Fin whales (Balaenoptera physalus) foraging on daytime surface swarms of the euphausiid Nyctiphanes simplex in Ballenas Channel, Gulf of California, Mexico

We identified specific areas where fin whales (Balaenoptera physalus) forage on daytime surface swarms of euphausiids in Ballenas Channel, Gulf of California. During 2003 and 2004, 4 field trips per year were carried out (winter-spring, spring, summer, and autumn). Systematic line transects and random surveys for whales were conducted in small boats (6 m) and euphausiid swarms were sampled with a zooplankton net. Fin whales foraged mostly in shallow waters (10-100 m) off the west and south coast of Coronado Island and were observed feeding on 18 of 19 swarms of Nyctiphanes simplex (5 in 2003 and 14 in 2004). Swarms occurred from March to early August and were almost absent from October to November. Although fin whales were sighted throughout the study area, there was a characteristic distribution pattern of feeding activity, mainly during spring and summer seasons. Examination of our data shows that the adjacent waters off Coronado Island constitute a unique seasonal foraging habitat for fin whales in the Gulf of California.