Where Do Brackish Water Habitats Exist?

The environments from which brackish water fishes are collected are diverse. Estuaries are the best known. An estuary is the part of a river where it meets the sea. Typically, estuarine waters are slow and sluggish, and often very silty and fertile. As a result they are not always as attractive to look at as the clear waters of a mountain stream, but they are tremendously productive. One characteristic of estuarine ecosystems, and brackish water habitats in general, is that while productivity (the amount of food available) is high, diversity (the number of species) can be quite low compared with rivers or the sea. This apparent contradiction is because relatively few fish and invertebrates can tolerate the fluctuations in salinity. On the other hand, those animals that can live there do so in enormous numbers. Gobies, flatfishes and catfish are characteristic fishes of estuaries, ranging from the fresher waters upstream right down to the sea. A few freshwater fishes may occur in the least saline parts of the estuary (for example garpike, ropefishes and cichlids). Many marine fishes, especially as juveniles, inhabit the saltier part of the estuary (such as sea bass, flatfishes, tarpon and herring).

Another important brackish water habitat is the mangal (or mangrove swamp). Mangals are characteristic of the tropics. Some mangals develop in estuaries, while others fringe islands. Mangrove plants are some of the most remarkable plants to be found anywhere in the world. They form dense forests which support a tremendous variety of animals both above and below the surface of the water. In the canopy, monkeys, snakes and a huge variety of birds are to be found; while the aquatic roots support oysters and barnacles, as well as many fish, snails and crustaceans which live in and around the thick knots of roots and stems. Mangals have provided many good fish for the aquarist, such as mudskippers and archer fishes. The mud that collects within the mangrove roots is a prime habitat for fiddler crabs, various clams, and snails.

The temperate equivalent of the mangal is often said to be the salt marsh. These are tidal habitats, periodically covered by the sea. Salt marshes are also important habitats, especially as the feeding grounds for seabirds and fishes. Instead of trees, the major plants are grass-like. Gobies and sticklebacks are especially common in salt marshes, which are quite easy to explore and a good place to collect native fish and invertebrates for the aquarium. Unfortunately many salt marshes are threatened by agricultural or commercial development.

Common to both the tropical and temperate zones, seagrass meadows are found in shallow water. Seagrasses are true plants and not algae, but have proved to be difficult to keep in aquaria. There are many fishes and invertebrates found in sea grass meadows, perhaps the best known is the seahorse. The salinity of a seagrass meadow is rarely much below that of full strength seawater, but seagrasses do occur in lagoons and estuaries where the water is brackish.