"This is a huge advantage for these animals," Fritsches said in an e-mail, "and while we use a fairly artificial stimuli (flickering light),we know that this reflects the animals' ability to see fast movement -- unlike their squid and fish prey, which have cold eyes."
Understanding how vision works in swordfish and other marine species is not just of academic interest. The U.S. swordfish catch is worth more than $18 million a year, and fishermen's light sticks play a crucial role in attracting them -- and in the unwanted, accidental "bycatch" of such endangered species as loggerhead and leatherback turtles.
The good news is that after years of overfishing, swordfish populations are much healthier. Rebecca Lent, deputy director of the National Marine Fisheries Service, said in an e-mail that international efforts have reduced fishing quotas and the undersized swordfish catch.
"North Atlantic swordfish stocks are close to the level that produces maximum catch, i.e., just where we want them to be. Pacific swordfish are also at optimum levels," she added.
But the huge, long-lived pelagic turtles remain at risk.
In commercial long-line fishing -- the method used to catch many big fish -- boats let out miles of line, studded at intervals with shorter lengths attached to a chemically illuminated stick and a baited hook. Fritsches said that the light sticks do not illuminate the bait so much as provide a visual cue, much the way that phosphorescent sea life would: Food is here.
But the long-line method attracts other creatures that get tangled in or caught by the gear.
"Most of the turtle bycatch," Fritsches said, "happens in the swordfish fisheries."
One of Fritsches's research partners is Richard W. Brill, director of the Virginia Institute of Marine Science's Cooperative Marine Education and Research program. Brill is working with about 10 other researchers to try to reduce turtle bycatch.
"We're hoping to find something simple and relatively cheap . . . that we can then spread throughout the world fisheries pretty quickly," he said. One possibility is a chemical that tastes offensive to turtles and that fishermen could apply to swordfish bait. "If you have the yet-to-be-discovered magic chemical, the bait could be treated," Brill said -- then fishermen could go into areas that had been restricted to protect sea turtles.
Another potential way to protect the turtles might be more precise use of light. The fisheries service has financed Fritsches's study and others to try to make fishing more species-specific by determining what other creatures can and cannot see underwater.
For example, Fritsches noted, "fish can't see in the ultraviolet at all." But sea turtles can see ultraviolet light.
"We now have to find out how sea turtles actually respond to ultraviolet light -- whether they are attracted to it, whether they are repelled by it," she said. Potentially, fishermen could use UV light in a way that scares turtles away from the fishing line. "If they are attracted to it, you could still design something to attract the sea turtles to the line high up" -- away from the baited hooks intended for fish.
Beyond protecting turtles, Fritsches said her studies are part of a bigger picture.
"Vertebrate eyes, from fish to humans, are actually surprisingly similar," she said. "Unlike humans, however, fish have a much less complicated brain. . . . So fish are actually a very good model to study vertebrate vision, since the principles are the same, but the 'hardware' [is] much less complicated.
"So while my research is unlikely to lead to a cure for human blindness in the near future," she said, "it adds a small but important piece to the huge puzzle of how vision works."
