NOTE Ahrenholz et al.: Periodicity of increment formation in otolitfis of Brevoortia tyrannus 



423 



a 



a. 



E 



25 



20 



15 



10 



5 - 



40 



35 



~ 30 



25 



20 



15 



10 



20 



40 



60 



80 



100 



120 



140 



Duration (days) 



Figure 1 



Environmental conditions experienced by two groups of ALC- 

 marked larval-juvenile Atlantic menhaden (Brevoortia tyrannus) 

 during the otolith validation study. The upper panel depicts tem- 

 perature histories and the lower panel, the salinity histories. The 

 high-salinity (low-temperature) rearing experiment ended after 

 75 days owing to high mortality rates. 



tion of the study; larval menhaden advanced well 

 into their transformation into juveniles. Through day 

 60 of rearing, water temperatures steadily declined 

 to low winter minima. During the lower tempera- 

 ture period, the growth rates of fishes were reduced 

 (Fig. 2). The mean width of the otolith growth incre- 

 ments for fish from the high-salinity group gradu- 

 ally dropped below 1 mm following the severe drop 

 in temperature. During the same period, the incre- 

 ments for the low-salinity group reflected the higher 

 growth rates for this group and were wider, with all 

 mean widths above 1 pm (Fig. 3). 



Similarity between regression parameters of incre- 

 ment counts versus days since marking between the 

 high- and low-salinity regimes was tested over the 

 shared range of duration (75 days). Tests for homo- 

 geneity of slopes showed no differences (P=0.679). 

 A subsequent ANCOVA analysis revealed no differ- 



ences between intercepts (P=0.712). Thus, the incre- 

 ment counts for the two rearing regimes were pooled 

 and validation hypotheses were tested with a regres- 

 sion analysis. Results of this analysis revealed that 

 the intercept was not significantly different from zero, 

 and the slope was not significantly different from one 

 (Fig. 4, Table 1). The confidence interval (95%) for 

 estimates of daily age obtained from otoliths of fish 

 which encountered conditions similar to those tested 

 here, would be ±9 to 10 days over a range of 140 days 

 (approximately 50 to 190 days of age). 



Discussion 



Growth increments on sagittal otoliths of fall- 

 spawned Atlantic menhaden formed daily, even when 

 experimental water temperatures declined to 3°C 



