As far as I know the devs don't want you to think of creatures as things you can harvest and get rewards from. The idea seems to be that you find what you need from the environment, rather than "looting" living creatures for your own personal gain. …
A lot of people have this issue to some extent or another. I've had it happen to me numerous times. As Slim says, usually a complete reboot works. Quit to dashboard, close Subnautica, load up something else like Netflix. Boot up Subnautica again and…
Is the brine in underwater rivers not made up partly of Hydrogen Sulfide? I seem to remember seeing something about it years ago. Perhaps that would explain the toxicity?