Estimating the number of fish in the sea is a wet, cold, and inexact business. To gauge how populations are faring—a critical part of managing fisheries—researchers typically drag a large net behind a ship, counting and measuring what they catch. But these trawl surveys only provide a rough indication of fish populations and they cost tens of thousands of dollars a day. Many researchers are hoping sampling loose bits of DNA that float in seawater can improve the surveys and extend them into places where trawls can’t go: sensitive habitats like coral reefs, wind farms, and stretches of rocky sea floor that are treacherous for heavy nets.