Reconnaissance study of Colola and Maruata, the nesting beaches of marine turtles along the Michoacan coast in southern Mexico

Marine turtles of the species Chelonia agassizi navigate hundreds of kilometers across the open sea between their reproductive and nesting areas every year. This task is thought to be carried out by utilizing information from the earth's magnetic field and other physical cues associated with natal beaches. Hence, geomorphology, topography and geomagnetic field anomalies might be important for marine turtle homing. This study describes such features in Colola and Maruata, the nesting beaches of C. agassizi in the southern Pacific Ocean margin of Mexico. Colola, on the west, is an elongated linear open beach of 4.8 km length and 150 m wide, limited at the north by coastal fluvial deposits. Maniata, on the east, is an arcute 2.3 km long and 40 m wide beach bordered by coastal fluvial deposits on the northwest and by colluvial slope volcano-sedimentary deposits on the southeast sector. Topographic surveys and magnetic field observations in Colóla (Fall 1998/Spring 2000) documented that beach profiles change during a given and in between breeding seasons. In Maniata, magnetic anomalies range from ~50 nT at sea level to > 350 nT over the Vigía Mountain. Geomagnetic field vectors vary in inclination and intensity with latitude from the horizontal at Galapagos Islands to about 45° in southern Mexico, and from 30,000 nT to over 40,000 nT, respectively. Locally, inclination and intensity at Colola and Maruata are 44.87° and 40,364 nT and 44.85° and 40,353 nT, respectively. These parameters may impose constraints to the turtles' sensory ecology.