Monday, July 29, 2013

Apple #645: Where do Fish Go in Winter and in Drought?

The park where I like to go walking has a little walking bridge that goes over a stream.  I like to pause there and look into the stream because after a while, I usually see a few small fish in the water.  I find it a hopeful sight.  So much so that I usually wait until a fish or two swims into view.

The fish I see in this particular stream are rather small and non-descript, kind of like this lady here, who is a chub.
(Photo from Rugbymadad's Blog)

But lately, I haven't been seeing any fish in that stream.  There were some rainstorms that changed the configuration of the shallows and deep spots, so I thought maybe the fish had moved to some other location in the stream.  But it's also been quite dry lately.  In fact, much of the stream these days seems so shallow, I wonder if it's even enough water for those fish.

All of which made me wonder, when the water gets shallow, where do the fish go?  Do they hide in the mud until the water comes back?  And for that matter, where do the fish go during the winter when the water that is their habitat freezes over?

It turns out, what fish do during dry weather and what they do during winter are very similar.  Let's start with the winter situation first.

Winter Cold & Ice

  • As a river or a pond freezes, the ice forms at the surface first.  As the weather gets colder, the ice gets thicker from the top down.  The warmer water is at the bottom or deepest parts.  So the first line of defense against ice is for the fish to go to the deepest waters.
  • If you're ever able to see through the ice to spot fish in the water, you'll probably notice that they're grouped together.  You might be tempted to think they're huddling together for warmth, but since they're cold-blooded creatures, that concept doesn't really apply to them.  They're close-packed like that because that deep, warmer spot isn't that big and they're all trying to fit themselves in that little space.

Koi just below the surface of the ice. Since they're hanging out together, I'd guess they've found a warm spot.
(Photo from Silver Pines Lodge. This page tries to launch an application that you're probably better of refusing to allow.)

  • So now you've got a cluster of fish packed into a small space.  Kind of like a lot of people stuck in a small closet--after a while, you'll notice that everybody's been breathing up the oxygen.  The same thing happens to fish under the ice.
  • Not only are they packed together in those deeper pockets, but the ice at the surface is also cutting off the supply of fresh oxygen to the water.  There are also fewer water plants doing their photosynthesis thing, so there's less oxygen coming from the plant source, too.  
  • Because fish are cold-blooded, their body temperature drops when the water temperature drops, and they become less active.  So they don't need as much oxygen in the winter as they do in the summer.  

More koi under ice.
(Photo from Koi Fish Care)

  • However, if the ice gets too thick or too expansive, the oxygen level can drop to a point that is problematic enough that the fish have to find other solutions to the problem of winter.
  • Some fish migrate.  They go to warmer waters, or deeper waters, or they go to places where the water is more turbulent -- near rapids or other incoming streams or springs, or even near waterfalls.  The churning water keeps ice from forming and also allows the water to be replenished with oxygen.
  • Other types of fish enter a state called torpor, or diapause.  It isn't true hibernation because they can be roused, but the concept is similar.  Everything drops to a state of very low activity -- respiration, heart rate, digestion, growth -- pretty much everything slows way down or even stops.  This further reduces the amount of oxygen they need to survive and helps them get through the iciest, lowest-oxygen part of winter.
  • Some fish, like carp, do bury themselves in the mud, as well as going into diapause.
  • Still, in spite of their efforts, fish sometimes just can't hold out in such oxygen-deprived icy waters, and they die.  When lots of fish die in the winter, it's called winterkill. It can take 3 to 4 years for a fish population to recover from a severe winterkill.

A school of fish struggling to get to the oxygen bubbles in the ice in a pond in St. Petersburg.
(Photo from AtBreak)

  • Don't be too alarmed, though.  Some amount of winterkill is par for the course.  As one fish expert says, "In some waters, partial winterkill is just a natural and beneficial process that results in faster growth rates for the survivors."
  • Even though some fish have to take lots of measures to survive winter's cold and ice, other fish are so well-adapted to cooling temperatures that their big feeding time is the beginning of winter.  Lake trout, brown trout, whitefish, and panfish take advantage of the fact that their summertime competitors have moved to deeper waters, or they have less vegetation to hide in, or they have slowed down so much that they become prey.  Put simply, these cool-friendly fish dig winter.
  • Here's one last thing about what can happen to fish during winter: they can get "drunk" on oxygen. After the first ice has formed on a body of water but before enough snow has fallen to block the sunlight, the plants growing in the water are still using that sunlight to photosynthesize and produce more oxygen.  The ice traps that extra oxygen so the water becomes saturated with it.  So some fisherman occasionally see fish just below the clear ice, swimming sort of goofily on their sides, as if they're drunk.  In fact, they've imbibed too much oxygen. 

These fish are hiding out under a dock in Long Lake, MN.  It's a whole different world under the water, one that fish know very well.
(Photo from Fargo-Moorhead Dive News)

Summer Heat & Drought

  • In the summer, fish are not so much bothered by the heat directly as they are by the heat's effects, which is to say the heat causes the water to evaporate, which means less water in which to swim and breathe. That's a much bigger deal for fish than air temperature.
  • Well, warmer air temperature eventually becomes warmer water temperature, and the fish that like the cold waters don't do well when the water gets warm.  Some coldwater fish such as brook trout can become stressed from warming waters.  They may not grow very large, or they may not reproduce in their usual numbers, or they may die.
  • Warmer temperatures do mean less oxygen, and the reduction in oxygen is the real problem for fish.  But just as fish seek out deeper waters in the winter, they also seek out deeper waters in the summer.  Where the deep water was warm in the winter, in the summer, the deep water stays cooler.  So the fish will move to deeper, cooler waters if they need to when the temperature rises.
  • These deeper, cooler waters may be little pools within a stream, or waterholes lining a streambed, or eddies along a riverbank.

Rivers have all sorts of geographies to them.  The fish know the lay of the land and where to go when it's too warm, or too cold.  Fisherpeople study diagrams like these so they know where to go to catch the fish.
(Diagram from Wikibooks)

  • Again, a few species of fish bury themselves in the mud and wait for the waters to come back.  The mud-minnow is uniquely adapted to low-water situations.  It can either extract oxygen from water through its gills as all other fish do, or it can also get oxygen from the air.  

Mud minnows or mudfish are really common, and lots of fisherpeople use them to catch other, larger fish. But perhaps the reason they're so common is they're able to cope with adverse conditions of all sorts, and produce plentiful offspring.
(Photo by blackmagic on 2CoolFishing)
  • But when the air temperature climbs so high that the water doesn't just get warm but starts evaporating (or if a dam has been constructed that blocks the flow of water), then the fish have a real problem.  Not only do the oxygen levels drop, but the shallow areas turn completely dry, so rivers and streams that used to be connected to each other can get broken up by dry patches (or made impassible by dams).  
  • Thus the fish may not be able to get to important places like bountiful feeding grounds, or they may not be able to reach the place where they go to spawn. Or sometimes fish lay the eggs in one place, counting on the current to carry the eggs to another place where food for the young is plentiful.  But with the river broken up, the eggs can't get there.  The eggs hatch in a less friendly environment and the young starve or get eaten.
  • Also during a drought, the vegetation that some species of fish use for hiding may recede, which means the fish are more exposed and thus more vulnerable to their predators, a.k.a. they get eaten.  
  • Conversely the vegetation underwater can grow so thick on a glut of sunshine that sunlight eventually can't penetrate deep enough to reach them anymore.  Throw in a few warm, still, and cloudy days, and you've got no sunlight reaching those water plants at all.  They can't do their photosynthesis thing, which means they're producing less oxygen.  With no wind or motion in the water to stir things up and aerate the place, the oxygen levels drop, and pretty soon the fish are gasping for breath.  If that situation continues for too long, the fish will die.

This is what you want a riverbank to look like, if you're a fish. Some vegetation that reaches into the water and provides cover and oxygen, but not too much. Also enough wind and current to make ripples and keep the water nicely aerated.  This is in Devon, England.
(Photo from Barry's Musings and Family News)

  • In fact, if any of these low-water situations continues long enough or is pervasive enough, the fish will die.  Just as winterkills can happen, so do summerkills.  A partial summerkill is not uncommon.  But widespread, large-scale summerkills are more of a concern.  
  • Places in Nevada in recent years have seen summerkills as high as 20,000 fish.  Recent droughts in the Plains states have resulted in the die-off of enough fish that some native species of fish such as the silver chub have all but completed disappeared.

What You Can Do

  • People who have ponds stocked with fish can do a lot to prevent winterkills or summerkills by aerating their ponds.  That means basically stick a fountain in the middle of it and keep it running summer and winter. 
  • Or if you don't have a fountain and you see fish acting sluggish at the surface of the water and visibly gulping at the air, that's the equivalent of the fish calling 911.  You can aerate the water yourself by using a 2- to 3-inch pump, like a pump you might carry on a bicycle, and pumping and spraying water back into the pond.  That's not an ideal solution because the aeration needs to continue until the oxygen levels are restored, and that usually takes at least over night.
  • There are a ton of websites that say their aerators are the best.  For some background information on types of aerators and how aerator performance is measured, check out the Southern Regional Aquaculture Center's information sheet on Pond Aeration.

There are lots of different types of aerators that make the fish happy. This one happens to be a floating fountain.
(Photo from

  • You could also do what we used to do when we were kids, which is dig little trenches to connect one body of water to another.  We were just playing in the creek when we did that, but it turns out that's pretty important.

[Edit:] In the days since I put this post together, it's occurred to me that when I go swimming in a lake, splashing and kicking around, I'm actually doing the fish a favor.  All that splashing is aerating the water. That's nice to think about.

You might also be interested in: Where Birds Go When it Rains; Rivers

Alisa Santiesteban, Wisconsin DNR, A cold world with an icy ceiling, December 2009
Minnesota DNR, Fish in Winter, 2010
The Ohio State University School of Natural Resources, Winter and Summer Fish Kills in Ponds
eWater Australia, Where do fish go in a drought?
Environment Agency UK, How does drought affect fish? April 2012
Full Service Aquatics, What Happens to Pond Fish in Winter? January 25, 2011
Rob Neumann, Department of Natural Resources Management and Engineering, University of Connecticut, Impacts of Drought on Fish
Drought, heat lower reservoirs, impact boating, fish in Northern Nevada, Reno Gazette-Journal, June 24, 2013

Drought, River Fragmentation Forcing Endangered Fish out of Water, Biologist Finds, Science Daily, June 6, 2013


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