Age estimation on long-beaked common dolphins, Delphinus capensis, from the Gulf of California

Abstract

Introduction: We estimated the age of 41 long-beaked common dolphins, Delphinus capensis, found stranded in Sonora and Baja California beaches in the Gulf of California from 1981 to 2000. The aim of our work was to know the age and if possible the size of stranded dolphins and the possible causes of their stranding.Methods: Dentinal growth layers in teeth were revealed using an acid-etching technique; it involves preparation of half-etched teeth by grinding and polishing with sandpaper (220-600 grit) and demineralization in 5 % formic acid. A stereo-microscope was used for counting annual growth layer groups (GLGs). Four different readers of dentinal layers assisted to obtain the estimate of age.Results: Age category estimates were: 17 individuals were adults (12-21 GLGs), 10 individuals were subadults (7-12 GLGs), 10 individuals were juveniles (2-7 GLGs), and 4 individuals were nursing calves (<1 GLG). 15 individuals were measured and sexed on the field (6 calves, 6 subadults and 3 adults). Larger dolphins were the older ones: the longest male of 264.7 cm presented 21 GLGs, and the longest female of 252.4 cm was 18 GLGs (one GLG corresponding to one year). Independent of the sex of the dolphins, the regression of age on length was significant with an r2 = 0.7068, P < 0.0001. Discussion: Age estimates can serve to know the vulnerability and incidence of long-beaked common dolphins of different ages due to toxic algae effects (Domoic acid) or to the biomagnification of man made toxins used for agriculture, due to the ingestion of small pelagic fishes that accumulates these toxins. It also can provide insight in fishery interactions and mortality on D. capensis in the Gulf of California.