James H Schmitz's "Trouble Tide" (novelette, smuggling, free)

Last third is action packed, but first two thirds is a much better read - a detective story that pieces together several apparently disjointed data sets on a human settled non-earth world:

1. A sea animal herd ("frayas") suddenly dies during its annual migration.
2. A coastal sea animal herd ("sea beefs") is behaving in a very weird way - a lot of animals have simply vanished; others that need to come up to surface for breathing every few minutes now seem to have developed some equivalent of gills.
3. Flow rate of a sea current somehow ties up the relative times of the two events.
4. ... 

Collected in.

1. James H Schmitz's "The Hub: Dangerous Territory".

See also.

1. L Sprague de Camp's "The Merman": Another story where mammals transform into fish.

Fact sheet.

First published: Analog, May 1965.
Download full text from Baen Books.
Caution: This is a long novelette - nearly novella length.
Rating: A.
Among the stories from Astounding/Analog issues edited by John Campbell.
Related: Stories of James H Schmitz.