The North Atlantic Right Whale (NARW) is one of the world’s most imperiled large whale species. With a population numbering only a few hundred, understanding its diet and foraging habits is crucial for conservation efforts. This species has a highly specialized diet, linking its survival directly to the health and distribution of its prey.
The Primary Food Source
The diet of the North Atlantic Right Whale is highly specialized, consisting almost entirely of tiny marine crustaceans called copepods. While they may occasionally consume other small zooplankton, their survival depends on finding dense concentrations of these minute organisms. The definitive food source is the copepod genus Calanus, particularly the species Calanus finmarchicus.
This specific copepod species is especially important because of its high lipid content, which provides the necessary energy for a whale’s massive caloric needs. Calanus finmarchicus accumulates significant fat reserves, especially the Stage 5 (CV) copepodites, which are the second most energetically dense of the life stages. These lipid stores are maximized before the copepods enter a resting phase called diapause, making them a rich target for the whales.
Other copepod species, such as Pseudocalanus spp. and Centropages spp., are also consumed, particularly early in the feeding season or when C. finmarchicus is less available. However, these secondary prey generally offer less caloric value. The ability of the right whale to forage successfully depends on locating patches of prey that offer sufficient quantity and energetic quality to meet its daily requirements.
The energy demands of these whales are substantial, with a lactating female requiring the highest amount of daily energy. The reliance on a few species of high-energy copepods makes the right whale a specialist feeder. This strategy is successful when prey is abundant but precarious when prey distribution shifts. The entire life history of the whale is intricately linked to the predictable, seasonal abundance of its preferred copepod.
How They Filter Feed
The North Atlantic Right Whale employs a unique foraging strategy known as “skim feeding” to capture its tiny prey. Unlike other large whales that gulp massive amounts of water in a single lunge, the right whale swims slowly with its mouth partially open. This allows water and prey to continuously flow into the oral cavity.
This method is facilitated by the whale’s distinctive anatomy, including a highly arched upper jaw and long baleen plates. These plates are made of keratin and hang in rows from the upper jaw, reaching up to eight feet in length. Their inner edges are lined with fine, hair-like fringes.
As the whale moves forward, the water passes through these baleen fringes, which act like a sieve. The fringes trap the tiny copepods and zooplankton inside the mouth. The water is then expelled, and the concentrated mass of copepods is swallowed.
The efficiency of this continuous-flow filtration system is dictated by the density of the prey patch. Skim feeding is an energetically economical method, but it requires the whale to find highly aggregated prey. The whale’s large mouth is perfectly adapted for collecting prey that is individually minuscule but collectively abundant.
Locating Dense Prey Patches
Since individual copepods are minuscule, the North Atlantic Right Whale must locate and consume extremely dense aggregations to satisfy its nutritional requirements. A single whale can consume over a ton (approximately 2,000 pounds) of copepods daily when actively feeding. This necessitates finding prey patches with a concentration exceeding an estimated feeding threshold of around \(10^3\) copepods per cubic meter.
The distribution of these dense patches directly determines the whale’s annual migration and foraging locations. Historically, the whales have concentrated in specific areas off the coast of New England and Atlantic Canada, including Cape Cod Bay and the Great South Channel. These regions offer predictable, seasonal blooms of the high-energy Calanus finmarchicus.
The formation and location of these dense copepod patches are influenced by complex oceanographic factors, including currents and water temperature. Environmental changes, such as marine heatwaves, can disrupt the copepod life cycle, leading to shifts in their abundance and distribution. When the prey field in traditional feeding grounds becomes less concentrated or less energetically valuable, the whales are forced to move.
The whales are known to follow their food source, and changes in prey availability have led them to new foraging areas, such as the Gulf of St. Lawrence. This reliance on dense, localized prey means the whale’s presence in a given area is a strong indicator of a healthy, concentrated zooplankton community. Researchers now focus on mapping the distribution of these prey aggregations to better predict the whale’s movements for conservation.