Ecological patterns, distribution and population structure of Prionace glauca (Chondrichthyes: Carcharhinidae) in the tropical-subtropical transition zone of the north-eastern Pacific

Abstract

Regional ecological patterns, distribution and population structure of Prionace glauca were analyzed based on samples collected on-board two long-line fleets operating in oceanic waters (1994–96/2000–02) and in coastal oceanic waters (2003–2009) of the eastern tropical Pacific off México. Generalized additive models were applied to catch per unit of effort data to evaluate the effect of spatial, temporal and environmental factors on the horizontal distribution of the life stages (juvenile, adult) and the sexes at the estimated depth of catch. The presence of breeding areas was explored. The population structure was characterized by the presence of juveniles' aggregations and pregnant females towards coastal waters and the presence of adult males' aggregations towards oceanic waters. The species exhibited horizontal segregation by sex-size and vertical segregation by sex. Distribution of the sex-size groups at oceanic waters was seasonally affected by the latitude; however, at coastal oceanic waters mainly females were influenced by the longitude. Latitudinal changes on the horizontal distribution were coupled to the seasonal forward and backward of water masses through the study area. Adult males showed positive relationship with high temperatures and high-salinities waters (17.0°–20.0 °C; 34.2–34.4) although they were also detected in low-salinities waters. The distribution of juvenile males mainly occurred beyond low temperatures and low-salinities waters (14.0°–15.0 °C; 33.6–34.1), suggesting a wide tolerance of adult males to explore subartic and subtropical waters. At oceanic areas, adult females were aggregated towards latitudes <25.0°N, mainly associated to subtropical waters during summer. The distribution of juvenile females indicated its preference by lower temperatures and more saline waters. Presence of pregnant females suggests that the eastern tropical Pacific off México represents an ecological key region to the reproductive cycle of P. glauca.