Rivers Meet Oceans: How Do Fish Know?

Just wondering how fresh water fish react when getting near salt water ocean, or salt water fish know when trying to swim up a fresh water river?

Do they suddenly start having problems and turn around?

Salt water tastes different than non-salt water. I’m sure you’ve noticed this yourself.

The interface zone is called “brackish” water, and certain fish have evolved to thrive in it. Others stay on their own side of the zone.

In addition to the difference in taste, the density of salt water is different from the density of fresh water. Fish who are adapted to have roughly neutral buoyancy in one type of water will therefore have more trouble controlling their depth in the other type.

And there are some who cross it at will.

The interface zone is called an estuary

Interesting tidbit, kind of goes along with this, is that fish that change from salt to fresh or the other way have to really go through some changes in their osmoregulation. Because saltwater is saltier than the internal fluids of fish, fish in saltwater have to drink a lot of water and excrete salt (mostly through special cells on the gills), otherwise they would dry out because of the natural loss of water through osmosis to the more saline environment. Fish in fresh water are saltier than their environment, so water moves into the fish osmotically, and they have to pee nearly constantly, and they never drink water. They cannot just shut off this osmotic relationship through impermeable membranes, especially because gills have to be pretty permeable. There is a big energy consumption in this moving of salt and water. Part of the reason when you move freshwater fish it is good to add a little salt to the water - just takes some of the background stress off the fish, because they have to expend less on osmoregulation.

It might be, but I believe that more rivers meet the sea in deltas rather than estuaries.

I think rivers that meet the sea through deltas also have estuaries.

They might sometimes, but I do not think any river mouth, where a river flows into the sea, automatically counts as an estuary. More than one river can flow into a single estuary, and they might (or might not) flow into it through a delta; or the mouths of a delta can flow directly into the sea.


River delta

How do salmon do it?

Most salmon are anadromous[sup]1[/sup] – that is, they are adapted to a lifestyle that involves hatching in fresh water, migrating to the ocean to live, and returning to fresh water to spawn and die. Adult salmon cannot survive for extended periods in fresh water; they must mate and spawn before their salinity becomes too low by osmosis and their tissues start to decay in consequence.

  1. There are a few species, relatively rare, that live out their entire life cycle in fresh water. The overwhelming majority of fish termed ‘salmon’, however, are anadromous.

You might also ask how bull sharks do it. Unlike salmon, which transfer from fresh to salt water during their life cycle, bull sharks may swim far upriver from the oceans. Look at the map of their range in the wiki article, which clearly shows several rivers as well as coastal oceanic waters (sharks in the Mississippi? Well, yes, occasionally.):

As noted by Carptracker, they cope by peeing a lot:

If I may chime in, I think you’re both basically right, I might give njtt a slight edge…
There are two basic types of estuaries. Semi-enclosed estuaries are bodies of water connected to the ocean but diluted with incoming freshwater. Chesapeake Bay, the Baltic Sea, and San Fransisco Bay are good examples of this type of estuary. This is sort of the “classic” estuary.
Estuarine scientists (of which I am one, or at least will be when I defend my thesis) generally also recognize “coastal estuaries” which are not semi-enclosed, basically where very large rivers dump directly into the ocean (the Amazon, Mississippi, and Nile rivers being examples, note that each of these rivers also has a delta at the mouth). In this instance there will be a plume of low-salinity water that extends out into the sea. Semi-enclosed and coastal estuaries are very different systems, but if you include them both in your tally I’d be willing to bet that most LARGE rivers that make it to the ocean have an estuary. However, in small rivers with discharge low enough that it can be quickly mixed with full-salinity seawater there will be no real estuary: What really counts when defining an estuary is a salinity gradient - you need low salinity at one point, high salinity at another, and intermediate in between.
You’re right about multiple rivers entering a single estuary. Additionally, estuaries can dump into other estuaries - the Potomac River estuary empties into Chesapeake Bay, itself the largest estuary in the US.