A recent joint study by the Minnesota DNR and groups from Wisconsin, Utah, Michigan, Illinois, and Canada focused on the diets of various gamefish. Its purpose was mainly to guide management decisions in issues like how successful fingerling stocking is and how vulnerable native species are to introduced predators. When the scientists studied the diets of crappie, bass, rock bass, muskie, pike, and walleye, though, they made a discovery that anglers can take advantage of.
Natural selection pushes populations towards maximizing energy gain while minimizing energy loss. Optimal foraging theory predicts a linear relationship between predator size and prey size, simply meaning that bigger fish should eat bigger, more energy-rich prey. In theory, the only limit to how big a prey item can be is the predator’s gape – or how wide it can open its mouth.
Unsurprisingly, the study showed that the largest prey items were consumed by the biggest fish. However, the diets of these larger individuals were comprised mostly of relatively small prey. The trend was represented in all species and in fish of all sizes. As fish grow, they appear not to shift their hunting focus to larger prey. Instead they merely expand their diet to encompass bigger prey in addition to the smaller prey that they’ve always hunted.
The reason for this is once again tied into evolution and energy tradeoffs. The most successful fish aren’t necessarily the ones who eat food items with the largest gross energy. The most successful fish are the ones whose overall diet provides the largest net energy.
Fish have evolved to eat mainly the most common and easiest to catch prey. As baitfish – and young of the year gamefish – get older and bigger, they become more experienced with escaping predators, they develop higher speed capabilities over short distances, and their visual acuity enhances. They are also much less common than their younger, dumber counterparts.
Fish that solely hunted these larger baitfish would most likely die very quickly. Between low encounter rates and high escape probability, bigger prey items are not usually the most energy efficient targets. Waiting around for them or even seeking them out could mean a catastrophic amount of energy wasted if none are found. Even if the predator finds large prey, it is more likely than not that they’d waste more energy trying to catch it than it’s worth.
The researchers also found that if the predatory fish did eat relatively larger fish, those prey items were more likely to be fusiform fish – like shiners and minnows – than laterally compressed fish – like sunfish and shad. Whether this is because tube-shaped fish are more common, easier to swallow, or not as good at avoiding predation is still unclear.
In most fishing situations, dialing down the size of your lure is likely to trigger more bites. This will ensure that you’re not excluding any experienced fish that have learned not to bother with big prey or any small fish that simply can’t open their mouths wide enough to suck in the lure. That isn’t to say that large lures don’t have a place, though.
One of the best times to use a big presentation is during the winter, when the water temperatures are about as low as they’re going to get. Fish with lowered metabolisms will be sluggish, and their top priority will be to conserve energy. If you’ve already attracted fish to the area with fluttering, reactive baits, and you’re not triggering any strikes with low-profile lures, switch to dead-sticking a bigger bait. An energy-rich prey item that doesn’t appear to be able to elude a predator makes for a tempting target.
To be published in Outdoor News Minnesota, January 2018.
Gaeta JW, Ahrenstoff TD, Diana JS, Fetzer WW, Jones TS, Lawsom ZJ, et al. (2018) Go big or…don’t? A field-based diet avaluation of freshwater piscibore and prey fish size relationships. PLoS ONE 13(3): e0194092.