Research throws light on fish vision

Horodysky, a VIMS graduate student working with faculty members Rich Brill, Rob Latour, and Jack Musick, is using electroretinography— a technique first developed for studying human vision—to explore how fishes see the underwater world of Chesapeake Bay.

Brill, an internationally recognized fish physiologist, has recently turned his attention to the sensory world of fish and other marine organisms.

The research is part of an emerging field called "visual ecology" that promises to throw new light on animal behavior and the interactions between predators and prey. Horodysky and his advisors are pioneers in applying this field to Bay fishes.

The researchers are focusing their initial studies on recreationally important Bay species such as striped bass, weakfish, croaker, and drum. This reflects the source of their funding, which comes from the Recreational Fishing Advisory Board of the Virginia Marine Resources Commission. The Board uses money from Virginia's saltwater fishing license to fund projects that improve the Commonwealth's recreational fisheries.

Horodysky also benefits from collaborations with Charter captains like Steve Wray, who provide him with the fish he needs for his experiments.

This graph shows early results from Horodysky's fish-vision experiments. The width of the black bar shows the visual range for each fish species; bar thickness indicates peak response to specific colors. Click image for larger version.

Horodysky's preliminary results provide basic insight into how Bay fishes see the world. The results show that some species, like striped bass, are adapted to see large, swiftly moving prey in daylight. Others, like weakfish, are adapted to see small, sluggish prey at night.

He is also comparing the types of prey that fishes are adapted to see with the prey items that are actually in their stomachs— with some surprising results that could hold important implications for fisheries management in coastal waters.

Horodysky's research shows that striped bass are most sensitive during daylight hours to a wide range of colors from blue to red, with a peak at chartreuse. They have a "flicker fusion frequency" (essentially the "shutter speed" of an eye) of around 50, relatively fast for a fish, which allows them to track large, quick-moving prey like menhaden.

Compared to striped bass, weakfish have slow vision (around 25 cycles per second) and are more sensitive to contrast than color. This allows them to see best under dim conditions, just right for detecting the small fish and shrimp that prowl the nighttime Bay. They also have the unusual ability to see in ultraviolet.

"Even though these two predators may swim side-by-side, they exist in different visual worlds," says Horodysky. "You've got two animals that are competing for the same food. How do they do it? Stripers use color to see and feed during the day. Weakfish use contrast sensitivity to see at night."

For the most part, study of stomach contents by VIMS researchers confirms what Horodysky's vision research predicts. Work by Dr. Rob Latour shows that the stomachs of weakfish are largely empty during the day, and then quickly begin to fill with small fishes and shrimp as evening falls. Work by graduate student Kathleen McNamee shows that striped bass have full stomachs during daylight hours, but that the stomachs gradually empty through the night.

One intriguing aspect of Horodysky's research is the disparity he's found between the prey items that striped bass are adapted to see— large, fast-moving fish like menhaden—and the items that actually occur in their stomachs—mostly small crustaceans like juvenile blue crabs and mysid shrimp.

Horodysky and his faculty advisors hypothesize that striped bass are living in a visual world very different from the one evolution prepared them for. That's because human activities in the Bay watershed and the demise of the native oyster have dramatically reduced the clarity of Bay waters.

"Chesapeake Bay used to be very clear," says Brill. "Now we've made it mucky. So we see the visual ecology of the Bay changing. Our argument is that over evolutionary time these fish have made certain visual choices, then suddenly find themselves in a visual environment they didn't evolve in."

This visual mismatch could have important implications for fisheries managers, who traditionally make management decisions based on the relative abundance of predator and prey—the number of striped bass or menhaden netted per unit area.

"What we're getting at," says Horodysky, "is that it isn't the number of prey per meter that's most important to these visual predators. It's the number they can see. Is there a visual issue, with the Bay being turbid, being murky? If you can't see very far, how is that affecting your ability to feed? These are larger questions we can begin to chip away at once we get our baseline data. We can't start to answer these questions until we know the limits of the eye."

In the meantime, Brill and Horodysky plan to expand their research to other popular recreational fish like summer flounder and cobia, and also to the forage fish—most notably menhaden—that so many recreational species depend on for food.

For Virginia's anglers, the most important question for Horodysky might be how a better understanding of fish vision can give them better luck on the water. "I can't guarantee that anyone who uses these data is going to catch more fish," responds Horodysky. "But they will be able to make more informed choices."

Horodysky, himself a fly-tier and avid angler, notes that his color research does confirm at least one common saying that Bay anglers use when selecting a lure for striped bass: "If it ain't chartreuse, it ain't no use."

"Nothing in the wild is ever chartreuse," says Horodysky, "but the color is right smack dab in the middle of a striper's visual range. They can see it really well."

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