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

Estimación de la edad en el delfín común de rostro largo, Delphinus capensis, del Golfo de California THERYA, agosto, 2014 Vol.5(2): 449-460 DOI: 10.12933/therya-14-198 Age estimation on long-beaked common dolphins, Delphinus capensis, from the Gulf of California Juan Pablo Gallo-Reynoso1*, Edna O. Francisco1,2 y Charles Leo Ortiz3 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. Key words: Age estimation, common strandings, Delphinus capensis, dolphins, Gulf of California, longbeaked. Resumen Se estimó la edad de 41 delfines comunes de rostro largo Delphinus capensis, encontrados varados en playas de Sonora y Baja California, entre 1981 y 2000. El objetivo de nuestro trabajo fue el de conocer la edad y estimar la longitud estándar, y en lo posible conocer las causas del varamiento. El crecimiento de las capas de dentina en dientes fueron obtenidas a partir de la técnica de rebajado con ácido, lo cual involucra la preparación de los dientes desgastados hasta la mitad con lijado y Pulido con lijas (grano de 220-600) y desmineralización con ácido fórmico al 5%. Se utilizó Centro de Investigación en Alimentación y Desarrollo, A. C. Unidad Guaymas. Laboratorio de Ecofisiología. Carretera a Varadero Nacional km 6.6. Las Playitas, Guaymas 85480, Sonora, México. E-mail: (JPGR) 2 Woodman Ave. 9235- J, Arleta, CA 91331. United States. E-mail: (EOF) 3 University of California Santa Cruz. Earth and Marine Sciences A 308. 1156 High Street Santa Cruz, CA 95064. United States. E-mail: (CLO) *Corresponding author. 1 LONG-BEAKED COMMON DOLPHIN AGE, GULF OF CALIFORNIA un microscopio estereoscópico para contar los grupos anuales de capas de crecimiento (annual growth layer groups o GLGs). Cuatro diferentes lectores asistieron para obtener las estimaciones de la edad. Las categorías de edad estimadas fueron: 17 individuos eran adultos (12-21 GLGs), 10 individuos fueron subadultos (7-12 GLGs), 10 individuos juveniles (2-7 GLGs), y cuatro individuos fueron crías lactantes (<1 GLG). Se obtuvieron las medidas y el sexado de 15 individuos en campo (6 crías, 6 subadultos y 3 adultos), los individuos más largos, fueron los más viejos: el macho más largo fue de 264.7 cm con 21 GLGs, y la hembra más larga con 252.4 cm con 18 GLGs (un GLG corresponde a un año). Independientemente del sexo de los delfines, la regresión de la edad sobre la talla fue significante con una r2 = 0.7068, P < 0.0001. La estimación de la edad puede servir para conocer la vulnerabilidad y la incidencia de la mortalidad a diferentes edades de los delfines comunes de rostro largo, debido por ejemplo a la mortalidad causada por efectos de algas tóxicas (ácido Domóico) debido a la ingesta de peces pelágicos menores que acumulan esas toxinas, o a la magnificación de toxinas hechas por los humanos y usadas en la agricultura. También puede proveer de información sobre las interacciones con las pesquerías y la mortalidad de D. capensis in el Golfo de California. Palabras clave: delfines comunes de rostro largo, Delphinus capensis, estimación de edad, Golfo de California, varamientos. Long-beaked common dolphin, Delphinus capensis, is the most common cetacean in the Gulf of California, with sightings in all seasons and throughout the entire Gulf (GalloReynoso and Alessio-Robles 1989; Gallo-Reynoso 1990; Gallo-Reynoso 1991; Vidal et al. 1993; Silber et al. 1994; Gallo-Reynoso 1998) with groups as large as 5,000 individuals (Gallo-Reynoso 1991). Much of the distribution of long-beaked common dolphins is over upwelling areas (Gallo-Reynoso and Alessio-Robles 1989; Gallo-Reynoso 1998), which influences the overall higher primary production rates of the Gulf of California (ÁlvarezBorrego 1983). This high primary production determines their prey distribution in coastal areas, consisting mainly of small pelagic species, such as sardines (Sardinops caeruleus), Pacific mackerel (Scomber japonicus), anchovy (Engraulis mordax), threadfin herring (Opisthonema libertate), herring (Harengula thrissina), schooling carangids (Decapterus macrosoma), hake (Merluccius angustimanus), lantern fishes (Triphoturus mexicanus and other myctophids), and neritic squids (Lolliguncula panamensis, Loliolopsis diomedae; Gallo-Reynoso 1991; Niño-Torres et al. 2006; Gallo-Reynoso unpublished data). The fact that dolphins are born with a permanent set of teeth (Myrick 1991) allows determining the approximate age of a particular individual with a high degree of accuracy. The technique for aging marine mammals consists on the counting of dentinal layers present on the teeth. This strategy has been used for aging large-toothed animals such as the California sea lions, Zalophus californianus (Tovar-Aguilar 1989), the sperm whale Physeter macrocephalus, and other cetaceans (Scheffer and Myrick 1980), the bottlenose dolphin Tursiops truncatus (Hohn 1990), and the short-finned pilot whales Globicephala macrorhynchus (Kasuya and Matsui 1984; Égido-Villarreal 1989); the teeth of these species have been used to obtain reliable measure of age (Hohn 1980; Kimura 1980; Hohn 1990). The aim of this study is to accurately estimate the age of long-beaked common dolphins from the Gulf of California on the base of counting dentinal layers. 450 THERYA Vol.5(2): 449-460 Introduction Gallo-Reynoso, Francisco y Ortiz Methods All analyze (...truncated)