“Why is any fish small?” asked Mark Abrahams, a zoologist and associate dean (research) in the Faculty of Science.
“It doesn’t make a lot of sense. There are fewer food items for you to consume and all sorts of predators to deal with. There’s got to be an upside. So that’s what we’re looking at in my lab: what’s the upside in being small.”
Funded by NSERC and Manitoba Hydro, his research into predator-prey relationships in various aquatic scenarios will aid conservation efforts, like those focused on revitalizing Lake Sturgeon population.
Small fish contend with some physiological factors that dim their survival hopes: they readily fit inside many mouths, they can’t swim as well as bigger fish, and they have less bodily space to invest in predator-sensing organs, like eyes.
But like soap operas of the terrestrial world, the underwater drama supplied by fathead minnows and yellow perch – Abrahams’ main experimental subjects – is one with counterintuitive plot lines playing out in unusual settings.
To learn more about the players, Abrahams built a contraption made of three interconnected plastic barrels with sensors that record each fish’s comings and goings. One barrel was a hypoxic environment (having low oxygen levels), and the other two were normoxic (having normal oxygen levels).
In marshes, like the one at the Delta Marsh Field Station, dissolved oxygen levels are often dangerously low. But given the choice, what environment would fish choose?
In his experiments, contrary to what fish physiology literature suggests, prey preferentially chose hypoxic environments while predators didn’t. Indeed, Abrahams observed adult yellow perch becoming dysfunctional in hypoxic water in about 15 minutes.
“They just didn’t look happy,” he said. “They looked like someone who’s been to a party and had too much to drink. Eventually they began to roll onto their sides. But the fathead minnows could care less. They don’t really get affected at all.”
Later experiments led Abrahams to conclude that both body size and species type determine a fish’s ability to tolerate hypoxia. Smallness, it turns out, allows for refuge in areas once considered valueless.
“Some of the critical environments may not be the ones you think are critical to sustaining your fish populations,” Abrahams said. “We seem to have a certain bias on what makes a nice fish habitat. I think this shows that is a very dangerous assumption to make.”
Abrahams’ lab was also surprised by another environment the little fish chose: turbid, rather than clear water. (It’s odd to spend much energy producing good eyes only to flee to low visibility areas.) A graduate student is presently mapping out detection probabilities for both predator and prey in turbid water to learn more about this.
So how does this work benefit Lake Sturgeon, an ancient fish with threatened existence despite having tremendous fecundity? Well, a brood consists of thousands of fish each varying in size and each capable of living upwards of 90 years. The smaller offspring were long considered the runts, but Abrahams now reckons it may be a parental strategy to ensure good diversification. After all, ecosystems can change a lot in 90 years and a petite physique could end up being the best asset.
Armed with such knowledge, conservationists can now better select what fish will succeed in certain environments allowing the aquatic soap opera another season to play out.