A Sei whale travels along the surface


A brief look at Sei whales 2009

John C. Conlon


On the morning trip of Monday, September 7 th, Capt. Todd Motta made the first tentative ID of a sei whale based primarily on his first (and good) look at the animal's tall and arced dorsal fin. The next two surfacings were at difficult distances and bad angles of sight. The animal had to surface several more times before I was able to get my first good look at it some 200 yards from the Dolphin Fleet’s, Portuguese Princess II. The whale allowed a great though still somewhat distant look. On that fourth sighting the animal lay profile to the boat at some 200 yards in the distance. The identification as a sei whale was then quick and straight forward.

The whale lay quite level in the water with its blowholes and dorsal fin simultaneously visible above the surface. This surfacing posture, allowing the blowholes and dorsal fin to be seen simultaneously, is typical of sei whales though it would not be expected from a fin whale which is the only other balaenopterid whale seen locally with which a sei whale is likely to be confused. The surfacing posture is crucial in sei whale identification because it would otherwise be very difficult to distinguish a large sei whale from a small fin whale. Sei whales might also be confused with Bryde’s whales though these are generally more subtropical and tropical in distribution. This allows for some range overlap in the lower temperate reaches of sei whale distribution. We would not, however, expect to find Bryde’s whales as far north as the Gulf of Maine.

Upon closer examination this particular sei whale’s left to right coloration was symmetric as opposed to the asymmetric left to right coloration of a fin whale. Having said this, sei whales do have similar blaze and chevron streaking to that of fin whales. The streaking on sei whales is more a blue-gray than the gray-white of fin whales. The streaking is also more evenly patterned on both sides of sei whales. In my experience the streaking has been difficult or impossible to see though this is likely due to the gray and overcast light conditions accompanying my sei whale sightings.

Sei whale respiration sequences and movement are also used by field workers to distinguish sei whales from other balaenopterids. In this case two or three rather diffuse spout sequences were followed by several minute duration dives. This is fewer breaths and shorter dive times than would be expected from a fin whale. This animal's motion through the water was also quite erratic. Its motion was regularly back and forth through a small area. This would generally be interpreted as subsurface feeding if it were a fin whale. In the case of sei whales, however, they are often reported as moving in erratic patterns when searching for food as well as feeding. It was thus not possible to make the searching or feeding determination in this case. The behavior combination of diffuse spout, respiration sequences, and erratic movement makes sei whales very difficult to observe.

krill and sand lance

While sei whales tend to stay farther offshore toward the edge of the continental shelf it is not surprising that we had this opportunity. Over the previous two weeks we had several days where we observed whales and pelagic birds feeding on large surface concentrations of krill. Sei whales are well known as a balaenopterid whale capable of skim feeding on planktonic copepods and krill. The dense patchiness of copepod and krill swarms necessitates these sei whales having to swim back and forth through the same small area of water before moving on to the next prey patch. Hence it is difficult to distinguish between sei whales searching for food versus their actual feeding if the food is any distance beneath the water’s surface.

sei whale’s dorsal fin

In our sighting area sei whales are rarely and unpredictably seen as singles, pairs or triplets. They are however known to arrive in larger groups and stay within the area for short periods of time. They then disappear not to be seen as individuals or groups potentially for many years. Their occasional movements into and out of our area are likely predicated by food availability.

Over these years my experiences with sei whales have been a single, a pair, and a triplet. And, on an evening trip some five years ago we observed some 20 sei whales skim feeding in the company of northern right whales. Planktonic copepods form a significant part of both of these species’ diets. In the case of those sei whales it was possible to watch the rhythmic pulsing of the rorquals and tongue as the animals skimmed the water’s surface. In all these cases my sightings were one time only regardless of the number of individuals or their activity. In all cases the animals spent their time in our area and then moved on to areas beyond our whale watch reach...

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