Amphipods, commonly known as scuds or freshwater shrimp, are small crustaceans that play an outsized role in aquatic food webs. They are typically under a centimeter long and possess a distinctive, laterally compressed body shape, earning them the nickname “sideswimmers.” Scuds thrive in freshwater environments, particularly in clear, clean, and often cold waters like spring creeks and temperate lakes. They are often found foraging among aquatic vegetation, leaf litter, and soft substrate.
Scuds function as primary consumers, feeding primarily on detritus, decaying plant matter, and algae. By consuming this organic material, they transfer energy from the bottom of the food web to higher trophic levels. Their presence indicates a healthy ecosystem, and they serve as a continuously available food source for many species of fish.
The Primary Game Fish Predators
Trout and char species often rely on scuds as a primary and consistent food source throughout the year. In certain spring-fed streams and lakes, particularly those with abundant aquatic vegetation, scuds can constitute a significant portion of a trout’s diet. These crustaceans do not “hatch” like mayflies or caddisflies, which means they are available for consumption even when other insect activity is dormant, such as during the winter months.
Scuds are so heavily consumed by trout that they directly influence the fish’s physical characteristics. The pink or orange flesh color commonly associated with wild trout, such as Rainbow, Brown, and Brook Trout, is often a direct result of ingesting carotenoid pigments found in scuds. In some productive stillwaters, scuds can account for up to one-third or one-half of a trout’s total diet. This rich, year-round forage enables fish to grow to larger sizes quickly.
Bass, including Smallmouth Bass, are highly opportunistic predators that also take advantage of scud populations. While larger bass may prefer more substantial prey like crayfish or smaller fish, juvenile and smaller adults actively target scuds. Scuds are frequently eaten when they are the most readily available benthic organism. This consumption is highest in the littoral zones of lakes and rivers where scuds congregate in weed beds and rocky substrate.
Opportunistic and Other Freshwater Eaters
Beyond the primary game fish, a broad array of freshwater species consume scuds, especially when other food sources are scarce. Panfish, such as Bluegill, Crappie, and Perch, are generalist feeders that regularly include these small crustaceans in their diet. Bluegill, for instance, are known to feast on scuds as they forage through vegetation.
Crappie and Perch also opportunistically graze on scuds, particularly smaller individuals that venture away from cover. These fish are highly adaptable and will adjust their feeding behavior to whatever prey is most numerous and easiest to catch. Bottom-feeding fish, including various species of Suckers and Catfish, will also inadvertently consume scuds as they sift through the substrate and detritus.
In controlled environments, such as aquariums, many smaller fish actively hunt scuds, which helps to keep the population in check. Species like Corydoras catfish and various small tetras forage for scuds in the substrate and among plants. This widespread consumption highlights the scud’s foundational role in the food web, serving as a food item for nearly any fish capable of ingesting them.
Nutritional Value and Prey Availability
The popularity of scuds as a food source is rooted in their exceptional nutritional profile. Scuds are packed with protein and fat, making them an excellent energy source for fish that need to build mass quickly. Studies indicate that scuds can contain up to 38% protein by dry weight, which is a significant concentration of nutrients for a small invertebrate.
Their value to fish is further enhanced by their consistent availability. Unlike many aquatic insects that undergo seasonal hatches, scuds are available year-round, continuing to reproduce even under the ice. This steady supply means fish do not have to rely on intermittent feeding opportunities. The scud’s movement, often a slow crawl or an erratic flutter when dislodged, also makes them a relatively easy meal for a hungry predator.