North Atlantic right whales eat tiny crustaceans called copepods, with one species in particular making up the bulk of their diet. Despite being one of the largest animals in the ocean, stretching up to 52 feet long, these whales survive almost entirely on organisms smaller than a grain of rice. An adult needs to consume roughly 425 to 570 kilograms (940 to 1,250 pounds) of copepods every single day to meet its energy demands.
Copepods: The Primary Food Source
The copepod genus Calanus, and specifically the species Calanus finmarchicus, is the cornerstone of the North Atlantic right whale’s diet. These tiny zooplankton are among the most abundant organisms in the North Atlantic and pack a surprising nutritional punch relative to their size. Right whales also eat other small invertebrates, including krill, smaller copepod species, pteropods (tiny swimming snails), and larval barnacles. But calanoid copepods remain the overwhelming dietary staple.
What makes Calanus finmarchicus so valuable is its fat content. These copepods build up large internal oil sacs filled with energy-rich lipids, essentially acting as tiny biological batteries. They accumulate fat from short-lived phytoplankton blooms and store it for months during a dormant phase spent in deep water. Right whales are particularly well adapted to digesting the specific type of fat (wax esters) these copepods carry. The energy packed into a single copepod varies significantly by region: individuals found in northern waters like the Gulf of St. Lawrence can contain two to three times the energy of those found further south in the Gulf of Maine. A copepod from the Gulf of St. Lawrence carries roughly 13 joules of energy, compared to about 4 to 5 joules for one from southern regions.
How Right Whales Feed
Right whales are skim feeders. They swim slowly through dense patches of copepods with their mouths wide open, letting water flow in and pass through hundreds of baleen plates that hang from their upper jaw. These plates are made of keratin, the same material as human fingernails, and act like a fine-mesh sieve. Water passes through, but the tiny copepods get trapped inside. The whale then uses its tongue to sweep the captured prey off the baleen and swallow it.
This feeding style looks deceptively simple, but it places strict demands on where and when right whales can eat. Skim feeding only works when prey is concentrated into dense patches. A diffuse scattering of copepods across open water isn’t worth the energy it takes to swim through it. Right whales need to find thick aggregations, sometimes containing tens of thousands of copepods per cubic meter, to make feeding profitable. Higher-fat copepods lower that threshold somewhat: a patch of large, lipid-rich individuals can be worth targeting even at moderate densities.
Where and When They Find Prey
Right whales feed both at the surface and at depth, depending on where copepods are concentrated. Copepods and other zooplankton undergo a daily cycle called diel vertical migration. They swim toward the surface at dusk to feed on phytoplankton under the cover of darkness, then sink back toward the seafloor before dawn to avoid fish and other visual predators. This synchronized movement creates dense layers of zooplankton near the bottom during daylight hours.
Research on the closely related southern right whale, using cameras mounted on tagged animals, has confirmed that right whales exploit these deep concentrations. Whales were recorded diving to depths of 73 to 100 meters to feed near the seafloor, where zooplankton biomass was five times higher than near the surface. Average dive times were just under seven minutes. At the surface, whales were observed skim feeding in the top 10 meters when copepods were abundant there, typically around dusk or dawn.
Historically, the Gulf of Maine and the Bay of Fundy were the primary summer feeding grounds for North Atlantic right whales. These areas supported massive concentrations of Calanus finmarchicus that the whales returned to year after year. But since around 2010, changing ocean conditions have disrupted these patterns. Warming waters have shifted where copepods are most abundant, and the whales have followed. Many now spend their summers further north in the Gulf of St. Lawrence, where denser, more energy-rich copepod patches are still forming.
Why Prey Quality Matters for Survival
For an endangered species with fewer than 350 individuals remaining, the quality of available food has consequences that go far beyond daily nutrition. Right whales need enormous caloric reserves to reproduce. Females must build up enough energy to sustain pregnancy and then nurse a calf for about a year. When prey is scarce or low in fat, females take longer between pregnancies, and calves may be born smaller and less robust.
The energy math is unforgiving. Bioenergetic models estimate that daily intake needs range from roughly 1,800 to 2,100 megajoules depending on the whale’s age, sex, and reproductive status. That translates to the 425 to 570 kilograms of copepods mentioned earlier, but only if those copepods are of average fat content. If the available copepods are smaller or leaner, the whale needs to find and consume even more of them, spending more energy in the process.
This is part of why the northward shift matters. Copepods in the Gulf of St. Lawrence are physically larger and carry substantially more fat than those in the Gulf of Maine. Even a modest increase in copepod body length, roughly 0.3 millimeters, can double or triple the energy each individual provides. For a whale that eats millions of copepods a day, that difference compounds enormously. The tradeoff is that the Gulf of St. Lawrence brings new risks, including ship traffic and fishing gear in areas where protections were not historically in place, because no one expected right whales to be there.