Oh yes it does.Pufferpunk wrote:Just because a fish "visits" BW occasionally, doesn't make it a BW fish.
As aquarists, we divide fish into "freshwater", "brackish", and "marine" categories. This is a totally artificial imposition onto the natural history of the fish. While you can probably argue that 90% of the time freshwater fish have minimal salt tolerance, and 90% of the time marine fish have minimal freshwater tolerance, there are still plenty of species in both groups that pass into brackish water.
To the best of my knowledge, there is no such thing as a "pure brackish water" fish. All brackish water fish are either freshwater or marine fish that have evolved some degree of tolerance for variations in salinity. In some cases this is connected with their life cycle. This is the case with salmon and eels for example, which pass through brackish water between their breeding and feeding environments. In other cases the fish use salt tolerance to exploit the rich food supply in estuaries. This is the case with many of the gobies and flatfish.
The only division that exists therefore is between freshwater fish that tolerate brackish *up* to a certain salinity, and marine fish that tolerate brackish water *down* to a certain salinity. So if you look at the Thames Estuary, you'll find cyprinids like roach and dace that are normally found in the river proper up in the estuary about as far as Central London where the water is around 25% normal seawater salinity. By contrast, the marine fish like pipefish and herrings swim in from the sea and up the estuary only as far as Essex, where the salinity is not much less than normal seawater, say around 75% normal salinity. In between you have mostly marine fish with high tolerance of freshwater, such as flounders and sea bass, though there are a few freshwater fish like eels and trout that can be found all over the estuary.
In the case of Tetraodon lineatus, it is probably fair to say that it is a freshwater fish with a high tolerance for salt. I don't know its precise level of tolerance, but would expect it to handle 25% seawater without any problems at all. This would be entirely in keeping with other salt-tolerant freshwater puffers, such as Colomesus asellus. One clue to the ecology of Tetraodon lineatus comes from seeing what fishes it is caught alongside in the mangroves of the Lower Volta in Ghana: Periophthalmus papilio, Sesarma sp. and Goniopsis sp. The first is a mudskipper, and the other two amphibious crabs.* The authors of that study actually make a point of saying that while the fishermen catch all the other fish for food, "P. papilio and T. lineatus are however of economic value as aquarium fishes". In other words, your so-called freshwater Tetraodon lineatus may well have been caught in brackish water!
It is fundamental to understanding brackish water fishes and their habitat not to see brackish water habitats as some sort of aquarium-like box filled with water. They are nothing of the sort. Fish move in and out of brackish water. Some fish may be common in the sea or in freshwater, and only sometimes found in brackish water. Others may have to pass through brackish water for some stage of their life cycle. Either way, the adapt to the conditions in a variety of ways, and that's what makes them so interesting.
Yours etc., Neale
* See: THE FISH AND FISHERIES OF THE LOWER VOLTA MANGROVE SWAMPS IN GHANA, Hederick R. Dankwa and Chris Gordon. African Journal of Science and Technology (AJST), Science and Engineering Series Vol. 3, No. 1, pp. 25-32.