Freshwater Shrimp




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GAMMARUS SHRIMP

article by Ron Newman


Scientific Name:

  • Class - Crustacea. Order - Amphipoda. Family - Gammaridae

Common Names:

  • Gammarus Shrimp, Shrimp, Scuds, Side Swimmers

General:
  • There are two types of freshwater Shrimp in our interior lakes. These are the Hyella shrimp and the Gammarus shrimp. At a casual glance, the only visible difference is in their size. The Hyella never grow as large as the Gammarus. Trout feed on both but a large Hyella imitation would be tied on size 18 to 20 hooks. Except for this size difference, the following discussion is applicable to both shrimp. Also note that these are not Fairy Shrimp.
Life Cycle:
  • The shrimp will mate several times per year. During this process the male carries the female on his back as they are swimming. The female carries about 50 fertilized eggs in her egg pouch and these are orange in color and show through her semi-transparent body. The young shrimp hatch within the egg pouch and emerge as fully developed young shrimp but microscopic in size. The young develop an exo-skeleton but molt this a number of times as they increase in size. Sometimes these discarded exo-skeletons can be seen floating on the lake surface. Since the shrimp have no natural defense mechanisms, except camouflage, they usually hide in the Chara weeds and are more active during periods of low sunlight or after dark.
Apperance:
  • The Gammarus Shrimp are semi-transparent and are laterally compressed. That is to say that they are thicker from top to bottom than they are from side to side. They have two pair of grasping legs near the head, five pair of legs for walking at mid-body, three pair of ciliated leg like appendages for swimming on what would be the abdomen, and one pair of hind legs to assist with eating in a curled position at the tail. They have eleven body segments (one for each set of appendages). The head has two longish antennas and the tail section has two short protrusions.

Size:

  • Up to 20 mm (3/4 inch) in length but adults usually average about 15 mm (1/2 inch) in length.

Colour:

  • What they have been eating and their immediate surroundings usually determine the coloration of shrimp. Being semi-transparent, the entire digestive tract of the shrimp shows through their shell. Although they scavenge on animal material, their primary food is often plant material and blue-green algae. The plants and algae causes most shrimp to appear as various shades of green, and sometimes into shades of bluish hues. However, the immediate surroundings can sometimes offset that color. For example, in lakes with reddish brown bottom mud, the mud is often ingested with the algae and the shrimp tend to take on the coloration of the mud. Whether plants or mud, the shrimp is usually well camouflaged with it's surroundings. Worthy of note is the orange coloration of the eggs carried within a pregnant female.
Movement:
  • The shrimp will swim for 5 to 10 inches and then stop to rest and breath. When stopped, they sink toward the bottom, usually in a curled position. After a few seconds they will uncurl and swim another short distance. This swimming and sinking is very erratic and is a good way to retrieve a fly representing the shrimp. The legs of the shrimp are constantly in motion to circulate water over their gills. When mating, the shrimp can be seen swimming in tandem, one on top of the other. Sometimes flies are tied to imitate a mating pair of shrimp.
Habitat:
  • Gammarus Shrimp are generally scavengers and feed on plant and animal material that have settled to the bottom. On occasion they are predacious and attack other injured or stressed aquatics. They tend to hide among the weeds and under rocks or debris when not traveling or mating. They can be found at most depths of the lake but are most common in water less than 35 feet deep. Shrimp need calcium to build their shells or exo-skeleton. Our lower elevation lakes tend to be high in dissolved salts and calcium and those lakes are generally very productive in shrimp. Lakes with Lily Pads tend to be more acidic and usually have lower populations of shrimp.

Importance to Fly Fishing:

  • Freshwater shrimp are the most important food source for trout in our interior lakes. Throughout the fly fishing season, feeding samples show that 35% of the trout's daytime feeding consists of shrimp. This increase to 42% for those fish feeding in the evening or at night. Whether in the day or evening, no other aquatic food source matches the shrimp in the trout's total diet.
  • The shrimp are Crustaceans and unlike the insects, they do not go through the visual transformations of larvae, pupa, and adult. They do not 'hatch' into something else and never have a terrestrial form. The newly hatched shrimp is just a smaller version of the adult. So, flies tied to imitate the shrimp should come in a variety of sizes for the same pattern and color combination.
  • Although they don't 'hatch', the shrimp do have 'peaks' as a food source for the trout. They are most abundantly fed upon in the fall. Beginning the first week of September, shrimp are eaten in large numbers and are, by far, the main component of the feeding samples. Quantities 'peak' about the second week of October and then steadily decline while still remaining the primary food component. Although I don't fish or have any records into the late fall (when ice is forming on the fly line in late November), I presume this trend continues until ice-on.
  • In terms of total numbers of shrimp consumed, there are three other 'peaks' that are about equal in importance. These are immediately after ice-off, the last week in May, and the third week of July. The fly fisher often ignores these three periods. Each period seems to have reasons why that particular period is ignored. Right after ice-off the Waterboatmen are steadily increasing in numbers. During the last week of May till the first week in June the Chironomid are reaching their peak hatches. During the third week of July, the Caddis hatches are sufficient in number to draw attention to them, and away from the shrimp. If, during these periods, what appears to be the main food source isn't working, then try a shrimp pattern.
  • The fourth most important 'peak' is when the numbers of shrimp are down but the percentage of shrimp consumed is fairly high. This occurs in mid-August, during the summer downers. The water is warmer, the fish are deeper, and the total numbers of aquatic 'bugs' are down. However, as a percent of feed consumed, the shrimp are at a peak with the exception of the late fall fishing.

Flies:

  • Tying a good shrimp pattern is a challenge for all fly fishers. I have only seen about two, maybe three, patterns that come close to a 'good' imitation of a shrimp. Try to imitate a laterally compressed body with lots of legs or at least the impression of legs. Believe me, it isn't easy to get the right look. Colors should usually be the greens or reddish browns and I often tie a 'back' over the top of the fly that is darker than the bottom of the body.

Fishing:

  • If you manage to come up with a good imitation of the shrimp, there are quite a number of ways to fish the pattern. The fly can be tied weighted or non-weighted. With a weighted fly, the imitation can be fished with a floating, or a sinking line. Retrieve the fly about 5 to 10 inches to bring it 'up' in the water column, then let sink for a second or two and then repeat. Try variations on this to see what is working. With a non-weighted fly, you will usually be fishing on a sinking line. My favorite is a slow sink (Type 1) which allows the slowest retrieve while still staying out of the sunken vegetation.

Hint:

  • Try tying your fly out of a buoyant material such as deer hair. Let your sinking line go right to the bottom while your buoyant fly rides slightly above the weed cover during the retrieve. With this approach your fly will even occasionally dip into the weeds. The combination gives an erratic movement and is a very effective way to fish shrimp patterns.

Last Notes:

  • Try fishing the shrimp patterns 'on' or near the bottom. This will normally produce success. Larger fish have been known to swim along the bottom, sweeping their tails to stir up the bottom materials, and then returning to see if there are delicacies (such as shrimp) that suit their fancy. More than once I have been broken-off with this 'on-bottom' retrieve. Freshwater shrimp are perhaps the least understood of the aquatics, while being the most important food source of the Kamloops Trout.

Recommended Fly Patterns:

  • Kevin's Orange Shrimp
  • Mylar Shrimp
  • Green Baggie Shrimp
  • Pregnant Baggie Shrimp
  • Crystal Shrimp
  • Orange Crystal Shrimp
  • Little Guy
  • Werner Shrimp

Be sure to read other articles by Ron Newman

Study Other Insects | Tip & Techniques | Study Fly Patterns


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Freshwater Shrimp