Secor and Piccoli: Oceanic migration rates of Mo/one soxotilis, determined by otolith microchemical analysis 



65 



Age class (yr) 



4-6 



7-9 



10-12 



13-18 



some degree of oceanic incidence (fish 

 identification [ID] number=98, 198, 

 260) throughout portions of their lives 

 and many females exhibited a pattern 

 of estuarine use (e.g., ID = 197, 263, 

 271, 272, 280, 281). A single instance 

 of freshwater residency was observed 

 for a fairly long-lived female (ID=271; 

 age = ll years; FL = 875 mm). Mean 

 lifetime salinity exposure differed sig- 

 nificantly between males and females 

 (Kruskal-Wallis ANOVA; P=0.01), 

 with females exhibiting an average 

 10% higher use of high-salinity habi- 

 tat during the mature portion (age>6) 

 of their lives (Fig. 2). 



The male sample, albeit small 

 (7! =10), did not exhibit age-dependent 

 patterns in salinity exposure (Fig. 2). 

 In part, high variance in older age 

 classes (8-12) obscured any pattern 

 of salinity exposure at ages <6 years. 

 Females showed a strong and nearly 

 linear trend of increased salinity ex- 

 posure with age (Fig. 2). Modal salin- 

 ity increased from a range of 20-25 

 to a range of 25-30 for ages 2 and 7, 

 respectively. Interestingly, both males 

 and females showed that a polyha- 

 line (salinity>18) habitat was used during the period 

 between age 1 and 2 years. Thus, yearlings may be 

 preferentially using polyhaline regions, followed by a 

 return of some individuals to lower salinity regions 

 (because slightly depressed salinities were observed for 

 ages 2 and 3 compared to age 1 yr in Fig. 2). We have 

 observed a similar pattern in Hudson River striped 

 bass (Zlokovitz et al., 2003). The ontogenetic trend of 

 increased salinity exposure with age in females could 

 be related to maturation (ages 6-8 years) — a pattern 

 observed in two-thirds of female fish (Fig. 3). Males 

 also showed a rise in salinity exposure with age, albeit 

 less consistently (Fig. 4). 



Spawning frequency 



Life history transects for the sample of ten males and 

 thirty females gave evidence of strong intra-annual pat- 

 terns in the salinity levels of their habitat (Figs. 3 and 

 4). Intra-annual trends often showed nadirs at or near 

 the opaque zone of the otolith, a pattern occurring in 

 both males (e.g., ID = 124, 196, 327) and females (e.g., 

 ID = 192, 297, 298). In some instances, seasonal cycles 

 indicated either less than annual (Fig. 3: ID = 320; Fig. 

 4: ID=298, 300) or greater than annual (Fig. 3: ID = 99: 

 Fig. 4: ID = 295, 300) cycles in patterns of salinity 

 exposure. 



Significant seasonal effects on patterns of salinity 

 exposure were observed for ages 6 and 8 and for pooled 

 ages 6-11 (Table 2), indicating that across individuals 

 there was a seasonal pattern in salinity exposure for 



Table 1 



Degree of oceanic incidence of Chesapeake Bay striped bass collected in 2000. 

 Data is presented for age classes by sex, and for size-class pooled sexes (the 

 latter to permit comparison with results of Dorazio et al., 1994). Lower (LCL) 

 and upper (UCL) confidence limits are presented for oceanic incidence by age 

 class. 



Oceanic incidence (% ) 



Males in; LCL-UCL) 



Females (n; LCL-UCL) 



20.0(5:0-41.3) 

 25.0(8:0-50.2) 

 50.0(18:37.4-62.5) 

 44.4(9:34.0-77.4) 



55.5(18:42.5-68.1) 

 58.8(51:56.8-62.5) 

 76.9(13:60.8-93.6) 



Oceanic incidence (%) 



Dorazio et al. (1994) 



mature age classes. The other analysis of five, rath- 

 er than four, seasonal points of measurement in the 

 otolith did not show strong evidence for seasonality, 

 although pooled age classes 6-11 and 7-11 showed 

 marginal significance at P=0.06. In the selection of 

 individuals that contained five seasonal points, sample 

 sizes were substantially reduced. This reduced sample 

 size in turn would have resulted in less statistical 

 sensitivity. Despite high variances among individuals 

 for each seasonal measurement, there was a trend for 

 a seasonal nadir in salinity exposure near the opaque 

 zone on the otolith (seasonal interval=0) (Fig. 5). This 

 trend was especially apparent in males, and in females 

 >6 years. 



There was a slight indication of sex-related differ- 

 ences in seasonality of salinity exposure. Significant 

 between-sex differences occurred for Sg at ages 8 and 

 11, and all mature age class groupings (Table 2). Sig- 

 nificant differences occurred for Sg 25 at ages 8 and 9 

 and mature age groupings 7-11 and 8-11; and for Sg 5 

 at age 8 and all mature age groupings. Males were 

 found at lower salinities at these seasonal points than 

 were females (Fig. 5). 



X-ray maps of Sr within otolith sections showed clear 

 concentric patterns of alternating high and low regions 

 of Sr in association with annuli (Fig. 6). X-ray maps 

 confirmed the cyclical patterns observed in the life 

 history transects (Figs. 3 and 4), but with greater ap- 

 parent difference between peak and nadir levels of Sr. 

 In one of the X-ray maps (Fig. 6, bottom panel), a high 

 level of Sr occurred within the first annulus). 



