Captive rearing can accidentally change animals so they may not survive in the wild

As we humans continue along our destructive path of destroying habitats and warming our climate, animals are increasingly getting squeezed out of the picture. That's led zoos, conservation centres, and hatcheries to the stopgap strategy of captive-breeding animals for reintroduction to the wild.

There's a problem with this strategy, that new research is underlining. Captive animals can inadvertently become domesticated — temporarily or even permanently adapted to a captive environment — making them less fit to survive in the wild when they're reintroduced. 

"Definitely in the last ten years, there was this change in thought process where [those of us in the captive breeding world] were better understanding the change in our animals," said Gabriela Mastromonaco, the curator of reproductive programs and research at the Toronto Zoo. 

Research into inadvertent domestication is a concern for many species, but is much more advanced in model species like mice and in hatcheries that raise fish to boost commercial stocks and help with conservation. 

"There are quite a lot of concerns surrounding hatchery raised salmon and one of the biggest ones is inadvertent domestication within hatcheries," said Jessy Bokvist, a University of Calgary master's student in ecology and evolutionary biology who's studying hatchery-raised fish at the federal Nitinat River Hatchery on Vancouver Island. "When we raise animals in a captive environment, they can tend to adapt to that environment at the cost of not being adapted to life in the wild."

That's exactly what the hatcheries are seeing — traditionally-raised hatchery fish are not as fit to survive in the wild as their wild counterparts.

Mechanisms behind inadvertent domestication 

Mastromonaco says the most obvious affects of inadvertent domestication are changes in behaviour. 

"Changes in tameness, so the attraction to human in the nearby environment, foraging behaviour, predatory behaviours, those are the most obvious to see that are slowly changing and the animals are perhaps not carrying out patterns they would see in the wild," she said.  But the changes go beyond behaviour.  "There are other underlying changes — morphological changes, changes in fat layer under the skin, changes in skeletal morphology, etcetera."

She also says in captive breeding situations, scientists often select mating pairs to retain as much genetic diversity as possible. And that can mean that captive breeding process itself can make the animal less fit to survive in the wild.

"Because we've removed some of the natural selection pressures and whether that is mate choice, sperm selection, and other factors," said Mastromonaco, "the overall fitness of the next generations are in a sense are impacted whether we meant to or not." 

There's something about a hatchery environment that's causing marked epigenetic changes in hatchery fish compared to [their] wild counterparts.- Jessy Bokvist, University of Calgary

The science coming out of hatcheries about underlying mechanisms is much more advanced than it is with zoo animals.

A 2017 study in the Proceedings of the National Academy of Sciences highlighted the epigenetic changes that can occur in fish that were raised in hatcheries. Epigenetics describes a range of ways that the environment can affect how animals express their underlying genetic code. 

"There's something about a hatchery environment that's causing marked epigenetic changes in hatchery fish compared to [their] wild counterparts," said biologist Jessy Bokvist. "These [changes] were playing a role in the expression of genes important to immune function, important to locomotion — so migration behaviours, and important to ion-regulation, which is obviously so important for salmon since they migrate from freshwater into marine phase." 

Making captive animals more street-smart

Since the early 2000s, the manager of the Nitinat River Hatchery, Robert Brouwer, has been conducting his own experiment. He's been raising coho pacific salmon in both raceway-type tanks, traditionally used in hatcheries, and in a new kind of tank that provides a richer and more complex environment.

In the new round tanks, the coho salmon get to swim against an artificial current. Brouwer has also added substrate to the tanks, giving the fish a bit of variability in their environment, so they have something to swim around and hide under. And he gives them a different type of food — rather than pellets, they get krill and plankton. 

"We're just trying to take those fish and give them life-skills training, just like you do with your kids." 

He says the difference between the fish that grew up in the different tanks is obvious when you look at them. 

We're just trying to take those fish and give them life-skills training, just like you do with your kids.- Robert Brouwer, Nitinat River Hatchery

"Just by having a few branches in the pond, these ones are way more shy," said Brouwer. The fish raised in the more traditional raceway tanks aren't as shy, making them easy pickings for predators. "When you let them go, they'll be looking to the surface for the first blue heron or the first duck and they'll think they're going to get food from it, but really they'll be eaten." 

Just before the coho salmon are released into the wild, they employ a similar strategy zoos use to give the animals they rear a bit more training before they're released. It's a stepwise release — otherwise known as a soft-release — where the captive-bred animals are put into a semi-natural or controlled environment to try and give the animals one last bit boost before their "training wheels" come off when they're reintroduced and left to fend for themselves. 

In the hatchery these are the large rearing channels — pool-sized bodies of water where the fish go before they're released. These rearing channels are much bigger, resulting in lower density of fish. Brouwer also adds branches and other natural materials to the water, and he will dip nets into the water with the intention of scaring the fish to make them less docile. These rearing channels are also open, so natural predatory birds can sweep down and capture a fish if they aren't smart enough to hide.

I do think it requires still further effort, like, we can't give up. And when there are some failures, it just means we have to learn some more and apply it again.- Dr. Gabriela Mastromonaco, Toronto Zoo

Brokvist has been studying the epigenetic effects of these enhanced rearing methods on the fish compared to traditional methods. She hopes to have results to her study in about a year's time, but she said, "This environmental enrichment we're using is resulting in higher returns from these fish as compared to fish that we don't give any stimuli or enrichment."
 
While scientists like Mastromonaco says the science for zoo animals still has to catch up to hatcheries and model animals, she's not willing to sit around without trying to improve endangered animal species' numbers in the wild.

"I think it's easy to say, 'All right, you know what? This isn't perfect, so let's just walk away from it,'" said Mastromonaco. "I do think it requires still further effort, like, we can't give up. And when there are some failures, it just means we have to learn some more and apply it again."