Henry Weed Fowler must have loved fish.
Ichthyology dominated his entire career. He started as a museum assistant at the Academy of Natural Sciences of Philadelphia in 1903. Other experts in his field soon recognized his prolific skill. In 1918, an assistant curator at the Smithsonian, Barton A. Bean, reached out to Fowler (then still an assistant) for help identifying fishes collected by the United States Exploring Expedition. Fowler dove into the work. He delivered a lengthy 750-page manuscript in two years, helping to discover 18 new species of fish in the process.
For reference, the average field book here at the Smithsonian Institution Archives is 110 pages.