Catadromous—Diadromous and Anadromous Fishes

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Catadromous—Diadromous and Anadromous Fishes

Diadromous fishes describe species that spend part of their lives in freshwater and part in saltwater. There are two categories of diadromous fishes, catadromous and anadromous .

Catadromous fishes hatch or are born in marine habitats, but migrate to freshwater areas where they spend the majority of their lives growing and maturing. As adults they return to the sea to spawn. The word "catadromous" means "downward-running," and refers to the seaward migration of adults. The best-known group of catadromous fishes are the true eels. In these species, females spend their lives largely in freshwater, while males live primarily in the brackish water of estuarine areas. Individuals breed in the seas and die after spawning once.

Anadromous fishes are the opposite of catadromous fishes in that hatching and a juvenile period occur in freshwater. This is followed by migration to and maturation in the ocean. Adult fish then migrate back up rivers— "anadromous" means "upward-running"—in order to reproduce in freshwater habitats. The length of the initial freshwater period and of the oceanic period vary greatly by species. Similarly, the length of the migration can vary tremendously. Some species travel hundreds of kilometers between their marine habitat and their breeding grounds, while others migrate only a short distance upstream from brackish water to reach freshwater spawning grounds.

There are approximately 100 known species of anadromous fishes. Several of these are well-known and of great commercial value, including many species of salmon along with striped bass, steelhead trout, sturgeon, smelt, shad, and herring. Salmon in particular have long been admired for their lengthy, arduous migrations up rivers to their original spawning grounds, as well as for the unusual homing ability that allows them to accomplish this. Their ability to navigate back to appropriate breeding areas is particularly impressive since migration often follows a lengthy period at sea, often as long as four or five years. Chemical cues are believed to guide them in this journey.

In some anadromous species, the majority of individuals die immediately after spawning, with only a few returning downstream and surviving to spawn again. In other species, multiple migrations and spawning bouts are common.

The Rigors of Making Freshwater-Saltwater Transitions

Diadromous fishes are of particular interest to physiologists because of the great challenges posed by freshwater-saltwater transitions. In particular, freshwater and saltwater environments make strikingly different demands on water-balance systems, so these fishes must make the necessary physiological adjustments whenever they pass from one type of aquatic habitat to the other. Every diadromous species migrates at least twice, once from freshwater to saltwater, and once in the other direction. Because of their ability to tolerate a variety of salinity regimes, diadromous species are also described as euryhaline , meaning "broadly salty."

Freshwater fish are in an environment in which they are hyperosmotic. That is, the concentration of salts and ions in their bodies is greater than that in the external aquatic environment. As a result, they have a tendency to lose important ions through diffusion across the skin and gills , and simultaneously to gain water from the environment. To maintain homeostasis , freshwater species have special adaptations for retaining ions and getting rid of excess water. First, they actively take in ions across their gills and skin, a process that requires energy. Second, to get rid of excess water they excrete nitrogenous waste products in great quantities, in the form of a highly diluted urine.

In marine environments the challenges are the opposite. Saltwater species must deal with an environment in which their salt and ionic concentrations are significantly lower than that of the surrounding aquatic environment. Saltwater species tend to lose water to the ocean and to gain ions from it. To obtain and conserve water, saltwater species increase their drinking rate, and excrete smaller amounts of a highly concentrated urine. In addition, they eliminate excess ions through specialized salt-excretion cells in the gills and in the lining of the mouth.

Euryhaline species must adopt the tactics of freshwater species while in freshwater environments, and those of marine species in saltwater environments. Frequently, physiological adjustments are made while organisms are in the intermediate, brackish waters of estuaries. These include changing their drinking rate, the degree of concentration of their urine, and the direction of ion-pumping in the gills and integument .

In addition to these physiological changes, associated with osmoregulation, other changes are made by diadromous species during transitions between freshwater and saltwater habitats. In some diadromous species, external features such as coloration change. For example, in some salmon species, individuals lose their typical red coloration before migrating to sea, where they take on a more silver-colored form. They regain their freshwater coloration when they reenter the freshwater environment.

Considering both the rigors of the long migratory journey and the serious physiological challenges faced by diadromous species, it makes sense to ask why these species have evolved a complex life cycle that requires multiple transitions between salt and freshwater environments. The likely answer is that species are able to take advantage of the benefits offered by each habitat, and that these benefits overshadow the burdens of the repeated migrations. For anadromous species such as salmon, for example, there appears to be a significantly greater safety for eggs in freshwater habitats yet the possibility for much faster growth in the ocean, where the food supply is more plentiful. The increase in growth rate that salmon exhibit once they have migrated to the ocean is dramatic.

The Benefits of Transitions

The rigors of the journey from saltwater to freshwater habitats, or vice versa, including the stresses related to physiological adjustment, is likely linked to the observation that many diadromous species are semelparous , that is, they reproduce in one large reproductive bout and then die. This is also known as "big-bang" reproduction. Semelparity is contrasted with the reproductive strategy of iteroparous species, which reproduce multiple times. Iteroparity characterizes numerous species, including humans.

Some formerly anadromous species have lost anadromy, having evolved to remain in freshwater habitats throughout the entire life cycle. For example, some species of salmon use lakes rather than oceans for the period of growth and maturation. However, they continue to migrate up rivers in order to find appropriate spawning grounds.

In other species, such as the steelhead trout, anadromy appears to be optional. Individuals that are spawned farther from the ocean have a tendency to remain in freshwater habitats during maturation, while those closer to river mouths have a tendency to retain the anadromous condition. This probably relates to differences in the costs of migration.

Perils to Diadromous Fishes

Diadromous fishes are particularly dependent on estuarine areas, the brackish areas linking freshwater rivers and saltwater environments. It is within the estuaries that diadromous species make the physiological adjustments necessary for transitioning between fresh and salt water. Unfortunately, many of these estuarine habitats are under threat. This is only one factor responsible for the dangerous declines in the populations of many anadromous species. Others include increasing river pollution that damages critical spawning habitats, the building of dams and other man-made barriers that make the upward migration difficult, and the overfishing of commercially important species. However, the release of young salmon into reclaimed rivers has met with some success, and in some areas special passages for migrating salmon allow individuals to get upstream to the spawning grounds.

see also Excretory and Reproductive Systems.

Jennifer Yeh