2.10 Where the river meets the sea: salty environment

The salty environments are coastal areas near estuaries where the freshwater mixes with the marine ones creating a very particular and unstable ecosystem. The coastal lagoons are composed of brackish water separated from the sea by sandy shores and, at the same time, communicating with it through interruptions. Thus, the tides ensure a partial change of water.
Salinity, temperature and oxygen concentration are environmental parameters that have important fluctuations both daily and seasonal in the coastal lagoons. This instability of environmental conditions makes life possible only for well-adapted species (euryhaline and eurytherm). These areas are ideal for valley fishing and aquaculture.

A very special fish that lives in salty environments is eel (FIG1_SES2.10).

Fig1_SES2.10 European eel. By GerardM – http://www.digischool.nl/bi/onderwaterbiologie/, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=284678

Adults live in freshwater by eating insects, worms, crustaceans or small fish. In autumn their color changes, the back becomes black. It is the signal to migrate, they leave the rivers for the sea but a particular sea: the Sargasso Sea (FIG2_SES2.10).

FIG2_SES2.10 Sargasso sea. Public Domain, https://commons.wikimedia.org/w/index.php?curid=691325

So the eels go out of the Po, for example, descend the Adriatic, cross the Mediterranean, come out of Gibraltar, cross the Ocean and reach their destination. Here, perhaps at a depth of about a thousand meters, the eels mate and then, exhausted by the journey, die; the females after spawning, some millions eggs. The hatchlings (the leptocephalic) immediately start the migration to the east and reach the European coasts only after 24 months. Near the coasts these larvae become small eels of less than 10 centimeters. They can stop in coastal waters or enter in the rivers where they change to become yellow eels. With the achievement of sexual maturity, 3-10 years for males, and 5-25 years for females, depending on the temperature and salinity of the areas in which they live, the silver eel phase arrives and the cycle starts again.