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Big Flies For Big Fish... right?

Big flies equals big fish, at least that's what we're all told. Is it actually true?

Due West Anglers

• 5 min read
When fly fishing for muskie, northern pike, largemouth bass and smallmouth bass, are larger flies better for catching big fish?

March 2025

Big flies = big fish. I've heard it from anglers reassuring themselves at the checkout line in fly shops, when all else fails on the river, and echoing off the walls of the pub after hard fought days. But, perhaps surprisingly, you'll also hear it from scientists researching how fish forage.

Unless you're directly able to study animals in a controlled laboratory setting, conclusions are difficult to draw about behavior in the wild. Why do certain animals choose the food they eat? And doubling down, why do they select item A over item B?

Optimal foraging theory emerged as the most likely answer to these types of questions; proposing that natural selection favors foraging strategies that maximize energy gain, while minimizing the energy acquisition costs. For gamefish, the best way to min/max calories is to be opportunistic and take advantage when meals present themselves. Meaning, fish can't be too picky. So, under optimal foraging theory, when a big meal swims by, a fish might be better off taking full advantage of the higher calorie count than ignoring the offering. In fact, this theory suggests that prey size should increase linearly with predator size. Anglers know this all too well, and that’s where we get the expression: big flies for big fish. We've seen trout pass on tiny midges in favor of stonefly nymphs, or go nuts over a salmonfly hatch, grasshopper emergence, or even a well skittered mouse fly after hours.

But, assuming that as predators grow larger, prey size also increases is still just an assumption. And some scientists now have evidence to suggest these assumptions are inaccurate for freshwater ecosystems. But in order to explore their findings, we've got to leave trout behind.

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Using diet data (stomach samples) collected between 1988-2003 for crappie, largemouth bass, muskies, pike, smallmouth, and walleye, scientists identified relationships between predator size and prey size. Based on their findings, for all fish, except large muskellunge, predators most commonly consumed prey between 0.5 to 3 inches. Maximum prey size ranged from 32-46% of the predator's length.

The study provides an interesting graphic, which summarizes things nicely. They show quantile trends in prey size for each species. You can think of a quantile much like a percentile, where the 50th quantile has just as many values below as above. Here, anywhere you see a flat 50th quantile line (slope = 0, aka is flat) this illustrates that small and large fish are eating prey about the same size. If the grey quantile lines show a positive slope (slope > 0, aka line goes up from left to right ) that suggests that bigger fish eat bigger prey.

Let's take a closer look at one species chart to explain what's going on more clearly. Exploring the trends in smallmouth bass, notice how the 50% quantile bass (graph on the left), around 375 mm (14") eats about the same number of prey items that are larger and smaller than 60 mm (2.25"). That's represented by the 50th quantile grey line. Now consider the 95th quantile prey size for the same size fish. The size of that prey/fly is somewhere around 100 mm (~4"). For a 14" bass, only 5 in 100 fish ate a fly larger than 4 inches... statistically speaking.

Comparing prey length for the average bass (left) compared to a trophy bass (right)

Now consider a fish at 475 mm, around 19 inches (graph on the right). That's a trophy smallmouth. That fish's 50th quantile prey length is approximately 70mm (~2.75"), and not much larger than the 50th quantile prey size of a 14 inch smallmouth, which remember was only 2.25". And looking at the 95th quantile, only 5 in 100 trophy smallmouth eat a fly that's 140 mm (5.5").

Summarized, based on these findings, a trophy smallmouth bass would eat a 2.5 inch fly 50 times out of 100, but a 5.5 inch fly only 5 out of 100 times.

What This Means for Anglers

Without getting too bogged down in the numbers and specifics of the study, the big take home for anglers depends on which fish you're chasing. If you're after muskie, big flies = big fish. The authors made special note to point that out. But for smallmouth bass and pike anglers, the largest fish still mostly eat about the same-sized prey as their smaller counterparts. So, putting on the big fly doesn't guarantee a big fish, it actually brings down your overall odds of catching something. But when you do hookup, it's less likely to be a dink. Alternatively, you may need to sift through many smaller fish when fishing smaller flies, but that doesn't necessarily reduce your odds of running into a larger predator. You've likely heard people suggest to increase your fly size to decrease the number of small fish that will have a run at your offering, but doing so drops your overall odds of catching any fish.

If able, you may want to focus your tactics on finding big fish water and worry less over fly size.

What This Means for Optimal Foraging Theory in Freshwater Fish

Why doesn't the optimal forage theory axiom 'big fly = big fish' seem to apply based on the data collected in this study? The authors suggest that search time, encounter rate, opportunity, and prey mobility all play restrictive roles. Plus, growing large, energetically favorable prey fish scarcely occurs in many ecosystems. Prey often gets eaten or growth is restricted by the environment before it can reach a certain size, as would be preferred for larger predators.

But you know when that does happen? Large energetically favorable prey fish show up when dumb hatchery rainbow trout are stocked. Watch for stocked rainbow trout, they offer that secret ingredient that supercharge how fish grow massive.

This study does have a survivorship bias, meaning fish may have tried to eat larger prey but couldn't fully swallow them. For fly anglers, that means these results skew to smaller flies, but doesn't necessarily mean that larger flies won't work.

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