Basking sharks and the spring bloom

The waters off the Maine coast warm slowly through late spring, and with that warming comes an explosion of life invisible from shore. Copepods, tiny crustaceans no larger than rice grains, multiply by the millions in the surface waters. Their numbers peak now, creating dense clouds of protein that drift with the currents. If you are near the water, step closer to the edge. The ocean holds more than it shows.

Basking sharks (Cetorhinus maximus) arrive to feed on this bounty. Earth's second-largest fish, they can stretch thirty feet from nose to tail, yet they sustain themselves entirely on these microscopic animals. A basking shark opens its cavernous mouth and swims forward, filtering seawater through gill rakers that trap copepods and other zooplankton. The shark's mouth can expand to nearly three feet across. Water flows in the front and out the gills, leaving behind everything the shark needs. They feed at the surface, their triangular dorsal fins cutting through calm water, sometimes in groups of a dozen or more.

The timing matters completely. Copepods reach peak abundance in late spring when water temperatures climb past fifty degrees Fahrenheit and phytoplankton blooms provide abundant food. The microscopic plants multiply first, then the copepods that graze on them, then the basking sharks that follow the copepod concentrations. This cascade happens predictably each year, but the exact timing shifts with water temperature and weather patterns. A cold spring delays everything. An early warm spell accelerates the bloom and brings the sharks sooner. The sharks track their food across hundreds of miles of ocean. Satellite tags show individual sharks moving from wintering grounds off the Carolinas to feeding areas from Cape Cod to the Bay of Fundy, following the zooplankton pulses northward as spring progresses. They filter feed for hours at a time, sometimes diving to depths of three thousand feet between surface feeding sessions. A single shark can filter over a million gallons of water per hour through its gill rakers. The relationship between predator and prey operates on a scale that dwarfs most terrestrial ecosystems, yet depends on organisms so small that thousands fit in a teaspoon of seawater.

Listen for the sound of water moving differently, the subtle splash of a large body breaking the surface tension. The dorsal fin appears first, cutting a steady line through the water. Then the snout, broad and blunt. Sometimes the tip of the tail fin shows as the shark feeds just below the surface. The water around a feeding shark often looks different, slightly disturbed, as millions of gallons flow through the animal's mouth and out its gills. Close your eyes if you are near the water now. The spring ocean carries its own rhythm, the sound of abundance feeding abundance in the warming currents offshore.