Sacramento, Calif. On frosty fall mornings and brilliant spring days, millions of Californians splash into the state's waterways seeking fish that they can hook, fight to possess and then set free.
In their drive to preserve fish for future generations, "catch-and-release" anglers may also be changing species in uncharted ways.
Biologists are certain that releasing fish helps sustain populations that would falter if those fish were eaten. But they know much less about how repeated releases may affect breeding, behavior and more.
"We are making more docile bass," smaller and less able to defend their nests, said Dave Philipp, a scientist at the Illinois Natural History Survey who studies the effects of angling on reproductive success.
Milton Love, a research biologist at UC Santa Barbara, calculated that for certain long-lived fish, even a few catch-and-release deaths can multiply into a serious problem if the same animals are caught over and over for years.
Love, who has fished since he was 5, worries when he hears recreational anglers, proud that they've released a fish for posterity, imply that they have no impact on fisheries.
"That's still an open question," Love said. He has urged more studies to help determine "at what point does the mortality rate, even if it's very small, begin to catch up with the population?"
Another open question which troubles some anglers as well as animal-rights activists is whether fish feel pain.
"I think they do feel pain. I'd be kidding myself if they didn't," said Kurt Bailey of Sacramento, who cares enough about fish to have trekked down to the Delta on a bitterly cold morning earlier this month to help save striped bass stranded on Prospect Island.
When he's not in rescue mode, Bailey fishes off the coast for rock cod and tuna, inland for black bass, and where he can for salmon and steelhead. He eats some and frees others, but he wonders about the ones he lets swim away.
"One thing I've thought about, when you let them go, do they survive? How messed up mentally are they? If someone did that to me, I'd be stressed out," Bailey said.
So are the fish.
Their hearts pound faster and pump more blood. Stress hormones such as cortisol increase. Lactate, the same waste product that can give humans muscle cramps after exercise, builds up.
For many species, the effect dissipates in two to four hours, said Cory Suski, a professor of fish physiology at the University of Illinois at Urbana-Champaign.
Suski compares the physical effects of angling to a human exercising hard. Just like a person who runs a marathon, a fish will recover, he said.
It is a comparison that does not sit well with People for the Ethical Treatment of Animals.
"Every parent who fishes is telling their kids that it's fun to torment and abuse animals," said Lindsay Rajt, manager of the group's factory farming and vegan campaigns.
"There are a variety of ways to enjoy the outdoors that don't involve hooking a live animal by the mouth and dragging them into an environment where they can't breathe," she said.
Yet there is no scientific certainty on what a fish actually feels.
"Even the world's most renowned expert in pain can't tell you if a fish feels pain," said Steve Jinks, a professor of anesthesiology and pain medicine at UC Davis Medical School.
"Pain is a very subjective sensation and involves other emotional processing," said Jinks, who has done research on an eel-like fish called a lamprey.
Those who argue against fish pain rely on a medical definition that describes it as an emotional experience.
You can't have emotion without consciousness, and fish don't have a complex enough brain for consciousness, said James D. Rose, a retired University of Wyoming neuroscience professor who is one of staunchest advocates of the idea that a fish can feel no pain.
When a fish flinches, grunts or moves away from something that we imagine might hurt, what we're seeing is simply a reflex that relies on nociception, the nerve system that detects and reacts to noxious stimuli, he said.
Yet fish clearly benefit from pain medication, said aquatic veterinarian Scott Weber, a UC Davis professor who has operated on fish tumors, cataracts and other disorders with and without analgesia.
The fish that got pain medication returned to feeding and normal behavior much sooner than those that didn't, Weber said.
Lack of data
In the wild, it's incredibly difficult to design experiments that can fully assess the impacts of being caught and released, say those who spend their careers studying fisheries.
We have really good data on only five species largemouth bass, walleye, striped bass, Atlantic salmon and rainbow trout, according to a 2005 study by Suski and co-author Steven Cooke.
That's out of an estimated 28,000 species of fish worldwide.
Mortality after release can range wildly, from zero to 89 percent, according to the 2005 study.
In California, about 95 percent of freshwater trout survive on release, with warm water fish such as catfish and bass doing better and anadromous fish such as salmon which migrate from the sea to breed in fresh water doing worse, said Dave Lentz, a senior fishery biologist with the state Department of Fish and Game.
"Catch and release has been successfully used for many decades," Lentz said, keeping fisheries far more robust than if all those fish had been taken home for food. In 2006, California issued more than 2 million recreational fishing licenses, and studies have estimated that recreational fishing pumps more than $5 billion into the economy.
Over the years, anglers have learned how to reduce death rates, choosing hooks, lures and other gear that put less stress on fish, and being especially careful when waters warm up and make fish more vulnerable.
"Fishers are keenly aware of these things," said Chris Lowe, a marine biology professor at Long Beach State University. "They go to the library, they go online, they do their homework. They tell me, 'I'm really worried about fish populations.' "
Al Kroeger, a Sacramentan who spent Saturday taking a friend's son on his first fishing trip, said he's constantly on the lookout for the best strategies to boost fish survival.
"I enjoy it so much, I want to pass it on to future generations," Kroeger said.
Many researchers say that passion among anglers is why more information about the effects of catch and release is especially important.
Among the cautionary tales is the one that Illinois scientist Philipp tells about largemouth and smallmouth bass. In both species, males make a nest and tend the fertilized eggs and young, hatched fry for several weeks. If the male is pulled from the water, predators can destroy half the offspring in five minutes and 90 percent in just 10.
A bass caught and released while it is caring for young often cannot return to its nest in time or abandons the few offspring left. That means it is removed from the gene pool for that breeding season. The bass fathers whose fry survive tend to be smaller, more docile fish that are less likely to strike at an angler's hook, Philipp has found in comparative studies of fished and less-fished lakes.
"We are putting selective pressure on every bass fishery around and selecting for the least aggressive fish," he said. "It probably means they're not as good defenders of their babies ... which can't be good for the population."