BIASES IN DOLPHIN AGE STRUCTURE DUE TO AGE ESTIMATION TECHNIQUE1

Authors

  • Aleta A. Hohn,

    1. National Marine Fisheries Services, Office of Protected Resources, 1335 East-West Highway, Silver Spring, Maryland 20910, U. S. A. E-mail: aleta.hohn@noaa.gov.
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    • 2

      Current address: NMFS, Southeast Fisheries Science Center, 101 Pivers Island Road, Beaufort, North Carolina 28516, U. S. A.

  • Stephanie Fernandez

    1. Dolphin Biology Research Institute, % Marine Mammal Program, NHB-108, Smithsonian Institution, Washington, DC 20560, U. S. A.
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  • 1

    As a young graduate student, I first met Ken Norris amidst the collection of marine mammal bones in the basement of the Smithsonian's Museum of Natural History. He revealed that he was visiting to test an off-beat idea about dolphin cranial structure that might explain some of their acoustic abilities. After a day or two, he concluded that he was off the mark but at least had pursued the possibility and could now reject the hypothesis. Later, in a book on spinnner dolphins, he eloquently described his approach to scientific inquiry. He strove to understand nature through observing and hypothesizing, and then testing and retesting. The approach works, he wrote “because nature is not reluctant to point out my mistakes, and because I have trained myself to discard fallen hypotheses. But one must reconcile oneself to creeping up on truth through a thicket of these fallen hypotheses.” Driven by this philosophy, Ken would appreciate the intent of this paper, to discard a previous hypothesis in favor of one that better reflects nature.

Abstract

The use of different tooth-preparation techniques resulted in widely different estimates of age in a sample of bottlenose dolphins, Tursiops truncatus. Teeth from 30 animals were prepared using the two most prevalent techniques reported in the literature for this species, unstained sections and decalcified and stained thin sections, and the resulting paired counts of growth layers were compared. Estimates from the two methods were identical or at least placed the specimen in the same age class in only five cases, ranging in age from 2 to 22 yr. Otherwise, the results fell into one of two categories: when the estimates were close (± 3-yr difference, n= 15), counts from unstained sections generally were higher (13 cases, age from unstained sections 2-20 yr); when the counts were more disparate, estimates from stained sections always were higher (6-31 yr difference, n= 10, age from unstained sections 12-27 yr and corresponding ages from stained sections of 27-47). Previous studies of age estimation in known-age bottlenose dolphins indicate that stained sections allow accurate estimates of age and demonstrate that maximum lifespan approaches or exceeds 50 yr. In contrast, the results herein suggest that using unstained sections for age estimation may result in imprecise or biased age-structure data.

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