A long-snouted, curious creature whose population is quickly dwindling, the sturgeon is an unlikely candidate for the title International Fish of Mystery. Prized for its caviar-potential roe, this ''gilded morsel'' is currently hot among both international smugglers and ecological preservationists, and right on their dorsal fins is Carey. A lyrical and humane writer, the ecojournalist patiently tracked down hatchery biologists, luxury-food buyers, wildlife agents, and others whose livelihood depends on the sturgeon. He weaves their intersecting stories into one engrossing, if dense, narrative. Carey is clearly enamored of the velvety black wonder he calls ''the pap of life, the milk of wonder, both the food of the gods and its seed.'' After finishing The Philosopher Fish, you'll be apt to agree.