Winter Fish Kill

(There were plenty of fish in this lake... they just didn't want to be caught.)

The only time I generally fish is on the summer backpacking trip I take with my husband every year. We go to remote places so that we won't have much company, but it's incredibly frustrating to spend two days backpacking up to some remote mountain lake only to find that there aren't any fish. We've had this happen even when oldtimers have assured us that there is indeed a fish population in a particular lake.

The first time this happened on one of our trips my husband just shrugged his shoulders and said, "I guess it was winter killed". I pictured it being so cold up there that the lake froze solid, killing the fish... not too hard to believe when you are up at dawn in July, in the mountains, wearing three layers, a stocking cap and gloves, and still a little too cold for comfort. Even so, it isn't likely that a fairly deep lake would freeze solid. So what kills fish during the winter?

When it gets cold the water on top of the lake freezes. This is handy, Ice floating like that, or it might really be likely to freeze solid. However, the ice on top of the lake restricts the ability of the lake to absorb oxygen. This isn't so bad because the fish's metabolism slows and they don't use as much oxygen as in the summer. As long as light can filter through the ice there will be some oxygen produced by the plants, but if it snows a lot then the snow will block the light. Eventually, if it doesn't melt off soon enough, the dissolved oxygen levels become so low that fish will start to die.

Unfortunately, it seems that trout (our fish of choice) are particularly sensitive to this. The population will eventually bounce back as long as there is a stream or something to connect the lake with other fish populations.

... and we can always spend the time taking lots of pretty pictures.

Links:

EPA's information about mercury in fish.

National Fish and Wildlife Foundation

The Nature Conservancy's program to map freshwater biodiversity.

Word Of The Day:

Biodiversity: The number of different kinds of living things in a given area.