Let the lunkers go: New science explains why big fish matter so much

PREDATION

I was in awe listening to Tufts and Baccante explain how smaller, younger fish acquire behavioural knowledge from BOFFFFs. It implies that if we remove the big fish from the population, we not only eliminate the highest quality reproductive potential, but also years of accumulated growth and critical fishy acumen.

As a case in point, Tufts points to a poorly run bass tournament in 2019 that resulted in 195 dead fish. After he and his team from Queen’s helped conservation officers investigate the incident, they aged the dead fish at their lab. The findings were disturbing. “We determined that the population had lost 1,700 years of growth.”

While he points out such incidents are rare, Tufts stresses the need for ensuring big fish are properly live-released. “I’ve spent a lot of time as a fish physiologist, trying to educate anglers on the value of those fish to the population, trying to get them to be extra vigilant,” he says. “Big fish are not just a commodity in a tournament. We’re lucky to use them and we need to look after them.”

The loss of accumulated growth aside, removing or greatly reducing the number of BOFFFFs in a fishery can also flip the table on the predator-prey relationship. In the absence of big fish, the prey they previously consumed can thrive and take over instead. And once that happens, Tufts says, it can be difficult for walleye, bass, trout, northern pike and muskellunge to regain control. The fish can even reverse roles, with the predators becoming prey in their earlier life stages—think hordes of smelt gobbling up precious young-of-the-year lake trout and whitefish.

In the absence of big fish, the prey they previously consumed can thrive and take over instead

Baccante recounts once discussing this same problem with the fisheries folks in Minnesota. They were desperately trying to restore the walleye population in a lake where the big fish had been overharvested, and the sucker population had since ballooned. “Some people blamed the suckers, but that wasn’t what made that fish community go out of whack,” he says. “The suckers dominated because their predator was gone.”

And again, restoring the balance in such a situation can be difficult. “Just removing the suckers, if you don’t have the conditions for the walleye to come back, doesn’t matter,” Baccante says. “Anyway, they weren’t very successful because suckers are very prolific, and when you start to remove them, they respond to exploitation and just produce more eggs.”