Pelicans and vultures in Baja California

In March of this year, we saw a large number of brown pelicans at the beach between the Pacific Ocean and the lagoon of the bird sanctuary in Todos Santos, Mexico. Pelicans are not the only birds in this area. Over 70 bird species are said to live here or come (like us) as seasonal visitors. Looking upward to the sky, we saw many other birds flying around and surveying the area: vultures. In fact, on the trail between the lagoon and the rocky slope, we passed a few spots covered with several dead pelicans. So, the vultures shouldn't be such a surprise.
Why were there so many dead pelicans. Natural Death? Pollution? Or domoic acid from toxic diatom blooms? In the literature, I found an article reporting and analyzing the massive death of Pelecanus occidentalis in January 1996, at the tip of the Baja California peninsula [1]. This study relates the described event of sea bird death to the fact that pelicans were feeding on mackerel contaminated by domoic acid-producing diatoms. Domoic acid, a neurotoxin, can bioaccumulate in marine organisms that feed on phytoplankton. Thus, sea food can become harmful for sea birds. But I don't know if this explains what we see in Todos Todos.
Fortunately, the blanket of healthy looking pelicans at the beach seemed much larger than the spots of dead ones.

Reference
[1] A. S. Beltrán, M. Palafox-Uribe, J. Grajales-Montiel, A. Cruz-Villacorta and J. L. Ochoa: Sea bird mortality at Cabo San Lucas, Mexico: Evidence that toxic diatom blooms are spreading. Toxicon 1997, 35 (3), pp. 447-453. DOI: 10.1016/S0041-0101(96)00140-7