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I've just read an interesting article from the December 2001 edition of Practical Fishkeeping, which illustrates that 'brackish water' is just as variable as fresh water, indeed even more so. Which, of course, leads to the interesting question, when keeping brackish water fishes, 'which brackish habitiat do I aim to reproduce?'.

Fresh water is tremendously variable - after all, Discus and Mbuna have starkly contrasting pH and hardness requirements - and the same applies to brackish water. Indeed, the salt content is but one factor in the equation! Brackish water occurs whenever a freshwater source (a river) meets the sea. There is a transition from fully fresh to fully marine water, as one progresses from the upper delta of the river (usually this is a totally freshwater habitiat), through the lower delta, and out to sea. Moreover, the boundaries marking the various transition points move, as tides cause the sea to push up the estuary and then recede. So, not only does one have to ask about the salinity levels required, but the chemistry of the freshwater body too. Is the river a soft, acidic one like the Amazon, or a hard, alkaline river flowing over limestone? This has an impact upon the kind of brackish fishes that can be mixed together!

Fishes can be divided into various groupings with respect to salt tolerance. Primary Freshwater fishes evolved in freshwater bodies from as far back as the Devonian era, and consequently have absolutely NO tolerance for salt whatsoever. Even tiny amounts of salt will kill these fishes, and examples include Corydoras and Loricariid catfishes, cyprinids and characins. Then, there are Secondary Freshwater fishes, fishes that evolved from marine ancestors that moved into freshwater to seek new niches and opportunities. These fishes still retain some (though not all) of the osmoregulatory machinery required to tolerate some salt in the water. Cichlids are, in numerous cases, secondary freshwater fishes (their closest taxonomic relatives are the marine Damselfishes) although some, such as Discus, have become specialised inhabitants of particular biotopes and MUST have water chemistry parameters conforming to a strict specification. An example of a brackish-tolerant Cichlid is the Orange Chromide, Etroplus maculatus, one of only three representatives of the Family in Asia. Killifishes and Poeciliid livebearers are other secondary freshwater fishes, which is why fishes such as Aphyosemion species can withstand salt (and in some cases, actually benefit from small quanitites). Then, there are the exclusively marine fishes, such as Chaetodon Butterfly Fishes, that MUST have marine water. Finally, there are the euryhaline fishes, fishes that have sophisticated osmotic equipment allowing them to migrate from fully marine to fully fresh water and back again. Several of these live in marine water as adults, and swim up rivers to spawn in almost completely fresh water - Monodactylus sebae is a prime example. The euryhaline fishes are further divided into those that change habitats for a specific reason (e.g., spawning), and those select few that migrate almost according to whim, or according to the availability of food in each habitat. Monodactylus sebae and the various Salmon species fall into the first category, while one or two adaptable Pufferfishes fall into the second.

Fresh water has no salt content (0 ppt - this stands for parts per thousand), while fully marine water contains 35 ppt of salt. Between these extremes are the brackish water values. The upper delta of a river is almost always a strictly freshwater habitat, containing fishes such as cyprinids, characins and mormyrids. Catfishes frequently live in upper delta regions, which can be turbid and murky because of sediment deposition, taking advantage of low visibility to hunt using long barbels as touch sensors, while mormyrids and other electric fishes create electric fields to detect prey. Others rely upon a very sensitive olfactory sense (equivalent to taste and smell in humans: in fishes, these two senses are somewhat merged) such as Erpetoichthys calabaricus, the Reed Fish, and the Bichirs.

The lower delta of a river begins to be influenced by seawater. Salinity here is of the order of 5 ppt, which is enough to kill most strictly freshwater plants and animals. Carps and characins are absent from such waters, and are replaced by killifishes and livebearers. A specialised insect hunter, the Four Eyed Fish, Anableps anableps, takes advantage of the rich insect life in these waters. There are virtually no Spiny Eels in such waters (although Mastacembelus erythrotaenia, the Fire Eel, is reputed to be an exception) and most Catfishes are absent from such waters, although an Amazonian species, Aspredo cotylephorus, can be found in brackish waters. I shall deal with the 'oddball' marine catfishes in due course!

Next, there are (in tropical regions anyway) the mangrove swamps. Here, tidal motion can cause salinity to vary from a mild 10 ppt to an almost marine 30 ppt. Fishes living here have to be adaptable to such changes, and consequently, the fauna of such regions is rather unusual. Archer Fishes live here, along with Mudskippers, the unique amphibious gobies whose captive maintenance requirements are highly specialised in any case.

Estuaries vary from 15 ppt to a fully marine 35 ppt depending upon tides, and here, we find two marine Catfish Families, the Plotosidae and the Ariidae. In the case of the Ariidae, several of its species are euryhaline, and Arius seemani is a well-known brackish member of the family. Among the euryhaline inhabitants of estuaries are Monodactylus species and Scats such as Scatophagus argus and Scatophagus tetracanthus. Scats are true euryhaline migrants, that will swim up rivers to seek new feeding opportunities, while Monos tend to migrate upriver to spawn. Several marine fishes spend their juvenile stages here, such as Platax Batfishes, the Dog-Faced Puffer Arothron hispidus, some Sharks and Rays, and one or two Snappers and Tuna species.

Finally, we reach the sea proper. Here, the fauna changes again, although some of the Puffers that are found here can also migrate into brackish estuaries, and of course there are 'pure brackish' Puffer species that never migrate out to sea. But here, the sole plant life is of the algal kind, apart from occasional beds of Eel Grass (Eel Grasses are the only flowering plants that can survive fully submerged in marine water) and the underwater vista changes to that of the coral reef. From here on, the gradation becomes one of different reef types, until the end of the continental shelf is reached, and the plunge to the abyssal depths begins.

Incidentally, the size and flow rate of a river can have an interesting effect upon where the boundaries lie. Small and slow-flowing rivers tend to have fairly sudden transitions from fresh to fully marine (i.e., within a few miles of each other) while large rivers with large flow rates can 'push back' the sea to several miles offshore. The classic case is the Amazon once more - the truly gargantuan volumes of fresh water that flow through the Amazon are such that it is possible, during the rainy season when floodwater courses through the river at a rate of tens of millions of gallons per second, for fresh water to be encountered as far as 200 miles offshore! Indeed, in the days of sail, this fact was a life-saver to sailors whose ships were lost in bad storms - if they knew that they were in that part of the sea affected by the Amazon's mighty outflow, then they knew that the water was safe to drink, and that they stood a resonable chance of surviving until making landfall or being picked up by another ship. While the Amazon is an exceptional case due to its sheer size (and because it receives influxes from numerous other large rivers over a huge land area), other rivers exhibit the same phenomenon. In the 19th century, the Nile 'pushed back' the Mediterranean Sea, upon account of its prodigious size and water flow, but since the construction of the Aswan High Dam and other man-made flow control measures, this effect is now much diminished.

So, more food for thought from my magazine trawls!



Panda Catfish fan and keeper/breeder since Christmas 2002