Few things are more frustrating than trying to learn something new, only to run into ridiculously complicated words. Not all scientific words can be found easily online or in a dictionary. They may be too new, describe extremely specific situations, have poorly-defined meanings, or there are disagreements on the proper usage of the word. Diadromy is a term that falls into several of this categories, but I’ll try to explain it in a way that makes sense to me.
Diadromy is a special type of migration for fish that spend parts of their life in the ocean and parts in fresh waters. However, there are many fish that can do this without actually needing to. An early definition limited it to “truly migratory fishes which migrate between the sea and fresh water.” (Myers 1949). So, fish that have to migrate to complete their life cycle. This doesn’t include species that are “facultative, euryhaline wanderers”, a complicated term for fish that choose to swim between salt and freshwater looking for food. They don’t need to go into saltwater, but they do it to get better access to food. A good example of this is the salter brook trout, which ventures out into saltwater seasonally. Breaking this down further, Myers (1949) excludes fish that emigrate (move away with no return) or are nomadic (aimless wanderers).
Others have refined this definition to fish that “normally, as a routine phase of their life cycle, and for the vast majority of the population, migrate between marine and fresh waters” (McDowall 1988). This breaks down to fish that “need” to move between freshwater and saltwater to complete a part of their life cycle (spawning), then return back again. And it still allows for the oddballs like landlocked populations (can’t or won’t migrate).
There are over 250 species of fish that are considered to be diadromous (McDowall 2008), with three distinct types of migration: anadromy, catadromy, and amphidromy (Myers 1949). Anadromy accounts for about 48% of diadromous species, with the remaining 52% split between catadromy and amphidromy (McDOWALL 1997).
Anadromy is the main type of diadromy in the north Atlantic. Species like salmon and river herring go on long migrations, returning to their home rivers to reproduce. It can be defined as diadromous fishes that are born in fresh water, migrate to salt water to grow into adulthood, then return back to fresh water to reproduce. The seaward migration may occur as eggs, larvae, or juveniles, but the key feature of anadromy is the return migration to freshwater for spawning by mature, ripe adults.
Catadromy is the opposite of anadromy. The eggs are spawned in the ocean and move to freshwater to grow into adulthood, then return back to the ocean to reproduce. Catadromous fish are seen less often than anadromous ones because the adults move out of the rivers alone and meet in the ocean to reproduce. The small juveniles that return to the rivers are so small that they are barely visible as they move upstream. American eels are catadromous, swimming from their home rivers out to the Sargasso Sea to reproduce and die. Catadromy is a more common strategy in tropical areas of the world, while anadromy is more common in temperate areas.
Most definitions of amphidromy that I’ve seen are difficult to understand. To start, a species can be either marine amphidromous (born in fresh water) or freshwater amphidromous (born in salt water). These diadromous fishes don’t migrate to reproduce, instead they migrate at a different life stage, usually as juveniles. Once the juveniles migrate back into freshwater they continue to feed and grow into adults, which will spawn in freshwater without migrating.
But, why diadromy.com?
Diadromy occurs in only 1% of the over 25,000 fish species, yet we have a greater connection to diadromous fish than to those living in the ocean or in lakes. There is good reason for this: much of human development has occurred around rivers and estuaries, the same places where diadromous fish aggregate in large numbers as they migrate to spawn. We see them swimming in huge numbers each year, we fish for them, eat them, and have made them an important part of our cultures. These same attributes that have made important to us have also made them vulnerable. Every dam on a river prevents fish from reaching their spawning grounds. The installation of fish passes on dams has improved the situation, but it is not enough. In Massachusetts there are thousands of dams with no fish passage that no longer serve any real function – dams that should come down. But only public support will improve the situation. The goal of this website is to help people recognize the difficulties that these miracles of evolution face, before it is too late.