Whatever solitude he sought in the woods of Concord, Massachusetts, writer and naturalist Henry David Thoreau was never really alone at Walden Pond. He had plenty of beaked, scaly, and leafy neighbors at the kettle pond—the state’s deepest natural puddle—and he chronicled his encounters with them in Walden: Or, Life in the Woods, his 1854 account of a stint in a humble cabin. Thoreau remarks on the fish—schools of small perch, flashing bronze in the greenish water, as well as pickerel and more, all “cleaner, handsomer, and firmer fleshed” than…