When trout sip naturals an inch from your delicately tied dry fly, deftly pushing it aside with the tip of their nose, they seem like wily, cunning animals, with the eyes of an eagle. But other times trout seem less wily, less cunning, feeding mindlessly like a Sumo wrestler on speed. I think this is in part due to the type of food available. A dense spinner fall of Tricos, for example, brings out the sharp-eyed, discriminating side of trout, while a large fall of ants turns them into reckless gluttons. Sometimes I like the challenge of fooling a cunning predator, other times I enjoy the ease of dangling hamburgers in front of a Sumo wrestler.

This column is about dangling hamburgers, fishing the type of food that trout find hard to pass up, and will often stuff themselves on. It’s about catching large trout! In short it’s about amphipods and isopods.

What do some of the most famous trout streams in North America have in common? Streams like the Big Horn, the Green, the Bow, the San Juan, the Madison, or spring creeks like Silver Creek and Armstrong. These streams produce not only a lot of trout, but also a lot of large trout. Why? First, they all have a good water supply of near ideal temperature for trout growth. Second, they all have water chemistry that promotes aquatic plant and algal growth. Third, they all have large numbers of aquatic insects. And last, they all have excellent populations of either amphipods or isopods.

Fishing guides on these famous rivers, and other rivers as well, are well aware of the importance of amphipods or isopods. After spending a few days on the Big Horn, for example, it was clear what the guides were doing to get their clients into trout - a ten foot leader, a strike indicator, a split shot, and at the end of it all a size 12 orange shrimp pattern, a.k.a. amphipod imitation. This formula brought more trout to the net then I cared to admit as I drifted size 18 floating nymphs over not so gullible rising fish.

Amphipoda (commonly called scuds or freshwater shrimp) and Isopoda (sowbugs or cressbugs) are two distinct orders in the class Crustacea. This means they are more closely related to lobsters then mayflies. The greatest diversity and abundance of crustaceans is found in marine waters. The wide variety of prawns, crabs, and lobsters are all marine forms that frequently end up on dinner plates. In freshwater, besides amphipods and isopods, common crustaceans include crayfish, copepods, and water fleas or cladocerans. Overall there are more than 1,000 species of freshwater crustaceans known in North America, most of which are small planktonic forms such as copepods, water fleas or seed shrimp. These can be extremely abundant and important elements in the aquatic food chain, but their small size make them unimportant to anglers.

Freshwater crustaceans can be easily recognized from aquatic insects. Crustaceans have two pairs of antennae instead of the single pair found on aquatic insects; they have numerous thoracic segments (from 4 to over 40 depending on the order) instead of just three thoracic segments like insects; and there are paired appendages, or legs, on each thoracic and most abdominal segments, instead of only three pair of legs (one pair on each thoracic segment) found on insects. The different number of legs is the single most obvious characteristic for distinguishing crustaceans from insects.

Scuds and sowbugs have similar body characteristics. Both have a head fused with the first thoracic segment to form a cephlothorax, seven free remaining thoracic segments, six abdominal segments and 13 pair of appendages, one on each thoracic and abdominal segment. They are easily distinguished from each other, however, by their different body shapes. Sowbugs are flattened dorso-ventrally (top to bottom) and their legs are held out to each side. Scuds, on the other hand, are compressed laterally (side to side) so that their legs hang down below the body. Adult sowbugs and scuds range in length from 5 to 20mm (1/4 to 3/4 inch) depending on water conditions and species. The Eastern and Midwestern states have a greater diversity of sowbug and scud species than the Western states. Approximately 100 species of sowbugs and 90 species of scuds are known from freshwaters of the United States. The most common sowbug, Asellus communis, is part of the family Asellidae, which contains the majority of sowbug species. The most common scud species is Hyalella azteca in the family Talitridae. This species is abundant across the entire continent. The greatest variety of scuds, however, is in the genus Gammarus of the family Gammaridae. This genus is particularly important in Midwestern and Eastern waters.

Both sowbugs and scuds have similar life cycles. Newly hatched young look like small imitations of the adult. As the young grow they molt their exoskeleton from five to nine times before becoming adults. Adults continue to molt another 15 to 20 times during their life. Periods between molts may be as short as 3 days or over 40 days depending on food, water temperature, and species. For most species mating and egg laying occurs sometime between spring and fall. The number of broods per year varies from one to over 15 depending on the species and water conditions. Hyalella azteca, one of the most common and prolific species of scud in North America, averages 15 broods in a single season. The total life span for most species is one year, though some may live two years or slightly more.

Scuds are much more abundant and common than sowbugs, especially in habitats where trout live. Sowbugs are generally restricted to springs or spring creeks and small streams where aquatic vegetation is abundant (hence the common name cressbug). They are rarely found in ponds or lakes. In contrast, scuds inhabit springs, spring creeks, small streams to large rivers, plus can be very abundant in ponds and lakes. Populations of both sowbugs and scuds can be very dense where conditions are ideal; populations over 10,000 individuals per square meter have been observed. Generally areas with moderate to slow currents and thick aquatic vegetation hold the largest concentrations of scuds and sowbugs. Scuds can also be abundant where decaying vegetation and debris collect. Scuds and sowbugs are scavengers, and consume a wide variety of dead animal and plant material along with leaves, stems, and most aquatic plants. Sowbugs and scuds rarely occur in deep water. Normally they prefer water of a meter or less in depth. Some people believe that scuds and sowbugs thrive in polluted water, however most species thrive best in well oxygenated, clean, unpolluted water.

The behavior of scuds is much more active than sowbugs. When foraging for food, or if washed off the bottom, they swim in a smooth gliding motion. While swimming their bodies are stretched out straight rather than in the common curved position they take when resting or when held in the hand. Sowbugs, in contrast, do not swim but crawl slowly through vegetation or debris as they feed. Both scuds and sowbugs dislike bright light, and will remain well hidden among vegetation during the day. At night, or on cloudy overcast days, they become more active and openly search for food.

Fishing scud or sowbug imitations can be extremely effective, but because their populations often vary widely between streams and lakes or within different sections of a stream or lake, some simple collecting is necessary before fishing. Pull up some weeds or swish an aquarium net through aquatic plants and debris to quickly determine if scuds or sowbugs are present and abundant. Also check the general size and color of the specimens to help select the best pattern.

Basic nymph fishing tackle and techniques work well when fishing scud or sowbug patterns. In streams use a floating line with an eight to ten foot leader. Slightly longer leaders may be necessary in very clear spring creeks. In lakes a floating line can often be used since scuds are normally found in shallow water near the shoreline, but a sink-tip line may help keep you fly near the bottom, where the scuds are, when fishing in water more than five or six feed deep. Leaders for lake fishing with a floating line may need to be 12 to 15 feet long to prevent spooking fish feeding in shallows. When using a sink-tip line a leader four to six feet long will normally be best.

For stream fishing an up and slightly across presentation usually works best. Fish the fly with a dead drift through channels in aquatic plants or along the edges of plant beds. When there are no aquatic plants to speak of, drift your fly along the bottom in areas where the current slows, like small pools, eddies, and quiet water near the bank. Strikes will often be subtle, and a strike indicator can help considerably in detecting a fish.

In lakes only amphipods are important. Cast your fly along the edges of aquatic plant beds or piles of debris near the shoreline. Let the fly sink close to the bottom then retrieve it in slow smooth 4 to 6 inch strips. Vary the speed of retrieve to find the best action, but be careful not to retrieve too fast. Strikes may come anytime, but they are most frequent during the pause between strips.

Many patterns have been developed for both scuds and sowbugs. Most are very simple to tie. Check a fly pattern book for specific versions of these flies. Because these patterns are normally fished in relatively slow, shallow water, weight them sparingly or not at all. Scuds and sowbugs vary widely in size and color, so be sure to check the naturals from the water you're fishing in order to select a matching pattern. One last point: Since scuds and sowbugs are underwater and available to fish throughout the year, you can give them a try anytime of year, especially when other insect activity is sparse.