I have been following the saltwater discussions with great interest.
We started working on salinity tolerance in tilapia back in the early 1980's mainly from a genetic perspective at Swansea University in the UK.
We collected a wide range of tilapia species from around the world to look at a range of parameters including salinity. This work was carried out over 20 years during which time Fishgen was established at the University and the team created the first YY supermales at the University.
Part of the salinity work was done in South Africa, specifically on Oreochromis mossambicus, and we collected many of the different strains found throughout the country for evaluation and to develop a synthetic strain of O. mossambicus for commercial aquaculture in regions where the faster growing Nile tilapia was prohibited. The genetic analysis showed that the different strains were related in different ways and that they had migrated into the different river systems by going out to sea and moving up the coast. It is known that O. mossambicus has considerable salt tolerance and can be cultured in full strength seawater (35 ppt) but growth rates at higher salinities were much lower than in freshwater or brackish water. Even though they were able to venture out to the sea, they did not colonise or establish populations in the marine environment, so probably do not pose an biodiversity risk as David raised, otherwise they would have historically cultivated this habitat by now.
During the research at Swansea, we also evaluated Oreochromis spiluris, which is closely related to O.niloticus but exhibits extreme salt tolerance having been cultured in up to 50 ppt in parts of the Middle East but regularly grown in 40 ppt in many arid regions using pumped ground water with very high dissolved minerals (not always Sodium Chloride). Again, growth rates were found to be slow compared to pure Nile Tilapia even when hybridised with Nile Tilapia. We were able to hybridise most of the tilapia species including the salt tolerant true tilapia, Tilapia zillii and T.rendalii which also exhibited considerable salinity tolerance. We also collected strains of Oreochromis niloticus subspecies vulcanii from Kenya which also exhibited high salt tolerance but again at the expense of growth rates.
We also looked at the Nile tilapia's salt tolerance with experiments conducted in Ecuador, and found that the Nile tilapia grew well up to 17 ppt but after that suffered all sorts of problems, including the flesh starting to drop off but the amazing fact was that the fish were still eating, showing how tough the tilapia are. The main advantage of using tilapia at that time in Ecuador was the positive benefits of using tilapia as part of a polyculture system with shrimp. It was when the WSSV ravaged the shrimp industry in Ecuador and it was reported at the time of survival of only 4% of the shrimp, in shrimp only ponds, but shrimp survival of up to 40% in tilapia polyculture shrimp ponds, so tilapia production increased dramatically during this time in Ecuador. This could be applied to the Brazilian reference mentioned yesterday on the list. There are many good strains of tilapia that were developed in Ecuador at that time and which have now been selected and improved.
Tilapia grown in brackish water certainly has advantages as mentioned already.
Ray mentioned the excellent work of Wade Watanabe who has published extensively on this subject and his team worked mainly with the Florida Reds. He found the fish were capable of reproduction at 36ppt but best reproduction was at 12ppt and declining markedly at salinities higher that 18ppt. I remember visiting Ray in California many years ago, and looking at the populations of O.mossambicus in the Salton Sea which were all dying as the lake was drying out and getting more and more saline. This was natural selection in the making and there could be a highly salt tolerant strain of tilapia living there by now unless it's all dried up!
CEO Fishgen Ltd