Sei whales (Balaenoptera borealis) are relatively rare in Hawaiian waters. In our study (22 years as of 2021) we’ve only encountered them on three different days. Our first two sightings were during our February/March 2018 Lāna‘i field project, and we encountered a single individual during our November 2021 Kona field project. The first known sighting of sei whales in Hawaiian waters was by Japanese whalers in March 1973 near French Frigate Shoals in the Northwestern Hawaiian Islands. A dozen or so other confirmed sightings have occurred between these encounters, ranging from offshore waters to the Northwestern Islands and the main Hawaiian Islands. Sei whales can be difficult to spot due to their tendency to be farther offshore and difficulty distinguishing them from other species. We have a small photo-ID catalog of sei whales from Hawaiian waters, so if you have or know of any photos of this species here please contact us!
Colin Cornforth and Shannon Harrison encountered three sei whales on February 16, 2018 when bringing the research vessel from Kaua‘i to Maui
Adult sei whales are about 12 to 17m long, roughly the same size as adult humpback whales, but with much more streamlined bodies (typical of the rorqual whales). They have a single ridge running down the center of their heads, similar to fin and blue whales but unlike the Bryde’s whale which has three. To distinguish fin and sei whales, first look at the angle at which the dorsal fin rises from the back. The sei whale has a steep rise before curving back, while the fin whale’s dorsal fin rises more gradually. The tip of a sei whale’s head is also downturned, and the coloration of their lower jaw is symmetrical (unlike the fin whale). Sei whales also have a very shallow surfacing profile.
We were able to get underwater footage of the sei whale on March 1, 2018
Sei whales were hunted heavily in the North Pacific by commercial whaling operations in the twentieth century. Nearly seventy-four thousand individuals were killed during this time, and sei whale populations are thought to have been reduced to 20 percent of their pre-whaling numbers.
Sightings are likely to remain infrequent, as there is no evidence that sei whale populations are recovering in the North Pacific. Estimated abundance from the 2010 NMFS survey is about 390 individuals in Hawaiian waters, though it is possible this number increases during winter when they are most likely to be in the area.
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Updated November 2021.