Sightings of Blue whale ( B. musculus ) calves and calving intervals for known females in the Sea of Cortez from 1984-2001 and subsequent observations off California

Abstract:

A photo-identification catalogue of 255 individual blue whales ( Balaenoptera musculus ) was compiled from 1984 to 2001 in the Sea of Cortez, of which 45 were determined to be females either through molecular analysis of sloughed skin samples or because they were accompanied by a calf. A total of 42 calves, estimated at between 35-45ft in length, was observed, which suggests that they were one to three months old. Four of these calves were seen again in the Sea of Cortez and photographed in subsequent years indicating some site fidelity. Two other calves first observed in the Sea of Cortez, but not re-sighted there, were later observed off California - one just four months after its initial March sighting off Loreto. Although very little is known about calving in blue whales, a minimum calving interval of two years is generally accepted, with gestation taking 10 to 11 months and nursing lasting approximately 7 months. In this study four females were observed with a second calf, with calving intervals ranging from 2 to 7 years. Matching of photographs revealed that a significant proportion of the known females (28) observed in the Sea of Cortez had been photo-identified off California (11). Nine of the 11 seen off California were known to have had calves. The high sighting rate of females and calves in the area, as well as the repetition of sightings suggests that the Sea of Cortez is regularly used as a nursery for blue whales. The relative high number of calves observed (16.1%) could be indicative of a recovering population with high recruitment rate, or that a higher proportion of reproducing animals move into the Sea of Cortez during winter months. A crude birth rate from the Sea of Cortez sightings ranges between 0,063 and 0,167 with an average of 0,1.

Sears, R., C. Ramp and J. Calambokidis. 2001. Sightings of blue whale ( B. musculus ) calves and calving intervals for known females in the Sea of Cortez from 1984-2001 and subsequent observations off California . Poster presented at the 14th biennial conference on the Biology of Marine Mammals, Vancouver, Canada, November 28 – December 3, 2001.