I was walking in a local park this weekend and went past a couple little waterfalls. There were no fish in the streams, but that may just be because no fish lived there. But that did make me wonder, in streams and rivers that do have fish and waterfalls, how do the fish know that the waterfall is there and keep from being swept over the side to their fishy death?
Well fish are smaller and less flabby than us so I’d guess that they don’t fall as fast due to air resistance and they don’t go all squelchy when they hit the water like a person would. Other than that, I would say fish probably have a pretty good awareness of waterfalls.
Why would their corpses remain at the bottom of the waterfall in a pile and not keep floating away downstream?
It’s not unknown for predators and/or scavengers to wait in the tailraces of dams and rapids to catch any wounded fish that come along.
It’s called natural selection.
Smart fish feels the current, and knows theres a waterfall. Smart fish swims away from waterfall, fucks other fish, and hopefully passes on those “smart” instincts to baby fish.
Stupid fish doesn’t know the difference, and gets swept away by the waterfall and turns into salmon salad on the rocks below. Dead dumb fish doesn’t get to fuck, and pollute fish population with his stupid gene.
fish will always avoid fast water. they’re smart enough to avoid coming too close to falls (from the top that is.) but hatched salmon do take the plunge and spawning ones somehow climb up.
And that is the simple answer. Any area that regularly produces dead and injured fish will be haunted by large numbers of predators. As noted, the downstream side of weirs and waterfalls are among the best places to fish for predators.
Most of the wounded fish will come though the power turbines, a fifty meter drop over a dam wall is nothing to most fish, some of the best fishing I’ve experienced is at the base of a one hundred and forty meter high dam with plenty of fish surviving when it tops over.
Out of interest, how do you know this? How do you know that all the fish that come over the top are not dead and that the fish you are catching are not fish that live at the bottom of the dam?
This is not what I learned in biology class…unless by “fucks other fish” you mean “performs an underwater money shot.”
But I thought that was exactly how salmon reproduced. Isn’t spawning technically a money shot that ends in death? :eek:
Here’s a better question – what happens to the fresh-water fish that flow down from the mountains in Utah and end up in the Great Salt Lake?
The fish can’t avoid getting swept downstream – it’s not a question of avoiding the “fast water” of the falls. Inevitably, the water all runs downstream, and unless the fish spends its life swimming upstream against the current, it ultimately ends up going downstream. So eventually the stream empties into the Salt Lake, where the high salinity kills the fish*. But the salt lake is so salty that the only things living in it are brine shrimp (Sea Monkeys!) and whatever bacteria or stuff the shrimp eat. It’s too salty for predators. So there ought to be vast quantities of pickled fish in the salt lake.
What I suspect happens is that predators from outside the lake (hawks, eagles, daring mammals, flies) scoop up the dead fish from the lake or from the shoreline. But you never hear about this.
*Many years ago, when Utah was having extremely heavy rain and snowfall that caused the lake to rise and threatened Interstate 80 and the airport, there were reports that parts of the lake near the mouths of the feeder streams coming down out of the mountains had enough fresh water that the fish were surviving in the Lake, at least for a time.
Wikipedia pretty much agrees:
I still suspect that there are large numbers of dead and pickled fish floating in the vastness of the Great Salt Lake, though – too far from the birds’ hunting grounds, too salty to eat even if caught, and too salty to decay rapidly.no Lake predators can eat them and there’s no outlet to the sea, so they just sort of float around until slow digestion by algae or something exhausts the organic material, or they become the nuclei for salt nodules, or something.
Link to old article about reduced salinity in the 1980s. It’s back up now, I believe:
http://www.nytimes.com/1986/08/11/us/fish-are-now-able-to-survive-in-low-salt-great-salt-lake.html
But even in fast-moving streams there are rocks and pools where the water flow is not as heavy. I’d imagine that since floods and yearly melts are an expected occurence, that fish seek shelter in those places during those times and do not move more than they need to. Sometimes of course they’ll get swept downstream.
Of course not. You think they want the whole world to see what really goes on? You think they want everyone to know about their dirty little secret?
How is this any different from any other river system in the world. All rivers run downhill. All rivers end up either in the sea or in high salinity inland seas.
Obviously most individuals of most freshwater fish species are to be able to maintain their positions in the stream for at least the time taken to reach maturity. If a fish species takes 5 years to mature, and yet most of them get swept into the ocean within the first 12 months, then the species would be extinct. Whether it ends up in the ocean or a salt lake makes no difference to the outcome. And if a fish can maintain position for 5 years then it can maintain it forever.
Quite simply, healthy fish don’t get swept downstream much. They are quite capable of fighting against the current usually, as Ludovic notes, by finding the sheltered backwaters that occur in even the most swiftly flowing rivers. That’s why there are still fish in rivers all around the world, despite the fact that they all end up in salt water somewhere.
Seabirds will travel thousands of kilometres to feed. Hawks will travel hundreds of kilometres. Neither is going to be worried by a lake less than 50 kilometres across, especially given that these dead fish will all be floating close to the outflows of the rivers, and thus within a few kilometres of shore.
First thing is that salt isn’t something that seabirds are too concerned about. They drink large amounts of seawater during the course of normal feeding and have special glands for excreting the salt.
Second thing is that the skin of a fish is a semi-permeable membrane. It lets water out but it won’t let salt in. So a fish floating in this brine would dehydrate, but it wouldn’t contain any more salt than it did when it was alive.
Those lakes would be full of halophilic microbes. I doubt if decay would be much slower than in any other lake.
Most oceans are filled with predators and the salinity isn’t as high. So I’m not surprised about the fish from rivers going into the ocean – something’s going to eat them very soon. Not true in the Great Salt Lake.
Fish don’t get swept downstream in general, that’s a ridiculous notion. Because a) species adapted to fast streams are efficient swimmers with very low drag facing upstream and b) the current speeds are not the same across the whole section of a river.
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