“If you’re a fisherman living in Ontario province bordering Lake Superior, there’s a good chance you know what a coaster is, otherwise, you’ll probably think it’s the thing your mother makes you put under your glass of iced tea.” says famed Colorado-based fly fishing writer John Gierach.

On one hand, he’s right: Anglers in the region know coasters are fat, bright brook trout that live on the bounty of Lake Superior’s coastal waters for part of the year before heading to spawn in the tributaries, where they dwarf the year-round stream fish. On the other hand, the question of just what a coaster is still has fish biologists hard at work.

One of those scientists is Rob McLaughlin, an associate professor in the Toronto’s Guelph University’s  Department of Integrative Biology in Ontario. Sure, some anglers might be satisfied with the mystique around an enterprising stream brook trout getting the nerve to move out to the big lake. But McLaughlin and his colleagues are after hard data on when coasters are migrating and what drives some individuals to do so while others are content to stick to one habitat or the other.

“I’ll be honest: We’ve sometimes felt that people didn’t want us research them because we might interfere with the lore,” McLaughlin said.

But scientists and managers need to learn more about coasters if they’re going to restore the flagging populations of these fish. Overfishing and habitat loss have wiped them out of the majority of their range, leaving a just few token populations left on either side of the border. An effort to list them as an endangered species in the U.S. failed in part because the U.S. Fish and Wildlife Service said there wasn’t enough to distinguish coasters from the region’s broader brook trout population, which is doing just fine.

That makes figuring out what makes a stream-born brook trout go coastal an important research question. McLaughlin’s recent work on that issue has involved crossing stream-resident and migratory brook trout and keeping a very close eye on their offspring in the lab. The idea was that observed differences in behavior (for example, whether a fish in an empty chamber moves or stays put) might be linked to a migratory or stream-resident parentage. That would hint at genetic difference in coasters, suggesting managers could raise more brook trout with coaster genes in hatcheries and let them loose in streams that formerly supported them.

But McLaughlin said they haven’t really seen any link between what the kids are doing in the lab and what the parents are doing in the real world, undermining the idea of a separate set of genetics. That suggests the trigger may be environmental, that something going on in the streams pushes the coasters out.

So what’s happening in the environment? McLaughlin and his research group turned to computer simulations to come up with a plausible-yet-unconfirmed explanation. When big female coasters come in from the lake to spawn in the tributaries, they’re carrying a lot of eggs that turn into a lot of baby brook trout. If the streams don’t have enough food to go around, the competition will drive some of those brook trout out into Lake Superior, where they adopt the better-eating coaster lifestyle. The flip side of that is if competition drops below a certain threshold (say, if fewer spawners return or a river beings producing more food), “the population collapses into just a purely tributary-resident population,” McLaughlin said.

And that’s no fun, especially for anglers who travel to the area to fish for a class of brook trout that — here comes the lore again — supposedly produced the species’ world record in Ontario’s Nipigon River. The compelling mythos around these big, secretive brook trout would likely play a role in an effort to build an economy around a recreational coaster fishery. But scientists are going to have to continuing uncovering the coaster’s secrets if that fishery is ever going to return.

“If we’re going to try to restore the migratory form out in the lake for a recreational catch-and-release fishery,” McLaughlin said, “how are we going to do that if we don’t understand what they are and the mechanisms that lead to their existence?”