length relation documented for the 

 Potomac River population (Houde and Rutherford 

 1992). 



Results 



Mean Sr/Ca ratios in Chesapeake striped bass were 

 three to four times greater than Santee-Cooper striped 

 bass (Table 3; Figs. 2, 3). This trend is consistent with 

 a salinity influence on the ratio, because Santee-Cooper 

 striped bass are confined to freshwater and both 

 Santee-Cooper females were sexually mature. Al- 

 though substantial variation occurred in Sr/Ca between 

 South Carolina samples, both samples were near the 

 electron microprobe's detection limit of Sr/Ca (Sr/Ca 

 = 0.0008). Instrumental precision decreases markedly 

 as the detection limit is approached, which may pro- 

 duce spurious variation. Peaks and nadirs in the Sr/Ca 

 ratios were apparent in Chesapeake striped bass, and 

 in fish >age-4, these patterns generally showed an an- 

 nual cycle (Fig. 2). This is most apparent for sample 

 MD-1. Because low Sr/Ca ratios can be associated with 



Figure 1 



Back-scatter electron micrograph of otolith from striped bass Morone saxatilis Sample 

 MD-1. Twenty-one annuli are clearly visible along the sulcus (s) and sulcal ridge. Transects 

 1 and 2 were performed at 20fim step size; Transect 3 began at the 7th annular check 

 and 13fjm step size. The probed transect previous to Transect 3 was performed at S^m 

 step size and resulted in no positive Sr counts. Note that individual probed points are 

 visible in a series of physical disruptions (pits) for Transects 1 and 2. 



freshwater excursions, results indicate yearly migra- 

 tion for this large female. 



Lack of agreement among replicate transects (Fig. 

 2) probably was due to the manner in which ages were 

 assigned, spatial resolution, and within-sample vari- 

 ability. Probed points of replicate transects could not 

 be directly "lined up" with respect to annuli. This 

 offset occasionally resulted in the interpretation that 

 an annuli was associated with a peak in one transect 

 and a nadir in the other transect (e.g., annuli 15, 18, 

 and 19 in Transect 1 vs. these annuli in Transect 2 

 for MD-1; Fig. 2). Transects 1 and 2 for Sample MD-1 

 (20;jm step size) sampled few points between succes- 

 sive annuli at older ages, and the accuracy with which 

 points could be assigned to annuli was less (Fig. 4). 

 Transect 3 for MD-1 (13fim step size) clearly shows 

 increased resolution of the ratio across annular in- 

 crements. Similarly, Transect 2 (13jjm) for MD-3 

 revealed several more peaks and nadirs after the 

 5th annulus than did Transect 1 (20^m). The overall 

 Sr/Ca ratio was significantly different between 

 Transects 1 and 2 for MD-3 (Table 4, t 3.06, p< 

 0.01). Replicate transects in MD-1, where step size 



