The sei whale, Balaenoptera borealis, is a streamlined member of the rorqual family. As a baleen whale, this giant relies on specialized structures within its mouth to filter vast quantities of small organisms from the water. Unlike many close relatives, the sei whale has developed a highly flexible feeding strategy that allows it to thrive across diverse marine environments. Understanding the diet of this fast-swimming animal requires an examination of the tiny organisms it targets. This article explores the specific prey categories and the adaptive foraging behaviors that define the sei whale’s sustenance.
Primary Prey Categories
Despite its massive size, the sei whale primarily consumes small organisms, eating an average of about 2,000 pounds of food each day. The diet depends on three main groups: crustaceans, small schooling fish, and, less frequently, cephalopods. The most significant component is zooplankton, including copepods and euphausiids (krill).
Sei whales exhibit a notable preference for copepods, particularly the species Calanus tonsus in certain regions, distinguishing them from other rorquals. Copepods are often the primary food source, with krill serving as a secondary choice when copepod populations are less abundant. By targeting these small organisms, the sei whale occupies a lower trophic level than many other large baleen whales.
The diet is supplemented by small schooling fish, which they opportunistically consume when available in dense patches. Examples include anchovies, sardines, Pacific saury, and juvenile rockfish. These fish represent a higher-calorie meal crucial for building the fat reserves needed for migration.
Cephalopods, such as squid, are an occasional part of the sei whale’s intake, though they are less common than the other two groups. This variety allows the sei whale to be highly adaptable to the fluctuating conditions of the open ocean.
The Sei Whale’s Specialized Feeding Technique
The anatomy of the sei whale’s mouth is highly adapted for a dual feeding strategy, but it is primarily known for “skimming” or “grazing.” This method involves the whale swimming continuously through dense patches of prey with its mouth partially open. Water and captured organisms flow into the mouth and are filtered out through the baleen plates.
The sei whale has physical features that favor this continuous filtration method. Its baleen plates are relatively longer and hang at a more vertical angle, with finer bristles on the inner fringe. This structure creates a tighter filter, which is more effective at trapping very small prey like calanoid copepods.
While skimming is characteristic, sei whales are also capable of “gulping” or lunge-feeding, similar to their rorqual relatives. Anatomical differences, such as shorter ventral grooves on the throat, limit the maximum expansion of the buccal cavity. This suggests their gulping technique is less voluminous than that of a blue or fin whale, making skimming more energetically favorable for their typical, smaller prey. The ability to switch between these two methods provides the species with significant ecological flexibility, allowing them to exploit different types of prey concentrations.
Geographic and Seasonal Shifts in Diet
The sei whale’s diet is highly opportunistic and tied to its long-distance annual migration patterns. These whales generally move from low-latitude wintering grounds to high-latitude, cooler summer feeding areas. This movement dictates a significant seasonal shift in the species’ primary food source.
During the summer months in productive subpolar waters, the whales gorge on the most abundant plankton, often euphausiids or dense concentrations of copepods. The availability of these organisms in the cold water is the main driver for their presence in these high-latitude feeding grounds. Prey availability can vary dramatically, which results in the sei whale’s distribution being highly nomadic and unpredictable.
The concept of “prey switching” is central to the sei whale’s survival, providing an ecological advantage. For instance, off central California, sei whales may feed predominantly on anchovies during the early summer before shifting to krill later in the season. This adaptability allows them to utilize whatever food source is locally concentrated, ensuring they build up required energy stores before migrating back to warmer waters.