New Online Tools Says to Catch the Big One, Think Small

When it comes to fish food webs, the basic narrative is that the bigger the fish, the bigger its prey. In fact, one of the measurements scientists often use to determine a predator’s potential diet is “gape size” which calculates, quite literally, the biggest possible thing a fish can fit into its mouth.

Jereme Gaeta with a largemouth bass.

But such measurements “don’t take the natural ecology of predation into account,” says Jereme Gaeta, an assistant professor of fish ecology at Utah State University.
In a paper he recently published in the journal, PLoS ONE, Gaeta and a team of researchers compiled and collected data on the diets of seven different species from across the Midwest – crappie, rock bass, smallmouth bass, largemouth bass, northern pike, walleye and muskellunge. What they found was that, with one notable exception, big fish are often quite content sticking to the diet of their younger years.
For example, the average size of the prey fish that a 10-inch walleye eats is in the neighborhood of 2 inches long, which is right in the ballpark of the preferred prey size for an older, trophy walleye of 30 inches or more.
“Most of the time these fishes really are eating small prey,” Gaeta says, explaining that the reasons for this are likely two-fold. “First, there’s a lot of small prey out there. Secondly, as prey get larger, their ability to evade predators increases. So not only are there fewer large prey, but they’re harder to catch. If you’re hungry and have to either go hunt for a pizza or there’s a box full of granola bars in front of you, you’re going to go for the granola bar.”
An Online Tool for Fishermen and Fishery Managers
Gaeta believes that this information could be illuminating for anglers and fisheries managers alike and has developed an online tool that lets users select the size of the species they’re interested in to see the size and body-shape of their preferred prey.
“If anglers are going out to catch walleye, maybe they should throw a smaller lure,” Gaeta says, advice that holds true for six of the seven species covered. The exception is the legendary muskellunge, or “musky.” If you select a 50-inch musky, the interactive model provides results that could be taken as an excuse for “going out and spending a lot of money on big expensive gear,” Gaeta jokes. “Because that’s what they eat. In case of musky it really is ‘go big or go home.’”
The Predator-Prey Interaction tool in action. Big walleye still prefer small meals. Courtesy: Lake Ecology Lab

Besides helping anglers land the big one, Gaeta hopes this model of predator-prey interactions can help inform management decisions.
Across the country, states spend millions of dollars every year raising various species of fishes in hatcheries and then stocking them into rivers and lakes. It’s much cheaper and easier to do this with younger, smaller fish, but if those fish are going to be at a length that is right in the “sweet spot” of the dominant predators in a particular water body, there is unlikely to be an acceptable return on investment.
In other words, “managers can say, ‘Okay, I want to stock walleye into this lake and I know there are largemouth bass and northern pike in this lake and I want to wait to stock walleye until my fish get bigger than what the predators eat,’” Gaeta explains. Using his predator-prey interaction model, those managers can see that, if they stock 4-5 inch walleye, they’re likely to make up, say more than 50% of the diet of those predators. But if hatcheries can hold on to those fish and wait for them to grow just an inch or two longer, that percentage drops and many more walleye will survive.
The online tool can also help “increase the reality” of fishery studies, Gaeta says. For example, northern pike are an invasive species in Western lakes and often present in habitat where managers are concerned about endangered native species. “Our model can say, ‘Okay if you’re a 35-inch northern pike, you’ll likely be eating certain ages of individuals of these species,’” he explains. Managers can then take the next step of calculating how many of those endangered species are likely to end up in pike stomachs or, in other words, the overall impact of predation on that endangered species’ population.
“My hope is that managers can use this to inform their stocking,” Gaeta says. “A lot of times, it’s trial and error and that’s why scientists use models, because we do the trial and error over the course of a week on our computers.” It’s an approach Gaeta prefers to the alternative – spending lots of money on a fish stocking program and then waiting years to see if it all pans out.

A copy of the PLoS ONE paper is here.

You can try out the Predator-Pery Interaction Tool here.