Changes in the relative abundance and distribution of gray whales (Eschrichtius robustus) in Magdalena Bay, Mexico during an El Nino event.

The oceanographic changes during the EI Nino and La Nina of 1998-1999 have provided an opportunity to document the effects of these events on the migratory patterns of gray whales (Eschrichtius robustus). Magdalena Bay is located at the southernmost point of the gray whale's annual breeding migration along the Baja California Peninsula in Mexico. Standardized surveys were conducted for three consecutive seasons (1997-1999) in Magdalena Bay, enabling an assessment: of the changes in the relative abundance of gray whales with altered temperature. The average temperature of the bay in 1998 was 4.2 degrees C higher than in 1997 and 5.8 degrees C higher than in 1999. The number of whale sightings during that time period was inversely related to temperature, with nearly 15 times greater relative abundance observed in 1999 than in 1998. It is hypothesized that the decreasing temperatures associated with the La Nina event of 1999 may have induced more whales to travel farther south along their migration route. Alternative hypotheses are also explored. Comparisons of the three years are discussed with consideration of the importance of environmental conditions versus area fidelity as determining factors in the location of gray whale calving and breeding activities.