The spring hunger of right whales

The waters off Casco Bay hold a quiet vastness this morning, the surface barely disturbed by the southwest breeze. Somewhere beyond the islands, in the deeper channels where the continental shelf drops away, the ocean carries a different energy. The water here runs colder and richer than it did a month ago, thick with microscopic life that has traveled north with the warming currents.

North Atlantic right whales (Eubalaena glacialis) return to these waters each late spring, following an ancient hunger that pulls them from their calving grounds off Georgia and Florida. They arrive as some of the ocean's most endangered giants, fewer than 340 individuals remaining in the entire North Atlantic. What draws them here is invisible to us but abundant beyond measure: dense swarms of copepods, primarily Calanus finmarchicus, tiny crustaceans no larger than rice grains that drift in massive clouds through the water column.

The whales feed by swimming forward with their mouths agape, their baleen plates acting as enormous sieves that strain millions of copepods from each gulp of seawater. A single right whale can consume over a ton of these tiny animals each day, and they must. The females especially arrive here thin from months of nursing calves on milk that contains forty percent fat, their own bodies depleted from the demands of reproduction. The copepods represent more than food; they are survival itself, dense packets of lipids and proteins that will sustain the whales through another year of migration, mating, and the long journey south.

The copepods themselves follow rhythms older than the whales. They rise and fall through the water column each day, ascending toward the surface at night to graze on phytoplankton, then sinking to deeper waters as dawn approaches. Their populations peak in these waters during late spring and early summer, timed to the warming temperatures and the spring bloom of microscopic algae. The whales have learned to track these movements, diving to depths where the copepod swarms concentrate most densely. When conditions align, when the currents carry the richest concentrations of prey into the feeding areas, a right whale can meet its daily energy needs in just a few hours of focused feeding.

This relationship between whale and copepod plays out against a backdrop of constant change. Warming ocean temperatures shift the timing and location of copepod blooms, sometimes pushing them farther north or deeper than the whales expect. Ship traffic increases through these same feeding areas, bringing noise that can mask the low-frequency calls whales use to communicate across vast distances. Fishing gear poses entanglement risks that have killed more right whales than any other human activity. Yet the whales return each spring, their massive bodies moving through water that holds both abundance and peril in equal measure.

If you stand at the edge of Casco Bay this morning and look toward the open Atlantic, the water stretches unbroken to the horizon. The wind carries the salt scent of exposed rockweed and the distant calls of herring gulls. Beneath that surface, in the cold currents that flow past these islands, the ocean's most endangered whale may be feeding on some of its smallest creatures, each mouthful a quiet act of persistence in waters that remember their ancestors.