Bait

When Tomás Torres arrives at a remote research facility to consult on an unsuccessful breeding program for a rare species, he finds his new patients are fish with oddly humanoid face and torso -- merfolk. 

The legend that eating a mermaid will grant immortality seems to be based loosely in fact, and early trials suggest their meat is a powerful curative with incredible potential. A company with a successful fish farm will command nutrition and pharmaceutical markets. But many of the fish are not reproducing. 

Before they can even begin to evaluate the breeding program, Tomás and fellow consultant Dr. Alyssa Cooper learn staff at the facility are dying, devoured after falling into tanks they had no reason to approach. Tomás and Alyssa set out to discover the abilities of the carnivorous mermaids -- but Nature will not wait for research. 

A novelette of 12,000 words.