A River Never Sleeps

In many trout lakes scuds and leeches can sometimes be seen moving through the shallow waters close to shore. They are easy to overlook, because they don't make their presence obvious with big surface hatches that draw rising trout. But they are far more important in the food chain than a casual observation might suggest.

Scuds are freshwater crustaceans, and are commonly referred to as freshwater shrimp. As far as the angler is concerned, the two most important kinds of scuds belong to the genera Gammarus and Hyalella. Gammarus scuds are the larger of the two, averaging 1.0 to 2.5 cm (1/2 to 1 inch) in length. Hyallela scuds average 2 to 8 mm (1/8 to 1/4 inch) in length. At rest, both Gammarus and Hyallela are flea-like in appearance.

Features useful in the identification of Hyalelela include the presence of two projecting spinules on the dorsal surfaces of its posterior body segment and primary antennae that are typically shorter than the second pair.

In gammarus scuds there are numerous hairy filaments on the dorsal surfaces of the last three abdominal segments, the primary antennae are longer than the secondary pair, and there are tiny posterior appendages on the abdomen.

It may seem difficult to tell the two types apart, but once you have sorted out a few of them the differences become apparent.

Scuds come in a variety of colors ranging from yellowish gray, olive, green, tan, grayish, pale brown, creamy yellow, pink and even bright red. Color in a lake depends on the surrounding habitat, the food eaten, and moulting status.

The colour of rainbow trout can often be a clue to a rich scud diet. Typically the flesh of the fish will be dark orange or red. The tips of fins are often colored a bright orange, and the sides of the trout may be a vivid scarlet red color. This profusion of orange and red in flesh, skin, and fin tips is attributed to a fatty organic compound called carotene. Carotene is a pigment, which is produced by scuds, and which accumlates in the fish after they eat the freshwater shrimp.

Scuds can be found wherever there is decaying plant material, and dead animal life for them to scavenge upon. They are most active from dusk to dawn. During day they tend to seek out dark shadowy areas to rest. Even so, one can almost always find a few clambering and darting among the leaves and stems of submerged grasses, rushes, bur-reed, bulrushes, pond-lily, and pondweed species (Potamogeton spp) - especially if you stir up the bottom or lift a rock or log. Scuds typically live in shallow waters up to 7 m deep but can be found down to depths of 20 metres. Greatest numbers are found in the first 1 - 2 metres.

Adults mate several times during the year. A female can produce up to 7 broods, and up to 20,000 offspring in a single year. Scuds hatch from eggs, the young being miniature versions of the adults. Growth is by series of moults. The entire life cycle is spent under water.

Locomotion is by swimming. At rest the back is humped, while swimming the body is completely outstretched. Legs located along the abdominal segments propel the scuds through the water. Typically, a scud will swim up from 15 to 30 cm, then rest in a curled position, slowly settling downward, before darting forward again. It is this erratic movement that a flyfisher must mimic when retrieving a shrimp imitation, like Werner's Shrimp pattern. The retrieve should be relatively slow, and steady for up to 15 to 30 cm, and there should be frequent stops interspersed with quick short movements. When fishing a scud imitation during bright daylight concentrate on shadowy areas along weed beds, around rocks and submerged logs.

Leeches are another common, abundant, and year-round animal which are available to trout.

They are a kind of worm or Annelid, and typically have a dorso-ventrally flattened, ribbon-like segmented bodies. The body is muscular, and they have anterior and posterior suckers, and a disc shaped mouth.

Size varies depending on whether the body is contracted or fully extended. For example, a leech which is only one inch long when contracted may reach four inches long when extended. At rest most leeches resemble a blob-like jelly mass. Locomotion is by crawling along the bottom in an inch-worm fashion, or by swimming in a wave-like undulating manner through the water.

Like scuds, leeches come in a wide variety of colors. The most common colors are black, brown and maroon. The dorsal surface is almost always darker than the ventral surface.

Leeches feed by preying on snails, insects, worms and other small invertebrates; or by scavenging on dead animal matter. They frequent shallows where plants, rocks or submerged debris offer food and hiding places.

Leeches are easily found by turning over submerged rocks. Leeches are most active from dusk to dawn, and instinctively seek out dark areas during the day. Large rainbow trout; ie ones that are 20 plus inches long, seem to love leeches and often hit leech imitations very aggressively.

My favorite pattern is a black Marabou Leech (#8 hook size). The soft fibres of marabou wave easily in water, and the action wonderfully simulates the snake-like undulations of a swimming leech. Let the fly sink to the bottom and then slowly began to retrieve it. I began with a short, sharp, jerky retrieve. . . ever hoping for the slashing take of a big rainbow.