Ocean salinity, euryhaline animals, and possible bluefin tuna aquaculture

 A map of ocean salinity - it ranges from about 33 to 38 parts per thousand

Thus, all cosmopolitan marine animals such as orcas and bluefin tuna can tolerate about +/- 5 parts per thousand of salinity.

https://animalia.bio/cosmopolitan

https://animalia.bio/atlantic-bluefin-tuna

For comparison, brackish water has a salinity range of 1 to 10 parts per thousand. Tide pools have a much greater salinity range.

https://www.ncesc.com/geographic-faq/which-condition-will-increase-the-salinity-of-the-water-in-a-tide-pool/

Because gentle gradients of salinity do not exist in nature, euryhaline species are somewhat rare. However, lionfish can live in freshwater if the salinity is gradually reduced. There may be other species that have the same ability.

Sixth grader credited with scientific breakthrough on lionfish

https://www.youtube.com/watch?v=bJZz9SM2Q8I

It would be a great boon to aquaculture and conservation if marine species like lobsters and bluefin tuna could be raised in freshwater. Saltwater aquaculture is much more difficult and vulnerable to natural disasters. If vulnerable freshwater species could be adapted to seawater, that would greatly increase their long-term chances of survival. 

There are a number of species of freshwater fish that can adjust to living in saltwater aquariums:

https://healthyhomeaquarium.com/biology/community-and-ecosystem/which-freshwater-fish-can-live-in-saltwater-aquarium/

***

First, it’s crucial to increase the salt in the water slowly. This gradual change prevents the fish from getting shocked or stressed.

To get started, use a freshwater tank. Add a bit of marine salt mix each day to raise the salt level. Watch how the fish react and make sure they’re okay. Adding no more than 0.002 specific gravity (SG) a day is recommended.

Keep testing the water often. Check the salt, pH, and ammonia levels to ensure they’re right for your fish. Adjust as necessary to keep things perfect.

***

Hmm. So adding 2 g of marine salt per liter per day is a rule of thumb. Good to know. Hopefully the opposite is true for my upcoming experiment involving clams.