A note on humpback whales off the coast of Ecuador during the 1997 ‘El Niño’ event

The southeastern Pacific humpback whale stock was studied for seven years (1991-1997) on the central coast of Ecuador (1°25’S, 79°55’W) during the breeding season (June-September). Boat trips were conducted from two different sites, Puerto López and Puerto Cayo, following well-defined routes offshore. In 1997, a strong El Niño affected the eastern Pacific area. In order to try and investigate possible climate-induced shifts, results obtained from 1996, a ‘normal’ year, were compared with those obtained in 1997, when water temperature was 4°C above its historical mean. Four factors were considered: whale encounter rate; distribution; group structure; and crude birth rate. The whale encounter rate decreased in 1997 for both sites: 11.2% for Puerto López and 8.7% for Puerto Cayo, although the differences were not statistically significant (p > 0.05). Group distribution in relation to water depth was not significantly different (p > 0.05), nor were the mean distances from sighting sites to port (p > 0.05). Group size was equal in both years for Puerto López, but in Puerto Cayo it was larger in 1997, although not significantly (p > 0.05). Group composition was not significantly different (p > 0.05). No difference in birth rate was found in Puerto López. Results for Puerto Cayo are difficult to interpret. No changes in the investigated parameters were found in the study area during El Niño 1997. Since humpback whales do not feed in tropical waters, they may not be as vulnerable to El Niño events as other marine mammals.