26. Grandes Islas del Golfo de California/Bahía de Los Ángeles

Site Overview

This region, called “the Galapagos of Mexico” by John Steinbeck in The Sea of Cortez, is located in the north-central portion of the Gulf of California. It is composed of the large island group in the center Gulf, frequently referred to as the Midriff Islands, and Bahía de Los Ángeles, along the eastern coast of the Baja California Peninsula. Bahía de Los Ángeles is comprised of hills and mudflats, while the Midriff Islands are lined with rocky reefs. Most of the islands were formed when they broke off from the peninsula early in its history, although some, such as Isla Partida, are volcanic.

Trenches and basins line the ocean floor in this area. The two basins along Isla Ángel de la Guarda and Isla San Lorenzo are 1,650 and 800 meters (5,410 and 2,620 feet) deep, respectively. Upwelling results from these deep pockets in the seafloor, such as the trench that runs from Isla Pelicano in the upper reaches of the Gulf of California to the north end of Wagner Basin in the Midriff Islands. This upwelling results in rich nutrient inputs. Strong tidal currents and winds are also present, resulting in increased productivity. Bahía de Los Ángeles experiences coastal currents and that, along with its shape, makes it a coastal retention zone. The area as a whole is extremely productive of biomass and contains a high degree of biological diversity and endemism.

Fish spawning aggregations and the high abundance of pelagic fishes, such as sardines and anchovies, which together represent thousands of tonnes of Mexico’s annual fish catch, are just two of the reasons for the successful fisheries here. Craveri’s murrelet (Endomychura craveri), which is endemic to Mexico and listed as vulnerable on the IUCN Red List, nests on Isla San Esteban and Isla Tiburón, among others. Isla Rasa is home to 90 percent of the world’s Heermann’s gull breeding population (approximately 150,000 pairs) and up to 95 percent of the world’s elegant tern breeding population (approximately 30,000 pairs) (BirdLife International 2000). More than 50 species of birds, including the brown pelican from as far north as the Columbia River in the United States, can be found in this area.

The bays and area surrounding the islands are also important feeding and overwintering grounds for marine turtles that spend part of the year in the Gulf (green, loggerhead and olive ridley). Whale sharks can be found in Bahía de Los Ángeles.

A unique biotic association of marine mammals occurs in this area, including the blue whale, the common dolphin (Tursiops truncatus), which forages in the area, and a large sea lion rookery on the Baja California Sur mainland, just west of Isla Ángel de la Guarda.

Along the mainland, resource over-exploitation, coastal alteration and pollution, and damaging recreational use are minimal but on the rise, while on Las Grandes Islas del Golfo de California, these threats are expected to remain low. A management plan for the Biosphere Reserve is available that discusses the biodiversity and threats to the area (Conanp 2003).