Introduce goldfish to Lake Victoria and/or breed them to be euryhaline?

Goldfish originate from Asia but have been become an invasive species in North America:

Goldfish overrunning Minnesota lakes to become snacks for zoo animals

How massive, feral goldfish are threatening the Great Lakes ecosystem

If goldfish were introduced to Lake Victoria, they could be a food source for Nile Perch and thus take pressure off cichlid fish. 

And if goldfish could be conditioned to saltwater, they could be a food source for tuna and swordfish.

Lionfish have been successfully conditioned to freshwater.

Sixth grader credited with scientific breakthrough on lionfish

  

Introduce brine shrimp to the Dead Sea?

Lake Mono in California is home to brine shrimp. The salinity of that lake is 81 g/l vs 342 g/l for the Dead Sea. Sea water salinity is about 34 g/l. 

Mono Lake - Wikipedia

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The whole food chain of the lake is based on the high population of single-celled planktonic algae present in the photic zone of the lake. These algae reproduce rapidly during winter and early spring after winter runoff brings nutrients to the surface layer of water. By March the lake is "as green as pea soup" with photosynthesizing algae.[34]

The lake is famous for the Mono Lake brine shrimp, Artemia monica, a tiny species of brine shrimp, no bigger than a thumbnail, that are endemic to the lake. During the warmer summer months, an estimated 4–6 trillion brine shrimp inhabit the lake. Brine shrimp have no food value for humans, but are a staple for birds of the region. The brine shrimp feed on microscopic algae.[35]

Alkali flies, Ephydra hians, live along the shores of the lake and walk underwater, encased in small air bubbles, for grazing and to lay eggs. These flies are an important source of food for migratory and nesting birds.[36]

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Even the Dead Sea comes to life sometimes:

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In times of flood, the salt content of the Dead Sea can drop from its usual 35% to 30% or lower. It temporarily comes to life in the wake of rainy winters. In 1980, after one such rainy winter, the normally dark blue Dead Sea turned red. Researchers from Hebrew University of Jerusalem found it to be teeming with an alga called Dunaliella. Dunaliella in turn nourished carotenoid-containing (red-pigmented) halobacteria, whose presence caused the color change. Since 1980, the basin has been dry and the algae and the bacteria have not returned in measurable numbers.

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a visual aid for such a lake:

Swallows Searching for Water in the Deadly Sahara Desert |Africa | BBC Earth

Every living thing is a carbon sink for those worried about CO2 emissions. 

Nauru inhabitants acclimatized milkfish (Chanos chanos) to freshwater

From the wiki article on Nauru:

Nauru - Wikipedia

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Inhabitants practised aquaculture: they caught juvenile milkfish (known as ibija in Nauruan), acclimatised them to freshwater, and raised them in Buada Lagoon, providing a reliable food source. 

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It seems any marine species found near estuaries is potentially euryhaline.

The key is to capture fry. It seems the juveniles adjust more easily. It most cases, it is best to gradually reduce the salinity though I doubt the Nauruans did that. 

Sixth grader credited with scientific breakthrough on lionfish

Bluefin Tuna and Alaska King Crab would be good candidates for experiments in euryhaline aquaculture. Angula eels (Anguilla anguilla) would also be worth studying given they are second most expensive seafood.  

I'm not sure how to get juveniles of any of those species, though baby Angula are commonly caught near Spain. 

Perhaps I should try my luck with clams (Mercenaria mercenaria), oysters (Crassostrea virginica), or lobsters. 

Lobsters need salt water!

Nile Perch vs Asian Carp or Lionfish or Green Crabs?

Nile Perch aren't picky eaters and are voracious predators. It would be worthwhile to conduct experiments on whether they would preferentially eat Asian Carp or other invasive species. The variety from Lake Maryut could be introduced to brackish waters and thence gain the ability to survive in the ocean.

Of course, a repeat of the rabbit or cane toad incidents in Australia is possible, but less likely in the ocean. 

 

Horseshoe crab blood, conservation, and euryhaline conditioning

Horseshoe crab blood began to be used to test for the presence of bacterial toxins in vaccines in the 1960s. Soon, it developed into a major industry, but with a devastating impact on horseshoe crab populations.

https://www.npr.org/2023/06/10/1180761446/coastal-biomedical-labs-are-bleeding-more-horseshoe-crabs-with-little-accountabi

The good news is that there is evidence that prehistoric ancestors of the crabs were able to adapt to freshwater a few times. Wikipedia says:

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Only four species of horseshoe crab are extant today. Most are marine, though the mangrove horseshoe crab is often found in brackish water. Additionally, certain extinct species transitioned to living in freshwater. Horseshoe crabs primarily live at the water's bottom but they can swim if needed. In the modern day, their distribution is limited, only found along the east coasts of North America and South Asia.

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Horseshoe crabs are often caught for their blood, which contains Limulus amebocyte lysate, a chemical used to detect bacterial endotoxins. Additionally, the animals are used as fishing bait in the United States and eaten as a delicacy in some parts of Asia. In recent years, horseshoe crabs have experienced a population decline. This is mainly due to coastal habitat destruction and overharvesting. To ensure their continued existence, many areas have enacted regulations on harvesting and established captive breeding programs.

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Perhaps marine horseshoe crabs can be conditioned to live in freshwater just as the lionfish was in this experiment:

Sixth grader credited with scientific breakthrough on lionfish

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

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/

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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.

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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.

Successful acclimation of saltwater fish to freshwater

Lionfish can be trained to live in freshwater 

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

There is a freshwater sardine in the Philippines which is vulnerable from overfishing. Perhaps it could be trained to live in saltwater.

https://en.wikipedia.org/wiki/Sardinella_tawilis

Salmon can famously survive in both environments at different times in their life cycle.

Freshwater fish account for about 40% of the species even though only about 2.5% of the earth's surface is freshwater. If vulnerable freshwater species could be trained to live in saltwater, that would ease conservation efforts, particularly for cavefish

Of course, the introduction of the Nile perch to Lake Victoria shows the dangers of invasive species.

https://en.wikipedia.org/wiki/Nile_perch#Lake_Victoria_introduction