SECTION n 

 AGE STRUCTURE 



Lany J. Hansen 



Southeast Fisheries Science Center 



Miami Laboratory 



75 Virginia Beach Drive 



Miami, FL 33149 



Hersh (1988) found that the age structiire of bottlenose dolphins stranded during the 

 1987-88 U.S. east coast dieoff differed significantly from the available pre-dieoff composite 

 sample. Specifically, a significantly larger proportion of 5 to 9 year olds stranded during the 

 dieoff. This may have reflected that the hypothesized disease epidemic (Geraci, 1989) caused 

 proportionally higher than normal rates of mortality among an age group which usually 

 exhibits a relatively low mortality rate (Hersh, 1988). 



Methods 



An age analysis was conducted on the available sample of teeth from animals that 

 stranded along the Texas coast to evaluate the age structure of the 1990 strandings as 

 compared to the composite sample of previous years. The teeth were examined for growth 

 layer groups (GLG or GLGS; see Perrin and Myrick, 1980) by Ms. S. Fernandez, with 

 assistance from Dr. A. Hohn, a leading expert on techniques for aging odontocete cetaceans. 

 The methods used followed those detailed in Myrick et al (1983) as modified for bottlenose 

 dolphins (Hohn et aL 1989). The sample included animals that stranded from 1983-1990. Of 

 this sample, 195 stranded during January- June, of which 70 were from 1990. 



Results and Discussion 



The cumulative distributions of ages of the two samples (animals from 1983-89 and 

 animals from 1990) are shown in Figure 1. These distributions were compared using a 

 Kolmogorov-Smimov two-sample test. There were proportionally more young animals in the 

 1990 sample, but the results of this test indicate that the cumulative distributions were not 

 significantly different (DN=0.186, p>0.10). 



The length data form a larger and probably more representative sample of the 

 stranded animals. A comparison of the cumulative distributions of the 1990 zmd 1983-89 

 length samples (Figure 2) indicates that proportionally more short (i.e., young) animals 

 stranded during 1983-89. The distributions were significantly different (DN=0.0987, p<0.04). 



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