In just about any trout stream in Minnesota, picking up a rock from the streambed will present a potentially impressive display of critters, wriggling and writhing – caddis flies, midges, and mayflies. But there might be more than just insects – if you look closely, you may see another type of organism – one with a rounded, flattened shape, yellowish (or sometimes olive or gray) body lined with antennae, gills, and legs – twisting and turning on its side –  about a quarter to half the size your pinky fingernail. A crustacean known as the freshwater shrimp or scud!

As a stream biologist, I’d often find these little crustaceans when exploring streams as I studied the ecology of trout. Related to marine shrimp, these “freshwater shrimp” are classified in the crustacean order Amphipoda, which includes over 10,000 species, mostly marine. In freshwater ecosystems of the upper Midwest, the only “relatives” they live among are crayfish, which are also crustaceans.

Scuds are fairly common in shallow and slower moving parts of Minnesota streams, often near springs – and especially in softer bottoms (like mud and sand) where leafy debris and vegetation accumulate. In areas with swifter flows, scuds tuck in between rocks to shield from the current. Scuds can be very tiny – often difficult to see with the naked eye – but reach up to 2 centimeters in length. As with all life under water, scuds play a unique role in their ecosystem, labeled as “shredders,” which means they feed on (or shred) decaying organic debris along the bottom. This breaks down and decomposes the organic matter, “recycling” essential nutrients, making them available for other aquatic plants and animals to use for growth and survival.

Scuds remain active year-round, making them a reliable food source for trout – even in colder months – so scud patterns are effective for winter trout fishing. In the streams where scuds do occur, they tend to be in high numbers – meaning that if a fish encounters one scud, it will encounter many – making it a smart and efficient meal. Also, trout are visual eaters, and scuds can be quite large relative to other tiny prey options. Plus, compared to other aquatic insects in the “benthic buffet”, scuds pack a caloric punch – providing energy and nutrition that is essential for growth. In fact, research shows that trout that feast on scuds throughout the year tend to be more robust in size than trout in streams where scuds don’t occur.

Scuds can also be an indicator of healthy aquatic ecosystems. Like all life, scuds depend on oxygen to survive, “breathing” underwater by absorbing dissolved oxygen through their gills. Highly polluted water often lacks enough oxygen to support scuds. And, unlike underwater insects which develop into flying adults that emerge from the water to reproduce and disperse, scuds aren’t highly mobile. Once they disappear, they may not return, sending a warning sign about diminished water quality.

When fishing scud patterns in winter, it’s important to present them near the streambed where scuds are typically found. Using a longer tippet – 5 feet or more of 5X – and adding weight to your line can help achieve the necessary depth. For instance, pairing a size 12 scud with a few wraps of non-lead wire, trailed by a smaller midge larva, can be effective. Placing a small split shot about 10 inches above the scud adds extra weight to reach deeper waters.

For a visual guide on tying good scud fly patterns, you might find this video helpful: