Startling....

By Bill Hauser

Earlier this month, as I scanned through a magazine, a headline caught my attention. It claimed, “Shade attracts fish but they swim away from shadows.” Think about that. Now stop and realize that shade just hangs around to create a cover for fish. A shadow moves and if it is moving above the water, it could be from a predator and a moving fish is a harder target for that predator.

Okay. I got it. but then, I immediately flashed on a Fish Talk that I wrote about ten years ago and I dug into my files and came up with the following article. I hope you will find it interesting.

During a recent steelhead fishing trip, we were treated to a variety of sensory treats… sights, sounds and odors. One special, over-arching sight-treat was the brilliantly clear water. No tannin stain and no silt. But this offered both advantage and disadvantage. On the one hand, we could see fish and we did enjoy a week of sight fishing. On the other hand, the fish could see us and head for cover. We also enjoyed an abundant variety of animals and birds that included phalanxes of migrating waterfowl as well as local birds that seemed to appear from nowhere. Adult and immature bald eagles were common. One event made a special impression.

We were intently focused on getting a good drift among a group of a nice fish that were holding in a “flat”, open reach of the stream. Abruptly, we were startled and our concentration was instantly blown when an immature bald eagle headed upstream, skimming by at alder-top level. We were not the only ones startled. The dozen or so steelhead that were spread out and holding so nicely in the clear, flat water were startled, too. The water seemed to erupt in a frenzied reaction to the over flight of that eagle. That made a real impression.

You have seen it happen. A little movement or, especially a moving shadow over a stationary fish causes an instant “startle response” by that fish.

What is that all about? And, why is this important? As with so many fish behaviors, the simple answer is “survival”. Fish live in an “eat or be eaten” world. Admittedly, mature steelhead should have little fear of birds and they have few predators in freshwater, but remember that, while they migrate at sea, they have one or more years of conditioning in a world with ample populations of large predators, including bigger fish (e.g., tuna), killer whales, porpoise, seals, and more. Smaller fish, of course, have more predators, including birds, so a quick response to any distraction may be even more useful as a survival tactic.

As it happens, the startle response is so important for survival that many fishes have a set of special innate behavior responses to some unexpected stimuli such as a passing shadow or a sudden noise. Different species have different automatic reactions, but the net result is simple. When a resting fish is startled, their body rapidly arches and the tail is flicked to propel the fish away from the source of the stimulus in much the same way as our automatic human response to withdraw our hand from a hot or sharp object before our brain has the opportunity to process the information.

Some species of fish have specialized nerve cells that allow very rapid transmission of nerve impulses. These cells, called “Mauthner cells” lie near the midline of the brain stem. Mauthner cells receive sensory input directly from an audio, visual or lateral line stimulus and a cause a response directly from the trunk muscles to bring about the startle response. The response is so fast, because the brain is bypassed and the nervous circuit is wired so that a stimulus on one side causes a reaction in the opposite direction! The automatic “flex and flick” startle response results in a instantaneous reaction and burst of movement at right angle to the original orientation of the fish.

Why is this important? Survival is so important that this built-in startle response is triggered when a predator approaches undetected and surprises a fish. The startle response automatically caroms the fish away in a burst of energy. The predator may lose the edge long enough for the fish to find escape cover.

Fish are sometimes categorized as “simple organisms” or “lower animals”; but, as it happens so often, the more we learn, the more we discover that they can be very complex.